Discoloration | Definition, Causes & Inspection Criteria

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Discoloration | Definition, Causes & Inspection Criteria

Last updated on Jul 22, 2026

(1) What is Discoloration? (Visual Defect)

Discoloration means the color of molded parts deviates from the normal standard color.

Plastics are chemical substances. When continuously heated above their melting point, they gradually decompose and degrade. Discoloration is a visual defect accompanying this degradation process.

Figure 1: Specimens with Discoloration

Specimen with Normal Color

Specimen Discolored due to Material Residence

📄 Definition

Yellowish, darkened appearance or local browning on parts.

🔍 Potential Causes

Material residence inside barrel, excessive temperature, polymer degradation, abnormal material blending.

👁 Inspection Focus

Compare against standard color chip; confirm whether color shift occurs locally or over the whole part.

Inspection Checklist

☑ Compare with standard reference sample

☑ Watch for residence traces

☑ Verify process temperature settings

☑ Keep photos of color difference for record

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Discoloration | Definition, Causes & Inspection Criteria