How to Solve Flash in Plastic Injection Molding
Flash (or burrs) is a common molding defect where molten plastic leaks from the mold cavity during injection or holding, forming a thin unwanted layer along parting lines, ejector pins, or vents. It increases secondary trimming costs, causes material waste, and risks tooling damage.
Key Causes of Injection Molding Flash
Flash occurs when plastic pressure inside the cavity exceeds the machine’s clamping force, or when gaps exist on the mold.
1. Tooling & Mold Issues
Parting Line Wear: Long production runs wear down mating surfaces, creating micro-gaps.
Plate Deflection: Lack of structural support causes mold plates to flex under pressure.
Improper Venting: Excessively deep or damaged vents allow plastic to escape alongside trapped air.
2. Process Parameter Misalignment
Excessive Pressure/Speed: High injection or hold pressure overrides clamp tonnage, forcing mold plates open.
Overfilling (Shot Size): Injecting excess resin creates extreme internal pressure during packing.
Overheated Resin: Excessive barrel temperatures reduce melt viscosity, letting plastic flow into tiny clearances.
3. Equipment Limitations
Underpowered Clamp Tonnage: Machine clamp force is insufficient for the part’s projected surface area.
Platen Misalignment: Unlevel platens or worn tie bars cause uneven clamping force.
Practical Solutions to Eliminate Flash
Resolving flash requires a systematic approach, starting with process adjustments before modifying physical tooling.
Step 1: Tune Process Parameters
Reduce Pressure: Lower hold pressure incrementally until flash vanishes while maintaining full part fill.
Lower Temperatures: Decrease barrel heat to raise melt viscosity, reducing leakage into micro-clearances.
Slow Injection Speed: Reduce velocity near the end of fill to prevent dynamic pressure spikes.
Control Cushion: Fine-tune shot size to fill the cavity without overpacking.
Step 2: Inspect and Maintain Tooling
Regrind and Align Parting Surfaces: Inspect mold halves for debris, wear, or damage. Clean parting lines thoroughly and re-machine if necessary.
Check Venting Channels: Ensure vents are within recommended depth specifications
Increase Support Pillars: Add structural support behind mold plates to prevent flexing under high tonnage.
Step 3: Press & Material Selection
Verify Required Clamp Tonnage: Ensure machine clamping tonnage exceeds the force required by the part's projected area (3-5tons per square inch for engineering resins).Select Suitable Resin Flow Rate: Switch to a lower melt-flow index resin grade if high flowability is unnecessary.
Summary
Flash is a manageable defect when addressed with structured troubleshooting. By balancing process temperatures, optimizing injection pressures, and maintaining precision tooling, manufacturers can achieve flash-free components, minimize scrap rates, and extend mold service life.
