Engineering Plastics for Flight and Ground Hardware

The high-performance plastics short list

Six polymers cover most machined plastic parts on launch, spacecraft and test hardware: seals and valve seats, insulators and standoffs, bushings, wear strips, connectors and fixtures. The table gives published nominal properties for unfilled material. Filled and reinforced grades differ substantially.

Sources for each row are listed at the end of the page. These are nominal values; for anything structural, get the producer data sheet for the exact grade and stock shape, because extruded rod, compression molded plate and direct-formed parts often have different properties.

Where each polymer earns its place

• PEEK is the default for machined plastic parts that must survive temperature, chemicals and vacuum: connector bodies, insulating bushings, pump and valve components, cable clamps. It machines well and holds dimensions better than most because it absorbs little moisture. Glass and carbon filled grades add stiffness and reduce expansion.
• PEI is amorphous, so it is dimensionally stable and machines to clean edges, but like most amorphous polymers it is more sensitive to solvent stress cracking, so control cleaning solvents. It is common for electrical insulators and fixtures.
• PAI has very high strength and wear resistance among melt-processable polymers. Molded parts and shapes are post-cured for several days at temperatures up to 260 C to build molecular weight and chemical resistance, so buy stock that is fully cured from a producer that certifies it.
• Polyimide parts such as Vespel are used where nothing else holds up to temperature and wear: thrust washers, seals and insulators near hot hardware. SP-1 is the unfilled grade, available as isostatically molded billet, direct-formed parts and pressed slab under ASTM D6456 and AMS 3644.
• PTFE is chemically inert, very low friction and stays usable down to cryogenic temperature, but it creeps. Seals and gaskets in PTFE often need spring energizers or Belleville washers to keep load on them.
• PCTFE trades some of PTFE's temperature capability for much better creep resistance, near-zero moisture absorption and a long history in cryogenic seals and oxygen regulator valve seats.

Outgassing: ASTM E595 and the NASA database

For vacuum and spacecraft hardware, the outgassing screen is ASTM E595. A sample is held at 125 C for 24 hours at below 5 x 10^-5 torr, and the test reports total mass loss (TML), collected volatile condensable materials (CVCM) on a 25 C collector, and water vapor regained (WVR). The standard notes that TML of 1.00 percent and CVCM of 0.10 percent have been used as screening levels for spacecraft materials, but acceptance is up to the user.

Practical points for plastics:

• Look up the exact grade, color and filler in the NASA Goddard outgassing database. A pigmented or lubricated grade can behave very differently from the natural grade.
• WVR matters for hygroscopic polymers. PAI and polyimide pick up water, which shows up as mass loss in vacuum and as dimensional change on the ground. A vacuum bake before integration is common practice for these materials.
• Machining coolants, mold releases and marking inks contribute to outgassing too. Specify dry machining or approved coolants and a cleaning process for parts headed to a clean or vacuum environment.

Oxygen service: what the tests mean

Nonmetals are usually the ignition point in oxygen systems, so they get more scrutiny than the metals around them. NASA-STD-6001B sets the test framework. Upward flammability (Test 1, and Test 17 in gaseous oxygen at use pressure) screens whether a material propagates fire. Pneumatic impact for nonmetals in pressurized gaseous oxygen (Test 14) addresses adiabatic compression heating, a key ignition mechanism for seals and seats downstream of fast-opening valves. Mechanical impact in liquid and gaseous oxygen (Tests 13A and 13B) is now listed as a supplemental test. Polymers that fail Test 1 or Test 17 are to be tested for autogenous ignition temperature and heat of combustion.

The standard summarizes the preference plainly: for oxygen service choose nonmetals that do not burn in the use environment and configuration, with high autogenous ignition temperature, low heat of combustion and high oxygen index. That is why fluoropolymers such as PCTFE and PTFE dominate oxygen seals and seats, and why PEEK, PEI and polyimide parts in oxygen systems need specific test data at the use pressure and temperature. Vespel SP-1 lists an oxygen index of 53 percent, which is useful context but not a substitute for the pressure-specific tests.

Test history is recorded in the NASA MAPTIS database. When requesting oxygen-service parts, give the supplier the exact grade, the use pressure and temperature, and the required cleaning level, and require the material cert to carry the same grade and lot. See precision and oxygen cleaning for cleaning and packaging.

Machining plastics to aerospace tolerances

Plastics are easy to cut and hard to hold. Thermal expansion of polymers is far higher than that of metals, residual stress in stock relaxes as material is removed, and some grades change size with humidity. Vespel SP-1, for example, lists a linear expansion coefficient of 54 microns per meter per degree C. What shops that hold tight plastic tolerances do:

• Buy stress-relieved or annealed stock, and anneal again between roughing and finishing for tight parts.
• Use sharp, polished tools with generous clearance and keep heat out of the part; dull tools smear and leave stress.
• Measure at a controlled temperature and state the measurement temperature on the drawing for tight tolerances.
• Condition hygroscopic materials (PAI, polyimide) before final machining and inspection, or define tolerances in a stated moisture condition.
• Do not specify metal-like tolerances on thin PTFE features; it will cold flow in the fixture and in service.

Callouts and certs for plastic parts

A good plastic callout names the polymer, the producer grade or a specification that controls it, the stock form if it matters, color, and any annealing, cleaning or bake requirements. "PEEK" alone allows glass-filled, carbon-filled, bearing grade or recycled material. Require a certificate of conformance naming the grade, lot and producer, and for oxygen or vacuum parts the same lot traceability you would require for metal. When a drawing calls a specific trade name, confirm whether an equivalent grade is acceptable before the buyer substitutes.

Get plastic parts quoted

Send drawings for machined PEEK, PEI, PAI, polyimide or fluoropolymer parts through a CNC quote request. We flow down the grade, cleaning and bake requirements, confirm the material cert matches the grade on the drawing, and ship with the cert package. See also the materials hub, nickel superalloys for oxygen-service metals, and controlled expansion alloys.

Questions

Related

• Aerospace Materials: Selection, Specs and Paperwork
• Nickel Superalloys and Nickel-Copper Alloys
• Controlled Expansion Alloys and Copper Alloys for RF
• Material Certs, Specialty Metals and Lot Traceability

Sources

• Polyether ether ketone, properties
• Polyetherimide, properties
• Polyamide-imide, properties and post-cure
• DuPont, Vespel SP-1 polyimide properties
• Polytetrafluoroethylene, properties
• Polychlorotrifluoroethylene, properties and oxygen service use
• ASTM E595 outgassing test method
• NASA Goddard outgassing database
• NASA-STD-6001B, Flammability, Offgassing, and Compatibility Requirements