Views: 0 Author: Site Editor Publish Time: 2025-04-29 Origin: Site
Sink marks are primarily caused by uneven material shrinkage during cooling. Here’s a breakdown of the key factors:
Material Shrinkage
Thermoplastics shrink as they cool, but if the outer layer solidifies while the inner core is still cooling, the contraction pulls the surface inward, creating a sink mark.
Crystalline plastics (e.g., PP, PA) shrink more than amorphous plastics (e.g., ABS, PC), making them more prone to sink marks.
Insufficient Packing Pressure or Time
If the holding/packing pressure is too low or applied for too short a time, the plastic cannot fully compensate for shrinkage.
High Melt or Mold Temperature
Excessive heat increases shrinkage, especially in thick sections where cooling is slower.
Poor Part Design
Thick sections or abrupt changes in wall thickness (e.g., ribs thicker than 50% of the main wall) lead to uneven cooling.
Increase packing pressure & time – Ensures more material fills the cavity to offset shrinkage.
Lower melt & mold temperatures – Reduces overall shrinkage and speeds up cooling.
Adjust cooling time – Longer cooling helps solidify thick sections uniformly.
Optimize gate location & size – Ensures proper material flow and packing.
Enhance cooling channels – Focus cooling near thick areas to minimize shrinkage.
Maintain uniform wall thickness – Avoid sudden changes; use gradual transitions.
Follow the "50% rule" for ribs – Rib thickness should be ≤50% of the main wall.
Use low-shrinkage resins or additives (e.g., glass fiber) to minimize contraction.
Sink marks vs. voids: Voids are internal air pockets, while sink marks are surface depressions.
Sink marks vs. warpage: Warpage is overall part distortion, whereas sink marks are localized.
Sink marks are a common but preventable issue in injection molding. By optimizing process parameters, improving mold and part design, and selecting the right material, manufacturers can achieve high-quality, defect-free plastic parts.
