Injection Molding Shot Size Calculator

Inputs

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Molded part weight per cavity, excluding runner, sprue and gate scrap.
oz
Mold cavities filled each cycle.
cavities
Cold runner, sprue and gate weight per shot. Use 0 for hot-runner molds if appropriate.
oz
Allowance for cushion, early estimating error and process reserve. It is applied after part plus runner weight.
%
Machine injection-unit shot capacity. Many machine data sheets quote this using polystyrene.
oz
Actual material capacity divided by the rated capacity. Leave 1.0 if the listed capacity is already adjusted for your resin.
Lower target utilization. General-purpose resins often use a wider 20-80% window; engineered resins may need a tighter window.
%
Upper target utilization. Staying below this leaves room for process variation, cushion and recovery limits.
%
Projected plan-view area of one cavity at the parting line.
in²
Projected plan-view area of runners/sprue that contributes to mold-opening force.
in²
Estimated average cavity pressure acting over projected area. Use a value from simulation, historical jobs or a conservative process estimate.
psi
Multiplier applied to the projected-area clamp force estimate.
Available clamp force of the press. Imperial display uses US short ton-force.
tonf

Results

Default result
Edit inputs
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)3.111oz

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

Adjusted shot capacity(C_adj)4.233oz

Rated shot capacity multiplied by the material capacity factor.

Margin to max use(m_margin)0.2751oz

Max recommended shot weight minus required shot weight.

Min recommended capacity(C_min)3.889oz

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

Max recommended capacity(C_max)15.56oz

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.

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