Flash (Burrs) | Definition, Causes & Inspection Criteria | Justway

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Flash (Burrs) | Definition, Causes & Inspection Criteria

Last updated on Jul 22, 2026

Formation Mechanism

Even though molds are manufactured with micron-level precision and clamped under high pressure during molding, small clearances still exist due to high resin filling pressure. Flash forms when molten resin squeezes into these gaps. Flash may appear on parting lines (PL), sleeves, slide core interfaces and vent slots.

Flash is the phenomenon where molten resin extrudes into the mold parting line, forming unwanted thin film on the component. This occurs when the parting line separates under resin pressure or gaps exist on the parting line.

Common Locations

Flash occurs at Parting Lines (PL), sleeves, slide core interfaces and vent slots.

Figure 1: Flash on the parting line of 120mm square flat plate

Inspection Criteria (Inspection Steps)

☑ Focus inspection on parting lines (PL)

☑ Check for thin film excess material along edges

☑ Pay attention to corners and joint interfaces

☑ Distinguish flash from burrs or damaged areas

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