(1) Injection pressure:
Ordinary injection molding machine: 147~177Mpa; precision injection molding machine: 216~243 Mpa; ultra-high pressure injection molding machine: above 243MPa, usually between 243~392MPa.
The advantages of precision injection molding machine for high pressure molding are:
1. Improve the precision and quality of precision products
The injection pressure has the most obvious effect on the molding shrinkage rate of the product. When the injection pressure reaches 392 MPa, the molding shrinkage rate of the product is almost zero. At this time, the accuracy of the product is only affected by the mold control or the environment. Experiments show that the mechanical strength of the parts can be increased by 3 to 33% when the injection pressure is from 98 to 392 MPa.
2. Reduce the wall thickness of precision products and increase the molding length
Taking PC as an example, the injection pressure of the ordinary machine is 177 Mpa, which can form products with a wall thickness of 0.8mm, while the precision injection molding machine can form products with a thickness of more than 0.45mm when the injection pressure is 392 MPa. The ultra-high pressure injection machine can obtain products with a larger flow ratio.
3. Increasing the injection pressure can give full play to the effect of the injection rate
In order to achieve the rated injection rate, there are two ways: one is to increase the maximum injection pressure of the system. The other is to modify the parameters of the injection device or injection system, including screw parameters.
(2) Injection speed:
The injection speed of the precision injection molding machine is required to be high. The injection speed of the precision hydraulic injection molding machine is required to reach more than 200mm/s, and the injection speed of the all-electric injection molding machine can reach more than 300mm/s.
(1) The repeatability (reproducibility) of precision injection molding parameters is high, and multi-stage injection feedback control should be adopted.
1. Multi-level position control;
2. Multi-level speed control;
3. Multi-level pressure holding control;
4. Multi-level back pressure control;
5. Multi-stage screw speed control.
The accuracy of the displacement sensor is required to be at least 0.1mm, so that the metering stroke, the injection stroke and the thickness of the expected pad (injection monitoring point) can be strictly controlled. Ensure accurate injection volume each time and improve product forming accuracy.
(2) The temperature control of barrel and nozzle should be precise
When the temperature control system heats up, the overshoot should be small, and the temperature fluctuation should be small. Precision injection molding adopts PID proportional, integral and differential temperature control, so that the temperature accuracy is within ±1°C, the temperature control accuracy of ultra-precision injection molding machine is ±0.1°C, and the temperature deviation can be stably controlled within ±0.2°C.
(3) The temperature of the hydraulic oil of the precision injection molding machine needs to be controlled
The change of the oil temperature of precision injection molding causes the fluctuation of the injection pressure. It is necessary to use a cooling device for the hydraulic oil, and it is advisable to stabilize the oil temperature at 50 to 55 °C.
(4) Control of mold temperature requirements
The mold temperature has a considerable influence on the later heat shrinkage of the part, and also affects the apparent quality and crystallinity of the part, and also affects the mechanical properties of the part. If the cooling time is the same, the thickness of the product with a lower mold cavity temperature is larger than that of the product with a higher temperature.