Injection molding

A thin layer of flash around the mold parting line may not look like much, but removing it is labor-intensive. For cosmetic parts, it means direct scrap; for self-tapping screw holes, the burr can jam an entire assembly line.

When flash occurs, most people's first reaction is to blame the material's high flowability or suspect insufficient drying. But after countless trips to the shop floor, I've come to the opposite conclusion—90% of flash problems are not about the material, but about the mold not being held shut tightly enough.

Clamping Force Issues

The principle of injection molding is simple: melt plastic and inject it into the mold under high pressure, while the machine uses its massive tonnage to keep the mold closed. Flash happens for one reason only—the cavity's expansion force exceeds the clamping force the machine can provide.

So where does the clamping force go missing?

The most common culprit: something stuck on the parting line. A dried pellet of regrind, a tiny speck of rust—barely visible to the eye, but it creates a gap when the mold closes. The moment high-pressure material hits it, it squirts right through the gap. That's why many experienced operators wipe the parting line with a cloth before starting a shift.

Another cause: the machine is "stealing" its own force. Toggle-type clamping mechanisms wear out over time—hinges degrade, and the screen may show 500 tons, but the actual force reaching the mold could be just over 400 tons. There's also a more subtle issue: uneven tension on the tie-bars. If the platen is skewed, all the clamping force goes to one side while the other side barely holds, and material will escape from the loose side.

Process Parameter Issues

Even if clamping is adequate, process settings can amplify the problem.

If injection pressure is too high or fill speed too fast, plastic rushes into the cavity like water, and the instantaneous peak pressure far exceeds the average. The mold gets pried open for a split second, and material shoots out. Some operators habitually set the V-P switchover point (the transition from injection to packing) too late—the mold isn't fully filled yet, but packing pressure gets slammed in, which essentially applies even higher pressure to force the mold open, making flash worse.

And then there's melt temperature. Raise the barrel temperature by just 10℃ for the same material grade, and flowability increases noticeably. A mold that never flashed before might start flashing. That's why the same parameters tend to cause flash in summer when the workshop is hot, but run fine in winter.

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