FIRST, WE NEED TO KNOW WHAT KIND OF MOULD IT IS COLD RUNNER OR HOT RUNNER.

If it is Hotrunner, No of cavities (layman words number of parts to be filled) multiplied by each part weight is Total shot weight or Shot Size.

if it is a cold runner, No cavities multiplied by each part weight added to Runner weight is Shot weight or Shot Size.

Remember the shot size mentioned by the Machine manufacturer for PS. If your material other than PS, then you have to identify the specific gravity of the material to be processed and then multiply it with the Total shot weight of the machine mentioned in the catalog.

Multiply machine shot by two if maximum injection screw stroke is less than four.
Multiply machine shot by 1.4 if maximum injection screw stroke is greater than or equal to four
Divide the result by the melt density for polystyrene.
Multiply the result by the cycle time of molding.

Theoretical injection capacity = (injection molding machine) cylinder cross-sectional area screw stroke, injection weight = injection capacity melting density of raw materials injection efficiency coefficient (generally 0.86).

Theoretical injection weight is 91% of PS theoretical injection capacity.