Sizing a Vertical Machine: Shot Volume and Clamping Force

Vertical injection molding machines vary widely in size, specification and quality. Select a machine based on the size and quality requirements of your parts, and on both current and long-term needs.

Start by defining the part's function, material, volume, shot weight and annual output. A plant molding silicone, for instance, should approach selection differently from one molding engineering plastics. Then check the following specifications.

Shot volume

Shot volume is normally rated for PS (polystyrene). If your material is not PS, convert the rated figure using the density of your material before comparing it with your part weight.

Clamping force

Clamping force is the final locking force the platens exert on the mold while melt is injected into the cavity. Without enough clamping force the part will flash, so make sure the force your part requires is below the machine's rating.

Required clamping force is usually calculated as average cavity pressure multiplied by the projected area of the cavity. Average cavity pressure is generally 20–40 MPa, depending on the material, part requirements and flow-length ratio. During mold design, computer simulation can give an accurate clamping force.