Design Matters (Part 1) – Shrinkage

Shrinkage is a physical phenomenon mostly happening to molding parts made by plastic injection and die casting.
Shrinkage is a physical phenomenon mostly happening to molding parts.

It is always exciting to carry out new projects. However, before moving on to the production process, you have to make sure that your design already includes all the factors that might have influences on your parts. This time, APPORO would like to share with you how material shrinkage happens, and how it can affect your parts.

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Cross Knurling Profile DIN 82-RGV

Standard specification of DIN 82 (link)


After CNC fabrication processing, the workpiece is usually with smooth metal surface. Knurling is a manufacturing process to feature straight, crossed, angled, diamond-like lines or pattern onto the CNC components. Generally speaking, knurling can perform better grip for finger/hand operation, plastic injection insert or decoration purpose. Sometimes, we also machined multiple shallow slots or polygonal for same above purpose. DIN 82 is most commonly used knurling spec in CNC turning machining field. For example, DIN 82-RGV is with cross knurling pattern. DIN 82-RBR/RBL is with right/left hand spiral. DIN 82-RGE is with diamond-like 30° cross male knurling.


Failure Cross Knurling

Our Belgium textile industrial customer had sent us the original sample made of stainless steel 304. The sample made by Belgium local prototyping manufacturer was with failure knurling surface. See the shorter item in below photo. The measurement of sample was precise and within tolerance, but with bad knurling which was with improper length, too light pattern, and also lack of lead in chamfer at the threaded hole.

Cross Knurling DIN 82-RGV
Cross Knurling DIN 82-RGV Compare Zoom-in


The improper length of knurling do bad for its appearance, a customized full length cross knurling tool can solve this issue. Too light patter may result from the CNC programming or limitation of CNC lathe. Moreover, our CNC production lines can easily machine the lead in chamfer on the edge of threaded hole.

Improved knurling

Therefore, we ordered a customized full length knurling tool to meet the required pattern spec of DIN 82-RGV 0.8. An automatic Japaneses CNC turn-mill machine also helps improving the quality of surface and dimensions. Above photo shows the longer workpiece machined by APPORO is with improved knurling pattern.


We can offer CNC precision parts with several types of knurling surface, such like straight knurling, cross knurling, diamond-like knurling. Our production lines are also capable of knurling on precision plastics parts. Should you have any inquiry for above knuring surface on your CNC workpieces, kindly send us RFQ for free project reviewing without hesitation.

Stress Marks on the Surface of Plastic Injection Parts

Residual stress is an inherent result of plastics injection molding process. The stress mark occurs when molten polymer is cooled and shaped. Sometimes, residual stress is purposely produced to obtain desirable characteristics, such as in oriented PET films and bottles. Instead, in most cases, these stresses could be invisible problem that will likely cause field failures.

How the stress mark occurred on plastic part?

Problems caused by residual stress in plastic part generally fall into some of the following categories:

1.) Cracking:

Cracking under normal load conditions can be strong evidence of internal residual stress. You will observe lots crazing or fine micro cracks from the surface appearance of product.

2.) Distortion:

Distortion occurs when plastic parts with excessive residual stress go through post-processing thermal cycles, such as sterilization, ultrasonic welding or heat sealing. Moreover, products exposed to various ambient temperatures in transportation or warehousing. The parts may warp or shrink, induced packaging failures that are visually unpleasant.

3.) Aggravation of optical properties:

Mark caused by residual stress changes refractive index, introducing optical distortion. This can be harmful for the surface appearance of products such as bottles, windows, plastic lenses and monitor screen.

Generally speaking, a good plastic injection depends on high quality polymer powder, proper temperature, pressure and time control. Below is a photo of precision thin-walled plastic part made of polypropylene. It shows how the residual stress causes mark on plastic part caused by too much holding time control. In this case, we had built a family shared mold, 1-cavity for each part. We intended to do a good injection for the other item, so we increased injection pressure up to 80 kgs/, holding pressure till 10 seconds. But, we observed a obviously white stress mark on the surface appearance without warning which may induce field failures.

Stress mark on plastic injection part
Residual stress causes damage mark on plastic injection part



How do we avoid stress mark on plastic injection parts in above case? This part was with damage as to cause cracks or stress mark on its surface. During trouble shooting, we had excluded the issues one by one. The issues can be differential cooling rates, non-uniform temperature or flow rate, material contaminants, mechanical loading, thermal cycling during transport and storage, inadequate or non-uniform annealing, and sharp corners or protrusions designs. Finally, we found the best way to avoid the cracks or marks on precision injection plastics parts is to reduce the duration of holding pressure down to 2 to 3 seconds.



Check out our previous plastic injection case in our website! Diagnose the residual stress before it ruins your parts. Feel free to send us your RFQ for project review.