The basis for determining the injection molding of load-bearing belts is the accuracy of the injection molded product, namely the dimensional tolerance, positional tolerance, and surface roughness of the product. To carry out precision injection molding, there must be many related conditions, and the essence is the four basic factors of plastic material, injection mold, injection molding process, and injection molding equipment.
When designing plastic products, engineering plastic materials should be selected first, and engineering plastics that can undergo precision injection molding must choose materials with high mechanical properties, stable size, good creep resistance, and resistance to environmental stress cracking. Secondly, suitable injection molding machines should be selected based on the selected plastic material, finished product size accuracy, part weight, quality requirements, and expected mold structure. During the processing, the factors that affect precision injection molded products mainly come from the accuracy of the mold, injection shrinkage, as well as the amplitude of environmental temperature and humidity changes in the product.
In belt injection molding, molds are one of the key factors used to obtain plastic products that meet quality requirements. The molds used for belt injection molding should truly meet the requirements of product size, accuracy, and shape. But even if the accuracy and size of the mold are consistent, the actual size of the plastic products it molds will still be inconsistent due to differences in shrinkage. Therefore, effectively controlling the shrinkage rate of plastic products is crucial in precision injection molding technology.
The rationality of mold design will directly affect the shrinkage rate of plastic products. As the size of the mold cavity is obtained by adding the estimated shrinkage rate to the size of the plastic product, and the shrinkage rate is a range of values recommended by the plastic manufacturer or engineering plastic manual. It is not only related to the gate form, gate position and distribution of the mold, but also to the crystal orientation (anisotropy) of engineering plastics, the shape and size of plastic products, and the distance and position to the gate.
The main factors affecting the shrinkage rate of plastics include thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage, and elastic recovery, which are related to the molding or operating conditions of precision injection molded products.
Therefore, when designing molds, it is necessary to consider the relationship between these influencing factors and injection molding conditions, as well as their apparent factors, such as injection molding pressure and cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure and gate form and distribution, as well as the influence of gate cross-sectional area, product wall thickness, content of reinforcing filler in plastic materials, crystallinity and orientation of plastic materials, and other factors. The influence of the above factors also varies due to different plastic materials, other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in injection molding machines.
