Technical serialisation - maintenance and servicing of plastic injection moulding machines (2)

1.4.2.3 Selection of injection pressure The injection pressure of the injection moulding machine is regulated by a pressure regulating valve. Under set pressure conditions, the injection pressure in the high pressure and low pressure oil channels is controlled to control the pre- and post-injection pressure.
There are three pressure options for normal medium-sized and above injection moulding machines, namely high pressure, low pressure and high pressure followed by low pressure. High pressure injection is achieved by injecting high pressure oil into the injection cylinder. Due to the high pressure, the plastic enters the mould cavity at high pressure and speed from the start. During high pressure injection, the plastic is injected into the mould quickly and the injection cylinder pressure gauge reading rises rapidly. Low pressure injection is achieved by injecting low pressure oil into the injection cylinder. The pressure gauge reading rises slowly during injection and the plastic enters the cavity at low pressure and speed. After high pressure, the pressure introduced into the cylinder is controlled in time to achieve low pressure, depending on the type of plastic and the actual requirements of the mould.
In order to meet the requirements of different plastics at different injection pressures, screw or plunger changes of different diameters can also be used. This meets the injection pressure and gives full play to the machine's production capacity. In large injection moulding machines, there are often multi-stage injection pressure and multi-stage injection speed control functions, which can better ensure the quality and precision of the products.