Flow Marks and Poor Surface Finish
Flow marks are wavy lines frozen into the part surface where the melt front hesitated or turned turbulent. They do not weaken the part. They ruin its appearance, which is enough to scrap a cosmetic housing. The fix starts at mold design, because gate position decides where flow marks land.
How flow marks form
Melt entering a mold follows nearly the same path every shot. From a sprue gate into a flat plate it spreads in a disc. From a well-designed side gate it spreads in an ever-widening half-disc. The melt always runs faster through wide, deep channels and slows when it must spread into thin sections. Hit an obstruction and the flow turns turbulent. The turbulence mark rides the flow front, sometimes all the way to the end of fill. Pigmented materials show this clearly. The color makes every hesitation visible.
Gate position decides everything
Flow marks belong in the mold design review, not the trial report. Gate location, gate type, and every obstruction downstream set the type and intensity of the marks. Where marks are unavoidable, put them where nobody looks. A tunnel gate near the underside of the part beats one near the top cosmetic face. The underside still carries the mark, but the customer never sees it.
Lettering on a cosmetic face is one of the most common causes. Embossed or engraved makes little difference. The melt trips over the letters either way and leaves a mark. If the text must sit on the show surface, expect the mark and plan the texture around it.
The mold surface sets the ceiling
A part can never look better than the steel it was molded against. A good finish degrades fast in a busy shop. Hard tools used to pry out a stuck part gouge the cavity. Rust pits it. Some chiller antifreeze sprays turn slightly acidic with age and etch any metal they touch. Hard water evaporating on mold steel leaves a chalky deposit that resists normal cleaning. Closing the mold on a stray piece of plastic dents both halves. A protective plate between the halves during bench work prevents the classic slipped-tool scar.
Surface damage also magnifies other defects. Weld line witness marks print deeper on a scarred cavity, and flow marks stand out more. If cosmetic defects keep appearing across different jobs, run a structured defect root-cause analysis before re-cutting steel. The steel is often innocent.
Process levers that actually move the needle
Melt temperature is the main lever. Lower viscosity wets the steel more completely, so the part copies the mold surface faithfully. Low-density polyethylene is the textbook case. It finishes best molded hot. The tradeoff is real: hotter melt needs longer cooling, and longer cooling cuts output. Chasing maximum output always costs some surface quality. Decide what the customer will accept and mold to that, not to the machine’s limit.
Filled materials play by different rules. Glass fiber, talc, or mineral-filled nylon, acetal, polycarbonate, and polypropylene finish best when the mold fills fast through a restricted gate. The high shear coats each filler particle with a thin film of plastic. That film keeps the filler below the part surface, where it cannot roughen the finish. Mineral-filled compounds show their lowest viscosity at the highest shear rates. The curve is far steeper than for unfilled grades. Slow, gentle filling is exactly wrong for these materials.
What to do when flow marks appear
First confirm the gate is where the design intended. A gate moved for tooling convenience can drag the flow front across the cosmetic face. Then raise melt temperature in small steps and watch the mark fade. Log the cycle time penalty so the costing stays honest. For filled grades, shorten fill time instead of raising temperature. Check mold temperature as well. A cold mold freezes the skin before the front finishes, which locks every ripple in place.
If the mark sits over lettering, a rib, or a thickness step, no process tweak will erase it. That is a design problem wearing a process disguise. The honest fix is moving the gate, softening the obstruction, or accepting the mark on a hidden face.
Prevention checklist
- Review gate position against the cosmetic faces before steel is cut, not after T1.
- Keep lettering and logos off show surfaces wherever the print allows.
- Protect polished cavities: no pry tools, no hard water, neutral chiller chemistry.
- Match the fill strategy to the material. Hot and steady for unfilled PE, fast and shearing for filled grades.
- When marks are unavoidable, hide them with a planned texture on the non-cosmetic side rather than fighting the flow front.
- Record the agreed surface standard with the customer at quoting. A defined SPI finish grade ends arguments about what “good” means, and it gives the mold maker a measurable target to hit.
