When making a decision based on shot capacity and dosage volume one must bear in mind that the ideal screw operating range is between 20 and 80 percent of the maximum possible injection volume. If operation is carried out at the lower end of the injection volume, increased process fluctuations can be expected. The reason for this is poorer dosage behaviour, too long dwell times and the closing characteristics of the non-return valve. On the other hand, too large a load results in an increase in trapped air and, due to lower dwell times, in inhomogeneous material. These can then be seen in the moulded part in the form of non-melted particles or poorer mechanical properties.

In addition to the required shot capacity, the melting capacity of the injection unit is another decisive selection criteria. It is determined by means of the screw pitch volume and the material-specific dwell time. The minimum dwell time required – approximately 30 seconds for solid plastics and approximately 60 seconds for engineering plastics – therefore limits the maximum possible melting capability of the plasticising unit.

With high injection flows and pressures, which, for example, are required for thin-wall items, a selection based on shot capacity and melting capability alone is often inadequate, as the maximum attainable injection pressure is reduced within the injection unit as the screw diameter increases. In cases such as this it is advisable to change up to the next size of unit.

Finally, a check is to be carried out as to whether the plasticising speed available is adequate in order to increase the dosage to the necessary shot capacity within the cooling time. The selection of a larger injection unit could also be required in this case as well.