Injection Molding

1
Manual Injection Molding Machine
Introduction:
Injection molding machines can fasten the molds in either a horizontal or vertical position. The
majority of machines are horizontally oriented, but vertical machines are used in some niche
applications such as insert molding, allowing the machine to take advantage of gravity. Some
vertical machines also don't reuire the mold to be fastened. There are many ways to fasten the
tools to the platens, the most common being manual clamps !both halves are bolted to the
platens"# however hydraulic clamps !chocks are used to hold the tool in place" and magnetic
clamps are also used. The magnetic and hydraulic clamps are used where fast tool changes are
reuired.
Parts of the Injection Molding machine:
Barrel:
In the heated e$truder barrel, which contains the reciprocating screw, the thermoplastic is
gradually melted by a combination of shear heating.
Screws:
The screw design consists of the basic design based on the premises of smooth conveyance of
pellet, plasticization for melting, dearation and compression, and measurement with a little
unevenness.
Nozzle:
It is used to inject melted thermoplastic material into the mold cavity under pressure.
Clamping unit:
The mold clamp, which holds the halves of the mold closed against the injection pressure of the
melt. The clamping system consists of a fi$ed platen and a movable platen, each of which has
half of the mold attached.
Heater:
2
Since %& is molded at high temperature, the heater with heat capacity can be heated to about
'( is used, and a band heater is usually used. 0000000000000 0
Injection capacity:
The proper injection capacity is found from the relationship of the molding machine capacity for
the weight.
Types of injection molding machines:
)achines are classified primarily by the type of driving systems they use* hydraulic, mechanical,
electric, or hybrid.
Hydraulic:
+ydraulic presses have historically been the only option available to molders. The clamping
force is achieved by the hydraulic pressure.
Mechanical:
)echanical type machines use the toggle system for building up tonnage on the clamp side of the
machine. Tonnage is reuired on all machines so that the clamp side of the machine does not
open !i.e. tool half mounted on the platen" due to the injection pressure. If the tool half opens up
it will create flash in the plastic product. ,eliability of mechanical type of machines is more as
tonnage built during each cycle is the same as compared to hydraulic machines.
Electric:
The electric press, also known as Electric Machine Technology (EMT), reduces
operation costs by cutting energy consumption and also addresses some of the
environmental concerns surrounding the hydraulic press Electric presses have been
shown to be !uieter, faster, and have a higher accuracy, however the machines are
more e"pensive
Hybrid:
+ybrid injection molding machines claim to take advantage of the best features of both hydraulic
and electric systems, but in actuality use almost the same amount of electricity to operate as a
#
standard hydraulic, - robotic arm is often used to remove the molded components# either by side
or top entry, but it is more common for parts to drop out of the mold, through a chute and into a
container.
Operation of Injection Moulding:
There are three phases in this operation.
• illing phase: .uring which the ma$imum filling of cavity that done volumetrically by
simple action of plunger.
• !ac"ing !hase* In which rela$ation tolerances are satisfied i.e. in which there is switch
over from transferring pressure to holding pressure.
• Holding phase* .uring which the material compensated the shrinkage in it at the holding
pressure. That means the solidification at the holding pressure.
Induction Heating:
Induction heating is a non/contact heating process. It uses high freuency electricity to heat
materials that are electrically conductive. Since it is non/contact, the heating process does not
contaminate the material being heated. It is also very efficient since the heat is actually generated
inside the workpiece. This can be contrasted with other heating methods where heat is generated
in a flame or heating element, which is then applied to the workpiece. 0or these reasons
Induction +eating lends itself to some uniue applications in industry.
Since S,%!Self ,einforced %olymers" consist e$clusively of non/conductive polymers, which do
not offer the possibility to be heated by an alternating magnetic field on their own, ferromagnetic
heating promoters are added.
Major defects and their remedial tactics:
The major defects are as follows*
#nder pac"ing:
It is due to early switching over from transferring to holding pressure. -s a result the cavity not
copletely filled. It is due to e$cess moisture and wrapping of material. It can be avoided by*
 &ontrolled dryness of material
 &hecking the shrinkage calculation
$%er pac"ing:
$
It occurs when there is delay in switching over from transferring to holding pressure. The e$cess
coming out material is &lash' It can be avoided by
 Increasing the locking force
 1y reducing the injection speed and melt temperature
Other Defects:
Defect Causes
0lash
• Injection pressure too high
• &lamp force too low
2arping
• 3on/uniform cooling rate
1ubbles
• Injection temperature too high
• Too much moisture in material
• 3on/uniform cooling rate
4nfilled sections
• Insufficient shot volume
• 0low rate of material too low
Sink marks
• Injection pressure too low
• 3on/uniform cooling rate
5jector marks
• &ooling time too short
• 5jection force too high
)any of the above defects are caused by a non/uniform cooling rate. - variation in the cooling
rate can be caused by non/uniform wall thickness or non/uniform mold temperature.
Polystyrene:
%olystyrene is one of the best materials for injection molding. The reasons are*
 &ycle times with polystyrene are usually short due to ease of melting, fluidity and
reasonable set up times.
 &omple$ tooling can be used.
%
 The surface finish of the final product is also good, and the low mould shrinkage of
polystyrene is a useful factor.
 %olystyrene resins are amorphous and well suited for yielding solid injection moldings
with low warpage.
 %olystyrene e$hibits low water absorption relative to other thermoplastics !.6'/.678" and
normally reuires no drying before use.
The cavity pressure curve:
&
Referances:
• 1ryce, .ouglas ). !lastic Injection Molding: Manu&acturing !rocess undamentals. S)5,
9::;.
• 1rydson, <, !lastics Materials, 1utterworths :th 5d !9:::".
• &allister, 2illiam ., Materials Science and Engineering: (n Introduction, <ohn 2iley and Sons