Several moulding parameters directly influence the injection of plastics into the mould, including the following:
· Injection speed
· Melt cushion
· Injection pressure
· Injection time
3.4.1 Injection Speed
Injection speed is a key processing parameter.
· For older machines
Older machines have only a simple flow control valve that regulates the amount of hydraulic oil going to the injection cylinder piston. Opening the valve will allow more oil to enter the piston at a greater rate and thus the plastics is injected faster.
With materials that flow with some difficulty it is recommended to use full injection velocity- (75% to 100% of the available injection velocity.) For moulding plastics which flow more readily it is recommended to start at 50% of the potential injection velocity and slowly work up as needed.
· For newer machines
More advanced machines control multiple stages of the injection velocity to more accurately control the process. Later model injection moulding machines allow you to better control your process and, as a result, will give you less part-to-part variation and better part performance.
3.4.2 Melt Cushion
The melt cushion is the material at the front of the screw when the screw is in the forward position. Always injection mould with a melt cushion of 1/8” to 1/4” to allow the part to pack out evenly. A pressure loss can result if the cushion is too high, and the parts will not mould consistently.
3.4.3 Injection Pressure
Pressure is created by a resistance to flow. As hydraulics are controlled by this property, injection pressure settings can be developed.
To establish first stage injection pressure, raise the pressure to a point where the part fills out without any packing. The screw moves forward and stops as it reaches the melt cushion.
At this point, second stage pressure is implemented to allow the cushion to pack out the part. Older machines usually have a combination hydraulic pump that includes a high-volume, low-pressure pump and a low-volume, high-pressure pump.
The high-volume pump allows for a fast, steady, forward movement of the screw, and the low-volume pump handles the packing of the part. As moulding machines are designed around their hydraulics, the high-volume pump consumes more energy than the low-volume pump. Therefore, it is more efficient to switch to the low-volume pump as soon as possible to reduce the amount of energy consumption. As a result, more energy is conserved by reducing the first stage time than by reducing the cooling time.
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The more advanced machines control multiple pressure settings and therefore more accurately control the moulding process and final moulded part quality.
3.4.4 Injection Time
Injection time is the amount of time that the screw remains forward. By controlling the rate of hydraulic oil to the injection cylinder piston, injection time is controlled.
Assuming a machine has only a first and second stage:
· The first stage is the amount of time it takes to fill the part.
· The second stage is the time required for the gate to freeze off.
In a multiple stage injection, the last stage is usually the packing stage because the sprue, runner, and parts are full. At this point, you will notice that the part weight remains constant from shot to shot.
