PC material has high viscosity, poor flowability, is extremely sensitive to moisture, is prone to stress cracking, requires high mold temperature, and is easily scorched.
I. Silver Streaks / Water Marks / Gas Marks (Most Common)
Phenomenon: Silver-white thread-like streaks, a misty appearance, or water-mark patterns on the surface.
Causes:
- PC material has absorbed excessive moisture and was either not dried or insufficiently dried.
- Material temperature is too high, causing material decomposition and the generation of gas.
- Injection speed is too fast, resulting in air entrapment.
- Screw back pressure is insufficient, preventing complete air expulsion.
- Poor mold venting.
Solutions:
- Ensure thorough drying: Dry at 110–120°C for 3–4 hours; for recycled material, extend drying time to 4–6 hours.
- Lower the barrel temperature to prevent thermal decomposition.
- Reduce the injection speed, particularly during the initial phase.
- Increase back pressure to ensure complete air expulsion.
- Improve mold venting by adding vent channels.
II. Cracking / Brittle Fracture / Stress Cracking
Phenomena:Cracking during demolding, cracking during assembly, cracking after storage, screw boss bursting.
Causes:
- Excessive internal stress (due to high injection speed, high shear stress, or large temperature differentials).
- Mold temperature is too low, or cooling is too rapid.
- Uneven ejection or "ejector whitening" leading to stress concentration.
- Material temperature is too low, resulting in poor plasticization.
- Insufficient draft angle, causing tearing or cracking during demolding.
- Contact with chemicals or greases inducing cracking.
Solutions:
- Increase mold temperature to 70–100°C to reduce internal stress.
- Reduce injection speed and minimize shear stress.
- Adjust ejector pins and increase the ejection area to prevent ejector whitening.
- Appropriately increase material temperature to improve flowability.
- Increase the draft angle.
- Avoid contact with organic solvents; perform an annealing treatment if necessary.
III. Short Shots / Uneven Flow / Insufficient Filling
Phenomenon: Corners, thin-walled sections, and deep ribs fail to fill completely.
Causes:
- PC material exhibits high viscosity and poor flowability.
- Melt temperature and/or mold temperature are too low.
- Injection pressure and injection speed are insufficient.
- Runners and/or gates are too small.
- Poor venting leads to trapped air and flow blockage.
- Insufficient back pressure results in uneven plasticization.
Solutions:
- Increase melt temperature to 280–310°C.
- Increase mold temperature to 80–100°C.
- Increase injection pressure and injection speed.
- Appropriately enlarge the gates and runners.
- Improve venting to eliminate trapped air.
- 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, resulting in poor weld strength.
- Injection speed is too slow, causing the flow front of the molten material to cool prematurely.
- Poor venting leads to air entrapment at the weld line junction.
- Improper gate placement results in an excessively long flow path.
Solutions:
- Increase the material temperature and mold temperature.
- Appropriately increase the injection speed.
- Add vents at the location of the weld lines.
- Optimize gate placement to shorten the flow path.
- Install a cold slug well if necessary.
V. Bubbles / Internal Voids
Phenomenon: Bubbles or voids present on the surface or within the part.
Causes:
- Raw material contains moisture, which vaporizes at high temperatures.
- Material temperature is excessively high, leading to thermal decomposition and gas generation.
- Injection speed is too fast, resulting in air entrapment.
- Insufficient holding pressure prevents proper compaction.
- Cooling shrinkage in thick-walled sections creates internal vacuum voids.
Solutions:
- Thoroughly dry the raw material.
- Reduce the barrel temperature.
- Reduce the injection speed during the initial phase.
- Increase the holding pressure and extend the holding time.
- For thick-walled parts, employ a slow injection speed combined with high holding pressure.
VI. Flash / Burrs
Phenomenon: Material overflow at the parting line or inserts.
Causes:
- Material temperature is too high, resulting in excessive fluidity.
- Injection pressure/speed is too high, leading to overfilling.
- Clamping force is insufficient.
- Excessive clearance in the mold mating surfaces.
- Holding pressure is too high; cooling is delayed.
Solutions:
- Appropriately lower the material temperature.
- Reduce the injection pressure and speed.
- Increase the clamping force.
- Modify the mold to reduce mating clearance.
- Lower the holding pressure to avoid overfilling.
VII. Black Spots, Black Streaks, and Scorching
Phenomena: Black spots, yellowing, scorching, burn marks
Causes:
- Excessive material temperature; thermal decomposition of PC material.
- Material residence time in the screw is too long.
- Poor venting; localized high-temperature compression leading to scorching.
- Carbon buildup in the barrel or nozzle.
- Excessive injection speed; shear overheating.
Solutions:
- Lower the material temperature; shorten the cooling cycle.
- Minimize downtime; purge the barrel before shutting down.
- Improve venting to prevent trapped air and compression burning.
- Clean the barrel, screw, and nozzle.
- Reduce injection speed to minimize shear heat .
VIII. Warpage and Deformation
Phenomenon: Bending, twisting, dimensional instability
Causes:
- Uneven cooling, high internal stress.
- Mold temperature too low or uneven.
- Unbalanced ejection.
- Excessively fast injection speed, severe molecular orientation.
- Uneven product wall thickness.
Solutions:
- Increase and equalize mold temperature; extend cooling time.
- Reduce injection speed to minimize molecular orientation.
- Adjust ejection balance.
- Optimize product wall thickness design.
- Perform annealing treatment if necessary.
IX. Poor Gloss, Matte Finish, Dull Surface
Phenomenon: Lack of luster, hazy appearance, absence of a mirror-like finish.
Causes:
- Mold temperature is too low.
- Material temperature is insufficient.
- Mold surface roughness is excessive.
- Poor venting.
- Raw material contains moisture.
Solutions:
- Increase mold temperature to 90–100°C.
- Appropriately increase material temperature.
- Polish the mold and optimize surface texturing.
- Improve venting.
- Ensure the raw material is dry.
Ⅹ. Ejector Whitening, Protrusion, and Bulging
Phenomenon: Whitening or raised bumps appear at the ejector pin locations.
Causes:
- Low mold temperature; the molded part is overly brittle.
- Uneven ejection; the ejector pins are too small.
- Ejection occurs before sufficient cooling is complete.
- Insufficient draft angle; high "clamping force" (part adhesion to the mold).
- Excessive internal stress within the part.
Solutions:
- Increase the mold temperature.
- Increase the diameter of the ejector pins and/or increase the number of pins.
- Extend the cooling time.
- Increase the draft angle.
- Optimize processing parameters to reduce internal stress.
