July 23, 2014

Sink mark on plastic products

1. Definition.

  Sink mark is a local surface depression that typically occurs in plastic products  when use injection molding process with thicker sections, locations above ribs, boss, and internal fillets.

sink mark

sink mark

 

2. Cause of sink mark defect.

 Sink marks appear when shrinkage of plastic materials is not compensated enough on a certain part of a product during cooling process. If the outer wall of a molding product is not stabilized enough due to insufficient cooling, the outer wall will be pulled inward due to cooling stress

The factors that lead to sink marks are

+ Low injection and packing pressure
+ Short hold time or cooling time
+ High melt temperature or mold temperature
+ Localized geometric features

3. Improve sink mark on plastics product.

3.1. change design of products.

We will make wall thickness thinner, change local of boss, rib,… you can see these post below for how to design plastics products when use injection molding process.

Design wall thickness of products.

 Design boss of products.

Design rib of products.

Design draft angle of products.

3.2 Improve mold structure.

+ Increase the size of gates and runners to delay the gate freeze-off time.
+ Add more vents or enlarge the vents.
+ Relocate the gate to or near a thicker section.

3.3 Change injection condition.

+ Increase the cushion at the end of the injection stroke.
+ Increase the injection pressure and the holding time.
+ Increase the screw forward time and decrease the injection rate.
+ Decrease the melt and mold-wall temperatures.
+ Increase the cooling time.
+ Check the non-return valve for possible material leakage.

for more change injection condition, please see picture below

injection conditon & sink mark

injection conditon & sink mark

 

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