Injection Molding Calculator
Batch Price Breaks
| Volume↕ | Price/Part↕ |
|---|---|
| 100 | $95.35 |
| 500 | $19.35 |
| 1,000 | $9.85 |
| 5,000 | $2.25 |
| 10,000 | $1.30 |
This injection molding calculator estimates the part volume, shot weight, tooling cost, cycle time, and per-piece cost for a plastic part, then finds the breakeven quantity against 3D printing. It approximates the part as a thin shell from your overall dimensions and wall thickness, applies resin density and price, and uses deposition-style cost factors for material, machine time, and amortized mold cost. The output helps screen tooling investment for a target production volume.
Formula
V = 2(LW + LH + WH)·t ; shot = V·ρ·cavities·1.15 ; cycle = 2·t² + max(1, V/50) + 5
- V
- Approximate part volume as a thin shell (cm³ after converting from mm³)
- t
- Wall thickness (mm); the cooling term scales with t²
- ρ
- Resin density (g/cm³) from the selected material
- shot
- Shot weight including a 15% runner and sprue allowance (g)
- cycle
- Estimated cycle time in seconds: cooling 2·t², fill max(1, V/50), and 5 s open/close
How it works
- Enter the part length, width, height, and wall thickness in millimeters, then choose the resin (which sets density and a relative material price).
- Set the number of cavities and the planned production volume. The tool estimates shell volume, shot weight (with a 15% runner allowance), and cycle time from a cooling-dominated model.
- Mold cost is built from a base tooling cost plus a size and cavity premium, then amortized over the production volume. The result reports piece-part cost, an optimal cavity count, batch price breaks, and the breakeven quantity versus 3D printing.
Worked example
A 100 × 50 × 30 mm ABS part with a 2 mm wall, single cavity, and a planned run of 10,000 pieces.
- Shell volume = 2 × (100·50 + 100·30 + 50·30) × 2 = 2 × 9,500 × 2 = 38,000 mm³ = 38 cm³.
- Shot weight = 38 × 1.04 × 1 × 1.15 = 45.4 g (ABS density 1.04 g/cm³).
- Cycle time = 2 × 2² + max(1, 38/50) + 5 = 8 + 1 + 5 = 14 s.
- Mold cost = 8,000 + (100·50/10,000)·3,000 = 8,000 + 1,500 = 9,500, amortized as 0.95 per part; with 0.11 material and 0.23 machine, piece cost ≈ 1.30.
Part volume 38 cm³, shot weight 45.4 g, cycle time 14 s, mold cost about $9,500, piece-part cost about $1.30, and breakeven versus 3D printing near 1,775 pieces.
Frequently asked questions
- How is the part volume estimated?
- The calculator treats the part as a thin-walled box and multiplies the total surface area by the wall thickness. This shell approximation is fast for screening but will differ from solid or heavily ribbed parts, which should use CAD-measured volume.
- When does injection molding beat 3D printing?
- Molding has a high upfront tooling cost but a very low per-part cost, while 3D printing has no tooling cost but a higher per-part cost. The breakeven quantity is the volume at which the amortized mold cost is offset by the per-part savings, often a few hundred to a few thousand parts.
- Why does wall thickness dominate the cycle time?
- Cooling is usually the longest phase of the molding cycle and scales with the square of the wall thickness. Doubling the wall roughly quadruples the cooling time, so thinner, more uniform walls are the most effective way to speed up the cycle.
- How accurate is the mold cost estimate?
- It is a parametric estimate built from a base cost plus size and cavity factors, not a quote. Real tooling cost depends on part complexity, steel grade, surface finish, action count, and supplier, so treat the figure as an order-of-magnitude planning number.