What are the causes of sink marks in injection molded parts?
Sink marks in injection molded parts are localized depressions on the surface. They often occur in areas far from the gate and thick sections or around inserts. Sink marks can adversely affect the appearance, performance, and structural integrity of a product.
Causes of Sink Marks:
Material Shrinkage:
All plastics shrink as they cool from a molten to a solid state.
The type and amount of fillers (e.g., calcium carbonate, talc) can influence shrinkage. Poor adhesion between fillers and the polymer matrix can lead to uneven shrinkage and sink marks.
Part Design:
Uneven Wall Thickness: Thicker sections are more prone to sink marks because they cool more slowly, allowing more time for shrinkage.
Improper Rib Design: Ribs, if too thick, can trap material and cause shrinkage.
Processing Parameters:
Insufficient Injection and Packing Pressure: Inadequate pressure can lead to incomplete filling and insufficient compensation for shrinkage.
Short Packing Time: A short packing time prevents the melt from fully solidifying, resulting in shrinkage.
Uneven Cooling: Uneven cooling rates can cause differential shrinkage and sink marks.
Mold Design:
Suboptimal Gate Location and Size: Poor gate location or size can hinder the flow of molten plastic and contribute to shrinkage.
Venting Issues: Trapped air can cause voids and sink marks, especially in thick sections.
