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Plastic forming is the process of making various forms of plastic (powder, particle, solution, and dispersion) into products or blanks of the desired shape. There are over thirty methods of forming.

The choice of plastic molding mainly depends on the type of plastic (thermoplastic or thermosetting), the initial form, and the shape and size of the product. The commonly used methods for processing thermoplastic plastics include extrusion, injection molding, rolling, blow molding, and thermoforming. The processing of thermosetting plastics generally involves molding, transfer molding, and injection molding. Plastic forming is the process of making various forms of plastic (powder, particle, solution, and dispersion) into products or blanks of the desired shape. There are over thirty methods of forming. Lamination, molding, and hot forming are the processes of forming plastic on a flat surface. The above plastic processing methods can be used for rubber processing. In addition, there are also casting processes using liquid monomers or polymers as raw materials. Among these methods, extrusion and injection molding are commonly used and are also basic molding methods.

Process characteristics

Shrinkage rate

The performance of plastic parts that undergo dimensional shrinkage after being taken out of the mold and cooled to room temperature is called shrinkage. Due to the fact that shrinkage is not only the thermal expansion and cold contraction of the resin itself, but also related to various forming factors, the shrinkage of the formed plastic part should be referred to as forming shrinkage.

1. The form of forming shrinkage is mainly manifested in the following aspects:

(1) The linear size shrinkage of plastic parts is caused by thermal expansion and contraction, elastic recovery during demolding, plastic deformation, and other reasons, resulting in the size reduction of plastic parts after demolding and cooling to room temperature. Therefore, compensation should be considered in the design of the mold cavity.

(2) When forming with directional shrinkage, molecules are arranged in the same direction, resulting in anisotropy of the plastic part. Along the direction of the material flow (i.e. parallel direction), the shrinkage is large and the strength is high, while in the direction perpendicular to the material flow (i.e. vertical direction), the shrinkage is small and the strength is low. In addition, due to uneven density and filler distribution in various parts of the plastic part during forming, shrinkage is also uneven. The occurrence of shrinkage differences makes plastic parts prone to warping, deformation, and cracking, especially in extrusion and injection molding where directionality is more pronounced. Therefore, when designing the mold, it is advisable to consider the shrinkage direction and select the shrinkage rate according to the shape and flow direction of the plastic part.

(3) When forming post shrinkage plastic parts, due to the influence of factors such as forming pressure, shear stress, anisotropy, uneven density, uneven distribution of fillers, uneven mold temperature, uneven hardening, plastic deformation, etc., a series of stresses cannot disappear completely in the viscous flow state. Therefore, there is residual stress when forming plastic parts under stress state. When the residual stress changes due to stress balance and storage conditions after demolding, resulting in re shrinkage of the plastic part, it is called post shrinkage. Generally, plastic parts undergo significant changes within 10 hours after demolding and are basically shaped after 24 hours, but stability takes 30-60 days. Usually, the post shrinkage of thermoplastic materials is greater than that of thermosetting materials, while those formed by extrusion and injection molding are greater than those formed by compression molding.

(4) Post processing shrinkage sometimes requires heat treatment after forming plastic parts according to performance and process requirements, which can also cause changes in the size of the plastic parts. Therefore, when designing molds for high-precision plastic parts, the errors of post shrinkage and post processing shrinkage should be considered and compensated.