Meaning
Surplus plastic resin extruding beyond the intended part cavity during the injection moulding cycle defines this phenomenon. When flash occurs, high injection pressure forces molten material into the parting line or between tool shut-offs. The thickness of this excess material depends on the clamp tonnage and the accuracy of the mould plates.
Excess material creates non-conforming parts that require secondary trimming operations to achieve the desired dimensions.
Injection Dynamics
Hydraulic pressure inside the mould often exceeds the clamping force holding the tool halves together. This imbalance separates the steel plates at the parting line, allowing polymer to flow into the unintended gap. Low clamp pressure or extreme injection velocity increases the probability of this occurrence during mass production.
Worn mould surfaces or uneven thermal expansion in the tool steel also contribute to this separation.
Material Economics
Regrind usage often worsens the presence of excess material on a part. Higher levels of regrind decrease the melt viscosity of the polymer, allowing the resin to flow more easily into thin gaps. Virgin resin provides more predictable rheological behaviour, but it cannot fully offset the mechanical deficiencies of a poorly maintained tool.
Trimming operations add significant labour costs to the production cycle, which reduces the overall profit margin per unit.
Tooling Integrity
Maintenance of the tool surfaces ensures that the shut-offs remain tight across the duration of a production run. Surfaces must be cleaned frequently to remove residual resin that prevents the plates from closing perfectly. Regular monitoring of the clamp tonnage during the start of a shift prevents the mould from opening under the peak injection pressure.
Rigid mould design acts as the primary barrier against the formation of unwanted resin protrusions.
