This post is the 11th article in our series about common issues that you can encounter during your plastic production. In this installment, we're talking about shrinkage marks. If you want to start from the beginning, you can check out our article on short shots.
What is a sink mark?
During the injection process, 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.
Some of these marks are pictured below. Pay attention to the change in wall thickness at the cross section on the first picture:
Reasons and solutions
1. Improper control of molding process.
The sink marks will appear at places where the melt is slower or the wall section is thicker. An example of this could be the edge between the rib, brace and surface of the product.
Under this condition, the following should occur :
The injection pressure and speed should be appropriately increased.
The compression density of the melt should be increased.
The time for maintaining the pressure of the injection should be extended.
The contraction of the plastic melt should be compensated for.
The injection buffer should be increased.
The pressure, however, should not be too high, otherwise, it will cause a bump. If the sunken and sink mark occurs near the gate, it can be solved by extending the holding time. Additionally, the cooling time of the plastic parts in the mold should be extended when the plastic parts are recessed at the thickness of the wall. If there are sunken and sink mark around the insert due to partial contraction of melt, which is mainly caused by the low temperature of the insert, we should try to increase the temperature of the insert. If the surface sunken is caused by insufficient feed, the amount of feed should be increased. In addition, the molds must be sufficiently cooled.
2. Mold defects
Whenthere are issues with the mold, the gate and runner section shouldbeappropriatelyexpandedaccording to the specific situation. The gate location shouldbe set as far as possible in a symmetrical place while the feedinletshouldbe set at the back wall of the plastic part. If the sunken and sink mark occur far from the gate, itisusually due to poormelt flow at one part of the mold structure thatimpedes the pressure transfer. In this regard, the structural dimensions of the mold casting system shouldbeappropriatelyexpanded, and itis best to extend the flow channel to the area where the sunken marks occur. For the plastic parts where the walls are thick, a winggateispreferred.
3. Raw materials do not conform to the molding requirements
For plastic parts withhigher surface requirements, plastics withlowshrinkagecharacteristicsshouldbeused as much as possible. A suitablelubricantcanbeadded in the rawmaterials to assistwiththese issues.
4. The shape and structure of plastic parts are not well designed
Whendesigning the shape structure of plastic parts, the wallthicknessshouldbe consistent as much as possible. If the wallthickness of the plastic part varies greatly, itcanbesolved by adjusting the structural parameters of the casting system or changing the wallthickness distribution. The followingpicture shows how a design could be improved:
Free Resource: 20 Most Common Problems in Your Plastics Manufacturing
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