Common Plastic Moulding Defects and the Additives That Fix Them

Before you blame the mould or the machine, check whether the material's additive package is actually built for the defect you're seeing.

By Siya Polymer & Speciality Chemicals Technical Team · Manufacturer & Supplier, Maharashtra, India

Not Every Defect Is a Process Problem

Plastic processors often first look at machine settings or mould design when a defect appears - but many recurring defects are actually material or additive issues that no amount of process tuning can fully solve. Here are the most common defects and the additive-side fixes worth checking.

Warping and Dimensional Instability

Warping often results from uneven shrinkage across the part, which can be worsened by inconsistent filler dispersion or an unbalanced resin-to-filler ratio. A well-dispersed mineral filler (talc) or glass reinforcement with proper coupling improves dimensional stability and reduces differential shrinkage.

Flow Lines and Poor Surface Finish

Flow lines usually indicate the melt is cooling unevenly as it fills the mould, often because the base resin's melt flow index is too low for the part geometry. An MFI flow improver raises melt flow without switching resin grades, letting the material fill thin sections and complex geometries more evenly before it cools.

Brittleness and Cracking

Parts that crack under impact - especially in cold conditions - typically need better toughness than the base resin provides. Impact modifiers (such as EPDM-g-MA or POE-g-MA for engineering plastics, or CPE/ACR for PVC) absorb impact energy and prevent brittle failure, without requiring a full resin change.

Sink Marks

Sink marks occur in thick sections where the outer surface cools and solidifies before the inner material has finished shrinking. While this is partly a design and process issue, a well-balanced filler package (talc or glass) that reduces overall shrinkage can meaningfully reduce sink mark severity.

Inconsistent Colour Batch to Batch

Colour inconsistency often traces back to masterbatch dosage variation or poor dispersion rather than the base resin. Verifying dosage percentage and using a well-dispersed masterbatch grade resolves most batch-to-batch colour drift.

Premature Degradation, Yellowing or Brittleness Over Time

Parts that degrade, yellow or become brittle after weeks or months of use or outdoor exposure are usually missing adequate stabilization. Antioxidants protect against thermal and processing degradation, while UV stabilizers protect against sunlight-driven breakdown - both should be specified based on the part's actual service environment.

Quick Diagnostic Table