Once the complete injection phase is finished, the cavities are filled with material having a solidified outer skin and a molten core.As the mould is considerably colder than the melt, the temperature of the material begins to fall and shrinkage starts. The total amount of
shrinkage varies between 7% and 18% of the volume of the mouldings depending upon the chemical composition and structure of the material. In the case of the semicrystalline materials, the cooling rate can also greatly affect overall shrinkage, which is usually quite high compared with amorphous materials.
Gate Freeze-off Phase
The material in the core of the moulding remains fluid for some time, as does that in the gates, runners and sprue. The gate itself will probably be hotter than the rest of the mould owing to the shear heat generated as the mould filled, as explained previously. As long as this fluid pathway through the gate remains open and the holding pressure is applied, the mould will be kept topped up with fresh material to replace the loss in volume as the melt cools.
The holding pressure can be effectively used in this way right up until the gate freezes off, and when making high-quality mouldings it is essential that the holding pressure time matches exactly the gate freeze-off time. Clearly, once the gates have completely frozen off, there is no point in any further pressure being applied as there is no pathway for material to enter the cavities.
Melt Compressibility and Shrinkage
The compensation phase provided by the holding pressure will substantially reduce the overall shrinkage of the mouldings. However, further reductions in shrinkage can be achieved by making use of the high melt compressibility property displayed by
most plastics.
The compressibility of a polymer melt depends upon several factors including the chemical nature and structure of the polymer, the melt temperature, the mould temperature and the pressure being applied. The compressibility varies between 2% to 15% by volume; this means that by compressing the melt with sufficient holding pressure it is possible to force additional material into the mould, giving further reductions in shrinkage.
However, this technique should be used with caution and not applied universally or at too high a holding pressure as this may cause the parts to flash or overpack and induce ejection problems as described earlier. Additionally, too high a holding pressure can often
lead to moulded-in stress.
Generally, for most materials the holding pressure should be set at the lowest value needed to give mouldings of acceptable quality and be maintained until the gate freezes off. However, semicrystalline materials like acetal, Nylon, etc. do require high holding pressure to achieve required dimensions and mechanical performance.
Walsh's Plastic consulting. 