Most of plastic moulding parts of car are injection-molded. Due to high design precision of these plastic moulding parts, conventional injection mold making cannot be used for these plastic moulding parts, and precision injection mold making technology must be adopted. In order to ensure performance, quality and reliability of precision plastic moulding parts of car, plastic injection molding products with high quality and meeting design requirements, plastic materials, injection mold making equipment, molds and plastic molding process must be continuously improved.

 

Basis for determining precision injection molding is precision of plastic injection molding products, that is, dimensional tolerance, shape tolerance and surface roughness of product. There are many relevant conditions for precision injection molding, and the most essential are four basic factors of plastic materials, injection molds, plastic molding process and injection mold making equipment. When designing plastic injection molding products, engineering plastic materials should be selected first, and engineering plastics capable of precision injection molding must use materials with high mechanical properties, dimensional stability, good creep resistance and environmental stress cracking resistance. Secondly, appropriate injection mold making machine should be selected according to selected plastic material, dimensional accuracy of finished product, weight of part, quality requirements and expected mold structure. During processing, factors affecting precision injection molding products mainly come from precision of mold, shrinkage of injection mold making, variation of ambient temperature and humidity of product.
In precision injection molding, mold is one of the keys to obtain precision plastic products that meet quality requirements. Molds for precision injection molding should meet requirements of product size, precision and shape. However, even if precision and size of mold are same, actual size of molded plastic article may be inconsistent due to difference in the amount of shrinkage. Therefore, effective control of shrinkage of plastic injection molding products is very important in precision injection molding technology.
Whether mold design is reasonable will directly affect shrinkage rate of plastic injection molding products. Since mold cavity size is determined by size of plastic injection molding products plus estimated shrinkage rate, shrinkage rate is a range of values recommended by plastics manufacturer or engineering plastics manual. It is not only related to gate form of mold, location and distribution of gate, but also related to crystal orientation of engineering plastic, shape and size of plastic moulding parts, distance and position of gate. Main factors affecting plastic shrinkage are heat shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery. These factors are related to molding conditions or operating conditions of precision injection molding products. Therefore relationship between these influencing factors, injection mold making conditions and their apparent factors must be considered when designing mold, such as injection pressure, cavity pressure, filling speed, injection melt temperature, mold temperature, mold structure, gate form and distribution, as well as gate cross-sectional area, wall thickness of product, content of reinforcing filler in plastic material, crystallinity and orientation of plastic material. Influence of above factors is also different due to different plastic materials, other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in injection mold making machine.

 

Because plastic molding process is process of converting plastic from solid powder or pellets to liquid melt and solid products. From pellets to melt, from melt to product, through temperature field, stress field, flow field and density field, different plastic thermoset or thermoplastic, crystalline or amorphous, reinforced or non-reinforced, etc. have different polymer structural morphology and rheological properties under combined action of these fields. Any factors affecting above field will affect physical and mechanical properties, size, shape, precision and appearance quality of plastic injection molding products. Thus, intrinsic link between process factors and polymer properties, structural morphology, and plastics is manifested by plastics. It is important to analyze these intrinsic links to rationally formulate injection mold making processes, rationally design and manufacture molds according to drawings, and even select injection mold making equipment. Precision injection molding and ordinary injection mold making also differ 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 small molding shrinkage. In view of above various reasons, in addition to considering the design elements of general mold, following points must be considered when designing precision injection molding: 1 use appropriate mold dimensional tolerance; 2 prevent mold shrinkage error; 3 prevent injection mold making deformation; 4 Prevent demoulding deformation; 5 minimize mold manufacturing errors; 6 prevent mold accuracy errors; 7 maintain mold accuracy; 8 Prevent molding shrinkage error.
Since shrinkage rate changes due to injection pressure, for a single cavity mold, cavity pressure in cavity should be as uniform as possible; as for multi-cavity mold, cavity pressure between cavities should be small. In the case of single cavity multi-gate or multi-cavity multi-gate, it must be injected at the same injection pressure to make cavity pressure uniform. To do this, you must ensure that gates are balanced. In order to make cavity pressure in cavity uniform, it is preferable to keep pressure at the gate entrance consistent. Equilibrium of pressure at the gate is related to flow resistance in runner. Therefore, before gate pressure reaches equilibrium, flow should be balanced first. Since melt temperature and mold temperature have an effect on actual shrinkage rate, in order to facilitate determination of molding conditions, it is necessary to pay attention to arrangement of cavities when designing precision injection molding cavity.