Injection moulding involves a high pressure injection of a polymer into a mould where it is shaped. The individual parts of this process are very short. The whole injection moulding process usual lasts from 2 seconds to 2 minutes. There are four stages in the cycle. These stages are the clamping, injection, cooling and ejection stages.
Clamping
Both of the mold's halves must be sealed up before material is injected into it. A clamping device is used to close them. Then, one half can slide once the two halves are linked to the injection molding machine. The clamping unit presses the parts together, and material is subsequently injected while both half are tightly clamped. Larger, more powerful machines require more time to shut and clamp the mold.
Injection
A hopper is used to feed plastic pellets into the injection molding process. The injection unit moves these pellets in the direction of the mold. The pellets are melted by the pressure and heat surrounding the barrel. The term “shot” refers to the amount of substance administered. When the mold is filled to a 95%–99% level or the injection time is complete. Because the plastic flow is dynamic and constantly changing, it is challenging to determine the insertion time precisely. Other characteristics, such as injection pressure, power, and shot volume, can be used to predict injection time. An injection pressure of 35 to 140 MPa is possible. The machine's hydraulic system determines and regulates the injection rate and the maximum pressure that can be applied.
Cooling
The cooling process involves the plastic inside the mould beginning to cool after it makes contact with the interior mould. As the plastic cools when it hardens it will take the desired shape. The part may shrink slightly during cooling. Only when the cooling period has elapsed can the mould be opened.
It is from the wall thickness of the part and the thermodynamic properties of the plastic that the cooling time can be estimated.
Ejection
The last stage is ejection from the machine. This is done with an ejection system. When the mould opens the part is pushed out, Force must be used because the part shrinks and sticks to the mould. The mould can be shut again after ejection and another shot can be injected for the process to begin again.
0
