Plastic water imbibition, swelling, and creep

Plastic water imbibition effect the precision of the use of plastic, injection molding processing products used in damp environment, caused by suction size increases, the amount of plastic water imbibition effect on dimensional accuracy than affect the contraction, plastic water imbibition size related to its molecular structure. Heat bilges cold shrink is the general rule of the object, to measure the coefficient of thermal expansion degree can be used for thermal said. Plastic expansion coefficient related to the melt flow direction, and the shrinkage is different, the longitudinal thermal expansion coefficient than lateral expansion coefficient is small, generally about 50% smaller. Plastic only if the temperature change of the environment will influence its dimension precision, and at constant temperature, thermal expansion can not influence the accuracy of the size of plastic.

plastic creep refers to the forming of plastic processing products in use process, due to the effect of stress and deformation of material increases gradually with the increase of time, to produce the phenomenon of permanent deformation, resulting in the dimension of plastic, influence its dimension precision. Plastic creep under stress, temperature and time of the three factors, the greater the stress, the higher the temperature, the longer the time, its corresponding creep increase.

processing conditions, appropriate control can reduce the plastic shrinkage rate, thus improve the precision of injection molding products processing. Influence of molding conditions on the products, however, no plastic itself affect the shrinkage and the precision of mould manufacturing, due to the plastic molding by the general dimensional tolerance just under a third of the total tolerance. The precision of different molding methods, manufacturing, general rules as follows, molding > > extrusion molding vacuum forming > pressure molding.

mold temperature, mold temperature control through, namely the cooling temperature, crystallization and orientation of can control products. Mold temperature and shrinkage rate, the relationship between the change in parabolic, namely there is a optimal cooling temperature, the products of the shrinkage rate, higher than the temperature and lower than the temperature, shrinkage rate is increased. Main causes of this change: higher mold temperature, melt cooling slowly, molecules are fully the relaxation time of crystallization, and thus high crystallinity, injection molding products processing more close-grained, make shrinkage rate increases, mold temperature is lower, melt cooling fast, products lower crystallinity, small shrinkage rate, but due to the cooling fast, products caused by internal stress, make shrinkage increases, the overall contract also increases.

cylinder temperature, as the cylinder temperature, shrinkage rate is falling, this is mainly because: cylinder temperature is higher, the lower the viscosity of the melt, the injection pressure increased, the pressure loss is reduced, make the products more close-grained, shrinkage rate decreases.

injection pressure, along with the increase of injection pressure, shrinkage rate of the products, but fell flat. The reason is: the injection pressure, the greater the injection molding products processing, the more dense, shrinkage rate. But the pressure is not too big, big products are prone to stress.

the holding pressure on the back pressure, its relationship with the shrinkage rate is similar with the injection pressure. With the increase of the holding pressure, shrinkage rate decline. Unlike injection pressure increased with the holding pressure, shrinkage rate down more quickly. Its principle is: with the holding pressure, more compact products. But the holding pressure is not too big, too big to produce internal stress, makes products easier to deformation and cracking.

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