Plastic part cost drivers: material, cycle, cavitation and yield

The four drivers behind a moulded part price

A moulded part price is built from material, machine time, tool amortisation and yield, plus finishing, packing and freight where they apply. Each one is set by a different decision, which is why a single line item in a quote rarely tells you where to focus. A heavy part is a material problem. A slow cycle is a wall and cooling problem. A high price at low volume is often a tool amortisation problem.

The useful way to read a quote is to separate the cost that repeats with every part from the cost that is spread across the programme. Material and machine time repeat. Tooling is spread. Yield sits in between, because it repeats but it should not.

Material: weight first, price second

The material cost per part is the shot weight times the resin price, adjusted for the runner and the scrap. Shot weight follows the part volume and the density of the grade, so a 2.5 mm wall and a 3 mm wall are different products, not the same product at a different price. Glass-filled and flame-retardant grades also cost more per kilogram and often need a higher injection pressure.

Colour and regrind change the number again. A pre-coloured grade carries a premium and a minimum order. Natural resin plus masterbatch is cheaper at volume but adds a mixing step and a colour risk. Regrind is cheaper than virgin resin, but it is only acceptable within the limit the drawing or the customer allows, and a flame-rated part may not allow any.

Cycle time: where the money is lost quietly

Machine time is charged by the hour, so the cycle decides how many parts share that hour. Cooling usually dominates the cycle, and cooling is driven by wall thickness. Doubling the wall does not double the cooling time, it increases it by roughly the square of the thickness change, which is why a thick cosmetic wall is expensive in a way that is not obvious on the drawing.

Ejection, opening and the robot also take time, but they are small compared with a hot core or an unbalanced circuit. The most common cycle loss is a hot spot: an isolated boss, a thick rib or a heavy pad that stays warm after the rest of the part is ready. Fix the hot spot and the cycle falls without changing the machine.

Cavitation: tool cost against machine time

A tool with more cavities costs more to build but produces more parts per shot, so the machine cost per part falls. The trade is not linear. A 16-cavity tool needs a bigger machine, a heavier tool, a longer changeover and a wider quality risk if one cavity runs badly. At low volume the extra tool cost is never recovered, and at high volume the machine time saving is large.

For illustration only, take a part with a 30-second cycle on a machine charged at £36 per hour, which is £0.30 of machine time per shot. A single-cavity tool gives £0.30 per part. A four-cavity tool gives £0.075 per part at the same cycle, before the tool cost is added. If the four-cavity tool costs £24,000 more and the volume is 50,000 parts, the extra tool adds £0.48 per part, which is more than the machine saving. At 500,000 parts it adds £0.048.

Yield and the scrap allowance

A quote usually includes a scrap allowance. If it is not stated, it is inside the price, and you cannot tell whether it is 2% or 12%. Ask for the assumed yield, the start-up loss and the regrind limit. A new tool with a difficult part may genuinely lose a shift of parts at start-up, and that cost has to land somewhere.

Yield also depends on the inspection rule. A cosmetic part that is inspected under bright light will reject more than a hidden bracket. Agree the acceptance criteria, the sample size and the rework route before the first production run, because a late change to the cosmetic standard changes the effective price.

Volume, amortisation and the break that is not a discount

A volume break is usually a tool amortisation effect, not a discount. At 10,000 parts a £30,000 tool adds £3.00 per part. At 100,000 parts it adds £0.30. The moulder has not become more efficient; the fixed cost is spread thinner. That is why a price that looks generous at one volume can be normal at another.

Long programmes also carry maintenance, tool refurbishment and a possible cavity insert. Ask how those are covered. A price that excludes tool maintenance will come back as a charge or as a quality problem, and a price that includes it is easier to compare with a competitor who has hidden it.

Reading the quote

A useful sanity check is the shape of the price. For illustration, a 100 g cover at 100,000 parts a year might split around 38% material, 30% machine time, 18% tool amortisation, 8% finishing and packing and 6% scrap.

Ask for the part weight, the cycle, the cavitation, the machine tonnage, the material grade and the yield assumption. Those six numbers let you check the price against a simple should-cost model, and they show which lever to pull if the price is too high. A supplier who will not share them is usually protecting a margin rather than a process secret.

• Shot weight and runner weight
• Cycle time and machine tonnage
• Cavitation and tool cost
• Material grade, colour route and regrind limit
• Yield assumption and start-up loss
• Finishing, packing and freight, stated separately