Analysis and Troubleshooting of Sink Marks in Custom Plastic Injection Molding¹

Sink marks (also known as depressions or sink marks) are one of the most common defects in the injection molding industry. For manufacturers of custom plastic parts², understanding the root cause is the first step toward quality control.

This guide analyzes why sink marks occur and provides actionable solutions to eliminate them from your production line.

What are Sink Marks?

A sink mark is a local inward contraction on the surface of a molded part. It typically appears as a shallow depression or a dimple.

These defects usually occur in areas where the wall thickness is greater, such as near bosses, ribs, or external corners. The fundamental cause is the thermal expansion and contraction of thermoplastic materials. As the plastic cools and solidifies, it shrinks. If the internal material shrinks more than the outer skin (which has already solidified), the surface gets pulled inward, creating a depression.

Key Factors Influencing Shrinkage:

• Material properties (Thermal expansion coefficient)
• Mold temperature and Melt temperature
• Holding pressure and Cooling rate
• Part geometry (Wall thickness variations)

1. Improper Molding Process Conditions

Often, sink marks are the result of incorrect machine settings. Adjusting the processing parameters is usually the first and easiest step to troubleshoot the issue.

• Injection Pressure & Speed: If the pressure is too low or the speed is too slow, the melt density decreases, leading to shrinkage. 솔루션: Increase injection pressure and speed to compress the melt.
• Holding Time: If the gate freezes before the part is fully packed, shrinkage occurs. 솔루션: Extend the holding time to compensate for volume shrinkage.
• Temperature Control: High melt or mold temperatures require longer cooling times. 솔루션: Lower the melt temperature and ensure the mold cooling system is efficient.
• Cooling Time: Ejecting a part while it is still too hot causes it to deform and sink. 솔루션: Increase the cooling time in the mold.
• Insert Temperature: If the temperature around metal inserts is too low, the melt shrinks rapidly upon contact. 솔루션: Preheat the inserts³.

2. 금형 설계⁴ Defects

If process adjustments fail, the issue may lie within the mold design itself.

• Gate and Runner Size: Small gates increase flow resistance. 솔루션: Enlarge the gate and runner cross-sections to improve flow.
• Gate Location: Gates should ideally be placed at the thickest section of the part. 솔루션: Redesign the gate location to ensure symmetrical filling and better pressure transmission.
• Flow Obstruction: Sink marks far from the gate indicate flow resistance. 솔루션: Remove "bottlenecks" in the runner system or extend the sprue closer to the defect area.
• Thick Wall Strategy: For thick parts, use a tab gate (wing gate). This attaches a small "wing" to the part, placing the gate on the wing rather than the part itself. The sink mark is transferred to the wing, which is cut off later.

3. Raw Material Issues

The choice of resin plays a critical role in surface finish.

• Shrinkage Rate: Materials with high shrinkage rates are prone to sink marks. 솔루션: For high-precision parts, select low-shrinkage resin grades.
• Moisture Content: Wet material can cause voids and surface defects. 솔루션: Always pre-dry hygroscopic materials (like Nylon or PC) before molding.
• Lubrication: Poor flow can lead to short shots and sinks. 솔루션: Add lubricants to improve melt flow index (MFI).

4. Part Geometry Design

The root cause often starts at the product design phase.

• Wall Thickness Uniformity: The most common cause of sink marks is uneven wall thickness. Thick sections cool slower than thin sections, pulling the surface inward.
• Design Rule: Maintain uniform wall thickness whenever possible.
• Coring Out: If a thick section is necessary, "core out" the thick area to reduce mass while maintaining structural integrity.

[Image Suggestion 4]

• Description: A cross-section diagram showing "Good Design" (uniform wall thickness) vs. "Bad Design" (thick ribs causing sinks).
• Alt Text: Plastic part design guidelines showing uniform wall thickness to avoid sink marks.

Summary Checklist for Troubleshooting