Injection Molding Machine
Plasticizing capacity is the amount of material the screw can melt and homogenize in one hour; it is given in kg/h and is usually measured with polystyrene as the reference. If the real requirement set by shot weight and cycle time exceeds this capacity, the screw cannot keep up: the melt is not homogeneous and the cycle lengthens. In jobs with short cycles and heavy shots such as PET preforms, the real bottleneck is plasticizing, not clamping force.
The capacity value in the catalog is measured under ideal conditions with a specific material. In real production the material, back pressure, screw speed and temperature profile lower this value; that is why a margin of 20–25% is added on top of the requirement.
The requirement calculation is simple: total shot weight (part + runner) × the number of cycles per hour. The calculator does the same and gives the recommended capacity.
The metering time exceeds the cooling time and the cycle lengthens; when the screw speed is raised, shear heat and the risk of degradation rise. The solution is a larger screw diameter or a screw with a higher L/D ratio.