Analysis of 13 Common Defects in PP Injection Molding

1. Short Shot

Phenomenon: The product fails to fully fill the mold cavity, with missing parts at corners or thin-walled areas.

Causes: Excessively long PP melt flow distance (exceeding the limit flow - length ratio of 120:1 - 150:1), insufficient injection pressure, or poor mold venting causing gas trapping.

Solutions: Increase injection pressure to 80 - 120 MPa and control the front barrel temperature at 200 - 235 °C to enhance fluidity. Enlarge the runner diameter to 4 - 8 mm, add 0.02 - 0.04 mm deep vents at the melt end, and set cold slug wells to collect cold front material.

2. Flash

Phenomenon: Excess plastic burrs appear at parting lines or insert gaps, with a thickness of 0.01 - 0.1 mm.

Causes: Insufficient clamping force (not meeting the "injection pressure × projected area × 1.2 safety factor" requirement), foreign matter on the parting line, or worn mold guide posts.

Solutions: First clean the parting line impurities. If flash still exists, calculate according to "3.5 - 5 MPa clamping force per square centimeter of projected area" and replace with a suitable tonnage machine. Repair worn guide posts to ensure mold closing accuracy.

3. Sink Mark

Phenomenon: Depressions occur in thick - walled areas (> 2.5 mm) or at the roots of ribs, with a depth usually < 0.5 mm.

Causes: Inadequate compensation for PP crystallization shrinkage (shrinkage rate 0.5% - 2.5%), mainly due to insufficient packing pressure.

Solutions: Set the packing pressure to 50% - 80% of the injection pressure and extend the packing time to 50% - 70% of the cooling time. Use conformal cooling channels in the mold, controlling the spacing of channels in thick - walled areas at 25 - 35 mm to reduce cooling temperature differences.

4. Air Hole

Phenomenon: Closed voids form inside the product. Surface air holes are mostly needle - tip shaped, and internal air holes can reach a diameter of 0.5 - 3 mm.

Causes: Excessive moisture content in raw materials (> 0.03%), air entrainment due to too fast injection speed, or uneven shrinkage caused by too fast cooling of thick - walled parts.

Solutions: Dry raw materials at 70 - 85 °C for 2 - 4 hours and adjust the back pressure to 0.3 - 1.5 MPa to compact the melt. Use "slow - fast - slow" segmented injection, control the injection speed of thick - walled parts at 20 - 50 mm/s, and increase the packing pressure to 80 - 100 MPa.

5. Silver Streak

Phenomenon: Silver - white streaks appear on the surface along the melt flow direction, especially obvious in transparent products.

Causes: Vaporization of moisture or volatile substances in raw materials when heated, or gas being pressed into the melt surface due to poor mold venting.

Solutions: Besides strictly drying raw materials, add vents near the gate with a spacing ≤ 50 mm. Increase the mold temperature to 40 - 70 °C and speed up the injection speed to 50 - 80 mm/s to shorten melt residence time.

6. Weld Line

Phenomenon: A linear trace formed by the convergence of multiple melts, with strength 10% - 30% lower than that of normal areas.

Causes: Temperature difference > 15 °C when melts converge, or gas retention at the interface due to poor venting.

Solutions: Move the gate to a non - appearance area and add 0.02 - 0.04 mm deep vents at the weld line position. Increase the mold temperature to 70 - 90 °C and increase the injection speed to 60 - 90 mm/s. Add 0.1% - 0.3% weld line improver if necessary.

7. Cracking

Phenomenon: Fine cracks appear on the surface, mostly concentrated at corners with insufficient draft angle, and can penetrate the product in severe cases.

Causes: Concentrated demolding stress, or reduced toughness due to PP degradation (material temperature exceeding 260 °C is prone to degradation).

Solutions: Increase the draft angle to 1° - 3° and polish the cavity surface to Ra0.8 μm or less. Reduce the injection pressure to 70 - 100 MPa and control the barrel temperature at 180 - 230 °C to avoid raw material degradation.

8. Deformation

Phenomenon: The product bends or twists after leaving the mold, with a dimensional deviation exceeding 0.5 mm/m.

Causes: Uneven cooling (temperature difference > 10 °C) or excessive difference in molecular orientation, and uneven wall thickness also aggravates deformation.

Solutions: Use 3D - printed conformal cooling channels to ensure uniform cooling speed in all areas. Extend the cooling time to 15 - 30 seconds and reduce the injection speed to 30 - 60 mm/s to reduce orientation stress.

9. Warpage

Phenomenon: The edges of flat products warp upward, with a radius of curvature > 500 mm.

Causes: Anisotropic crystallization of PP leads to uneven shrinkage, or the gate position deviates from the center of gravity causing uneven flow.

Solutions: Use a center gate or balanced gate layout to balance the melt flow path. Control the mold temperature difference within 5 °C and perform 0.5 - 1.5 hours of aging treatment after cooling, which can reduce warpage by 30% - 50%.

10. Jetting

Phenomenon: Snake - like patterns appear near the gate, usually 5 - 20 mm in length.

Causes: The melt is injected into the low - temperature cavity at high speed, and the surface layer solidifies rapidly to form ripples.

Solutions: Enlarge the gate diameter to 1.5 - 2.5 times the wall thickness or use a fan - shaped gate to disperse the material flow. Reduce the initial injection speed to 15 - 30 mm/s, then increase the speed after the melt contacts the cavity wall, and increase the mold temperature to 50 - 70 °C.

11. Burning

Phenomenon: Black spots appear in mold corners or poorly vented areas, accompanied by a burnt smell.

Causes: Gas trapping is compressed to above 300 °C causing thermal decomposition, or residual material in the barrel degrades and carbonizes.

Solutions: Add vents in gas - trapped areas, with the total venting area not less than 0.8% - 1.2% of the cavity volume. Clean residual material in the barrel, reduce the back pressure to 0.3 - 0.8 MPa, and control the screw speed at 30 - 60 r/min.

12. Sticking to Mold

Phenomenon: The product clings to the cavity and is difficult to remove, and forced ejection is prone to scratches or damage.

Causes: Insufficient cavity surface finish (Ra > 1.6 μm), or small draft angle and insufficient ejection area.

Solutions: Polish or chrome - plate the cavity and expand the ejection area to 25% - 35% of the product bottom. Reduce the mold temperature to 30 - 50 °C, reduce the injection pressure by 10% - 15%, and use silicone - based release agent (diluted to 2% - 5%) if necessary.

13. Vacuum Hole

Phenomenon: Irregular voids appear in the center of thick - walled parts, with a diameter of 0.5 - 2 mm.

Causes: Packing does not cover the gate freezing time, and internal shrinkage forms a vacuum.

Solutions: Determine the gate freezing time through mold flow analysis and extend the packing time to 1.1 - 1.3 times the freezing time. Increase the packing pressure to 80 - 110 MPa and reduce the barrel temperature by 5 - 10 °C to reduce melt volume shrinkage.

Conclusion

The solution to PP injection molding defects requires establishing a three - dimensional inspection system of "material - mold - process". The parameters in the article are all industry - general ranges. In actual operation, fine - tuning is required according to specific raw materials (such as glass fiber reinforced PP requiring adjustment of wear - resistant design), product structure and equipment characteristics. It is recommended to verify parameters through small - batch test molding, establish a defect case library, and formulate standardized processing procedures for high - frequency problems to achieve lean production.