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What Is Splay in Injection Molding?

Also known as: silver streaking

DefinitionSplay 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.

Explained

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. Molded parts from wet material can be 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. Residual volatiles are typically held below 500 ppm for good processing, and 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.

  • Appearance: Splay shows as silvery, streaky or splash-like marks; a weld line is a single fine line or shallow notch.
  • Location: Splay appears on the surface, often as a band of streaks; weld lines form where two flow fronts meet, such as behind holes, pins and inserts, or between gates.
  • Root cause: Splay is caused by moisture or other gas in the melt; weld lines result from two flow fronts fusing incompletely.
  • Main fix: Splay is fixed by drying the material properly and checking melt temperature and residence time; weld lines are addressed by moving the gate and raising mold temperature and injection speed.
  • Main concern: Splay affects appearance and may indicate voids and degradation; weld lines affect appearance and reduce strength.

How to Prevent Splay

  • Dry hygroscopic materials before molding, to the temperature and time on the resin supplier's data sheet.
  • 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.

Key takeaways

  1. Splay appears as silvery streaks or splash-like marks on molded part surfaces.
  2. It is most often caused by moisture in hygroscopic resin that was not dried properly.
  3. Other volatiles, residual solvents, and polymer degradation gases can also cause splay.
  4. Splay is mainly cosmetic, but it can signal internal voids, bubbles, or resin degradation.
  5. The main fix is proper resin drying before molding and controlling melt temperature and residence time.

Example

In one patent example, a polyimide sulfone resin was dried at 160 °C (320 °F) for 3 hours before molding, producing splay-free parts. Another study dried glass-filled nylon at 85 °C (185 °F) for 12 hours before injection molding to remove humidity and avoid moisture-related defects.

Related terms

Weld Line Flash Melt Flow Index (MFI)

References

  1. US Patent Application 2005/0070684 (polyimide sulfones) — patents.google.com
  2. European Patent Application EP0364060A2 (injection molding and extruding wet hygroscopic ionomers) — patents.google.com
  3. Weld Line Strength of Polyamide Fiberglass Composite at Different Processing Parameters in Injection Molding Technique — Polymers (PMC)
  4. MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — ocw.mit.edu