The short answer
Use PEEK when the part's critical geometry matches that process naturally. Use Delrin (acetal) when it reduces setups and holds the important features with less risk. The cheapest route is usually the one that keeps the part closest to its natural geometry, not the one with the lowest hourly rate. Buyers should choose based on datum structure, feature access, and secondary operations.
Which geometry favors each process
PEEK is the better fit when the part is driven by temperature, chemical resistance, creep resistance, or medical/semiconductor performance justify a premium engineering polymer. Delrin (acetal) is the better fit when the part is driven by you need a stable, easy-to-machine plastic for general wear parts, fixtures, and lower-cost components. Buyers get cleaner quotes when they classify the part by its functional features, not by the first operation that comes to mind.
A simple rule helps. If the critical dimensions revolve around one axis, start with Delrin (acetal). If the critical dimensions live across faces, pockets, patterns, or contours, start with PEEK. Mixed parts need a more honest conversation about combined processes, secondary operations, or whether one setup must control both feature families.
What moves cost and lead time
PEEK is the high-performance choice. Delrin is the efficient choice. Both machine well, but the material cost delta is massive, which means wrong material selection can dominate the quote before the spindle even starts.
This is why similar-looking parts can price very differently. Two suppliers may both be able to make the part. One may be able to make it in the natural process route. The other may be forcing the geometry through workarounds. That shows up in cycle time, tool life, fixture count, and inspection effort.
Tolerance and quality implications
PEEK earns its price in heat, chemicals, sterilization, and demanding environments. Delrin wins when the part just needs low friction, dimensional stability, and easy machining at sane cost.
Good sourcing teams separate true function from inherited drawing habits. If the tolerance callout is really about concentricity, runout, flatness, or hole position, the process choice should support that directly. Otherwise you end up paying for extra handling just to chase geometry that the wrong machine created in the first place.
The decision error that costs money
The bad habit is defaulting to PEEK because the design team wants 'premium polymer' without proving the environment demands it.
Related reading: Machining titanium Ti-6Al-4V: what makes it difficult and how to spec it and 4140 vs 4340 steel: which is better for high-stress machined components?.
Comparison table where relevant
| Priority | PEEK | Delrin |
|---|---|---|
| Temperature capability | Higher | Lower |
| Chemical resistance | Higher | Good but narrower |
| Material cost | Much higher | Much lower |
| Typical use | Medical, aerospace, semiconductor | Fixtures, bushings, general machine parts |
How to specify this in your RFQ
Specify resin family and grade, not just 'plastic.' If moisture, sterilization, vacuum, or outgassing matter, say so. For Delrin, note whether FDA or specific grade requirements exist. For PEEK, confirm whether filled or unfilled material is acceptable because that changes machining behavior.
If suppliers are free to propose an alternate route, say that explicitly. If one process is mandatory because of qualification, source control, or validated history, state that too.
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