Introduction
Choose POM when the specified grade meets the actual temperature, load, chemical and dimensional requirements of the assembly. Consider PEEK when a more demanding operating envelope justifies further material and process evaluation. Neither polymer name, a higher price nor an attractive machined surface proves that a finished part will work.
For OEM engineers and technical buyers, the useful comparison is between qualified grades in a defined application. A room-temperature guide roller, a heated insulating spacer and a chemically exposed valve component present different questions. Even an identical drawing can require different acceptance conditions when the operating environment changes.
This guide explains how to turn those questions into a CNC machining RFQ. The engineering scenarios are illustrative, rather than customer case studies or measured production results. Material availability, achievable tolerances and manufacturing scope must be confirmed for the specific project.
1. Start with the Exact Grade, Not the Polymer Name
POM means polyoxymethylene, commonly called acetal. POM-C identifies a copolymer family; POM-H identifies a homopolymer family. These descriptions narrow the material choice but do not specify a complete purchasing requirement. A drawing marked only “white plastic” leaves considerably more room for error.
Mitsubishi Chemical Group's POM overview distinguishes these families and describes acetal's general machinability and low moisture absorption. Its portfolio also includes modified grades. Those distinctions are a reason to name the approved stock material, rather than treating every acetal product as interchangeable.
PEEK means polyetheretherketone. Unfilled, fiber-reinforced and bearing formulations serve different purposes. Ensinger's PEEK material overview describes several variants; properties of one formulation should not be transferred to another. In particular, do not assume that every filled PEEK grade preserves the electrical behavior of an unfilled material.
Specify the manufacturer and designation where required, approved alternatives, filler system, color and stock form. If equivalent material is acceptable, define the properties and documentation needed for approval. A supplier should resolve a substitution before machining, rather than explain it after shipment.
Customer-Supplied ABS CUP Samples: Reviewing a Physical Reference
The two photographs below show ABS CUP samples sent by a customer, as identified by the customer. They are actual sample photographs, rather than the POM and PEEK illustrations elsewhere in this guide. They document incoming reference samples rather than establish their original manufacturer. Their appearance does not verify the polymer grade or production process.
The visible side openings, recessed circular features and opposite faces provide useful starting points for a drawing discussion. A sample review should establish which surfaces locate the component, which openings are functional, and which dimensions require measurement. Confirm the material specification, mating components and acceptance criteria before planning a replacement.
An ABS sample can support a geometry review without establishing that POM or PEEK is an approved substitute. A material change requires a separate assessment of the operating conditions and fit requirements described below.
2. Compare the Operating Envelope Before Comparing Prices
The following table is a purchasing framework, not a universal property chart. Use the current datasheet for the exact stock grade and the relevant test conditions.
| Decision | POM evaluation | PEEK evaluation |
|---|---|---|
| Service temperature | Check continuous exposure, peaks and mechanical load together | Evaluate the selected grade at the required temperature and duration |
| Sustained load | Check creep, support area and retained fit | Check grade-specific long-term behavior, not only room-temperature strength |
| Chemical contact | Review every fluid and cleaning stage | Review compatibility at actual concentration and temperature |
| Sliding contact | Examine counterface, lubrication and wear requirements | Compare unfilled and bearing formulations for the complete contact system |
| Close dimensions | Define measurement and service conditions | Define the same conditions; premium material does not remove machining risk |
| Procurement | Confirm stock, identity and acceptance records | Confirm grade, stock form, traceability and approved substitutions |
Separate temperature ratings from usable load capacity
Record the normal temperature, brief excursions, cycle frequency and exposure duration. Include heat generated by friction and cleaning, rather than reporting only room ambient temperature. The material must satisfy the mechanical requirement throughout that history.
Victrex's PEEK overview identifies thermal, mechanical and chemical performance as grade-selection factors. A published temperature capability is useful screening information, but it does not establish allowable bearing pressure, bolt preload or dimensional change in your assembly.
Ask what “strong enough” means over time
A static load can gradually change a polymer's shape through creep. A part that survives installation may still lose functional clearance or fastener preload during service. State the load direction, contact area, duty cycle and allowable displacement. Request applicable long-term data or an agreed application test when those effects govern selection.
Avoid using one short-term tensile value as an assembly design approval. The shape, support and loading history matter as well as the material.
3. Specify Fits at the Conditions That Matter
A CNC plastic part needs a dimensional requirement that can be reproduced at inspection and interpreted during use. Define critical bores, locating diameters, mounting faces and hole positions individually. Leave nonfunctional features at a suitable general tolerance instead of applying the tightest requirement everywhere.
Distinguish three questions: what dimension is accepted at inspection, what fit is needed during installation, and what clearance remains during operation. They can have different answers. A polymer bushing running on a metal shaft is a useful example because both components contribute to the fit.
For preliminary reasoning, linear thermal expansion can be expressed as change in length equals coefficient of expansion multiplied by original length and temperature change. Use grade-specific data over the relevant range. This simplified calculation does not include moisture, restraint, creep or directional effects in reinforced stock, so it cannot replace the complete fit assessment.
Ask how dimensions will be measured, at what temperature and after what conditioning or equilibration. Record the part's free or restrained state and the measurement method. A close bore assessed with excessive contact force may not represent the unloaded geometry. A go/no-go check and a measured diameter also answer different questions.
The MCG guide to machining plastic parts treats tolerances by material group and size. Such guidance helps frame a manufacturing discussion; it is not a tolerance guarantee from every machine shop. Agree the requirement against the actual geometry before placing the order.
4. Evaluate Chemicals, Cleaning and Sliding Contact
“Chemical resistant” is incomplete without the chemical, concentration, temperature, exposure time and mechanical stress. List process fluids, lubricants, cleaning products and incidental spills. Include rinsing and drying stages when they affect the assembly.
Ensinger's chemical-resistance guidance provides a starting point for matching polymers to media. A compatibility rating should be checked against the exact formulation and intended conditions. It does not automatically qualify a finished component exposed to mixed fluids, repeated cleaning or simultaneous loading.
For a sliding component, specify the mating material, surface condition, speed, load, lubrication and acceptable wear. Friction and wear belong to a contact system, rather than to the polymer in isolation. An insulating support and a bearing sleeve should not be selected from the same single property ranking.
If the application requires food-contact documentation, electrical characteristics or another controlled property, identify the required evidence at quotation. A polymer family name or material color does not establish compliance. Approval must cover the selected grade and intended conditions; machining a part does not create a qualification that the material lacks.
Send Your Plastic Part Requirements for Review
Send the drawing revision, approved material grade, quantity and operating conditions. Identify critical fits, cleaning exposure and the required inspection state. Ask YILIU to review the manufacturing scope and documentation needed before quotation. Use the CNC RFQ preparation checklist to organize the package.
Share your drawing, exact polymer grade, quantity, service temperature, chemical exposure and critical fits. Define the inspection condition and any traceability requirements.
Open the full RFQ form →5. Plan Machining Around Heat and Workholding
Good stock selection does not eliminate manufacturing risk. Local heating, insufficient support or an unsuitable machining sequence can affect a plastic part independently of its service suitability. The buyer's role is to define functional requirements and ask how the proposed route protects them.
Control support without distorting the part
Broad contact and appropriate backing can help support compliant geometry. Clamping a thin wall aggressively may make the setup feel secure while changing the shape being cut. After release, that wall can move away from its restrained position.
Ask which surfaces locate the part, where clamps act and whether inspection is performed free of manufacturing restraint. Do not prescribe a universal jaw pressure from a photograph. The approach must account for wall thickness, unsupported length and accessible cutting directions. Our thin-wall machining guide discusses the distinction between clamped geometry and free-state acceptance.
Review heat, chip removal and material compatibility
Sharp tooling, suitable engagement and effective chip evacuation are process-planning topics. Packed chips or rubbing can add unwanted heat and damage a surface. Cooling methods and fluids require compatibility review, particularly when cleanliness or chemical exposure is controlled.
Ensinger's machining FAQ explains the importance of supporting plastic workpieces and managing machining-related effects. Ask for a credible route for the actual part, rather than a cutting-speed claim detached from tool diameter, grade and geometry. This article intentionally provides no universal speed, feed or coolant recipe.
6. Address Stock Stress and Geometry Changes
Removing material changes the balance of a stock shape. Deep pockets, large differences in wall thickness and one-sided removal can leave a component susceptible to movement. A premium resin does not make these geometries automatically stable.
Discuss whether the route needs staged roughing and finishing, a controlled pause, intermediate dimensional checks or a material-supplier-approved stress-management step. These are options to evaluate, not operations that every POM or PEEK part requires. If annealing is proposed, agree its purpose, sequence and applicable material instructions; do not invent a time-temperature cycle.
Consider the datum strategy at the same time. If a pocket moves after roughing, a later operation must still establish the intended relationship between functional surfaces. Rechecking only the outside length may miss a displaced mounting bore or a warped seating face.
An illustrative review might compare a solid spacer with a deeply pocketed bracket made from the same nominal grade. The bracket introduces unsupported walls and asymmetric removal that the spacer lacks. That difference can justify a different route and inspection plan without claiming either part has already been manufactured or tested.
7. Treat Holes, Threads and Surface Requirements Separately
Small features can determine whether the part is practical to machine and assemble. Mark the holes that locate a component, carry a screw, guide motion or provide fluid passage. Specify their function before tightening every tolerance.
For tapped holes, state the thread designation, useful engagement and assembly requirements. Distinguish direct plastic threads from an approved insert arrangement. Do not carry over tightening torque from a metal part without design validation. Repeated assembly, sustained preload and surrounding wall thickness may change the preferred solution.
For blind threads, allow the manufacturing review to distinguish usable thread depth from the drilled hole and tool-clearance requirements. The blind tapped-hole guide explains why those dimensions should be stated separately.
Define surface requirements by function: a sliding face, a sealing interface and a cosmetic exterior need different acceptance language. A photograph cannot verify roughness, freedom from contamination or suitability for a seal. Specify the required parameter and measurement location where numerical surface acceptance matters.
Edges also deserve explicit instructions. Loose slivers or a raised edge can interfere with assembly, but an excessive edge break can remove a functional land. Use the deburring and edge-break guide to frame those requirements without assuming metal and polymer finishing routes are identical.
8. Define Inspection, Traceability and Shipment Protection
Agree acceptance before manufacturing starts. Identify the dimensions to report, measurement state, sampling plan, instruments or functional gauges where required, and treatment of nonconforming results. “Factory testing” should name a test, method and criterion; it should not be a label added to an ordinary product photograph.
Request material identification and batch traceability at the level the application needs. A certificate supplied with stock is different from a dimensional report for the finished part. Decide what records must connect those two stages, and which supplier documents will accompany the shipment.
Use a first-article review when the geometry, material route or application warrants it. A dimensional first article establishes an agreed set of part measurements; it does not prove wear life, chemical compatibility or long-term creep performance. Define separate functional validation where necessary. Our first-article inspection guide helps distinguish those scopes.
For shipment inspection, check identification, quantity, visible edge condition and protection of functional surfaces against the approved plan. Arrange separation to prevent rubbing and avoid loading delicate walls during packing. Keep different grades and revisions distinguishable. Photographing a packed tray documents appearance, not polymer chemistry or validated cleanliness.
9. Compare Complete Quotations and Prepare Installation
Compare the same scope across suppliers: exact grade, stock form, quantity, dimensional requirements, edge treatment, inspection, documentation, packing and delivery basis. A price difference may reflect an omitted requirement rather than a more efficient machining route.
Do not request a POM prototype and assume it validates a later PEEK production part. It may help evaluate access or assembly geometry, but a material change can affect fit, handling and service behavior. Record which conclusions remain valid and which require renewed verification.
For installation preparation, confirm mating-part condition, orientation, cleanliness and approved assembly method. Review press fits, fastener sequences and support surfaces with the design owner. If a trial assembly is required, define its acceptance and record the configuration. Avoid forcing a close-fit component to compensate for an unresolved dimension or material assumption.
Retain the approved drawing, material designation and inspection plan for repeat orders. Our prototype-to-production checklist provides a broader framework for controlling those changes.
FAQ
Is PEEK always better than POM for CNC parts?
No. Choose against documented service requirements. PEEK deserves evaluation for demanding thermal, chemical or mechanical conditions, but its name alone does not qualify the assembly. POM can be a suitable choice when the specified grade satisfies the complete requirement.
Are POM-C and POM-H interchangeable?
Do not assume so. They are different acetal families, and individual grades can also contain modifiers. Approve a substitution against the design, material documentation and application conditions before manufacturing.
Does PEEK automatically allow tighter machining tolerances?
No. Geometry, stock condition, process route and inspection conditions still matter. Obtain a feature-specific manufacturing review and define which dimensions are critical to function.
Must every machined plastic part be annealed?
No. Evaluate the stock, material instructions, removal pattern and stability requirements. If an additional step is proposed, agree an approved procedure and sequence rather than applying a generic recipe.
Can color verify which polymer was supplied?
No. Color is not reliable proof of polymer identity, filler content or grade. Use the agreed identification, procurement and traceability records. Four images in this guide are AI-generated illustrations; two photographs show customer-supplied ABS CUP samples.
What should a plastic CNC machining RFQ include?
Provide the drawing revision, exact grade, quantity, service conditions, critical features and inspection requirements. Include documentation, cleaning, packing and approved-alternative rules, so the quotation covers the intended part.
Conclusion
POM versus PEEK is an application and procurement decision supported by material evidence and a realistic manufacturing plan. Define the operating envelope, select an exact grade, separate inspection conditions from service fits, and agree the records needed for acceptance. Then compare quotations on that complete basis.
Get a Quote for Your CNC Plastic Part
Send your drawing and customer requirements to YILIU for a project-specific manufacturing review. Include temperature, loading, chemical exposure, approved grade, order quantity and critical fits. Contact us to discuss the quotation scope, inspection and delivery requirements before production is agreed.
Start with your requirements.
Share your drawing, exact polymer grade, quantity, service temperature, chemical exposure and critical fits. Define the inspection condition and any traceability requirements.
helen.han@yiliuprecision.comManufacturing evidence you can review
Equipment and material examples refer to the supplied inventory, company profile and manufacturer literature. General guidance and illustrative images are not customer delivery case studies or a substitute for drawing review. Agree on inspection, reporting and delivery scope for each project.
Need to verify a capability or request a correction? Contact the YILIU team.
Engineering scenarios and four AI-generated illustrations are conceptual. Two additional photographs show customer-supplied ABS CUP samples, with material identification provided by the customer; they are not presented as YILIU-manufactured parts or POM/PEEK samples. No tested service life, material certification, stocked grade or guaranteed machining tolerance is claimed. Final suitability and manufacturing scope require project-specific confirmation.