Silver Streaks and Splay in Injection Molding — What Causes the Fine Silvery Lines That Radiate from the Gate, and How Do You Separate Moisture, Degradation and Air from Contamination?
Last updated: 2026-09-04 · Reading time: about 6 min
Silver streaks (splay) are fine silvery lines fanning from the gate — tiny gas voids or degraded material smeared along the melt path. Four root causes look similar but are fixed differently: moisture in the resin (dominant on PC, PA, ABS, PET), thermal degradation, air entrapment, and contamination. Fix order: dry first, then reduce thermal stress, then vent, then isolate contamination.
See also related article.
Key facts
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Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.
What are the four causes — and how do you tell them apart on the shop floor?
Splay is a symptom, not a single defect. The four causes look similar on the part but arrive through different mechanisms and require different corrections. The identifying clues in the table below are what separate them in practice.
| Cause | Mechanism | Identifying Clues | Primary Fix |
|---|---|---|---|
| Moisture in resin | Water in pellets flashes to steam in the barrel | Streaks from gate across whole part; worse on PC/PA/ABS/PET; micro-bubbles; seasonal | Dry to spec (PC ~120 °C / 4 h to <0.02 %); verify dew point |
| Thermal degradation | Resin held too hot or too long breaks down and chars | Streaks darker at core; tied to a hot spot or dead zone; burnt smell | Lower melt temp; purge dead zones; right-size machine |
| Air entrapment | Fast fill pushes air ahead that cannot vent | Streaks near end of fill or around cores; burn-mark companion | Slow fill stage; add vents; vacuum-assist |
| Contamination | Foreign resin, release agent or moist regrind enters melt | Intermittent; after material/hopper change; regrind discolored | Isolate and clean material path; control regrind |
Splay (silver streaks). A surface defect in injection molding where fine silvery or whitish lines fan out from the gate or along the flow path. The streaks are tiny gas voids or degraded material smeared on the surface. Splay is always a gas-in-the-melt problem — the gas comes from moisture, degradation, air or contamination — and the fix follows that source, not the mold.
What is the diagnostic and correction sequence?
Four steps, in the order they should be executed. Each step eliminates one cause before the next is tested.
- Verify and correct drying first. Measure resin moisture at the machine — do not rely on the dryer timer. A working dehumidifying dryer should deliver air at a dew point of −30 to −40 °C for PC and PA. If moisture is above spec (PC above ~0.02 %, PA above ~0.15 %), the run will splay no matter what you set on the barrel. Fix the dryer: correct temperature and time, confirm airflow passes through the resin bed, and check that pellets are not re-wetting in an unsealed hopper or transfer line.
- Reduce thermal stress and check residence time. If dry material still splays, lower the melt temperature toward the bottom of the processing window. Inspect for degradation: burnt smell, darkening, streaks that fade after a purge (char in the screw or a nozzle dead zone). Right-size the machine — a small shot on a large barrel means long residence and breakdown. Clean any charred screw, barrel or nozzle, and eliminate dead zones.
- Manage air with speed profile and venting. When splay sits near end of fill or around cores and pairs with burn marks, the cause is trapped air. Slow the injection speed at the specific fill stage where streaks originate — not the whole shot. Add or deepen vents at last-to-fill areas (typically 0.02–0.05 mm deep). This is a mold-and-process fix that the DFM and trial stages should have caught.
- Isolate the material path from contamination. If streaks appear intermittently or after a material change, purge the barrel and hopper, then run fresh, correctly dried stock. If the streaks stop, the cause was contamination. Control regrind: store covered, dry with virgin if hygroscopic. Remove oil or release-agent carryover. Never let a dissimilar resin enter the dryer or loader.
“Splay is always a gas-in-the-melt problem. The gas comes from moisture, degradation, air or contamination — in that order. The disciplined response is to prove the material dry at the machine before touching a single barrel temperature.”
— Process Engineer, aCatMould
Frequently asked questions
Is splay the same as a flow mark or jetting?
No.
Splay is a fan of fine silvery lines radiating from the gate — its root is gas or degraded material in the melt. A flow mark is a wavy surface pattern tied to the melt front stalling against a cold wall. Jetting is a snake-like squiggle from an unrestrained gate.
The field test: lines fanning from the gate with micro-bubbles point to splay (check drying first); surface ripples tied to a cold region point to flow marks; an isolated squiggle from a gate points to jetting. Getting the classification right prevents chasing the wrong fix.
If I dried the resin, why does it still splay?
Because “the dryer ran” and “the resin is dry at the machine” are different claims. Common gaps: wrong dryer temperature or time for the grade; pellets re-absorb moisture in an ambient hopper or transfer line; the dryer filter is clogged or the desiccant wheel degraded; damp regrind reintroduced moisture downstream.
Measure the actual moisture and the dryer dew point at the machine. If both are genuinely in spec and splay persists, the cause is not moisture — move to degradation, air and contamination in that order.
Can raising injection speed fix splay?
Usually it makes it worse. Splay from moisture or degraded melt gets smeared more aggressively at higher speed — more shear heating, more air dragged into the front. A knee-jerk speed increase often deepens the streaks and adds burn marks.
The narrow exception: if splay is specifically end-of-fill air entrapment, then slowing the final fill stage (not speeding the whole shot) plus venting is the right move. The reliable first levers are drying, melt temperature and residence time. Injection speed is fine-tuning for air-related streaks at a specific stage, not a blanket cure. If your program needs a splay diagnosis at DFM, tell us before the deposit.
Key takeaways
- Splay is always a gas-in-the-melt defect. Four causes — moisture, degradation, air, contamination — look similar but are fixed differently.
- The diagnostic sequence is fixed: prove dry → reduce thermal stress → manage air → isolate contamination. Skipping step one wastes the other three.
- Splay is not a flow mark or jetting.
The field test: fan pattern from gate = splay; wavy surface = flow mark; snake squiggle = jetting.
Silver streaks are not a mold problem — they are a material and process problem. The moisture, the degradation, the air and the contamination all arrive before the melt reaches the cavity.
At aCatMould we verify resin moisture, thermal history and venting at the trial stage and hand you the process sheet that shows the splay was eliminated before T1, not chased after.
Sources
Plastics Technology — splay marks, resin drying and molding defects
Society of Plastics Engineers — injection molding defect diagnosis literature
Plastics Industry Association — molding process troubleshooting guides
Polymer Processing — splay, moisture and thermal degradation mechanisms
MoldMaking Technology — mold venting and surface defect troubleshooting
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet