In terms of injection pressure, ordinary machines: 147-177MPa; precision machines: 216-243MPa; ultra-high pressure machines: 243-392MPa. Reason why precision injection molding machine must be high pressure is:
(1) To improve precision and quality of precision products, injection pressure has the most obvious influence on molding shrinkage rate of products. When injection pressure reaches 392MPa, molding shrinkage 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 98MPa to 392MPa, 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 ordinary machine is 177MPa, which can form products with wall thickness of 0.2-0.8mm, while injection pressure of precision machine is 392MPa, which can form products with thickness between 0.15-0.6mm. Ultra-high pressure injection machine can obtain products with a larger flow ratio.
(3) Increasing injection pressure can fully obscure effect of 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 aspect ratio. Injection rate of precision injection machine is required to be high. Taking German-made DEMAG precision injection molding machine (60-420 tons) as an example, its injection speed can reach 1000mm/s, and screw can obtain an acceleration of 12m/s2.

 

(1) Repetition accuracy (reproducibility) of injection molding parameters is high, and multi-stage injection feedback control should be adopted:
a. Multi-level position control;
b. Multi-level speed control;
c. Multi-level holding pressure control;
d. Multi-level back pressure control;
e. Multi-stage screw speed control.
Accuracy of displacement sensor is required to reach 0.1mm, which can strictly control metering stroke, injection stroke and thickness of residual material pad (injection monitoring point) to ensure accuracy of each injection and improve molding accuracy of product. Temperature control of barrel and nozzle should be precise, overshoot should be small when heating, and temperature fluctuation should be small. Precision injection molding should be controlled by PID, so that temperature accuracy should be within ±0.5℃.
(2) Plasticizing quality requirements, uniformity of plastic plasticizing not only affects molding quality of injection molded parts, but also affects resistance of molten plastic when it passes through gate. In order to obtain uniform plasticizing, a special screw is designed. And use of dedicated plasticizing technology is essential. In addition, temperature of barrel should also be precisely controlled. At present, temperature of screw and barrel is mostly controlled by PID (proportional, differential, integral), and accuracy can be controlled within ±1 ℃, which can basically meet requirements of precision injection molding. If FUZZY control is used method, it is more suitable for precision injection molding.
(3) Temperature control of working oil should cause fluctuation of injection pressure due to change of high oil temperature. A closed-loop device for heating and cooling must be used for working oil, and it is advisable to stabilize oil temperature at 50-55 ℃.
(4) Influence of holding pressure, pressure holding has a great influence on precision plastic parts. To be precise, holding pressure can be better fed, reduce deformation of plastic parts, control accuracy of plastic parts, and determine stability of holding pressure. In order to ensure molding accuracy of plastic parts, constant end position of screw is decisive factor to determine pressure holding effect.
(5) For mold temperature control requirements, if cooling time is same, thickness of product with low mold cavity temperature is larger than that of product with high temperature. Such as POM and PA materials, products with a thickness of 50 to 100 μm when mold temperature is 50 ℃, thickness is reduced to 20 to 40 μm at 80 ℃, and only 10 μm at 100 ℃. Room temperature also affects dimensional tolerances of precision products.

 

(1) Oil circuit system needs to use a proportional pressure valve, a proportional flow valve or a proportional system of a servo variable pump.
(2) In direct pressure clamping mechanism, separate oil circuit of clamping part and injection part.
(3) Due to high speed of precision injection molding machine, response speed of hydraulic system must be emphasized.
(4) Hydraulic system of precision injection molding machine should fully reflect machine-electricity-hydraulic-instrument integration project.

(1) Due to high injection pressure of precision injection machine, it is necessary to emphasize rigidity of clamping system. Parallelism of movable and fixed mold plates is controlled within range of 0.05-0.08mm.
(2) It is required to protect low pressure mold, control size and accuracy of clamping force. Because size of clamping force affects degree of mold deformation, and ultimately affects dimensional tolerance of workpiece.
(3) Speed of mold opening and closing should be fast, generally around 60mm/s.
(4) Plasticizing parts: screw, screw head, non-return ring, barrel, etc., should be designed into a structure with strong plasticizing ability, good homogenization degree and high injection efficiency; screw driving torque should be large, and can be stepless variable speed.
Conclusion: No matter what kind of precision injection molding machine, in the end, it must be able to stably control repeatability of product size and quality.

 

Manufacturers representing world's advanced level in development of precision injection molding machines mainly include Germany's KraussMaffei, Demag, Albemarle, Arburg and Japan's Nissei, Nippon Steel, Toshiba Machinery and Sumitomo Heavy Machinery, etc. enterprise. Clamping mechanism of precision injection machine introduced by German Arburg Company adopts a box-type design to improve clamping accuracy.
Since front plate and rear plate of three-plate injection machine are fixed on frame, when clamping force is applied, elongation of four tie rods is constrained by frame, so that tie rods tend to be "arch-shaped" and affect clamping accuracy. As a result of pattern design, clamping accuracy is improved from perspective of limiting undesirable deformation. Injection cylinder adopts two-way pressure servo control to precisely locate screw position, which doubles injection volume control accuracy; frequency converter is used to optimize control of main pump motor, which not only improves control accuracy of hydraulic system, but also has a significant energy saving effect;
In addition, with modular design, each motion system of injection molding machine can be combined with hydraulic and electric power according to actual needs of users, relative positions of clamping system and injection system can also have various configurations.
Nissei has adopted new control system TACT, which improves response speed, operation stability, and multi-language control interface displayed on LCD screen; New injection mechanism: screw length-diameter ratio is increased to enhance plasticizing effect, five-stage temperature zone optimizes plasticizing temperature control, and hydraulic circuit is optimized; pressure loss of oil circuit is reduced, clamping efficiency is improved, low-speed motion stability is improved, a new hydraulic FN series precision injection molding machine is developed, hydraulic small precision injection molding machines NP7RealMini, AU3E, AS15 and other machines type are successfully developed. Main processing materials of this precision injection molding machine are liquid crystal polymer (LCP), polyamide (PA), polyphenylene sulfide (PPS) and other engineering plastics.
Nissei NEX150 precision injection machine integrates Ultimate micro-injection technology, which can be used for high-volume injection molding and processing of tiny precision plastic parts. It is equipped with an optimized design of precise metering and rapid response injection system, high-precision and high-rigidity mold clamping mechanism, can process precision micro-plastic parts weighing 0.1-0.3g, including digital cameras, mobile phones, hard drives, and fine pinholes. connectors, etc.
Japan Steel Works has also developed J-ELII-UPS ultra-high-speed precision injection molding machine. Under standard pressure-holding condition, fluctuation range of product weight is 0.022 grams. After adopting new pressure-holding control method, fluctuation range of product weight can be reduced to 0.006 grams.