Precision injection molding solutions

Date：2011-12-06 18:02:19

Precision injection molding injection molding process is the size of repeated high precision products that use the common injection molding machine, the conventional injection molding process is difficult to meet the requirements of an injection molding method. "Precision Injection Molding," The concept is mainly distinguished from "conventional injection molding", which is based on the rapid development of polymer materials, in instrumentation, electronics, precision plastic components used in high-precision metal parts instead of the technology. Currently defined for the target precision injection products, there are two, one product size repeatability, the second is product quality repeatability. This article from the product size expound repeatability precision injection molding. However, due to the nature of various materials and processing technology, you can not put the plastic parts of the accuracy and precision metal parts equated.

Precision injection molding is a performance related to raw materials, formulation, molding technology and equipment and other aspects of integrated technology, precision plastic products, including DVD digital disc, DVD laser head, digital camera parts, computer connectors, light guide, and other non-spherical lens precision products, such products are not only distinctive feature of high dimensional accuracy requirements, but also the intrinsic product quality and yield requirements are very high. Forming mold is to determine the products of the product can meet the design requirements of an important condition for tolerance, and precision injection molding machine is to ensure products always in the required tolerance range forming, and to ensure a high yield of key equipment. Plastic products is three highest level of accuracy.

1.1 The characteristics of precision injection

(1) parts of the high dimensional accuracy, tolerances, precision that is the size limit; (2) Product weight high repetition accuracy, requiring a day, month, year dimensional stability; (3) the mold material is good, rigid enough, the cavity of the dimensional accuracy, finish and high positioning accuracy between the template; (4) replace the conventional use of precision injection machine injection machine; (5) the use of precision injection molding process; (6) Select to adapt precision injection molding material.

Assessment of the most important technical indicator products, plastic products, is precision (dimensional tolerances, geometric tolerances and surface finish products). China's use of standard SJ1372-78, with Japan's precision plastic mold and precision levels very close. For injection molding of precision plastic products, to be from the material selection, mold design, injection molding process, the operator's skill level, four major factors strictly controlled.
Precision injection molding machine is generally required product dimensional accuracy within 0.01 ~ 0.001mm, many also require precision injection molding machine with high injection pressure, high injection speed; required clamping system has enough rigidity and clamping accuracy is high enough, Accuracy refers to the so-called mode-locking clamping force uniformity, adjustable, high stability and reproducibility, high precision clamping position; requirements for pressure, flow, temperature, measurement, etc. can be precisely controlled to the appropriate precision, multi- Class or no class injection molding process to ensure reproduction of the conditions and product sizes of repeat accuracy.

1.2 Factors affecting the product dimensional accuracy

(1) precision molds; (2) mold shrinkage; (3) products with the environment temperature, humidity, and volatility.

Precision injection molding material 2 the principle of selection

High mechanical strength, dimensional stability, good creep resistance, a wide range of environmental adaptation. There are four commonly used materials:

(1) POM and carbon fiber (CF) or glass-reinforced (GF); this material is characterized by good resistance to creep, fatigue resistance, weather resistance, good dielectric properties, flame retardant, easy to mold release lubricant added.
(2) PA and glass fiber reinforced PA66, which is characterized by: strong impact resistance and wear resistance, flow properties, and can be molded 0.4mm wall thickness of the products. Heat resistant glass fiber reinforced PA66 (melting point 250 ℃), the drawback is hygroscopic, the general shape after going through the conditioning process.
(3) PBT reinforced polyester, forming a short time. Molding time are as follows: PBT ≤ POM ≈ PA66 ≤ PA6.
(4) PC and GFPC characteristics: good wear resistance, enhanced rigidity improved dimensional stability, weather resistance, flame retardant and good processability.

3 in precision injection molding shrinkage

There are four factors affecting shrinkage: shrinkage, contraction of the phase transition, orientation contraction and compression shrinkage.

3.1 Shrink molding material is inherent with the mold material thermophysical properties. Mold temperature, product temperature is high, the actual contraction rate will increase, so precision injection mold temperature is not too high.

3.2 As the contraction phase change in the orientation of crystalline resin process, with the polymer crystallization, due to the contraction caused by a decrease in hematocrit, which is called phase transition contraction. Mold temperature, high crystallinity, shrinkage rate is high; other hand, the crystallinity of products will increase density, reduce the coefficient of linear expansion, contraction rate. Therefore, the actual rate of contraction may be combination of both.

3.3 orientation contraction, due to the molecular chain in the flow direction of the tensile force, so in cooling of the molecules have to re-curl recovery trend in the orientation direction of the contraction. The degree of molecular orientation and injection pressure, injection speed, mold temperature and resin temperature and so on. But the main thing is the injection speed.

3.4 contraction and elastic compression reset, generally have plastic compression. That under high pressure a significant change in hematocrit. In general, the temperature, increasing pressure molding products will reduce the specific volume, density will increase, reducing the coefficient of expansion, contraction rate decreased significantly. Corresponding to the compression molding material flexible reset function, so that shrinkage decreases. Factors affecting the molding shrinkage and molding conditions and operating conditions.

4 precision injection molds

4.1 precision molds, depending on size and high precision mold cavity, the cavity or sub-surface, accurate positioning accuracy meets the requirements. General precision injection mold tolerance, tolerance should be controlled in products of 1 / 3 less.

4.2 machinability and rigid, in the mold design, the cavity number should not be too much, and bottom, bearing plates, type of cavity wall to be thicker to avoid parts in high temperature, high pressure severe elastic deformation occurs.

4.3 products, mold release, mold cavity to try to take less number of small and short flow channel and a higher than normal mold finish, it is a good mold.

4.4 Precision mold material, selection of high mechanical strength alloy steel. Produced cavity, runner material to go through rigorous heat treatment, the choice of high hardness (HRC52 molded parts to reach around), wear resistance, corrosion resistance of the material.

5 characteristics of precision injection machine and choose

5.1 Technical aspects of the characteristics

Divided from the injection pressures, general machine: 147 ~ 177MPa; precision machine: 216 ~ 243MPa; high pressure machine: 243 ~ 392 MPa. High precision injection molding machine must be on the grounds that:

(1) to improve the accuracy and quality precision products, injection molding shrinkage pressure on the most significant impact. When the injection pressure of 392MPa, the molding shrinkage rate is almost zero. And then only by the accuracy of product control or mold environment. Experiments show that: injection pressure from 98MPa to 392MPa, the mechanical strength increased by 3% to 33%.

(2) to decrease the thickness of precision products to improve the molding length. The PC, for example, ordinary machine injection pressure of 177MPa, 0.2 ~ 0.8mm wall thickness can be formed of the products, and precision machine injection pressure of 392MPa can be formed in the 0.15 ~ 0.6mm in thickness between the products. High pressure injection machine available stream length greater than the product.

(3) increase the injection pressure can be muddy injection rate into full effect. To achieve the rated injection rate, there are only two ways: First, improve the system the maximum injection pressure; the second is the transformation screw parameters, improved aspect ratio. Precision injection machine for high injection rate. Precision manufactured in Germany DEMAG injection molding machine (60 ~ 420 tons), for example, injection speed can reach 1000mm / s, the screw can be 12m/s2 acceleration.

5.2 precision injection machine control features

(1) of the injection molding parameters repeatability (reproducibility) for high, should adopt the multi-stage injection feedback control: a. Multi-level position control; b. multi-speed control; c. control of multi-level packing; d. more level back pressure control; e. multi-level screw speed control. Precision displacement sensor to 0.1mm, so you can strictly control the measurement of stroke, stroke, and injecting more than the thickness of the pad material (injection control point), the amount of each injection to ensure accuracy, improve the molding accuracy. Barrel and nozzle temperature control to be precise, when the temperature overshoot is smaller, the temperature fluctuations to be small. Precision Injection should be used PID control, the temperature accuracy of ± 0.5 ℃ in between is appropriate.

(2) quality plastics, plasticizer not only affect the uniformity of the injection molding quality parts, but also affects the molten plastic through the gate resistance suffered when the size, in order to get uniform plastics, special screw design by using special rods and plastic technology is essential. In addition, the barrel temperature should be precisely controlled, and now the screw, barrel temperature more than the use of PID control (proportional, derivative, integral), the precision can be controlled within ± l ℃, the basic requirements to meet the precision injection molding, if the use of FUZZY control method, more suitable for injection molding of precision.

(3) working oil temperature control high oil temperature changes lead to the injection pressure fluctuations, the use of heating oil must be working, closed-loop cooling device, the oil temperature stabilized at 50 ~ 55 ℃ appropriate.

(4) the impact of holding pressure, holding pressure on the impact of great precision plastic parts, to be precise, pressure can be better feeding, reducing the plastic deformation, the control precision plastic parts, the stability of the decision holding pressure the precision molding of plastic parts, screw termination is to determine the location of the same determinants of the effect of holding pressure.

(5) of the mold temperature control requirements if the same cooling time, the mold cavity temperature lower than the thickness of products, high temperature product thickness size large. Such as POM, PA class of materials, mold temperature 50 ℃, the thickness of 50 ~ 100μm of products, at 80 ℃ thickness is reduced to 20 ~ 40μm, 100 ℃, when reduced to only 10μm. Room temperature also affect the tolerance of precision products.

5.3 precision injection molding machine hydraulic system

(1) The hydraulic system requires the use of proportional pressure valve, proportional flow valves or servo-proportional variable pump system. (2) direct pressure type mold institutions, some of the mold and injection part of the oil separated from oil. (3) The precision injection molding machine with high speed, this must be emphasized that the response speed of the hydraulic system. (4) precision injection molding machine hydraulic system, but also to realize the machine - electric - hydraulic - instruments of integration projects.

5.4 Structural characteristics of precision injection molding machine

(1) Due to the high precision injection machine injection pressure, it is necessary to emphasize the stiffness of the mold system. Move, set the template control in parallel within 0.05 ~ 0.08mm.
(2) requires the protection of low-pressure mold clamping force size and precision control. Because the size of clamping force to influence the degree of deformation of the mold, and ultimately to affect parts of the dimensional tolerances.
(3) Kai, closed die faster, usually in 60mm / s or so.
(4) plastics parts: screw, the screw head, check ring, cylinder, etc., should be designed to plasticizing capacity and the degree of both good and high injection efficiency of the structure; screw drive torque to a large, and can continuously variable transmission.

Conclusion: No matter what kind of precision injection molding machine, must ultimately be able to stability control of product size and quality repeatability repeatability.

6 the main representative of precision injection machine model

Precision injection molding of the development on behalf of the world advanced level in the main German manufacturer Krauss-Maffei, Demag, Albemarle, Arburg and Nissei of Japan, Nippon Steel, Sumitomo Heavy Machinery Toshiba Machine and other enterprise. Germany, Arburg launched precision injection mold machine design institutions to take a box, in order to improve the locking accuracy. As the three-plate injection machine front panel and back panels are fixed in the rack, when the clamping force is applied, the four-rod elongation bound by the rack, so that the bar tends to be "arch-shaped" and affect the clamping accuracy, Box style design results from the limited perspective of improving poor deformed clamping accuracy. Two-way injection cylinder pressure servo control, precise positioning screw, and make the double-precision injection volume control; using optimal control of the main pump motor inverter, not only improve the accuracy of the hydraulic system control and energy saving effect is remarkable; In addition, the use of modules design, injection molding machine according to the user of the motor system actually needs to take a combination of both hydraulic and electric clamping system and injection system relative position can also have a variety of configurations.

Nissei Corporation has adopted a new control system TACT, improve response time, operational stability, multi-language LCD display control interface; new injection institutions: screw length increased to enhance plasticizing effect, optimization of the five sections of plastic-zone temperature control, and the hydraulic circuit is optimized; oil pressure loss reduction, clamping efficiency, low-speed motion stabilization to improve, develop a new series of precision injection molding machine hydraulic FN also developed a hydraulic type of small precision injection molding NP7RealMini, AU3E, AS15, and other models. The main processing precision injection molding material for the liquid crystal polymer (LCP), Polyamide (PA), polyphenylene sulfide (PPS) and other engineering plastics.

Nissei NEX150 precision injection molding machine combines Ultimate micro technology can be used for high-volume precision injection molding small plastic parts, configured to optimize the design of accurate measurement and rapid response to injection system, high precision and high rigidity of the mold body, can be processed 0.1 ~ 0.3g weight of precision miniature plastic parts, use of digital cameras, mobile phones, hard, fine needle connector and so on. Japan Steel Works has developed the J-ELII-UPS ultra high-speed precision injection molding machine. In the standard packing conditions, products, variation in weight 0.022 g, packing a new control method, the product weight fluctuation can be reduced to 0.006 grams.

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