Splay in injection molding appears as silver or streak-like marks on the molded part surface. It is commonly caused by moisture in the resin, excessive melt temperature, shear heating, trapped gas, poor venting, material contamination, or an unstable screw and injection profile.
Splay is easy to mistake for a simple cosmetic issue, but it often reveals a process or material control problem. For transparent PC, cosmetic ABS housings, nylon components and flame-retardant materials, splay can affect appearance, mechanical properties and customer acceptance.
Main Causes of Splay in Injection Molding
| Cause / material | Typical sign or use | Buyer or engineering action |
|---|---|---|
| Moisture | Silver streaks, brittleness, random surface marks | Dry resin and protect dried material from reabsorbing moisture |
| Excess melt temperature | Streaks with discoloration or burning risk | Lower barrel temperature and reduce residence time |
| High shear | Splay near gates or thin restrictions | Review gate size, injection speed and screw speed |
| Poor venting | Streaks near flow end or trapped gas zones | Improve vents and balance filling |
| Contamination | Irregular streaks, black specks or color defects | Clean hopper, barrel, regrind and material handling path |
How to Separate Moisture Splay from Shear Splay
- Moisture splay often changes after drying conditions are corrected.
- Shear splay is commonly concentrated near the gate, ribs or thin flow restrictions.
- Material degradation can appear with odor, discoloration or brittle parts.
- Venting-related splay may repeat at the end of fill or in enclosed flow areas.
- A stable troubleshooting plan changes one variable at a time instead of changing resin, mold and machine settings together.
Material-Specific Splay Risks
| Material / case | Main concern | Selection note |
|---|---|---|
| Polycarbonate | Moisture and high temperature | Dry thoroughly and control melt temperature |
| Nylon | Moisture absorption | Use correct drying time and sealed material handling |
| ABS | Moisture, overheating and contamination | Check dryer, residence time and regrind |
| TPE / TPU | Moisture and shear sensitivity | Review drying, gate and injection speed |
| Filled materials | Shear, fiber orientation and venting | Balance process and gate design |
Buyer RFQ Details for Splay Troubleshooting
For a useful review, send the 3D model or 2D drawing, resin grade or target material, part photos if the defect already exists, machine and mold information if available, expected annual volume, cosmetic requirements, critical dimensions, application environment and any inspection standard. Nylon Plastic can review the design, material and molding route before tooling or production changes are made.
Related Engineering Guides
- Polycarbonate injection molding guide
- ABS plastic injection molding guide
- Custom molded plastic parts guide
Request a Quote or Design Review
Share your plastic part drawing, material requirements, production quantity and defect or DFM concern. Nylon Plastic can help compare material choices, tooling changes, process adjustments and production risks for custom molded plastic parts.
Câu hỏi thường gặp
What is splay in injection molding?
Splay is a silver or streak-like surface defect on molded plastic parts, often caused by moisture, gas, excessive heat, high shear or contamination.
How do you know if splay is caused by moisture?
Moisture-related splay often improves after correct resin drying and sealed material handling. It is common with PC, nylon, TPU and other moisture-sensitive plastics.
Can high injection speed cause splay?
Yes. Excessive shear near gates or thin restrictions can create splay, especially when the gate is small, the flow path is restricted or the melt temperature is already high.
Which materials are more sensitive to splay?
Polycarbonate, nylon, TPU, some TPE grades and flame-retardant materials need careful drying, temperature control and material handling to reduce splay risk.
