KECACATAN PRODUK PLASTIK

28. Stringing
Definition:
Stringing is the appearance of a thin plastic string coming from the sprue.

Some possible causes:
a. Nozzle temperature too high.
b. Back pressure too high.
c. Sprue break function not used.

Remedies, to check:
a. Decrease temperature.
b. Decrease back pressure.
c. Use sprue break function.
d. Use suck-back.

STRINGING

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29. Thermal degradation

Definition:
Light brown to very dark streak in mould part.

Some possible causes:
a. Melt temperature too high
b. Hot runner temperature too high
c. Resident time of melt in cylinder too long
d. Injection speed too high

Remedies, to check:
a. Decrease cylindertemperature
b. Decrease hot runner temperature
c. Use cylinder with smaller diameter
d. Reduce injection speed : use steeped profile : i.e. fast to slow.
e. Use screw with smaller compression ratio

THERMAL DEGRADATION

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30. Warpage

Definition:
Moulded part is twisted or bent from the intended shape after ejection. This is
common in thin walled containers and large flat moulded parts.

Some possible cause:
Differential shrinkage within the component.

Remedies, to check:
a. Check mould temperatures - both halves of the mould.
b. Shot weight - may need to reduce slightly.
c. Injection rate - may be too slow (or too fast).
d. Mould cooling - use differential cooling.

PICTURE 1
EXAMPLE OF WARPAGE

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PICTURE 2
EXAMPLE OF WARPAGE

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31. Ejector Marks
Marks found at the direction of the movement at the ejector pin usually on core side.
Three types of ejector marks:-

a. Protruding Out
- Ejector project above the core surface. Ejector mark will be formed sunken on the

moulding.

b. Sunk Inside
- Ejector pin below the core surface. Ejector mark will be form protruding out of the
moulding.

c. Ejector Stress Mark
- Moulding is deformed at the ejectors location due to excessive force required to
push out the moulding.

Countermeasure
- For (a) and (b), make sure the ejector pins are flush with the core face.
- The top surface of the ejector pin must have same finish as the rest of the core.
- For Stress Mark:

a. Add number of ejectors.
b. Polish the core better.
c. Increase draft angle.
d. Reduce undercut at core.

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EJECTOR MARK
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EJECTOR MARK - SUNK INSIDE

EJECTOR STRESS MARK

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32. Bubbles
Definition:
Air induced in the plastic moulding.

Some possible causes:
Outward shrinkage, i.e. due to the hard outer surface of the component it cannot
‘ink’inward, therefore a vacuum (void) is formed inside the component, where the
material is still soft. Back pressure is too low. Volatiles are present in the material
(eg. gases created during melted).

Remedies, to check:
a. Increase back pressure
b. Follow-up pressure - may need to be increased.
c. Follow-up pressure time - increase.
d. Mould temperature - may be too low.
e. Melt temperature - may be too high.
f. Gate - may be too small.
g. Check material of contamination.

BUBBLES

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BUBBLES

AIR BUBBLES (SOLIDIFIED WHILE FLOWING AROUND THE CORE)
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