Splash Marks, Mica Marks, Silvering

Silver streaks radiating from the gate, like mica flakes frozen in the plastic — that is splash marking, also called silvering. The streaks are volatiles that vaporized at the melt front and condensed on the part surface. The cure is always the same: get the volatiles out of the process. The detective work is finding which volatile and where it came from.

What makes the streaks

As the melt front advances into the cavity, any volatile matter vaporizes, then condenses on the cooler surface as liquid droplets. The incoming melt drags those droplets along and stretches them into the familiar streaky fan. The volatiles have three origins. Water absorbed by the plastic before molding. Decomposition or depolymerization products from overheating. Moisture condensed on a cold mold. A mold chilled below the shop’s dew point sweats, and that sweat streaks exactly like wet resin.

Moisture: know your resin’s tolerance

Some resins forgive dampness. Some forgive nothing. Nylon and acrylic tolerate a small amount of moisture. Under 0.1% will usually still mold cleanly. ABS and polycarbonate tolerate almost zero. Those two must be dried immediately before use and charged hot into the hopper. Nylon and acrylic should live in airtight containers or sealed film sacks. Rework must be kept dry. Damp rework is the most common source of surprise silvering on an otherwise dialed-in job.

The chemistry explains the ranking. Acrylics, nylons, and cellulosics carry hydrophilic groups in their molecular structure, so they absorb water readily. Some outdoor weathering grades of polyolefin pick up moisture thanks to finely divided carbon black. ABS and polycarbonate do not absorb deeply, but moisture adsorbs onto the surface pores of the pellets. That is enough to streak the part. Do not assume a resin is dry because the bag was sealed at the factory. Verify with a moisture analyzer when silvering appears. On high-cavitation jobs the check pays for itself fast. One wet batch can scrap thousands of parts before anyone looks at the dryer.

Decomposition: the hotter suspect

When drying changes nothing, look at temperature. Decomposition is hard to spot on the part but easy to spot in the purge. Pull material from the nozzle and look for bubbling or frothing. Any froth means the cylinder is too hot somewhere. Check for hot spots: a failed heater band zone, or a thermocouple reading low while its band overruns. Excessive mechanical heating during screw recovery is another classic. If screw-back time drifted longer or more erratic before the fault appeared, shear heat is your volatile source. Drop the melt temperature and stabilize recovery before touching anything else.

One theory deserves retiring: shear heating in the gate. Splash marks radiate from the gate, so the gate looks guilty. With a properly designed gate system, gate shear cannot generate the volatiles. The residence time is far too short. A gate that actually degrades material is either far too small or damaged, and it will show other symptoms first. Do not enlarge a good gate to chase silvering. You will only move the problem and slow the fill.

Fixes in order

Dry the material first, because moisture is the most common cause and the cheapest to rule out. Dry to the resin supplier’s specification. Typical practice is 2–4 hours at 80°C for ABS and 3–4 hours at 120°C for polycarbonate. Mold promptly after drying; dried resin left standing reabsorbs moisture from shop air. If streaks persist, drop cylinder temperatures stepwise and inspect purges for frothing. If the mold is the moisture source, raise mold temperature above the shop dew point rather than drying the resin harder. Keep rework dry and capped. Every damp handful of rework reintroduces the problem you just dried away.

For critical cosmetic parts, some shops add a final safeguard. A short purge and a fresh dryer load go in before the production run. Then the first shots run on known-dry resin. It costs twenty minutes and has saved more than one launch.

Tell silvering from splay in one shot

Both streak from the gate, and shops confuse them weekly. The one-shot test: dry a small batch thoroughly and run it. If the streaks vanish, it was moisture. If they stay, it is degradation or mold condensation. Next, pull a purge and look for froth. Froth means the cylinder is the source. No froth and dry resin points at the mold. Check whether the mold runs below the shop dew point, especially on humid mornings after a weekend shutdown. Three checks, one answer, no guessing.

Prevention checklist

• Dry hygroscopic resins per spec and charge the hopper hot. Near-zero tolerance for ABS and polycarbonate.
• Store nylon and acrylic sealed. Segregate and dry all rework.
• Watch nozzle purges for bubbling. It is the earliest sign of decomposition.
• Keep screw recovery consistent, and investigate drift immediately.
• Hold mold temperature above the shop dew point in humid months.
• Do not blame gate shear without evidence. Check drying and temperatures first.

Silvering shares a family resemblance with splay marking. Both streak from the gate. But the causes diverge: splay is lost pressure, silvering is volatiles. Confuse them and you will tune the process in the wrong direction.

When streaks survive drying and temperature cuts, stop treating every streak as wet resin. A defect root-cause analysis separates moisture, degradation, and mold condensation into three different fixes.