A drawing lands on the desk, the title block says "POM" or "PEEK," and somewhere in the shop a quiet assumption kicks in: plastic is soft, soft is easy, easy is cheap. That assumption costs money in both directions. Some plastics cut like butter and let you run feeds a steel job could never survive; others move on you the moment the clamp comes off, char if you push the speed, and cost more per kilo than the aluminum sitting next to them. Getting the machining plastics cost for Delrin, PEEK and nylon right means pricing the material and its behavior separately, the same way you would for a metal grade.

This guide breaks down what the common engineering plastics actually cost to buy, how they behave at the tool, and how to turn those differences into a number you can defend instead of a flat "it's just plastic" rate.

Plastics are not one price band

The biggest quoting error with plastics is treating them as a single cheap category sitting below metal. The raw-material spread inside "engineering plastic" is enormous — wider than the spread across common aluminum and steel grades. Nylon and acetal are genuinely inexpensive. PEEK, PEI and the fluoropolymers are some of the most expensive stock you will ever load.

Here is how the materials a job shop actually sees line up on the things that hit the quote. Treat the cost multipliers as typical ranges, not fixed truth — they move with grade, stock form, fillers and quantity.

Material	Common name	Relative stock cost	Machinability	Notes for the estimator
Acetal (POM-C/H)	Delrin	1.0x (baseline)	Excellent	The workhorse. Crisp chips, tight tolerances, clean finish.
Nylon (PA6/PA66)	Nylon	~0.9–1.3x	Good	Absorbs moisture and moves; gummy, can burr badly.
HDPE / UHMW	Polyethylene	~0.8–1.2x	Good	Very soft, low cost; springs back, hard to hold tight tolerances.
Polycarbonate	PC / Lexan	~1.3–1.8x	Fair	Cracks and crazes; needs sharp tooling and care.
PTFE	Teflon	~3–6x	Fair	Soft but creeps badly; cost is in the material, not the cut.
PEI	Ultem	~8–15x	Good	High-temp, stable; expensive stock dominates the price.
PEEK	PEEK	~15–40x	Good	Among the priciest stock in the shop; cuts well but burns if pushed.

The relationship is the point. A PEEK blank can cost twenty to forty times the same volume of acetal before a single chip is cut. On that material, a careless stock-size choice or a high scrap rate is not a rounding error — it is the quote.

Why "soft" does not mean "fast" — or "cheap"

Softness helps cutting speed and hurts almost everything else. The traps that eat margin on plastic jobs are rarely the metal-removal rate:

• Heat and melting. Plastics carry heat away far worse than metal. Push the speed on PEEK or polycarbonate and the chip welds back onto the part, the edge gums up, and you get a burnt, gummy surface that fails inspection. You often run plastic slower and with air or coolant specifically to keep the cut cool.
• Dimensional movement. Nylon absorbs moisture and grows. Many plastics have a thermal expansion roughly ten times that of steel, so a part that measures perfectly warm is out of tolerance once it cools. Tight tolerances on plastic can mean stress-relieving the stock, machining in stages, and letting parts normalize — all time the flat rate never captured.
• Work-holding. Soft, springy stock deflects under clamping and cutting load. Thin walls bow away from the cutter. Holding a plastic part rigidly without marking or distorting it is its own setup problem, and it shows up as setup time, not cut time.
• Burrs and finishing. Gummy grades like nylon and UHMW throw burrs that a metal part of the same geometry would not, and they have to be removed by hand. That deburring time is invisible on the cycle-time estimate and very visible on the clock.

None of these are captured by a machinability percentage. They are why two parts with identical geometry — one in acetal, one in nylon — can carry meaningfully different prices even though both are "cheap plastic."

Delrin, PEEK and nylon: the head-to-head

These three are the engineering plastics estimators confuse most, because all three show up on real drawings and all three look like unremarkable bar or plate on the rack.

Acetal (Delrin) is the default and the one you want when you can have it. It machines beautifully — crisp chips, no gumming, excellent dimensional stability, and a clean finish straight off the tool. It holds tight tolerances better than almost any other plastic. If a part is mechanical but not high-temperature or chemically aggressive, acetal is usually the right call and the cheapest to both buy and cut. Quote it at your baseline plastic rate. Watch one thing: acetal has poor resistance to some acids and is flammable, but for machining purposes it is the friendly grade.

Nylon (PA6/PA66) looks similar on the rack and machines reasonably, but it absorbs moisture and moves with it — a part can grow several tenths of a percent after machining as it equilibrates with humidity. It also runs gummier than acetal, so chip control and burrs are worse. The material is cheap; the behavior is what costs you. Tight-tolerance nylon parts deserve a movement and deburring allowance that acetal does not.

PEEK is the opposite trade. It cuts well — close to acetal on the tool — but the stock is the most expensive thing on the rack, often fifteen to forty times the acetal price. The risk on PEEK is never the cut rate; it is scrap and stock yield. A scrapped PEEK part can wipe out the margin on the whole order. It also burns if you run it too hot, so the honest quote carries the real material line plus a higher scrap allowance and careful, cooler cutting parameters.

Putting a number on a plastic part

The cost build-up for a plastic part follows the same skeleton as any CNC machined part — it is the inputs that shift. A workable formula:

1. Material cost = (stock volume including waste) × (grade price per volume). On expensive grades, this line dominates everything, so the waste fraction matters far more than on metal.
2. Machine time = cycle time × your shop hourly rate. Plastics often cut faster than metal, but cooling, staged machining and gentle finish passes claw some of that back.
3. Setup time = work-holding for soft, deflecting stock — usually higher per part than the geometry alone suggests.
4. Finishing = deburring, annealing or stress-relieving, and any required normalization for moisture or tolerance. Real time, routinely forgotten.
5. Scrap allowance = a percentage that scales with material cost. On acetal it is a rounding error; on PEEK it is a line item you cannot skip.

The arithmetic is the same as metal. The mistake is reusing a metal scrap allowance and a metal finishing assumption on a part where the material costs twenty times as much and behaves nothing like steel.

Tolerances and stock yield change the answer again

Two factors swing a plastic quote harder than they do on metal: tolerance and yield.

Tolerance is dangerous on plastic precisely because the material moves. A tight callout that is routine on steel can force stress-relieved stock, multi-stage machining and a controlled cool-down on a plastic part. The hourly rate is the same; the part just takes longer and risks more scrap. The logic for pricing tolerance is identical to metal — see the tolerance cost guide — but the penalty lands harder because the material works against you.

Stock yield is the other lever, and it is multiplied by the grade premium. Twenty percent waste on acetal is nothing. The same twenty percent on a PEEK billet is real money — sometimes more than the entire machining cost. When you quote a turned plastic part, the stock-and-cutoff math in the turning guide applies on top of the grade premium, not instead of it, and on PEEK it can be the difference between a profitable order and a loss.

Reading the material off the drawing — and pricing it

None of this helps if the material never makes it cleanly from the title block into the estimate. The failure mode is mundane: the drawing says PEEK, the estimator is busy, "plastic bracket" goes into the quote at the acetal rate, and an order that should have been priced in the hundreds-per-part ships at a fraction of its cost. The same gap swallows the moisture note on nylon and the stress-relieve callout on a tight-tolerance part.

A disciplined plastic quote pulls four things off every drawing before pricing, the same discipline you apply when you build a full CNC machining cost breakdown:

1. The exact material and grade — "PEEK," "acetal copolymer," "PA66" — not just "plastic."
2. The stock cost for that grade in the form the part needs, including realistic waste.
3. The behavior allowances — cooling, moisture movement, deburring, stress relief — implied by the grade.
4. A scrap allowance that scales with the material price, not a flat metal default.

From drawing to material-aware price, automatically

The bottleneck is rarely the arithmetic — it is the reading. Catching "PEEK" instead of "plastic," remembering that nylon moves and PEEK scrap is expensive, applying the right allowances under deadline pressure: that is the part that gets shortcut when the quote is due at five o'clock.

QuoteBuddy reads the technical drawing and surfaces the material callout — the grade, the tolerances and the features that drive the cut — so the estimator works from what the drawing actually specifies rather than a hurried "plastic." It builds on AI drawing interpretation, then applies your material prices and your per-grade machining rates the same way every time, so a PEEK part is priced as a PEEK part and an acetal part is not overcharged to cover it.

Start a 30-day trial and run a few of your real plastic drawings through it — mix some acetal, some nylon, some PEEK — and see whether the material-aware price it builds matches what you would have quoted with the title block in front of you and an hour to think.