Publish Time: 2021-09-21 Origin: Site
When a custom plastic part is cooled to room temperature after being removed from the mold, the shrinkage in size is called shrinkage. Since the shrinkage of the custom plastic part is not only the thermal expansion and contraction of the resin itself but also related to each molding factor, the shrinkage of a custom plastic part after molding should be called molding shrinkage. So what is included in the shrinkage of the custom plastic part? Let's take a look at it together.
This is the content list:
l Forming shrinkage in the form of forming shrinkage is mainly manifested
l Factors that affect the change of shrinkage rate
The shrinkage of the custom plastic part is due to thermal expansion and contraction, elastic recovery of the custom plastic part when it is de-molded, plastic deformation, etc., which leads to the reduction of the size of a custom plastic part after it is de-molded and cooled to room temperature, which must be considered when designing the cavity to compensate. custom plastic part Shrinkage directional molecules are arranged in the direction of forming, so that the plastic parts show anisotropy, along the direction of material flow (parallel direction), the shrinkage is large, high strength, and right-angle direction with the material flow (vertical direction), the shrinkage is small, low strength. In addition, custom plastic part molding due to the density of each part of the plastic parts and filler distribution is not uniform, so the shrinkage is also not uniform. custom plastic part shrinkage difference makes the plastic parts prone to warping, deformation, cracking, especially in extrusion and injection molding is more obvious direction. Therefore, custom plastic part mold design should consider the shrinkage direction according to the shape of the custom plastic part, the direction of the flow of material to select the shrinkage rate is appropriate. After shrinkage plastic parts forming, due to forming pressure, shear stress, anisotropy, density uneven, filler distribution uneven, uneven mold temperature, uneven hardening, plastic deformation, and other factors, causing a series of stress, in the viscous flow state cannot all disappear, so custom plastic part in the stress state forming residual stress exists. When the mold is released, due to the stress tends to equilibrium and the influence of storage conditions, the residual stress changes and makes the plastic part shrink again is called post-shrinkage. Generally custom plastic part changes the most within 10 hours after demolding, and after 24 hours it is basically set, but finally stabilized after 30-60 days. Usually, the post-shrinkage of thermoplastic is larger than that of thermosetting, and the extrusion and injection molding is larger than that of compression molding. Post-treatment shrinkage sometimes plastic parts according to the performance and process requirements, after forming the heat treatment, treatment will also lead to custom plastic part size changes. Therefore, the mold design of high-precision plastic parts should be considered after the shrinkage and post-treatment shrinkage of the error and be compensated.
Plastic varieties of various plastics have their shrinkage range, the same type of plastic filler, molecular weight and ratio of different, custom plastic part shrinkage rate and anisotropy is also different. custom plastic part characteristics of the shape, size, wall thickness, with or without inserts, the number of inserts, and layout of the shrinkage rate size also has a great impact. custom plastic part mold structure mold parting surface and pressure direction, the form of the pouring system, layout and size of the shrinkage rate, and directional impact is also greater, especially in extrusion and injection molding more obvious. custom plastic part molding process extrusion, injection molding process generally shrinkage rate is larger, directional obvious. Preheating, forming temperature, custom plastic part forming pressure, holding time, filling the form and hardening uniformity have an impact on shrinkage and directionality. custom plastic part mold design should be based on the shrinkage range provided in the specifications of various plastics, and according to the shape of the plastic part, size, wall thickness, with or without inserts, parting surface, and The shrinkage rate should be selected according to the shape, size, wall thickness, inlay or not, parting surface and pressure forming direction, custom plastic part mold structure and inlet size and location, custom plastic part molding process and other factors. For extrusion or injection molding, it is often necessary to select different shrinkage rates according to the shape, size, wall thickness, and other characteristics of each part of the custom plastic part. In addition, custom plastic part forming shrinkage is also affected by various forming factors but is mainly determined by the plastic variety, shape, and size of the plastic part. Therefore, the shrinkage of the custom plastic part can be changed by adjusting the forming conditions.
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