Precision injection molding refers to an injection molding method in which dimensional repeatability of processed injection molding products is so high that it is difficult to use a general injection molding machine and a conventional injection molding process to meet requirements.
Concept of "precision injection molding" is mainly different from "conventional injection molding". It is based on rapid development of polymer materials, technology of replacing high-precision metal parts with precision plastic parts in the fields of instruments and electronics.
At present, there are two defining indicators for precision injection products, one is repeatability of product size, and the other is repeatability of product quality.

 

This paper mainly expounds precision injection molding from aspect of product size repeatability. However, due to different properties and processing techniques of various materials, accuracy of plastic parts cannot be equated with accuracy of metal parts.
Precision injection molding is a comprehensive technology involving raw material properties, formulations, molding processes and equipment.
Precision plastic products include DVD digital discs, DVD laser heads, digital camera parts, computer connectors, light guide plates, aspherical lenses and other precision products. Yield requirements are also extremely high.
Mold of molded product is an important condition to determine whether product can meet dimensional tolerance required by design, precision injection molding machine is the key equipment to ensure that product is always molded within required dimensional tolerance range and to ensure a very high yield. The highest precision level for plastic products is level three.

(1) Dimensional accuracy of workpiece is high and tolerance is small, that is, there is a high-precision size limit;
(2) Weight repeatability of product is high, dimensional stability of day, month and year is required;
(3) Material of mold is good, rigidity is sufficient, dimensional accuracy, smoothness of cavity and positioning accuracy between mold plates are high;
(4) Use precision injection machine to replace conventional injection machine;
(5) Adopt precision injection molding process;
(6) Select materials suitable for precision injection molding.
The most important technical index for evaluating products is accuracy of injection molded products (dimensional tolerances, geometric tolerances and surface finish of products). Standard used in my country is SJ1372-78, which is very close to precision and mold precision grade of Japanese plastic products.
In order to inject precision plastic products, it is necessary to strictly control four major factors, including material selection, mold design, injection molding process, and operator's technical level.
Precision injection molding machine requires that dimensional accuracy of product is generally within 0.01 ~ 0.001mm, many precision injection molding machines also require high injection pressure and high injection speed;
Clamping system is required to have sufficient rigidity and high clamping precision. So-called clamping precision refers to uniformity, adjustment, stability and repeatability of clamping force, high precision of opening and closing positions;
It is required that pressure, flow rate, temperature, measurement, etc. can be accurately controlled to corresponding accuracy, multi-stage or stepless injection is used to ensure reproduction conditions of molding process and repeatability of product size.

(1) Mold accuracy;
(2) Molding shrinkage;
(3) Temperature, humidity and fluctuation range of environment where product is used.

 

High mechanical strength, good dimensional stability, good creep resistance and wide environmental adaptability. There are four commonly used materials:
(1) POM and carbon fiber reinforced (CF) or glass reinforced (GF); this material is characterized by good creep resistance, fatigue resistance, weather resistance, good dielectric properties, flame retardant, and easy demoulding by adding lubricants.
(2) PA and glass fiber reinforced PA66, its characteristics: strong impact resistance and wear resistance, good fluidity, and can form products with a wall thickness of 0.4mm. Glass fiber reinforced PA66 has heat resistance (melting point 250℃), but its disadvantage is that it has hygroscopicity. Generally, it must be processed by humidity adjustment after molding.
(3) PBT reinforced polyester has short molding time. Molding time comparison is as follows:
PBT≤POM≈PA66≤PA6.
(4) Features of PC and GFPC: good wear resistance, enhanced rigidity, good dimensional stability, weather resistance, flame retardancy and good molding processability.

There are four factors that affect shrinkage: thermal shrinkage, phase transition shrinkage, orientation shrinkage, and compression shrinkage
3.1 Thermal shrinkage is a thermophysical property inherent in molding materials and mold materials. Temperature of mold is high, temperature of product is also high, and actual shrinkage rate will increase, so temperature of mold for precision injection should not be too high.
3.2 Phase change shrinkage. Since crystalline resin is in process of orientation, shrinkage caused by reduction of specific volume along with crystallization of polymer is called phase change shrinkage. Mold temperature is high, crystallinity is high, and shrinkage rate is large; but on the other hand, increase in crystallinity will increase density of product, reduce linear expansion coefficient, and reduce shrinkage rate. Therefore, actual shrinkage rate is determined by combined effect of the two.
3.3 Orientation shrinkage. Due to forced stretching of molecular chain in flow direction, macromolecules have a tendency to re-curl and recover during cooling, and shrinkage will occur in orientation direction. Degree of molecular orientation is related to injection pressure, injection speed, resin temperature and mold temperature. But main thing is injection speed.
3.4 Compression shrinkage and elastic reset, generally plastics are compressible. That is, specific volume changes significantly under high pressure. Under normal temperature, specific volume of pressure-molded product will decrease, density will increase, expansion coefficient will decrease, and shrinkage rate will decrease significantly. Corresponding to compressibility, molding material has an elastic restoring effect, which reduces shrinkage of product. Factors affecting molding shrinkage of product are related to molding conditions and operating conditions.

4.1 Accuracy of mold mainly depends on whether mold cavity has high dimensional accuracy, accurate cavity positioning or whether parting surface accuracy meets requirements. Dimensional tolerance of general precision injection molds should be controlled below 1/3 of dimensional tolerance of product.
4.2 Machinability and rigidity, in mold structure design, number of cavities should not be too many, and bottom plate, support plate, cavity wall should be thicker to avoid severe elastic deformation of parts under action of high temperature and high pressure.
4.3 Demoulding property of product, mold should adopt as few cavities as possible, few and short runners and a higher finish than ordinary molds, which is conducive to demoulding.
4.4 For material of precision mold, choose alloy steel with high mechanical strength. Materials for making cavity and runner should undergo strict heat treatment, select materials with high hardness (molding parts should reach about HRC52), good wear resistance and strong corrosion resistance.