Precision injection molding typically refers to the appearance accuracy of injection molded products meeting strict dimensional tolerances, form and position tolerances, and surface roughness.
Many relevant conditions must exist to perform precision injection molding. The most important are the four basic factors of plastic materials, injection molds, injection molding process, and injection molding equipment.
Engineering plastic materials should be chosen first when designing plastic products, and engineering plastics that can be precision injection molded must choose materials with high mechanical properties, dimensional stability, good creep resistance, and environmental stress cracking resistance.
Secondly, the appropriate injection molding machine should be selected according to the plastic material chosen, finished dimensional product accuracy, part weight, quality requirements, and expected mold structure.
In the process of processing, the factors that affect precision injection molded products mainly come from the temperature of the mold, the control of the injection molding process, the range of changes in the environmental temperature and humidity of the production site, and the annealing treatment of the late product.
In terms of precision injection molding, the mold is one of the keys to obtaining precision plastic products that meet quality requirements. The mold for precision injection molding should meet the product size, accuracy, and shape requirements, and the mold material should be carefully chosen. However, even if the precision and size of the mold are the same, the actual size of the molded plastic products will vary due to shrinkage differences. As a result, controlling the shrinkage rate of plastic products in precision injection molding technology is critical.
The shrinkage rate of the plastic product is directly affected by whether the injection mold design is reasonable or not. The shrinkage rate is determined by the plastic manufacturer or the engineering plastics manual because the mold cavity size is calculated by adding the estimated shrinkage rate to the size of the plastic product. The recommended value within a range is affected not only by the gate form, gate position, and mold distribution, but also by the crystal orientation (anisotropy) of engineering plastics, the shape and size of the plastic product, and the distance to the gate. It is related to the location as well as the mold cooling distribution system.
The main factors that affect plastic shrinkage are thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage, and elastic recovery. These influencing factors are related to the molding conditions or operating conditions of precision injection molded products.
As a result, when designing the mold, the relationship between these influencing factors and injection conditions, as well as their appearance factors, such as injection pressure and cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure, and gate form and distribution, and the influence of factors such as gate cross-sectional area, product wall thickness, reinforcing filler content in plastic materials, crystallinity a must be considered.
Because of differences in plastic materials, other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in the injection molding machine, the influence of the above factors varies.