Among the related factors affecting precisioninjection molding, mold is the key to obtain precision plastic products that meet the quality requirements.
Design of mould
Whether the mold design is reasonable or not will directly affect the quality of plastic products. The mold cavity size is obtained from the required size of plastic products plus the shrinkage rate of materials used, and the shrinkage rate is often a value within a range recommended by plastic manufacturers or engineering plastics manuals, which is not only related to the mold gate form, gate position and distribution, but also related to the crystallization orientation (anisotropy) of engineering plastics, the shape and size of plastic products and the distance and position from the gate.
The main factors affecting the shrinkage of plastics include thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, which are related to the molding conditions or operating conditions of precision injection molding products.
Therefore, mold designers must have rich experience in design and injection molding, and must consider the relationship between these factors and injection conditions and their apparent factors, such as injection pressure, cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure, gate form and distribution, gate cross-sectional area, product wall thickness, reinforcing filler content in plastic materials, crystallinity and orientation of plastic materials, etc.
The influence of the above factors is also different due to different plastic materials or other molding conditions such as temperature, humidity, continuous crystallization, internal stress after molding, and changes of injection molding machine.
Because the injection molding process is a process of transforming plastics from solid (powder or granule) to liquid (melt) and solid (product). From granular materials to melt, and then from melt to products, the effects of temperature field, stress field, flow field and density field are needed. 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 factors affecting the above-mentioned "field" will definitely affect the physical and mechanical properties, size, shape, precision and appearance quality of plastic products.
In this way, the intrinsic relationship between process factors and the properties, structure and morphology of polymers and plastic products will be shown through plastic products. Analyzing these internal relations clearly is of great significance for drawing up injection molding process, designing and manufacturing molds according to drawings, and even selecting injection molding equipment reasonably. There are also differences in injection pressure and injection rate between precision injection molding and ordinary injection molding. High-pressure or ultra-high-pressure injection and high-speed injection are often used to obtain smaller molding shrinkage.
Based on the above reasons, in addition to the design elements of general molds, the following points should be considered when designing precision injection molds:
1. Adopt proper die size tolerance;
2. Prevent molding shrinkage error;
3. Prevent injection molding deformation;
4. Prevent demoulding deformation;
5. Minimize mold manufacturing errors;
6. Prevent the error of mold precision;
7. Keep the precision of the mold.
Prevent molding shrinkage error from occurring
Because the shrinkage will change due to the injection pressure, the cavity pressure in the single cavity mold should be consistent as much as possible. As for the multi-cavity mold, the cavity pressure difference between cavities should be very small. In the case of single cavity with multiple gates or multiple cavities with multiple gates, it is necessary to inject at the same injection pressure to make the cavity pressure consistent. Therefore, it is necessary to make sure that the gate position is balanced.
In order to make the cavity pressure in the cavity consistent, it is best to keep the pressure at the gate entrance consistent. The pressure balance at the gate is related to the flow resistance in the runner. Therefore, the runner should be balanced before the gate pressure is balanced.
Because melt temperature and mold temperature have an impact on the actual shrinkage rate, attention must be paid to the arrangement of mold cavities in order to determine the molding conditions. Because molten plastic brings heat into the mold, and the temperature gradient distribution of the mold is generally around the cavity, taking the shape of concentric circles with the main runner as the center.
Therefore, the design measures such as runner balance, cavity arrangement and concentric circle arrangement with the main runner as the center are necessary to reduce the shrinkage error among the cavities, expand the allowable range of molding conditions and reduce the cost. The cavity arrangement of precision injection mold should meet the requirements of balanced runner and centering on the main runner, and the cavity arrangement with the main runner as the symmetry line must be adopted.
