Matching a machine to a mold: shot volume, screw diameter and mold dimensions beyond tonnage

When choosing a machine for a mold, clamping force (tonnage) is usually the first number people look at. Tonnage is necessary but not sufficient: machines with the same tonnage can have very different screw diameters, shot volumes and mold mounting dimensions. A poor match means degraded material, an unstable cushion, short shots, or a mold that simply cannot be mounted. For the tonnage calculation see: How to calculate clamping force

1. Shot volume and utilisation

Machine catalogs give the screw's theoretical shot volume (cm³). To convert it to grams use the material's melt density, not its room-temperature density. Approximate melt densities: PP ≈ 0.75, PE ≈ 0.75–0.78, PS ≈ 0.95, ABS ≈ 0.95, PA66 ≈ 0.97, PC ≈ 1.05, POM ≈ 1.15 g/cm³. Check the data sheet for exact values.

Shot utilisation = required shot volume (parts + runner) ÷ machine's theoretical shot volume.

• In practice 20–80% is accepted; 30–70% is the comfortable target.
• Too low (below 20%): material sits in the barrel for a long time and can degrade thermally (especially PVC, POM, PC and flame-retardant grades); screw position control becomes difficult on short strokes.
• Too high (above 80%): no room left for a cushion, melt homogeneity suffers, the risk of air and cold spots rises.

2. Screw stroke: the 1D–3D rule

Metering stroke is also checked as a multiple of the screw diameter (D). Common practice: between 1D and 3D is good, up to 4D acceptable. Below 1D, metering and the V/P switchover point are squeezed into a tiny distance; above 4D, melt temperature and mixing homogeneity suffer.

3. Residence time in the barrel

How long the material stays in the barrel can be estimated roughly as follows (assuming the barrel — screw flights plus the front volume — holds about twice the maximum shot):

Residence time ≈ 2 × (machine max. shot ÷ part shot) × cycle time

This is a rough estimate; the exact value depends on screw design. Compare the result with the maximum residence time recommended by the material supplier. For heat-sensitive materials, values of more than a few minutes are risky.

4. Plasticising capacity

The screw must prepare the next shot within the cooling time. Divide the required shot weight (g) by the cooling time (s) and compare it with the machine's plasticising rate (g/s) for that material. If screw recovery takes longer than cooling, the cycle is being set by the screw, not the mold.

5. Does the mold fit the machine?

• Distance between tie bars: the mold width and height must pass between the tie bars (or the mold must be loadable from the side).
• Mold height: the closed mold thickness must lie between the machine's minimum and maximum mold height.
• Opening stroke: there must be enough opening for the part to drop or be picked by a robot. Rule of thumb: at least twice the part depth plus the runner; extra opening for three-plate molds.
• Ejector stroke and connections: ejector stroke, core pulls (hydraulic), number of hot runner zones, water circuits and locating ring diameter must match.
• Nozzle: nozzle radius and orifice must suit the sprue bushing.

Example

4-cavity PP closure mold: 35 g per part, 20 g runner → 160 g in total. Melt density ≈ 0.75 g/cm³ → required volume ≈ 160 ÷ 0.75 ≈ 213 cm³.

• Suitable theoretical shot volume for a 30–70% utilisation target: between 213 ÷ 0.70 ≈ 305 cm³ and 213 ÷ 0.30 ≈ 710 cm³.
• For a machine with a 450 cm³ shot volume and a 50 mm screw, utilisation ≈ 47% → suitable. Metering stroke ≈ 213 cm³ ÷ (π × 2.5² cm²) ≈ 10.8 cm ≈ 2.2D → within the good range.
• Mount the same mold on a machine with a 1,200 cm³ shot volume and utilisation drops to ≈ 18%; even if tonnage is enough, the material stays in the barrel longer than it should.

You can find tonnage, cooling and cycle values for your own part in the calculation tool, and save your machine list on the My machines page.