How is resin cost per part calculated with runners, rejects and regrind?

Shot weight is part weight times cavities plus runner: 46 g x 4 + 22 g = 206 g in the example. With 1.5 percent rejects there are 3.94 good parts per shot, so 24.76 g per shot is runner and rejects. The print allows 10 percent regrind (20.6 g), so 4.16 g is lost. Virgin resin per good part is (206 - 20.6) / 3.94 = 47.06 g, which costs 0.1223 at 2.60 per kg.

Why does screw recovery not always add to the cycle?

The screw recovers while the part cools, so the cycle uses the longer of the cooling timer and recovery time. In the example cooling is 16 s and recovery 9 s, so the cycle is 1.8 + 5.0 + 16.0 + 6.0 = 28.8 s. If recovery grew to 18 s, the cycle would grow by 2 s.

How is press cost per part worked out?

Press rate times cycle time in hours, divided by good parts per shot. At 64 per hour, 28.8 s and 3.94 good parts per shot that is 64 x 28.8 / 3600 / 3.94 = 0.1299 per part. Dividing by good parts, not cavities, means you are paid for the shots that make rejects.

How should the mold cost be handled?

Choose Amortise to add mold cost divided by the amortisation quantity to every part, for example 38,000 over 300,000 parts is 0.1267 per part. Choose Customer pays separately when the customer buys the mold, and the piece price leaves it out.

Why is the piece price higher for small orders?

Each run carries a changeover: in the example 2.5 hours of press time at 64 per hour plus 6 kg of purge resin, 175.60 per run. That adds 0.0878 per part on a 2,000-part order but only 0.0035 on 50,000 parts, so the price falls from 0.928 to 0.812.

Does it agree with the Molded Part Cost calculator?

Yes where they overlap. The calculator divides press time by cavities; this workbook divides by good parts per shot so rejected shots are paid for. At a 0 percent reject rate they are identical, and the build checks the calculator's own example (0.095 resin, 55 per hour, 22 s, 8 cavities, 0.017 tooling) against its engine.

Industry Cost Kits

Injection Molding Part Cost Excel Template

Cost a molded part from resin, cycle time and press rate, add secondary work, changeover and mold amortisation, and price five order quantities against your target margin.

Overview

This injection molding cost template is an Excel estimating tool for custom molders and for buyers who want to check a molded-part quote. It ships with a worked example for a fictional molder, a 4-cavity ABS instrument housing with heat-staked brass inserts and a pad-printed logo, priced at five order quantities from 2,000 to 50,000 parts.

Resin is costed the way it is bought. The shot is part weight times cavities plus the cold runner; rejects reduce the good parts per shot; runner and rejects are reground up to the share of the shot the customer allows, and only the rest is lost. That gives the virgin resin bought per good part and its cost. Cycle time adds fill, pack and hold, the longer of the cooling timer and screw recovery, and mold movement; the press rate times cycle time, divided by good parts per shot, is the press cost per good part.

Secondary operations and packaging add labour seconds at your labour rate and bought-in items. The mold is amortised over a quantity you agree with the customer, or billed separately, and the changeover (mold change, start-up and purge resin) is spread over each order. Overhead and your target margin on price give a piece price rounded to your step, and press hours and press days show how long each order occupies the press. The resin, cycle-time and part-cost formulas match the calculators on this site, and the build checks them against the calculators' engines.

What this template includes

Shot and resin block: part and runner weights in g or oz, cavities, reject rate and the regrind allowed, giving virgin resin bought per good part and the runner-and-reject loss
Cycle time from fill, pack and hold, cooling or screw recovery (whichever is longer) and mold movement, with good parts per hour and press cost per good part
Press rates and resin prices on Setup, with prices per kg or per lb
Secondary operations and packaging table: labour seconds at your labour rate plus bought-in inserts, ink and cartons
Mold cost amortised over a quantity you set or paid separately by the customer, and a changeover per run with purge resin
Five order quantities with shots, cost lines, overhead, margin on price, a piece price rounded to your step, press hours and press days
Dashboard for the quantity you pick: piece price, cost, margin against target, good parts per hour, two charts and the cost build-up
Verify sheet that recomputes resin, cycle time and the price build-up on fixed inputs against an independent reference

Suggested use case

Use it to quote a new molded part, to check a supplier's piece price, or to see what a cavity, a cycle-time saving or a regrind allowance is worth per part.

How to use it

On Setup, type your company name, currency, labour rate, overhead, margin and price step, then your press rates and resin prices.
On Cost Builder, choose the resin, enter part and runner weights, cavities, reject rate and the regrind allowed.
Choose the press and enter fill, pack and hold, cooling, screw recovery and mold movement times.
List secondary operations and packaging, then the mold cost, how it is paid, the changeover per run and five order quantities.
Read cost, piece price, margin and press days per quantity; on the Dashboard, pick a quantity to see the tiles, charts and cost build-up.

Frequently Asked Questions

How is resin cost per part calculated with runners, rejects and regrind?: Shot weight is part weight times cavities plus runner: 46 g x 4 + 22 g = 206 g in the example. With 1.5 percent rejects there are 3.94 good parts per shot, so 24.76 g per shot is runner and rejects. The print allows 10 percent regrind (20.6 g), so 4.16 g is lost. Virgin resin per good part is (206 - 20.6) / 3.94 = 47.06 g, which costs 0.1223 at 2.60 per kg.
Why does screw recovery not always add to the cycle?: The screw recovers while the part cools, so the cycle uses the longer of the cooling timer and recovery time. In the example cooling is 16 s and recovery 9 s, so the cycle is 1.8 + 5.0 + 16.0 + 6.0 = 28.8 s. If recovery grew to 18 s, the cycle would grow by 2 s.
How is press cost per part worked out?: Press rate times cycle time in hours, divided by good parts per shot. At 64 per hour, 28.8 s and 3.94 good parts per shot that is 64 x 28.8 / 3600 / 3.94 = 0.1299 per part. Dividing by good parts, not cavities, means you are paid for the shots that make rejects.
How should the mold cost be handled?: Choose Amortise to add mold cost divided by the amortisation quantity to every part, for example 38,000 over 300,000 parts is 0.1267 per part. Choose Customer pays separately when the customer buys the mold, and the piece price leaves it out.
Why is the piece price higher for small orders?: Each run carries a changeover: in the example 2.5 hours of press time at 64 per hour plus 6 kg of purge resin, 175.60 per run. That adds 0.0878 per part on a 2,000-part order but only 0.0035 on 50,000 parts, so the price falls from 0.928 to 0.812.
Does it agree with the Molded Part Cost calculator?: Yes where they overlap. The calculator divides press time by cavities; this workbook divides by good parts per shot so rejected shots are paid for. At a 0 percent reject rate they are identical, and the build checks the calculator's own example (0.095 resin, 55 per hour, 22 s, 8 cavities, 0.017 tooling) against its engine.