In a cold-runner moulding shop, every shot makes plastic you didn't sell: the runner and sprue, and a few rejected parts. Some runner material comes back as regrind, which costs money to grind and handle. The rest, and every reject, is virgin material paid for and lost. On this calculator's defaults (20,000 shots a day, an 8 g runner, 70% of it reused, 2% rejects on a 25 g part, ₹120/kg) that comes to about ₹23.57 lakh a year.
How the calculator works
The calculator takes eight inputs:
- Shots a day. Total shots across the machines you are looking at.
- Runner weight per shot (g). Sprue plus runners, weighed off the mould. On a hot-runner mould this is close to zero.
- Runner regrind you can reuse (%). The share of runner material that actually goes back into production. The rest is contaminated, mixed-colour, over your regrind limit, or sold as scrap.
- Part weight per shot (g). The weight of the good parts from one shot, across all cavities.
- Rejected parts (%). Short shots, flash, sink, burns and colour streaks that are scrapped.
- Material cost (₹/kg). Landed cost of the virgin resin, including masterbatch if you colour in-house.
- Regrind processing cost (₹/kg). Grinding, drying, handling, dust loss and labour per kg reused.
- Working days a year.
The formulas:
Runner material a day (kg) = shots × runner weight ÷ 1,000
Runner material lost (kg) = runner material × (1 − reuse %)
Runner material reused (kg) = runner material × reuse %
Rejected material a day (kg) = shots × reject % × part weight ÷ 1,000
Cost a day = lost runner × material price + reused runner × regrind cost + rejects × material price
Cost a year = cost a day × working days
Lost runner and rejects are valued at the full virgin price. That is the cost of the material you bought and didn't sell. If you sell scrap, the true loss is lower by the scrap price; if a reject also wasted machine time, it is higher. Rejects are assumed not to be reground. If you grind them too, the material cost is overstated.
Worked example (India)
A PP housewares or closures shop on a cold-runner mould. All values are assumptions for illustration; they are the calculator's defaults.
| Input | Value | Basis |
|---|---|---|
| Shots a day | 20,000 | assumption |
| Runner weight per shot | 8 g | assumption |
| Runner regrind reused | 70% | assumption |
| Part weight per shot | 25 g | assumption |
| Rejected parts | 2% | assumption |
| Material cost | ₹120/kg | assumption; see the PP price below |
| Regrind processing cost | ₹8/kg | assumption |
| Working days a year | 300 | assumption |
| Runner material a day | 160 kg | 20,000 × 8 g |
| Runner reused a day | 112 kg | 160 × 70% |
| Runner material lost a day | 48 kg | 160 × 30% |
| Rejected parts a day | 10 kg | 20,000 × 2% × 25 g = 400 shots' worth |
| Cost a day | ₹7,856 | 48 × ₹120 + 112 × ₹8 + 10 × ₹120 |
| Cost a year | ₹23.57 lakh | ₹7,856 × 300 |
Where the money goes:
| Loss | Kg a day | ₹ a day | ₹ a year |
|---|---|---|---|
| Runner material lost | 48 | ₹5,760 | ₹17.28 lakh |
| Regrind processing | 112 | ₹896 | ₹2.69 lakh |
| Rejected parts | 10 | ₹1,200 | ₹3.60 lakh |
| Total | ₹7,856 | ₹23.57 lakh |
The runner that never comes back is nearly three-quarters of the bill. A 2% reject rate, which gets most of the attention on the shop floor, is about 15%.
One check on the blend: the shop moulds 500 kg of parts and 160 kg of runner a day, about 660 kg of material. The 112 kg of regrind is about 17% of that. That is under the 25% limit in UL 746D, but above the 15% that one hot-runner vendor says is typical (see below). If your customer's drawing sets a lower limit, the surplus can't go back in, and the reuse input should come down.
What typical numbers look like
- Runners can be heavier than the parts. Hot-runner maker Mold-Masters writes that a cold runner "can equal 50% to 250% of the mold part weight with regrind typically limited to 15% at most". It is a vendor selling the alternative, so treat the range as an upper bound for scale. The default here, 8 g on 25 g, is 32%.
- The common regrind ceiling is 25%. UL Solutions explains that under UL 746D, in-house sprue and runner regrind "can be re-introduced into the molding hopper to comprise up to 25%", with the rest the same virgin grade. Automotive, food-contact and pharma buyers often set their own lower limits.
- Resin price. When checked on 13 October 2026, one price index showed India polypropylene at US$1.42/kg for September 2026, about ₹125/kg at an assumed ₹88 to the dollar. An earlier reading of the same index, quoted in our moulding guide, showed US$1.28 for that month: index values get revised. Domestic list prices change monthly, so use your last invoice.
- Reject rates and regrind processing cost. We found no credible public benchmark for Indian moulding shops. Weigh your own scrap bins for a week.
What this number does not tell you
- Machine time lost on rejects. Every rejected shot also used a cycle of machine time, power and an operator. The calculator only counts material. The rework cost calculator and the machine hour rate calculator cover the time side.
- Whether regrind is hurting quality. Each melt shortens polymer chains. Material that has been reprocessed several times can give brittle hinges or colour drift that show up as rejects later. The full guide to runners, purge and rejects covers the evidence.
- Startup and purge scrap. Colour changes, mould changes and cold starts produce purge and setup shots that aren't in this calculator. On a high-mix shop they can be larger than steady-state rejects. Add them as extra rejects if you weigh them.
- Whether a hot runner pays. A hot-runner mould removes most of the runner loss but costs more and needs its own maintenance. Use the runner-lost line here as the yearly saving to set against the quote.
- What a camera can and cannot see. A camera can show how long a machine stood after a mould change, how often an operator cleared a jam, and how many purge cycles a colour change took. It cannot weigh a runner, see a short shot inside a closed bin, or tell you a part is out of tolerance. Weights and reject rates come from the scale and from QC.
Related guides
- Runners, Purge and Rejects: What Regrind and Scrap Really Cost an Indian Moulding Plant (₹)
- Cameras in Injection-Moulding Units: Short Shots, Cycle Drift and Mould Changes
- Machine Hour Rate in India: Count the Hours You Never Sold
- Changeover time cost calculator (₹)
- Rework cost calculator (₹)
FAQ
How do you calculate the cost of runner scrap in injection moulding? Multiply shots a day by runner weight to get kg of runner. Value the share you can't reuse at the virgin material price, and the share you reuse at your regrind processing cost. At 20,000 shots, an 8 g runner, 70% reuse, ₹120/kg and ₹8/kg, that is ₹6,656 a day before rejects.
How much regrind can you add to virgin material? UL 746D allows in-house sprue and runner regrind up to 25%, with the rest the same virgin grade. Many customers set a lower limit in their drawings, and one hot-runner vendor says 15% is typical in practice. Validate the blend on your actual part.
Is regrind free material? No. Grinding, drying, handling and dust loss cost money, and repeated melting weakens some materials. The calculator charges a processing cost on every kg reused for that reason.
Do rejects or runners cost more? On a cold-runner mould, usually the runner. In the default example, lost runner costs ₹17.28 lakh a year against ₹3.60 lakh for a 2% reject rate. A heavy runner on a light part makes the gap wider.
When does a hot runner pay for itself? When the yearly runner loss and regrind cost it removes are larger than its extra cost spread over the mould's life, plus its maintenance. Use the runner lines of this calculator as the saving.
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