Concept of precision injection molding
Precision injection molding is the processing of molded products of high dimensional repeatability, so that the use of universal injection molding machine, the conventional injection molding process is difficult to achieve the requirements of an injection molding method.
The concept of 'precision injection molding', which is different from 'conventional injection molding', is based on the rapid development of polymer materials and the replacement of high-precision metal parts with precision plastic parts in instrumentation and electronics.
There are currently two definitive targets for precision injection products:
First, product size repeatability, and second, product quality, repeat accuracy.
This article mainly elaborates precision injection molding from the aspects of product size repeatability, but due to the different nature of materials and processing technology, the precision of plastic parts can not be equated with the precision of metal parts.
Molding mold is to decide whether the product can meet the design requirements of the dimensional tolerance of the important conditions, and precision injection molding machine is to ensure that the product is always within the required dimensional tolerance within the framework of molding, and to ensure the high yield of the key equipment.
Precision injection features:
Parts of high dimensional accuracy, small tolerance, that is, high-precision size limits;
Repeated product weight high precision, requiring a day, month, year dimensional stability;
Mold material is good, rigid enough, the cavity size accuracy, finish and template positioning accuracy;
The use of precision injection machine replacement conventional injection machine;
Using precision injection molding process;
Choose to adapt to precision injection molding materials.
The most important technical indicators to assess the product, is the accuracy of injection molding products (dimensional tolerances, geometric tolerances and surface finish) To plastic injection precision plastic products, from the material selection, mold design, injection molding process, the operator's Technical level and other four factors strictly controlled.
Precision injection molding machine requirements of product size accuracy is generally less than 0.01 ~ 0.001mm, many precision injection molding machines also require high injection pressure, high injection speed;
Requires clamping system has a sufficiently large rigidity and high enough accuracy of the clamping mode, the so-called clamping accuracy refers to the uniformity of clamping force, adjustable, stable and repeatable, the opening and closing die position high accuracy;
Requirements of the pressure, flow, temperature, measurement, etc. can be precisely controlled to the corresponding accuracy, using multi-level or stepless injection, molding process to ensure reproducibility conditions and product size repeatability.
Factors affecting the dimensional accuracy of products:
Mold precision
Molding shrinkage;
Product environment temperature, humidity and the extent of fluctuations.
Injection molding precision molding material selection principle
High mechanical strength, good dimensional stability, good creep resistance, 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, dielectric properties, flame retardant, lubricants easy demolding.
PA and glass fiber reinforced PA66, its characteristics: impact resistance and wear resistance, good flow properties, forming 0.4mm wall thickness of the product. Glass fiber reinforced PA66 has heat resistance (melting point 250 ℃), the disadvantage is that with Hygroscopicity, the general shape after conditioning by humidity treatment.
PBT reinforced polyester, forming a short time molding time is as follows: PBT ≤ POM ≈ PA66 ≤ PA6.
PC and GFPC Features: Good abrasion resistance, increased rigidity after reinforcement, good dimensional stability, weatherability, flame retardant and good molding processability.
Shrinkage problems in precision injection molding
There are four factors that affect shrinkage: heat shrinkage, phase change shrinkage, orientation shrinkage, and compression shrinkage.
Heat Shrinkage is the inherent thermal physical properties of molding materials and mold materials, high mold temperature, high product temperature, the actual shrinkage rate will increase, so precision injection mold temperature should not be too high.
Phase change shrinkage Crystalline type resin in the orientation process, accompanied by the crystallization of polymers, due to reduced specific volume caused by shrinkage, called phase change shrinkage. Mold temperature is high, high crystallinity, shrinkage rate; the other hand , The crystallinity will increase the product density, linear expansion coefficient decreases, the shrinkage decreased, so the actual shrinkage by the combined effect of both.
Orientation shrinkage, due to the molecular chain in the flow direction of the forced stretching, so that the cooling of the macromolecules have a tendency to re-curl in the direction of orientation will have contraction. The degree of molecular orientation and injection pressure, injection speed, resin temperature and mold Temperature, etc. But the main thing is injection speed.
Compression shrinkage and elastic reset, the plastic is generally compressive, that is, a significant change in specific volume at high pressure at normal temperatures, to enhance the pressure forming product specific volume will be reduced, the density will increase, the expansion coefficient decreases, the shrinkage rate will be Which is corresponding to the compressibility, the elastic deformation of the molding material reduces the shrinkage of the product, and the factors affecting the shrinkage of the product are related to the molding conditions and operating conditions.
Precision injection mold
Mold accuracy, mainly depends on the high precision mold cavity size, cavity positioning accuracy or sub-surface accuracy to meet the requirements. General precision injection mold size tolerances should be controlled in the product size tolerance of 1/3 below.
Machinability and rigidity, in the mold design, the number of cavities should not be too much, while the floor, back plate, cavity wall thicker to avoid parts under high temperature and high pressure under the dramatic elastic deformation.
Mold release, the mold should try to take less number of cavities, short and short runner and a higher finish than the average mold, which is conducive to mold release.
Precision mold material, choose high mechanical strength alloy steel. The production of cavity, runner material to undergo a rigorous heat treatment, the choice of high hardness (molding parts to achieve about HRC52), good wear resistance, corrosion resistance of the material .
Precision injection machine features and selection
Technical parameters of the features:
Divided from the injection pressure, the general machine: 147 ~ 177MPa; precision machine: 216 ~ 243MPa; ultra-high pressure machine: 243 ~ 392MPa.
Precision injection molding machine must be high pressure reasons are:
(1) to improve the precision and quality of precision products, the injection pressure on the molding shrinkage rate has the most obvious impact when the injection pressure reaches 392MPa, the product shrinkage rate is almost zero .At this time the accuracy of the product only by the mold control or Environmental impact.Experiments show that: the injection pressure increased from 98MPa to 392MPa, the mechanical strength increased by 3% to 33%.
(2) The wall thickness of precision products can be reduced and the molding length can be increased, taking PC as an example, the injection pressure of common machine is 177MPa, the product with wall thickness of 0.2 ~ 0.8mm can be formed, while the injection molding pressure of precision machine is 392MPa, 0.15 ~ 0.6mm between the products. EHV injection machine can get a larger flow length products.
(3) increase the injection pressure can be fully fat the injection rate of the effect To achieve the rated injection rate, there are only two ways: First, to increase the maximum injection pressure system; the second is to modify the screw parameters to improve the aspect ratio injection of precision injection machine High speed requirements.
Precision injection machine in the control of the features:
(1) Repeatability (reproducibility) requirements for injection molding parameters are high, and multi-stage injection feedback control should be adopted:
a. Multi-level position control;
b. multi-speed control;
c. Multi-level packing control;
d. multi-level back pressure control;
e. Multi-stage screw speed control.
Displacement sensor accuracy requirements of 0.1mm, so you can strictly control the measurement stroke, injection stroke and the thickness of the material pad (injection monitoring point), to ensure accurate injection volume, improve product molding accuracy. Cylinder and nozzle temperature control to be accurate , When the temperature rise overshoot to be small, temperature fluctuations should be small.Precision injection should be used PID control, the temperature accuracy of ± 0.5 ℃ between appropriate.
(2) plasticization quality requirements, the plasticization of plastic uniformity not only affects the quality of injection molding parts, but also affect the molten plastic through the gate when the resistance suffered by the size, in order to obtain uniform plasticization, the design of special screw And the use of proprietary plasticizing technology is essential.
In addition, the barrel temperature should be precisely controlled, and now screw, barrel temperature PID control (multi-proportional, differential, integral), the accuracy can be controlled within ± l ℃, the basic injection molding to meet the requirements, if using FUZZY control Method, it is more suitable for precision injection molding.
(3) the temperature control of the working oil To change the high oil temperature caused the injection pressure fluctuations, the working oil must be heated, cooled closed-loop device, the oil temperature stabilized at 50 ~ 55 ℃ appropriate.
(4) The impact of packing pressure, packing on the accuracy of the plastic parts of the great impact, accurately speaking, packing can better shrink, reducing plastic deformation, plastic parts control accuracy, holding pressure stability of the decision The molding precision plastic parts, screw termination position is the decisive factor in determining the packing effect.
(5) Mold temperature control requirements If the cooling time is the same, the mold cavity temperature lower product thickness than the high temperature product thickness size.Such as POM, PA type material, the mold temperature 50 ℃, the thickness of 50 ~ 100μm products , The thickness is reduced to 20-40 μm at 80 ° C and only 10 μm at 100 ° C. Room temperature also affects the dimensional tolerances of precision products.
Precision injection molding machine hydraulic system:
The oil system needs a proportional system with a proportional pressure valve, a proportional flow valve or a servo variable pump.
In the direct pressure clamping mechanism, the clamping part of the oil path and the injection part of the oil separated.
Due to the high speed of precision injection molding machines, the response speed of the hydraulic system must be emphasized.
Precision injection molding machine hydraulic system, but also to fully reflect the integration of mechanical and electrical engineering.
Precision injection molding machine structural features:
Due to the high injection pressure of the precision injection molding machine, it is necessary to emphasize the rigidity of the clamping system. The parallelism of the moving and the fixed template is controlled within the range of 0.05-0.08mm.
Requirements for the protection of low-pressure mold and mold clamping force size accuracy control because the size of mold clamping force to affect the extent of mold deformation, the final to affect the size of the workpiece tolerances.
Kai, mold closing speed faster, generally about 60mm / s.
Plasticization components: screw, screw head, check ring, barrel, etc., to be designed into plasticization ability, a good degree of homogeneity, high injection efficiency of the structure; screw drive torque to large, and can be continuously variable speed.
No matter what kind of precision injection molding machine, eventually must be able to control the product size repeat accuracy and quality repeat accuracy.