Injection Moulding
Content
INJECTION MOULDING
INTRODUCTION
It is a process manual, semiautomatic or fully automatic for the manufacture of plastic articles
and it is the basic, fast, economical and major processing method and require little or no
subsequent finishing operation or rework for the product.
Plastic moulding especially thermoplastic items may be produced by compression moulding
methods, but since they are soft at the temperature involved, it is necessary to cool down the
mould before they may be ejected. This would not be convenient since on a run of production, it is
desirable to keep up the temperature to avoid the slow and protracted operation of alternative
heating and cooling . Besides much time has to be spend for accurate charging. This has resulted
in a process of injection moulding technique.
Injection moulding differs from compression moulding is that the plastic material is rendered
fluid in a separate chamber or barrel, out side the mould is then forced into the mould cavity by
external pressure. The mould is cooled and split, the two halves being locked by pressure to resist
injection force during moulding and opening automatically when the mould cavity has been filled
by the molten plastic. The cooled, hardened or vulcanized article is then ejected from the mould
and the process repeated.
In simple machines, it consist of a heated barrel with hydraulically, pneumatically or manual
operated ram, an opening in the rear of the barrel there is a hopper into which measured amounts
of plastic material generally in the form of granules are fed. The material is then forced under
high pressure by the plunger into the heating zone of the barrel, pushing out an equal weight of
molten plastic into the mould. A torpedo shaped distributor and the nozzle to heat the plastic
uniformly and to feed the mould.
In a typical injection moulding machine, the plastic is feeding through a hopper which is loaded
using a hopper loader fitted outside the machine. The material is then feed to the heating zone by
the rotating screw whose rotation is controlled by an electrical device and the screw not only
intake required volume of material but also acts as a ram as described earlier. The groves of the
screw distribute the material uniformly around the barrel and forces the heat plasticized material
in to the mould cavity. Higher capacity is achieved by the incorporation of plasticizing units,
which increases shot capacity and by operating at low temperature reduces decolouration and
degradation of the material. All of these machines are self contained in so far as heating and
pressure pump units are concerned, and they have control system that make either automatic or
semiautomatic. The mould locking is achieved by toggle or hydraulic clamping mechanism. The
nozzle may be fitted with a device known as non-return valve to prevent premature leakage of
molten plastic and to permit high pressure build up before the shot begins.
Basic Principles of Injection Moulding
A. A good injection moulding can be achieved by the correct selection of machinery, proper mould
design, mould, right material, and best processing method.
1. A best injection moulding machine incorporated with all control will not give
optimum result until the following weak points are existing.
1. The material is not pure, preheated, dried or the percentage of reground
material is more than recommended.
2. The material is very tough for processing.
3. The operation and speed of mould are not suitable to the machine.
4. The mould design is poor.
5. Mould temperature can not be controlled properly.
6. Machine capacity is not matching with mould capacity.
7. Processing datas incorporated are not correct.
8. Low quality mould.
2. A better designed mould will not give best result if the following things are as
under;
1. Poor quality machine.
2. Improper selection of material.
3. Processing method is wrong.
4. Quality of mould is poor.
5. Material is contaminated.
6. Heating and cooling controls are weaker.
7. Wrongly selected machine or right machine is not available.
3. A best quality mould will not produce quality product unless,
1. Gating and runners are correct.
2. Have proper cooling and cooling channels.
3. Have proper heating and heater positions.
4. Proper processing parameters are used.
5. The material has been correctly prepared.
6. It has good design and stronger sections.
7. Shrinkage calculation along the flow, traverse and across are correct.
8. The machine is of best quality.
4. The best grade of plastic will not allow optimum, economical and quality product
if;
1. The design is poor.
2. The machine is improperly selected .
3. The processing parameters are wrong .
4. The material is difficult to process by this method.
5. Quality of the mould is poor.
6. Quality of the machine is poor.
7. Further treatment like quenching and annealing is not done properly.
8. Conventional moulds used in the place of hot runner moulds.
5. Only by the selection of correct processing method, quality and economical
moulding is possible.
As mentioned earlier almost all the items can be produced by compression moulding. But it is not
an economical method in most cases. Similarly a conventional mould can not offer uniform and
economical production in the place of a hot runner mould, if the quantity of the moulding is very
high. A number of hand mould is equal to a machine mould and a number of machine mould is
equal to a hot runner mould in the case of small parts. When we switching to the latter method
much time, manpower and material can be saved and also higher quality and uniformity can be
achieved.
Similarly the processing parameters and setting methods are equally important. When we use
solid inserts, direct cooling to the insert is better than nest cooling. A nest can give 80% of the
cooling effect, a back plate can give 50% of the cooling effect and the pipe ring fit can give 65%
cooling and tubular coils can give 10%. Every machine settings must be based on the
consideration of the mould profile and the size of the moulding than standard setting parameters
prescribed by the manufacturer. Always remember to add only the right percentage of ground
material.
When using hot runner, temperature control to each cavity is necessary so that even if one cavity
fails it will not affect production run. Similarly for large 'mouldings, valve gate instead of valve pin
offer faster cycle. The gate system must be suitable to the moulding and material, considering the
type of mould, and number of cavities.
B. Correct injection speed is necessary for filling the mould.
1. A low injection speed is necessary 1. For a good surface quality
2. To avoid excessive shearing at sharp edges, change of flow and at wall
thickness variation points
3. To avoid overheating of the gate
4. To fill thick walled moulding gently
5. To avoid flashes
6. To reduce warpage
7. To avoid inferior surface finish
8. To avoid sticking on cavities
9. To avoid decolouration of sprue
10. To avoid brittleness
11. To avoid worm lines
12. To avoid air bubbles
13. To avoid streak formation.
2. A high injection speed is essential ;
1. To shorten injection time
2. To reduce internal strain
3. To fill all parts with the same melt viscosity
4. To fill all unsuccessful areas
5. To reduce locking force
6. To avoid flow seams
7. To avoid wavy condition of the moulding.
All parts of the mould should be filled in correct injection pressure to avoid jetting and formation
of weld lines.
The mould must be filled with correct pressure. It must be as low as possible but as high as
necessary. There is no solution if the material jammed due to solidification of the sprue, runner or
by partly filled moulding.
C. Plastic melt should not suffer degradation. Correct plasticisation is a must for good moulding.
For this;
1. Material should be in an amorphous thermoplastic condition.
2.
3.
4.
5.
6.
7.
Volume utilization should not be less than 45% but at any case less than 20%.
Residence time should be as less as possible.
If possible higher capacity machines should be avoided.
Frictional heat from screw should be taken into consideration.
Speed of rotating screw should be controlled.
Back pressure should be adjusted to avoid excessive heat. [/ol]
D. The mould must be controlled for better quality product. Bear in mind that the mould
is setting the shape. The points to be noted are :
1. Maintain correct mould temperature for the particular plastic material.
2. The cooling must be uniform with respect to the profile.
3. Use releasing agents for smooth ejection of part if sticking; in the
beginning.
4. For parts have undercuts, threads etc., employ a faster and earlier ejection.
5. Furring-up of mould is harmful to temperature controllers.
6. When the cycle must be interrupted also interrupt the temperature controls
so that the mould temperature will not drop heavily.
7. Each moulding should be solidify in the direction of sprue, so that followup pressure intended for compensating the shrinkage can remain effective
in the last stage.
8. For moulds have side cores, use coupling system for ejection to avoid
mistakes.
9. Operate separately complete system before the commencement of
moulding cycle.
10. Check the gating whether it is applicable for the particular material when
moulding in another material.
E. Logical consideration of moulding profile and material is important than standard
setting guide lines.
11. The important points to be noted are ;
1. The material temperature should be as low as possible to avoid
degradation.
2. The cylinder utilization should be as high as possible to avoid
degradation,
3. The residence time should be as low as possible to avoid
degradation.
4. Use correct screw size and speed to suit moulding .
5. Use correct back pressure to suit moulding .
6. The cycle time should be in proportionate to the part,
7. The screw should be suitable for the particular material.
12. There is a connection between injection pressure, quality of the moulding,
cooling system, and pressure built up.
13. There is an interlinking between injection speed, material, cylinder
temperature, gating and runners, shape of moulding, surface quality, flash
and clamping force etc.
14. There is an interaction between back pressure, material, screw size and
type, and plasticizing capacity of the machine.
F. Economical setting of the machine.
For an economical, quality and faster production, the following points should be noted.
15. Instead of using trial and error, use stored data, if available.
16. If the mould is a new one or first time loading for production, it should be
handled by experienced personnels.
17. Check the screw, valve system etc. before loading the mould. For example
for acetal ordinary screw is not sufficient. Similarly for nylon and PPS
flow control valves are necessary,
18. Avoid break in cycle to get better quality products,
19. Do not start with maximum force and speed; start from minimum possible
values.
20. Avoid hard impact of mould halves,
21. Extreme speed charges should be avoided,
22. When flashes noticed first up all check mating surface instead of
increasing mould locking force,
23. Do not eject part at extreme speed.
G. Proper maintenance of machine;
24. Use correct tools, mould, and material.
25. Spend a few minutes for oiling and greasing the machine and mould.
26. Open the bag for material only at the time of loading to the hopper.
27. Use only pure material and recommended percentage of ground material.
28. When changing inserts in a production running mould, it should be done
by the recommended person,
29. When moulding using same material for long period, frequent cleaning of
material will improve quality of product,
30. Avoid reground of blended material especially with rubbish materials.
H. Safety Operations.
31. Keep the floor and machine clean, so that the operator will not slip.
32. Before starting the operation, see whether the safety guards are in place;
there is every chance for damaging the hand.
33. When plastic material jammed in moulds, use softer materials to take it
out.
34. Heating the mould with electric heaters before the commencement of
production can solve the above problem,
35. If the mould have side cores, make it sure that it will not 'foul' or use
coupling system for ejection.
36. When squirting out material, protect the face and hands,
37. When moulding corrodable material take necessary steps to safe guard the
mould and machine.
PRELIMINARY CHECKING FOR MOULDING
A. Material
38. Which is the material, grade and manufacturer.
39. Processing data's for the particular material.
40. Pre-dying is necessary or not ?
41. Allowed percentage of reground
42. Which machine can be used; and the type of screw required.?
43. Whether the material is corroding type or not.?
44. Where is the material kept and volume is sufficient or not.?
45. Previous experience.
B. Component
46. The part drawing and tolerance.
47. Quantity requirement and allotted time.
48. Have it previously produced or not ?
49. Time gap required between production and Q.C. check for the particular
material.
50. Annealing required or not.?
51. Other finishing operations.
52. Assembly operation and inserts.
53. Special fixtures and checking equipment's.
54. Time required for rework, if necessary.
55. Weight and size of moulding.
C. Mould
56. Where is the mould.
57. Mould size.
58. Type of mould and number of cavities.
59. Mounting, cooling or heating facilities.
60. Mould have side core or not, if so safety measures like core pullers
required or not?
61. What temperature control is necessary and what preparation is required.
62. Additional attachments required or not?
63. Previous experience.
D. Machine
64. Suitable machine and its specification.
65. Available date and condition of machine.
66. Any additional equipment like hydraulic ejectors or core puller required or
not?
67. Type of nozzle required.
68. What cylinder required for the process.?
69. Previous experience.
Injection Moulding Technique
The capacity of moulds to produce parts is completing only, when it is fitted to the correct
injection moulding machine. So it is essential that the mould should be designed to suit
the moulding machine of sufficient capacity and its cooling system and ejection system
should be suitable for the machine. " THE MOULD MAKING THE SHAPE OF PARTS
WHERE AS THE MOULDING MACHINE SETTING THE SHAPE"
The Various Type of Injection Moulding Machines are.
70. Hand injection moulding machines.
71. Horizontal injection moulding machines.
72. Vertical injection moulding machines.
73. Universal injection moulding machines.
74. Multi colour injection moulding machines.
75. Special purpose injection moulding machines.With hydraulic, pneumatic or toggle joint clamping system in the fashion of
semiautomatic/fully automatic/ progra-mmable/ Multi-microprocessor control machines
without or with independent temperature control to each cavity for sprueless or
runnerless moulding.
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2. Platen Mounting of Moulds
Moulding machines have two halves for mounting the mould; the fixed half and the
movable half. Standard threads at standard distance is provided in them to facilitate easy
mounting of the mould. These two platens are aligned with the tie bars or by side frame
construction. For both halves a center bore is provided to locate the mould. This center
bores and nozzle of the machine are in one axis.
For hand moulding machine the clamping is against the frame and the machine available
only in the vertical model for direct injection.
3. Locating Spigots
There are two types of locating spigots also known as locating ring / register ring. The
first model have around 0.5 mm clearance with the sprue bush and around 0.05 - 0.1 mm
clearance with the machine bore. An H7/g6 fit is provided between the clamping plate
bore on mould and the register ring. This method is adopted where the sprue bush have
separate heating and the clamping plate have insulation. The main advantage here is that
the heat will not conduct to much to the machine platen. So it is widely used for hotrunner moulds and for bigger moulds. The second method is with out bore for mounting
plate. The spure bush head is directly located with the spigot using H7/g6 fit and the
spigot have around 0.05 - 0.1 mm clearance to the machine bore.
4. Mould Clamping
Two methods for securing the mould to the press platens are in general practice, namely
direct clamping and clipping. In the first method the bolts are passing through the hole or
milled slots on the clamping plate to the platen. In second method a separate metal
clamp/dog is used. It can be flat or "Z" type to suit the mounting plate. Second method is
more popular because the position of clamping holes and thickness of plates are
unimportant.
Press Capacity
Before the mould loading for production ; it is necessary to determine the press capacity
that will be required for successful operation, and hence the particular press to be
employed. When the actual moulding machine is known that the information necessary
in respect of design data for mounting, platen area, tie bar clearance; can be obtained as
previously indicated. The essential consideration are;
76. Shot capacity
77. Plasticizing rate
78. Clamping force
79. Injection pressure
80. Ejection force
1. Shot Capacity
Injection pressures are normally rated by the maximum weight of moulding material that
can be injected per shot. Originally this was given as the weight of cellulose acetate, but
present practice is to express the weight in ounce /gm of polystyrene (PS). Thus to
determine the shot capacity of a machine with any other material B from a machine
specification that is based on material A. (PS) is,Shot capacity with material B = Shot
capacity with material A ´ Density of B / Density of A.
If the bulk factors of the plastics are not equal then the calculation must be multiplied by
the factor bulk factor A / bulk factor of B. This method widely using for plunger
machines. For screw machines the formula is, Shot capacity (g) = Swept volume (cm3) ´
density of melt at plasticizing temperature and pressure (g / cm3).
If the effect of pressure is ignored then,
Shot capacity (g) = Swept volume (cm3) ´ e ´ c
Shot capacity (oz) = Swept volume (in3)´0.5776 e´c
Where e = density of plastic at normal temperature in g/cm3, and c = Correction factor
( % volume expansion at moulding temperature).
Correction factor for amorphous materials C = 0.93 Correction factor for
crystalline materials C = 0.85
Shot capacity can also be calculated roughly by using standard values (F). Where, F = 0.5
for PS and ABS etc. = 0.4 for PE and PP etc.
Thus Shot Capacity = Swept Volume of Cylinder ´ F.
2. Plasticizing Capacity
Plasticizing rate is expressed as the number of pounds of material that the machine can
bring per hour or grams per hour. Polystyrene is the standard material taking for
calculations. The plasticizing capacity is a function of the heating capacity of the machine
and hence the amount of material that can be brought to moulding condition in a given
time is dependent upon the moulding temperature necessary and the specific heat of the
material.
Plasticizing rate with material A = Specific heat of A /Specific heat of B X Moulding
temperature of A / Moulding temperature of B.
If the total heat content per / lb or per / gram or kg. of the material is known and is equal
to Q , (B.Th.U or J) the plasticizing rate may be determined from;
Plasticizing rate with material B = Plasticizing rate with material A ´ Qa / Qb.
It is necessary, therefore that the moulding machine selected should be capable of
plasticizing sufficient material to maintain the moulding cycle expected with the tools,
and this may be determined from ; Plasticizing rate kg /lb / hr = Weight of moulding
(kg / lb) X Number of moulding per hour.
For maximum efficiency it is generally considered that an injection press should not
operate at above 80% of its rated performance as either shot weight or plasticising
capacity.
To calculate the quantity of heat Qc (B.Th.U) or (J / Kg ) to be extracted from mould per
hour, the formula is;
Qc = M(Cp(T1-T2) + L) where,
M = Mass (Ib / Kg ) of plastic material injected per hour into the mould.
Cp = Specific heat of material.
T1 = Injection temperature of material (°F or °C)
T2 = Mould temperature (°F or°C)
L = Latent heat of fusion of material (B.Th.u / lb or J / Kg )
Then Qc = Alternatively, if the total heat per pound or per kg. of the plasticised material is
known then;
Qc = MxAwhere,
M = Mass of plastic material injected in to the mould (Ib or kg)
A = Total heat content in B.Th.u/ lb or J/ kg. of the plasticized material.
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3. Clamping Force
The clamping tonnage of the press controls the maximum projected area of moulding that
can be produced. The injection pressure exerts with in the mould cavity is a force which
tends to open the mould. This force is proportional to the projected area of the moulding
and runner, and must be opposed by the clamping force. Only a proportion of the
pressure produced by the injection cylinder is transmitted to the cavity, various losses
occurring in the heating cylinder, nozzle and gate. The actual cavity pressure to be
counteracted by the clamping force is, only a fraction of the injection gauge pressure, and
it is usual to take this fraction as between 50% - 33% so;
Clamping force = Projected area of moulding including runners ´ 1/2 -1/3 of Injection
pressure.
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4.
Injection Pressure
The exact proportion of injection pressure to be adopted is dependent upon the thickness
of the moulding section and the ease of the flow of moulding material used. Thin and
uneven section require a high injection pressure to fill, and need more clamping force.
Injection pressure may be found from the formula,
injection pressure = Injection hydraulic line gauge pressure ´ di 2 ¸ dp2
Where, di = Diameter of injection cylinder
dp = Diameter of heating cylinder ram
As mentioned earlier the injection pressure necessary on the machine is dependent upon
the section thickness, length and size of runners, gates, and the material to be employed.
It is quite frequent practice however to fit where necessary a machine with a special high
pressure injection cylinder. This is done by reducing the diameter of the ram and the base
of the heating cylinder, thus increasing the specific pressure although it will reduce shot
capacity.
5. Ejection Force
The ejection of the moulding is performed by the ejector pins, core ejectors, plate ejection
or by air ejection. These ejection units are secured to and operated by, an ejection plate or
grid with in the mould. Pneumatic ejectors are incorporating for multi colour insert
mouldings to facilitate additional ejection between cycles as well as for large mouldings
like luggage shell, to release vacuum sticking to the core.
The ejection force determines mainly based on the following factors. (1) Area of contact.
(2) Surface quality (3) Shrinkage. (4) Vacuum formation. (5) Flexibility of the material.
(6) Under cuts. (7) Draft for the core. (8) Stiffness and rigidity of the moulding. (9)
Ejection unit balance. (10) Condition of machine.
To find out ejection force a number of formulas are available. But when considering the
above points a clear value is impossible to achieve especially in the case of vacuum
sticking where a slight passage of air is enough to release the total ejection force.
MOULD COOLING
It is necessary to reduce the temperature of the hot plastic material injected into the
mould cavity to a point when the material freezes into a sufficiently rigid state to allow the
moulding to be extracted. Thus the temperature of the mould must be maintained
sufficiently low to cause the hot material to give up both its sensible and latent heat of
fusion to the tool surfaces. Cooling time increases in proportion to the wall thickness and
size of the moulding. If the heat input to the tool from the hot material is greater than can
normally be dissipated by the mould through conduction, etc, then other means have to
be introduced to remove the excess heat. Such additional heat removal is usually doing by
water cooling, warm oil cooling, and by high pressure air cooling.
Cooling doesn't mean that mould temperature should be brought down to minimum
possible. To set best result and reduce post moulding shrinkages etc. a particular
temperature should be maintained depending upon the plastic. For those values refer the
table No.8 at the end ot this part
Water Cooling
Most of the moulds are water cooled; the water mixed with anticorrosion chemicals,
being directed through passage provided in the mould.
All the injection moulding machines have valves for water, and these are connected to the
inlet and outlet of the mould by means of flexible pipes, the valves being adjusted to
control the flow of water to maintain optimum mould temperature. Water cooling allows
an increase in the production rate, especially when the shot weight-moulding weight ratio
is high. Cooling circulation passages can be made either by drilling, by milling slots
between plates, by copper tubing or by making partition for center bores.
The following particular points should be observed by mould designer.
81. The weld like formation area of moulding, should cool at last ; so that for
the next cycle these areas will be hotter than the injection points.
82. Large difference in cooling water temperature between the inlet and outlet,
or across subsidiary water circuits should be avoided, since these lead to
differing temperatures in various parts of the mould and may cause
moulding difficulties.
83. The amount of cooling provided must be sufficient to maintain the
temperature of the mould when the latter is running at maximum
production. So additional cooling if necessary should be provided on back
plates.
84. The inlet and outlet should be on the same side and normally at the rear of
the machine, in order to prevent restriction for the operator.
85. Where ever possible use extended nozzle to mould inserts so that optimum
result can obtained.
86. The design should be such that there should not be any leakage between
plates or inserts ; use 'O' rings or gaskets.
2. Air Cooling.
There are occasions where water cools too rapidly or water cooling is difficult to apply to
particular mould piece. In such case air cooling at standard air line pressure (6 to 10 bar)
may be used.
3. Oil Heating/Cooling.
A number of thermoplastic materials like PMMA, PA, PC, POM, PBT etc. will not mould
satisfactory in cold moulds, and hence the tool temperature must be raised before sound
moulding can be achieved.
This can be done by continuing to inject the material until the mould temperature rises,
but this is slow and wastage of time. So external method of heating the mould to the
operating temperature are adopted.
This heating may be effected electrically or employing strip or catridge heaters similar to
those used for compression mould heating and operated at main voltage.
Nowadays portable mould heaters known as temperature - controllers are very popular
which not only raise the mould to the required temperature but also hold this
temperature to the close limits now demanded by high class moulding technique. The
controller normally comprises a unit in which the heating liquid (oil or water) is heated
and pumped to the mould the same way as explained earlier.
Mould temperature is very important to avoid post moulding shrinkage.
4. Mould Cooling Time
S = t2/2pa log e [p (Tx-Tm) ¸ 4 (Tc-Tm)] where,
S = Minimum cooling time in seconds.
t = Thickness of moulding (in or cm)
a = Thermal diffusivity of material (in2 / s or cm2 /s)- refer table No.25
Tx = Ejection temperature of moulding (ºF or ºC)
Tm = Mould temperature (ºF or º C)
Tc = Cylinder temperature (ºF or °C).
p = 3.14
Melt Processing
Different materials have different processing parameters and tips. Here Delrin a
polyacetal, one of the latest material is taken as an example, which require special care.
Acetal resins may be processed without difficulty on conventional injection moulding,
blow moulding, and extrusion equipment. The main points should be noted are:
87. Overheating leads to the production of formaldehyde gas and if produced
in sufficient quantities with in the container of an injection cylinder or
extruder barrel, the gas pressure may become sufficiently high that there is
a risk of damage or injury and the gas effect is similar to tear gas. A typical
copolymer maybe kept in an extruder barrel for 110 minute at 190°C
before serious discolouration occurs. Dead spot must be carefully avoided.
88. Although less hygroscopic than the nylons, it must be stored in dry place.
89. With most homopolymers and copolymers the apparent viscosity is less
dependent on temperature and sheer stress (up to 10 6 dyn / cm2) than the
polyolefins thus simplifying mould design. On the other hand the melt has
a low elasticity, and strength and so extruded sections need to be
supported and brought below the melting point as soon as possible .
90. The high crystallinity which develops on cooling, results to a shrinkage of
2%. Because of low glass transition temperature, crystallization can take
place quite rapidly at room temperatures and after shrinkage is usually
complete within 48 hours of moulding or extrusion. In processing
operations injection moulds, blow moulds and sizing dies should be kept
at about 80 - 120º C in order to obtain best result.
91. Because of low glass transition temperature (- 13 °C and - 73 °C ) it is not
possible to make clear film, stable at room temperature, by quenching.
Some improvement in, clarity may be obtained by cold rolling as this
tends to dispose the crystal structure into layers.
Both homopolymers and copolymers are available in a range of molecular weights 20,000
- 100,000. The material are normally characterized by the melt flow index (MFI) using
basically the same tests as employed for polyethylene. For high precision work and
complex moulding low molecular weight polymers should be used (MFI as high as 27).
For general purpose work MFI of about 9 are employed. For extrusions and thick walled
moulding a polymer with MFI about 2.5 (molecular weight 45,000) is often employed,
although for extrusion blow moulding the special polymers used have an MFI of about
1.0.
Acetal is a crystalline polymer which requires a greater heat input for melting than
amorphous resins. This additional heat is needed to destroy the ordered crystalline
structure of the solid, and it is called the heat of fusion.
EQUIPMENTS FOR INJECTION MOULDING
1. Heating Cylinder
To deliver the necessary amount of uniformly melted resin of good quality, the adapter,
non-return valve (screw check valve), screw, and cylinder must be properly designed to
ensure maximum streamlining. This is necessary to avoid hold-up of resin, which will
lead to resin degradation. Relatively insignificant hold up spots can cause degradation
and a reduction in quality of the parts moulded. Poor seating or matching of diameters as
well as poor maintenance can also cause hold up spots.
2. Nozzle
The nozzle of the open design (not shut off) are suitable for acetals. The heater bands
should extend as close to the nozzle tip as possible and cover as much of the exposed
surface as practical to counteract loss. It is particularly important to cover the small
orifice by heat bands, where the material separation occurs. If a thermocouple is used the
well should be located over this orifice. Improper location of heater bands of
thermocouple will results in over heating of certain areas in an effort to prevent freezing
of the tip.
Screw decompression or suck back is frequently used to make control of drool easier with
these open nozzles. This feature is available with most machines.
3. Non-Return Valve
Although shut off nozzle have been used successfully with acetals they tend to cause hold
up of resin which results to brown streaks or gassings especially after some wear has
occurred in the moving parts of the nozzle. The non-return valve or check ring prevents
melt from flowing backward during injection. A non-return valve should meet the
following requirements:1. No hold-up spots. 2. No flow restriction. 3. Good seal. 4. Control of wear.
4. Adapter
The adapter have a standard tapered short cylinder sections to match with the cylinder as
well as the nozzle inner diameters and it is the intermediate between the nozzle and
cylinder. In addition to its mechanical functioning of reducing the diameter, the adapter
acts to isolate the nozzle thermally from the front of the cylinder for better control of
nozzle temperature. It also protect the cylinder from damaging that can occur due to
frequent change of nozzles, and it should be out of thermally stable good quality medium
hardened steel.
5. Screw
The length of the screw will affect the melt quality and consequently the output usable for
good parts. The preferred length is about 20 times the screw diameters (L /D = 20/1) or
20 turns when the pitch and diameter are equal. The screw should be divided as follows
45 - 50% (9-10 turns) feed section. 20 - 30 % (4 - 6 turns) transition section and 25 - 30%
(5 - 6 turns) metering section. Normal division of screw is 10 - 5 - 5. In the screws less
than 16 turns, the feed section should have 7 or 8 turns.
ﾧ
The shallow metering high compression ratio screws suggested for acetals are designed to
increase the heat input by mechanical working of the resin. Since the energy for this
increase comes from the screw motor, additional horse power must be available if an
increase in melting capacity is to be realised. The suggested screw also have to be
operated at a higher RPM to deliver equal or greater melt output during a specified screw
retraction time, because the amount displaced by one turn is less. Neither a general
purpose nor high compression screw should be operated at rotational speed so high that
unmelted particles or degradation occurs.
Usually the moulding machine injection unit chosen so that the shot size is 50% or less of
the injection stroke. For thin parts moulded at more than four shots per minute, as little
as 10 - 20 % of injection stroke capacity may be used to obtain adequate melting capacity.
On the other hand for thicker parts moulded on long cycles more than 50% of the stroke
may be used. These considerations determine the average holdup time of the resin in the
moulding cylinder.
ﾧ
6. Cylinder Temperature
Inter zone temperature control should be used and thermocouples should be placed near
the centre of each zone. Burn out of one or more heater bands with in a zone may not be
apparent from the temperature controllers, so some moulders have used ammeters in
each zone to detect changes. In latest machines PID temperature controls are available
which can give an accuracy of ±1.5°C.
7. Injection Rate
The moulding machine should be able to inject its rated capacity in less than 2 seconds
with lower viscosity acetal resins. Highly viscous acetals may have slower fill rates.
Control of injection rate, however will often be necessary to correct certain molding
problems; a slower injection rate may be required to prevent warping or jetting of resin
into some cavities. Now a days latest machines have the facility for two stage injection ie,
initially with low clamping force to release any gas or air, then with high clamping force to
prevent flashes. Latest machines have an injection time setting range of 0.1 second and
screw position digital indicator range of 0.1mm.
8. Clamping Force
The force required to hold mould closed against injection pressure depends on;
92. Injection pressure to fill the mould and pack out parts.
93. Injection speed required for filling.
94. The dimensions of parts and runners at the mould parting line (projected
area of cavities and runners)
95. Side cores, core pull requirements etc.
96. Precision and tolerance requirements.
Most well built moulds can be adequately clamped if the machine has 48-69 MPa (3.5-5.0
tons / in2) based on the projected area, since the injection pressures are seldom over 100
MPa (14,500 Psi) and fill rates are moderate. Higher available clamping pressure is
preferred and will give more freedom in choosing the most suitable molding conditions
for some molds. Very thin, long flow parts may require clamp forces as high as 100 MPa
(7 tons / in2 ) because extremely fast fill rate and high injection pressures may be
required.
START UP AND SHUT DOWN PROCEDURES
1. Start Resin Change/Purging
The following procedure is designed to prevent over heating of the resin and
contamination of the cylinder with material from previous runs.
Before loading the acetal resins to the hopper the cylinder which contains another resin
must be purged at temperatures above the melting temperature for the other resins with a
clear PS or natural colour low melt index PS (MFI 1-2) until the cylinder has been cleared.
The cylinder temperatures should be then to be adjusted to 205ºC (400°F) while
continuing to process the purge resin through the machine. When the cylinder has
reached 205°C acetal resin can be added to the hopper. After the purge resin has been
cleared from the cylinder, the moulding operation with acetal may begin.
Both PS and PE are chemically compatible with acetal resins where as PVC is not.
Contamination of acetals with such foreign materials can cause objectionable odour or
even a violet blow back. Very thorough purging is necessary to remove the last traces of
PVC from the cylinder. In the worst cases like after glass reinforced resins or severe
degradation of previous material, it may be necessary to pull the screw and clean the
equipment manually to prevent contamination of the mouldings.
2. Temporary Shut Down
A moulding machine with acetals in the cylinder at operating temperatures should not be
allowed to stand idle. When it is necessary to interrupt cycle for a minor repair, it is good
practice to take periodic air shots or shut the hopper feed gate and empty the cylinder and
screw, if the interruption will be more than a few minutes. The cylinder temperature
should be reduced to about 150 °C (300 °F), if the interruption is prolonged.
3. Normal Shut Down
The best practice is shut off the hopper feed gate, turn off the cylinder heaters leaving the
nozzle heater on and continue moulding or purging until the cylinder became empty. The
machine is then turned off with the screw in the forward position and is ready for
reheating by the procedure mentioned earlier. If another resin will be moulded next, the
acetal should be purged from the cylinder with PE or PS
INTRODUCTION
It is a process manual, semiautomatic or fully automatic for the manufacture of plastic articles
and it is the basic, fast, economical and major processing method and require little or no
subsequent finishing operation or rework for the product.
Plastic moulding especially thermoplastic items may be produced by compression moulding
methods, but since they are soft at the temperature involved, it is necessary to cool down the
mould before they may be ejected. This would not be convenient since on a run of production, it is
desirable to keep up the temperature to avoid the slow and protracted operation of alternative
heating and cooling . Besides much time has to be spend for accurate charging. This has resulted
in a process of injection moulding technique.
Injection moulding differs from compression moulding is that the plastic material is rendered
fluid in a separate chamber or barrel, out side the mould is then forced into the mould cavity by
external pressure. The mould is cooled and split, the two halves being locked by pressure to resist
injection force during moulding and opening automatically when the mould cavity has been filled
by the molten plastic. The cooled, hardened or vulcanized article is then ejected from the mould
and the process repeated.
In simple machines, it consist of a heated barrel with hydraulically, pneumatically or manual
operated ram, an opening in the rear of the barrel there is a hopper into which measured amounts
of plastic material generally in the form of granules are fed. The material is then forced under
high pressure by the plunger into the heating zone of the barrel, pushing out an equal weight of
molten plastic into the mould. A torpedo shaped distributor and the nozzle to heat the plastic
uniformly and to feed the mould.
In a typical injection moulding machine, the plastic is feeding through a hopper which is loaded
using a hopper loader fitted outside the machine. The material is then feed to the heating zone by
the rotating screw whose rotation is controlled by an electrical device and the screw not only
intake required volume of material but also acts as a ram as described earlier. The groves of the
screw distribute the material uniformly around the barrel and forces the heat plasticized material
in to the mould cavity. Higher capacity is achieved by the incorporation of plasticizing units,
which increases shot capacity and by operating at low temperature reduces decolouration and
degradation of the material. All of these machines are self contained in so far as heating and
pressure pump units are concerned, and they have control system that make either automatic or
semiautomatic. The mould locking is achieved by toggle or hydraulic clamping mechanism. The
nozzle may be fitted with a device known as non-return valve to prevent premature leakage of
molten plastic and to permit high pressure build up before the shot begins.
Basic Principles of Injection Moulding
A. A good injection moulding can be achieved by the correct selection of machinery, proper mould
design, mould, right material, and best processing method.
1. A best injection moulding machine incorporated with all control will not give
optimum result until the following weak points are existing.
1. The material is not pure, preheated, dried or the percentage of reground
material is more than recommended.
2. The material is very tough for processing.
3. The operation and speed of mould are not suitable to the machine.
4. The mould design is poor.
5. Mould temperature can not be controlled properly.
6. Machine capacity is not matching with mould capacity.
7. Processing datas incorporated are not correct.
8. Low quality mould.
2. A better designed mould will not give best result if the following things are as
under;
1. Poor quality machine.
2. Improper selection of material.
3. Processing method is wrong.
4. Quality of mould is poor.
5. Material is contaminated.
6. Heating and cooling controls are weaker.
7. Wrongly selected machine or right machine is not available.
3. A best quality mould will not produce quality product unless,
1. Gating and runners are correct.
2. Have proper cooling and cooling channels.
3. Have proper heating and heater positions.
4. Proper processing parameters are used.
5. The material has been correctly prepared.
6. It has good design and stronger sections.
7. Shrinkage calculation along the flow, traverse and across are correct.
8. The machine is of best quality.
4. The best grade of plastic will not allow optimum, economical and quality product
if;
1. The design is poor.
2. The machine is improperly selected .
3. The processing parameters are wrong .
4. The material is difficult to process by this method.
5. Quality of the mould is poor.
6. Quality of the machine is poor.
7. Further treatment like quenching and annealing is not done properly.
8. Conventional moulds used in the place of hot runner moulds.
5. Only by the selection of correct processing method, quality and economical
moulding is possible.
As mentioned earlier almost all the items can be produced by compression moulding. But it is not
an economical method in most cases. Similarly a conventional mould can not offer uniform and
economical production in the place of a hot runner mould, if the quantity of the moulding is very
high. A number of hand mould is equal to a machine mould and a number of machine mould is
equal to a hot runner mould in the case of small parts. When we switching to the latter method
much time, manpower and material can be saved and also higher quality and uniformity can be
achieved.
Similarly the processing parameters and setting methods are equally important. When we use
solid inserts, direct cooling to the insert is better than nest cooling. A nest can give 80% of the
cooling effect, a back plate can give 50% of the cooling effect and the pipe ring fit can give 65%
cooling and tubular coils can give 10%. Every machine settings must be based on the
consideration of the mould profile and the size of the moulding than standard setting parameters
prescribed by the manufacturer. Always remember to add only the right percentage of ground
material.
When using hot runner, temperature control to each cavity is necessary so that even if one cavity
fails it will not affect production run. Similarly for large 'mouldings, valve gate instead of valve pin
offer faster cycle. The gate system must be suitable to the moulding and material, considering the
type of mould, and number of cavities.
B. Correct injection speed is necessary for filling the mould.
1. A low injection speed is necessary 1. For a good surface quality
2. To avoid excessive shearing at sharp edges, change of flow and at wall
thickness variation points
3. To avoid overheating of the gate
4. To fill thick walled moulding gently
5. To avoid flashes
6. To reduce warpage
7. To avoid inferior surface finish
8. To avoid sticking on cavities
9. To avoid decolouration of sprue
10. To avoid brittleness
11. To avoid worm lines
12. To avoid air bubbles
13. To avoid streak formation.
2. A high injection speed is essential ;
1. To shorten injection time
2. To reduce internal strain
3. To fill all parts with the same melt viscosity
4. To fill all unsuccessful areas
5. To reduce locking force
6. To avoid flow seams
7. To avoid wavy condition of the moulding.
All parts of the mould should be filled in correct injection pressure to avoid jetting and formation
of weld lines.
The mould must be filled with correct pressure. It must be as low as possible but as high as
necessary. There is no solution if the material jammed due to solidification of the sprue, runner or
by partly filled moulding.
C. Plastic melt should not suffer degradation. Correct plasticisation is a must for good moulding.
For this;
1. Material should be in an amorphous thermoplastic condition.
2.
3.
4.
5.
6.
7.
Volume utilization should not be less than 45% but at any case less than 20%.
Residence time should be as less as possible.
If possible higher capacity machines should be avoided.
Frictional heat from screw should be taken into consideration.
Speed of rotating screw should be controlled.
Back pressure should be adjusted to avoid excessive heat. [/ol]
D. The mould must be controlled for better quality product. Bear in mind that the mould
is setting the shape. The points to be noted are :
1. Maintain correct mould temperature for the particular plastic material.
2. The cooling must be uniform with respect to the profile.
3. Use releasing agents for smooth ejection of part if sticking; in the
beginning.
4. For parts have undercuts, threads etc., employ a faster and earlier ejection.
5. Furring-up of mould is harmful to temperature controllers.
6. When the cycle must be interrupted also interrupt the temperature controls
so that the mould temperature will not drop heavily.
7. Each moulding should be solidify in the direction of sprue, so that followup pressure intended for compensating the shrinkage can remain effective
in the last stage.
8. For moulds have side cores, use coupling system for ejection to avoid
mistakes.
9. Operate separately complete system before the commencement of
moulding cycle.
10. Check the gating whether it is applicable for the particular material when
moulding in another material.
E. Logical consideration of moulding profile and material is important than standard
setting guide lines.
11. The important points to be noted are ;
1. The material temperature should be as low as possible to avoid
degradation.
2. The cylinder utilization should be as high as possible to avoid
degradation,
3. The residence time should be as low as possible to avoid
degradation.
4. Use correct screw size and speed to suit moulding .
5. Use correct back pressure to suit moulding .
6. The cycle time should be in proportionate to the part,
7. The screw should be suitable for the particular material.
12. There is a connection between injection pressure, quality of the moulding,
cooling system, and pressure built up.
13. There is an interlinking between injection speed, material, cylinder
temperature, gating and runners, shape of moulding, surface quality, flash
and clamping force etc.
14. There is an interaction between back pressure, material, screw size and
type, and plasticizing capacity of the machine.
F. Economical setting of the machine.
For an economical, quality and faster production, the following points should be noted.
15. Instead of using trial and error, use stored data, if available.
16. If the mould is a new one or first time loading for production, it should be
handled by experienced personnels.
17. Check the screw, valve system etc. before loading the mould. For example
for acetal ordinary screw is not sufficient. Similarly for nylon and PPS
flow control valves are necessary,
18. Avoid break in cycle to get better quality products,
19. Do not start with maximum force and speed; start from minimum possible
values.
20. Avoid hard impact of mould halves,
21. Extreme speed charges should be avoided,
22. When flashes noticed first up all check mating surface instead of
increasing mould locking force,
23. Do not eject part at extreme speed.
G. Proper maintenance of machine;
24. Use correct tools, mould, and material.
25. Spend a few minutes for oiling and greasing the machine and mould.
26. Open the bag for material only at the time of loading to the hopper.
27. Use only pure material and recommended percentage of ground material.
28. When changing inserts in a production running mould, it should be done
by the recommended person,
29. When moulding using same material for long period, frequent cleaning of
material will improve quality of product,
30. Avoid reground of blended material especially with rubbish materials.
H. Safety Operations.
31. Keep the floor and machine clean, so that the operator will not slip.
32. Before starting the operation, see whether the safety guards are in place;
there is every chance for damaging the hand.
33. When plastic material jammed in moulds, use softer materials to take it
out.
34. Heating the mould with electric heaters before the commencement of
production can solve the above problem,
35. If the mould have side cores, make it sure that it will not 'foul' or use
coupling system for ejection.
36. When squirting out material, protect the face and hands,
37. When moulding corrodable material take necessary steps to safe guard the
mould and machine.
PRELIMINARY CHECKING FOR MOULDING
A. Material
38. Which is the material, grade and manufacturer.
39. Processing data's for the particular material.
40. Pre-dying is necessary or not ?
41. Allowed percentage of reground
42. Which machine can be used; and the type of screw required.?
43. Whether the material is corroding type or not.?
44. Where is the material kept and volume is sufficient or not.?
45. Previous experience.
B. Component
46. The part drawing and tolerance.
47. Quantity requirement and allotted time.
48. Have it previously produced or not ?
49. Time gap required between production and Q.C. check for the particular
material.
50. Annealing required or not.?
51. Other finishing operations.
52. Assembly operation and inserts.
53. Special fixtures and checking equipment's.
54. Time required for rework, if necessary.
55. Weight and size of moulding.
C. Mould
56. Where is the mould.
57. Mould size.
58. Type of mould and number of cavities.
59. Mounting, cooling or heating facilities.
60. Mould have side core or not, if so safety measures like core pullers
required or not?
61. What temperature control is necessary and what preparation is required.
62. Additional attachments required or not?
63. Previous experience.
D. Machine
64. Suitable machine and its specification.
65. Available date and condition of machine.
66. Any additional equipment like hydraulic ejectors or core puller required or
not?
67. Type of nozzle required.
68. What cylinder required for the process.?
69. Previous experience.
Injection Moulding Technique
The capacity of moulds to produce parts is completing only, when it is fitted to the correct
injection moulding machine. So it is essential that the mould should be designed to suit
the moulding machine of sufficient capacity and its cooling system and ejection system
should be suitable for the machine. " THE MOULD MAKING THE SHAPE OF PARTS
WHERE AS THE MOULDING MACHINE SETTING THE SHAPE"
The Various Type of Injection Moulding Machines are.
70. Hand injection moulding machines.
71. Horizontal injection moulding machines.
72. Vertical injection moulding machines.
73. Universal injection moulding machines.
74. Multi colour injection moulding machines.
75. Special purpose injection moulding machines.With hydraulic, pneumatic or toggle joint clamping system in the fashion of
semiautomatic/fully automatic/ progra-mmable/ Multi-microprocessor control machines
without or with independent temperature control to each cavity for sprueless or
runnerless moulding.
ﾧ
2. Platen Mounting of Moulds
Moulding machines have two halves for mounting the mould; the fixed half and the
movable half. Standard threads at standard distance is provided in them to facilitate easy
mounting of the mould. These two platens are aligned with the tie bars or by side frame
construction. For both halves a center bore is provided to locate the mould. This center
bores and nozzle of the machine are in one axis.
For hand moulding machine the clamping is against the frame and the machine available
only in the vertical model for direct injection.
3. Locating Spigots
There are two types of locating spigots also known as locating ring / register ring. The
first model have around 0.5 mm clearance with the sprue bush and around 0.05 - 0.1 mm
clearance with the machine bore. An H7/g6 fit is provided between the clamping plate
bore on mould and the register ring. This method is adopted where the sprue bush have
separate heating and the clamping plate have insulation. The main advantage here is that
the heat will not conduct to much to the machine platen. So it is widely used for hotrunner moulds and for bigger moulds. The second method is with out bore for mounting
plate. The spure bush head is directly located with the spigot using H7/g6 fit and the
spigot have around 0.05 - 0.1 mm clearance to the machine bore.
4. Mould Clamping
Two methods for securing the mould to the press platens are in general practice, namely
direct clamping and clipping. In the first method the bolts are passing through the hole or
milled slots on the clamping plate to the platen. In second method a separate metal
clamp/dog is used. It can be flat or "Z" type to suit the mounting plate. Second method is
more popular because the position of clamping holes and thickness of plates are
unimportant.
Press Capacity
Before the mould loading for production ; it is necessary to determine the press capacity
that will be required for successful operation, and hence the particular press to be
employed. When the actual moulding machine is known that the information necessary
in respect of design data for mounting, platen area, tie bar clearance; can be obtained as
previously indicated. The essential consideration are;
76. Shot capacity
77. Plasticizing rate
78. Clamping force
79. Injection pressure
80. Ejection force
1. Shot Capacity
Injection pressures are normally rated by the maximum weight of moulding material that
can be injected per shot. Originally this was given as the weight of cellulose acetate, but
present practice is to express the weight in ounce /gm of polystyrene (PS). Thus to
determine the shot capacity of a machine with any other material B from a machine
specification that is based on material A. (PS) is,Shot capacity with material B = Shot
capacity with material A ´ Density of B / Density of A.
If the bulk factors of the plastics are not equal then the calculation must be multiplied by
the factor bulk factor A / bulk factor of B. This method widely using for plunger
machines. For screw machines the formula is, Shot capacity (g) = Swept volume (cm3) ´
density of melt at plasticizing temperature and pressure (g / cm3).
If the effect of pressure is ignored then,
Shot capacity (g) = Swept volume (cm3) ´ e ´ c
Shot capacity (oz) = Swept volume (in3)´0.5776 e´c
Where e = density of plastic at normal temperature in g/cm3, and c = Correction factor
( % volume expansion at moulding temperature).
Correction factor for amorphous materials C = 0.93 Correction factor for
crystalline materials C = 0.85
Shot capacity can also be calculated roughly by using standard values (F). Where, F = 0.5
for PS and ABS etc. = 0.4 for PE and PP etc.
Thus Shot Capacity = Swept Volume of Cylinder ´ F.
2. Plasticizing Capacity
Plasticizing rate is expressed as the number of pounds of material that the machine can
bring per hour or grams per hour. Polystyrene is the standard material taking for
calculations. The plasticizing capacity is a function of the heating capacity of the machine
and hence the amount of material that can be brought to moulding condition in a given
time is dependent upon the moulding temperature necessary and the specific heat of the
material.
Plasticizing rate with material A = Specific heat of A /Specific heat of B X Moulding
temperature of A / Moulding temperature of B.
If the total heat content per / lb or per / gram or kg. of the material is known and is equal
to Q , (B.Th.U or J) the plasticizing rate may be determined from;
Plasticizing rate with material B = Plasticizing rate with material A ´ Qa / Qb.
It is necessary, therefore that the moulding machine selected should be capable of
plasticizing sufficient material to maintain the moulding cycle expected with the tools,
and this may be determined from ; Plasticizing rate kg /lb / hr = Weight of moulding
(kg / lb) X Number of moulding per hour.
For maximum efficiency it is generally considered that an injection press should not
operate at above 80% of its rated performance as either shot weight or plasticising
capacity.
To calculate the quantity of heat Qc (B.Th.U) or (J / Kg ) to be extracted from mould per
hour, the formula is;
Qc = M(Cp(T1-T2) + L) where,
M = Mass (Ib / Kg ) of plastic material injected per hour into the mould.
Cp = Specific heat of material.
T1 = Injection temperature of material (°F or °C)
T2 = Mould temperature (°F or°C)
L = Latent heat of fusion of material (B.Th.u / lb or J / Kg )
Then Qc = Alternatively, if the total heat per pound or per kg. of the plasticised material is
known then;
Qc = MxAwhere,
M = Mass of plastic material injected in to the mould (Ib or kg)
A = Total heat content in B.Th.u/ lb or J/ kg. of the plasticized material.
ﾧ
3. Clamping Force
The clamping tonnage of the press controls the maximum projected area of moulding that
can be produced. The injection pressure exerts with in the mould cavity is a force which
tends to open the mould. This force is proportional to the projected area of the moulding
and runner, and must be opposed by the clamping force. Only a proportion of the
pressure produced by the injection cylinder is transmitted to the cavity, various losses
occurring in the heating cylinder, nozzle and gate. The actual cavity pressure to be
counteracted by the clamping force is, only a fraction of the injection gauge pressure, and
it is usual to take this fraction as between 50% - 33% so;
Clamping force = Projected area of moulding including runners ´ 1/2 -1/3 of Injection
pressure.
ﾧ
4.
Injection Pressure
The exact proportion of injection pressure to be adopted is dependent upon the thickness
of the moulding section and the ease of the flow of moulding material used. Thin and
uneven section require a high injection pressure to fill, and need more clamping force.
Injection pressure may be found from the formula,
injection pressure = Injection hydraulic line gauge pressure ´ di 2 ¸ dp2
Where, di = Diameter of injection cylinder
dp = Diameter of heating cylinder ram
As mentioned earlier the injection pressure necessary on the machine is dependent upon
the section thickness, length and size of runners, gates, and the material to be employed.
It is quite frequent practice however to fit where necessary a machine with a special high
pressure injection cylinder. This is done by reducing the diameter of the ram and the base
of the heating cylinder, thus increasing the specific pressure although it will reduce shot
capacity.
5. Ejection Force
The ejection of the moulding is performed by the ejector pins, core ejectors, plate ejection
or by air ejection. These ejection units are secured to and operated by, an ejection plate or
grid with in the mould. Pneumatic ejectors are incorporating for multi colour insert
mouldings to facilitate additional ejection between cycles as well as for large mouldings
like luggage shell, to release vacuum sticking to the core.
The ejection force determines mainly based on the following factors. (1) Area of contact.
(2) Surface quality (3) Shrinkage. (4) Vacuum formation. (5) Flexibility of the material.
(6) Under cuts. (7) Draft for the core. (8) Stiffness and rigidity of the moulding. (9)
Ejection unit balance. (10) Condition of machine.
To find out ejection force a number of formulas are available. But when considering the
above points a clear value is impossible to achieve especially in the case of vacuum
sticking where a slight passage of air is enough to release the total ejection force.
MOULD COOLING
It is necessary to reduce the temperature of the hot plastic material injected into the
mould cavity to a point when the material freezes into a sufficiently rigid state to allow the
moulding to be extracted. Thus the temperature of the mould must be maintained
sufficiently low to cause the hot material to give up both its sensible and latent heat of
fusion to the tool surfaces. Cooling time increases in proportion to the wall thickness and
size of the moulding. If the heat input to the tool from the hot material is greater than can
normally be dissipated by the mould through conduction, etc, then other means have to
be introduced to remove the excess heat. Such additional heat removal is usually doing by
water cooling, warm oil cooling, and by high pressure air cooling.
Cooling doesn't mean that mould temperature should be brought down to minimum
possible. To set best result and reduce post moulding shrinkages etc. a particular
temperature should be maintained depending upon the plastic. For those values refer the
table No.8 at the end ot this part
Water Cooling
Most of the moulds are water cooled; the water mixed with anticorrosion chemicals,
being directed through passage provided in the mould.
All the injection moulding machines have valves for water, and these are connected to the
inlet and outlet of the mould by means of flexible pipes, the valves being adjusted to
control the flow of water to maintain optimum mould temperature. Water cooling allows
an increase in the production rate, especially when the shot weight-moulding weight ratio
is high. Cooling circulation passages can be made either by drilling, by milling slots
between plates, by copper tubing or by making partition for center bores.
The following particular points should be observed by mould designer.
81. The weld like formation area of moulding, should cool at last ; so that for
the next cycle these areas will be hotter than the injection points.
82. Large difference in cooling water temperature between the inlet and outlet,
or across subsidiary water circuits should be avoided, since these lead to
differing temperatures in various parts of the mould and may cause
moulding difficulties.
83. The amount of cooling provided must be sufficient to maintain the
temperature of the mould when the latter is running at maximum
production. So additional cooling if necessary should be provided on back
plates.
84. The inlet and outlet should be on the same side and normally at the rear of
the machine, in order to prevent restriction for the operator.
85. Where ever possible use extended nozzle to mould inserts so that optimum
result can obtained.
86. The design should be such that there should not be any leakage between
plates or inserts ; use 'O' rings or gaskets.
2. Air Cooling.
There are occasions where water cools too rapidly or water cooling is difficult to apply to
particular mould piece. In such case air cooling at standard air line pressure (6 to 10 bar)
may be used.
3. Oil Heating/Cooling.
A number of thermoplastic materials like PMMA, PA, PC, POM, PBT etc. will not mould
satisfactory in cold moulds, and hence the tool temperature must be raised before sound
moulding can be achieved.
This can be done by continuing to inject the material until the mould temperature rises,
but this is slow and wastage of time. So external method of heating the mould to the
operating temperature are adopted.
This heating may be effected electrically or employing strip or catridge heaters similar to
those used for compression mould heating and operated at main voltage.
Nowadays portable mould heaters known as temperature - controllers are very popular
which not only raise the mould to the required temperature but also hold this
temperature to the close limits now demanded by high class moulding technique. The
controller normally comprises a unit in which the heating liquid (oil or water) is heated
and pumped to the mould the same way as explained earlier.
Mould temperature is very important to avoid post moulding shrinkage.
4. Mould Cooling Time
S = t2/2pa log e [p (Tx-Tm) ¸ 4 (Tc-Tm)] where,
S = Minimum cooling time in seconds.
t = Thickness of moulding (in or cm)
a = Thermal diffusivity of material (in2 / s or cm2 /s)- refer table No.25
Tx = Ejection temperature of moulding (ºF or ºC)
Tm = Mould temperature (ºF or º C)
Tc = Cylinder temperature (ºF or °C).
p = 3.14
Melt Processing
Different materials have different processing parameters and tips. Here Delrin a
polyacetal, one of the latest material is taken as an example, which require special care.
Acetal resins may be processed without difficulty on conventional injection moulding,
blow moulding, and extrusion equipment. The main points should be noted are:
87. Overheating leads to the production of formaldehyde gas and if produced
in sufficient quantities with in the container of an injection cylinder or
extruder barrel, the gas pressure may become sufficiently high that there is
a risk of damage or injury and the gas effect is similar to tear gas. A typical
copolymer maybe kept in an extruder barrel for 110 minute at 190°C
before serious discolouration occurs. Dead spot must be carefully avoided.
88. Although less hygroscopic than the nylons, it must be stored in dry place.
89. With most homopolymers and copolymers the apparent viscosity is less
dependent on temperature and sheer stress (up to 10 6 dyn / cm2) than the
polyolefins thus simplifying mould design. On the other hand the melt has
a low elasticity, and strength and so extruded sections need to be
supported and brought below the melting point as soon as possible .
90. The high crystallinity which develops on cooling, results to a shrinkage of
2%. Because of low glass transition temperature, crystallization can take
place quite rapidly at room temperatures and after shrinkage is usually
complete within 48 hours of moulding or extrusion. In processing
operations injection moulds, blow moulds and sizing dies should be kept
at about 80 - 120º C in order to obtain best result.
91. Because of low glass transition temperature (- 13 °C and - 73 °C ) it is not
possible to make clear film, stable at room temperature, by quenching.
Some improvement in, clarity may be obtained by cold rolling as this
tends to dispose the crystal structure into layers.
Both homopolymers and copolymers are available in a range of molecular weights 20,000
- 100,000. The material are normally characterized by the melt flow index (MFI) using
basically the same tests as employed for polyethylene. For high precision work and
complex moulding low molecular weight polymers should be used (MFI as high as 27).
For general purpose work MFI of about 9 are employed. For extrusions and thick walled
moulding a polymer with MFI about 2.5 (molecular weight 45,000) is often employed,
although for extrusion blow moulding the special polymers used have an MFI of about
1.0.
Acetal is a crystalline polymer which requires a greater heat input for melting than
amorphous resins. This additional heat is needed to destroy the ordered crystalline
structure of the solid, and it is called the heat of fusion.
EQUIPMENTS FOR INJECTION MOULDING
1. Heating Cylinder
To deliver the necessary amount of uniformly melted resin of good quality, the adapter,
non-return valve (screw check valve), screw, and cylinder must be properly designed to
ensure maximum streamlining. This is necessary to avoid hold-up of resin, which will
lead to resin degradation. Relatively insignificant hold up spots can cause degradation
and a reduction in quality of the parts moulded. Poor seating or matching of diameters as
well as poor maintenance can also cause hold up spots.
2. Nozzle
The nozzle of the open design (not shut off) are suitable for acetals. The heater bands
should extend as close to the nozzle tip as possible and cover as much of the exposed
surface as practical to counteract loss. It is particularly important to cover the small
orifice by heat bands, where the material separation occurs. If a thermocouple is used the
well should be located over this orifice. Improper location of heater bands of
thermocouple will results in over heating of certain areas in an effort to prevent freezing
of the tip.
Screw decompression or suck back is frequently used to make control of drool easier with
these open nozzles. This feature is available with most machines.
3. Non-Return Valve
Although shut off nozzle have been used successfully with acetals they tend to cause hold
up of resin which results to brown streaks or gassings especially after some wear has
occurred in the moving parts of the nozzle. The non-return valve or check ring prevents
melt from flowing backward during injection. A non-return valve should meet the
following requirements:1. No hold-up spots. 2. No flow restriction. 3. Good seal. 4. Control of wear.
4. Adapter
The adapter have a standard tapered short cylinder sections to match with the cylinder as
well as the nozzle inner diameters and it is the intermediate between the nozzle and
cylinder. In addition to its mechanical functioning of reducing the diameter, the adapter
acts to isolate the nozzle thermally from the front of the cylinder for better control of
nozzle temperature. It also protect the cylinder from damaging that can occur due to
frequent change of nozzles, and it should be out of thermally stable good quality medium
hardened steel.
5. Screw
The length of the screw will affect the melt quality and consequently the output usable for
good parts. The preferred length is about 20 times the screw diameters (L /D = 20/1) or
20 turns when the pitch and diameter are equal. The screw should be divided as follows
45 - 50% (9-10 turns) feed section. 20 - 30 % (4 - 6 turns) transition section and 25 - 30%
(5 - 6 turns) metering section. Normal division of screw is 10 - 5 - 5. In the screws less
than 16 turns, the feed section should have 7 or 8 turns.
ﾧ
The shallow metering high compression ratio screws suggested for acetals are designed to
increase the heat input by mechanical working of the resin. Since the energy for this
increase comes from the screw motor, additional horse power must be available if an
increase in melting capacity is to be realised. The suggested screw also have to be
operated at a higher RPM to deliver equal or greater melt output during a specified screw
retraction time, because the amount displaced by one turn is less. Neither a general
purpose nor high compression screw should be operated at rotational speed so high that
unmelted particles or degradation occurs.
Usually the moulding machine injection unit chosen so that the shot size is 50% or less of
the injection stroke. For thin parts moulded at more than four shots per minute, as little
as 10 - 20 % of injection stroke capacity may be used to obtain adequate melting capacity.
On the other hand for thicker parts moulded on long cycles more than 50% of the stroke
may be used. These considerations determine the average holdup time of the resin in the
moulding cylinder.
ﾧ
6. Cylinder Temperature
Inter zone temperature control should be used and thermocouples should be placed near
the centre of each zone. Burn out of one or more heater bands with in a zone may not be
apparent from the temperature controllers, so some moulders have used ammeters in
each zone to detect changes. In latest machines PID temperature controls are available
which can give an accuracy of ±1.5°C.
7. Injection Rate
The moulding machine should be able to inject its rated capacity in less than 2 seconds
with lower viscosity acetal resins. Highly viscous acetals may have slower fill rates.
Control of injection rate, however will often be necessary to correct certain molding
problems; a slower injection rate may be required to prevent warping or jetting of resin
into some cavities. Now a days latest machines have the facility for two stage injection ie,
initially with low clamping force to release any gas or air, then with high clamping force to
prevent flashes. Latest machines have an injection time setting range of 0.1 second and
screw position digital indicator range of 0.1mm.
8. Clamping Force
The force required to hold mould closed against injection pressure depends on;
92. Injection pressure to fill the mould and pack out parts.
93. Injection speed required for filling.
94. The dimensions of parts and runners at the mould parting line (projected
area of cavities and runners)
95. Side cores, core pull requirements etc.
96. Precision and tolerance requirements.
Most well built moulds can be adequately clamped if the machine has 48-69 MPa (3.5-5.0
tons / in2) based on the projected area, since the injection pressures are seldom over 100
MPa (14,500 Psi) and fill rates are moderate. Higher available clamping pressure is
preferred and will give more freedom in choosing the most suitable molding conditions
for some molds. Very thin, long flow parts may require clamp forces as high as 100 MPa
(7 tons / in2 ) because extremely fast fill rate and high injection pressures may be
required.
START UP AND SHUT DOWN PROCEDURES
1. Start Resin Change/Purging
The following procedure is designed to prevent over heating of the resin and
contamination of the cylinder with material from previous runs.
Before loading the acetal resins to the hopper the cylinder which contains another resin
must be purged at temperatures above the melting temperature for the other resins with a
clear PS or natural colour low melt index PS (MFI 1-2) until the cylinder has been cleared.
The cylinder temperatures should be then to be adjusted to 205ºC (400°F) while
continuing to process the purge resin through the machine. When the cylinder has
reached 205°C acetal resin can be added to the hopper. After the purge resin has been
cleared from the cylinder, the moulding operation with acetal may begin.
Both PS and PE are chemically compatible with acetal resins where as PVC is not.
Contamination of acetals with such foreign materials can cause objectionable odour or
even a violet blow back. Very thorough purging is necessary to remove the last traces of
PVC from the cylinder. In the worst cases like after glass reinforced resins or severe
degradation of previous material, it may be necessary to pull the screw and clean the
equipment manually to prevent contamination of the mouldings.
2. Temporary Shut Down
A moulding machine with acetals in the cylinder at operating temperatures should not be
allowed to stand idle. When it is necessary to interrupt cycle for a minor repair, it is good
practice to take periodic air shots or shut the hopper feed gate and empty the cylinder and
screw, if the interruption will be more than a few minutes. The cylinder temperature
should be reduced to about 150 °C (300 °F), if the interruption is prolonged.
3. Normal Shut Down
The best practice is shut off the hopper feed gate, turn off the cylinder heaters leaving the
nozzle heater on and continue moulding or purging until the cylinder became empty. The
machine is then turned off with the screw in the forward position and is ready for
reheating by the procedure mentioned earlier. If another resin will be moulded next, the
acetal should be purged from the cylinder with PE or PS
