Sink marks and voids

What are sink marks and voids?

A sink mark is a local surface depression that typically occurs in moldings with thicker sections, or at locations above ribs, bosses, and internal fillets. A void is a vacuum bubble in the core.

Causes of sink marks and voids
Sink marks and voids are caused by localized shrinkage of the material at thick sections without sufficient compensation when the part is cooling. A sink mark almost always occurs in extrusion on a surface that is opposite to and adjoining a leg or rib. This occurs because of unbalanced heat removal or similar factors.

Factors that lead to sink marks and voids are:

After the material on the outside has cooled and solidified, the core material starts to cool. Its shrinkage pulls the surface of the main wall inward, causing a sink mark. If the skin is rigid enough, as in engineering resins, deformation of the skin may be replaced by formation of a void in the core. Figure 1 illustrates this phenomenon.


FIGURE 1. Sink marks and voids are created by material shrinkage without sufficient compensation.

Remedies
Sink marks and voids can usually be alleviated by fine-tuning some combination of your part and mold design and the conditions under which the part is molded. Use the suggestions below to pinpoint and fix the problem.

Alter the part design


Conceal sink marks by adding a design feature, such as a series of serrations on the area where they occur.
Figure 2 illustrates this technique.

FIGURE 2. Sink marks can be eliminated by creating a design, rib, serrations.



Modify the part thickness design as suggested to minimize the thickness variation.


Re-design the thickness of the ribs, bosses, and gussets to be 50 to 80 percent of the attached (base) wall thickness.
Figure 3 shows the dimensions we prescribe.

FIGURE 3. Recommended dimensions for ribs, bosses, and gussets

Alter the mold design


Increase the size of gates and runners to delay the gate freeze-off time.
This allows more material to be packed into the cavity.


Add more vents or enlarge the vents.
Vents allow air trapped inside the cavity to escape.


Relocate the gate to or near a thicker section.
This allows them to be packed before the thinner sections freeze off.

Adjust the molding conditions


Increase the cushion at the end of the injection stroke.
You should maintain a cushion of approximately 3 mm (0.12 inches).


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.