Splay and Silver Streaks in Injection Molding: Causes and Fixes

Splay—often called silver streaks or splash marks—appears as fine, glossy streaks on a part surface, typically fanning out from the gate. While it is a common cosmetic defect, it is also a diagnostic one. Because the most frequent cause is improper material handling, splay provides a window into a supplier’s process discipline. This guide covers how to interpret splay and what questions to ask suppliers during process validation. (See injection molding defects).

The Mechanics of Splay

Splay occurs when gas or vapor is caught in the polymer melt and smeared against the cavity wall during fill. For many engineering resins, the source of that gas is unevaporated moisture.

Hygroscopic resins absorb moisture from the atmosphere. If the material is not dried to the manufacturer’s specification before molding, trapped moisture flashes to steam at melt temperatures. This steam drags along the surface of the part, leaving silver streaks.

Other sources of gas or vapor can produce similar-looking marks:

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

Because several variables can cause streaks, reviewing a supplier’s drying records is a necessary verification step, rather than proof of the diagnosis on its own.

Evaluating Supplier Discipline Through Splay

For a buyer, persistent splay warrants a review of the supplier’s material handling. A single occurrence may just be a process deviation. A repeatable pattern—especially one that fluctuates without clear cause—suggests poor moisture control.

When evaluating suppliers (as outlined in the supplier capability checklist), how they dry and handle moisture-sensitive materials is a differentiating factor.

Ask About Dew Point and Measurement

Asking “Do you dry your material?” yields a universal yes. Supplier capability is revealed in the specifics.

SABIC’s processing guide for LNP compounds establishes a baseline of at least 3–4 hours at a dew point of 0 °F (−18 °C) or lower. More importantly, it divides resins into two categories:

1. Drying required: Resins like 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: Resins like polyethylene, polypropylene, acetal, and PPS. Here, drying primarily prevents cosmetic surface blemishes.

This distinction dictates how you handle splay on a first article. On a required-drying resin, streaks indicate a chemical degradation risk.

When discussing 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 (e.g., using a moisture analyzer) rather than relying solely on dryer timers. BASF, for instance, 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, meaning it absorbs moisture faster. SABIC notes that regrind should be blended with virgin before entering the dryer. Introducing undried regrind directly 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 to confirm if drying is required or recommended.
• Request moisture data, not just dryer settings. If splay appears on a required-drying resin, ask the supplier for the actual moisture measurement taken prior to the run.
• Define cosmetic standards. Specify exactly which surfaces are cosmetic and how splay will be evaluated.
• Review regrind handling. If splay is intermittent, audit how and where regrind is introduced into the feed system.

Unlike sink or warpage, splay is rarely solved through part design changes. It is a process and handling issue. The buyer’s role is to verify that the supplier has the equipment and discipline to handle the specified resin.

Buyer FAQs

What causes splay or silver streaks in injection molding?

The most frequent cause is moisture. Hygroscopic engineering plastics absorb water; if not dried properly, this moisture turns to vapor at melt temperatures, creating silver streaks. Other causes include material degradation from excessive heat or shear, and contamination.

Is splay a material problem or a process problem?

It is primarily a material-handling and process problem. Because it is rarely caused by part geometry, splay serves as an indicator of a molder’s discipline regarding moisture control, regrind handling, and process stability.

Should I ask suppliers how they dry material?

Yes, particularly for hygroscopic resins. Asking for their moisture verification method and dew point capabilities provides far more insight than asking if they use a dryer.

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.