PP material has good flowability, high crystallinity, large shrinkage, and is prone to water shrinkage and warping. It is one of the most commonly used general-purpose plastics.
1. Shrinkage / Depression
Symptom: Localized depressions or unevenness on the product surface
Causes：
- Insufficient holding pressure; short holding time
- Injection pressure is too low
- Uneven wall thickness; slow cooling in thick-walled sections
- Gate is too small; premature freezing of the gate during cooling
- Melt temperature / mold temperature is too high, resulting in excessive shrinkage
- Injection speed is too fast, leading to internal air entrapment that prevents shrinkage compensation
Solutions：
- Increase holding pressure; extend holding time
- Appropriately increase injection pressure
- Optimize product structure to avoid abrupt changes in wall thickness
- Enlarge the gate / runner; extend the gate freezing time
- Appropriately lower the melt temperature and mold temperature
- Adjust injection speed; use a slower speed for thick-walled sections
2. Warpage and Deformation
Symptom: Product bending, warping, or dimensional instability
Causes：
- High crystallization shrinkage of PP material; uneven cooling
- Uneven wall thickness, resulting in inconsistent shrinkage across different sections
- Unbalanced ejection force; improper placement of ejector pins
- Excessive internal stress (due to excessively high injection speed or strong shear forces)
- Uneven mold temperature; improper layout of cooling channels
- Insufficient draft angle, leading to deformation caused by ejection drag
Solutions：
- Lower the mold temperature and extend the cooling time to ensure uniform crystallization
- Optimize the product design regarding wall thickness
- Adjust the ejector pin layout to balance the ejection force
- Reduce the injection speed to minimize shear stress
- Improve the mold cooling system to ensure uniform mold temperature
- Increase the draft angle to prevent drag-induced deformation
3. Weld Lines / Knit Lines
Symptom: Fine lines and poor strength observed at the weld line (point of material stream convergence).
Causes:
- Material temperature and/or mold temperature are too low, causing the material to cool prematurely upon convergence.
- Injection speed is too slow, resulting in cooling of the flow front.
- Poor venting leads to air entrapment at the weld line.
- Gate placement is suboptimal, resulting in an excessively long flow path.
- Excessive use of mold release agent contaminates the weld interface.
Solutions:
- Increase material and mold temperatures.
- Appropriately increase injection speed to minimize cooling of the flow front.
- Improve vent design to eliminate trapped air.
- Optimize gate placement to shorten the flow path.
- Reduce the usage of mold release agent.
4. Bubbles / Voids
Phenomenon: Surface bubbles or internal voids.
Causes:
- PP raw material contains moisture (although PP has low water absorption, recycled or filled materials are prone to retaining water).
- Melt 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.
- Thick-walled parts cool too rapidly; internal shrinkage creates vacuum voids.
Solutions:
- For recycled or filled PP materials, drying at 80°C for 2–3 hours is recommended.
- Reduce barrel temperature to prevent overheating and decomposition.
- Reduce injection speed during the initial phase to prevent air entrapment.
- Increase holding pressure to ensure thorough compaction.
- For thick-walled parts: use a slower injection speed, apply extended holding pressure, and prolong the cooling time.
5. Flow Marks / Ripple Marks
Phenomenon: Wave-like patterns appear in the vicinity of the gate.
Causes:
- Melt temperature and/or mold temperature are too low.
- Injection speed fluctuates erratically (unstable).
- The gate is too small, resulting in excessive shear stress.
- The mold lacks a sufficient cold slug well, allowing cold material to enter the cavity.
Solutions:
- Increase the melt temperature and mold temperature.
- Stabilize the injection speed, avoiding abrupt step changes.
- Appropriately enlarge the gate size to reduce shear stress.
- Add or enlarge the cold slug well to prevent cold material from entering the cavity.
6. Flash / Burrs
Symptom: Material leakage at the parting line or insert interfaces.
Causes:
- Insufficient clamping force.
- Material temperature is too high, resulting in excessive fluidity.
- Injection pressure and/or injection speed are too high.
- Excessive clearance or wear in the mold assembly.
- Overfilling; excessive injection volume.
Solutions:
- Increase clamping force.
- Appropriately lower the material temperature.
- Reduce injection pressure and injection speed.
- Repair the mold to reduce assembly clearance.
- Adjust metering settings to prevent overfilling.
7. Silver Streaks / Mottling
Phenomenon: Silver-white streaks or a cloudy, nebulous appearance on the surface.
Causes:
- Raw material is damp or contains moisture.
- Barrel temperature is excessively high, causing PP degradation.
- Poor venting, leading to gas entrapment and release.
- Screw back pressure is too low, resulting in inadequate mixing.
Solutions:
- Dry the raw material.
- Lower the material temperature to prevent thermal degradation.
- Improve venting.
- Appropriately increase back pressure.
8. Ejector Whitening / Cracking
Phenomenon: Whitening or cracking occurs at the ejector pin locations.
Causes:
- Insufficient draft angle.
- Mold temperature is too low, making the molded part overly brittle.
- Uneven ejection force; ejector pins are too small.
- Excessive internal stress.
Premature ejection; the part has not yet cooled and set.
Solutions:
- Increase the draft angle.
- Appropriately raise the mold temperature.
- Adjust the ejector pin layout and/or increase the surface area of the ejector pins.
- Reduce injection speed to minimize internal stress.
- Extend the cooling time before ejection.
9. Brittleness / Insufficient Strength
Symptoms: Prone to breakage, notch cracking
Causes:
- Melt temperature is too high, leading to PP degradation
- Proportion of recycled material is excessively high
- Internal stress is excessive
- Excessive filler content (talc / calcium carbonate) or poor dispersion
- Mold temperature is too low, resulting in poor crystallization
Solutions:
- Control melt temperature to avoid overheating
- Control the proportion of recycled material
- Optimize processing parameters to reduce internal stress
- Improve compounding to enhance dispersion
- Appropriately increase mold temperature to improve crystallization
10. Poor Gloss / Rough Surface
Phenomenon: Surface appears dull and lacks luster.
Causes:
- Mold temperature is too low.
- Material temperature is insufficient.
- Mold surface roughness is high.
- Poor venting.
- Excessive use of mold release agent.
Solutions:
- Increase mold temperature and material temperature.
- Polish the mold cavity.
- Improve venting.
- Reduce the amount of mold release agent.
