Introduction Of Plastic Fine Injection Molding Process

1. The concept of precision injection molding

Precision injection molding refers to an injection molding method in which the dimensional repeatability of the 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 the requirements.

The concept of "precision injection molding" is mainly different from "conventional injection molding". It is based on the rapid development of polymer materials. In the fields of instruments and electronics, precision plastic parts are used to replace high-precision metal parts. At present, there are two defining indicators for precision injection products: one is the repeatability of product size, and the other is the repeatability of product quality. This paper mainly expounds precision injection molding from the aspect of product size repeatability. However, due to the different properties and processing techniques of various materials, the accuracy of plastic parts cannot be equated with the accuracy of metal parts.

The mold of the molded product is an important condition to determine whether the product can meet the dimensional tolerance required by the design, and the precision injection molding machine is the key equipment to ensure that the product is always molded within the required dimensional tolerance range and to ensure a very high yield.

The highest precision level for plastic products is level three.

1.1 Features of precision injection

(1) The dimensional accuracy of the workpiece is high and the tolerance is small, that is, there is a high-precision size limit;

(2) The weight repeatability of the product is high, and the dimensional stability of day, month and year is required;

(3) The material of the mold is good, the rigidity is sufficient, the dimensional accuracy and smoothness of the cavity and the positioning accuracy between the templates 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 the accuracy of injection molded products (dimensional tolerances, geometric tolerances and surface finish of 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 machines require that the dimensional accuracy of products is generally within 0.01 ~ 0.001mm. Many precision injection molding machines also require high injection pressure and high injection speed; the clamping system is required to have sufficient rigidity and high enough clamping accuracy. The so-called clamping accuracy refers to the uniformity, adjustment, stability and repeatability of the clamping force, and high precision of the opening and closing position; it is required to accurately control the pressure, flow, temperature, measurement, etc. to the corresponding accuracy. Step or stepless injection to ensure the reproduction conditions of the molding process and the repeatability of the product size.

1.2 Factors affecting the dimensional accuracy of products

(1) Mold accuracy;

(2) Molding shrinkage;

(3) The temperature, humidity and fluctuation range of the environment in which the product is used.

2. What is the ultimate goal?

High mechanical strength, good dimensional stability, good creep resistance and wide environmental adaptability.

There are four commonly used materials:

(1) Polyoxymethylene (POM) and carbon fiber (CF) reinforced or glass fiber (GF) reinforced POM. This material is characterized by good creep resistance, fatigue resistance, weather resistance, good dielectric properties, flame retardancy, and easy demoulding by adding lubricants.

(2) Nylon (PA) and GF enhance PA66. Features: Strong impact resistance and wear resistance, good fluidity, and can form products with a wall thickness of 0.4mm. GF reinforced PA66 has high heat resistance (melting point 250°C), its disadvantage is that it has hygroscopicity, and it is generally processed by humidity adjustment after molding.

(3) Reinforced polyester, such as polybutylene terephthalate (PBT). The molding time is short. The molding time is compared as follows: PBT≤POM≈PA66≤PA6.

(4) Polycarbonate (PC) and GF reinforced PC. Features: good wear resistance, enhanced rigidity, good dimensional stability, weather resistance, flame retardancy and good molding processability.

3. Shrinkage problems in precision injection molding

There are four factors that affect shrinkage: thermal shrinkage, phase transition shrinkage, orientation shrinkage, and compression shrinkage.

3.1 Heat shrinkage. Thermal shrinkage is a thermophysical property inherent in molding and mold materials. The temperature of the mold is high, the temperature of the product is also high, and the actual shrinkage rate will increase, so the temperature of the mold for precision injection should not be too high.

3.2 Phase transition shrinkage. During the orientation process of crystalline resin, the shrinkage caused by the reduction of specific volume along with the crystallization of the polymer is called phase transition shrinkage. The mold temperature is high, the crystallinity is high, and the shrinkage rate is large; but on the other hand, the increase in the crystallinity will increase the density of the product, reduce the linear expansion coefficient, and reduce the shrinkage rate. Therefore, the actual shrinkage rate is determined by the combined effect of the two.

3.3 Orientation shrinkage. Due to the forcible stretching of the molecular chains in the flow direction, the macromolecules have a tendency to re-curl and recover during cooling, and shrink in the orientation direction. The degree of molecular orientation is related to injection pressure, injection speed, resin temperature and mold temperature. But the main thing is the injection speed.

3.4 Compression contraction and elastic reset. All plastics are compressible. That is, the specific volume changes significantly under high pressure. Under normal temperature, the specific volume of the pressure-molded product will decrease, the density will increase, the expansion coefficient will decrease, and the shrinkage rate will decrease significantly. Corresponding to the compressibility, the molding material has an elastic restoring effect, which reduces the shrinkage of the product. The factors affecting the molding shrinkage of the product are related to the molding conditions and operating conditions.