PVC is classified into rigid PVC (UPVC) and flexible PVC. Its characteristics include extreme thermal sensitivity, susceptibility to decomposition, poor flow properties, and strong corrosiveness; furthermore, plasticizers contained within it tend to leach out, and it cannot be subjected to prolonged exposure to high temperatures. Consequently, its molding presents a significantly greater challenge compared to ordinary plastics.
I. Yellowing, Scorching, Black Spots, and Black Streaks (Most Typical)
Phenomena:Yellowing of the finished product, brown streaks, black spots, scorching, and a pungent odor.
Causes:
- Material temperature is excessively high, leading to rapid thermal decomposition of the PVC.
- Screw rotation speed is too fast, resulting in excessive shear heating.
- Residence time within the barrel is too long.
- Material accumulates and carbonizes in "dead zones" within the screw or barrel.
- Stabilizer content is insufficient or unevenly dispersed.
Solutions:
- Strictly control the material temperature: maintain it between 160–190°C; strictly prohibit exceeding 200°C.
- Reduce the screw rotation speed to minimize shear heating.
- Shorten the molding cycle; the barrel must be completely purged before shutting down the machine.
- Regularly clean the screw and barrel.
- Ensure that the stabilizer is uniformly dispersed in the mixture, and avoid using expired materials.
II. Silver Streaks, Gas Marks, and Flow Marks
Phenomena: Silver streaks, cloudy patches, or linear striations on the surface.
Causes:
- Raw material contains moisture, which vaporizes at high temperatures.
- Slight thermal decomposition generates hydrogen chloride gas.
- Injection speed is too high, causing air entrapment.
- Poor venting in the mold.
- Insufficient back pressure results in uneven plasticization.
Solutions:
- Dry the raw material at 60–70°C for 1–2 hours.
- Lower the material temperature to prevent decomposition.
- Reduce the injection speed during the initial phase to ensure smooth mold filling.
- Improve mold venting.
- Appropriately increase the back pressure.
III. Short Shots, Uneven Flow, and Insufficient Filling
Symptoms: Unfilled sharp corners, thin-walled sections, and extremities.
Causes:
- Material temperature and/or mold temperature are too low, resulting in poor flowability.
- Insufficient injection pressure and/or injection speed.
- Runners and/or gates are too small.
- Poor mold venting.
- Uneven distribution of plasticizers in flexible PVC, leading to localized viscous flow issues.
Solutions:
- Appropriately increase the material temperature and mold temperature.
- Increase injection pressure; fill the mold at a medium-to-low injection speed.
- Enlarge the gates and runners.
- Improve mold venting.
- Ensure thorough mixing of the material to guarantee proper plasticization.
IV. Prominent Weld Lines and Poor Strength
Symptoms:Deep and discolored (whitish) weld lines; prone to fracture.
Causes:
- Low melt temperature and/or mold temperature, resulting in poor fusion.
- Slow injection speed, causing the melt flow front to cool prematurely.
- Poor venting, leading to air entrapment at the weld line.
- Improper gate location.
Solutions:
- Increase melt temperature and mold temperature.
- Appropriately increase injection speed.
- Add vent grooves at the weld line location.
- Optimize gate placement to shorten the melt flow path.
V. Shrinkage and Sink Marks
Phenomenon: Depressions occurring at rib locations, bosses, and 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 is too high, resulting in excessive shrinkage.
- Uneven wall thickness.
Solutions:
Increase holding pressure; extend holding time.
- Appropriately increase injection pressure.
- Enlarge the gate size.
- Appropriately lower the melt temperature.
- Design the product with as uniform a wall thickness as possible.
VI. Flash / Burrs
Phenomenon: Material overflow at the parting line or inserts.
Causes:
- Material temperature is too high, resulting in increased fluidity.
- Injection pressure and/or injection speed are excessive.
- Clamping force is insufficient.
- Excessive clearance in the mold assembly.
- Overfilling.
Solutions:
- Lower the material temperature.
- Reduce the injection pressure and injection speed.
- Increase the clamping force.
- Modify the mold to reduce clearance.
- Reduce the injection volume.
VII. Warpage and Deformation
Phenomenon: Bending, twisting, dimensional instability
Causes:
- Uneven cooling; high internal stress
- Inconsistent mold temperature (front vs. rear)
- Significant variations in wall thickness; uneven shrinkage
- Excessively high injection speed; severe molecular orientation
- Unbalanced ejection
Solutions:
- Ensure uniform mold temperature; extend cooling time
- Reduce injection speed to minimize internal stress
- Optimize product wall thickness
- Adjust ejection balance
- For rigid PVC, the mold temperature may be appropriately increased
VIII. Cracking, Brittleness, and Chipping
Phenomena:Cracking during demolding, cracking during assembly, chipped corners/edges.
Causes:
- Rigid PVC is inherently brittle.
- Excessive internal stress.
- Mold temperature is too low; cooling is too rapid.
- Uneven ejection; "ejection whitening" (stress whitening).
- Insufficient plasticizer content or uneven distribution.
Solutions:
- Increase mold temperature to 30–50°C.
- Reduce injection speed and minimize shear stress.
- Adjust ejector pins to increase the ejection contact area.
- Appropriately increase the proportion of plasticizer (for flexible grades).
- Avoid excessive cooling.
IX. Surface Fogging, Poor Gloss, and Oily Residue
Phenomena: Surface haziness, lack of gloss, greasy feel, blooming (frosting).
Causes:
- Migration (exudation) of plasticizers or lubricants.
- Material temperature is too low, resulting in poor plasticization.
- Mold temperature is too low.
- Rough mold surface finish.
- Uneven mixing of raw materials.
Solutions:
- Control the proportion of additives to avoid excess.
- Appropriately increase the material temperature to improve plasticization.
- Increase the mold temperature.
- Polish the mold surface.
- Ensure thorough mixing of materials before commencing production.
X. Bubbles and Voids
Phenomenon: Bubbles or voids present on the surface or within the part.
Causes:
- Moisture content in the raw material.
- Excessively high material temperature, leading to decomposition and gas generation.
- Excessively fast injection speed, resulting in air entrapment.
- Insufficient holding pressure, preventing proper compaction.
- Shrinkage in thick-walled parts, creating vacuum voids.
Solutions:
- Dry the raw material.
- Lower the material temperature to prevent decomposition.
- Use a slower injection speed during mold filling.
- Increase the holding pressure.
- For thick-walled parts, extend the holding and cooling times.
