Breaking Down the Injection Moulding Cycle

A single injection moulding cycle can be broken down into three distinct stages: plastication, mould filling and cooling with solidification.

1.3.3.1 Plastication

This stage is carried out in the injection unit and is similar to the process of extrusion.

The polymer flow rate is governed by the material processing conditions of the plastication stage: a combination of material rheology, barrel temperature and shear, back pressure and screw speed. The basic aim is to produce a homogeneous melt for the next stage where the material enters the mould.

Moulding parameters which control the plastication stage are cylinder temperature, screw back temperature and back pressure.

1.3.3.2 Filling

Here the injection unit delivers a pre-set amount of molten polymer to the mould tool.

The parameters of mould filling are of great importance to the end result especially when considering factors such as warpage (orientation effects) and surface finish (skin formation). Filling dynamics are also thought to be the major factor in affecting the levels of residual stress. It is important that injection speeds are reproducible as slight changes can cause variations in the end product. Injection speeds that are too high can cause jetting and degradation and thus affect mechanical properties. A low speed may cause an increase in pressure requirements due to a thicker frozen layer and short shots (incomplete filling of the mould).

Thinner sections will generally need faster injection speeds than thick walled parts, mainly because of the decrease in the importance of the relationship between mould filling time and cooling time with a thicker section. The important thing is that the speeds are reproducible from one shot to the next.

Important moulding parameters for filling are the injection speed and injection pressure.

1.3.3.3 Packing and Solidification

Once the material is in the tool, filling must be completed (tool packing), the part cooled and finally ejected. The purpose of the packing stage is to add extra material to compensate for the shrinkage caused by the decreasing density of the solidifying polymer. If the additional polymer were not injected the component would shrink and warp due to nonuniform cooling. Ideally the packing and cooling stages should be such that the final dimensions are maintained as close as possible to design tolerances. Variables during this stage are packing pressure, packing time and the

mould temperature. Bad mould design can lead to inconsistent cooling along the dimensions of the mould surface which can cause increased residual stresses. Once the material has cooled sufficiently, the component can be injected and the injection cycle continues. The cycle does not occur sequentially, while one part is cooling, plastication of the next cycle has already begun.

A breakdown of the cycle and the relative time for each stage is shown in Figure

The injection moulding process and the parameter effects will be further discussed in Chapter 8.

A brief introduction to injection moulding has now been given. Before moving on to consider it in greater depth, a quick summary of other commercial methods of polymer processing will now be considered for comparison.