Sink marks and voids: thick regions, ribs and holding

Plastic shrinks as it cools. The thickest region of a part cools last and shrinks the most; if its surroundings have already solidified, that region either pulls the surface inward (sink mark) or leaves a void inside (void).

Telling the two apart

• Sink mark: The surface is dented. The mold surface is relatively warm or the wall is soft; the surface gets pulled inward.
• Void: The outer surface is smooth but there's a bubble inside the part. The surface has already solidified first, and a void forms when the material inside shrinks. This is usually seen when the mold is too cold or holding ended too early.

Taking a cross-section of the thick part clarifies the diagnosis.

The process side

1. Holding pressure and time: Holding must continue until the gate freezes. To measure the gate-freeze time, apply the gate-freeze study described in our holding article.
2. Melt temperature: Melt that's too hot shrinks more. Try the lower-middle part of the range.
3. Mold temperature: For sinking, cooling the mold usually helps (the surface layer hardens); for voids, heating the mold may be needed. The right direction depends on the symptom.
4. Cooling: Extra cooling in the thick region (a local channel, a beryllium-copper insert) reduces sinking.
5. Gate location and size: If the gate is close to the thick region, holding reaches it; if the gate is small, it freezes early.

The design side (the lasting fix)

• The ratio to wall thickness for ribs and bosses is generally kept between about half and two-thirds of the nominal wall thickness (a common rule of thumb). If a rib is kept too thick, a sink mark appears on the exact opposite face.
• Avoid sudden thickening of the wall; give transitions a radius.
• On parts where surface appearance matters, giving the opposite face a texture (grain) visually hides the sink.

Suggested order

If you see sinking, change holding first, then mold temperature, and melt temperature last. Don't change more than one variable at a time — you won't know which one worked.

Note: On thick-walled parts, process adjustment only reduces the problem; the full fix is usually in geometry or in a process change such as gas-assisted or foam injection.

You can take the required holding pressure and time from the calculation tool as a starting value.