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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