What Is Splay in Injection Molding? Definition

Splay is an injection molding defect that shows up as silvery streaks or splash-like marks on the surface of a molded part. It is caused by gas in the molten plastic, most often moisture from resin that was not dried properly, and is also known as silver streaking.

In short: splay is what water or other gases in the melt leave behind on the part's surface.

What Causes Splay?

Many engineering plastics absorb moisture from the air. When wet pellets are melted, that water turns to vapor. As the melt is injected, the vapor is carried to the flow front and smeared against the mold surface, where it freezes into the part's skin as streaks. One patent on moisture-absorbing ionomer resins describes molded parts from wet material as unacceptable because of moisture defects such as splaying and voids.

Moisture is not the only source. Any volatile material in the melt can do the same: residual solvent left over from making the resin, or water and other gases released when the polymer reacts or starts to break down during melting. One patent on high-temperature polyimide sulfone resins notes that residual volatiles are typically held below 500 ppm for good processing, and that escaping water causes splay and silver streaking.

Splay is mainly a cosmetic problem, but it is a warning sign. The same gas that marks the surface can also leave voids or bubbles inside the part, and resin that is degrading may have lost some of its properties.

Splay vs Weld Line

Both appear as lines on the surface of a molded part, so they are often confused. They have different causes and different fixes.

	Splay	Weld line
What it looks like	Silvery, streaky or splash-like marks	A single fine line or shallow notch
Where it appears	On the surface, often as a band of streaks rather than one line	Where two flow fronts meet: behind holes, pins and inserts, or between gates
Root cause	Moisture or other gas in the melt	Two flow fronts fusing incompletely
Main fix	Dry the material properly; check melt temperature and residence time	Move the gate; raise mold temperature and injection speed
Main concern	Appearance, plus possible voids and degradation	Appearance and reduced strength

How to Prevent Splay

Dry hygroscopic materials before molding, to the temperature and time on the resin supplier's data sheet. Drying conditions vary widely by material: one study dried glass-filled nylon at 85 °C (185 °F) for 12 hours, while one patent example dried a polyimide sulfone at 160 °C (320 °F) for 3 hours before molding splay-free parts.
Keep dried material covered or in a closed hopper so it does not pick up moisture again.
Avoid overheating the melt or leaving it too long in the barrel, which can make it release gas.
If splay appears in production, check the dryer and the material's moisture level before changing other process settings.

Material handling and process settings are fixed during first-article trials (T1, T2) on every plastic injection molding project, before pilot production.

Frequently Asked Questions