Injection Molding Process

From a practical point of view, injection molding can be described very simply with the following steps:

• Take a certain amount of plastic material in pellets.
• Melt it inside a barrel.
• Apply a certain pressure.
• Let it cool down.
• Eject the part from the mold.

The polymer therefore goes through two changes of state:

• From the solid state to the fluid state.
• From the fluid state to the solid state.

The change of state takes place inside the mold, which has been specifically designed to give the part the desired shape.

The characteristic parameters describing the behaviour of the material in the injection molding process are therefore:

• The amount of material, which defines the flow rate.
• The temperature at which the material can flow: melting temperature for crystalline polymers and glass transition temperature for amorphous ones.
• The pressure applied to the melt.
• The cooling rate.
• The ejection temperature.

The correlation between the machine settings and the parameters describing the behaviour of the material is not straightforward.

Injection molding machines have electronic devices that control from a few to many parameters affecting the movement of the molten polymer from the plasticizing barrel to the mold.

As an example, the influence of the various machine parameters on the behaviour of the material can be summarised in a simple table:

Most machines have a closed-loop control based on the V/P switch-over.

During the filling phase the screw moves forward at constant speed but, as the cavity fills, the melt exerts a certain pressure on the screw. For the screw to keep moving at constant speed, the machine must provide a higher hydraulic pressure on the piston.

When the pressure becomes very high, the forward movement of the screw becomes minimal, so it is more convenient to switch to pressure control at a given value and for a given time (holding phase).

The main stages of the injection molding process can be summarised as follows:

Besides validating a design, process simulation makes it possible to "play" with the various machine parameters without carrying out any physical work.