The concept of "precision injection molding" is mainly different from "conventional injection molding". It is based on the rapid development of polymer materials and uses precision plastic parts to replace high-precision metal parts in the fields of instruments and electronics.
1. Features of precision injection molding:
1.1 The dimensional accuracy of the workpiece is high and the tolerance is small, that is, there is a high-precision dimensional limit;
1. 2 The weight of the product has high repeatability accuracy, requiring daily, monthly, and yearly dimensional stability;
1.3 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;
1.4 Replace conventional injection machines with precision injection machines;
1.5 Adopt precision injection molding process;
1.6 Select materials suitable for precision injection molding.
The most important technical index for evaluating products is the precision of injection molded products (dimensional tolerance, shape tolerance and product surface finish). In order to inject precise plastic products, it is necessary to strictly control four factors including material selection, mold design, injection molding process, and operator's technical level.
Precision injection molding machines require that the dimensional accuracy of the product is generally within 0.01-0.001mm, and many precision injection moldings also require injection molding machines to have high injection pressure and high injection speed;
The clamping system is required to have sufficient rigidity and high enough clamping precision. The so-called clamping precision refers to the uniformity, adjustment, stability, and high repeatability of the clamping force, and the high precision of the mold opening and closing position;
It is required to accurately control the pressure, flow rate, temperature, measurement, etc. to the corresponding accuracy, and use multi-stage or step-less injection to ensure the reproduction conditions of the molding process and the repeatability of the product size.
2. Factors affecting the dimensional accuracy of precision injection molded parts:
2.1 Mold precision;
2.2 molding shrinkage;
2.3 The temperature, humidity, and fluctuation range of the environment where the product is used.
3. Selection principles of injection molding precision molding materials
High mechanical strength, good dimensional stability, good creep resistance, and wide environmental adaptability.
There are four commonly used materials:
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 demolding by adding lubricant.
PA and glass fiber reinforced PA66, its characteristics: strong impact resistance and wear resistance, good fluidity, and can be formed into products with a wall thickness of 0.4mm. Glass fiber reinforced PA66 has heat resistance (melting point 250°C), but its disadvantage is that it is hygroscopic, and generally needs to be subjected to humidity control after molding.
PBT-reinforced polyester with short molding times. The molding time is compared as follows: PBT≤POM≈PA66≤PA6.
PC and GFPC. Features: Good wear resistance, increased rigidity after reinforcement, good dimensional stability, weather resistance, flame retardant, and good molding processability.
4. Shrinkage problem in precision injection molding
There are four factors that affect shrinkage: heat shrinkage, phase change shrinkage, orientation shrinkage, and compression shrinkage.
Heat shrinkage is an inherent thermophysical property of molding materials and mold materials. The mold temperature is high, the temperature of the product is also high, and the actual shrinkage rate will increase, so the mold temperature for precision injection should not be too high.
Phase change shrinkage is due to the shrinkage caused by the decrease in specific volume during the orientation process of crystalline resins, accompanied by the crystallization of polymers, which is called phase change shrinkage. The mold temperature is high, the crystallinity is high, and the shrinkage rate is large; but on the other hand, the increase of the crystallinity will increase the density of the product, the linear expansion coefficient will decrease, and the shrinkage rate will decrease. Therefore, the actual shrinkage rate is determined by the combined effect of the two.
Orientation shrinkage, due to the forced stretching of molecular chains in the flow direction, the macromolecules tend to recurl and recover when cooled, and shrinkage will occur 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.
Compression shrinkage and elastic reset, generally plastics are compressible. That is, the specific volume changes significantly under high pressure. At normal temperatures, 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 reset effect, which reduces the shrinkage of the product. Factors that affect product molding shrinkage are related to molding conditions and operating conditions.
5. Precision injection mold
The precision of the mold mainly depends on the high dimensional accuracy of the mold cavity, the accurate positioning of the cavity or whether the precision of the parting surface meets the requirements. Generally, the dimensional tolerance of precision injection molds should be controlled below 1/3 of the product's dimensional tolerance.
Machinability and rigidity, in the design of the mold structure, the number of cavities should not be too many, and the bottom plate, support plate, and cavity walls should be thicker to avoid severe elastic deformation of the parts under high temperature and high pressure.
In product molding, the mold should try to use as few cavities as possible, few and short runners, and a higher finish than ordinary molds, which is conducive to molding.
The material of the precision mold is alloy steel with high mechanical strength. The materials for making the cavity and the sprue must undergo strict heat treatment, and materials with high hardness (molding parts should reach about HRC52), good wear resistance and strong corrosion resistance should be selected.
6. Characteristics and selection of precision injection machine
Features in terms of technical parameters:
In terms of injection pressure, ordinary machine: 147 ~ 177MPa; precision machine: 216 ~ 243MPa; ultra-high pressure machine: 243 ~ 392MPa.
7. The reason why the precision injection molding machine must be high pressure is:
(1) To improve the precision and quality of precision products, injection pressure has the most obvious impact on the shrinkage rate of products. When the injection pressure reaches 392MPa, the molding shrinkage of the product is almost zero. At this time, the precision of the product is only affected by mold control or the environment. Experiments have proved that: after the injection pressure is increased from 98MPa to 392MPa, the mechanical strength increases by 3% to 33%.
(2) It can reduce the wall thickness of precision products and increase the molding length. Taking PC as an example, the injection pressure of the ordinary machine is 177MPa, and products with a wall thickness of 0.2-0.8mm can be formed, while the injection pressure of the precision machine is 392MPa, and products with a thickness of 0.15-0.6mm can be formed. Ultra-high-pressure injection machines can obtain products with a larger flow length ratio.
(3) Increasing the injection pressure can fully realize the effect of a muddy injection rate. To achieve the rated injection rate, there are only two ways: one is to increase the maximum injection pressure of the system; the other is to modify the screw parameters and increase the aspect ratio. The injection rate of the precision injection machine is required to be high.
8. The characteristics of the precision injection machine in terms of control:
8.1 The requirements for repeatability (reproducibility) of injection molding parameters are high, and multi-stage injection feedback control should be adopted:
a. Multi-level position control;
b. Multi-level speed control;
c. Multi-stage pressure holding control;
d. Multi-level back pressure control;
e. Multi-stage screw speed control.
The precision of the displacement sensor is required to reach 0.1mm, so that the metering stroke, injection stroke and the thickness of the remaining material pad (injection monitoring point) can be strictly controlled to ensure the accuracy of each injection and improve the molding accuracy of the product. The barrel and nozzle temperature control should be precise, the overshoot should be small when the temperature rises, and the temperature fluctuation should be small. Precision injection molding should adopt PID control, so that the temperature accuracy is preferably within ±0.5°C.
8.2 Plasticization quality requirements. The uniformity of plasticization not only affects the molding quality of injection molded parts, but also affects the resistance of the molten plastic when it passes through the gate. In order to obtain uniform plasticization, special screw and use Dedicated plasticizing technology is essential.
In addition, the temperature of the barrel should also be precisely controlled. Now the temperature of the screw and the barrel is mostly controlled by PID (proportional, differential, integral), and the accuracy can be controlled within ±1°C, which can basically meet the requirements of precision injection molding. If FUZZY control is used method, it is more suitable for precision injection molding.
8.3 The temperature control of the working oil should be high. The change of the oil temperature will cause the fluctuation of the injection pressure. It is necessary to adopt a closed-loop device for heating and cooling the working oil, and it is advisable to stabilize the oil temperature at 50-55 °C.
8.4 Influence of holding pressure, holding pressure has a great influence on precision plastic parts. To be precise, holding pressure can better feed shrinkage, reduce the deformation of plastic parts, and control plastic parts precision, and the stability of holding pressure determines plastic parts. The forming accuracy of the parts and the constant end position of the screw are the decisive factors that determine the pressure-holding effect.
8.5 Requirements for mold temperature control If the cooling time is the same, the thickness of products with low mold cavity temperature will be larger than those with high temperatures. Such as POM and PA materials, products with a thickness of 50-100 μm at a mold temperature of 50°C, the thickness is reduced to 20-40 μm at 80°C, and reduced to only 10 μm at 100°C. Room temperature also has an effect on the dimensional tolerances of precision products.
9. Hydraulic system of precision injection molding machine:
The oil circuit system requires a proportional system using proportional pressure valves, proportional flow valves, or servo variable pumps.
In the direct pressure mold clamping mechanism, the oil circuit of the clamping part is separated from the oil circuit of the injection part.
Due to the high speed of precision injection molding machines, the response speed of the hydraulic system must be emphasized.
The hydraulic system of the precision injection molding machine should fully reflect the integrated engineering of electromechanical hydraulic instruments.
Structural features of precision injection molding machine:
Due to the high injection pressure of the precision injection machine, it is necessary to emphasize the rigidity of the mold clamping system. The parallelism of moving and fixed templates is controlled within the range of 0.05-0.08mm.
It is required to protect the low-pressure mold and control the precision of the clamping force. Because the size of the mold clamping force affects the degree of mold deformation, and ultimately affects the dimensional tolerance of the workpiece.
The speed of opening and closing the mold should be fast, generally around 60mm/s.
Plasticizing components: screw, screw head, non-return ring, barrel, etc., should be designed to have a structural form with strong plasticizing ability, good homogenization degree, and high injection efficiency; the driving torque of the screw should be large, and it can be continuously variable.
No matter what kind of precision injection molding machine, it must be able to stably control the repeatability of product size and quality in the end. It is also the most important thing for every injection molding company. Our company has many years of experience in precision injection molding. Welcome to communicate.