Overcoming Sink Marks Injection Molding

Sink marks are the bane of injection molders everywhere. I’ve seen perfectly good parts rejected because of a slight depression on an A-surface that you can barely see, but the customer can feel it. And I’ve spent countless hours chasing sink marks that seemed to appear out of nowhere. Here’s everything I’ve learned about why sink marks happen and how to eliminate them.

Key Takeaways

What Causes Sink Marks

Key Point: At its core, a sink mark is simple: it’s a depression in the surface caused by the material shrinking more in one area than the surrounding plastic.

This happens when:

• Thicker sections cool slower and shrink inward after the surface has solidified
• Insufficient packing pressure fails to compensate for shrinkage
• Premature gate freeze-off prevents additional material from entering

The math is straightforward: plastic shrinks 0.5-3% as it cools. In a uniform wall, this shrinkage is consistent. But add a rib, boss, or thick section, and you’ve created a differential shrinkage situation.

The Physics of Sink Marks

Sink Depth ≈ (Shrinkage × Wall Thickness Difference) × Material Factor

For a rib that’s 80% of wall thickness (let’s say 2.4mm rib on 3mm wall):

• The rib-wall intersection is effectively 5.4mm thick
• It shrinks about 1.8× more than the nominal wall
• Result: visible sink mark

Visual Identification Guide

Troubleshooting Flowchart

Step 1: Where is the sink mark?

Near the gate → Gate or packing issue

• Check gate size (may be too small)
• Increase pack pressure
• Extend pack time

Far from gate → Pressure transmission issue

• Increase injection pressure
• Check for flow restrictions
• Consider additional gates

Opposite a feature (rib/boss) → Design issue

• Evaluate rib/boss thickness
• Add coring or reduce mass

Random location → Cooling issue

• Check mold temperature uniformity
• Look for hot spots
• Verify cooling circuit flow

Step 2: Process Adjustments (Try First)

Step 3: If Process Doesn’t Solve It

Move to design or tooling changes:

Design Guidelines to Prevent Sink Marks

Rib Design Rules

Maximum rib thickness by material:

Boss Design Rules

Wall Thickness Transitions

When you must have thickness changes:

Material-Specific Solutions

Semi-Crystalline Materials (PP, PE, Nylon, POM)

These materials have higher shrinkage (1.5-3%) and are more prone to sink marks.

Amorphous Materials (ABS, PC, PS, PMMA)

Lower shrinkage (0.4-0.8%) but still susceptible, especially PC.

Glass-Filled Materials

Glass fiber reduces shrinkage but creates anisotropic behavior.

Advanced Solutions

Gas-Assist Injection Molding

For thick sections or parts with heavy ribs, gas assist can eliminate sink marks entirely.

How it works:

• Partially fill cavity with plastic
• Inject nitrogen gas to hollow out thick sections
• Gas pressure maintains surface against mold wall

Best applications:

• Handles and grips
• Structural parts with thick ribs
• Parts requiring weight reduction

Cost impact: Adds $5,000-15,000 to mold cost plus gas equipment

Foam Injection (Chemical Foaming)

Low-pressure structural foam naturally resists sink marks.

Advantages:

• No sink marks on thick sections
• 10-20% weight reduction
• Lower clamp tonnage required

Disadvantages:

• Swirl surface finish
• Lower strength than solid
• Longer cycles

External Gas Pressure

Apply gas pressure to cavity during cooling to force plastic against mold surface.

When to use:

• Class A cosmetic surfaces
• Parts where design changes aren’t possible
• High-volume production (justifies equipment cost)

Quality Control for Sink Marks

Measurement Methods

Acceptance Criteria

Typical standards for sink mark depth:

Troubleshooting Case Study

Problem: Sink marks appearing opposite ribs on a PC housing, Class A surface

Initial Condition:

• Rib thickness: 70% of wall (2.1mm on 3mm wall)
• Pack pressure: 800 psi
• Pack time: 4 seconds
• Mold temp: 180°F

Step 1: Process adjustments

• Increased pack pressure to 1,000 psi → 20% improvement
• Extended pack time to 6 seconds → 10% improvement
• Still visible under paint

Step 2: Design evaluation

• Rib intersection creates 5.1mm effective thickness
• Too much mass to pack out

Step 3: Tooling modification

• Reduced rib thickness to 50% (1.5mm)
• Added coring inside boss at rib intersection
• Cost: $2,400, 4-day turnaround

Result: Sink marks eliminated, passed paint inspection

Prevention Checklist

Use this checklist during design review:

Part Design

• Rib thickness ≤60% of wall (≤50% for crystalline)
• Boss walls ≤60% of part wall
• No abrupt thickness transitions
• Thick sections cored where possible
• Gate located to pack thick sections

Tooling Design

• Adequate cooling near thick sections
• Gate sized for proper packing
• Venting at end of fill
• Steel allow for rib reduction if needed

Process Validation

• Gate seal study completed
• Pack pressure optimized
• Cooling balanced confirmed
• Sink marks measured and documented

The Bottom Line

Sink marks are mostly a design problem, occasionally a process problem, and rarely a mystery. The physics are clear: where you have more material, you get more shrinkage, and shrinkage pulls the surface inward.

Your first line of defense is designing ribs, bosses, and wall thickness according to proven guidelines. Your second line is process optimization—pack pressure, pack time, and cooling. And when those don’t work, you’ve got advanced options like gas assist or tooling modifications.

The best approach? Catch potential sink marks in the design phase with mold flow analysis. It’s a lot cheaper to fix a CAD model than to modify a mold.