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| Plastic sheet extruder |
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Source：admin Popularity：5086 Release time：2019-05-01
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The forming characteristics of the sheet for molding, whether single or multi-layer composite sheet, must have the following properties: 1. Plastic memory, that is, when the sheet is stretched and softened, it has the tendency to tighten and resist the tensile force, and There is a tendency to stretch as evenly as possible. This feature allows the shaped article to revert to its original flat shape if it is reheated to its original molding temperature. This property has an important influence on the stretching of the molding process. 2. Hot stretching, that is, the sheet can be stretched when heated, and this characteristic has a great influence on the shape and quality of the product. Some can be stretched by 15% to 20%, and some can even be stretched to 500% to 600%. Third, the heat strength, that is, the heat-softened sheet, as long as it is slightly stressed, will form a clear outline on the mold. On the other hand, if too much pressure is required to form, and the vacuum suction molding provides a limited pressure difference, it is difficult to display a subtle pattern. Fourth, the molding temperature, that is, the formed sheet needs to have a suitable processing temperature range. It is easy to form at its heat-softening temperature, and has a certain distance from its melting temperature. The molding temperature range is wide; it can not be molded only in a small specific temperature range. When the temperature is high or low, the molding is easy to tear. , collapse and other phenomena. In order to understand the characteristics of the sheet for plastic forming, we will analyze it from the following aspects. 1. Thermoplastic Chemical Composition and Structure Thermoplastics consist of macromolecules (polymers) with a molecular chain length of 10-3 mm. These macromolecules can be linear, such as HDPE, or branched, such as LDPE. The macromolecules are completely disordered and we call them amorphous thermoplastics. Macromolecules of uniform structure, such as linear polyethylene or polyoxymethylene, can form part of the regular arrangement, and the macromolecules are partially crystallized according to certain rules. We call it a partially crystalline thermoplastic. The difference between amorphous and partially crystalline thermoplastics, amorphous thermoplastics are not crystalline due to their asymmetric structure or large pendant groups, and are transparent without modification and coloration. Amorphous thermoplastics should be used at temperatures below their glass transition temperature jaws. Partially crystalline plastics contain regions in which molecular chains are regularly arranged, referred to as crystalline regions. Because of crystallization, partially crystalline thermoplastics are generally opaque and the transparency decreases with increasing crystallinity. Partially crystalline thermoplastics are used at temperatures between a few minutes and a melting point Tm.
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