2015年9月15日星期二

Characteristics of plastic commonly used in transparent materials(a)

Because plastic is light weight, good toughness, molding and easy. Low cost, and therefore in the modern industrial and household products, more and more plastic instead of glass, especially for optical instruments and packaging industry, develop rapidly. However, due to its transparency requirements better, higher abrasion resistance, impact toughness is better, so the plastic components, injection of the process technology, equipment, molds, they have made a lot of work to ensure that they be used instead of glass plastic (hereinafter referred to as transparent plastic), good surface quality, so as to achieve the requirements of use.

Currently on the market transparent plastic commonly used are poly (methyl methacrylate) (commonly known as acrylic or Plexiglas, codenamed pmma), polycarbonate (code pc). Polyethylene terephthalate resin (codenamed pet), transparent nylon. as (acrylonitrile-styrene copolymer), polysulfone (codenamed psf), of which we have the most contact is pmma, pc and pet three plastic, because of space limitations, the following three plastic as an example to discuss Characteristics of transparent plastic and injection molding process. A transparent plastic properties of transparent plastics must first have a high transparency, and secondly to have a certain strength and wear resistance can impact resistance, heat resistance and better, excellent chemical resistance to water absorption should be small, the only way to use , and can meet the requirements of transparency and long-term deformation

Second, transparent plastic injection molding process should pay attention to common problems

Transparent plastic due to high light transmittance, a necessary requirement the plastic surface strict quality requirements, can not have any markings, pores, whitening. Halo fog, black spots, discoloration, poor gloss and other defects, and thus the entire injection molding process for raw materials and equipment. Mold, and even product design, we must pay great attention to and propose strict and even special requirements. Second, because the transparent plastic, mostly high melting point, poor mobility, so as to ensure the surface quality of the products, often in machine high temperature, injection pressure, injection speed and other parameters for fine adjustments to make when both full of plastic injection molding, without It will produce internal stress caused by deformation and cracking products.

Here its preparation of raw materials. Requirements for equipment and tooling, raw material injection molding process and product handling aspects, to talk about the matter should be noted.

Preparation and drying (a) the raw material at any point because it contains impurities in plastics, are likely to affect the transparency of the product, so the storage, transportation, feeding process, we must pay attention to the seal, to ensure a clean raw material. Especially raw materials containing water, can cause deterioration of raw materials is heated, so be sure to dry, and at the time of injection, the feeding must use the drying hopper. Also note that the drying process, the air inlet should preferably be filtered, dehumidified to ensure that does not contaminate raw materials.

(Ii) Clean barrel, screw and its annexes to prevent contamination of raw materials and there are old material or impurities in the screw and accessories depression, especially poor thermal stability of resin exist, so before use, screw cleaning agents are applied after the shutdown clean all parts, so that it can not stick with impurities, when there is no screw cleaning agent, available pe, ps and other resin cleaning screw. When the temporary shutdown, in order to prevent material at high temperatures to stay a long time, causing degradation should be drier and barrel temperature decreases, such as pc, pmma, etc. barrel temperature should be reduced to below 160 . (Hopper temperature for pc should be reduced to below 100 )

(C) the issue in the mold design should pay attention (including product design) in order to prevent back flow of poor or uneven cooling caused by bad plastic molding, surface defects and deterioration in general in the mold design should note the following points. a) wall thickness should be uniform, stripping slope be large enough;
b) should be gradually transition. Smooth transition to prevent any sharp corners. Produce sharp edges, especially pc product must not have a gap;
c) gate and runner as large, chunky, and the location of the gate should be set according to the contraction condensation process, if necessary add cold material well;
d) the mold surface should be smooth, roughness low (preferably less than 0.8);
e) vent grooves must be sufficient, and when the exhaust gas in the air and melt;

f) In addition to pet, the wall thickness of not too thin, usually not less than 1mm.

more information www.cnmoulding.com 

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