Injection molding defects in PC/ABS parts usually originate from three areas: material condition, processing parameters, or mold design. Because several defects can share similar visual symptoms, troubleshooting should follow a structured approach instead of adjusting multiple settings at once.
PC/ABS combines the heat resistance and stiffness of polycarbonate with the processability of ABS, but the blend is also more sensitive to moisture, cooling balance, and residual stress than standard ABS. Small variations in drying or mold temperature can noticeably affect surface quality and dimensional stability.
Polycarbonate Properties
Provides heat resistance, stiffness, and dimensional stability — but increases sensitivity to moisture and residual stress.
ABS Processability
Improves melt flow and processing window, but the blend demands careful attention to drying time and cooling uniformity.
Diagnose Before Changing Parameters
Before modifying machine settings, first identify the most likely source of the defect. Changing several parameters simultaneously often makes root-cause analysis more difficult and can introduce secondary defects.
Material Condition
Check moisture levels, drying consistency, and the proportion of regrind in the mix.
Processing Conditions
Review melt temperature, injection speed, holding pressure, and cooling time settings.
Mold Design
Evaluate gate size, venting, cooling channel balance, and wall-thickness transitions.
Sink Marks
Sink marks appear as surface depressions, usually opposite ribs, bosses, or thick sections. Insufficient holding pressure or early gate freeze commonly prevents the melt from compensating for volumetric shrinkage during cooling.
- Increase holding pressure and holding time gradually
- Confirm the gate remains open long enough for proper packing
- Reduce excessive rib thickness if geometry allows
Warpage
Warpage occurs when uneven cooling creates differential shrinkage across the part. Flat housings and large interior panels are particularly sensitive.
- Keep core and cavity temperatures as consistent as possible
- Extend cooling time before ejection
- Reduce excessive residual stress caused by over-packing
Flow Lines
Flow lines appear as streaks or wave-like patterns along the melt-flow direction. The defect usually develops when the melt front cools too quickly before the cavity is fully filled.
- Raise mold temperature within the recommended processing range
- Increase injection speed to maintain melt-front temperature
- Verify melt temperature is appropriate for the selected PC/ABS grade
Silver Streaks & Splay
Silver streaks near the gate area are commonly associated with moisture contamination or unstable melt flow. Moisture vapor generated during injection can leave visible streaks on the part surface.
- Dry material thoroughly before molding
- Verify dryer performance and material exposure time
- Lower excessive melt temperatures if degradation is suspected
Weld Lines
Weld lines form where two melt fronts meet inside the cavity, often around holes or multi-gate areas. Poor venting or insufficient melt temperature can weaken fusion at the meeting point.
- Increase melt temperature and injection speed moderately
- Improve venting near the weld-line location
- Adjust gate position if the weld line falls in a high-stress area
Troubleshooting Summary
Use this table as a first-pass diagnostic guide when a defect is identified on the production floor.
| Defect | Check First | Typical Cause |
|---|---|---|
| Sink marks | Holding pressure | Shrinkage during cooling |
| Warpage | Cooling balance | Differential shrinkage |
| Flow lines | Mold temperature | Premature melt cooling |
| Silver streaks | Drying conditions | Moisture contamination |
| Weld lines | Venting & melt temp | Poor melt fusion |
