KECACATAN PRODUK PLASTIK

Topic 10

Trouble-Shooting For Injection Moulding

Introduction ........................................................................................ 2

Introduction ................................................................................................................ 3
Part/Mould Design ..................................................................................................... 4
Materials .................................................................................................................... 7
Methods ..................................................................................................................... 9
Machine ................................................................................................................... 11

Identifying Defects, Causes and Remedies.................................... 15

Black Specks ........................................................................................................... 15
Blisters ..................................................................................................................... 17
Blush ........................................................................................................................ 18
Brittleness ................................................................................................................ 19
Voids ........................................................................................................................ 20
Burn Marks .............................................................................................................. 21
Cloudy Appearance ................................................................................................. 23
Cold Slug ................................................................................................................. 24
Contamination ......................................................................................................... 26
Cracking .................................................................................................................. 28
Crazing..................................................................................................................... 30
Delamination ........................................................................................................... 31
Discoloration ............................................................................................................ 32
Flash ........................................................................................................................ 34
Flow Lines ................................................................................................................ 35
Glossiness ............................................................................................................... 36
Jetting ...................................................................................................................... 37
Knit lines / weld lines ................................................................................................ 39
Matt Patches ............................................................................................................ 41
Short shot ................................................................................................................. 42
Shrinkage ................................................................................................................ 43
Silver Streaks ........................................................................................................... 44
Sink Marks ............................................................................................................... 45
Splay marks ............................................................................................................. 47
Sticking sprue .......................................................................................................... 48
Stick at cavity side .................................................................................................... 49
Stick at core side ...................................................................................................... 50
Stringing ................................................................................................................... 51
Thermal degradation ............................................................................................... 52
Warpage .................................................................................................................. 53
Ejector Marks ........................................................................................................... 55
Bubbles .................................................................................................................... 58

TROUBLE-SHOOTING FOR INJECTION MOULDING
Terminal Objective:
Identify different sections of the moulding machine and their functions.
Introduction
Enabling Objectives
Knowledge:
1. To identify the various defects (names) in injection moulding.
2. To detect how each of these are caused (fish bone charts, etc)
Skills :
1. Identify these defects when they occur.
2. Troubleshoot to eliminate/prevent defects.

2

Introduction

In the Plastics Injection Moulding industry, there are great demands for quality product
of plastics in shorter development times.

However during the plastic processing, most of the potential part problems are not
recognized until the final stages of product development.

There are always a fire-fighting situation in order to find a quick solution. There are
factors that need to be analysed prior to checking a specific part defect.

These are the 4 areas that should be focused when addressing typical injection
moulding machine defect.
1. Part / Mould Design
2. Materials
3. Methods
4. Machine

EXAMPLE OF DEFECTED PRODUCTS

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1. Part/Mould Design
In altering the design, it should be made one at a time. Repeat the procedure until
satisfactory results are obtained.

Design improvement should be done during the drawing phase, of the product and
the mould parts.

Examples of some of the faults caused by moulds are:
a. Inadequate cooling channels
b. Sprue bush sizes too small or taper absent.
c. Inadequate cold slug well.
d. Incorrect runner size or type.
e. Incorrect Gate size or position.
f. Varied Gate size in multiple cavity tools.
g. Inadequate or poor venting.

PICTURE A
POOR AND GOOD DESIGN FEATURES

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PICTURE B
POOR AND GOOD DESIGN FEATURES

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PICTURE 3: 4 CAVITY MOULD - MOULD DESIGN

PICTURE 4: SPLIT MOULD - MOULD DESIGN

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2. Materials
Moulding faults caused by material can be due to basic problems of moisture or
the use of an incorrect grade or type of material.
To avoid defect by moisture, pre-dry the material and check the correct grade
against drawing specification. Use reground materials (regrind) in correct
proportions.

POLYPROPYLENE

POLYSTYRENE

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POLYAMIDE

ACRYLONITRILE BUTADIENE STYRENE

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3. Methods
Here the assumption made is the mould’ function’ is designed correctly for the
product intended. Therefore to resolve faults like short-shots, sink marks, streaks,
weld line, bubbles, etc moulding parameters are evaluated (and corrected when
necessary).

Examples of moulding parameters are: -
a. Injection speed and filling time.
b. Injection and holding pressures.
c. Cooling time and mould temperature.
d. Ejection speed and time.

PICTURE 1: 4 CAVITY MOULD

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PICTURE 2: SPLIT MOULD
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4. Machine

Machine induced faults can be many. One such fault is flashing, where a thin film of
material terms outside the design. This generally occurs along the split line. Flash -
A thin film of unwanted/excess material formed outside the design. It generally
occurs along the split line or around undersized ejector pins.

Machine induced flash, could be caused by one of the following reasons:
a. There is an obstruction in the mould.
b. The toggle lock is wrongly adjusted.
c. In a hydraulic machine, the clamp force is too low. Check the pressure and if

correct, check for a faulty valve.

If there is flash consistently on one side of the component, this will indicate:
a. Obstruction in pillar and guide of mould.
b. The tie-bars are of uneven tension.
c. Insufficient lock-up pressure.

PICTURE 1
MACHINE INDUCED FAULTS -FLASH

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PICTURE 1
MACHINE INDUCED FAULTS -FLASH

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Ways of analysing defects:
1. Fish bone diagram.
2. Refer to table on possible causes of defects.

PICTURE A: FISH BONE DIAGRAM -
SHOWING POSSIBLE CAUSES OF DEFECTS IN INJECTION MOULD

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PICTURE B: TABLE 'POSSIBLE CAUSES DEFECT '
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Identifying Defects, Causes and Remedies
1. Black Specks

Definition:
Degraded plastic material or contaminants like paper, cotton threads; that has
carbonised which appears as small black dots.

Some possible causes:
a. Excessive resident time in barrel.
b. Sprue bush cracked or not seat properly.
c. Contaminated raw material.
d. Inconsistent cycles.

Remedies, to check:
a. Inject the moulding in a proper sized machine.
b. Ensure the nozzle tip opening has an equal or smaller diameter than sprue

bush, which ensures a proper seal.
c. Use only high quality material, maintain good house keeping practice and

properly training material handling personnel.

BLACK SPECK (BLACK DOT)

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BLACK SPECK (BLACK DOT)
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2. Blisters
Definition:
A raised defect on the surface of product cause by trapped gas.

Some possible causes:
a. Back pressure too low.
b. Mould temperature too low.
c. Use of regrind that is too coarse.

Remedies, to check:
a. Ensure that back pressure should be increased in small increments until the

problem is solved.
b. Increase the temperature of the mould.
c. Limit the usage of regrind.

BLISTER

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3. Blush
Definition:
Dull discolored or whitish area on the surface of the part, usually at the gate.

Some possible causes:
a. Injection speed is too fast.
b. Mould temperature is too cold.
c. Excessive moisture.

Remedies, to check:
a. Ensure that the injection speed must be adjusted (decreased) until the

optimum.
b. Increase the mould temperature.
c. Use properly drying procedures.

BLUSH

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4. Brittleness

Definition:
The tendency of a product to break or crack under conditions in which it would
not normally do so.

Some possible causes:
a. Improper screw design.
b. Condensation in the mould.
c. Excessive moisture.

Remedies, to check:
a. Use an injection screw with higher compression ratio.
b. Raise the mould temperature to prevent condensation from forming.
c. Use proper drying procedures.

BRITTLENESS

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5. Voids
Definition:
An empty space (vacuum), often spherical or sausage-shaped, in the centre of the
thickest part of the moulding.

Some possible causes:
Outward shrinkage, i.e. due to the hard outer surface of the component it cannot
‘ink’inwards therefore a void is formed inside the component, where the material is
still soft.

Remedies, to check:
a. Follow-up pressure - may need to be increased.
b. Follow-up pressure time - increase.
c. Mould temperature - may be too low.
d. Melt temperature - may be too high.
e. Gate - may be too small.

VOID

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6. Burn Marks
Definition:
Discolored, often burnt areas on or in the past part of the cavity to fill, occasionally
as a surface bubble or blister after moulding has been ejected.

Some possible causes:
Air is trapped in the cavity. Being compressed by the melt, the air temperature
rises and get so hot that it oxidises the surrounding plastics.

Remedies, to check:
a. Mould vents - could be blocked.
b. Injection speed - may be too high.
c. Mould clamping force - may be too high.
d. Mould temperature - may be too high.
e. Melt temperature - may be too high.

BURN MARKS

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BURN MARKS
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7. Cloudy Appearance
Definition:
Area of imperfection resembling a cloud formation.

Some possible causes:
a. Mould surface may have an area of imperfection, resembling a cloud

formation.
b. Contaminated material.
c. Partially melted material.
d. Excessive moisture.

Remedies, to check:
a. Ensure that mould surface.
b. Ensure no contaminated material.
c. Increase barrel temperature.
d. Dry the material properly.

CLOUDY APPEARANCE

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8. Cold Slug

Definition:
Cold piece of plastic that has been forced into the part along with the melt.

Some possible causes:
a. Plastic from last shot left in nozzle solidifies between shots. The tool

designer usually is able to allow for a “cold slug well”in the runner to catch this
piece.
b. Cold slug effects can also occur at the end of a long runner.

Location:
If allowed to enter the part it can move anywhere.

Cure:
Add a cold slug well at each intersection in the runner. Addition of a shortened
ejector pin on the runner very close to the gate may divert the cold slug. For direct
sprue gating try to make a feature in the part to catch the slug or use a heated
nozzle.

PICTURE 1: COLD SLUG

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PICTURE 2: COLD SLUG (NEAR THE SPRUE)
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9. Contamination

Definition:
Foreign material has contaminated the melt, either in the form of bubbles, craters,
marks or lumps.

Some possible causes:
If raw material bins or hoppers have been left uncovered, it is possible for foreign
particles to enter the melt. This can also cause damage to the mould and the screw.

Remedies, to check:
a. Material in hopper for contamination.
b. Hopper magnet for ferrous metals.
c. Raw materials from manufacturers.
d. Melt temperature - may be high.
e. Screw tip, screw and nozzle for evidence of ‘dead spot’due to local

overheating.

PICTURE 1
CONTAMINATION

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PICTURE 2
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10. Cracking
Definition:
A fracture in the material of a moulded part, usually found in weld line, such as
at a mould insert, boss, and hole.

Some possible causes:
a. Moulded-in stresses.
b. Insufficient draft angle or insufficient polish.
c. Excessive moisture.

Remedies, to check:
a. Reduce the injection pressure and speed.
b. Increased draft angle.
c. To ensure proper drying of material.

CRACKING

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CRACKING
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11. Crazing
Definition:
Because crazing is simply a very fine network of cracks, the same causes and
remedies apply that are mentioned under “racking,”in the previous example.

CRAZING

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12. Delamination
Definition:
Thin skin peeling off the surface of the moulding.

Some possible causes:
a. Contamination of material (with different type of material)
b. Injection rate too fast / slow
c. Mould temp too low

Remedies, to check:
a. Material has not been contaminated from another polymer.
b. Filling rate - may need increase/decrease.
c. Mould temperature - may need to increase.

DELAMINATION

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13. Discoloration

Definition:
A change in the original colour of a plastic material.

Some possible causes:
a. Excessive barrel resident time.
b. Contaminated with wrong regrind, fabric strands, thermally degraded

material and foreign particles.

Remedies, to check:
a. Optimize the shot to barrel ratio.
b. Maintain a proper house keeping.

PICTURE A
DISCOLORATION

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PICTURE B
DISCOLORATION

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14. Flash
Definition:
Flash is a thin film of unwanted material forming outside the designed cavity. It is
frequently found along the split line or around the undersized ejector pins.

Some possible causes:
The mould is forced open due to the injection pressure being too high for the mould
locking force or the mould tool may be worn.

Remedies, to check:
a. Mould clamping pressure - may be too low.
b. Injection pressures may be too high for mould lock.
c. Change over point from injection to ‘old-on’ pressure is too late.
d. Mould - For wear damage or an obstruction.

FLASH

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15. Flow Lines

Definition:
Visible mark on the finished items due to the direction of flow in the cavity.

Some possible causes:
a. Runners too small.
b. Improper gate size and/or location.
c. Mould temperature too low.
d. Inadequate cold slug well.

Remedies, to check:
a. Increase runner size.
b. Increase gate size and/or relocate as needed.
c. Increase mould temperature.
d. Increase in size or number.

FLOW LINE

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16. Glossiness

Definition:
An unwanted shining patch on the dull surface of product.

Some possible causes:
a. Inadequate injection pressure.
b. Mould temperature too low.
c. Improper flow rate.

Remedies, to check:
a. Increase injection pressure.
b. Increase mould temperature.
c. Increase/decrease injection speed.

GLOSSINESS

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17. Jetting

Definition:
A worm shaped area opposite and away from a gate with flow or weld lines

at the furthest edges.

Some possible causes:
Uncontrolled flow in the resin melt flow caused byundersized gate, abrupt

change in cavity volume, or too high injection speed.

Remedies, to check:
a. Injection speed - may be too high.
b. The gate - may need to alter or change its position.

JETTING

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JETTING
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18. Knit lines / weld lines

Definition:
A surface mark, often a line where cold fronts of material meet e.g. after

flowing round a core pin or along thicker sections.

Some possible cause:
Lack of fusion between cold fronts of material.

Remedies, to check:
a. Injection speed - too low.
b. Mould temperature - may be too low.
c. Mould vents - may be blocked.
d. Short volume - may be too small.
e. Barrel temperatures - may be too low.

PICTURE A: KNIT LINES/WELD LINES

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PICTURE B
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19. Matt Patches

Definitions:
Unwanted matt patches on a glossy surface product.

Some possible causes:
a. Material temperature low
b. Material contaminated
c. Material cavity not polished evenly
d. Condensation on mould surface
e. Wet material

Remedies, to check:
a. Increase temperature
b. Improve housekeeping and material handling
c. Re-polish mould
d. Increase temperature of mould
e. Dry the material

MATT PATCH

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20. Short shot

Definition:
A ‘hort’is an incomplete moulded product. Moulded products with narrow sections
or ribs are vulnerable to shorts shots. Frequently the moulded product is unfilled at the
furthest point from the gate.

Some possible cause:
Insufficient material injected into the cavity.

Remedies, to check:
a. Shot size.
b. Injection speed - may need to be increased.
c. Injection time - may need to be increased.
d. Melt temperature - may be top low.
e. Gate - may be too small.

SHORT SHOT

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21. Shrinkage

Definition:
Excessive dimensional change in a part after processing, or the excessive

decrease in dimension in a part through cooling.

Some possible causes:
a. Injection and holding pressure too low.
b. Melt temperature inadequate.
c. Mould temperaturemtoo low.

Remedies, to check:
a. Analyze conditions, raise or lower as required.
b. Adjust cylinder temperature to match requirements for specific resin.
c. Lower mould temperature.
d. Correct mould temperature variation. Reduce variation in wall thickness.

SHRINKAGE

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22. Silver Streaks
Definition:
The appearance elongated silvery streaks or the surface of a moulding.

Some possible causes:
a. Damaged check ring.
b. Material contamination by other material.

Remedies, to check:
a. Check for worn or damaged check ring.
b. Check for material contamination.
c. Check that barrel is properly purged.
d. Check for overheated cylinder heater bands.
e. Check for overheated nozzle heater band.

SILVER STREAK

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23. Sink Marks
Definition:
Depression in a moulded product caused by shrinking or collapsing of the resin
during cooling.

Main cause:
As the resin changes from a molten state to a solid state, it occupies a
smaller volume (this is called shrinkage). As more and more of the molten resin
solidifies a vacuum is formed in the thicker sections and this tends to pull the
surface of the moulding inwards and forms a depression called a sink mark.

Remedies, to check:
a. Hold-on pressure - may be too low.
b. Hold-on pressure time - may be too short.
c. Screw return position (shot size) too low.
d. Mould temperature - may be too low.
e. Gate - may need to be increased.
f. Part section may be to thick.

SINK MARKS

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SINK MARKS
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24. Splay marks

Definition:
Marks or droplet type imperfections formed on the surface of a moulded product.

Some possible causes:
a. Screw rpm too high.
b. Melt temperature too high.
c. Gates too small.
d. Insufficient venting.
e. Resin contaminated.
f. Excessive moisture.

Remedies, to check:
a. Decrease screw rpm.
b. Decrease temperature of plastic melt.
c. Increase gate dimensions.
d. Provide additional mould vents.
e. Check materials for contamination
f. Test for moisture. Increase drying time or temperature.

SPLAY MARKS

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25. Sticking sprue

Definition:
At the end of cycle, the sprue does not release from mould but stick in the sprue
bushing.

Some possible causes:
a. Injection pressure too high.
b. Too much material feed.
c. Nozzle frozen.
d. Nozzle diameter too large for sprue.
e. Inadequate draft angle.
f. Drool from nozzle.
g. In effective sprue pulled.

Remedies, to check:
a. Decrease injection pressure.
b. Decrease injection hold time.
c. Increase mould close time.
d. Check incorrect seat between nozzle sprue bush.
e. Check polishing of sprue.

NOOZLE DIAMETER TOO LARGE FOR THE SPRUE

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26. Stick at cavity side

Definition:
At the end of the cooling cycle, the moulding does not release from the mould but
sticks at the cavity during mould opening.

Some possible causes:
a. Injection pressure too high.
b. Injection speed too high.
c. Cavity has undercuts.
d. Poor mould polish.
e. Material melt too hot.
f. Too much material feed.

Remedies, to check:
a. Check cooling time.
b. Decrease injection pressure.
c. Decrease injection speed.
d. Check for poor mould finish or corrosion on mould surface.
e. Check mould for undercut and / or insufficient draft and taper.

STICK AT CAVITY SIDE

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27. Stick at core side

Definition:
During the ejection cycle, the moulding does not release from the mould but sticks
at the core side.

Some possible causes:
a. Injection pressure too high.
b. Core too hot.
c. Core bent.
d. Formation of vacuum.

Remedies, to check:
a. Decrease injection pressure.
b. Check cooling time.
c. Check mould for bending.
d. Check mould for undercut and / or insufficient draft and taper.
e. Increase the number of ejectors.

STICK AT CORE SIDE

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