Flash (also known as burr or excess material) is one of the common surface defects in injection molding production. Its main cause is: molten plastic under injection pressure overflows from fitting gaps or structural gaps such as mold parting surfaces, slides, ejector pins, inserts, guide pillars, and forms excess material after cooling. It usually appears as thin flakes, filaments, or irregular protrusions. Flash not only impairs appearance and precision but may also cause assembly interference, out-of-tolerance dimensions, increased post-processing costs, and lower production yield.
Based on practical injection molding experience, the various causes, appearances, and solutions for flash are summarized below, following the troubleshooting logic:
Process → Equipment → Mold

I. Process Parameter Causes (Priority Adjustment, No Mold Modification, Fast Improvement)
Process parameter deviation is the most common and easiest-to-resolve cause of flash. Prioritize parameter adjustment without mold or equipment changes to quickly restore production and reduce waste.
1. Excessively High Material Flowability
Cause: The material itself has high flowability (e.g., PA, PP), excessive plasticizer, or high regrind ratio, resulting in low melt viscosity. The melt easily penetrates tiny mold gaps under injection pressure.
Typical Appearance: Fine filamentary or flaky soft flash, mainly around parting surfaces and ejector pins, easy to peel off.
Practical Solution: Optimize material formulation, reduce plasticizer, control regrind ratio (≤30%, clean and non-degraded). If material cannot be changed, lower barrel temperature and slow down injection speed to reduce flowability, avoiding short shots from excessive pressure reduction.
2. Excessively High Melt Temperature
Cause: Excessively high barrel temperatures or long residence time cause over-plasticization, degradation, sharply reduced viscosity, and abnormally high flowability. May also cause yellowing or brittleness.
Typical Appearance: Thick and widely distributed flash on parting surfaces, ejectors, and inserts; abnormal gloss or yellowing (e.g., ABS).
Practical Solution: Lower barrel temperature step by step (5–10°C each time), especially nozzle temperature to prevent drooling. Shorten cycle to reduce residence time. Sample 3–5 shots and check for short shots or sink marks.
3. Excessively Fast Injection Speed
Cause: Rapid filling causes instant pressure surge in the cavity, exceeding the sealing capacity of mold gaps, especially for thin-walled or complex parts.
Typical Appearance: Flash concentrated near the gate or complex corners, sharp edges; may accompany whitening or burning near the gate.
Practical Solution: Use segmented injection, reduce initial speed (20%–40%), avoid instant high pressure. Adjust speed by 10%–15% each time.
4. Excessively High Holding Pressure
Cause: Excessive holding pressure forces melt into gaps; also increases internal stress and may cause deformation or ejector whitening.
Typical Appearance: Flaky flash tightly attached at parting surfaces and ejector pins; may accompany sink marks from stress concentration.
Practical Solution: Reduce holding pressure by 5%–10% each time, adjust holding time to avoid pressure build-up.
5. Late Holding Pressure Switchover
Cause: Switchover too late leads to over-filling, excessive cavity pressure, and gap opening. Often accompanied by oversized parts or surface bulging.
Typical Appearance: Uniform flash along the entire parting line; thicker parts with uneven gloss.
Practical Solution: Advance switchover position by 1–2 mm, switch at 95%–98% filling. Avoid early switchover causing short shots.
6. Excessively High Mold Wall Temperature
Cause: High mold temperature slows cooling, maintains low viscosity, and allows melt to flow into gaps; also extends cooling time and reduces efficiency.
Typical Appearance: Thin, soft flash at ejectors and inserts; parts prone to sink marks and deformation.
Practical Solution: Lower mold temperature (ABS: 50–70°C, PP: 30–50°C) by 5–10°C each time. Optimize cooling circuits for uniform temperature.
II. Equipment and Clamping Issues (Checked When Process Adjustment Fails)
If flash remains after parameter tuning, check the injection molding machine and clamping system, which usually require maintenance rather than mold modification.
1. Insufficient Clamping Force
Cause: Clamping force too low or system aging cannot resist cavity pressure, parting surface opens.
Typical Appearance: Thick, uniform flash along the entire parting surface, not improved by process changes.
Practical Solution: Calculate and increase clamping force (5%–10% each time). Repair leaking cylinders or worn seals.
2. Incomplete Mold Closing
Cause: Platen not fully closed or improper clamping stroke leaves gaps.
Typical Appearance: Local flash (edge, opposite gate), uneven thickness.
Practical Solution: Adjust clamping stroke, check parting gap with feeler gauge (<0.02 mm). Lubricate or replace worn guide pillars.
3. Clamping Pressure Relief Mid-Cycle
Cause: Oil leakage or check valve failure leads to dropping clamping force during injection/holding.
Typical Appearance: Unstable, intermittent flash, irregular distribution.
Practical Solution: Inspect and replace seals and check valves. Verify stable clamping pressure throughout the cycle.
4. Non-Parallel Tie Bars / Uneven Loading
Cause: Misaligned, worn, or deformed tie bars cause platen tilting and local gaps.
Typical Appearance: Persistent flash on one side or corner, cannot be eliminated by process tuning.
Practical Solution: Have professionals adjust parallelism and tighten nuts. Replace worn or deformed tie bars.
5. Foreign Objects Between Platens
Cause: Contamination, iron chips, or dust on parting surfaces prevent full closing.
Typical Appearance: Local flash, sometimes containing foreign material; reoccurs easily.
Practical Solution: Regularly clean parting surfaces and platens. Improve material filtering.
III. Mold Issues (Require Mold Modification for Root Cause Resolution)
If flash persists after process and equipment checks, the problem lies in the mold and requires modification to improve precision and structure.
1. Insufficient Mold Strength / Plate Deformation
Cause: Thin plates, low-strength material, or poor design lead to deformation under high pressure.
Typical Appearance: Full-width uniform flash that remains despite clamping and process changes.
Practical Solution: Thicken plates, use higher-strength mold steel (Cr12MoV, S136), add ribs to improve rigidity.
2. Poor Parting Surface Machining / Unmatched Shut-Offs
Cause: Rough, scratched, or uneven parting surfaces; oil or burrs affect sealing.
Typical Appearance: Fine flash at defects, temporarily improved by cleaning but not eliminated.
Practical Solution: Grind and polish parting surfaces to achieve 0.01–0.02 mm fit. Repair deep depressions via welding and polishing.
3. Excessively Deep Vent Grooves
Cause: Vent depth >0.05 mm allows melt overflow; also causes gas marks or short shots.
Typical Appearance: Filamentary flash at vents (corners, near gate), soft texture.
Practical Solution: Adjust vent depth to 0.02–0.05 mm (lower for high-flow materials). Optimize vent position and width.
4. Worn Ejector Pins, Inserts, or Bushings / Excessive Clearance
Cause: Worn or deformed moving parts increase fitting clearance beyond 0.02 mm.
Typical Appearance: Local filamentary flash around pins/inserts, worsening over time.
Practical Solution: Replace worn components, control clearance to 0.01–0.02 mm. Lubricate regularly to reduce wear.
IV. On-Site Troubleshooting and Handling Notes
Troubleshooting Order: Strictly follow Process → Equipment → Mold to avoid unnecessary mold modification.
Trial Verification: After each adjustment, run 3–5 shots and check for flash, short shots, sink marks, and deformation.
Material Adaptation: For high-flow materials (PA, PP), control temperature and speed. For brittle materials (PS, ABS), avoid excessive clamping force or speed.
Daily Maintenance: Regular cleaning of parting surfaces, oil circuits, and moving parts effectively reduces recurring defects.
Record & Trace: Document parameters, equipment status, mold condition, and trial results for quick future troubleshooting.

Summary

The root cause of flash is molten plastic overflowing from gaps under pressure.
The core solution logic:
Reduce melt flowability → Control injection pressure → Eliminate fitting gaps.
By following the workflow:
Process Tuning → Equipment Maintenance → Mold Modification
flash can be efficiently resolved, improving yield and reducing waste for all types of injection molded products.