Part Ⅱ

3. Features and selection of precision injection machine

3.1 Features in technical parameters

According to injection pressure,

ordinary machine: 147 ~ 177MPa

precision machine: 216 ~ 243MPa

ultra-high pressure machine: 243 ~ 392 MPa

The reasons why precision injection molding machines must be high pressure are:

(1)  To improve the precision and quality of precision products, injection pressure has the most obvious impact on the shrinkage of products. When the injection pressure reaches 392MPa, the molding shrinkage rate of the product is almost zero. At this time, the accuracy of the product is only affected by mold control or the environment. Experiments show that after the injection pressure is increased from 98MPa to 392MPa, the mechanical strength is increased by 3% to 33%.

(2)  It can reduce the wall thickness of precision products and increase the molding length. Taking PC as an example, the injection pressure of an ordinary machine is 177MPa. It can form products with a wall thickness of 0.2~0.8mm. While the injection pressure of a precision machine can form products with a thickness of 0.15~0.6mm when the injection pressure is 392MPa. Ultra-high pressure injection machine can obtain products with larger flow length ratio.

(3)  Increasing injection pressure can fully develop the effect of muddy injection rate. To achieve the rated injection rate, there are only two ways. One is to increase the maximum injection pressure of the system. The other is to modify the screw parameters to increase the aspect ratio. The injection rate of the precision injection machine is high. Take the DEMAG precision injection molding machine (60-420 tons) made in Germany as an example. Its injection speed can reach 1000mm/s, and the screw can get an acceleration of 12m/s2.

precision injection molding

3.2 Features of precision injection machine in terms of control

(1) The repetition accuracy (reproducibility) of injection molding parameters is high. So we should adopted multi-level injection feedback control:

a. Multi-level position control.

b. Multi-level speed control.

c. Multi-level pressure holding control.

d. Multi-level back pressure control.

e. Multi-stage screw speed control.

The accuracy of the displacement sensor is required to reach 0.1mm. Then, it can strictly control the metering stroke, injection stroke and the thickness of the remaining material pad (injection monitoring point). So it can ensure the accuracy of each injection and improve the molding accuracy of the product. The temperature control of the barrel and nozzle should be precise. And the overshoot should be small when heating up with a small temperature fluctuation. Precision injection molding should be controlled by PID, so that the temperature accuracy can be within ±0.5℃.

(2) Plasticization quality requirements.

The uniformity of plasticization not only affects the molding quality of injection molded parts, but also affects the resistance of molten plastic through the gate. To obtain uniform plasticization, designing a special screw and using dedicated plasticizing technology is essential. In addition, the temperature of the barrel should also be accurately controlled. At present, the temperature of the screw and barrel is mostly controlled by PID (proportional, differential, integral). And the accuracy can be controlled within ±l℃, which can basically meet the requirements of precision injection molding. The method is more suitable for precision injection molding if adopting the FUZZY control method.

(3) The temperature control of working oil

The temperature control of the working oil requires high oil temperature changes to cause fluctuations in the injection pressure. A closed-loop heating and cooling device must be used for the working oil. Besides, the oil temperature should be stabilized at 50-55°C.

(4) The influence of holding pressure

Holding pressure has a great impact on precision plastic parts. To be precise, holding pressure can better feed and shrink and reduce plastic part deformation. What’s more, it can control plastic part accuracy. The stability of holding pressure the molding accuracy of the plastic parts. The end position of the screw remains unchanged is the decisive factor in determining the pressure holding effect.

(5) The mold temperature control

If the mold temperature control requires the same cooling time, the thickness of the product with a lower mold cavity temperature is larger than that of the product with a higher temperature. Like POM and PA materials, the thickness of products is 50-100μm at a mold temperature of 50℃. But it will be reduced to 20-40μm at 80℃ and only 10μm at 100℃. Indoor temperature also affects the dimensional tolerances of precision products.

3.3 Hydraulic system of precision injection molding machine

(1) The oil circuit system needs to adopt the proportional system of proportional pressure valve, proportional flow valve or servo variable pump.

(2) In the direct pressure mold clamping mechanism, separate the oil passage of the clamping part and the oil passage of the injection part.

(3) Due to the high speed of the precision injection molding machine, the response speed of the hydraulic system must be emphasized.

(4) The hydraulic system of a precision injection molding machine must fully embody the machine-electricity-hydraulic-instrument integration project.

3.4 Structural characteristics of precision injection molding machine

(1) For the high injection pressure of the precision injection machine, we must emphasize the rigidity of the clamping system. The parallelism of the moving and fixed templates is controlled within the range of 0.05~0.08mm.

(2) The protection of low-pressure molds and the control of the precision of the clamping force are required. Because the size of the clamping force affects the degree of mold deformation. And it ultimately affects the dimensional tolerance of the part.

(3) The opening and closing speed of the mold should be fast, generally around 60mm/s.

(4) Plasticizing parts: screw, screw head, non-return ring, barrel, etc., should be designed with a structure with strong plasticizing ability, good homogenization, and high injection efficiency. Besides, screw drive torque should be large and with stepless variable speed.

Conclusion: No matter what kind of precision injection molding machine, in the end, it must be able to stably control the dimensional repeatability and quality repeatability of the product.