It is determined that the injection molding is based on the precision of the injection molding product, the size tolerance of the products, the tolerance of the shape and the surface roughness. For precision injection there must be many related conditions, and the most essential is the plastic material, injection mold, injection molding process and injection molding equipment, the four basic factors.

Design of plastic products, should be first selected engineering plastic material, and can perform a precision injection molding engineering plastics and must choose those high mechanical properties, dimension stability, good creep resistance, environmental stress cracking resistance of the material. The plastic injection molding machine should be chosen according to the selected plastic material, the precision of the finished product, the weight, the quality requirement and the desired mold structure. In the process of machining, the factors influencing the precision of injection molding products mainly comes from the precision of the mold, injection molding shrinkage, and the change of environmental temperature and humidity range, etc.

In load with injection molding, the mold is used to obtain the key to meet the requirements of the quality carrier with plastic products, one of the carrier with injection with mould should be practical and meets the requirement of product size and shape, precision. But even if the accuracy and size of the molds are identical, the actual size of the molded plastic products will vary by the amount of shrinkage. Therefore, it is very important to effectively control the shrinkage of plastic products.

Mould design is reasonable or not will directly affect the plastic shrinkage rate, due to the mold cavity dimension is estimated by the plastic product size and shrinkage rate is obtained, and the shrinkage is or manual of engineering plastics as recommended by the plastic manufacturer of a range of values, it not only with the mold gate forms, gate location and distribution, and with the crystal orientation of engineering plastics (anisotropic), plastic products shape, size and the position and the distance to the gate.

Influence mainly include thermal contraction of plastic shrinkage rate, phase change shrinkage, orientation, compression contraction with factors such as elastic recovery, and the influence factors and precision injection parts molding conditions or operating conditions.

Therefore, these factors must be considered when designing the mold and injection molding conditions and the relationship between apparent factors, such as injection pressure and cavity pressure and mold filling speed, melt temperature and mold temperature, injection mold structure and gate forms and distribution of products as well as the gate area, wall thickness, plastic materials, plastic materials content of reinforcing filler in the impact of factors such as crystallinity and orientation. The influence of above factors for different plastic materials, and other processing conditions such as temperature, humidity, continue to crystallization, the change of the internal stress of after molding, injection molding machine.

The process of making plastic from a solid (powder or particle) to a liquid (melt) to a solid (product). Expect the melt from grain, again by the melt to the products, through middle temperature field, stress field, flow field and density field, in the field, under the joint action of different plastic (thermoset or thermoplastic, crystalline or non-crystalline, enhanced or not enhanced, etc.) with different morphology and rheological properties of polymer structure. All the factors affecting the "field" will affect the physical and mechanical properties, size, shape, accuracy and appearance of plastic products.

In this way, the interconnections between the process factors and the properties of the polymer, the structure form and the plastic products are expressed through the plastic products. Analysis clear these inner link, to reasonably to injection molding processing technology, reasonable design and manufacture of moulds according to the drawing, and choose the injection molding processing equipment has important significance. Precision injection with normal injection molding in the injection pressure and injection rate also has difference, precision injection often adopts high voltage or super high pressure injection, high-speed injection for smaller molding shrinkage rate.

In addition to considering the design elements of general mould, the following factors should be considered as follows:

Adopting proper mould size tolerances;

To prevent the formation shrinkage error;

To prevent injection molding;

To prevent deform deformation;

To minimize the manufacturing error of the die.

To prevent the accuracy of the molds;

The system keeps the mould accuracy.