In the mold shop, we often say that flow marks, burn marks, and silver streaks on an injection-molded part are like clouds in the sky—if you can read them, you can predict the "weather" inside your mold before a batch gets scrapped. Take flow marks: those wavy, concentric lines near the gate usually point to insufficient injection speed, low melt temperature, or a cold mold. If the mold temperature sits below the recommended range for the resin—say, 60°C on a mold that really wants 80°C—the melt freezes off too early and leaves visible record grooves. Bumping injection speed in stages and raising barrel temperatures by 10–15°C often clears them up. From a tooling standpoint, check whether the gate is undersized or the runner has a sharp step; a restricted gate acts like a nozzle, and no amount of process tweaking will fully hide it.

Burn marks are the storm clouds. They show up as dark brown or black streaks at the end of fill or in blind pockets, and they mean trapped gas is being compressed and dieseling. Venting is the first thing to verify: vent depth on most commodity resins should sit around 0.02–0.03 mm, and if the cavity has no vent at the last-fill location, you are asking for trouble. Silver streaks—those silvery streaks radiating from the gate—usually come from moisture or shear. Dry the resin properly (PC at 120°C for 3–4 hours, for example), and check for excessive screw decompression that pulls air into the shot. In short, these surface defects are free diagnostic data. Read them early, adjust process and tooling together, and you avoid scrapping a whole run.

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