Troubleshooting focus: Sinks and voids

Sink marks appear as surface depressions (seen as dimples or grooves); voids are less obvious and may appear as bubbles in clear parts. Both compromise aesthetics and can reduce the confidence a customer has in a part-but the impact is much greater on applications such as fluid management, where leakage from a Luer or stopcock can result in loss of sterility and contamination.

In opaque medical device housing applications where aesthetic is critical, the matte finish and pastel colors that are often used make sinks and voids obvious.   

Sinks and voids are caused by localized shrinkage of the resin at thick sections during the following steps:   
  1. When excessively heated material expands to fill the mold cavity, it results in excess space between the plastic molecules.
  2. The skin of the material in the mold solidifies (freezes) first.
  3. As the remaining resin core cools and shrinks, it pulls the solidified skin with it away from the main mold wall.
  4. If the skin is sufficiently stiff, core shrinkage may not cause surface deformation but a void can form within the core of the resin as it shrinks.
Here are some of the most common causes of sinks and voids and corrective actions.

Possible cause 1—Insufficient packing
The proper amount of pressure held for the proper amount of time helps ensure consistency throughout the mold while the molten resin cools and solidifies.
Corrective actions:
•  Increase packing time and/or pressure.
•  Check gate, sprue, and tip dimensions to make sure their size is adequate.
•  Check part dimensions (packing thick sections through thin walls).
 
Possible cause 2—Excess wall thickness 
Corrective action: Reduce thickness if possible.
 
Possible cause 3—Hot spots in mold 
Corrective action: Improve cooling.
 
Possible cause 4—Injection speed too fast
Corrective action: Reduce speed to allow more uniform fill and pack.  
 
Possible cause 5—Melt temperature too low
Corrective action: Check and adjust temperature upward if needed. 
 
You can see how Eastman uses mold-filling simulation to predict the fill pattern of a proposed part design by opening the “Reasonable fill pattern” and/or “Eliminating areas of excessive shrink” in the Medical Part Design section of TritanMoldIt.com.

If you have additional questions about sink marks and voids in your parts, talk with your Eastman technical service representative—and ask how to receive a free copy of our Injection Molding Troubleshooting Guide