Mold Plastic

Mold Plastic Set textbook for Website

JETRO SUPPORTING INDUSTRY PROGRAM
June 2006

SET C
1. Mold design chapter
1-1 Gate method
1-2 Guidelines for steel materials and mold life
1-3 Guidelines for steel material selection intended for plastic injection
materials
1-4 Comparison table of International Standards for steel materials of mold
1-5 Heat treatment method of steel materials
1-6 General data on steel materials used for mold making
1-7 Computation of deflection on moveable side
1-8 Computing necessary thickness of side walls of rectangular shaped
cavities(if bottom surface is integrated)
1-9 Computing necessary thickness of side walls of rectangular shaped
cavities(if the bottom surface is separated from the others)
1-10 Computing necessary thickness of side wall of cylindrical cavities
1-11 Mold temperature control method
1-12 Cross sectional shape design for “runners”
1-13 Guideline for designing tunnel gate
1-14 Head shape design for pinpoint gate
1-15 Runner lock structures for Pinpoint gate
1-16 Polishing, finishing of Plastic mold parts
1-17 Outline for food container mold
1-18 Outline for medical utensils mold
1-19 Outline for blow molding

2. Plastic injection materials chapter
2-1 predrying condition of plastic injection materials
2-2 tables showing material properties of plastic injection materials
2-3 trouble shooting for rejected works (bubbles)
2-4 trouble shooting for rejected works (sink mark)
2-5 trouble shooting for rejected works (flow mark)
2-6 trouble shooting for rejected works (burn)
2-7 trouble shooting for rejected works (silver streak)
2-8 trouble shooting for rejected works (defective brightness)
2-9 trouble shooting for rejected works (short shots)
2-10 injection materials and purpose of usage (polyamide)
2-11 injection materials and purpose of usage (polyacetal)
2-12 injection materials and purpose of usage (polycarbonate)
2-13 injection materials and purpose of usage (thermoplastic elastomer)

From page 3…

1 ‒ 1 Gate method
Gate Method
[Submarine gate (tunnel gate)]
∅d=
θ=
α=
l =
runner

Parting surface

gate tip diameter
penetrating angle
angular aperture
gate land length

==Characteristics==
① gates are automatically cut
when mold open
② gate marks are small and not
obvious
③ gating are possible on both
movable and fixed side
④ pressure keeping is difficult
to apply due to fast gate sealing

[Tab gate]
Tab
b= gate width
l= gate length
h= gate depth

runner

Parting surface

cavity

core

==Characteristics==
① gate mark is same as that of
side gate
② after staying in the tab area
for a moment, melted resins
will enter into cavities. This
is the reason why flow marks
tend to generate at the area.

[Disc gate]

l= gate land length
h= gate depth
sprue
Disc gate

cavity

parting surface

core

==Characteristics==
①gate mark appears toroidally
at the inner surface of
molded items
② it is necessary to include
cutting out process of gate
③ cylindrical filling becomes
homogeneity

[Film gate]

b= gate length
l= gate land length
h= gate depth

sprue
cavity
runner
parting surface

core

==Characteristics==
① gate mark will remain on the
side surface of the product
② it is necessary to include
cutting process of film
shaped gate
③ it is suited to filling of
thin plate shaped products

[Side gate]

b= gate width
h= gate depth
l= gate length

side gate

cavity

Runner

Parting surface

core

==Characteristics==
① gate mark will remain on the
side surface of products
② it is necessary to include
gate cutting process
③ machine fabrication of
cavities at gate areas are
easy
④ filling of resins is
relatively easy

[Under gate]

under gate

cavity

runner
parting surface

core

b= gate width
h= gate depth
l= gate length
l1= inflow area
l2= gate land length
l3= runner overlapping length
==Characteristics==
①gate mark will remain at bottom
surface of products but not on
side surface
②it is necessary to include gate
cutting process
③gates are done by scrive
process on the core side
④it is relatively easy to fill
resins

[Direct gate]

∅D= gate diameter
θ= sprue removal pitch

sprue
cavity
parting surface

core

==Characteristics==
① large gate mark will remain
at the bottom surface of
product
② it is necessary to include
gate cutting process
③ it is relatively easy to
fill resins

[fan gate]
α= opening angle
l = gate land length
h = gate depth

runner

cavity

parting surface

core

==Characteristics==
① gate mark will remain
on the side surface of
products
② it is necessary to
include gate cutting
process, but cutting is
difficult
③ it is suited for thin
plate shaped items

[Pinpoint gate]

runner lock part

runner lock pin
runner plate

cavity

parting surface

core

∅d= gate diameter
l= gate land length
α= opening angle
SR= runner tip radius
θ= runner removing taper
h1= gate relief depth
h2= depth of the basin
SR = radius of basin
∅dr1= runner lock pin diameter
∅dr2= runner lock area diameter
h3= runner lock pin undercut length
h4= runner lock area depth
==Characteristics==
① one can locate gate at random area at flat surface of product
② gate mark appears small, and not obvious
③ tend to use multiple point gate
④ scrive process of gate area is relatively difficult
⑤ countermeasures for gate cutting remains must be establish

1-2 Steel materials and mold life guidelines

Guidelines of steel
materials and mold
life sample

appliances /
general items

automobile / OA

electronic parts /
mass production /
optical parts

marageing steel

Hardness(HRC)
prehaiden steel

mold life(expected shots) (×10,000 shots)

- non heat treated type
of steel
- prehaiden steel
- age hardening type of
steel
- quenching and tempered
type of steel

1-3 Guidelines for selecting steel materials depends on types of plastic injection
materials
Examples of
selecting steel
materials
depends on
types of plastic
injection
materials

Thermoplastic
types of
resins

Types of
Resins

Forming
sample

expected
specifications
to resins

PP
ABS

Bumpers
OA enclosures

Impact
resistance

PS
PMMA
ABS

Lighting fixture
Miscellaneous
goods
Cosmetic
containers

POM
PA

Gears
Shafts

PC
PMMA

Creping
control

Recommended
steel type
S50C
SCM440

Creping
control
Mirror finish

SKD61

Abrasion
resistant

Abrasion
resistant

SKD61

Lens
Photo
conductor

Transparency
Optical
transparency

Mirror finish
easiness

SUS420J2
precipitatio
n hardening
types of
steel

PC
PMMA

CD Discs
DVD Discs
Gutter
Pipes

Mirror finish
easiness
Corrosion
resistant
Corrosion
resistant

SUS420J2

PVC

Optical
Transparency
Light
retractivity
Heat
resistant

Fire
resistan
ce ABS

TV Cabinet
Appliances part

Heat
resistant

Corrosion
resistant

SUS420J2p

PBT-GF
PA-GF

Camera
Enclosure
Electrical
Equipment

resistance

Abrasion
resistant

SKD11

Magneti
c
powder
containi
ng PA
Mg
forming

design

expected
specificatio
n to steel
materials

prehaiden
steel

prehaiden
steel

SUS

rehaiden
steel

prehaiden
steel

Printer rollers
Sensor parts

Mold ability
Magnetic
characteristic

Nonmagnetic
Abrasion
resistant

Nonmagnetic steel

Computer
enclosure
Cellular phone
enclosures

Heat
resistance
Light weight

Heat
resistant
Abrasion
resistant

Nonmagnetic steel

Thermo
setting
type of
resins

Phenol resin
Melamine resin

Dishes
Ashtray

Heat
resistant

Heat
resistant
Abrasion
resistant

prehaiden
steel
SKD11

Phenol resin
unsaturated
polyester
Epoxy resin

Switches
Connectors

IC seal
Transistor

Heat
resistant
Fire
resistant
Electric
insulation

abrasion
resistant
corrosion
resistant
Abrasion
resistant
Corrosion
resistance

SUS420J2
SKD11

SUS420J2
SKD11

1-4 Comparison tables of International Standards for steel materials of mold

Comparison tables of Standards for steel materials of mold in different countries
JIS

Carbon
steel
for
mecha
nical
structu
res

AISI SAE

AISI ASTM

BS
970
Part 1.3

DIN 17210,
17220

NFA35-551
NF EN100831,2

CK10
C10

XC10

CK22
C22

S10C

1010

S12C
S15C

1012
1015

040A10
040A10
040M10
040A12
055M15

S17C
S20C

1017
1020

070M20

S22C
S25C

1023
1025

S28C
S30C

1029
1030

S33C
S35C

1035

S38C
S40C

1038
1039
1040

S43C

1042
1043
1045
1046

S45C
S48C
S50C
S53C
S55C
S58C
S09CK
S15CK
S20CK

080M40

1049
1050
1053
1055

070M55

1059
1060

XC18
1C22
2C22
3C22
1C25
2C25
3C25

CK30
C30
CK35
C35

1C35
2C35
3C35

CK40
C40

1C40
2C40
3C40

CK45
C45

1C45
2C45
3C45

CK50
C50

1C50
2C50
3C50

CK55
C55

CK10

1C55
2C55
3C55
1C60
2C60
3C60
XC10

CK15
CK22

XC12
XC18

CK60
C60
045A10
045M10

C10

C15E4
C15M2

080A42

080A47
080M50

ISO
683/1,10
,11

XC12
CK15
C15

CK25
C25

080A30
080M30

ΓOST
14959,
4543

C25
C25E4
C25M2

Carbon
tool
steels

SK1
SK2
SK3
SK4
SK5
SK6

Y8

SK7

Y7

JIS
High
speed
tool
steel

Y13
Y12
Y11
Y10
Y9

W1-11 1/2
W1-10
W1-9
W1-8

AISI
SAE
SKH2
SKH3
SKH4
SKH10
SKH51
SKH52
SKH53
SKH54
SKH55
SKH56
SKH57
SKH58
SKH59

AISI
ASTM

BS

DIN

NF

OST

TC140
TC120
TC 105
TC 90
TC90
TC80
TC80
TC70

ISO

Alloy
tool
steel

SKS11
SKS 2
SKS21
SKS 5
SKS51
SKS7
SKS8
SKS4
SKS41
SKS43
SKS44
SKS3
SKS31
SKS93
SKS94
SKS95
SKD1
SKD11
SKD12
SKD4
SKD5
SKD6
SKD61
SKD62
SKD7
SKD8
SKT3
SKD4

F2
105WCr6

105WC13

XB4

105WCR1

JIS: Japan Industrial Standards
AISI: American Iron and Steel Institute
SAE: Society for Automotive Engineers
ASTM: American Society for Testing Material
BS: British Standards Institution
DIN: Deutche Industrie Normen
NF: Normes Francaises
Гost:
ISO: International Organization for Standardization
(Reference literature) JIS Handbooks

1-5 Heat treatment methods of steel materials
Heat treatment methods of mail steel materials
Types
of steel

Heat tempering method

Quenching method

Heat tempering
hardness (reference)

Standard quenching
Tempering
℃

Oil quenching

none

Air cooling
marquenching

air
cooling

Standard tempering

Standard quenching

air cooling

Air
cooling

preheat preheat
High temperature tempering

marquenching

cool down
preheat preheat

air cooling

Air
cooling

Tempering
℃
none

Tempering
℃
none

Air
cooling
Primary
preheat

Secondary
preheat

Cool down
Main
temperature

Air
cooling

1-6 General data on steel materials used for mold making

General data on steel materials used for mold making

Specific
ation
Material
quality
mild
steel

modulus of
longitudinal
elasticity
(Young's
modulus)

modulus ofmaximum
transverse elasticity
elasticity

E
Kgf/cm2
210 x 104

G
Kgf/cm2
81 x 104

ultimate strength
tension

com
press

shear
ing

Specific coefficient
gravity of linear
expansion

210 x 104

81 x 104

SKD11

210 x 104

81 x 104

prehaiden 210 x 104
steel

thermal
conductiv
ity

S
(1800)

(1900)

3400

1900

2900

11.7
7.85

S55C

(SCM440)

YP (yield
point)

(2500)

(2800)
7000
10000

4500
6600

(2800
)

3800
(4000
)

39

7.86

11.7

8500

7.85

11.7

12000
10800

7.80

11.5

(39)

(25)

nickel
steel
(2~3%
Ni)
Cast steel

209x104

84x104

215x104

83x104

210x104

84x104

75105x104
63x104

35007000
900012000

(2800)

2740x104

12002400

7000 8500

24x104

1500

Ni-Cr-Mo
steel
Cast steel
Brass
(rolling)
Copper
(Casting)

105x104

(2000)

(2100)
8000
–
10000

420
620

-

12501800

(4000)

1300
–
2600
1500

7.85

(11.7)

7.75

17-18

7.30
8.50

18-23

8.88–
8.95

16.5

Aluminum
(casting)
extra super
duralumin

68x104

26x104

930

2.56

(23)

197

73x104

27.5x104

5100

2.80

23.4

(202)

2600

6.9

27.4

(97)

880015000

4.51

8.2

(Alcuin300)
Zinc alloy
third type
(ZAS)

13x104
68x104

Titanium
( ) indicates reference value

850012500

Computation of deflection on moveable side

Cavity
Projected area

Cavity
mold plate
Backing plate
spacer block
Attachment plate
on movable side

B: width of mold plate (mm)
L: space inside spacer block (mm)
h: thickness of backing plate (mm)
l: length of part that receives internal pressure(p)of the cavity
b: width of part that receives internal pressure(p)of the cavity
p: internal pressure of cavity (Kgf/㎠)
E: modulus of longitudinal elasticity of materials (Young's modulus) (Kgf/㎠)
δmax ： max deflection of backing plate (mm)

<Computation sample>

Question: How thick must be bottom part(h) for the following
movable side plate using carbon steel used in
mechanical structure(S55C)?

Based on Table1
A : E of S55C is E=210 ×10

kgf/㎠

Therefore
h=3√⎯5x500x30x1144
32x210x104x250x0.01
= 42.2 (mm)
Answer : at least 42.2mm is needed
In case one allows up to σmax=0.025
h=3√

5x500x30x1144
32x210x104x250x0.025

=31.1 (mm)

In this case, at least 31.1mm is needed

[Simplified computation when we assume l= L]

δmax = 5.p.b.L4
32.E.B.h3

(mm)

h =3√5.p.b.l4
32.E.B.δmax

(mm)

Table 1. List of value “E”
material
mild steel
prehaiden steel
(SCM440system)
extra super
duralumin

E (kgf/ cm2)
210x104
230x104
(75-105)x104
73x104

Table 2. rule of thumb for cavity internal pressure(p)

mold
condition
lower setting
of injection
pressure
higher setting
of injection
pressure

P (kgf/ cm2)
200 - 400

400 – 600

Table 3. rule of thumb for maximum permissible deflection considering
parting burrs

molding
materials

maximum
permissible
deflection σmax
(mm)

good flow
ability (such as
PA,PP etc.)
those which
with general
flow ability
products which
are not affected
by burr
generation

0.025

0.03 – 0.05

0.1 – 0.2

[Variation of maximum deflection by insertion of supporting blocks]
Position of supporting block

Maximum deflection σ

σ1 generates when position
is X= 0.421 × L/2

One portion at
the center for
supporting
block

※ δmax = maximum
deflection when there is no
supporting blocks

(Note) above computation may applied when supporting block is inserted with the
width equivalent to mold plate width B. On the other hand, above computation will
not applied once there is (are) insertion of supporting pins
Supporting
Supporting block
pin

Diagram 1 Case that above
computation applies

Diagram 2 Case that above
computation will NOT apply

Altered condition
Once one creates 1/2 of
inner support block space

once backing plate
thickness h become h1

Maximum deflection σ

Computing necessary thickness of side walls of rectangular shaped cavities (if
bottom surface is integrated)

core

cavity

l/a
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
3.0
4.0
5.0

C
0.044
0.053
0.062
0.070
0.078
0.084
0.090
0.096
0.102
0.106
0.111
0.134
0.140
0.142

p: cavity internal pressure (kgf/㎠)
l: cavity inner length (mm)
h: thickness of cavity side surface (mm)
a: sidewall height of cavity internal pressure (p) receiving area (mm)
E: modulus of longitudinal elasticity(Young's modulus) (kgf/㎠)
c: constant derived from l/a ratio
σmax: maximum deflection (mm)

<Computation sample>
Question: How much should be a minimum cavity side wall thickness
“h”for the following bottom surface integral hard steel mold?

380
381

From Table 1. E= 220 × 10⁴kgf/㎠
yh, l/a value is considered as modulus of longitudinal elasticity of
hard steel

382

l/a= 40/10 = 4

from the table it becomes
384 therefore
385
386

Answer : at least 2.95 is needed
at least approx.5mm is needed including over design thoughts

Computing necessary thickness of side walls of rectangular shaped cavities (if the
bottom surface is separated from the others)

Core

Cavity
Bottom bushing

p: cavity internal pressure (kgf/㎠)
l: cavity inner length (mm)
h: thickness of cavity side surface (mm)
a: sidewall height of cavity internal pressure (p) receiving area (mm)
b: cavity height (mm)
E: modulus of longitudinal elasticity(Young's modulus) (kgf/㎠)
σmax: maximum deflection (mm)
h = 3√12.p.l4.a
384.E.b.σmax
(note) this formula does NOT apply if it is bottom attached type of cavity

Table 1 Table for value “E”
Materials
mild steel
hard steel
prehaiden
steel
(SCM440system)
cast steel
extra super
duralumin

E (kgf/cm2)
210x104
220x104
230x104
(75-105)X104
73x104

Table 2 Guidance for cavity internal pressure (reference value)
forming condition
lower injection
pressure setting
higher injection
pressure setting

200 – 400
400 - 600

<Computation sample>
Question: How much minimum thickness “h”do you have to provide on the side wall of
hard steel cavity which the bottom surfaces are divided as drawing?

A: if the maximum deflection of cavity side wall is σmax= 0.01mm, we will know
the modulus of longitudinal elasticity of hard steel is
E = 220 × 10⁴kgf/㎠ based on Table 1.
Therefore
at least 7.83mm is needed
hopefully 10mm is given including some allowance purpose

Computing necessary thickness of side wall of cylindrical cavities

p: cavity internal pressure (kgf/㎠)
R: cavity external radius dimension (mm)
r: cavity internal radius dimension (mm)
σℷ：tangential allowable stress of cavity
(kgf/㎠)

Table 1. value ofσℷ

material quality
mild steel
hard steel
cast steel

900 ‒ 1500
1200 ‒ 1800
500 ‒ 750

<Computation sample>
Question: How much external radius of the following cylindrical
cavity shape made of SKD11 is needed?

Answer : SKD11 is hard steel materials, and therefore value of σ ℷ from
Table1.becomesσ₁= 1200 ∼ 1800 kgf/㎠. Just to give allowance on the
computation, we use smaller value to setσℷ= 1200 kgf/㎠.
430 therefore

Answer: At least 7.07mm is necessary. Giving some extra space, we will make it
10mm.

1-11 Mold temperature control method
Some of the engineering plastic and super engineering plastic generate
cavity temperature over 100 .
It is difficult to raise the cavity temperature by water temperature control,
once cavity surface temperature exceeds 90 .
Following methods are used generally for countermeasure.
1. Oil temperature control
Temperature is kept constant by oil exhaled from recycling pump passes
channels in mold and cavities, passes joint hose and circulate. Once
temperature will raise up to setting temperature, it is relatively easy to
sustain stable temperature.
But the weak point is, it takes time to startup temperature.
Also, there are other concerns like potential danger of burns, and difficulties
of post treatment.
2. Electric heater
Electric heater (cartridge heater) temperature control may be done with
temperature sensor (thermo-couple etc.) so that temperature will be kept
constant.
Since the heat capacity is big, startup temperature is fast.
Difficulty is that heater surroundings become high temperature, but low
temperature at isolated area from heater which means keeping heat
distribution constantly.
Heaters have product life, and therefore we must replace heater periodically.
For the heater attachment, it is important to provide right clearance at
attachment hole. Too big clearance will cause a situation like boiling without
water, and reduce heater life.
MISUMI M-HTM3021 (Temperature controller for cartridge heater) can
control temperature by PID control method, therefore quite stable
temperature control than “ON-OFF type controller” is possible.
It is recommended to provide heat intercepting board in between platen and
mold attachment plate of injection machine. It is further recommended to
provide heat intercepting board surrounding mold plate.

[Table] Example of molding materials with high cavity surface
temperature
Name of plastic

cavity surface temperature(

PPS Glass fiber 30%
(polyphenylene sulfide)

130∼150

LCP Glass 40%
(Liquid crystal polymer)

70∼110

PET
(polyethylene terephthalate)

130∼150

PA 46
(Polyamide/46 Nylon)

80∼120

PC
(Polycarbonate)

80∼120

heat resistant PLA
(heat resistant polylactic acid)

110∼120

PEEK
(polyether ether ketone)

120∼160

PI
(polyimide)

170∼200

)

In injection molding of PPS resins and liquid crystal polymer, one need to
maintain the cavity surface temperature above 100 . Therefore it is
necessary for one to control temperature thru oil or cartridge heater.
Commercially available “Cartridge heater” utilize [ON-OFF control] by power
ON and OFF to control temperature.
“ON-OFF Control” is simple structure to shift switch that the cost is cheap.
On the other hand, it has weakness like,
(ア)
cavity surface temperature gap is big
(イ)
instability
Instability cavity surface temperature will cause defects such as shrinkage
of dimension, unevenness of surface brightness on precision products.
To stabilize cavity surface temperature, we use “PID control”.
PID control is
P: Proportional
I: Integral
D: Derivative
PID control applies above to reduce lead time to stabilize temperature.
Heat intercepting board is very important parts considering that it stabilizes
temperature of injection mold, materialize energy conservation while
keeping the temperature.
Heat intercepting board has following usage
1. fixed to platen of injection mold machines and use
2. fixed to back of attachment plate of mold and use

Following are selection standard of Heat intercepting board.
1. Heat resistant temperature
Following are recommended working temperature as guidance
●
●
●
●
●

recommended
recommended
recommended
recommended
recommended

working
working
working
working
working

temperature
temperature
temperature
temperature
temperature

below
below
below
below
below

100
180
220
400
500

2. Material
Materials are related to recommend working temperature and crushing
strength.
There are following types of materials.
●
●
●
●
●
●

cotton cloth ＋ Phenol resins(bakelite)
craft paper ＋ Phenol resins(bakelite)
Glass fiber ＋ silicate binder
Glass fiber ＋ super heat-resistant epoxy resin
Glass fiber ＋ phosphate binder
Glass fiber ＋ borate binder

1-12 Cross sectional shape design of runner
Runner is channel to let molted resins to flow from sprue to product.
Selection of cross sectional shape of runner depends upon product size,
resin types, expected molding condition etc.
Following are basic standard for the selection of runner cross section shape.
[Diagram] shows main runner cross section shape
parting surface
movable side
angle
thin

fixed side
angle
inscribed circle

One may select runner from following 3 types
1) carve on movable side
2) carve on fixed side
3) carve on both movable and fixed side
Selection depends on shape restriction and mold parting position.
Symbols such as ◎: excellent , ○: proper, ×: inadequate
Important roll of runner is to let molten resin flow under minimal pressure
loss condition. Unnecessary large sized runner will increase scraps, worsen
material costs, prolonged molding cycle, and increase volume of disposing
waste.
Index to indicate runner efficiency is “inscribed circle diameter area” at
cross sectional shape. The bigger the space of inscribed circle, wider the
area that hot resins will flow, and therefore one can expect better flow of
molten resins.
So the ideal is the runner on round cross section shape. But cost wise this
becomes more expensive for mold cost because one must carve runner on
both movable and fixed side.
To comply on such nonconformity, further biting of trapezoidal shape or
semicircle cross section are utilize.
It is better to provide angles on the side of runner so that mold release
becomes better. On the other hand inner surface of runner must be polished
by rubber wet stone or rapping to avoid pressure loss.

1-13 Guideline of tunnel gate design
Tunnel gate (submarine gate) is widely used gate method where the
product and gate are automatically cut every time parting surface opens
and close.
It is necessary for one to know the basics such as shapes and dimension to
design tunnel gate. But for this time, we are introducing to you the basic
variation of molded products and gate, runner relation.
In the [Diagram], shows basic patterns of tunnel gate.
Put parting surface between, there are 4 patterns of gate-runner
combination on fixed side and moving side.

Fixed side tunnel gate
Fixed side runner

Movable side tunnel gate
Movable side runner

Fixed side tunnel gate
Movable side runner

Movable side tunnel gate
Fixed side runner

Movable side tunnel gate
Arrangement for hub
Fixed side runner

In case if tunnel gate is provided on the fixed side, cutting of product and
gate will be done while parting surface opens. Therefore, gate cutting
condition varies depending on mold opening speed.
On the other hand, in case if tunnel gate is provided on the fixed side,
cutting of product and gate will be done when runner ejector pin pushes
runner. This means, gate cutting condition depends on protruding speed of
runner ejector pins.
In case runner is provided on the fixed side, it is necessary to make
structure that the lock pins will pull runner toward movable side, because
there is a chance that runner itself may be left on fixed side.
In case runner is provided on the movable side, we must set ejector pin to
push out runner properly.
In special case, there are times that boss-shaped (sharpen up ejector pin)
tunnel gate is provided on the movable side, and fill resins from back
surface of top panel.
In actual practice, we must decide what type of gate and runner to use on
mold design based on product characteristics, and material properties.
If specification prohibits any trace of gate mark on side panel or top panel,
we rarely have to provide gate on back surface of products.
In such a case, “Curved tunnel gate” is rarely used.

Runner

Curved tunnel gate

Cavity

Ejector pin

Gate holding hub
Ejector pin

In the structure of curved tunnel gate, bulging gate shape extends from
parting surface to inner movable core. Therefore, gate opening locates at
top plate of core.
Ejector pins are provided on runner near the gate area, and boss is
provided at the top part of pin part to hold gate. Overall length of boss is
“H”. During the ejection, so as to undergo smooth gate cutting, it will hold
gate until product will be cutout from core completely.
Final shape of bulging part is usually corrected after several trial-and-error.
It is wise to set nested split type structure from very beginning, so that the
correction of mold become easy.
Gate passage is always affected by mold cooling time and pressure keeping
time. Try several variation as sample for prototype.
Whichever method one may choose, cutting condition at cutting area and
scum become common problem upon cutting product and gate.
Following factors may affect cutting condition
1. Gate tip shape design
2. gate size (cross-sectional shape)
3. distance from gate cutting part to runner lock part
4. acting condition of pressure keeping
5. orientation of high molecules at gate part
6. cutting timing of gate and products
Careful analysis must be made before entering into preparing mold for
precision mold and multi-cavity mold.

1-14 Pinpoint gate tip shape design
Following are potential problems on pinpoint gate structure.
1) Gate tip portion protrude and remain on the product surface, or pluck off
part of product
2) In comparison to high filling pressure and keeping pressure, filling cant
be done smoothly
Above are common problems and headaches of mold designer upon using
“Pinpoint gate”.
Following are technical trouble shooting.
[Diagram1.] Shows general pinpoint gate structure. Gate design without
any consideration usually looks like this.
On the other hand, [Diagram2.] shows contrive design to minimize above
problems.

2. Gate
Opening angle
1. gate land

3. thickness relief

Length

4. Dimple

(basin)

Point 1 Gate land length L
If the gate land is unnecessary long length, it may remain as protrusion,
because gate will be cut in the middle portion.
From the study, we recommend that Gate length(L) size be 1∼2 times the
diameter of gate tip.
Point 2 Gate open angle A
One must provide tapering to open angle and create cone shape. As long as
it is cone shape, strong chance that contact point of minimal cross section
gate area and product shall be the cutting point.
Mold release becomes easy too. Value A is usually 15°∼30°. Bigger the
value, more assurance on cutting, but wearing out at the tip portion tends
to become faster.

Point 3 Slot
Once “Slots” are provided, they will encroach 1 step into the surface of
molded products. Because of this, protrusion will not exceed the surface
level of products even if it remains at cutting area.
One must first acquire “Approval” of product designer in prior to set slots on
the drawing.
Point 4. Dimple
Dimples are spherical shape concave provided at the opposite side of the
gate which has almost same wall thickness to product, so that molten resin
will flow stable when slots are created.
One must also first acquire “Approval” of product designer in prior to set
dimples on the drawing.

457

1-15 Runner lock structures for Pinpoint gate

Runner lock shapes in pinpoint gate structures have several patterns,
however usually applied pattern is indicated in Diagram 1.

Diagram 1.
Product

Gate

Runner
Fixed side mold plate +
Cavity

Narrow
Runner

Runner plate

Fixed side attachment plate

Runner lock pin

In the [Diagram1.], locate runner lock pin which has undercut shape at the
head area into the runner foundation. When fixed side of mold plate and
runner plate create a space, they will compulsively separate runner and
gate.
Runner lock pin will be engage by clearance fit to runner plate, and fixed to
fixed side attachment plate by plate and screw plug.
In this method, if one will work on thin product or resin with higher
pressure loss, channel within runner from the head portion of runner lock
pin becomes narrower. In this case, it is necessary for one to provide higher
filling pressure or pressure keeping as molding condition.
In this case, as indicated in [Diagram2.], one may set lock area one step
lower within runner plate. In this manner, one is necessary to create
coniform curving on top of runner plate. For this process, mold cost may
become higher, but be able to improve pressure losses during molding.

Place runner
locks one step
down

If incase one wish to reduce the molding cycle, create a pointed conifom
shape at tip of runner lock pin shown in [Diagram3.], which will also act
as slot like lock shape. In this way, excess materials at the runner
center will be reduced, therefore more efficient cooling is done, then
contribute to reduction of cycle.
But one must take care of coniform shape portion balance, because
poor balance at the area will cause fracture of cuneiform shape from the
base. One can also provide corner radius to improve strength.
One must select proper runner lock, by first determining expected roll of
runner lock in prior to design mold.

Encroach into inner side
also for the purpose for
relief thickness

1-16 Polishing finish of plastic mold parts
Cavity surface of plastic injection mold undergo mainly hand polish or
machine polishing after milling, EDM, wire cut process to smoothen the
surface.
Transcription surface for products be dull, and surface quality may
become poor if polishing condition is not good.
凹凸 surface may become potential undercut upon separating product
from cavity, and cause defective mold release.
Hard wetstone and abrasive grain are utilized for cavity polishing. You
may use rough mesh number (larger grain) to finer mesh number. Use
grinding fluid while making sure that clogging and galling will not take
place.
Polishing is not only done one direction but relative position and
circumferential direction so that polishing surface will be uniform.
Following are main polishing materials.
Natural polishing agent - silica group (agate, alcansas)
- Corundum group (emery, garnet)
- Diamond powder
Synthetic polishing agent - alumina (Al2O3)
- Silicon carbide (SiC)
- Boron carbide (BC)
- Synthetic diamond
Grain size of polishing materials ranges from #10∼#20000. Smaller the
value, rougher the mesh.
(Ex. Using #10∼#30 polishing results rough finish, but #1000∼#2000
polishing results finer finish)
In case one will use fine abrasive grain, we mix olive oil or vegetable oil
into maple, pine, willow, balsa like trees and polish.

1-17 Food container mold overview
Many plastics are used as food container. PET bottles, food cups,
wrappings are mostly plastics.
It is necessary to give extra care on food container quality, so that man
can enjoy foods not worrying hygienics, at the same time not to
damage lips and tongues by burrs.
Any pores or cracks on plastic will allow bacteria to infiltrate into foods
and spoil them. Such quality defects are not allowed too.
There are following major food containers
●
●
●
●
●
●

margarine container : PP
ice cream container : HIPS, HDPE
lactic acid bacteria beverage : HIPS
pudding container : PS, PP
confectionary container : HIPS, PP, HDPE
soft drink container : PET

Necessity of function depends on food purposes such as thermostability
(for foods necessary to heat up), low thermal resistant(for food
necessary to freeze, refrigerate), gas barrier(for those foods that can
not be exposed to oxygen).
For food containers, sales of the foods will also depends on product
design(beautiful finish), which means 3 dimensional curved surface
shape and design become significant. Therefore, in product design and
mold design, one needs 3 dimensional solid data.
There are always chances of burrs to generate at parting surface,
therefore location of parting surface must be paid attention.
This is same for gate position and method.
Cavities, and cores must be fabricated by corrosion resistance steel
materials. It is better that one will not apply grease nor slide promoting
oil. This means non lubricating mold mechanism is recommended.
Electric injection machines are ideal, further ideal to work inside clean
rooms.
Its worth trying using valve gate and hot runners. If mass production is
expected, one can also use scrap press effects and high cycling.

1-18 Medical utensils mold overview
Many plastics are used in medical utensils. In many cases, injection
molding method is used for production.
This means many parts of medical utensils have been produced thru
mold making.
There are following major plastic medical utensils.
1.
2.
3.
4.
5.
6.

syringe (syringe body) : PP, PE
piston of syringe : PP
pipet chips : PP
catheter : PVC, PC
blood collecting test tubes : PC
dish for culturing : PS

For plastic medical utensils materials, only those which are allowed by
drug legislation and quality standard prescribed by Ministry Of Labor
and Health can be utilize. Those materials must withstand exposures of
UV lights and gamma rays used for sterilization, at the same time
passed the clinical test for blood coagulation reaction, allergies etc.
Basically, medical utensils are disposable. Therefore those materials
must be environmental friendly upon incineration.
It is recommended that mold materials shall be non rusting type. This is
the reason why many cases, medical utensil molds are made of
stainless, ion plating film, hard chrome plating so as to prevent
corrosion of core, cavity.
Valve gate molds are used in mass production items.
Needless to say that burrs for products are defects, therefore use
precision guide to locate movable side and fixed side, and also try to
apply contraption preventing the abnormal worn out of core pins of
centering location structure.
There must be delicate temperature setting for cavities and cores to
produce stable quality products. For this point, one must pay attention
to the structure of cooling circuit and heat pipes.
Cooling structure in core becomes very important for cylinder body of
syringe.
Removal of gases and resins on the mold surface is related to quality.
Gas vent settings and enforced ventilation structures are applied.

1-19 Blow molding mold overview
Blow molding method is usually applied to pet bottle containers for
juices (PET : polyethylene terephthalate resin) which blow methods are
used.
It is familiar in all over the world because it is used as shampoo
container, soy sauce and other seasoning container, detergent container
etc.
Polyethylene terephthalate, polypropylene, PVC, nylon, polycarbonate
may be used for blow molding method.
Exclusive blow machine is used for blow molding. In the mold, there is
only cavity on female mold, but no core on male mold.
Instead, there is a nozzle prepared to blow air. Balloon like performing
shape part called “Parison” blows, stick to cavity and transcribe shape.
Generally speaking, soft metal materials are used for cavities in blow
molding. These are aluminum alloy, bronze, and special steels.
Usually beautiful brightness surface is required to product, therefore
polishing of inner cavity surface must be done properly.
In beverage containers blowing, multi-cavity moldings are practice
generally which means contraption on cooling and temperature control
are important to comply high-cycle operation.
Also design of products that incorporated recycle is important, because
most of those blow items will be disposed after the consumption.

2-1

pre-drying condition of plastic molding materials

Usually, plastic molding materials comes in pellet form in the paper bag
upon their delivery.
Pellets absorbs moisture in the air, therefore hydrolysis may take place
on some resins in the process of molding if pellet still contains moisture,
or degrade physical properties of materials. There are also cases that
silver streak may appear on the surface of product, or short shots may
occur due to gases, and burns may be generated too.
Many cases, materials are first placed into box shape pre-drying device
before placing them into the hopper dryer.
It is recommended that proper drying temperature and time are take
into consideration. Moisture can not be eliminated completely even
placed for long period, if drying temperature is below the proper setting.
Quick consumption of materials after pre-drying is recommended. If
there will be any left over of materials in a daily production, you must
undergo pre-drying again before usage.

Pre-drying temperature of plastic molding materials
Name of materials

symbol

predrying temperature℃ drying time(H)

Liquid crystal polymer

LCP

110 – 150

4-8

Polyetherimide

PEI

120 – 150

2-7

polyamide imide

PAI

150 – 180

8 - 16

thermoplastic elastomer

TPE

120

3-4

polyether ether ketone

PEEK

150

8

polyphenylene sulfide

PPS

140 – 250

3-6

polyalylate

PAR

120 – 150

4-8

polysulfone

PSU

120 – 150

3-4

ABS resin

ABS

70 – 80

2-3

Acryl

PMMA

70 – 100

2-6

Polycarbonate

PC

120

4-6

Nylon 6

PA6

80

8 - 15

Nylon 66

PA66

80

8 - 15

Nylon 11

PA11

70 – 80

8 - 15

Nylon 46

PA46

80

8 - 10

Polyacetal

POM

110

2-3

PBT

PBT

120

4-5

Pellets of Plastic molding materials usually absorbs certain degrees of
moisture in the air. If there will be too much moisture absorption,
hydrolysis may take place (some resins cause chemical decomposition
by water) in the cylinder of injection machine during molten and
kneading process. Sometimes, silver streak, bubbles, defective
brightness, defective transcription may occur during injection.
To avoid above problems, we must first place the pellets in the drying
machines to remove water content.
Variation of flow of materials, degrading physical properties, defective
molding may happen if we do not give out predrying properly.

2 – 2 Tables showing material properties of plastic molding materials
Main plastic material physical properties list
Thermoplastic type of Plastic
Resin name
Grade

JIS testing
method

Mold
ability

High steel
ability

Heat resistance
Glass fiber
20 – 40%

Filling materials
Abbreviation

ABS

Drying temperature

70~80

70~80

70~80

Drying time

2

2

2

Injection mold cylinder
temperature

200~260

250~300

200~260

Injection mold temperature

50~80

50~80

50~80

Injection molding pressure

560~1760

560~1760

1050~2810

Compression mold
temperature

160~180

160~260

Compression mold pressure

0.7~506

0.7~5.6

Mold contraction rate

0.4~0.9

0.4~0.9

0.1~0.2

K6911.
K7112

D792

Specific gravity (density)

1.03~1.06

1.05~1.08

1.22~1.36

K6911.K7113

D638

Tensile strength

400~530

400~560

570~740

Elongation percentage

3.0~20.0

5.0~25.0

2.5~3.0

Compression strength

127~879

505~702

844~1550

Bending strength

773~914

703~1050

1120~1900

Impact strength (Izod)

10.9~33.7

10.9~35.4

5.4~13.1

Hard ness (Rockwell

R107~115

R100~115

M65~100

Heat resistance (continuos)

71~93

88~165

93~110

101~112

85~107

107~122

93~119

Mechanical
properties

Thermal
Characteristic

A.S.T.M.
Testing
method

ABS resin

Heat distortion (℃) a)bending
stress
K7206.
K7207

b) bending stress

99~108

Thermoplastic type of plastic
Etylene-vinyl
acetate

AS Resin

Polyamide (Nylon)

General

Nylon 6
Glass
fiber 20 –
30%
SAN

Nylon 66

Glass fiber
30%

EVA

Nylon 11.12

Glass fiber
30%

-

PA6

PA66

PA11.12

85

85

80

80

80

80

70~80

2~4

2~4

8~15

8~15

8~15

8~15

8~15

200~260

200~260

120~230

240~290

240~290

260~300

260~300

190~270

50~80

50~80

20~60

40~120

40~120

40~120

40~120

20~100

710~2320

1050~2810

562~1410

150~200

90~150

70.3~703

0.04~1.76

0.2~0.7

0.1~0.2

0.7~1.2

0.5~1.5

0.4~0.6

0.8~1.5

0.5

0.3~1.5

1.07~1.10

1.20~1.46

0.92~0.95

1.12~1.4

1.35~1.42

1.13~1.15

1.38

1.03~1.08

600~840

600~1440

95~200

700~850

1650

770~850

1850

530~550

1.5~3.7

1.1~3.8

500~900

200~300

3~6

150~300

3

300~500

984~1200

1550

914

1340

1050

2070

984~1340

1550~1830

1270~2320

429~1200

471~1260

Non
destructive

3.3~5.4

16

4.3~5.4

12

10~30

D17~45

R119

M101

R120

M100

R106~109

82~121

93~149

82~121

82~121

82~149

M80~90

M100~E60

60~96

93~104

88~104

88~110

33.7

88.1

210

74.8

77

54.2

101~115

77~80

203

239

208

236~239

167

Thermoplastic type of Plastic
Methacrylate
resin (acryl)

Polyacetal
General

-

General
Glass fiber
20%

PMMA

General

High
impact

Thermos
ability

Glass
Fiber 40%

-

POM

PP

PS

HIPS

PS

70~100

110

110

2~6

2

2

190~290

180~230

180~230

200~300

200~300

170~280

190~280

199~280

40~90

60~120

60~120

20~90

20~90

20~60

10~80

20~80

703~1410

703~1410

703~1410

703~1410

703~1410

703~2110

703~2110

703~2110

149~218

171~288

171~288

129~204

121~204

129~204

141~703

0.35~0.70

0.35~0.70

70.3~703

70.3~703

70.3~703

0.1~0.4

2~2.5

0.4

1.0~2.5

0.2~0.8

0.4~0.7

0.4~5.7

0.2~0.6

1.07~1.20

1.41~1.42

1.61

0.90~0.91

1.22~1.23

1.03~1.05

1.03~1.06

1.05~1.09

470~770

580~800

1250~1300

210~400

560~1000

350~840

200~350

350~530

2~10

25~75

3

100~800

2~4

3~4

13~50

2~60

844~1270

1270

1200

260~562

387~492

809~1120

281~633

914~1340

991

1970

352~492

492~773

562~984

211~844

1.6~2.7

5.4~13

10

2.2~110

7.6~11

1.4~2.2

3.3~20

M85~105

M78~94

M79

R50~111

R102~75

M60~75

M10~80

59.8~93

90

104

88~115

121~138

65.3~76.5

59.3~79.2

73.7~99

124

110

45.9~59.8

59.8~93

104

90

79.2~107

170

158

103~130

117~161

82~110

82~104

2.2~19
M70

90

Thermoplastic type of Plastic
Polycarbonate
Low
density

General
Glass fiber
less than
10%

Polyethylene
Moderate
density

polybutylene
terephthalate
High
density

Glass Fiber
10 – 40%

PC

Glass fiber
20 – 30%
LDPE

MDPE

HDPE

PBT

120

120

120

120

120

>4

>4

>4

4

4

270~380

270~380

270~380

150~270

200~300

200~300

230~280

230~280

80~120

80~120

80~120

20~60

10~60

10~60

40~80

40~80

700~1410

700~1410

1050~2810

562~2110

562~2110

703~1410

562~1800

562~1800

249~326

135~176

149~190

149~232

0.7

7.03~56.2

7.03~56.2

0.35~0.56

0.5~0.7

0.2~0.5

0.1~0.2

1.5~5.0

1.5~5.0

2.0~6.0

1.5~2.0

0.2~0.8

1.19~1.20

1.27~1.28

1.24~1.52

0.91~0.925

0.926~0.940

0.941~0.965

1.31~1.38

1.52

550~700

630~675

840~1760

42~161

84~246

218~387

550~540

110~1340

100~130

5~10

0.9~5.0

90~800

50~600

20~130

50~300

2~4

844

984

914~1480

190~253

605~1020

1270~1650

949

1050

1200~2250

844~1170

1830

75~100

5.5

11

Non
destructive

2.7~87

2.7~110

4.4~5.4

7.0~8.7

R115~125

M75~85

M88~95

D41~50

D50~60

D60~70

M68~78

M90

121

135

135

82~100

48.7~121

121

49.8~121

115~176

129~140

142

143~149

40.3~48.7

40.3~48.7

43.1~54.2

49.8~85

220

132~143

146

149~154

48.7~73.7

48.7~73.7

59.8~88

115~193

225

337~492

Thermoplastic type of plastic
Polystyrene

Polyvinyl chloride
Flexible

rigid

Glass fiber
20 -30%

-

-

PS

SPVC

HPVC

Fluororesin

Polyphenylene chloride

Crystal liquid polymer

Glass fiber
40 %
FEP

Glass
fiber 40%

PPS

LCP

120~140

120~140

140~160

140~160

3~5

3~5

4

4

170~280

160~190

170~210

370~430

315~330

315~360

250~310

250~310

20~80

10~20

10~60

95~230

130~150

130~150

70~110

70~110

1050~2810

562~1760

703~2810

352~1410

300~1000

300~1000

150~500

150~500

140~176

140~204

315~399

35.2~141

52.7~141

70.3~140

0.1~0.3

1~5

0.1~0.5

2~3

0.6~0.8

0.2~0.4

0.1~0.8

0.1~0.55

1.20~1.33

1.16~1.35

1.30~1.58

2.15~2.17

1.3

1.60~1.67

1.35

1.7

633~1050

100~240

400~500

180~210

630

1500~1550

1060~1335

900

1~2

200~450

40~80

250~330

1~2

0.9~4

1.3~4.5

1.8

949~1270

63~120

562~914

155

1.4~2.2

2.2~100

703~1120
Largely
change

Non
destructive

<2.7

5~8

13~21

8.7

M70~95

A50~100

D68~85

D60~80

R123

R123

R60~63

R79

82~93

54.2~79.2

204

90~104

59.8~76.5

97~110

57.0~82

135

250~265

332~335

319

738~1410

69.8

2-3 Countermeasure on defective molding
“Bubble” “Void” are phenomenon where air bubbles are generated within
molded products. These bubbles or voids will be considered rejected on a
product such as lens, prism because appearance and optical characteristic
be disturb by such defect. It will also reduce strength and become cause of
destruction of machine once those defective items are used as machine
parts.
There are 2(two) major cause of bubbles.
One cause is that, air was mixed into molten plastic. This is called bubbles.
Other one is caused in the process of shrinkage of molded articles known as
“Void”. In the void, insufficient pressure keeping was applied to thick wall
area, and this caused abnormal shrinkage just like this happened in the
process of generation of “Sink mark”.
Following can be possible countermeasures for above problems.
Countermeasures for bubbles
1. Countermeasures related to mold
1. There are no air vent or lack of numbers of air vent
2. There is no cold slag well, or it may be too small

2. Countermeasures related to injection molding condition
1. too fast screw rotational speed
2. too high cylinder temperature
3. too fast injection speed
3. Countermeasures on product design
1. insufficient predrying of mold materials
Countermeasures on void
1. Countermeasures on molds
1.
2.
3.
4.

There is no air vent, or lacking
There is no cold slag well, or it is too small
too small sprue, runner
too small gate

2. Countermeasure related to injection mold condition
1. Too high cavity surface temperature
2. Too low pressure keeping
3. lack of pressure keeping time
3. Countermeasure on product design
1. insufficient predrying of molding materials
2. too thick molded product wall thickness

2-4

Countermeasures on defective molding (sink mark)

“Sink mark” is a phenomenon which generate slight concave due to
shrinkage of product surface.
It may become defective quality in case product used for surface
appearance.
There are following methods to control sink mark.
1. Countermeasures related to mold
1.
2.
3.
4.
5.
6.

Set the cavity surface temperature slight lower
Make gate size bigger
Make runner size larger
Use larger sprue
Review cooling channels of mold to improve cooling efficiency
Improve the structure where difficult cooling area to become easier
to cool. (Ex. Baffle plate structure, cooling pipe structure, heat pipe,
non-ferrous bushing)
7. Increase the number of gate
8. Relocate gate position to thick wall area

2. Countermeasures related to injection mold condition
1.
2.
3.
4.
5.
6.
7.
8.

Set pressure keeping time longer
Set pressure keeping to higher setting
Set the injection speed faster
Set nozzle temperature lower
increase indiscrete value
Increase cushion volume
Change the injection mold machine
Exchange the “backflow prevention ring” of injection unit

3. Countermeasure on product design
1. remove the thick wall area of products
(Ex. Material relief shape, alter into another parts)
2. apply amorphous resins

2-5

Countermeasure for defective molding (flow mark)

“Flow mark” is a phenomenon where melted resin flowing marks remains
on the surface of products. This may become a cause of reject depends
upon degree of such mark appearance. Products such as electric appliances
or cosmetic product casing are particular about such appearance.
Flow mark is generated in the process that molted resins contact metal
surface in the mold will encounter different degree of cooling at the tip of
resins.
Following methods are considered to improve the flow mark.

1. Countermeasures related to molds
1.
2.
3.
4.

set the cavity surface temperature lower
expand the size of the gate
enlarge the size of the runner
secure enough cold slag well

2. Countermeasures related to injection molding condition
1.
2.
3.
4.
5.
6.

set the injection pressure higher
set the injection speed faster
secure enough measurement to increase the cushion volume
set the pressure keeping time longer
set the resin temperature higher
enlarge the nozzle tip diameter larger

3. Countermeasures related to molded article design
1. set the variation of wall thickness of molded articles smaller

2-6 Countermeasures on defective molding (Burn)
“Burn” is a phenomenon where black burning materials are generated on
the surface of molding articles. Air which remained in the cavity while
compressed will generate heat, and this heat burns the plastics when
molten plastics are filled into cavity.
Burn can be appearance defect, causing missing parts, and decreasing
material properties.
Following are considered countermeasures on burns.

1. Countermeasures related to mold
1.
2.
3.
4.

provide the air vent
prepare deeper air vent and secure enough vent passage
dismount mold and wash give out maintenance to air vent
use insertion mold structure or bushing structure so that air will escape
from parting surfaces
5. Use vacuum aspirator device
6. change the gate position
2. Countermeasures related to injection molding condition
1.
2.
3.
4.

set the injection speed slower
set the cylinder temperature lower
make smaller measurement
avoid staying of resins in the cylinder

3. Countermeasures related to molded article design
1. try to apply design that avoids thinner wall areas of molded articles
2. change the wall thickness of molded articles, and flow of plastics
3. alter the design that allows air to stay

2-7 Countermeasures on defective molding (Silver streaks)
Silver streak (silver film) is a phenomenon where shinny stripe marks are
generated on the surface of molded articles. This may become potential
rejects as an exterior parts for electric appliances, automobile, bicycle for its
defective appearance quality.
Silver streaks occur because the air and volatile gas in the molding
materials come out on the surface of products.
Following are potential countermeasures on silverstreaks.

1. Countermeasures related to mold
1. improve the functions of air vent
2. enlarge the gate size
3. enlarge the cold slag well
2. Countermeasures related to injection molding condition
1. confirm the preheating condition(temperature, drying time) of
molding materials and apply proper drying.
2. set the injection speed slower to slower filling
3. set the cylinder temperature lower
4. reduce the screw rotation speed
5. avoid staying in the cylinder
3. Countermeasures related to molded article wall thickness
1. prepare as much as possible the uniform wall thickness

2-8 Countermeasures on defective molding (defective brightness)
Ideal surface of molded articles must be the transcribed surface appearance
of cavities, but there are times that surface of molded articles exhibit
obscured or uneven brightness surfaces.
This may become major defect cause on the product exterior surface which
the appearance quality is considered significant.
Following are potential countermeasures on defective brightness.

1. Countermeasures related to mold
1.
2.
3.
4.
5.
6.

there might be NO air vent or lack of air vent
small gate
small and narrow sprue and runner
cavity plating is NOT good condition
cavity surface polishing is NOT good condition
deposits are attached to the surface of cavity

2. Countermeasures related to injection mold condition
1.
2.
3.
4.
5.
6.

lack of measurement
too short cushion surface
too low pressure keeping
too short pressure keeping time
low cavity surface temperature
insufficient predrying of molding materials

2-9 Countermeasures on defective molding (Short shots)
Short shot is a phenomenon where incomplete filling takes place at the part
of molded articles.
There are 2(two) potential cause of short shots.
First cause is, cooling down and solidifies of tip of materials while molten
resins flow.
Second cause takes place in the process of flowing. Because air traps are
generated depends on the flowing condition.
To take countermeasures on short shots, one must first find out the cause
of problems from the above cause.

■ Short shots caused by flow tip solidification
1. Countermeasures related to mold
1.
2.
3.
4.
5.
6.
7.

Expand further the gate size
Expand further the runner size
use the larger sized sprue
too small cold slag well
provide heat insulating plate at bottom surface of mold plate
increase the number of gate
alter the location of gate

2. Countermeasures related to injection mold condition
1. set the resin temperature higher
2. set the cavity surface temperature higher
3. increase the filling pressure
4. set the pressure keeping higher
5. set the pressure keeping time longer
6. increase the measurement value
7. increase the number of cushon
8. change the injection machines
9. exchange the backflow prevention rings of injection unit
10.exchange the injection nozzle tip diameter of injection machine larger

3. Countermeasures related to molded article design
1. increase wall thickness of molded articles
2. provide ribs at the area where there is poor flowing
■ Short shot due to air trap
1. Countermeasures related to mold
1.
2.
3.
4.

provide an efficient air vent at air trap generating areas
change the gate position
change the runner balance
review and improve the structure into heatup possible structure where
there is poor flow
5. change the structure to insertion or bushing structure where there is
poor flow

2. Countermeasures related to injection mold condition
1.
2.
3.
4.
5.

change the injection speed, to change the flowing patterns
relayout the position of screw speed pressure exchange position
set the injection speed slower
set the cavity surface temperature higher
set the mold clamping pressure slight lower

3. Countermeasures related to mold article design
1. review and apply uneven wall thickness of molded articles
2. increase the wall thickness of molded articles

2-10

Molding materials and its usage (polyamide)

Polyamide is also known as “Nylon(trademark)”. There are PA6,PA66,PA46,
and aromatic polyamides.
Characteristics of polyamide are that it is outstanding in friction abrasion
character. Because of this characteristics, materials will not cause much
noise, stable sliding. Its also has superior resistant character to organic
solvents and oils.
On the other hand, it has great water permeability and hygroscopic property.
This means, one can expect dimensional change of products,
There will be an improvement of heat resistant and strength by filling glass
fibers.
Following are main usage:
■ Automobile parts
● throttle cam
● intake control valve
● door mirror bracket
● electric oil sensor
● intake manifold

● throttle body housing
● shifting lever set
● engine cover

■ electric / electronics parts
● FPC connector
● Coil bobbins
● Relay
● Switch parts
■ electric appliances / residential related
● surfing board
● electric shaver moving parts
● residential window handle levers
● electric tool housing
■ others
● fastener
● chair legs

2-11 Molding material and its usage (polyacetal)
POM resins (Polyacetal, polyoxymethylene) are superb in mechanical
specification like breaking strength, and worn out resistance property so
they are known as engineering plastics.
In POM, there are “homopolymer” and “copolymer”.
There are differences in strength, thermal resistance, molding condition
between homopolymer and copolymer.
Biggest feature of “POM Resins” are self-lubrication. It is valued functions
when its applied as gear, bearing like parts where there are always facing
frictions.
Crystallinity is high, therefore is shows good result in strength and thermal
resistance.
In injection molding, one must take care not this materials to stay long
period in the cylinder, because they will undergo thermal decomposition.
Followings are main usage.
■ Automobile parts
● fuel gauge
● fuel chamber
● door mirror warm gear
● mirror stay
● fuel pump
● radiator drain cock
■ Electric / Electronics parts
● CD pickup unit
● Switch stem
● DVD drive unit / pulley. Cum
● Magnetic memory device roller
■ precision parts
● watch gears
● bearing
■ others
● shower bib
● fastener
● gas meter parts
● flushing toilet parts

2-12 Molding material and its usage (polycarbonate)
Polycarbonate is a transparent strong thermal resistant material applied in
diverse areas.
Recently alloy type of usage of polycarbonate by mixing it with ABS resins
and used for industrial purposes.
It is amorphous and exhibit good light transmittance therefore applied as
lens and cover materials. It also exhibits outstanding strength especially
shock resistance among other plastics.
However it will be corroded by organic solvents. This is the reason why one
must pay great attention, if ever grease group of chemicals and solvents
are applied on the material.
In injection molding, this material exhibits poor liquidity. Therefore filling
pressure must be set higher. One shall also necessary to increase the
temperature up to approx 80 .
Following are usage of the materials.
■ Automobile parts
● meter panel
● head lamp lens
● door handle
● sun roof
● instrumental panels
● wheel cover
■ Electric / Electronics
● cellular phone case
● memory sticks
● CD disc
● DVD disc
● Digital camera case
● Printer chassis
● PC casing
■ optical components
● camera lens
● aspheric surface lens
● prism
● light conductor
● protection goggle
● dome shape roofing
● window glasses

2-13 Molding material and its usage (Thermoplastic elastomer)
Thermo plastic elastomer (TPE) is a synthetic resin which has rubber like
properties.
Automobile tire is one good example of rubber, but those rubbers are
solidify thru chemical reaction and therefore generally speaking it is not
possible to fabricate by injection process.
On the other hand, TPE can fabricate thru injection molding. It is
thermoplastic and therefore one can fabricate without burrs under relatively
fast cycle time.
In many cases now, this material is applied to products such as sticker parts,
sports items, toys, writing materials. 1.43M tons(2001) has been consumed
in the world.
TPE shows rubber like flexible hypertrophic properties because it contains
soft segments and hard segments.
There are following types of TPE available, and being used accordingly.
(1) SBC (styrenic TPE)
Sole, automobile parts, food container, writing material grip, sports
items
(2) TPVC (vinyl chloride TPE)
Electric wire coating, automobile parts, electric appliance
(3) TPO (olefine TPE)
Automobile parts, construction/civil work parts, electric appliance
(4) TPE (Polyurethane TPE)
Watch band, sole, automobile parts. etc
(5) TPEE (polyester TPE)
Automobile parts, electric appliance, industrial materials. etc.
(6) nitryl TPE
(7) TPAE (polyamide TPE)
(8) Fluoro-chemical TPE
(9) Silicone TPE

<reference/citation list>
1) c Michio Komatsu : “Molds for beginners. Introduction to plastic injection
mold, molding technology.” Mold technology
April 2004, special issues, [Daily industrial journals]2004
2) c Michio Komatsu : “Plastic injection molding design manual, pin gate
structure” mold technology. March 1998 special issue, Daily industrial
journals 1998
3) c Michio Komatsu : “Plastic injection design manual” Daily industrial
journals (1996)
4) “Plastic mold standard parts 2005.6 ∼2007.4”MISUMI Inc. (2005)
5) Technology course, MISUMI online, MISUMI Inc, www.mol.ne.jp