Splay and Silver Streaks in Injection Molding: Causes and Fixes

Splay (silver streaks or splash marks) shows as fine, glossy streaks on the surface, usually fanning out from the gate. It is cosmetic, and it is also diagnostic. The most common cause is bad material handling, so persistent splay is a signal about supplier process discipline, not just a wipe-and-ship mark. See injection molding defects.

The Mechanics of Splay

Splay forms when gas or vapor in the melt is smeared against the cavity wall during fill. For many engineering resins, that gas is unevaporated moisture.

Hygroscopic resins absorb moisture from the air. If the material is not dried to the manufacturer’s specification before molding, trapped moisture flashes to steam at melt temperatures. That steam drags along the surface and leaves silver streaks.

Other sources of gas or vapor can look similar:

• Material degradation: Overheated or excessively sheared resin can break down and release volatiles.
• High injection speed: Pushing material through a restrictive gate too fast can create shear heat and degrade the melt.
• Contamination: Foreign material or incompatible resin in the barrel can vaporize or degrade.

Because several variables can cause streaks, drying records are a verification step, not proof of the diagnosis by themselves.

Evaluating Supplier Discipline Through Splay

Persistent splay warrants a look at material handling. One occurrence may be a process deviation. A repeatable pattern that fluctuates without a clear cause suggests poor moisture control.

When you evaluate suppliers (see the supplier capability checklist), how they dry and handle moisture-sensitive materials is a real differentiator.

Ask About Dew Point and Measurement

“Do you dry your material?” always gets a yes. Capability shows up in the specifics.

SABIC’s processing guide for LNP compounds sets a baseline of at least 3–4 hours at a dew point of 0 °F (−18 °C) or lower. It also splits resins into two categories:

1. Drying required: Nylons, ABS, PBT, TPU, polycarbonate, PET, PEEK, and PEI. Moisture at melt temperature attacks the polymer chain (hydrolysis). On these materials, splay is a visible symptom of compromised mechanical properties.
2. Drying recommended: Polyethylene, polypropylene, acetal, and PPS. Here drying mainly prevents cosmetic surface blemishes.

That split changes how you handle splay on a first article. On a required-drying resin, streaks indicate a chemical degradation risk.

When you discuss moisture control, ask:

• How is residual moisture measured? A hopper with a heater warms material but may not dry it. Capable suppliers measure actual moisture content (for example with a moisture analyzer) rather than relying only on dryer timers. BASF specifies a maximum of 0.02% moisture before processing Elastollan TPU grades.
• How is regrind introduced? Reground material has a higher surface-area-to-volume ratio than virgin pellets, so it absorbs moisture faster. SABIC notes that regrind should be blended with virgin before entering the dryer. Introducing undried regrind at the hopper throat can cause intermittent splay in an otherwise stable process. See regrind and recycled content.

Buyer Action Plan

• Identify the material requirement. Check the resin datasheet: drying required or recommended.
• Request moisture data, not just dryer settings. If splay appears on a required-drying resin, ask for the actual moisture measurement taken before the run.
• Define cosmetic standards. Specify which surfaces are cosmetic and how splay will be evaluated.
• Review regrind handling. If splay is intermittent, audit how and where regrind enters the feed system.

Unlike sink or warpage, splay is rarely fixed by part design. It is a process and handling issue. Your role is to verify that the supplier has the equipment and discipline for the specified resin.

Buyer FAQs

What causes splay or silver streaks in injection molding?

Moisture is the most frequent cause. Hygroscopic engineering plastics absorb water; if not dried properly, it turns to vapor at melt temperatures and leaves silver streaks. Degradation from heat or shear, and contamination, can look the same. On a drying-required resin, treat streaks as a hydrolysis risk until moisture data says otherwise.

Is splay a material problem or a process problem?

Primarily material-handling and process. Geometry rarely causes it, so use splay as a signal of moisture control, regrind handling, and process stability. Do not open a design ECO for splay until drying and regrind are ruled out in writing.

Should I ask suppliers how they dry material?

Yes, especially for hygroscopic resins. Ask for moisture verification method and dew point capability, not whether they “use a dryer.” If they cannot show dew point or measured moisture on drying-required resins, treat that as a qualification gap before award.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.