Injection Molding Shot Size Calculator

Inputs

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Part weight each (m_part)

Molded part weight per cavity, excluding runner, sprue and gate scrap.

Number of cavities (n)

Mold cavities filled each cycle.

cavities

Runner / sprue weight (m_runner)

Cold runner, sprue and gate weight per shot. Use 0 for hot-runner molds if appropriate.

Shot allowance (a)

Allowance for cushion, early estimating error and process reserve. It is applied after part plus runner weight.

Rated shot capacity (C_rated)

Machine injection-unit shot capacity. Many machine data sheets quote this using polystyrene.

Material capacity factor (f_mat)

Actual material capacity divided by the rated capacity. Leave 1.0 if the listed capacity is already adjusted for your resin.

Minimum barrel use (U_min)

Lower target utilization. General-purpose resins often use a wider 20-80% window; engineered resins may need a tighter window.

Maximum barrel use (U_max)

Upper target utilization. Staying below this leaves room for process variation, cushion and recovery limits.

Projected area each (A_part)

Projected plan-view area of one cavity at the parting line.

mm²

Runner projected area (A_runner)

Projected plan-view area of runners/sprue that contributes to mold-opening force.

mm²

Cavity pressure (P_cav)

Estimated average cavity pressure acting over projected area. Use a value from simulation, historical jobs or a conservative process estimate.

bar

Clamp safety factor (N)

Multiplier applied to the projected-area clamp force estimate.

Machine clamp force (F_clamp)

Available clamp force of the press. Imperial display uses US short ton-force.

Results

Default result

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Shot utilization(U_shot)

0.735

Pass

Target window: 20% to 80% of adjusted shot capacity.

Required shot weight divided by material-adjusted machine shot capacity.

Also computed

Required shot weight(m_shot)88.2g

(part weight * cavities + runner weight) with the shot allowance applied.

Adjusted shot capacity(C_adj)120g

Rated shot capacity multiplied by the material capacity factor.

Margin to max use(m_margin)7.8g

Max recommended shot weight minus required shot weight.

Min recommended capacity(C_min)110.3g

Machine capacity that would put this shot at the maximum target utilization.

Max recommended capacity(C_max)441g

Machine capacity that would put this shot at the minimum target utilization.

Max cavities by shot(n_max)4

Integer cavity count that fits the max barrel-use target with the same part and runner weights.

Method notes 4 notes

Shot weight = (4 cavities * part weight + runner/sprue weight) * (1 + shot allowance).
Shot utilization is checked against your selected barrel-use window. General-purpose resins often tolerate a wider window than heat-sensitive or engineered resins.
If the machine data sheet is PS-rated, adjust the material capacity factor for the actual resin or use a capacity already converted by the machine/resin supplier.
Clamp force is a projected-area estimate. Thin walls, long flow length, slides, parting-line geometry, venting, fill speed and pressure profile can change the required press tonnage.