PC injection molding defects have three root causes, and nearly every one traces back to them: moisture sensitivity, high melt viscosity at high temperature, and notch sensitivity. This guide covers the ten PC injection molding defects that account for most rejections, what causes each, and the countermeasure that actually works.
This guide covers the ten defects that account for most PC rejections, what causes each, and the countermeasure that actually works.
Why PC is harder to mold than ABS
| Property | PC | ABS | Consequence |
|---|---|---|---|
| Melt temperature | 280 – 320 °C | 210 – 260 °C | Higher thermal load, more degradation risk |
| Melt viscosity | High | Moderate | Higher injection pressure, more shear |
| Moisture sensitivity | Must dry to 0.02% | Must dry to 0.10% | Tighter drying spec |
| Notch sensitivity | High | Low | Stress concentration causes cracking |
| Processing window | Narrow | Wide | Less tolerance for setup error |
The combination of high viscosity and notch sensitivity is what makes PC unforgiving: it needs high pressure to fill, which builds stress, in a material that cracks when stressed.
The ten PC injection molding defects that matter
| Defect | Appearance | Primary cause |
|---|---|---|
| Silver streaks / splay | Silvery lines from the gate | Moisture in the resin |
| Black spots / brown streaks | Dark specks or streaks | Degradation from overheat or residence time |
| Bubbles / voids | Internal or surface blisters | Moisture, or gas from decomposition |
| Stress cracking | Cracks days or weeks after molding | Molded-in stress plus chemical exposure |
| Sink marks | Surface depressions at thick sections | Insufficient packing |
| Warpage | Part bends after ejection | Differential shrinkage, uneven cooling |
| Weld line weakness | Visible or breaking line where fronts meet | Low mold temperature, gate position |
| Flow marks | Wavy or rippled surface | Uneven fill speed, cold mold |
| Yellowing | Colour shift, especially at the gate | Overheating, long residence time |
| Ejection marks / drag | Scuffing or white marks at ejection | Insufficient draft, small ejector area |
Drying: the defect that causes three others
PC absorbs 0.15 to 0.35 percent moisture and must be dried below 0.02 percent. Miss it and you get silver streaks, bubbles and a measurable drop in impact strength — all from one cause.
| Parameter | Requirement |
|---|---|
| Dryer type | Desiccant, dew point below −20 °C |
| Temperature | 120 °C |
| Time | 4 – 6 hours |
| Target moisture | below 0.02% |
| Hopper | Sealed, on-machine dry |
| Re-drying after exposure | 1 – 2 hours if exposed to air over 30 minutes |
A hot-air oven does not work for PC. It circulates moisture rather than removing it, and the part emerges hot, dry-looking and wet.
Our guide to ABS water absorption explains the mechanism, and the PC injection molding guide covers the grade side.
Degradation: black spots and yellowing
| Cause | Mechanism | Countermeasure |
|---|---|---|
| Melt temperature too high | Thermal decomposition | Keep below 320 °C; verify with a purge test |
| Long residence time | Cumulative heat history | Size the shot to the machine; avoid small shots on big barrels |
| Screw speed too high | Shear heating | Moderate back pressure and screw speed |
| Regrind at high ratio | Repeated heat history | Limit regrind to 20%, re-dry it |
| Contamination | Mixed materials, cleaning residue | Purge properly between materials |
| Dead spots in hot runner | Material sits and degrades | Balanced hot runner, no dead ends |
The residence-time point is the one most often missed: a 200 g part on a 1,000 g barrel sits at temperature for five times as long as it should, and no temperature setting fixes that.
Stress cracking: the defect that appears weeks later
PC parts crack after molding because molded-in stress plus a chemical or mechanical load exceeds the material's resistance. It is the most expensive defect because it escapes inspection.
| Contributor | Effect | Fix |
|---|---|---|
| Low mold temperature | Freezes stress into the part | 80 – 120 °C, as high as the part allows |
| Fast injection | High orientation and shear | Slow to moderate, profiled fill |
| Small gate | High local shear | Larger gate, gate away from stressed areas |
| Excessive packing | High residual stress | Pack to gate freeze, not beyond |
| Sharp corners | Stress concentration | Radius 0.5 mm minimum |
| Chemical contact | Solvent stress cracking | Verify compatibility; anneal |
| Insufficient annealing | Stress remains | 120 °C for 2 – 4 hours |
Annealing is the standard mitigation: it relaxes frozen-in stress and reduces the risk of later cracking. It does not fix distortion from orientation, but it does keep the crack from appearing.
Warpage and sink: the packing problem
| Defect | Cause | Fix |
|---|---|---|
| Sink at ribs and bosses | Insufficient packing, rib too thick | Raise pack pressure and time; rib at 50 – 60% of wall |
| Warpage in flat parts | Differential shrinkage, uneven cooling | Uniform wall, symmetric cooling, balanced gates |
| Warpage at glass-filled grades | Directional shrinkage | Balanced flow, symmetric gating |
| Sink at gate | Gate freezes before packing completes | Larger gate |
PC's high shrinkage (0.5 to 0.7 percent unfilled) and high mold temperature make warpage a common issue on large flat parts. The fix is almost always thermal: cooling balance matters more than pressure.
Weld lines and flow marks
| Defect | Cause | Fix |
|---|---|---|
| Weld line visible | Gate position forces two fronts | Move the gate, or hide with texture |
| Weld line weak | Low mold temperature at the merge | Raise mold temperature 10 – 20 °C |
| Flow marks | Uneven fill speed, cold mold | Profile the injection, raise mold temperature |
| Gate blush | High shear at a small gate | Larger gate, slower entry |
PC's high viscosity makes it prone to flow marks at low mold temperature — the melt freezes against the wall and the next material slides over it. A 20 °C rise in mold temperature usually removes them.
Our guides to mold etching and weld lines and molecular orientation cover the mechanism.
Process setup that prevents most defects
| Parameter | Range | Note |
|---|---|---|
| Melt temperature | 280 – 320 °C | Mid-range unless filling is difficult |
| Mold temperature | 80 – 120 °C | Higher for optics, stress resistance, gloss |
| Injection speed | Slow to moderate, profiled | Fast fill causes stress and flow marks |
| Packing pressure | 50 – 80% of injection | Higher than ABS because of shrinkage |
| Packing time | Until gate freeze | Measure, don't guess |
| Cooling time | Long enough to avoid ejection stress | PC needs more than ABS |
| Back pressure | Low to moderate | High back pressure degrades the melt |
| Clamp force | Adequate to prevent flash | PC flows easily at high temperature |
Material selection as a defect fix
| Problem | Material answer |
|---|---|
| Cracking under chemical exposure | PC with better ESCR, or PC/ABS |
| Warpage on large parts | PC/ABS (lower shrinkage, easier flow) |
| Optical requirement | Optical-grade PC, annealed |
| Flame retardancy | FR PC — check for stress cracking tendency |
| Outdoor exposure | PC with UV stabiliser, or PMMA-capped |
| Cost pressure | PC/ABS |
PC/ABS is the pragmatic answer for many housing applications: easier flow, lower processing temperature, less notch sensitivity, at the cost of some heat resistance and transparency.
Inspection that catches what visual cannot
| Check | What it finds |
|---|---|
| Visual under oblique light | Silver streaks, flow marks, black spots |
| Polarized light viewing | Molded-in stress, birefringence |
| Solvent wipe test (acetone or IPA) | Stress cracking tendency |
| Dimensional check after 24 h | Sink and warpage |
| Drop or impact test | Embrittlement from moisture or degradation |
| Cross-section under magnification | Voids, weld line quality |
The solvent wipe is the cheapest stress test available: wipe with acetone or IPA and look for crazing within seconds. A part that crazes immediately will crack in service.
Related guidance
Our guides to PC injection molding, PC optical distortion, PC lens molds and PC/ABS injection molding cover the specific cases, the general defect catalogue is in our injection molding defects guide, and the material comparison is in the materials overview.
Property and process data in this guide follows the test methods published by ASTM International and ISO, with typical values cross-checked against MatWeb and Engineering Toolbox. Processing practice reflects Plastics Technology guidance and PLASTICS industry standards, with material characterisation maintained through SPE and NIST reference data. For the polymer chemistry, see polycarbonate.
What causes silver streaks in polycarbonate?
Moisture. PC must be dried to below 0.02 percent at 120 °C for 4 to 6 hours in a desiccant dryer. A hot-air oven will not do it — it circulates moisture instead of removing it.
Why do PC parts crack weeks after molding?
Molded-in stress plus a chemical or mechanical load. Low mold temperature, fast fill and small gates all build stress. Annealing at 120 °C for 2 to 4 hours relieves it and prevents the crack.
What causes black spots in PC parts?
Degradation from excessive melt temperature, long residence time, high screw speed or dead spots in the hot runner. Keep melt below 320 °C and size the shot to the machine.
How do I reduce warpage in PC parts?
Cooling balance first: even core and cavity temperature, symmetric gates, uniform wall. Then packing. PC's high mold temperature makes thermal uniformity more important than pressure.
What mold temperature should PC run at?
80 to 120 °C. Low mold temperature causes flow marks, stress and poor surface; high mold temperature reduces stress and improves surface but lengthens the cycle. For optical parts, run at the top of the range.
Can PC be molded with regrind?
Yes, up to about 20 percent, provided the regrind is re-dried and is not degraded. Regrind has a longer heat history, so impact strength drops and black spots become more likely.
Conclusion
Most PC defects come from the three properties that make the material useful: moisture sensitivity causes silver streaks and bubbles, high viscosity and high temperature cause degradation and stress, and notch sensitivity turns stress into cracks. Dry properly, keep the melt in range, run the mold hot, and anneal where stress matters — those four habits prevent most of the list.
At Rui Cheng we mold PC, PC/ABS and optical grades on 80 to 1,400 tonne machines with desiccant drying to 0.02 percent, mold temperature control to ±2 °C, documented annealing and ISO 9001 / IATF 16949 processes. Send us the drawing plus the failure you are seeing, and we will come back with a process window and a corrective action.
