shrinkage and fluidity of plastic
The main factors that usually affect the change in shrinkage rate are the following four aspects:
- Plastic variety
Various plastics have their own shrinkage range, but even the same plastics have different shrinkage and anisotropy due to their relative molecular mass, filler and ratio.
- Plastic structure
The shape, size, wall thickness, number and layout of the inserts have a great influence on the shrinkage value. Generally, the larger the wall thickness of the plastic part, the larger the shrinkage rate, and the plastic part with a complicated shape is smaller than the shrinkage rate of the plastic part with a simple shape. Plastics with inserts are hindered by inserts and have small chilling shrinkage
- Mold structure
The parting surface of the mould, the direction of the pressurization and the structure of the casting system, the layout and the dimensions directly affect the flow direction, the density distribution, the pressure-preserving and shrinking action and the molding time, which have a great influence on the shrinkage rate and the directionality, especially Extrusion and injection molding are more prominent.
- Molding process conditions
Molding temperature, injection pressure, holding time and other molding conditions have a great influence on the shrinkage rate of plastic parts. The mold temperature is high, the melt cools slowly, the density is large, and the shrinkage is large. Especially for crystalline plastics, their shrinkage is greater due to their large volume change. The uniformity of the mold temperature distribution also directly affects the size and directionality of the shrinkage of each part of the press. The injection pressure is high, the melt viscosity difference is small, the elastic recovery is large after demolding, and the shrinkage is reduced. The shrinkage is reduced when the holding time is long.
Generally, the fluidity of thermoplastics can be divided into three categories:
Good mobility plastic
For example, polyamide, polyethylene, polystyrene, polypropylene, cellulose acetate, and the like.
Such as modified polystyrene, ABS, AS, polymethyl methacrylate, polyoxymethylene and chlorinated polyether, etc.
Poorly flowing plastic
Such as polycarbonate, hard polyvinyl chloride, polyphenylene ether, polysulfone, polyarylsulfone and fluoroplastic.
The main factors affecting the fluidity of plastics are:
- Temperature. When the material temperature is high, the fluidity of the plastic increases, but the temperature of the material has different effects on the fluidity of different plastics.
For example, the fluidity of polystyrene, polypropylene, polyamide, ABS, AS and other plastics is greatly affected by temperature changes; the fluidity of polyethylene and polyoxymethylene is less affected by temperature changes.
- injection pressure. When the injection pressure is increased, the melt is subjected to a large shearing action and the fluidity is also increased, especially polyethylene, which is very sensitive to pressure changes. However, excessive pressure can stress the plastic and reduce the viscosity of the melt to form a flash.
- Mold structure. The shape, size, arrangement, cavity surface roughness, runner interface thickness, cavity form, exhaust system, cooling system design, melt flow resistance and other factors of the casting system directly affect the fluidity of the melt.
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