Plate-Out (Mold Surface Deposits)

How to Recognize It

• Deposits on the mold surface, mostly near the GATE and VENT areas; they can form within just a few thousand cycles.
• It shows up on the part as a rough surface or local differences in gloss — the cavity surface cannot be replicated fully.
• With white pigments, the deposit is called "chalking"; it is linked to oxidative degradation of the plastic.
• The materials where it is seen most often: POM, PP, PET, ABS, PC, PSU, PBT, PE.

Physical Causes

• High material stress: excessive melt temperature, shear stress and shear rates.
• High shear in the plasticizing unit (excessive screw speed) and excessive residence times.
• High shear inside the mold (excessive injection speed).
• Insufficient mold venting (sometimes because of excessive clamping force).
• Material with excessive residual moisture; excessive drying times.
• Wrong or excessive use of lubricant; incompatibility between the base material and the colorant/additive — the colorant is carried on the lubricant like a "sled" and settles on the surface.
• Flame-retardant materials can react chemically when stress rises and cause corrosion in the mold.

Solution Steps at the Machine

1. Measure the melt temperature; if it is above the permitted values, lower the barrel temperature, screw speed and back pressure.
2. If the residence time is long, reduce the cycle time, add a plasticizing delay, consider a smaller unit.
3. Measure the residual moisture; dry according to the manufacturer's instructions and DO NOT EXCEED the permitted drying times.
4. Reduce the injection speed; avoid sharp edges and small flow cross-sections.
5. Reduce the clamping force and improve venting (check the channels for contamination).

Mold, Design and Material Solutions

• Review the use of lubricant: the right type, the right dosage; an anti-friction layer on moving elements. Avoid silicone-based release agents and lubricants — they both feed plate-out and transfer to the part surface, spoiling subsequent coating, painting and bonding operations.
• Check the compatibility and mixing ratio of base material / colorant / additive; verify the thermal stability of the additives.
• Use corrosion-resistant steel or a coating for flame-retardant materials.
• DRY ICE is an effective method for mold cleaning (it can be applied on the machine).
• With certain POM grades, pre-drying at about 80 °C for 2 hours reduces plate-out and improves surface quality.

Important Notes

• ⚠️ Plate-out often cannot be eliminated with a single measure; in processes running at the limits, many optimization steps need to be applied together. The goal is not to eliminate it completely but to extend the cleaning intervals.