Mold Cavity Count
Compute the maximum number of mold cavities the machine can fill per cycle by dividing the machine's injection capacity by each part's mass (rounding down). More cavities increase productivity but require a larger machine and a more expensive, complex mold. It is a key calculation in production planning and mold selection. Enter the machine injection capacity and each part's mass.
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Mold cavity count
Why build a mold with a single cavity when the machine can fill several at once? The cavity count multiplies productivity: an 8-cavity mold yields 8 parts per cycle instead of one. The basic limit comes from the machine shot capacity divided by the mass of each part (plus the runners), rounded down. Other ceilings apply as well: the clamping force (more cavities = more projected area = more force), the plasticizing capacity and the physical space inside the mold. The decision is economic: more cavities demand a larger machine and a far more expensive and complex mold (with balanced runners so every cavity fills evenly), yet they spread the cost per part across high volumes. For short runs, a simple 1- or 2-cavity mold pays off better. Enter the machine shot capacity and the mass of each part.
Related Tools
Mold Clamping Force
Compute the clamping force needed on a plastic injection machine, F = projected area · cavity pressure, to keep the mold closed against the molten plastic pressure. If the force is insufficient, the mold opens during injection and plastic leaks out at the parting lines (flash). It is the parameter that defines the machine tonnage required for a part. Enter the part's projected area (mm²) and the cavity pressure (MPa).
Mold Cavity Pressure
Estimate the pressure that actually reaches the mold cavity by multiplying the injection pressure (at the screw tip) by the pressure transmission factor, which accounts for pressure losses along the runners, nozzle and gates to the cavity. Typically only 40–60% of the machine pressure reaches the part. It is the cavity pressure that defines the clamping force and fill quality. Enter the injection pressure and the transmission factor.
Plastic Mold Shrinkage
Compute the final dimension of a plastic part after molding shrinkage, part_dim = mold_dim · (1 − shrinkage%/100). Every thermoplastic shrinks as it cools and solidifies in the mold — from ~0.5% (amorphous like ABS) to 2–3% (semicrystalline like PP and PA). That is why the mold cavity is machined larger than the final part, compensating exactly for this shrinkage. Getting it wrong ruins an expensive mold. Enter the mold dimension and the material's shrinkage rate.
Hedström Number
Calculates the Hedström number (He), a dimensionless group combining density, yield stress, pipe diameter and plastic viscosity of a Bingham fluid. Used together with the Bingham Reynolds number, it locates the laminar-to-turbulent transition for drilling fluids, mineral slurries and cement pastes. Enter the four quantities.
Plastic Injection Flow Rate
Compute the injection flow rate by dividing the injected volume by the fill time, in cm³/s. It is the speed at which the molten plastic enters the mold — a parameter that controls the shear rate, molecular orientation, surface finish and defects such as jetting or flow marks. High flow fills fast but may degrade; low flow may solidify before filling. Enter the injected volume and the fill time.
Injection Cycle Time
Compute the total cycle time of a plastic injection by adding the injection time (fill + pack), the cooling time and the mold open/eject time. Cooling is usually the largest share (50–80% of the cycle). The cycle time directly determines productivity: parts/hour = 3600/cycle × cavities. Reducing it is the constant focus of optimization. Enter the injection, cooling and opening times.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.