What is the cause of shrinkage of injection parts?

Large Shrinkage Rate: Different plastic materials have different shrinkage rates. Some crystalline plastics such as polypropylene (PP) and polyethylene (PE) will have regular arrangement of molecular chains during the crystallization process, resulting in large volume shrinkage and making shrinkage holes prone to occur.

High Water Content: Plastic raw materials may absorb a certain amount of water during storage and transportation. When the water content exceeds the standard, the water will turn into water vapor during the injection molding process, occupying a certain space and forming shrinkage holes after cooling. Meanwhile, water will also affect the fluidity and thermal stability of plastics, further aggravating the generation of shrinkage holes.

Impure Material or Presence of Impurities: If other impurities or different types of plastics are mixed into the plastic raw materials, it will make the material properties uneven. During the molding process, different components will shrink differently, thus generating shrinkage holes.

Uneven Cooling System: The design of the mold's cooling water channels is unreasonable. For example, the uneven distribution of cooling water channels or inconsistent distances from the cavity surface will cause different cooling rates at various parts of the plastic part during the cooling process. The parts that cool quickly shrink less, while the parts that cool slowly shrink more, thus producing shrinkage holes.

Improper Gate Design: The size, quantity and position of the gate have an important impact on the flow and filling of plastics. If the gate size is too small, the resistance of the plastic melt during cavity filling is large, resulting in insufficient cavity filling and making shrinkage holes prone to occur in the parts far from the gate. Insufficient number of gates or unreasonable positions will also make it impossible for plastics to fill the cavity evenly, causing local material shortage and the formation of shrinkage holes.

Surface Roughness of the Mold Cavity: If the surface roughness value of the mold cavity is too large, it will increase the friction between the plastic melt and the cavity surface, affecting the flow and filling of plastics, making the surface of the plastic part uneven and prone to defects such as shrinkage holes inside.

Insufficient Injection Pressure: Injection pressure is an important driving force for the plastic melt to fill the cavity. If the injection pressure is not enough, the plastic melt cannot fully fill the cavity, and shrinkage holes will be formed due to volume shrinkage during the cooling process. Especially for some injection molded parts with complex structures and large sizes, sufficient injection pressure is required to ensure complete cavity filling.

Too Short Holding Pressure Time: The function of the holding pressure stage is to continuously replenish plastics during the cooling and shrinkage of the plastic melt to compensate for the volume reduction caused by shrinkage. If the holding pressure time is too short, the plastic melt will not be sufficiently replenished during the cooling process, and shrinkage holes are likely to be formed inside the plastic part.

Too High Melt Temperature: Excessively high melt temperature will reduce the viscosity of plastics and increase fluidity. After filling the cavity, the volume change is large during cooling and shrinkage, making shrinkage holes prone to occur. At the same time, high temperature may also lead to the degradation of plastic molecular chains, affecting the performance and quality of plastic parts.

Too Short Cooling Time: Insufficient cooling time means that the internal heat of the plastic part cannot be fully dissipated. After demolding, it will continue to cool and shrink, resulting in shrinkage holes. Moreover, too short cooling time will also affect the dimensional stability and mechanical properties of plastic parts.

Insufficient Plasticizing Capacity of the Injection Molding Machine: Serious wear of the plasticizing components of the injection molding machine or unreasonable design of the plasticizing screw will lead to insufficient melting and uniform mixing of plastics during the plasticizing process, making the quality of the plastic melt entering the cavity uneven and generating shrinkage holes during cooling and shrinkage.

Malfunction of the Hydraulic System: If there are malfunctions in the hydraulic system of the injection molding machine, such as unstable pressure or insufficient flow, it will affect the stability of the injection and holding pressure processes, resulting in defects such as shrinkage holes in injection molded parts.