Precision injection molding will be affected by many related factors and environmental conditions, and the most basic are four basic factors of plastic materials, injection molds, injection molding process and injection molding equipment. In the early stage of designing plastic products, engineering plastics with corresponding performance requirements should be selected according to application environment.

 

Secondly, appropriate injection molding machine should be selected according to selected plastic material, finished product dimensional accuracy, part weight, quality requirements, and expected mold structure.
Among related factors that affect precision injection molding, molds are key to obtaining precision plastic products that meet quality requirements.

Whether mold design is reasonable will directly affect quality of plastic products. Since mold cavity size is derived from required size of plastic product plus shrinkage rate of material used, shrinkage rate is often a value within a range recommended by plastic manufacturer or engineering plastics manual. It is not only related to mold gate type, gate location and layout of mold, but also related to crystal orientation (anisotropy) of engineering plastics, shape and size of plastic product, distance and location of gate.
Main factors affecting plastic shrinkage include thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, etc. These influencing factors are related to molding conditions or operating conditions of precision injection molded products.
Therefore, mold designer must have rich design and injection molding experience, must consider relationship between these influencing factors and injection conditions, their apparent factors, such as injection pressure, cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure, gate form and distribution, as well as gate cross-sectional area, product wall thickness, reinforcing filler content in plastic materials, crystallinity and orientation of plastic materials.
Influence of above factors is also different due to different plastic materials, or other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in injection molding machines.
Because injection molding process is a process of transforming plastic from solid (powder or pellets) to liquid (melt) to solid (product). From pellets to melt, then from melt to product, temperature field, stress field, flow field, and density field must be affected in the middle. Under combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline or amorphous, reinforced or non-reinforced, etc.) have different polymer structural morphologies and rheological properties.
All factors that affect above-mentioned "field" will definitely affect physical and mechanical properties, size, shape, precision and appearance quality of plastic products.

 

In this way, 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 process, rationally designing and manufacturing molds according to drawings, and even rationally 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.
Based on above reasons, in addition to general mold design elements when designing precision injection molds, following points must be considered:
1 Use appropriate mold size tolerances;
2 Prevent molding shrinkage error;
3 Prevent injection deformation;
4 Prevent demoulding deformation;
5 Minimize mold manufacturing errors;
6 Prevent errors in mold accuracy;
7 Maintain mold accuracy.　　
Prevent errors in molding shrinkage
Since shrinkage rate will change due to injection pressure, for a single cavity mold, cavity pressure in cavity should be as consistent as possible. As for multi-cavity mold, cavity pressure between cavities should be very small. In the case of single cavity with multiple gates or multiple cavity with multiple gates, injection must be made with same injection pressure to make cavity pressure consistent. For this reason, it is necessary to ensure that gate positions are balanced.
In order to make cavity pressure consistent in cavity, it is best to keep pressure at gate entrance consistent. Pressure balance at gate is related to flow resistance in runner. Therefore, before gate pressure reaches equilibrium, runner should be balanced.
Because melt temperature and mold temperature have an effect on actual shrinkage rate, when designing a precision injection mold cavity, in order to facilitate determination of molding conditions, attention must be paid to arrangement of cavity. Because molten plastic brings heat into mold, temperature gradient distribution of mold generally surrounds cavity, which is in the shape of concentric circles with main runner as center.
Therefore, design measures such as runner balance, cavity arrangement and concentric arrangement centered on the sprue are necessary to reduce shrinkage error between cavities, expand allowable range of molding conditions, and reduce costs. Cavity arrangement of precision injection mold should meet requirements of balance of runner and arrangement with main runner as center, and cavity arrangement with main runner as line of symmetry must be adopted.