Summary
Flash is a thin layer of excess plastic that forms outside the intended part boundary, typically along the parting line, at ejector-pin locations or at other mould interfaces. It occurs when plastic melt is forced into gaps in the mould under injection or clamping pressure. Flash may affect appearance, dimensional accuracy, assembly fit and, in some cases, function or safety if it is in a critical area.
What It Looks Like
Flash appears as a thin fin, film or ridge of plastic extending beyond the part boundary. It may be translucent or the same colour as the part.
Appearance may vary with:
- Flash along the parting line typically appears as a continuous or intermittent fin around the part perimeter.
- Flash at ejector pins appears as small raised discs or rings on the ejector-pin contact surfaces.
- Severity ranges from a barely perceptible film to a pronounced fin several millimetres wide.
- Thin flash may be flexible; thicker flash is more rigid.
- Some materials produce more brittle flash that breaks off during ejection, leaving a rough edge.
Where It Commonly Appears
- Along the parting line of the mould.
- At ejector-pin locations on the ejection side of the part.
- At side-action or lifter interfaces.
- At insert interfaces in insert-moulded parts.
- At vents if vent depth is excessive.
Possible Causes
The following are possible contributing factors, not confirmed diagnoses. Multiple causes may be present simultaneously.
Part design
- Part geometry that creates high local pressure at the parting line.
- Projected area that requires clamping force beyond the machine's capacity.
Mould and tooling
- Worn, damaged or poorly fitted parting-line surfaces.
- Insufficient clamping force for the projected area and injection pressure.
- Mould damage or contamination on the parting-line surfaces.
- Excessive vent depth allowing material to enter the vent.
- Worn ejector-pin bores allowing material to flow around the pins.
- Mould deflection under injection pressure opening gaps at the parting line.
Material handling and selection
- Low-viscosity materials or high-flow grades are more prone to flashing into small gaps.
- Processing at temperatures above the recommended range reduces viscosity and increases flash risk.
Moulding process
- Injection pressure that is too high, forcing material into parting-line gaps.
- Melt temperature that is too high, reducing viscosity.
- Insufficient clamping force relative to the injection pressure and projected area.
- Overpacking the cavity.
- Injection speed that is too high, generating excessive pressure at the flow front.
Machine or equipment
- Insufficient clamping force for the mould and part.
- Clamping system wear or inconsistency.
- Platen parallelism issues causing uneven mould closure.
Practical Checks
Begin with observation and verification before recommending changes.
- 1Confirm whether flash occurs consistently or intermittently.
- 2Identify the exact location of the flash: parting line, ejector pins, vents or other interfaces.
- 3Check whether flash started after a mould, machine or settings change.
- 4Inspect the parting-line surfaces for wear, damage or contamination.
- 5Review process records: injection pressure, clamping force, melt temperature and injection speed.
- 6Confirm the clamping force is appropriate for the projected area and injection pressure.
- 7Check ejector-pin bores for wear.
- 8Inspect vents for excessive depth.
- 9Assess whether the defect is visual only or affects dimensional requirements, assembly or function.
Possible Corrective Directions
These are possible directions for investigation, not guaranteed solutions. Process changes must be evaluated against other part-quality and validation requirements. Do not change several variables simultaneously without first identifying the likely cause.
Design considerations
- Review the part geometry to minimise projected area if clamping force is a constraint.
Tooling considerations
- Repair or re-fit worn or damaged parting-line surfaces.
- Clean and inspect parting-line surfaces for contamination.
- Reduce vent depth if vents are flashing.
- Replace worn ejector pins or re-bore worn ejector-pin holes.
- Consider mould stiffening if deflection under pressure is contributing.
Material considerations
- Avoid processing at temperatures above the recommended range.
- Consider whether a higher-viscosity grade is appropriate if flash is persistent, noting that material changes require validation.
Process considerations
- Reduce injection pressure to the minimum needed to fill the part.
- Reduce melt temperature within the material's processing window.
- Increase clamping force if the machine capacity permits.
- Reduce injection speed.
- Reduce holding pressure.
- Process changes must be evaluated against other part-quality and validation requirements.
Risks and Trade-Offs
- Reducing injection pressure to eliminate flash may result in short shots or sink marks.
- Reducing melt temperature to increase viscosity may reduce flow length and cause short shots.
- Repairing parting-line surfaces requires mould downtime.
- Reducing vent depth to prevent flash may worsen short shots or burn marks from trapped gas.
- Increasing clamping force beyond the machine's rated capacity is not appropriate.
When to Involve Your Moulding Partner
Consider requesting an engineering review when:
- Flash is in a location that affects assembly fit, function or safety.
- The defect is recurring despite process adjustments.
- Mould repair or parting-line rework is being considered.
- The clamping force requirement is uncertain.
- Regulated or validation-controlled parts are involved.
Sources and Further Reading
- BASF. Injection-Molding Problems in Engineering Thermoplastics: Causes and Solutions. https://download.basf.com/p1/8a8082587fd4b608017fd6631d5a24b1/en/Injection-Molding_Problems_in_Engineering_Thermoplastics_-_Causes_and_Solutions(accessed )
- Autodesk Moldflow. Molding Problems and Troubleshooting. https://help.autodesk.com/cloudhelp/2017/ENU/MoldflowInsight/files/GUID-59A81F21-DC33-489D-B0A9-9436E5A9A32C.htm(accessed )