The hottest precision injection molding solution

Precision injection molding solution

1 concept of precision injection molding

precision injection molding refers to an injection molding method in which the dimensional repetition accuracy of injection molded products is so high that it is difficult to meet the requirements using general injection molding machines and conventional injection molding processes. The concept of "precision injection molding" is mainly different from "conventional injection molding". Based on the rapid development of polymer materials, it is a technology that uses precision plastic parts to replace high-precision metal parts in the field of instruments and electronics. At present, there are two defining indicators for precision injection products, one is the repetition accuracy of product size, and the other is the repetition accuracy of product quality. This paper mainly expounds precision injection molding from the aspect of product size repetition accuracy. However, due to the different properties and processing technologies of various materials, the accuracy of plastic parts cannot be equated with that of metal parts

precision injection molding is a comprehensive technology involving raw material performance, formula, molding process and equipment. Precision plastic products include DVD digital discs, DVD laser heads, digital camera parts, computer connectors, light guide plates, aspheric lenses and other precision products. The remarkable feature of these products is that they not only have high dimensional accuracy requirements, but also have high requirements for the internal quality and yield of products. 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 and ensure a very high yield. The highest accuracy level of plastic products is level 3

1.1 characteristics of precision injection

(1) the dimensional accuracy of the workpiece is high and the tolerance is small, that is, there is a high-precision dimensional limit; (2) The weight repetition accuracy of products is high, and the dimensional stability of day, month and year is required; (3) The material of the mold is good, the rigidity is enough, the dimensional accuracy and finish of the cavity and the positioning accuracy between the templates are high; (4) Replace conventional injection machine with precision 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 tolerance, geometric tolerance and product surface finish). The standard used in China is sj1372-78, which is very close to the accuracy grade of Japanese plastic products and molds. In order to produce precise plastic products, it is necessary to strictly control four factors: material selection, mold design, injection molding process, and the technical level of operators

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 enough rigidity and high clamping precision. The so-called clamping precision refers to the uniformity, adjustability, stability and repeatability of clamping force, and the precision of opening and closing position; It is required that the pressure, flow, temperature, measurement, etc. can be accurately controlled to the corresponding accuracy, and multi-stage or stepless injection is adopted to ensure the reproduction conditions of the molding process and the repeated accuracy 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 selection principle of injection precision molding materials

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 (g, otherwise cavitation phenomenon f will occur); This material is characterized by good creep resistance, fatigue resistance, weather resistance, good dielectric properties, fire resistance, and easy demoulding with lubricant

(2) Pa and glass fiber reinforced PA66 are characterized by strong impact resistance and wear resistance, good flow performance, and can form products with a wall thickness of 0.4mm. Glass fiber reinforced PA66 has heat resistance (melting point 250 ℃), and its disadvantage is that it has hygroscopicity. Generally, it should be treated by humidity regulation after molding

(3) PBT reinforced polyester, with short molding time. The comparison of forming time is as follows: PBT ≤ POM ≈ PA66 ≤ PA6

(4) characteristics of PC and gfpc: good wear resistance, increased rigidity after reinforcement, good dimensional stability, good weather resistance, fire resistance and molding processability

3 shrinkage problem in precision injection molding

there are four factors affecting shrinkage: thermal shrinkage, phase change shrinkage, orientation shrinkage and compression shrinkage

3.1 thermal shrinkage is an inherent thermophysical property of molding materials and mold materials. The mold temperature is high, the product temperature is also high, and the actual shrinkage will increase, so the mold temperature of precision injection should not be too high

3.2 phase transition shrinkage is the shrinkage caused by the reduction of specific volume due to the crystallization of polymer during the orientation of crystalline resin, which is called phase transition shrinkage. High mold temperature, high crystallinity and high shrinkage; On the other hand, the increase of crystallinity will increase the density of products, reduce the coefficient of linear expansion and the shrinkage rate. Therefore, the actual shrinkage is determined by the combined action of the two

3.3 orientation contraction, due to the forced stretching of the molecular chain in the flow direction, the macromolecules have a tendency to re curl 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

3.4 compression shrinkage and elastic reduction, general plastics have compressibility. That is, the specific volume changes significantly under high pressure. Under normal temperature, increasing the specific volume of pressure molded products will reduce, the density will increase, the expansion coefficient will decrease, and the shrinkage will decrease significantly. Corresponding to the compressibility, the molding material has the function of elastic reset, which reduces the shrinkage of the product. The factors affecting the molding shrinkage of products are related to the molding conditions and operating conditions

4 precision injection mold

4.1 mold accuracy mainly depends on whether the mold cavity dimension accuracy is high, the cavity positioning accuracy or the parting surface accuracy meets the requirements. Generally, the dimensional tolerance of precision injection mold should be controlled below 1/3 of the dimensional tolerance of WIP

4.2 machinability and rigidity. In the mold structure design, the number of cavities should not be too large, while the bottom plate, support plate and cavity bentler Sigri will develop in the direction of self-control production line, and the wall should be thicker, so as to avoid severe elastic deformation of parts under high temperature and high pressure

4.3 for the demoulding property of products, the mold should adopt as few cavities as possible, fewer and shorter flow channels, and have a higher finish than ordinary molds, which is conducive to demoulding

4.4 alloy steel with high mechanical strength shall be selected as the material of precision die. The materials used to make the cavity and sprue shall be subject to strict heat treatment, and the materials with high hardness (the molding parts shall reach about hrc52), good wear resistance and strong corrosion resistance shall be selected

5 characteristics and selection of precision injection machine

5.1 characteristics of technical parameters

from the aspect of injection pressure, ordinary machine: 147 ~ 177mpa; Precision machine: 216 ~ 243mpa; Ultra high pressure machine: 243 ~ 392 MPa. The reason why precision injection molding machines must be high-pressure is:

(1) improve the precision and quality of precision products, and the injection pressure has the most obvious effect on the molding shrinkage of products. When the injection pressure reaches 392mpa, the molding shrinkage of the product is almost zero. At this time, the accuracy of the product is only affected by the mold control or the environment. Experiments show that the mechanical strength increases by 3% - 33% when the injection pressure is increased from 98mpa to 392mpa

(2) it can reduce the wall thickness of precision products and improve the molding length. Taking PC as an example, ordinary machine can mold products with a wall thickness of 0.2 ~ 0.8mm with an injection pressure of 177mpa, while precision machine can mold products with a thickness of 0.15 ~ 0.6mm with an injection pressure of 392mpa. Ultra high pressure injection machine can obtain products with larger flow length ratio

(3) increasing the injection pressure can fully muddy the injection rate. There are only two ways to achieve the rated injection rate: one is to increase the maximum injection pressure of the system; The second is to reform the screw parameters and improve the length diameter ratio. Precision injection machine requires high injection rate. Taking Demag precision injection molding machine (60 ~ 420 tons) made in Germany as an example, its injection speed can reach 1000mm/s, and the screw can obtain an acceleration of 12m/s2

5.2 characteristics of precision injection machine in control

(1) it requires high repetition accuracy (reproducibility) of injection molding parameters, and multi-stage injection feedback control should be adopted: A. multi-stage position control; b. Multistage speed control; c. Multistage pressure maintaining control; d. Multi stage back pressure control; e. Multi stage screw speed control. The accuracy of the displacement sensor is required to reach 0.1mm, which can strictly control the metering stroke, injection stroke and the thickness of the residual pad (injection monitoring point), ensure the accuracy of each injection volume and improve the molding accuracy of the product. The temperature control of the barrel and nozzle should be accurate, the overshoot should be small when heating up, and the temperature fluctuation should be small. Precision injection molding should adopt PID control, so that the temperature accuracy is between ± 0.5 ℃

(2) plasticization quality requirements. The uniformity of plastic plasticization not only affects the molding quality of injection molded parts, but also affects the resistance of molten plastic when it passes through the gate. In order to obtain uniform plasticization, it is necessary to design special screws and use special plasticization technology. In addition, the temperature of the barrel should also be accurately controlled. Now the screw and barrel temperature are mostly controlled by PID (proportion, differentiation and integration), and the accuracy can be controlled within ± 1 ℃, which can basically meet the requirements of precision injection molding. If fuzzy control method is used, it is more suitable for precision injection molding

(3) the temperature of working oil should be controlled. The change of high oil temperature leads to the fluctuation of injection pressure. The closed-loop device of heating and cooling must be used for working oil, and the oil temperature should be stabilized at 50 ~ 55 ℃

(4) the influence of holding pressure. Holding pressure has a great influence on the precision of plastic parts. To be precise, holding pressure can better feed, reduce the deformation of plastic parts, and control the precision of plastic parts. The stability of holding pressure determines the molding precision of plastic parts, and the constant end position of screw is the decisive factor to determine the effect of holding pressure

(5) for mold temperature control requirements, if the cooling time is the same, the thickness of products with low mold cavity temperature is larger than that of products with high temperature. For example, for POM and PA materials, the thickness is 50 ~ 100 when the mold temperature is 50 ℃ μ M, and the thickness is reduced to 20 ~ 40 at 80 ℃ μ m. Reduced to only 10 at 100 ℃ μ m。 Room temperature also affects the dimensional tolerance of precision products

5.3 hydraulic system of precision injection molding machine

(1) the oil circuit system needs to adopt the proportional system of proportional pressure valve, proportional flow valve or servo variable pump. (2) In the direct pressure clamping mechanism, the oil circuit of the clamping part is separated from the oil circuit of the injection part. (3) Due to the high speed of precision injection molding machine, the reaction speed of hydraulic system must be emphasized. (4) The hydraulic system of precision injection molding machine should fully embody the engineering of machine electric hydraulic instrument integration

5.4 structural characteristics of precision injection molding machine

(1) due to the high injection pressure of precision injection molding machine, it is necessary to emphasize the stiffness of clamping system. The parallelism of movable and fixed formwork shall be controlled within the range of 0.05 ~ 0.08mm

(2) it is required to protect the low-pressure die and control the accuracy of the clamping force. Because the clamping force will affect the degree of mold deformation, and ultimately affect the dimensional tolerance of the workpiece

(3) the mold opening and closing speed should be fast, generally about 60mm/s

(4) plasticizing parts: screw, screw head, check ring, barrel, etc. should be designed into a structural form with strong plasticizing ability, good homogenization degree and high injection efficiency; Screw drive torque