Polybutylene terephthalate (PBT) has advantageous combination properties such as a convenient source of raw materials, and mature polymerization process has gotten easier, low cost and easily forming and processing. Nylon, polycarbonate (PC), polyoxymethylene and modified polyphenylene oxide are five major engineering plastics.
PBT Injection Molding (Moulding):
PBT plastic material is a kind of crystalline polymer with a sharp melting point (225-235 Celsius degree). Crystallinity can reach 40%. When the temperature rises above the melting point, the liquidity of melt will become obvious. However, when changing from the molten state to the gruel state, it not only has the fast rate of cooling rate but has a fast crystalline speed.
In the state of melting for PBT plastic, because of its low viscosity, perfect liquidity (inferior to the nylon) and better mold shrinkage, it needs to prevent the phenomena of edge flowing and flashing of products.
Influenced by factors of crystallinity, technology, mould, PBT plastics has anisotropy to makes products warping and distorted during the molding process. Therefore, it should pay attention to proper process conditions and mold design.
PBT contains esters gene and it will hydrolyze under the high temperature with the presence of moisture. Therefore, before starting the processing, it has to conduct the drying process.
For PBT plastic injection molding parts, most gates are applicable to melt’s rapid mold filling, which just requests the bigger caliber to prevent the contraction of the melt to reduce the loss of pressing force.
In the actual operation, the temperature of PBT ranges from 240 to 260 Celsius degree.
Owning to PBT’s fast cooling speed, injection speed should be slightly faster in order to prevent short shot of the products, obvious welding, and poor surface fineness caused by premature condensation during the filling process. However, it needs to prevent inadequate air exhausting and the flashing of products.
The revolving speed of screw should be controlled to some extent to avoid friction heat impacting on the resin and screw speed generally doesn’t exceed 80r/min, mostly ranging from 25 to 60r/min. In order not to obstruct plasticizing of materials and make sure the air among particles of fabric materials can be discharged in time, during PBT’ shaping, the screw packing should be controlled at 10-15% of the injection pressure.
To get products with better performance and good fineness as well as the smaller deformation in the molding, PBT’s appropriate temperature is 70 -80 Celsius degree.
In view of PBT’s low viscosity, good fluidity, fast cooling and solidifying in the molding, most products don’t need a long molding cycle.
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