Slip Marks (Ejector Pin Scuff Marks)

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Slip Marks (Ejector Pin Scuff Marks)

Last updated on Jul 22, 2026

Definition

A slip mark refers to the phenomenon where the partially solidified surface layer shifts under subsequent pressure.

The initially solidified surface slides laterally under injection or packing pressure and is pressed against the mold surface again, leaving patterned marks on the molded part.

Figure 1: Slip marks on appearance evaluation specimen (Pin hole diameter in photo: approx. 2 mm)

Figure 2: Slip marks near gate on 120mm square flat plate

📖 Definition Summary

The partially solidified surface layer is pushed by subsequent pressure and generates lateral displacement.

⚙️ Formation Mechanism

Sliding occurs under injection / packing pressure; the surface is re-pressed onto the mold surface and forms visible patterns.

👁 Key Inspection Focus

Check areas around ejector pins, gates, circular holes, and regions with dragging streaks.

Inspection Criteria (Inspection Steps)

☑ Observe the orientation of dragging streaks

☑ Inspect areas around ejector pins

☑ Pay attention to gate regions

☑ Distinguish slip marks from scratch marks

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Slip Marks (Ejector Pin Scuff Marks)