Injection Unit Specifications Every Process Engineer Should Know

Shot volume

Shot volume is a key performance figure that indicates the weight of part the machine can produce. Depending on definition, it is expressed as theoretical shot volume, theoretical shot weight or actual shot weight. It is determined by screw diameter and stroke.

Injection pressure

The pressure the screw applies to the material during injection. It overcomes flow resistance from barrel to cavity, gives the melt the necessary speed and packs it. It is closely linked to part quality and dimensional accuracy, and is influenced by the plastic, the screw design and the processing temperature. Flow length and part weight increase with pressure, while shrinkage in the flow direction decreases.

Injection rate

To obtain parts with uniform density and stable dimensions, the cavity must be filled in a short time. Injection rate is the mass of melt injected from the nozzle per unit time. It is proportional to screw travel speed and inversely proportional to injection time, and can be calculated from screw diameter, injection speed, stroke and injection time.

Injection power

Power demand varies greatly across the cycle. The hydraulic system's peak power occurs during injection and is determined by injection speed and injection pressure.

Plasticizing capacity and recovery

Plasticizing capacity is the amount of plastic the injection unit can melt uniformly per unit time at maximum screw speed and zero back pressure. It depends on screw diameter, L/D ratio, screw speed and the material, and is governed mainly by the conveying capacity of the metering zone. Screw recovery time should be shorter than the part's cooling time; otherwise the cycle is extended.