In this guide 6 sections
Cost a plastic part from five inputs: cycle time, cavity count, machine hour rate, resin weight with yield loss, and tool amortisation spread over expected volume. Calculate each per part, add labour and packing, then test the quote against actual runs.
Start with parts per hour
Everything else divides by this figure. Take the cycle time in seconds, from mould close to the next mould close. Divide 3,600 by it, then multiply by the number of cavities. That gives shots and parts per hour at full speed.
Then be honest about efficiency. A press never runs flat out. Mould changes, colour changes, short stops and waiting for material all take time. Use the efficiency your own shift records show for similar jobs, not the figure from the quotation sheet of the machine maker.
Trial shots give the cycle. History gives the efficiency.
Set a machine hour rate for each press
A 50 tonne press and a 500 tonne press do not cost the same to run. Build a rate per press from its own costs: depreciation, power, floor space, operator share, maintenance and auxiliary equipment such as the chiller, dryer and robot.
Divide by the hours the press actually earns, not by the hours in a year. An idle press still carries its costs. If it runs two thirds of available hours, the rate is higher than the sum divided by the calendar.
Review these rates every half year. A single rate for the whole shop is the usual reason quotes are wrong.
- Depreciation and finance cost of the press
- Power for press, chiller and dryer
- Operator and setter time share
- Maintenance and spares
- Productive hours per year, from shift records
Cost the resin by shot weight and yield
Weigh a shot from the trial, including runners. Resin per part is the shot weight divided by cavities. A hot runner mould sends less to the runner than a cold runner one, so the comparison matters.
Add the losses. Start up shots, purge, rejects and the regrind that cannot go back into the hopper all reduce yield. Use a yield from similar past jobs and keep it visible on the quote sheet.
If the customer allows regrind, credit what you reuse. If not, that runner weight is a plain cost.
Spread the tool over what it will really make
When the customer pays for the mould, tool amortisation drops out. When you carry it, divide the tool cost by the volume the customer commits to, not by what the mould could make in its lifetime.
Add the upkeep. A shared tool needs maintenance and eventually refurbishment. Include a provision per thousand shots so the refurbishment is funded when it falls due.
Where one mould serves two customers or two part variants, split by volume and write the split on the quote.
- Tool cost and who owns it
- Committed volume in the customer’s agreement
- Maintenance provision per thousand shots
- Mould changeover time and setter cost per change
Add the extras, then check against actuals
Packing, inserts, secondary operations such as ultrasonic welding or pad printing, freight and inspection come on top. Each has its own time and material.
After the job, compare. Actual cycle, actual efficiency, actual resin consumed and actual rejects against the quote. Write the differences on the costing sheet.
This is how last year’s rate gets replaced. Update one input per job, starting with your largest account.
Part costing worksheet
The worksheet opens with inputs: cycle time, cavities, efficiency, press and shot weight. Its middle calculates parts per hour, machine cost, resin cost with yield, tool amortisation and extras per part. Actuals from the finished job sit beside each quoted figure at the foot.
Open a blank worksheet to printQuestions owners ask
Why does the same part cost more on a different press?
A larger press carries a higher hour rate, and a mould may also run a slower cycle on it. Quote on the press that will actually run the job.
Should we include setup in the part cost?
Yes. Divide mould change and colour change time by the order quantity. Short runs carry more setup per part than long ones.
How often should machine hour rates be updated?
Review them every six months, or when power cost, wages or the press mix changes. A rate nobody revisits drifts away from reality.
What if the customer supplies the mould?
Leave tool amortisation out of the part cost. Keep the maintenance provision, because upkeep still falls on your tool room.