Injection Moulding

Injection Moulding Principles

For thermoplastics the dominant process for producing complex shapes is injection moulding in which the polymer melt is produced efficiently in one part of the machine.

In a separate function, the measured volume of melt is then forced into the cavity between the two mould plates. These are locked together to prevent separation under the high hydrostatic pressure of the melt in the cavity.

The final stage is to extract heat from the moulding using a heat exchange system incorporated into the mould plates, before the mould halves are opened and the finished moulding ejected. For thin walled parts cycle times can be less than 10 seconds but for thicker sections the cycle times may be well over 60 seconds.

Melt temperatures can be as low as 150oC but for some thermoplastics they have to be over 350oC. In all cases injection pressures can be up to 120 MPa, consequently creating large hydrostatic opening forces in the mould.

Mould locking (clamping) forces can be as low as 100 kN (10 tonnes force) for small area mouldings and more than 25,000 kN (2500 tonnes) for very large mouldings.

The injection moulding process can be conveniently divided into four phases.

To keep the cycle time short, some of these phases can operate in parallel, eg plasticisation for the next cycle can occur at the same time as cooling and ejection.

To optimise the filling process injection times are short (less than 5 seconds and sometimes less than 1 second). This accounts for the high injection pressures (up to 120 MPa).

As soon as the melt starts to cool in the cavity it will contract and lose compression. Volumetric shrinkage during the cooling phase will result in a moulding that will be slightly smaller than the mould cavity and lacking the detail of the mould cavity.

To minimise the mould shrinkage effect, pressure is maintained for a few seconds after the injection stage to pump more melt in to compensate for the shrinkage.

A high hold pressure will also minimise mould shrinkage but it cannot be eliminated. Typical mould shrinkage values range from 0.5% or less for amorphous thermoplastics and filled thermoplastics to more than 1% for semi-crystalline thermoplastics.

To meet dimensional specifications mould cavity dimensions have to be adjusted to allow for the shrinkage. Because the final mould shrinkage value depends on process variables such as melt temperature, injection pressure, hold pressure and hold time, this presents a major problem to toolmakers to anticipate the shrinkage factor.