Precision injection molding refers to an injection molding method in which dimensional repeatability of processed injection products is so high that it is difficult to meet requirements using general injection molding machines and conventional injection molding processes.

 

Concept of "precision injection molding" is mainly different from "conventional injection molding". It is based on rapid development of polymer materials and uses precision plastic parts to replace high-precision metal parts in the fields of instrumentation and electronics.
There are currently two defining indicators for precision injection products:
One is repeatability of product dimensions, and the other is repeatability of product quality.
This article mainly describes precision injection molding from aspect of product size repeatability. However, due to different nature and processing technology of various materials, accuracy of plastic parts cannot be equated with accuracy of metal parts.
Mold of molded product is an important condition to determine whether product can meet dimensional tolerance required by design, precision injection molding machine is key equipment to ensure that product is always molded within required dimensional tolerance and to ensure a high yield.
Features of precision injection: Dimensional accuracy of part is high and tolerance is small, that is, there is a high-precision dimensional limit; Weight repeatability of product is high, dimensional stability of day, month and year is required; Material of mold is good, rigidity is sufficient, dimensional accuracy, smoothness of cavity and positioning accuracy between mold plates are high; Use precision injection machine to replace conventional injection machine; Using precision injection molding process; Choose materials suitable for precision injection molding.
The most important technical index for evaluating products is 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 factors including material selection, mold design, injection molding process, and technical level of operator.
Precision injection molding machines require product dimensional accuracy within 0.01～0.001mm. Many precision injection molding machines also require injection molding machines with high injection pressure and high injection speed;
Clamping system is required to have sufficient rigidity and high enough clamping accuracy. So-called clamping accuracy refers to uniformity, adjustment, stability and repeatability of clamping force, high accuracy of opening and closing positions;
It is required that pressure, flow, temperature, measurement, etc. can be accurately controlled to corresponding accuracy, multi-stage or stepless injection is used to ensure reproduction conditions of molding process and repeat accuracy of product size.
Factors affecting dimensional accuracy of products: Mold accuracy; Molding shrinkage rate; Temperature, humidity and fluctuation range of environment in which product is used.

High mechanical strength, good dimensional stability, good creep resistance, and a wide range of environmental adaptation.
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 resistance, and easy demoulding with lubricant.
PA and glass fiber reinforced PA66, its characteristics: strong impact resistance and wear resistance, good fluidity, can form products with a wall thickness of 0.4mm. Glass fiber reinforced PA66 has heat resistance (melting point 250℃), but its disadvantage is that it has hygroscopicity. Generally, it must be processed through humidity control after molding.
PBT reinforced polyester, short molding time. Molding time comparison is as follows: PBT≤POM≈PA66≤PA6.
PC and GFPC. Features: good wear resistance, increased rigidity after enhancement, good dimensional stability, weather resistance, flame retardancy and good molding processability.
Shrinkage problems in precision injection molding
There are four factors that affect shrinkage: thermal shrinkage, phase change shrinkage, orientation shrinkage, and compression shrinkage.
Thermal shrinkage is inherent thermophysical properties of molding materials and mold materials. Mold temperature is high, product temperature is also high, actual shrinkage rate will increase, so mold temperature for precision injection should not be too high.
Phase change shrinkage is called phase change shrinkage due to shrinkage caused by decrease of specific volume due to crystallization of polymer during orientation process of crystalline resin. Mold temperature is high, crystallinity is high, and shrinkage rate is high; but on the other hand, increase of crystallinity will increase density of product, reduce linear expansion coefficient, and reduce shrinkage rate. Therefore, actual shrinkage rate is determined by combined effect of two.
Orientation shrinkage, due to forcible stretching of molecular chains in flow direction, macromolecules have a tendency to re-curl and recover during cooling, shrinkage will occur in orientation direction. Degree of molecular orientation is related to injection pressure, injection speed, resin temperature and mold temperature. But main thing is the injection speed.
Compression shrinkage and elastic reset, general plastics are compressible. That is, specific volume changes significantly under high pressure. Under normal temperature, increasing specific volume of pressure-molded products will decrease, density will increase, expansion coefficient will decrease, and shrinkage rate will decrease significantly. Corresponding to compressibility, molding material has an elastic reset function, which reduces shrinkage of product. Factors that affect molding shrinkage of products are related to molding conditions and operating conditions.

Accuracy of mold mainly depends on whether dimensional accuracy of mold cavity is high, positioning of cavity is accurate or accuracy of parting surface meets requirements. Generally, dimensional tolerance of precision injection molds should be controlled below 1/3 of product dimensional tolerance.
Machinability and rigidity, in mold structure design, number of cavities should not be too much, bottom plate, support plate, and cavity walls should be thicker to avoid severe elastic deformation of parts under action of high temperature and high pressure.
For product demoldability, mold should use as few cavities as possible, fewer and short runners, and a higher finish than ordinary molds, which is conducive to demolding.
Material of precision mold is alloy steel with high mechanical strength. Materials for making cavity and runner must undergo rigorous heat treatment, materials with high hardness (molded parts must reach about HRC52), good wear resistance and strong corrosion resistance are selected.

Features in technical parameters: Divided from injection pressure, ordinary machine: 147 ~ 177MPa; precision machine: 216 ~ 243MPa; ultra-high pressure machine: 243 ~ 392MPa.

 

Reasons why precision injection molding machines must be high pressure are:
(1) To improve precision and quality of precision products, injection pressure has the most obvious impact on molding shrinkage rate of products. When injection pressure reaches 392MPa, molding shrinkage rate of product is almost zero. At this time, accuracy of product is only affected by mold control or environment. Experiments show that after injection pressure is increased from 98 MPa to 392 MPa, mechanical strength is increased by 3% to 33%.
(2) It can reduce wall thickness of precision products and increase molding length. Taking PC as an example, injection pressure of an ordinary machine is 177MPa, which can form products with a wall thickness of 0.2-0.8mm, while injection pressure of a precision machine can form products with a thickness of 0.15-0.6mm when injection pressure is 392MPa. Ultra-high pressure injection machine can obtain products with a larger flow length ratio.
(3) Increasing injection pressure can fully develop effect of muddy injection rate. To achieve rated injection rate, there are only two ways: one is to increase maximum injection pressure of system; the other is to modify screw parameters to increase ratio of length to diameter. Injection rate of precision injection machine is high.
Features of precision injection machine in terms of control:
(1) Repetition accuracy (reproducibility) of injection molding parameters is high, and multi-level injection feedback control should be adopted: Multi-level position control; Multi-level speed control; Multi-level pressure holding control; Multi-level back pressure control; Multi-stage screw speed control.
Accuracy of displacement sensor is required to reach 0.1mm, so that metering stroke, injection stroke and thickness of remaining material pad (injection monitoring point) can be strictly controlled to ensure accuracy of each injection, improve molding accuracy of product. Temperature control of barrel and nozzle should be precise, overshoot should be small when heating up, temperature fluctuation should be small. Precision injection molding should be controlled by PID, so that temperature accuracy should be within ±0.5℃.
(2) Plasticization quality requirements. Uniformity of plasticization not only affects molding quality of injection molded parts, but also affects resistance of molten plastic when passing through gate. In order to obtain uniform plasticization, it is necessary to design a special screw and use a special plasticization technology.
In addition, temperature of barrel should also be accurately controlled. At present, temperature of screw and barrel is mostly controlled by PID (proportional, differential, integral), accuracy can be controlled within ±1℃, which can basically meet requirements of precision injection molding. If FUZZY control method is adopted, it is more suitable for precision injection molding.
(3) Temperature control of working oil requires high oil temperature changes to cause fluctuations in injection pressure. A closed-loop heating and cooling device must be used for working oil, and oil temperature should be stabilized at 50-55℃.
(4) Impact of holding pressure, holding pressure has a great impact on precision plastic parts. To be precise, holding pressure can better feed and shrink, reduce plastic part deformation, control plastic part accuracy. Stability of holding pressure determines molding accuracy of plastic part, end position of screw is decisive factor that determines holding pressure effect.
(5) If mold temperature control requires same cooling time, thickness of product with a lower mold cavity temperature will be larger than that of product with a higher temperature. Such as POM and PA materials, thickness of products with a thickness of 50-100μm at a mold temperature of 50℃ will be reduced to 20-40μm at 80℃ and only 10μm at 100℃. Room temperature also has an impact on dimensional tolerances of precision products.
Hydraulic system of precision injection molding machine: Oil circuit system needs to adopt proportional system of proportional pressure valve, proportional flow valve or servo variable pump. In direct pressure type mold clamping mechanism, oil path of mold clamping part is separated from oil path of injection part. Due to high speed of precision injection molding machine, response speed of hydraulic system must be emphasized. Hydraulic system of a precision injection molding machine must fully reflect integration of electromechanical and hydraulic instruments.

 

Structural features of precision injection molding machine: Due to high injection pressure of precision injection machine, rigidity of clamping system must be emphasized. Parallelism of moving and fixed mold plates is controlled within range of 0.05～0.08mm. Protection of low-pressure molds and control of precision of clamping force are required. Because size of clamping force affects degree of mold deformation, ultimately affects dimensional tolerance of part. Opening and closing speed of mold should be fast, generally around 60mm/s.
Plasticizing components: screw, screw head, non-return ring, barrel, etc., should be designed with a structure with strong plasticizing ability, good homogenization, and high injection efficiency; screw drive torque should be large, and it can be continuously variable.
No matter what kind of precision injection molding machine, in the end, it must be able to stably control dimensional repeatability and quality repeatability of product.