A clear inspection window, a sliding guide and a hot electrical spacer do not need the same plastic. Starting each drawing with “PEEK preferred” can add cost without solving the controlling problem; starting with the cheapest stock can miss a requirement that only appears after assembly.
Select a CNC plastic by eliminating unsuitable candidates, then validate the remaining grades under the intended conditions. PEEK, Ultem (PEI), Delrin (acetal), nylon, PTFE, PMMA and polycarbonate occupy different roles. No single ranking captures sustained load, water uptake, chemical exposure, friction and optical performance at once.
This is a functional shortlisting guide, not a catalogue of machining services or a replacement for grade-specific design data. The broader CNC machining materials framework covers the earlier choice between material families.
Write a duty profile with pass/fail limits
Replace broad adjectives with conditions an engineer can evaluate. “Heat resistant” does not say whether the part experiences a brief cleaning cycle or carries a load continuously near a heat source. “Chemical resistant” omits the fluid, concentration, temperature and contact duration.
Define genuine disqualifiers before scoring price. If a window must transmit a specified wavelength with controlled distortion, an opaque high-performance grade is not a runner-up: it fails that requirement. If a spacer cannot lose preload over its service life, short-term tensile strength is an incomplete screening metric.
Do not substitute heat-deflection temperature, melting point or a catalogue maximum service temperature for an allowable loaded operating temperature. They describe different tests or limits, not the same design condition.
Seven candidates, different reasons to stay on the list
The table below identifies starting candidates and the question that can overturn each choice. It describes common unfilled material families unless stated otherwise; exact grades and stock forms must be compared separately.
For a hot, loaded component, PEEK machining is a route to investigate, not an automatic specification. Ensinger’s unfilled PEEK stock-shape data distinguish short-term mechanical properties, thermal limits and low water uptake. Those advantages still need to be tested against the part’s load and exposure.
For a precision guide operating in a moderate environment, Delrin CNC machining may be a more relevant starting point than a high-temperature polymer. However, “acetal” includes homopolymer and copolymer products; they should not be substituted without reviewing the intended conditions.
Test the requirements together
Load plus time plus temperature
A component can pass a short proof load and still move gradually during service. Creep is time-dependent deformation under load. Evaluate the relevant grade’s data at the intended stress and temperature, and define how much movement the assembly permits.
PTFE illustrates why thermal capability and structural rigidity must remain separate questions. Carrara’s PTFE guidance discusses loss of sealing force through creep. A material may tolerate the environment chemically yet need a different support or sealing arrangement to maintain its function. A low coefficient of friction alone does not qualify it as a bearing material.
Moisture plus fit
For nylon, define the moisture state in which dimensions are accepted and the conditions the assembly will later experience. Ensinger’s machinable-plastics guidance notes that moisture uptake and release can alter dimensions. “Dry at inspection” is therefore not equivalent to “stable in a humid installation.”
Consider a hypothetical close-running roller. A small clearance established in a dry room may not represent the running clearance after environmental exposure. The design decision may involve a different grade, conditioning, revised clearance or another polymer—not simply tighter machining.
Transparency plus impact plus cleaning
PMMA and PC should be compared using the finished optical requirement, not photographs of raw transparent stock. Curbell distinguishes acrylic’s optical appeal from polycarbonate’s toughness. Machining marks, polishing, thickness, scratches and the chosen stock grade affect the final viewing surface.
For machined polycarbonate parts, include the cleaning agents and fastening arrangement in the review. A clear part that looks acceptable before cleaning may not represent its condition after repeated use. Do not claim a protective or safety function from the polymer name alone; the complete design and applicable tests govern that use.
Sliding contact plus the mating surface
Friction and wear belong to a contact pair. Counterface material and finish, pressure, speed, lubrication, contaminants and temperature can change the result. Compare candidates in a representative contact test instead of selecting the smallest friction number from unrelated datasheets.
A polymer name is not a complete material callout
State the manufacturer or approved equivalent, product grade, filler content, colour where relevant, and required stock form. Delrin is a trade name associated with acetal homopolymer; Ultem identifies a PEI product family. Neither is a substitute for a complete procurement specification.
For Ultem machining, an unfilled grade and a reinforced grade are not interchangeable. Fillers can alter stiffness, dimensional response, electrical behaviour, wear and the machined edge. A stronger filled grade may also be unsuitable for a required insulating or clean-contact function.
Keep stock-shape data separate from injection-moulding resin or 3D-printing data. Manufacturing route, reinforcement orientation and material condition can matter. An attractive number measured on a moulded test bar is not automatically a guaranteed property of the supplied machined component.
Compliance is also grade- and application-specific. Avoid generic callouts such as “FDA plastic” or “medical PEEK” without identifying the intended contact, relevant declarations and validation requirements. A material declaration and approval of the finished assembly answer different questions.
Turn the shortlist into a defensible decision
Retain two plausible candidates when important data are missing. Ask for comparable stock and machining proposals using the same geometry, quantities, inspection state and acceptance requirements. Compare the finished component—not just price per kilogram.
A short decision record should explain why one candidate remains and another was rejected. That record is more useful at the next drawing revision than a generic label such as “premium engineering plastic.”
Does choosing PEEK remove the need for prototype testing?
No. It does not establish the behaviour of a thin wall, a sliding pair, a stressed chemical interface or a long-term loaded assembly. Test the features and environments that control the design.
Can an inexpensive plastic prototype validate a more expensive production grade?
It may validate selected aspects of geometry or assembly access. It cannot automatically validate friction, creep, thermal movement, chemical resistance or electrical performance in the production material. Record what the prototype test does—and does not—demonstrate.