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The process of plastic shrinkage

by:CSSSLD     2021-01-23
The holding stage of contraction, the shrinkage of the holding stage, this stage, the continuously added to the mold liquid phase, until to the gate of solidification, the causes of the shrinkage rate for cooling contraction caused by temperature drops rapidly. At this stage of shrinkage affected by the holding pressure and holding time, holding pressure, the greater the pressure, the longer the smaller is the hot and cold contraction. Cooling stage of the contract, this stage for the gate agent at the parting. In this stage, for crystalline polymer, its the reason of shrinkage are mainly caused by crystalline phase transition of contraction, crystallization make injection molding products processing internal more closely and orderly, cause a decrease in size. All factors, which affect the crystallization to influence its shrinkage rate, one of the most important for the mold temperature, mold temperature is higher, the slower cooling rate, molecular has sufficient relaxation time, crystallization tend to be more complete, high shrinkage rate. For amorphous polymers, due to no crystallization phase change, at this stage for the temperature drop of hot and cold contraction.

after the mold shrinkage, this stage for the products after stripping over a period of contraction, also known as the free shrinkage and shrinkage, and the other two also known as molding contraction. Compared with the molding shrinkage, shrinkage is generally small. The causes of shrinkage on the one hand is plastic processing products caused by internal stress relaxation function. For polymer molecular chain is rigid, products have a larger shrinkage, rigid molecular chain speed slow, this is mainly because the molecular chain to shift from one state to another state, need a longer process of rearrangement and contraction after time is longer, and the shrinkage rate is larger. Rigid molecular chain, on the other hand, easy to produce stress, stress relaxation after molding will be slowly.

the cause that causes contraction after another to crystallization. For crystalline polymer, injection molding products processing, the mold and to achieve complete crystallization, at room temperature can still crystallization, that cause the product's trace contraction. In addition, such as annealing treatment after the mold plastic products, products of stress relaxation and crystallization will further increase, causing shrinkage increases.

due to plastic shrinkage size affected by processing conditions is larger, therefore the shrinkage rate is not a fixed value, but a range. Shrinkage is not precise, changed a lot because of the different processing condition, it is difficult to measure accurately fixed value. In the specific design for a mould designer, generally can choose average shrinkage rate. Plastic water imbibition effect the precision of the use of plastic, that is, products used in damp environment, caused by suction size increased. Plastic water imbibition contraction to the influence of dimensional accuracy is better than quantity. Plastic thermal expansion coefficient and the melt flow direction, and the shrinkage is different, the longitudinal thermal expansion coefficient than lateral expansion coefficient is small, generally about 50% smaller. Thermal expansion of the plastic only in the temperature change of environment will affect the precision of its size, 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, resulting in a permanent deformation phenomenon, 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.

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