How to Improve Excessive Flash & Large Burrs on Rubber Products

Essence of rubber flash/burrs: Excess rubber compound, insufficient clamping force, large mold‑matching clearance, mismatched temperature, pressure and curing time, as well as overly good compound fluidity. Troubleshooting and rectification shall be conducted from four dimensions: mold, process, rubber compound and equipment.

1. Excessive clearance on parting surface

• Inspect the clearance of mold cavity parting surfaces, inserts, ejector pins, guide pillars and guide bushes. For rubber processing, the clearance is generally controlled within 0.02‑0.05 mm. For soft compounds with excellent fluidity, keep it ≤ 0.03 mm.
• If the parting surface is worn, bruised or pitted, grind and polish the surface. Retighten and re‑fit loose or misaligned inserts.

Large clearance: Rubber squeezes into gaps and forms thick flash. Small clearance produces thin flash which is easy to trim.

2. Unreasonable design of flash trap and vent slots

• Avoid overly deep flash traps; recommended depth is 0.1‑0.2 mm. Excessively deep traps will generate large‑size thick flash. Keep flash channels short to discharge surplus compound timely.
• Blocked or shallow vent slots raise internal cavity pressure and force rubber into gaps, aggravating flash. Regularly clean carbon deposits and rubber residues inside molds.

3. Mold parallelism & deformation
   Under long‑term high‑temperature compression, mold plates may warp and deform. Poor closure of parting surfaces causes local rubber leakage. Repair plate flatness and perform stress‑relief tempering for molds if necessary.

1. Insufficient clamping force

When clamping force ＜ total rubber pressure inside cavity, the mold will be pried open and heavy flash occurs directly.

• Calculation reference: Clamping force ＞ projected area of finished part × unit curing pressure. Increase press clamping force accordingly.
• Check press platen parallelism. Uneven platens lead to heavy flash on one side while no flash on the opposite side.
• Inspect oil cylinder and hydraulic system for pressure drop. Pressure loss during holding stage will slightly open the mold and cause flash.

2. Uneven hot‑plate temperature
   Large local temperature difference on hot plates induces mold thermal warpage, poor contact of parting surfaces and local rubber leakage. Calibrate hot‑plate temperature.

1. Charged quantity for compression / injection molding

• Over‑charging: Surplus compound converts into flash directly. Properly reduce feeding weight so that the cavity is fully filled with moderate overflow only.
• For injection molding: Excessive injection pressure or too‑fast injection speed squeezes rubber into gaps. Lower injection pressure and injection speed.

2. Curing temperature
   Higher temperature improves compound fluidity and increases flash tendency. Avoid excessively high mold temperature; lower mold temperature appropriately on the premise of full curing.

High temperature reduces rubber viscosity and makes it easier to flow into tiny gaps, resulting in thicker flash.

3. Pressure & holding time
   Compression molding: Apply full clamping force only after mold closure to prevent flash caused by early high‑pressure rubber squeezing.
   Injection molding: Do not set excessively high holding pressure or prolonged holding time, which may cause continuous material overflow.

1. Over‑good fluidity (excess plasticizer / oil content) easily triggers heavy flash.

• Reduce process oil and plasticizer appropriately without sacrificing product performance; raise Mooney viscosity of the compound.

2. Too‑short scorch time causes rapid viscosity drop upon mold entry and heavy overflow. Adjust accelerator system to moderately prolong scorch time.
3. Impurities and particles trapped on parting surfaces prevent tight mold closure. Filter rubber compound regularly.

1️⃣ Observe symptoms: Heavy flash on all parts or only partial positions?

• Flash on all parts: Insufficient clamping force, over‑charging, excessive compound fluidity.
• Unilateral / partial flash: Mold deformation, uneven platens, worn insert clearance.

2️⃣ Priority actions: Reduce feeding weight → increase clamping force → lower injection pressure → remove foreign materials and rubber residues on parting surfaces.

3️⃣ If ineffective: Grind and repair mold parting surfaces and adjust clearances; then re‑evaluate compound Mooney viscosity.

• Flash thin and continuous: Acceptable, to be removed in post‑trimming.
• Flash thick, hard and bulky: Abnormality requiring rectification according to above items.
• Parting surfaces wear out in mass production. Regular grinding and maintenance is one of the most effective measures for flash reduction.

If you adopt compression or injection molding, or work with specific rubber types (NR / NBR / EPDM), I can provide parameter references tailored to your actual production.