- molds and plastics  injection troubleshooter -

• Streaks - plastic injection troubleshooter

• Description:

Splay marks, silver streaks, splash marks are the result of:
(a) moisture on the pellets which should be removed under recommended drying times and temperatures
(b) products of degradation due to overheating
(c) residual non-aqueous volatiles in material

(a) and (c) will produce fine lines emanating from the gate all over the part whereas (b) will show up as coarse lines, lumps in sections of the moulded parts.

• Causes:

· degraded material (hot spot in cylinder and/or material hang-up area at nozzle tips or adaptors)
· injection pressure too low or too high
· injection speed too low or too high
· back pressure too low

mould · frictional burning at gate, in machine nozzle or hot runner
· trapped volatiles
material · melt too hot
· contamination in resin
· excessive moisture
· degradation of material due to too long pre-drying at high temperatures

• Actions:

1st. check pre-drying: dry material before use
2nd. check moisture content after pre-drying
3rd. check effectiveness of drying equipment, both temperature and time
4th. lower nozzle temperature
5th lower material temperature by:
- lowering cylinder temperature
- decreasing screw speed
- lowering back pressure
6th decrease injection speed
8th raise mould temperature
9th shorten or eliminate screw decompression
10th shorten overall cycle
11th increase back pressure; in case of drooling, reduce back pressure
12th check for drooling
13th check for contamination (e.g. water or oil leaking into mould cavity)
14th barrel purging (hang-ups)
15th allow for adequate mouldventing
16th open gates
17th move mould to smaller shot-size press

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