Mold injection: What causes the precision injection molding of the mold?

December 24, 2021

Precision injection molding is affected by many related factors and environmental conditions, and the most basic ones are the basic factors of plastic materials, injection molds, injection molding processes and injection molding equipment. In the early stage of designing plastic products, first select the engineering plastics that can be needed according to their environment. Secondly, the appropriate plastic injection molding machine is selected for the selected plastic material, finished product dimensional accuracy, workpiece weight, quality requirements and expected mold structure.

Among the factors affecting precision injection molding, molds are the key to obtaining precision plastic products that meet quality requirements.

The design of the mold, whether the mold design is reasonable will directly affect the quality of plastic products. Since the mold cavity size is derived from the required size of the plastic product plus the shrinkage of the material used, the shrinkage rate is often a value within the range recommended by the plastics manufacturer or engineering plastics manual, not only with the gate form of the mold. The position of the gate is related to the distribution, and is related to the crystal orientation (anisotropic) of the engineering plastic, the shape and size of the plastic product, and the distance and position of the gate.

The main factors affecting plastic shrinkage include heat 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 articles. Therefore, the mold designer must have rich experience in design and injection molding, and must consider the relationship between these influencing factors and injection molding conditions and their apparent factors, such as injection pressure and cavity pressure and filling speed, injection melt temperature and mold temperature. , mold structure and gate form and distribution, as well as the cross-sectional area of the gate, the thickness of the product, the content of reinforcing filler in the plastic material, the crystallinity and orientation of the plastic material and other factors. The influence of the above factors also differs depending on the plastic material or other molding conditions such as temperature, humidity, continued crystallization, and changes in the injection molding machine after molding.

Because the injection molding process is the process of converting plastic from solid (powder or pellet) to liquid (melt) to solid (product). From the pellet to the melt, and then from the melt to the product, the temperature field, flow field and density field are used.

Under the combined action of these fields, different plastics (thermoset or thermoplastic, crystalline or amorphous, reinforced or unreinforced, etc.) have different polymer morphological and rheological properties. Any influence on the above factors will affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products.