How to use this calculator
- Enter part and cavity data. Use the molded part weight per cavity and the number of cavities filled each cycle.
- Add runner and allowance. Enter cold-runner/sprue weight and a process allowance for cushion or early estimating uncertainty.
- Set the machine capacity. Enter rated shot capacity and adjust it with a material factor if the data sheet is PS-rated.
- Choose the utilization window. Use a broad or tight barrel-use range based on resin sensitivity and process guidance.
- Check clamp force. Enter projected area, cavity pressure and available press clamp force, then compare utilization.
How it works
Injection molding press selection has two separate checks: the injection unit must deliver the shot without running too small or too large a percentage of barrel capacity, and the clamp must hold the mold closed against cavity pressure. This calculator keeps both checks visible instead of hiding them inside a quote form.
The shot weight starts from the molded part weight, cavity count and runner/sprue weight:
m_base = n · m_part + m_runner
m_shot = m_base · (1 + allowance)
Machine utilization compares that required shot with the material-adjusted machine capacity:
U_shot = m_shot / (C_rated · f_mat)
If the capacity is quoted for polystyrene and the real resin has a different
output factor, use f_mat to convert the rated capacity. For resin
weight data from CAD volume, the material density
chart can help with first-pass mass estimates.
Clamp force is estimated from projected area, cavity pressure and the safety multiplier:
F_clamp = A_proj · P_cav · N
For finished part costing, pair this with the machining cost calculator workflow as a quoting model, or use the weight calculator for simple stock and plastic-mass estimates.
Worked example
Verified against the live calculator
A 4-cavity mold has 18 g parts, 12 g of cold runner per
shot and a 5% shot allowance. The machine is rated at
120 g adjusted capacity, and the target barrel-use window is
20% to 80%.
Base shot weight is 4 × 18 + 12 = 84 g. With the allowance,
required shot weight is 88.2 g. Utilization is
88.2 / 120 = 0.735, inside the target window. The same shot suggests a
machine capacity range of 110 g to 441 g. With
7,800 mm² total projected area, 4,000 psi cavity pressure
and a 1.10 safety factor, required clamp force is about
26.6 tonf.
Frequently asked questions
How do you calculate injection molding shot size?
A practical first-pass shot weight is part weight times cavities, plus runner and sprue weight, then any process allowance: m_shot = (n*m_part + m_runner)*(1 + allowance). The calculator compares that shot weight with the material-adjusted machine shot capacity.
What shot capacity utilization should I target?
For general-purpose resins, a common machine-sizing window is about 20% to 80% of adjusted shot capacity. Some TPE and engineered-resin guidance uses tighter windows such as 40% to 80% or 30% to 50%. That is why the calculator makes the minimum and maximum utilization editable.
Why is there a material capacity factor?
Many injection-unit data sheets quote shot capacity using polystyrene. If your actual resin has a different melt density or output factor, the same screw volume delivers a different weight. Use the factor to convert the rated capacity to the resin you will actually run.
How is clamp force estimated?
Clamp force is estimated from total projected area times cavity pressure times a safety factor: F = A_proj * P_cav * N. This is a screening estimate; wall thickness, flow length, pressure profile, slides and parting-line details can change the real press requirement.
Should I include runner and sprue weight?
Yes for cold-runner molds. Runner, sprue and gate material is part of the shot even if it is not part of the finished part. For a hot-runner mold, enter only the material that is actually injected each cycle.
Does this replace molding simulation or machine trials?
No. It is a machine-selection and quoting estimator. Final settings and press choice should be validated with resin data, mold design, simulation, process history and sampling.
Method & assumptions
- Shot weight is based on molded part weight, cavity count, runner/sprue weight and the entered allowance. It does not predict screw position, recovery time or melt temperature.
- The barrel-use window is a process guideline. Adjust it for resin family, thermal sensitivity, residence-time limits, filler content and machine supplier guidance.
- Clamp force uses projected area and an estimated cavity pressure. Flow length, wall thickness, gate location, venting, fill speed, slides, core pulls and parting-line geometry are not modeled.
- Machine shot capacity is often PS-rated. Use the material factor or manufacturer data for the actual resin and screw.
- Validate final press selection with the molder, resin supplier, moldmaker, machine data sheet and sampling results.