Causes and Solutions of Shrinkage in PP Products
PP plastics and recycled PP materials may experience shrinkage issues due to various external factors after molding. Proper cooling temperatures and post-processing can help reduce this shrinkage during production. Shrinkage problems in PP products are mainly manifested in the following aspects:
Causes and Solutions of Shrinkage in PP Products
1. Mold Aspect
For multi-gate molds, adjust the filling speed of each gate. And it’s better to symmetrically set up the gates.
Critical areas of the mold should have effectively designed cooling channels to ensure that mold cooling has a significant effect on eliminating or reducing shrinkage.
The entire mold should be free of burrs and have a good mold sealing ability, capable of withstanding the filling of high-pressure, high-speed, low-viscosity melt.
2. Equipment Aspect
(1) Insufficient Supply
Severe wear of the screw or plunger can result in leakage of molten material during injection and holding pressure, reducing mold filling pressure and material volume, causing insufficient molten material.
(2) Overly Large or Overly Small Nozzle Hole
If it is too small, it is prone to block the feed channel. While if it is too large, it will reduce the injection force, making mold filling difficult.
3. Process Aspect
Increasing injection pressure, holding pressure, and extending injection time.
Raising the injection speed can conveniently ensure that the part is fully filled, eliminating most of the shrinkage.
Extending the dwell time of the part in the mold for cooling, maintaining a uniform production cycle, increasing back pressure, and retaining a certain buffer at the front of the screw are all beneficial for reducing shrinkage.
– For plastics with high fluidity, high pressure can cause flash and sink marks, so the material temperature should be appropriately reduced. Lowering the temperature of the barrel front section and nozzle reduces the volume change of the molten material entering the cavity, making it easier to solidify.
– For high-viscosity plastics, the barrel temperature should be increased to facilitate mold filling. When shrinkage occurs in the gate area, the holding pressure time should be extended.
– Thin-walled parts should increase the mold temperature to ensure smooth material flow.
– Thick-walled parts should reduce the mold temperature to accelerate the solidification and setting of the surface.
– Low-precision products should be demolded early and allowed to slowly cool in the air or hot water, which can make the shrinkage depression smooth without affecting use.
4. Material Aspect
If the raw material is too soft, it is prone to sink marks. An effective method is to add a nucleating agent to the plastic to accelerate crystallization.
