The compression molding process
preloading is refers to the compression molding, at room temperature or slightly higher than that of room temperature conditions, will loose powder, granular, fragmental, flake or long fibrous material compacted into a certain weight, shape consistent plastic parison, can make it easier to add to the compression modulus feeding chamber. Preloading blank shape for wafer or disc, commonly can also be pressed into the shape of a similar to the plastic parts. Usually in the 40 - preloading stress 200 mpa range selection, after preloading of billet density best can reached the density is about 80% of the plastic parts, to ensure the billet has certain strength.
thermosetting plastic, compression molding materials emit a considerable amount of water vapor in the cavity, low molecular volatiles and fumes, crosslinking reaction and volume shrinkage, therefore, after the mold die sometimes need to uninstall to rule out the gases in the cavity, otherwise, will extend the material heat transfer process, extend the molten material curing time, and will also appear on the surface of the plastic good burning, burning phenomenon such as bubbles, surface gloss is not good. The frequency and time should be according to the need of the exhaust, usually 1 - Each time, 3 times for 3 - time For 20 seconds.
mould mount after the press to be preheated. If the plastic with embedded parts, parts should be preheated before loading the insert into the mold cavity. Thermosetting plastic moulding process in general can be divided into charging, mold closing and exhaust, curing and demoulding several stages. Charging is joined in the mold cavity has quantitative materials, preheating it is an important link in production of compression molding. Charging is accurate, it will directly affect the density of plastic parts and dimensional accuracy. Commonly used methods have quality standard, capacity, volume and count three.
the volume quality standard must be weighing apparatus weighing the volume of material quality, and then added to the mold, using the method can accurately control the feed rate, but is not convenient to operate. Volumetric method is to make a certain volume or volume drop ah scale container loading to the mould, this method is simple, but control and capacity is not enough accurate. Counting method is suitable for the preloading billet. For large or complex cavity shape, still should according to the material flow in the mould and parts of the cavity with capacity is much, reasonable stacking materials, lest cause plastics phenomenon of uneven density or the lack of material.
before molding, heating should be made for thermosetting plastics. Heating purpose has two: one is to preheating of plastic, so as to provide a certain temperature for compression modulus thermal material, make plastic heated evenly inside the mold, shorten the molding cycle; 2 it is to dry the plastics, prevent the plastic with too much moisture and low volatile matter, to ensure the quality of machining parts of injection molding. Preheating and drying of commonly used equipment are oven and infrared heating furnace.
to die after completion of loading, forming parts by pressure to make the mould closed into a cavity shape consistent with plastic parts. When before punch contact materials, should try to make the mold closing speed, to shorten the molding cycle and many plastic hardening and degradation. And after punch contact material, mold closing speed should be slow down, to avoid the insert in the mould and molding displacement and damage of the bar, but also conducive to the smooth discharge of the air, avoid the material being outside the air discharge mode caused by a lack of material. Closing time is usually a few seconds to tens of seconds.
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