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Precision Injection Molding ---The concept of precision injection molding and specific requirements on plastic injection molding machine With the development of information technology and automatic control technology, the market for precision injection molding machines have a higher requirements, and the demand amount is also increasing rapidly. General precision injection molding machine has two indicators: first is the repetitive error of product dimension, and the other is the product’s repetitive weight error. The former is difficult to compare because of different size and thickness. The latter represents the plastic injection molding machine’s level of aggregation. An ordinary injection molding machine’s repetitive weight error is about 1%, a better machine can reach 0.8%, precision injection molding machine is less than 0.5%, and it is called ultra-precision machine when the error is less than 0.3%. The specific requirements on precision molding machine: 1. Injection pressure ≧ 25 MPa; 2. Injection speed ≧ 300 mm/s; 3. Product size precision 0.001mm; 4. Product quality standard deviation coefficient (velocity of variation) ≦ 0.1%; 5. Opening and closing mould position accuracy: opening ≦ 0.03 mm, closing ≦ 0.01 6. Injection position accuracy (holding pressure end point) ≦ 0.03 mm; 7. Equilibrium degree of force in tie bar ≦ 1%; 8. Pre-plastic position accuracy ≦ 0.03mm; 9. Stationary mould and movable mould’s degree of balance A. ≦ 0.03 mm when clamping force is zero; B. ≦ 0.005 mm when clamping force is max; 10. Temperature control accuracy of barrel and screw ≦ ± 0.5 ℃;
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The focus of precision injection molding technology: z
Clamping force In precision injection molding process, machine generally requires a high injection pressure (>250 MPa), or it would be difficult to molding. However, note that, in general, precision injection molding products have relatively small dimensions and thickness, so not all products have high requirements on clamping force. Conversely, if the clamping force is too large, or uneven in four corners, or instable, it will lead to uneven molding on mould and then ultimately affect the quality of molding product. Precision injection molding process requires adjusting clamping force reasonable, conveniently and accurately, overcoming the length and size differences caused by the clamping force unit deformation and the problem of unstable clamping force caused by room temperature and mould temperature’s changes.
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Plasticizing quality Uniformity of plasticizing not only affects the product molding quality, but also affects the resistance when melting plastic flow through the gate. In order to get a uniform plasticizing, it is necessary to design special screw and plasticization technology. In addition, the barrel temperature should be precisely controlled.
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Measurement accuracy To make the plastic product maintains a high stability, the injection plastic inject into the cavity each time must be equal, that means the screw reset accuracy when feeding must be the same. But in fact, the screw reset positions are unable to maintain the same completely, the larger diameter of the screw, the larger of system inertia, and the more difficulty for controlling position accuracy. We generally control the screw reset accuracy in 0.3 mm, perfectly in 0.2 mm.
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Injection speed Injection speed is an important indicator in precision injection molding machine as there is a time difference from the hot melting plastic start
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injecting into cavity to finish injection. During this time, the hot melting plastic begin cool, to make the material at the gate and in the mould cavity cool uniformly, it should choose high speed injection. And to improve injection speed, you must configure one or more accumulator which can release energy quickly to shorten the time difference. It is advantageous for precision injection molding to improve the original injection speed more than 3 times, reaching 300~800 mm/s. z
Injection pressure Precision injection molding requires a large injection pressure to increase the plastic density, reduce shrinkage, and ensure the product precision. However, the injection pressure is not the bigger the better, over pressure will have flash. In the best injection pressure, the injection cycle time should not more than 0.05 seconds.
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Holding pressure Holding pressure has a great impact on precision plastic. Accurately, holding pressure can make a nice feeding, reduce the plastic deformation and control the product precision. The stability of holding pressure decide the molding precision, unchangeable screw end position determine the holding effect.
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Mould temperature For precision injection molding, the mould cavity temperature should be controlled in Âą1℃, which must use PID control. But for ultra-precision injection molding, this is not enough, more advanced method is to use FUZZF control (fuzzy control). (edit by Dakumar Machinery Co.,Ltd market02@sinogroupe.com )
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