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Plastic mould defects-sink marks

Nov. 03, 2016

Mould defects-Sink marks

Sink marks occur during the cooling process, if the thermal contraction (shrinkage) of the plastic cannot be compensated in certain areas. If the outside walls of the molded part are not stable enough, due to insufficient cooling, the outer layer is drawn inside by cooling stresses.
There are three fundamental cases:
solidification too slow
effective holding pressure time too short
not enough holding pressure transfer, because flow resistances in the mold are too high
For optimum holding pressure transfer the molded part should be gated to the largest cross-section. In order to avoid premature solidification of the sprue and gate system, sufficient dimensioning is necessary.

Molded part with sink marks (gating at the thin wall)

Residual melt cushion too small?
If yes, then try:
(1) increase metering stroke
(2) check non-return valve
Sink marks near the gate or thick-wall areas?
If yes, then try:
(1) optimize holding pressure time
(2) increase holing pressure (maybe short overpacking)
(3) change mold wall temperature (-)
(4) change melt temperature (-)
(5) change injection rate (-)
Sink marks away from the gate or in thin-wall areas?
If yes, then try:
(1) optimize holding pressure time
(2) increase holding pressure (maybe short overpacking)
(3) change injection rate (+)
(4) change melt temperature (+)
(5) change mold wall temperature (+)
Sink marks directly after demolding?
If yes, then try:
(1) check ventialtion
(2) check sprue and gate dimensions
(3) check granules condition
(4) adapt mold temperature control
(5) remove material accumulations
(6) consider wall thickness/rib ratio
(7) add blowing agent
(8) use plastic with low shrinkage
If no, then try to increase cooling time

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