Plastic are mainly composed of high molecular weight synthetic resins. According to the performance of the surface after heating, plastic can be divided into two categories: thermosetting plastic and thermoplastic. The chemical structure of the former changes after hardening, the texture is hard, insoluble in solvents, and no longer softens when heated. If the temperature is too high, it will decompose. The characteristic of the latter is that the physical state changes after heating, from solid softening or melting to viscous fluid state, but it can harden and become solid after cooling, and the process can be repeated many times, and the molecular structure of the plastic itself does not change. Different varieties and brands of plastic have different usage and process characteristics.

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Plastic Forming Performance Chapter 1

Plastic Forming Performance Plastic are mainly composed of high molecular weight synthetic resins. Under certain conditions (such as temperature, pressure, etc.), they can be molded into a certain shape and keep the shape unchanged at room temperature. According to the performance of the surface after heating, plastic can be divided into two categories: thermosetting plastic and thermoplastic. The characteristic of the former is that it hardens by chemical reaction after heating, pressurizing or adding hardener at a certain temperature for a certain period of time. After hardening, the chemical structure of plastic changes, the texture is hard, it is insoluble in solvents, and it will no longer soften when heated. If the temperature is too high, it will decompose. The latter is characterized by a physical change after heating, from a solid softening or melting to a viscous fluid state, but it can harden again after cooling and become a solid, and the process can be repeated many times, and the molecular structure of the plastic itself does not change. Plastic are made of synthetic resins as basic raw materials, and various auxiliary materials such as fillers, plasticizers, dyes, stabilizers, etc. are added. Therefore, different varieties and brands of plastic have different uses and process characteristics due to the different properties, components, proportions and plastic production processes of the selected resins and auxiliary materials. For this reason, the process characteristics of the plastic used must be understood when designing molds.

Section 1 Thermosetting Plastic Commonly used thermosetting plastic include phenolic, amino (melamine, urea-formaldehyde) polyester, poly(dipropylene phthalate), etc. Mainly used for compression molding, extrusion molding, and injection molding. Plastic such as silicone and epoxy resin are currently mainly used for low-pressure extrusion packaging of electronic components and casting molding.

I. Process characteristics (I) Shrinkage After the plastic part is taken out of the mold and cooled to room temperature, it shrinks in size. This property is called shrinkage. Since shrinkage is not only the thermal expansion and contraction of the resin itself, but also related to various molding factors, the shrinkage of the plastic part after molding should be called molding shrinkage. 1. Forms of molding shrinkage Molding shrinkage is mainly manifested in the following aspects: (1) Linear size shrinkage of plastic parts Due to thermal expansion and contraction, elastic recovery and plastic deformation during demolding of plastic parts, the size of plastic parts shrinks after demolding and cooling to room temperature. Therefore, compensation must be considered when designing the cavity. (2) Directionality of shrinkage During molding, molecules are arranged in a direction, making the plastic part anisotropic. The shrinkage is large and the strength is high along the direction of the material flow (i.e. parallel direction), while the shrinkage is small and the strength is low in the direction perpendicular to the material flow (i.e. vertical direction). In addition, during molding, the density and filler distribution of various parts of the plastic part are uneven, so the shrinkage is also uneven. The shrinkage difference makes the plastic part prone to warping, deformation, and cracking, especially in extrusion and injection molding, the directionality is more obvious. Therefore, the shrinkage direction should be considered when designing the mold. The shrinkage rate should be selected according to the shape of the plastic part and the direction of the material flow. (3) Post-shrinkage When the plastic part is formed, due to the influence of factors such as forming pressure, shear stress, anisotropy, uneven density, uneven filler distribution, uneven mold temperature, uneven hardening, and plastic deformation, a series of stresses are caused. They cannot all disappear in the viscous flow state. Therefore, there are residual stresses when the plastic part is formed under stress. After demolding, due to the influence of stress equilibrium and storage conditions, the residual stress changes and the plastic part shrinks again, which is called post-shrinkage. Generally, the plastic part changes the most within 10 hours after demolding, and is basically fixed after 24 hours, but it takes 30 to 60 days to stabilize. Usually, the post-shrinkage of thermoplastic is greater than that of thermosetting plastic, and that of extrusion and injection molding is greater than that of compression molding. (4) Post-processing shrinkage Sometimes, according to performance and process requirements, plastic parts need to be heat treated after forming, which will 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. 2. Calculation of shrinkage rate The shrinkage of plastic parts can be expressed by shrinkage rate, as shown in formula (1-1) and formula (1-2). Qreal = (ab)/b¡Á100

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