Silver Streaks at the Gate Are a Drying Record, Not a Mold Problem

Silver streaks fanning out from the gate are usually blamed on the tool. On a hygroscopic resin they are a drying record, and unless somebody wrote that record down, the fix will be guessed at for weeks.

There are two kinds of moisture damage. Splay — the fine silver streaks radiating from the gate — is the visible half. Hydrolysis is the other: water in the melt breaks polymer chains, molecular weight drops, and impact strength goes with it. The part still measures in tolerance and still looks acceptable under shop lighting. Then it fails a drop test at the customer, or cracks at a screw boss six months on, and by then the material and the tool have both been blamed and nobody has looked at the dryer.

Which resins actually care

The useful split is chemistry, not brand. Polycondensation polymers — PA, PC, PET, PBT, and to a lesser degree POM and TPU — hydrolyze. Addition polymers — PP, PE, PS — do not, in any practical sense. ABS and ASA sit in the middle: mildly hygroscopic, worth drying, rarely the cause of a mystery.

The question is not whether the material is dry, but which polymer it is: a PP housing can be molded from a bag on the floor, a PC/ABS housing cannot.

Filled grades need the same attention: filler does not absorb water, so the percentage falls, but the polymer still hydrolyzes — and a brittle failure in glass-filled PA is the least likely to be caught in inspection.

What drying actually requires

Three conditions have to be true at once: temperature high enough, air dry enough, time long enough. They do not compensate for each other — a dryer at the right temperature with humid air produces warm wet resin.

The starting points below assume standard unfilled or lightly filled grades, a desiccant dryer and normal plant conditions. The grade datasheet overrides every one of them.

• PC: around 120 °C for three to four hours, with dew point at the hopper inlet at −20 °C or lower; target moisture near 0.02%.

• PA66 / PA6: 80–100 °C for four to six hours, targeting 0.1–0.2%. A window, not a maximum — PA over-dried at high temperature goes brittle, a separate failure with the same symptom.

• POM: 80–90 °C for two to three hours. Hotter or longer releases formaldehyde, which you can smell in the parts and which attacks the tool.

• PET / PBT: around 120 °C for four to six hours. The least forgiving of the group.

A hot-air oven heats resin but does not dry the air. On a humid day the equilibrium moisture inside it can sit above what PA or PET needs, which is why a molder who always dries for four hours gets splay in August and none in January. Only desiccant or vacuum drying moves the dew point.

The gap between the dryer and the screw

Most wet-material defects happen after the material has been dried correctly.

Dried granules tipped into an open bin, conveyed through an open line, or dropped into a hopper without a dry-air blanket pick moisture back up at the surface within minutes. A reading at the dryer outlet looks perfect; the granule that reaches the screw is not the granule that was measured.

Residence time is the second gap: dry material sitting hot in a hopper for a shift degrades thermally even though it is chemically dry, and the two problems look alike in the part.

Regrind is the third. Ground scrap has far more surface area per kilogram and reabsorbs moisture much faster: a sealed bin opened at the end of a shift is not the same material the next morning, and a 20–30% regrind ratio that ran well in winter can explain everything in summer.

The sequence that holds is short and boring: dry, convey closed, hold under dry air, shoot, with the shortest hopper residence the machine allows. Every stop of more than a few minutes gets a purge; a tool down for an hour no longer holds the material that was qualified.

Separating moisture from the other causes

Streaks at the gate have four possible sources, and only one of them is water.

• Moisture splay: fine silver streaks fanning from the gate, spread across the whole shot including thick sections, unchanged by injection speed.

• Trapped gas or burning: dark brown or black, at the last point to fill or behind a rib.

• Shear marks: dull, following flow lines, no silver sheen, and they move when fill speed changes.

• Gate blush: a localized ring at the gate, stress rather than moisture, unchanged when the resin is dried.

The cheap test is a drying trial: dry the same material properly in a lab oven, mold a short run on the same tool and settings. Streaks gone means moisture and the argument is over; streaks still there means venting, gate size or fill speed, and you are looking at the tool with evidence instead of a suspicion.

What to ask for before you approve the process

On any PC, PA, PET or PBT part, ask for the drying record as part of the process sheet:

1. Dryer type — desiccant, vacuum or hot air.

2. Set and actual temperature.

3. Dew point at the hopper inlet, measured during the run.

4. Hopper residence time at the running cycle.

5. Regrind percentage, and how regrind is stored.

6. One moisture reading from the hopper outlet during that shift.

Six lines. If the molder cannot produce them, the process is running on the bag label rather than on a measurement, and the first brittle failure will be diagnosed by whoever is least able to fix it. Put the six lines on the sheet before T1, and they stay there as long as the tool runs.