1, the concept of precision injection molding
Precision injection molding refers to the processing of molded injection molded parts with high repeatability, resulting in the use of universal injection molding machine, conventional injection molding process is difficult to meet the requirements of an injection molding method. The concept of ' precision injection molding ' is mainly distinguished from ' conventional injection molding ', which is based on the rapid development of polymer materials, and the use of precision plastic parts in instrumentation and electronics to replace high-precision metal parts technology. At present, there are two defining indexes for precision Injection products: one is the repetition precision of the product size and the other is the repeatability of the product quality. This paper mainly elaborates the precision injection molding from the aspect of the product dimension repetition precision.
However, due to the different nature of various materials and processing technology, the precision of plastic parts can not be equated with the precision of metal components.
The mold of the molded article is to determine whether the product can meet the design requirements of the dimensional tolerances of the important conditions, and precision injection molding machine is to ensure that the product is always in the required size tolerance within the molding, and ensure very high yield of the key equipment.
The highest accuracy grade for plastic products is three levels.
1.1 Characteristics of precision Injection
(1) The size of the workpiece is high precision, small tolerances, that is, high-precision size limits;
(2) The product weight repetition precision is high, the requirement has the day, the month, the year's dimensional stability;
(3) Mold material good, rigid foot, the dimensional accuracy of the cavity, the smoothness and the positioning accuracy between the template is high;
(4) The use of precision injection machine to replace the conventional injection machine;
(5) The use of precision injection molding process;
(6) Select materials suitable for precision injection molding. The most important technical index for evaluating products is the precision of injection molded products (dimensional tolerances, geometric tolerances and surface finish of the products).
To inject precise plastic products, we need to strictly control 4 factors, such as material selection, mold design, injection molding technology, operator's technical level and so on. Precision Injection molding machine requirements of the product size accuracy generally within 0.01~ 0.001mm, many precision injection molding machine also requires high injection pressure, high injection speed, requiring a clamping system with sufficient rigidity and high enough clamping accuracy, the so-called clamping accuracy is the uniformity of the clamping force, adjustable, stable and high repeatability,
Opening and closing die position accuracy is high, the requirements for pressure, flow, temperature, metering and so can be accurately controlled to the corresponding accuracy, the use of multistage or stepless injection, to ensure that the molding process reproduction conditions and product size repeatability and so on.
1.2 Factors affecting the dimensional accuracy of products
(1) Mold accuracy;
(2) Molding shrinkage rate;
(3) The temperature, humidity and fluctuation amplitude of the product's use environment.
2. What is the ultimate goal?
High mechanical strength, good dimensional stability, good creep resistance and a wide range of environmental adaptability.
There are four kinds of materials commonly used:
(1) Pom (POM) and carbon fiber (CF) reinforced or glass fiber (GF) reinforced poly. This material is characterized by good creep resistance, fatigue resistance, weathering resistance, dielectric properties are good, difficult to burn, adding lubricants easy demoulding.
(2) Nylon (PA) and GF reinforced PA66. Features: Strong impact resistance and wear resistance, good flow performance, can be molded 0.4mm wall thickness of the products. GF Reinforced PA66 has a high heat resistance (melting point 250 ℃), the disadvantage is that it has hygroscopic, generally after the molding through the wet treatment.
(3) Reinforced polyester, such as Polybutylene terephthalate (PBT) . Short molding time. The molding time is compared as follows: Pbt≤pom≈pa66≤pa6.
(4) Polycarbonate (PC) and GF Enhanced PC. Features: Good abrasion resistance, enhanced rigidity, good dimensional stability, weathering resistance, flame retardant and molding processability.
3, shrinkage problem in precision injection molding
There are four factors that affect shrinkage: Heat contraction, phase change shrinkage, orientation contraction, and compression contraction.
3.1 Heat shrink. Thermal shrinkage is the inherent thermal physical properties of molded materials and die materials. Mold temperature is high, the product temperature is also high, the actual shrinkage rate will increase, so the precision injection mold temperature should not be too high.
3.2 Phase change shrinkage. As the crystalline resin in the orientation process, with the crystallization of the polymer, less than the reduction of shrinkage caused by contraction, that is called phase change contraction. The mold temperature is high, the crystallinity is high, the shrinkage rate is big; on the other hand, the increase of crystallinity can increase the density of products, decrease the coefficient of linear expansion and decrease the shrinkage rate. The actual shrinkage rate is therefore determined by the combination of the two.
3.3 Orientation contraction. Due to the forced stretching of the molecular chain in the direction of flow, the large molecule in the cooling time has a tendency to re-curl, and in the orientation direction will produce contraction. The degree of molecular orientation is related to injection pressure, injection speed, resin temperature and mold temperature. But the main is the injection speed.
3.4 Compression contraction with elastic reset. General Plastics are compressible. That is, under high pressure, the specific capacitance changes significantly. Under the general temperature, the increase of the pressure molding product's specific capacity will be reduced, the density will increase, the expansion coefficient will decrease, the shrinkage rate can be significantly reduced. corresponding to compressibility, the molded material has an elastic reduction effect, which reduces the shrinkage of the product. The factors influencing the molding shrinkage of products are related to the forming conditions and operating conditions. Plastic Precision Injection molding concepts, characteristics, materials and shrinkage problems