- molds and plastics  injection troubleshooter -

• Sink Marks- plastic injection troubleshooter

• Description:

Visible defects resulting from insufficient cooling before removal from the mould. A heavy rib intersecting a thin wall may show up sink marks, and these are very difficult to eliminate by varying processing conditions. Too high holding pressure – useless when gating is too small – creates very high stresses in gate areas.

• Causes:
machine · injection pressure too low
· injection pressure time too short
· short of shot capacity
mould · mould temperature too high: too high shrinkage
· gate too small - leads to early cooling/freezing at the gate, holding pressure cannot help anymore to compensate for the shrinkage
· land length too long
· wrong dimensions rib vs wall
material · melt too hot

• Actions:

1st. increase injection speed to maximum range
2nd. sometimes lower injection speed: crystalline materials
3rd. increase injection hold time
4th. increase injection pressure
5th reduce melt temperature
6th reduce mould temperature
7th check for hot spots: separate water channels in cooling system / add heat pipes such as thermal pins or beryllium copper slugs for spot cooling
8th enlarge and/or add vents to mould parting line
9th increase size of sprue and/or runners
10th increase gate size and reduce gate land length
11th relocate gate next to heavy or thicker areas
12th core out heavy wall sections where possible
13th incorporate textured surfaces

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