Common Issues, Causes, and Solutions in PS Injection Molding - Knowledge

Polystyrene (PS) is classified into two main grades: General Purpose Polystyrene (GPPS)-which is transparent, hard, and brittle-and High Impact Polystyrene (HIPS)-which is opaque and possesses good toughness. Common characteristics shared by both grades include good flowability, low moisture absorption, high brittleness, susceptibility to cracking, tendency toward shrinkage and yellowing, and sensitivity to shear stress. PS is widely used in the manufacture of transparent daily necessities, lampshades, toys, housings, and similar products.

I. Silver Streaks / Water Marks / Air Marks
Phenomenon: Silver-white streaks, hazy patches, or water-like patterns on the surface.
Causes:

Raw material is damp or contains moisture.
Material temperature is excessively high, leading to decomposition and gas generation.
Injection speed is too fast, resulting in air entrapment.
Poor venting leads to trapped air.
Insufficient back pressure results in uneven plasticization.

Solutions:

Dry the raw material at 70–80°C for 1–2 hours.
Appropriately lower the barrel temperature.
Reduce the injection speed during the initial phase to prevent air entrapment.
Improve mold venting.
Appropriately increase the back pressure.

II. Cracking / Brittle Fracture / Chipping
Phenomena: Cracking during demolding, cracking during assembly, brittle chipping at corners/edges, bursting of screw bosses.
Causes:

GPPS material is inherently brittle.
Excessive internal stress (due to high injection speed or high shear).
Mold temperature is too low, resulting in overly rapid cooling.
Uneven ejection force, causing "ejector whitening" (stress marks).
Insufficient draft angle, leading to drag marks/surface tearing during demolding.
Melt temperature is too low, resulting in poor plasticization.

Solutions:

Increase mold temperature to 40–60°C to reduce internal stress.
Decrease injection speed and reduce shear stress.
Adjust ejector pin layout to increase the total ejection area.
Increase the draft angle.
Appropriately raise the melt temperature to improve material flowability.

III. Short Shots / Insufficient Fill
Symptoms: Unfilled sharp corners, thin-walled sections, or distal areas.
Causes：

Material temperature / Mold temperature is too low.
Injection pressure and injection speed are insufficient.
Runners / Gates are too small.
Poor venting; trapped air obstructs material flow.
Back pressure is insufficient.

Solutions：

Increase material temperature (180–220°C).
Increase injection pressure and injection speed.
Enlarge gates and runners.
Improve venting.
Increase back pressure to ensure proper plasticization.

IV. Prominent Weld Lines and Poor Strength
Phenomenon:Weld lines are deep, discolored (whitish), and prone to breakage.
Causes:

Material temperature and/or mold temperature are too low.
Injection speed is too slow, causing the flow front to cool prematurely.
Poor venting leads to air entrapment at the weld interface.
Gate placement is inappropriate.

Solutions:

Increase material temperature and mold temperature.
Appropriately increase injection speed.
Add vents at the weld line locations.
Optimize gate placement to shorten the flow path.

V. Shrinkage / Sink Marks
Phenomenon: Depressions occurring at rib locations, bosses, or thick-walled sections.
Causes:

Insufficient holding pressure; holding time is too short.
Injection pressure is too low.
Gate is too small, leading to premature freezing.
Melt temperature and/or mold temperature are too high.
Uneven wall thickness.

Solutions:

Increase holding pressure; extend holding time.
Appropriately increase injection pressure.
Enlarge the gate.
Appropriately lower the melt temperature and mold temperature.
Optimize the product's wall thickness.

VI. Flash / Burrs
Symptom: Material overflow at the parting line or inserts.
Causes:

PS material exhibits high fluidity, or the melt temperature is excessively high.
Injection pressure and/or injection speed are too high.
Clamping force is insufficient.
Excessive clearance within the mold.
Overfilling.

Solutions:

Reduce the melt temperature.
Decrease the injection pressure and injection speed.
Increase the clamping force.
Modify the mold to reduce clearance.
Reduce the injection volume.

VII. Black Spots, Black Streaks, Scorching, and Yellowing
Phenomena: Black spots, scorched patches, yellowing, burnt corners
Causes：

Material temperature is too high; PS undergoes thermal decomposition.
Screw residence time is excessively long.
Poor venting; trapped air is compressed and scorched.
Carbon buildup in the barrel.
Injection speed is too high, causing shear overheating.

Solutions：

Lower the material temperature; shorten the cycle time.
Purge the barrel before shutting down.
Improve venting.
Clean the barrel and screw.
Reduce the injection speed.

VIII. Bubbles / Voids
Phenomenon: Bubbles or voids present on the surface or within the interior.
Causes:

Moisture content in raw materials.
Excessive material temperature, leading to thermal decomposition and gas generation.
Excessively high injection speed, resulting in air entrapment.
Insufficient holding pressure; material not fully compacted.
Shrinkage in thick-walled sections, creating vacuum voids.

Solutions:

Dry the raw materials.
Reduce the material temperature.
Use a slower injection speed during the initial phase.
Increase the holding pressure.
For thick-walled parts, extend the holding and cooling times.

IX. Warpage and Deformation
Phenomena: Bending, twisting, dimensional instability
Causes:

Uneven cooling; high internal stress.
Mold temperature too low, or significant temperature difference between the front and rear mold halves.
Uneven wall thickness; differential shrinkage.
Excessively high injection speed; severe molecular orientation.
Unbalanced ejection.

Solutions: