When Is Polishing Needed in Injection Moulding?

1. After Initial Machining (First-Time Polishing)

When a mould is manufactured using CNC, EDM, or wire cutting, the cavity surface often has machining marks, tooling lines, or spark erosion textures. These imperfections can transfer to plastic parts, causing visible defects.

• Solution:
  Progressive polishing using oil stones, sandpaper (e.g., #400 to #1500 grit), or diamond paste to achieve a smooth finish.

2. After Trial Moulding Reveals Surface Defects

If test shots show issues like:

• Scratches or flow marks on the product

• Sticking or ejection problems

• Uneven gloss/texture

• Solution:
  Localized polishing in problem areas, following the demoulding direction to improve release.

3. Regular Maintenance During Mass Production

Over time, mould cavities wear down due to:

• Abrasion from filled materials (e.g., glass fibers)

• Plastic additive buildup

• Minor corrosion

• Solution:
  Periodic polishing of cores, cavities, and ejector pins to restore smoothness.

4. Changing Product Surface Requirements

If the customer requests:

• A shift from matte to glossy (or vice versa)

• Adding or removing texture (e.g., leather grain, geometric patterns)

• Solution:
  Re-polish before applying new textures or finishes.

5. Preparing for Special Materials

Some plastics demand ultra-smooth mould surfaces:

• Optical-grade materials (PMMA, PC): Mirror finish (Ra ≤ 0.05μm) to avoid haze.

• Sticky materials (TPU, PVC): High polish to reduce flow resistance.

Key Considerations

• Avoid over-polishing: It may alter critical dimensions.

• Uniformity: Complex curves need consistent polishing to prevent uneven gloss.

Polishing is a balance between aesthetics, functionality, and mould longevity. By timing it right, manufacturers can ensure high-quality plastic parts with fewer defects.