Plastic Injection Molding Cost Calculation
Knowing how to calculate injection-molding cost lets you check profitability, run what-if scenarios, and judge whether a vendor’s quote is fair. This guide covers the four cost items — material, labour, machine rate and tooling — and the rules of thumb that keep the numbers honest.
- Material, labour, machine, tooling
- Cost per cubic inch, not per pound
- 15% regrind & runner rule
- Clamp tonnage sizing
Why calculate molding cost
Understanding how to determine molding cost is useful in three ways:
- Profitability — determine whether making the product will be profitable.
- What-if analysis — change materials, cavitation, volume or press size to see if more profit can be generated.
- Vendor check — monitor whether an outside molder’s costs are reasonable.
The four cost items
Material
Costed by volume (per cubic inch of the part), not just per pound of resin.
Labour
The operator and handling time attributable to each part.
Machine rate
Set by press size, which is driven by clamp tonnage.
Tooling
The amortised mould cost across the production volume.
Material cost and regrind
Material is usually the largest variable cost, so the rules around it matter.
- Cost material by volume (per cubic inch), not by weight alone.
- Use regrind where possible — often about half the price of virgin material.
- Safe regrind is usually up to 15% by weight (though 100% regrind can match virgin for physical properties).
- Unless a hot runner is used, keep runner and sprue under 15% of shot weight, so no more than 15% regrind is generated.
Clamp tonnage and machine selection
Clamp tonnage determines the machine size needed for molding. It is calculated from the projected area of the part (and runner) times the injection pressure. The resulting tonnage sets the press, and therefore the machine rate used in the cost calculation — which is why part layout and cavitation feed straight into cost.
Want the real cost per part?
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- Honest, itemised cost breakdowns
- What-if on material & cavitation
- Vendor-quote sanity checks
Cost calculation FAQ
What are the four items in an injection molding cost calculation?
Material, labour, machine rate and tooling cost. Analysing all four tells you whether a product will be profitable, lets you run what-if scenarios on materials, cavitation, volume and press size, and lets you check whether an outside molder’s quote is reasonable.
How should material cost be calculated?
By volume — cost per cubic inch of the part, not just cost per pound of raw resin. Part geometry decides how much material each shot uses, so a volume basis gives the true material cost.
How much regrind can be used safely?
Usually up to about 15% of total weight is safe as a rule of thumb, though tests show 100% regrind can match virgin material for final physical properties. Unless a hot runner is used, keep runner and sprue under 15% of shot weight so no more than 15% regrind is generated.
How is machine size chosen?
By clamp tonnage — the projected area of the part(s) times injection pressure sets the tonnage, which determines the press size and therefore the machine rate in the cost calculation.
Why calculate molding cost at all?
For three reasons: to check whether the product will be profitable, to play what-if games with materials, cavitation, volume and press size to find more profit, and to judge whether an outside molding vendor’s price is fair.
