Precision mold injection will be affected by many related factors and environmental conditions. And the most basic are the factors of plastic materials, injection molds, injection molding processes and injection molding equipment. In the early designing plastic products, first select the necessary engineering plastics according to the environment. Secondly, according to the selected plastic material, finished product dimensional accuracy, part weight, quality requirements and expected mold structure, select the appropriate injection molding machine.
The mold is an important factor
Among the related factors, molds are the key to produce precision plastic products that meet quality requirements. The design of the mold and its reasonability will directly affect the quality of plastic products.
The mold cavity size is decided by the required size of the plastic product plus the shrinkage rate of the material used. So the shrinkage rate is often a value within a range recommended by the plastic manufacturer or engineering plastics manual. It is not only related to the mold gate type, gate position and distribution. It is also related to the crystal orientation (anisotropy) of engineering plastics, the shape and size of the plastic product, and the distance and location of the gate.
The main factors that affect plastic shrinkage include heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, etc. These influencing factors are related to the molding conditions or operating conditions of precision injection molded products.
Therefore, mold designers must have rich experience in design and injection molding. And they must consider the relationship between these influencing factors, injection conditions and their apparent factors, such as:
- Injection pressure, cavity pressure and filling speed
- Injection melt temperature and mold temperature
- Mold structure and gate form and distribution
- Gate cross-sectional area, product wall thickness, reinforcing filler content in plastic materials, crystallinity and orientation of plastic materials.
The influence of the above factors is also different due to different plastic materials, or other molding conditions like temperature, humidity, continued crystallization, and changes in the injection molding machine after molding.
The injection molding process
Because the injection molding is a process of transforming plastic from solid (powder or pellets) to liquid (melt) and finally to solid (product). From the pellets to the melt, and then from the melt to the product, they must go through the temperature field, flow field and density field and be affected. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline or non-crystalline, reinforced or non-reinforced, etc.) have different polymer structures and rheological properties.
All the factors that affect the above “field” will definitely affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products.
In this way, the internal connection between process factors and polymer properties, structural morphology and plastic products will be expressed through plastic products. A clear analysis of these internal connections is of great significance to rationally formulating injection molding processes, designing and manufacturing molds according to drawings, and even selecting injection molding equipment.
Precision injection molding and ordinary injection molding are also different in injection pressure and injection rate. Precision injection molding often uses high-pressure or ultra-high-pressure injection and high-speed injection to obtain a smaller molding shrinkage rate.