After analyzing the short shot issue, We decided to come back to another problem you can encounter during your injection molding plastic production: the Sink Mark.
WHAT IS a SINK MARK?
During the injection, there are times when parts of the mold don't get enough pressure to set the plastic. When this happens, it can cause additional stress on that area where the rate at which the plastic shrinks is slower than other areas. The sink marks appear when the surface of the plastic is pulled in by this stress because the surface isn't solid and the melt isn't complete (see image below). The sink marks will appear at the place where the melt is slower or the wall is thicker. An example of this could be the edge between the rib, brace and surface of the product.
As you can tell, the sink marks will not only make the appearance bad but also the strength low on the surface. The sink mark has close relationship with the material of plastic product, injecting manufacturability, and the structure of plastic product or mould.
1. The material of the plastic product
Different materials have different shrinkage rate. The materials that are most likely to have sink mark mostly belong to crystalline plastic products, such as polyamide (PA) and polypropylene (PP). When the crystalline plastic is heated to be fluid in the injecting, the volume shrink of the product can be higher than expected. This can cause the size to be irregular and cause sink marks.
2. Injecting manufacturability
There are many reasons that can lead to a sink mark with respect to injection manufacturability. Issues such as insufficient holding pressure, insufficient injection speed, low temperature of the mold or material, and insufficient pressure holding time are all elements that can contribute to sink marks. The structure of the plastic product or the mold can have an impact as well.
Non-uniformity of the plastic parts’ wall thickness is the primary cause of sink mark. One typical example is found at the plastic surface with rib or brace.
How to solve it?
According to the above analysis, you can see the reason and improving method following the table below:
Insufficient injection into the mold
Increase the amount of melt injection
Improper material temperature
（too high or too low）
Adjust material temperature
Improper mould temperature
（too high or too low）
Adjust mould temperature
Insufficient cooling time
extend cooling time before ejecting from the mold
Bad mold exhaust at sink mark
Modify the exhaust vent design
Wall thickness is too thick at rib or brace
Adjust the mold to make the wall thickness as uniform as possible within the design
Improper placement of gate or the process too long
Increase gate number or design gate at a place where the wall is thicker
The passage is too narrow or too large
Correct the size of the passage appropriate to the size of the part and plastic used
Because the shrinkage rate is important so there is another table to introduce the shrinkage rate of material, as follows
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