Injection-Molded PEEK/PEI vs CNC Aluminum for Aerospace Parts: Which Wins on Weight and Strength

Does the polymer grade or the process decide strength-to-weight?

Density alone favors polymer, but specific strength — tensile strength divided by density — is the number that changes a metal-versus-plastic decision¹⁶. In the same comparison table, unfilled PEEK sits below 6061-T6 while 30% carbon-fiber PEEK sits above it, so the reinforcement grade, not the choice of molding, sets the verdict¹⁶.

Treat the table as a screening input: the values are published typical ranges that vary by supplier, orientation and processing method, and should be confirmed against a current material datasheet before final design¹⁶.

How much weight can the part actually save?

Two different savings get quoted, and you should keep them apart. The first is density-only: unfilled PEEK is roughly 48% the density of aluminum, which for a geometrically identical part can deliver a 50-85% mass reduction before any design freedom is counted¹⁹.

The second is consolidation. One molder describes a structural bracket that needed 5 machined aluminum parts and 15 fasteners being molded as a single structure, with a total weight reduction of 30-50%¹⁴.

That second figure is smaller because it describes the whole assembly, not a like-for-like swap of one part. A separate claim puts aerospace brackets converted from aluminum to PEEK or CF-PEEK at 60-75% part-weight savings with no loss of certified performance¹⁹.

Ask which basis a supplier's number uses before you put it in a trade study.

Is the operating temperature inside the polymer's range?

Continuous service temperature can settle the choice before cost is discussed. PEEK and PEI are reported at 170-250C continuous operating temperature⁹, and the PEEK column of the comparison table gives 250-260C¹⁶.

Aluminum 6061-T6 is listed at 150-200C in that same table¹⁶. If your component runs hot, check the polymer's continuous rating against the actual soak temperature rather than a short-term peak.

And note that the sources give no property-retention curves across that range — so ask the material supplier for retention data at your operating temperature.

Does volume justify a PEEK mold?

PEEK molding economics are driven by resin price and tooling. Raw material is reported at $60-120/kg unfilled and $80-200/kg filled, tooling carries a 20-40% premium over standard P20 molds, and per-part production cost runs 8-15x an ABS equivalent²⁵.

CNC machining, by contrast, is described as cost-effective for one-off and medium-volume runs, with limits on complex part geometry³. The practical question is how many parts you will buy: no source in this set states a break-even volume between molded PEEK and machined aluminum.

So build your own comparison from quoted tooling, quoted per-part price and your annual quantity.

What tolerance and finish does the drawing demand?

Set the tolerance and finish requirement before you choose a process. PEEK is difficult to machine: its melt temperature of about 341C means cutting heat does not dissipate quickly, and its semi-crystalline structure can release stress so parts move after material is removed²⁰.

Parts may pass inspection and then warp or lose flatness hours or days later, and carbon- or glass-filled grades can micro-crack or chip at edges³². If you mold and then machine critical features, plan for that behavior in the routing and inspection timing.

• ✓Confirm the achievable molded tolerance and surface finish for your specific PEEK/PEI grade with the molder — the sources give no molded-PEEK tolerance data.
• ✓If post-machining is needed, ask how the shop controls cutting heat and stress relief on PEEK.
• ✓Specify when dimensions are measured — immediately after machining or after stress relaxation — and require the inspection to match.
• ✓For filled grades, ask how edge micro-cracking and chipping are inspected and dispositioned.
• ✓Compare the drawing's tightest features against aerospace CNC benchmarks such as ±0.0127 mm true position and Ra 0.4 um on bearing bores, and confirm the chosen route can hold them.

Which qualification documents must the supplier supply?

AS9100D is named as the foundational aerospace quality management standard, with ITAR registration required for defense articles, and a supplier should be able to give certification numbers and expiry dates for verification²⁹.

On the molding side, one molder claims AS9100 compliance with a Certificate of Conformity and Material Test Reports for every production batch⁹. Ask for the documents your program actually requires, and treat the polymer route's traceability claims as that supplier's position until you see the certificates.

• ✓Request the supplier's AS9100D certificate number and expiry date.
• ✓Confirm whether the part is a defense article requiring ITAR registration.
• ✓For molded parts, require CoC and MTR per batch and check they name your resin lot.
• ✓Ask separately for flammability, smoke, toxicity and material-allowable data — the sources in this set do not cover them for either route.

What durability risk does each route carry?

The sources describe process risk more than service-life risk. On the polymer side, poorly controlled PEEK machining shows up as warping, edge damage, internal stress and dimensional drift, which the material supplier links to rework, scrap and longer lead times³².

On the aluminum side, one molder claims traditional CNC machining of aerospace aluminum leaves about 80% of the raw material as scrap, against near-net-shape molding¹⁴. Neither figure tells you how a part behaves in fatigue, impact, moisture, UV or creep — this source set carries no comparative durability data for PEEK/PEI against aluminum.

So obtain it from the material supplier or your own test programme before you commit a flight-critical part.

Where the strength-to-weight numbers disagree

The apparent conflict over whether PEEK beats aluminum is not a dispute between sources — it is one comparison table showing two different grades¹⁶. Read the rows as grade-specific, and pick the row that matches the material you intend to specify.

Before you request a quote

• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.