Precision injection molding, as commonly understood, refers to injection-molded products that meet stringent requirements for dimensional tolerances, geometric tolerances, and surface roughness. Achieving precision injection molding requires many contributing factors, but the most essential are the four elements of plastic raw materials, injection molds, injection molding process, and injection molding equipment (injection molding machine).
Precision injection-molded plastics
Plastics suitable for precision injection molding should possess characteristics such as high mechanical strength, good dimensional stability, excellent creep resistance, and a wide range of environmental adaptability. The following four types of plastics are commonly used for precision injection molding:
① POM and carbon fiber (CF) reinforced POM or glass fiber (GF) reinforced POM. This material is characterized by good creep resistance, fatigue resistance, weather resistance, and dielectric properties, and is flame-retardant. The addition of lubricants facilitates mold release.
② PA and glass fiber reinforced PA66. Features: strong impact resistance and wear resistance, good flow properties, and can be molded into products with a wall thickness of 0.4mm. Glass fiber reinforced PA66 has good heat resistance (melting point 250℃), but its disadvantage is hygroscopicity; generally, it requires moisture conditioning after molding.
③ PBT reinforced polyester. Features: short molding cycle. The molding time comparison is as follows: PBT ≤ POM ~ PA66 ≤ PA6.
④ PC and glass fiber reinforced PC. Features: good wear resistance, improved rigidity after reinforcement, good dimensional stability, good weather resistance, flame retardancy, and good moldability.
Precision injection molding molds
The molds used for precision injection molding should possess the following characteristics:
① High mold accuracy.
This primarily depends on whether the mold cavity dimensions, cavity positioning, or parting surface accuracy meet the requirements.
② Good machinability and rigidity.
In mold structure design, the number of cavities should not be excessive, and the base plate, support plate, and cavity walls should be thicker to prevent severe elastic deformation of the parts under high temperature and high pressure.
③ Good product demolding performance.
The mold should ideally have a small number of cavities, fewer and shorter runners, and a higher surface roughness than ordinary molds, which facilitates demolding.
④ Steel for precision molds.
High-strength alloy steel should be selected; the materials used for the cavities and runners must undergo strict heat treatment, and materials with high hardness (forming parts should reach approximately 52 HRC), good wear resistance, and strong corrosion resistance should be chosen.
Precision injection molding machine
Characteristics in terms of technical parameters
Based on injection pressure, conventional injection molding machines operate at 147–177 MPa; precision injection molding machines operate at 216–243 MPa; and ultra-high-pressure injection molding machines operate at 243–392 MPa. Precision injection molding machines must use high-pressure injection molding machines for the following reasons:
①
To improve the accuracy and quality of precision products, as injection pressure has the most significant impact on the molding shrinkage rate of the product. When the injection pressure reaches 392 MPa, the molding shrinkage rate of the product is almost zero. At this point, the accuracy of the product is only affected by the mold or the environment. Tests have shown that increasing the injection pressure from 98 MPa to 392 MPa increases mechanical strength by 3%–33%.
②
To reduce the wall thickness of precision products and increase the molding length. Taking PC as an example, a conventional machine with an injection pressure of 177 MPa can mold products with a wall thickness of 0.2–0.8 mm, while a precision machine with an injection pressure of 392 MPa can mold products with a thickness between 0.15 and 0.6 mm. Ultra-high-pressure injection molding machines can produce products with a larger flow length ratio.
③
Increasing the injection pressure can fully utilize the effectiveness of the injection speed. To achieve the rated injection speed, there are only two ways: one is to increase the maximum injection pressure of the system; the other is to modify the screw parameters to increase the length-to-diameter ratio. Precision injection molding machines require high injection speeds. Taking the German-made DEMAG precision injection molding machine (60–420t) as an example, its injection speed can reach 1000 mm/s, and the screw can achieve an acceleration of 12 m/s².
Characteristics of precision injection molding machines in terms of control
① Requirements for the repeatability (reproducibility) of injection molding parameters.
② Requirements for plasticization quality.
③ Temperature control of the working hydraulic oil.
④ Requirements for holding pressure.
⑤ Requirements for mold temperature control.
The hydraulic (oil circuit) system of a precision injection molding machine
- ① The hydraulic system needs to utilize proportional components such as proportional pressure valves, proportional flow valves, or servo variable pumps.
- ② In direct-pressure clamping mechanisms, the hydraulic circuits for the clamping and injection parts should be separated.
- ③ Due to the high speed of precision injection molding machines, the response speed of the hydraulic system must be increased.
- ④ The hydraulic system of precision injection molding machines should fully embody the integrated engineering of mechanical, electrical, hydraulic, and instrumentation systems.
Structural characteristics of precision injection molding machines
① Due to the high injection pressure of precision injection molding machines, the rigidity of the clamping system is crucial, and the parallelism of the moving and stationary platens must be controlled within the range of 0.05 to 0.08 mm.
② Low-pressure molds must be protected, and the accuracy of the clamping force must be strictly controlled, as the magnitude of the clamping force affects the degree of mold deformation, ultimately impacting the dimensional tolerances of the molded parts.
③ The mold opening and closing speed should be fast, generally around 60 mm/s.
④ Plasticizing components: The screw, screw head, non-return ring, and barrel should adopt a structural form that provides strong plasticizing ability, good homogenization, and high injection efficiency; the screw drive torque should be large and capable of stepless speed regulation. Based on this, precision injection molding machines often utilize modular units to adapt to the production needs of different precision plastic parts, as shown in the figure.
We know that, regardless of the type of precision injection molding machine, it must ultimately be able to stably control the dimensional repeatability and quality repeatability of the molded products.
Major manufacturers of precision injection molding machines
In the field of precision injection molding machine development, leading manufacturers representing the current world's advanced level include German companies such as KraussMaffei, Demag, and Arburg, as well as Japanese companies such as Nissei, JSW, and Sumitomo Heavy Industries. Among them, the precision injection molding machine launched by Arburg of Germany features a box-type design for its clamping mechanism, which significantly improves clamping accuracy.
Shrinkage problems in precision injection molded plastic parts.
There are four factors that affect the shrinkage of molded plastic parts: thermal shrinkage, phase transition shrinkage, orientation shrinkage, and compression shrinkage.
The influence of each shrinkage factor on precision injection molding is analyzed below:
① Thermal shrinkage
② Phase transition shrinkage
③ Orientation shrinkage
④ Compression shrinkage
① Thermal shrinkage: Thermal shrinkage is an inherent thermophysical characteristic of the molding material and mold material. Higher mold temperatures result in higher product temperatures, leading to increased actual shrinkage. Therefore, the mold temperature should not be too high in precision injection molding.
② Phase transition shrinkage: In crystalline resins, during the orientation process, the crystallization of macromolecules occurs, causing shrinkage due to a decrease in specific volume. This is called phase transition shrinkage. Higher mold temperatures lead to higher crystallinity and greater shrinkage; however, on the other hand, increased crystallinity increases the density of the product, reduces the coefficient of linear expansion, and thus reduces shrinkage. Therefore, the actual shrinkage rate is determined by the combined effect of these two factors.
③ Orientation shrinkage: Due to the forced stretching of molecular chains in the flow direction, the macromolecules tend to re-coil and recover during cooling, resulting in shrinkage in the orientation direction. The degree of molecular orientation is related to injection pressure, injection speed, resin temperature, and mold temperature, but mainly to the injection speed.
④ Compression shrinkage: Most plastics are compressible, meaning that their specific volume changes significantly under high pressure. At normal temperatures, increasing the injection pressure reduces the specific volume of the molded product, increases its density, reduces the coefficient of linear expansion, and significantly reduces shrinkage. Corresponding to compressibility, the molding material has an elastic recovery effect, which reduces product shrinkage.