CNC Plastic Machining: Compare Suppliers for US Part Buyers

Compare Xometry and Protolabs by sending the same controlled drawing, material specification, quantities, and acceptance requirements—not by comparing resin datasheets or headline prices. Xometry explicitly includes standard inspection; Protolabs’ plastic-machining page leaves documentation scope to confirm with your quote. [1][2]

Compare the suppliers on three decisive dimensions

Xometry

• Drawing inputs: Xometry accepts technical drawings and describes the CAD model as supplying final-part geometry. Tapped holes not explicitly identified as features on the quoted model may be machined to their modeled diameters. [1]
• Tolerance information: Its page describes drawing-defined tight tolerances. Ask for written acceptance of your critical callouts rather than treating that capability statement as acceptance of the drawing. [1]
• Inspection/material documentation: Standard inspection is included in the quoted part price and lead time. Detailed First Article Inspection Report (FAIR) and CMM inspection options, material certifications, certificates of conformance (CoCs), and hardware traceability are optional. [1]

Protolabs

• Drawing inputs: The plastic-machining page provides an instant-quote entry point. Ask Protolabs to confirm the drawing and CAD submission requirements for your package. [2]
• Tolerance information: The page directs buyers to separate milling guidelines for limits including material-specific sizes and threading; request numerical plastic-part tolerances and written acceptance of critical callouts. [2]
• Inspection/material documentation: Ask Protolabs to confirm inspection-report options, material-certificate or CoC availability, and lot-traceability scope. Have the agreed deliverables included in the quote. [2]

This is a difference in published detail, not proof either supplier lacks a capability. Xometry expressly addresses US orders; ask Protolabs to confirm delivery to your US location. [1][2]

Send both suppliers one controlled RFQ package

Use an identical request for quote (RFQ) package:

• Part identity: Part number, drawing and revision, CAD file and revision, and units.
• Material: Exact producer and grade, conditioning requirements, and required lot traceability.
• Order stages: Separate quantities for samples, initial production, and anticipated repeat orders.
• Acceptance scope: Critical dimensions, inspection plan, and required inspection and material documents.
• Delivery: Destination and requested timing.

This checklist adapts resin-buying guidance from Syntex America, which recommends documenting geometry, exact grade, volume, conditioning, critical dimensions, inspection, traceability, and delivery requirements. [5]

Name the governing revisions and propose a hierarchy—for example, drawing requirements govern acceptance while CAD defines otherwise unspecified geometry. Require written acknowledgment and resolution of discrepancies before manufacture. Ask Xometry and Protolabs to confirm drawing precedence and approval authority in writing. [1][2]

Make material substitutions subject to your written approval as a purchasing requirement, not an assumed supplier promise.

Specify edge condition and finish

Xometry’s general table describes sharp edges as broken and deburred by default, but its as-milled description allows potentially sharp edges or burrs unless removal is requested. [1] Don’t interpret that conflict as proof deburring is included—or excluded. Xometry also says surface treatment is not applied unless purchased and specifically acknowledged. [1]

Put acceptance requirements on the drawing: required edge condition, allowable burrs or a no-burr requirement, functional edges that must remain unchanged, surface finish, and any treatment. Identify where each note applies rather than relying on “finish as standard.”

Ask each supplier to acknowledge these notes and confirm achievable edge limits, finishing feasibility, and any added operation or charge.

Turn critical tolerances into an inspection commitment

Flag dimensions determining fit and function. Ask each supplier to identify exceptions, proposed deviations, and unresolved measurement questions before you order. Keep supplier default tolerances separate from explicit acceptance of your drawing.

Syntex recommends recording conditioning state, critical dimensions, allowable movement, and a drawing-based inspection plan. [5] Build an acceptance record linking each critical characteristic to:

• Its drawing identifier and acceptance limits.
• The agreed measurement method and conditioning state.
• The sampling agreement—which parts will be checked.
• The required result format, such as recorded measurements rather than pass/fail alone.

Name the buyer representative authorized to approve changes, and route proposed deviations through that person rather than leaving them in unacknowledged quoting assumptions.

Order the records needed to release parts

Do not treat Xometry’s optional “hardware traceability” as a promise of plastic resin-lot traceability; request the exact linkage between the material lot and delivered parts. [1][5]

Add a deliverables list to the RFQ:

• Name each required report or certificate and its required contents.
• Identify the applicable lot or quantity.
• Specify delivery timing and whether documents must accompany shipment.

Syntex recommends lot documentation and identifying required technical data sheets (TDS), certificates of analysis (CoA), CoCs, and safety data sheets (SDS). Select what your release process needs, then ask both suppliers to price and acknowledge the same documentation scope. [5]

Approve a representative part against a checklist

Make representative-part approval a purchasing-and-quality release gate. Syntex recommends a controlled machine/tool trial, an approved representative part, lot documents, grade-specific compliance records, and a drawing-based inspection plan. [5]

Use this acceptance checklist:

• Correct drawing and CAD revision.
• Exact approved material grade and agreed material evidence.
• Recorded results for critical dimensions.
• Conformity to agreed edge and finish requirements.
• Written resolution of deviations before release.

Ask each supplier to agree on sample quantity, sample charges, inspection coverage, approval authority, and whether remaining production waits for written release.

Keep application testing separate from routine dimensional acceptance; choose tests for the intended use rather than imposing a universal qualification program. Syntex ties approval to the exact formulation, documented test conditions, process, and representative part. [5]

What to confirm with each provider

Neither supplier publishes order-specific prices, setup charges, sample-signoff procedures, cancellation terms, repeat-order controls, or nonconformance remedies on these pages. [1][2] Resolve them in the quote and applicable purchasing terms before issuing the purchase order:

• Total cost and timing: Request itemized setup/programming, tooling, finishing, inspection, certificates, freight, and taxes; confirm quantity pricing, quote validity, payment terms, and committed delivery.
• Changes and cancellation: Confirm revision-change charges, cancellation stages, and charges at each stage.
• Repeat orders: Agree which drawing, CAD, material, and inspection records will be retained and for how long. Establish substitution approval and whether reorders require renewed review or representative-part approval.
• Nonconformance: Confirm notification deadlines, required evidence, disposition authority, available rework/replacement/refund remedies, and freight responsibility.

Choose on accepted scope

Xometry describes service from prototypes through production, with optional inspection reports and material documentation; Protolabs offers plastic prototypes and production parts from stocked thermoplastics. [1][2] Greater published detail is not evidence of better manufacturing performance.

Shortlist quotes acknowledging the same revision, material, critical dimensions, finish, documentation, sample-release requirements, and commercial terms. Then compare total quoted cost and committed delivery. Retain written answers to unresolved questions with the purchase order.

References

1. Custom Online CNC Machining Services – Metal and Plastic Instant Quotes (xometry.com)
2. Plastic CNC Machining Service | Order Custom CNC Parts (protolabs.com)
5. How to Choose Plastic Resin: A Buyer's Material Selection Guide | Syntex America (syntexamerica.com, 2026)