Thermoplastic polyurethane, silicone/LSR and rubber: hardness, overmolding and printing

Elastomers return to shape after large deformation: Shore A 60–95 hardness covers soft seals to rigid gaskets. Thermoplastic polyurethane (TPU) is injection-moldable and 3D-printable; liquid silicone rubber (LSR) molds to medical- and food-grade parts; neoprene/EPDM/Viton are thermoset rubbers for seals. The choice hinges on temperature, chemicals, hardness and process.

1. Elastomer families

TPU is a thermoplastic: it flows when heated, so it can be injection molded and FDM printed. Shore hardness 60A–75D covers soft gaskets to semi-rigid bumpers; UTS ≈34 MPa at 95A with elongation up to 500%+ and excellent abrasion resistance — the standard for phone cases, wheels and dampers.

Silicone (LSR) resists -60 to +200 °C, is biocompatible and food-safe, but must be molded/cured — it cannot be FDM printed. Thermoset rubbers (EPDM, Nitrile, Viton) are vulcanized; choose by chemical compatibility (oils → NBR/Viton, weather → EPDM).

2. Hardness and design

Shore A is the common hardness scale: 30–40A soft gaskets, 50–70A general seals and bumpers, 80–95A stiff rings and covers. Softer compounds need thicker sections to avoid extrusion under pressure and are harder to demold — draft angles of 2–5° are common.

Overmolding bonds an elastomer to a rigid substrate (TPE over PC/ABS, silicone over metal) in a second shot; bond strength depends on material pairing and surface prep. In printed TPU, infill density tunes stiffness over a wide range.

3. Performance limits

Temperature: TPU continuous use -40 to +80 °C; silicone -60 to +200 °C; Viton (FKM) to +200 °C with chemical resistance; EPDM excellent for ozone/weather but poor with oils. Compression set — permanent deformation under sustained load — favors silicones and some TPU grades for sealing.

Abrasion and tear: TPU is the best abrasion-resistant elastomer; silicone tears more easily, so design thicker sections. UV resistance favors silicones and EPDM; TPU needs UV-stabilized grades outdoors.

4. Selection and quoting

For 3D-printed prototypes use TPU (95A is a good balance); for molded production seals, LSR suits medical/food and extreme temperature, TPU suits mechanical damping and abrasion, EPDM suits weather, NBR/FKM suit oils. State the Shore hardness and any regulatory need (FDA, USP Class VI, food contact) on the drawing.

Elastomer parts are quoted by compound cost and mold/cure cycle; LSR and Viton are premium materials. Tolerances are inherently looser (±0.25–0.5 mm) because elastomers deflect during inspection — use functional gages rather than tight numbers.

Quick reference

Material	Hardness	Temp range °C	UTS MPa	Elong %	Best for
TPU 95A	95A	-40–80	34	350–500	Abrasion, damping, print
TPU 60A	60A	-40–80	25	600+	Soft bumpers, gaskets
LSR silicone	20–70A	-60–200	7–11	400–700	Medical, food, extreme temp
EPDM	40–80A	-45–150	8–15	300–600	Weather seals, hoses
Nitrile NBR	40–90A	-30–120	12–20	300–500	Oil and fuel seals
Viton FKM	60–90A	-20–200	10–16	150–300	Chemicals, fuel, high temp

Sources & references

• ASTM D2240 — Durometer hardness
• ASTM D412 — Rubber tensile
• ISO 7619 — Durometer hardness
• Xometry Learning Center — TPU & Elastomers Guide (public engineering reference)

C110/C101, brass 360, bronze and beryllium copper: conductivity, machinability, cost

Copper alloys are selected for electrical conductivity (C101/C110), machinability (C360 free-machining brass) and bearing/wear duty (bronzes). Pure copper is an outstanding conductor but gummy to machine; brasses machine beautifully and dominate plumbing and terminals; aluminum bronze and phosphor bronze serve heavy wear. Beryllium copper adds spring and strength at premium cost.

1. Alloy families

Pure copper C110 (ETP) conducts ~101% IACS and is standard for busbars, terminals and heatsinks; C101 (OFHC) adds purity for vacuum and electronics. Copper anneals easily, so machined parts are typically supplied in half-hard or hard temper for dimensional stability.

Brasses (copper-zinc): C360 free-cutting brass (UTS ≈400 MPa) machines like butter — the default for high-volume turned fittings; C260 cartridge brass for stamping and springs; C230 red brass for plumbing. Bronzes: C510 phosphor bronze (springs, bearings), C954 aluminum bronze (heavy wear, UTS ≈550 MPa+), C932 bearing bronze.

2. Machining and finishing

Free-cutting brass C360 gives excellent finish, tight tolerances (±0.05 mm routine) and long tool life; pure copper C110 is gummy — sharp tools, controlled feeds and plenty of coolant. Copper work-hardens; avoid fine chips jamming in automated machines.

Finishes: electroplating (tin, nickel, silver, gold on copper/brass), clear lacquer to prevent tarnishing, polishing to mirror, and passivation for copper. Beryllium copper can be heat-treated to 38–42 HRC for springs and tooling.

3. Applications

Electrical: busbars, contacts, heat sinks (copper conducts ~60% better than aluminum per volume but weighs 3.3× more). Plumbing: brass fittings and valves. Marine: silicon bronze and aluminum bronze for propellers and bearings. Tooling: beryllium copper for spark-safe tools and molds that need fast heat extraction.

Cost: copper ≈3–4× steel per kilogram and heavy (density 8.9 g/cm³); beryllium copper costs significantly more. Specify material condition (temper) because annealed parts distort during machining.

4. Selection guidance

Choose C110/C101 for conductivity, C360 for machined fittings and terminals, C260 for stampings, C510 for springs, C954 for wear. Avoid brass in food/acid contact without verification (dezincification), and avoid beryllium copper where an alternative metal works — it needs special handling in grinding (beryllium dust).

For quoting, copper alloys are priced by weight with a machining multiplier; tight tolerances and fine finishes are economical on brass but costly on pure copper.

Quick reference

Alloy	UTS MPa	Density g/cm³	Conductivity %IACS	Best for
C110 ETP copper	220–270	8.89	101	Busbars, heatsinks, terminals
C101 OFHC	220–270	8.94	101	Vacuum, electronics
C360 brass	400	8.53	28	Machined fittings, terminals
C510 phosphor bronze	440–560	8.78	15	Springs, contacts
C954 aluminum bronze	550–650	7.45	13	Heavy wear, bearings
C17200 BeCu	1240 (HT)	8.25	22	Springs, spark-safe tooling

Sources & references

• ASTM B187 — Copper bar
• ASTM B16 — Free-cutting brass
• ASTM B139 — Phosphor bronze
• C17200 per ASTM B194
• Xometry Learning Center — Copper & Brass Guide (public engineering reference)

A decision framework: process first, then environment, strength, cost and supply

Material selection is a funnel, not a lookup: (1) fix the manufacturing process, (2) list service conditions — temperature, chemicals, loads, regulatory — (3) screen candidates on strength and stiffness, (4) compare cost and supply, (5) validate with a datasheet and a prototype. Skipping the process-first step is the most common error: a material that cannot be machined, printed or molded in your geometry is not a candidate at all.

1. Start from the process

Machining supports nearly any metal and most plastics in solid form; 3D printing adds nylon (PA12), resins, TPU and DMLS metals; sheet metal is restricted to formable alloys; injection molding applies to virtually all thermoplastics and LSR. Your chosen process defines the material universe before any property comparison.

Quantity is the other gate: prototype volumes (1–100 pcs) favor machining and 3D printing; production volumes favor molding and stamping, which may justify a material change (e.g. machined 6061 bracket → molded nylon or die-cast A380 at scale).

2. Service conditions first

Build a requirement list: max continuous temperature, chemical exposure, UV, moisture, wear, fatigue, electrical, regulatory (food, medical, RoHS, flame). Rank them — the binding constraint picks the family: >150 °C → PEEK/PPS/PI; chlorides → 316/titanium; wear → POM/PA66/bronze; transparency → PC/PMMA; biocompatibility → Ti-6Al-4V/LSR/PEEK.

Do not over-specify: many failures are cost failures, not property failures. If the application peaks at 80 °C with mild chemicals, ABS/PC or 6061 aluminum will be dramatically cheaper than PEEK or titanium.

3. Strength, stiffness and density

Screen on specific strength (UTS/density) and specific stiffness (modulus/density) rather than absolute numbers: Ti-6Al-4V, 7075 and CFRP dominate specific strength; magnesium and aluminum compete on stiffness per weight; steel wins on absolute stiffness and cost.

For precision and stability, compare modulus, thermal expansion and creep: aluminum expands ~23 µm/m·K, steel ~12, plastics 70–200. A machined aluminum part will move dimensions with temperature where a steel or Invar part will not.

4. Cost, supply and validation

Rank finalists by price index (aluminum ≈1, steel ≈1.2, stainless ≈3–4, titanium ≈8–12, PEEK ≈30–60) and by availability in your required form (bar, plate, sheet, powder). Small quantities of exotic grades are dominated by material minimums — check stock sizes before finalizing.

Validate with the specific grade datasheet (exact temper/alloy/fill) and a prototype under real loads; datasheet properties vary with condition (printed vs molded, annealed vs hardened). Document the chosen grade on the drawing so the quote and inspection match the intent.

Quick reference

Step	Question	Typical outcome
1. Process	How will it be made, at what quantity?	CNC / 3DP / sheet / molding universe
2. Environment	Temperature, chemicals, UV, moisture, loads	Family shortlist (PEEK vs PA66 vs 316)
3. Strength/stiffness	UTS, modulus, density, creep, expansion	Finalists + grades
4. Cost & supply	Price index, stock forms, minimums	Selected grade
5. Validate	Datasheet + prototype under load	Confirmed choice on drawing

Sources & references

• ASTM D638 / ASTM E8 — Tensile test methods
• ASM International — Materials handbooks
• ISO 527 / ISO 6892 — Tensile standards
• Xometry Learning Center — Material Selection Guide (public engineering reference)

Wrought alloy families, tempers and machinability of 6061, 6063, 7075, 2024 and 5052

Aluminum alloys are grouped by alloying system: 1xxx pure, 2xxx copper, 5xxx magnesium, 6xxx magnesium-silicide and 7xxx zinc. The 6xxx series dominates machined parts because it combines good strength, excellent machinability, corrosion resistance and anodizability at moderate cost; 7075 and 2024 are chosen for high strength where weight matters most.

1. Alloy families and designations

Wrought aluminum is designated by the AA four-digit system: the first digit is the major alloying element, and the temper follows the alloy, e.g. 6061-T6 where T6 means solution heat-treated and artificially aged. Cast alloys use a three-digit plus decimal system (A356, A380) and behave differently in machining and finishing.

For machined parts the practical shortlist is 6061-T6 (general structural), 6063-T6 (extrusions and anodized trim), 7075-T6 (aerospace, high strength), 2024-T3 (fatigue-critical aircraft skins) and 5052-H32 (marine and sheet forming). All are available as bar, plate and billet.

2. Mechanical properties and tempers

6061-T6 delivers roughly 310 MPa ultimate tensile strength, 276 MPa yield and 12–17% elongation, with hardness about 95 HB; 7075-T6 nearly doubles the yield to ~503 MPa at the cost of toughness and weldability. Properties are stable in the T6 condition; soft (O, H112) tempers machine with stringy chips but cut faster.

Modulus of elasticity is ~68–70 GPa for all aluminum alloys — about one-third of steel — so stiffness-limited designs need thicker sections or ribs. Thermal conductivity is high (≈170 W/m·K for 6061), which matters for heat sinks but complicates welding and anodizing consistency.

3. Machining and finishing

Aluminum is the easiest structural metal to machine: high cutting speeds, excellent surface finish (Ra 0.8–1.6 µm readily), and no coolant requirement for most operations. Free-machining grades include 6061-T6511 (as extruded bar) and 2011-T3; 7075 machines well but produces fine chips that demand chip management.

Anodizing (Type II decorative, 5–25 µm; Type III hard coat, 25–75 µm) is the standard protective finish, dyed black or clear. Chromate conversion (chem film / alodine) is used under paint and for corrosion protection. ASTM B209 (sheet/plate) and B211 (bar) define supply specifications.

4. Selection guidance

Choose 6061-T6 unless the application needs higher strength (7075-T6), better fatigue (2024), corrosion/marine performance (5052) or maximum anodized aesthetics (6063). Weldability ranks 6061 (good with 5356 filler), 5052 (excellent), 7075 (poor — use mechanical joints).

For cost estimation, aluminum price index is among the lowest of the engineering metals; dimensional stability is good but thermal expansion (≈23 µm/m·K) must be considered in precision assemblies at temperature.

Quick reference

Alloy	UTS MPa	Yield MPa	Elong %	Density g/cm³	Best for
6061-T6	310	276	12–17	2.70	General machined parts
6063-T6	241	214	12	2.70	Extrusions, anodized trim
7075-T6	572	503	11	2.81	Aerospace high strength
2024-T3	469	324	19	2.78	Fatigue-critical, aircraft
5052-H32	228	193	12	2.68	Marine, sheet forming
1100-O	110	105	12–30	2.71	Deep drawing, foil

Sources & references

• ASTM B209 — Aluminum sheet and plate
• ASTM B211 — Aluminum bar, rod and wire
• AA Aluminum Alloy Designations
• Xometry Learning Center — Aluminum Guide (public engineering reference)

Carbon steels, alloy steels and stainless families: strength, machinability and corrosion

Plain carbon steels (1018, 1045) are the cheapest structural metals and machine well; alloy steels (4140, 4340) add strength and hardenability; stainless steels — austenitic 303/304/316, martensitic 410/440, precipitation-hardening 17-4 — trade cost and machinability for corrosion resistance. Grade selection is a four-way trade: strength, corrosion, cost and machinability.

1. Carbon and alloy steels

Low-carbon steels (1018: UTS ≈440 MPa, yield ≈370 MPa) are soft, weld easily and machine with good finish — the default for brackets and non-critical parts. Medium-carbon (1045: UTS ≈585 MPa) and alloy grades (4140: UTS ≈655 MPa, 4340: UTS ≈745 MPa) add strength and respond to heat treatment for gears, shafts and structural components.

Free-machining 12L14 (leaded, resulfurized) cuts with short chips at high speed but has reduced toughness and is not weldable; it is the default for high-volume turned parts when corrosion resistance is not needed. Structural plate A36 (yield 250 MPa) is specified for weldments and frames.

2. Stainless families

Austenitic 304/316 are the workhorses: 304 (UTS ≈586 MPa, yield ≈241 MPa) is general-purpose; 316 adds molybdenum for chloride/marine resistance (UTS ≈579 MPa). Both work-harden during machining — use sharp tools and positive feeds. 303 is the free-machining austenitic grade (UTS ≈690 MPa) for production turning, at a small corrosion penalty.

Martensitic 410/440 offer high hardness (440C to ~58–60 HRC) for bearings and cutters; precipitation-hardening 17-4 PH (H900: UTS ≈1255 MPa, yield ≈1170 MPa) combines stainless corrosion resistance with high strength — the standard for medical and aerospace fittings.

3. Machining and heat treatment

Carbon and alloy steels machine at moderate speeds with coolant; hardness below ~35 HRC keeps tool life economic. Heat-treatable grades are specified by hardness rather than strength: 4140 Q&T to 28–32 HRC for shafts, 4340 for high-fatigue components, tool steels (O1, A2, D2) hardened to 55–62 HRC for dies and cutters.

Surface treatments: zinc plating (corrosion, ~5–12 µm), black oxide (cosmetic, thin), nickel plating (hardness and wear), phosphate + oil, and for stainless, passivation (nitric/citric) restores the chromium oxide layer after machining.

4. Selection guidance

Start with 1018 or 12L14 for machined parts, A36 for weldments, 304 for food/medical/corrosion, 316 for marine and chloride, 4140/4340 for high-strength components, 17-4 for high-strength stainless. Stainless costs roughly 3–5× carbon steel per kilogram and machines slower — avoid it unless corrosion or hygiene demands it.

Watch galvanic corrosion when coupling stainless with aluminum or carbon steel; specify passivation on all machined stainless to prevent embedded-iron rust spots.

Quick reference

Grade	Type	UTS MPa	Yield MPa	HRC / note	Best for
1018	Low carbon	440	370	126 HB	Brackets, general
12L14	Free-machining	540	415	163 HB	High-volume turned parts
1045	Medium carbon	585	450	197 HB	Shafts, gears
4140	Alloy	655	417	Q&T 28–32	High-strength components
A36	Structural	400	250	–	Weldments
303	Austenitic SS	690	550	–	Production machining
304	Austenitic SS	586	241	–	Food, medical, general
316	Austenitic SS	579	241	–	Marine, chloride
17-4 H900	PH SS	1255	1170	–	High-strength stainless

Sources & references

• ASTM A36 — Structural steel
• ASTM A240 — Stainless plate
• ASTM A276 — Stainless bar
• ASTM A311 — Free-machining steel
• EN 10083 — Quenched and tempered steels
• Xometry Learning Center — Steel & Stainless Guide (public engineering reference)

Grades 2 and 5 (Ti-6Al-4V): properties, machining, anodizing and cost

Titanium combines the strength of steel at ~57% of its weight with outstanding corrosion resistance and biocompatibility. Grade 2 (commercially pure) is the formable, weldable workhorse; Grade 5 (Ti-6Al-4V) offers aerospace-grade strength (UTS ≈950 MPa) and is the most common alloy for machined components. The trade-off is cost and machining difficulty: low thermal conductivity and work-hardening demand careful process control.

1. Grades and properties

Commercially pure grades 1–4 differ in strength and formability; Grade 2 (UTS ≈344 MPa, yield ≈276 MPa) is the standard for sheet, pipe and chemical equipment. Ti-6Al-4V (Grade 5) reaches UTS ≈950 MPa with yield ≈880 MPa and ~14% elongation, density 4.43 g/cm³ and modulus ≈114 GPa.

Titanium is highly corrosion-resistant in seawater, chlorides and most acids, biocompatible for implants, and non-magnetic. Its specific strength (strength/density) beats steel and aluminum, which is why it dominates aerospace and high-performance motorsport.

2. Machining behavior

Titanium conducts heat poorly (~7 W/m·K vs 50+ for steel) and work-hardens: cutting generates high localized temperatures that weld chips to tools. Successful machining uses low speeds, high feed, copious coolant, sharp positive-rake tools and rigid setups — mirroring the practice for stainless but more extreme.

Standard tolerance of ±0.127 mm is readily achievable; precision features to ±0.025 mm are possible but tool wear increases cost. Surface finishes to Ra 0.8 µm are routine; grinding or polishing reaches Ra 0.2–0.4 µm.

3. Finishing and joining

Anodizing colors titanium electrolytically (gold, blue, purple, black) and can add wear resistance. Passivation, bead blasting and shot peening (for fatigue) are common; electropolishing gives a mirror finish. Coatings such as hard anodize or nitride are used for wear.

TIG welding is standard with inert gas shielding; weld zones are softer and lose some strength. Bolted assemblies should avoid galling — threads require anti-seize or lubricated assemblies, and titanium fasteners against titanium are prone to seizure.

4. Cost and selection

Titanium costs roughly 10–15× carbon steel and 5–8× aluminum per part for machined components; specify it only when weight, corrosion or biocompatibility justifies the premium. For extreme strength at minimum weight, Ti-6Al-4V is the reference material across aerospace and medical.

For cost control, minimize material removal (design near-net), avoid thin walls that chatter, and consolidate features to reduce setups. ASTM B265 (sheet), B348 (bar) and F136 (surgical implant) define supply specifications.

Quick reference

Grade	UTS MPa	Yield MPa	Elong %	Density g/cm³	Best for
Grade 2 (CP)	344	276	20	4.51	Chemical, sheet, marine
Grade 5 (Ti-6Al-4V)	950	880	14	4.43	Aerospace, medical, racing
Grade 23 (ELI)	900	860	15	4.43	Surgical implants
Ti-3Al-2.5V	620	520	15	4.48	Hydraulic tubing

Sources & references

• ASTM B265 — Titanium sheet and plate
• ASTM B348 — Titanium bar
• ASTM F136 — Surgical implant Ti-6Al-4V ELI
• ISO 5832-3 — Implants, Ti-6Al-4V
• Xometry Learning Center — Titanium Guide (public engineering reference)

Thermoplastics vs thermosets, crystalline vs amorphous, and commodity engineering plastics

Plastics are divided into thermoplastics (re-meltable: ABS, PC, nylon, POM, PEEK) and thermosets (cross-linked: epoxies, polyimides). Within thermoplastics, semi-crystalline grades (nylon, POM, PEEK) resist chemicals and wear but shrink more in molding; amorphous grades (ABS, PC, acrylic) are dimensionally stable and transparent but solvent-sensitive. Selection starts with temperature, chemicals and mechanical load.

1. Thermoplastic vs thermoset

Thermoplastics soften and flow when heated, which is why they can be injection molded, extruded and recycled. Thermosets cure irreversibly — they cannot be re-melted and are typically molded in sheet or casting form; examples are epoxy (electrical laminates, composites) and phenolic.

Within thermoplastics, commodity grades (ABS, PP, PS, PVC) cover most consumer and structural-light applications at low cost; engineering grades (nylon, POM, PC, PET) add strength, temperature and wear performance; high-performance polymers (PEEK, PPS, PEI, PTFE) serve demanding thermal and chemical duty.

2. Crystalline vs amorphous

Semi-crystalline polymers (PA, POM, PEEK, PP, PE) have ordered molecular regions: they resist solvents and fatigue, wear well, but shrink more on molding (1–2.5%) and are opaque. Amorphous polymers (ABS, PC, PMMA, PS) shrink less (0.4–0.7%), are dimensionally stable and can be transparent, but stress-crack in some solvents and have lower wear resistance.

The crystalline/amorphous split drives practical choices: gears and wear parts → POM or PA66; transparent covers → PC or PMMA; high-temperature electrical → PEEK or PPS; chemical tanks → PE/PP; snap-fit consumer housings → ABS or PC/ABS.

3. Key engineering properties

Typical benchmark values: ABS density 1.06 g/cm³, UTS ≈47 MPa, heat deflection ~98 °C at 0.46 MPa; PC density 1.20, UTS ≈70 MPa, impact strength ~2× ABS; PA66 density 1.14, UTS ≈83 MPa (dry), tough but moisture-absorbing; POM density 1.41, UTS ≈69 MPa, excellent wear and fatigue; PEEK density 1.30, UTS ≈97 MPa, continuous service to ~250 °C.

Creep matters at temperature: amorphous grades creep more than semi-crystalline ones, so spring/retention features favor PA and POM. UV degrades most plastics — add UV stabilizers or black pigments for outdoor parts; PE and PP require surface treatment before bonding or painting.

4. Selection and quoting notes

Plastic material cost is a small fraction of total part cost; geometry, quantity and process dominate. For machined prototypes of a future molded part, machine ABS/PC first (they machine cleanly) and verify with molded samples later.

Thermal expansion of plastics is 5–10× steel (70–200 µm/m·K): precision assemblies need generous clearances or metal inserts. Keep in-service temperature below the heat deflection temperature with a margin, and check chemical compatibility with cleaners, fuels and lubricants.

Quick reference

Material	Type	Density g/cm³	UTS MPa	HDT °C (0.46MPa)	Notable
ABS	Amorphous	1.06	47	98	Tough, cheap, snap fits
PC	Amorphous	1.20	70	132	Clear, impact 2× ABS
PMMA (Acrylic)	Amorphous	1.19	72	95	Optically clear, brittle
PA66 (Nylon)	Semi-cryst.	1.14	83	95 (1.8MPa)	Wear, tough, hygroscopic
POM (Delrin)	Semi-cryst.	1.41	69	158	Gears, bearings, low friction
PP	Semi-cryst.	0.91	33	100	Living hinges, chemical tanks
PEEK	Semi-cryst.	1.30	97	315 (glass)	High temp, medical, aerospace

Sources & references

• ISO 1183 — Density of plastics
• ASTM D638 — Tensile properties
• ASTM D648 — Deflection temperature
• Xometry Learning Center — Plastics 101 (public engineering reference)

PEEK, PPS, PEI, PTFE, Ultem and reinforced grades for demanding duty

High-performance polymers retain mechanical properties and dimensional stability where commodity plastics fail: continuous service above 150 °C, aggressive chemicals, radiation, wear and friction. The shortlist for machined and molded parts is PEEK, PPS, PEI (Ultem), PTFE, PI and PAI (Vespel), each with a distinct cost/performance profile — typically 10–100× the price of ABS.

1. The material shortlist

PEEK (polyether ether ketone) is the benchmark: UTS ≈97 MPa (unfilled), continuous use to ~250 °C, chemical resistance across almost all solvents, and excellent creep resistance; glass- or carbon-reinforced grades raise modulus and reduce wear. PPS (polyphenylene sulfide) offers similar temperature performance at lower cost with outstanding chemical resistance but is more brittle.

PEI (polyetherimide, Ultem) is an amorphous high-temperature polymer with UTS ≈105 MPa, good electricals and flame resistance. PTFE (Teflon) is the extreme low-friction, chemically inert material (use to ~260 °C) but is soft and creeps — often used as a liner or in compounds.

2. Machining considerations

High-performance polymers machine like engineering plastics but demand sharp tools and controlled heat: PEEK and PEI can be machined to ±0.05 mm with good finish; PTFE is gummy — sharp rakes, high speed and coolant are essential to avoid smearing and tool loading.

Anisotropy from processing (extruded rod vs molded) affects warpage and mechanical consistency; for precision parts, machine from stress-relieved or annealed stock and allow stabilization before final dimensions.

3. Applications and trade-offs

PEEK replaces metal in aerospace brackets, medical implants and semiconductor hardware where weight, corrosion and sterilization matter. PPS suits automotive and fuel-system components; PEI serves electrical connectors and aircraft interiors; PTFE seals, bearings and chemical liners; PAI (Vespel) for extreme wear at temperature.

The trade is cost and processability: PEEK costs roughly 30–60× ABS per kilogram; PTFE is cheap by comparison but dimensionally unstable. Each also has a weakness — PTFE creeps, PPS is brittle in thin sections, PEI stress-cracks in some solvents, PEEK is expensive and needs high molding temperatures.

4. Selection guidance

Choose the lowest-cost polymer that meets the peak service temperature, chemical exposure and wear duty. If the application only needs 100 °C and mild chemicals, PA66 or POM will outperform a high-performance polymer on cost and machinability.

When quoting, high-performance polymers are priced by material weight plus a machining/molding surcharge; small quantities are expensive because material minimums dominate. Get the datasheet of the specific grade (e.g. PEEK 450G vs GF30) before finalizing the design.

Quick reference

Material	Density g/cm³	UTS MPa	Max continuous °C	Cost index (ABS=1)	Key trait
PEEK 450G	1.30	97	250	30–60	Best all-round high-temp
PEEK GF30	1.49	165	250	35–65	Stiff, low creep
PPS	1.35	90	200	10–20	Chemicals, low cost
PEI (Ultem)	1.27	105	170	8–15	Electrical, flame
PTFE	2.20	27	260	3–8	Friction, inert, soft
PAI (Vespel)	1.45	130	275	60–100	Extreme wear

Sources & references

• ASTM D638 — Tensile
• ASTM D3418 — Transition temperatures
• ISO 527 — Tensile properties of plastics
• Xometry Learning Center — High-Performance Polymers Guide (public engineering reference)

PA6, PA66 and PA12: hygroscopicity, wear, and SLS/MJF printed properties

Nylon (polyamide) is the workhorse engineering plastic for wear, toughness and fatigue. PA6 and PA66 are molded/extruded grades with high strength but strong moisture absorption that changes dimensions and stiffness; PA12 absorbs far less water and is the standard SLS/MJF 3D printing material. For printed parts, PA12 delivers UTS ≈48 MPa with layer-dependent ductility and excellent chemical resistance.

1. Grades and moisture

PA66 (UTS ≈83 MPa dry) and PA6 (UTS ≈75–85 MPa) are the strongest commodity nylons, used for gears, bushes, fasteners and structural brackets. Their weakness is hygroscopicity: they absorb up to 8% moisture by weight, which plasticizes the material — strength drops and dimensions grow — so parts must be conditioned or designed with moisture in mind.

PA12 absorbs only ~1.5% water, giving stable dimensions and electrical properties; it is tougher and more chemical-resistant than PA6/66 at slightly lower strength. It is the default powder for SLS and MJF printing and common in fuel, pneumatic and cable applications.

2. Printed PA12 (SLS / MJF)

SLS/MJF nylon prints at layer heights 0.1–0.12 mm with typical tolerance ±0.3% (±0.3 mm). Printed PA12 has density ≈1.0 g/cm³, UTS ≈48 MPa, elongation roughly 15–40% depending on orientation and infill, and excellent fatigue and impact behavior — strong enough for functional brackets, ducts and snap fits.

Printed parts are porous on the surface (0.1–0.3 mm Ra); dyeing, media blasting and sealers improve cosmetics and reduce fluid permeability. Minimum wall is ~0.7–1.0 mm; unsupported overhangs are not a constraint because un-sintered powder supports the geometry.

3. Wear and friction

Nylon is self-lubricating and wears well against steel: PA66 with internal lubrication (MOS2, PTFE) is a standard gear material. Its low friction coefficient (0.2–0.4 against steel) and fatigue resistance make it preferable to POM where impact or noise matters.

For printed gears, PA12 fatigue life is good but inferior to molded PA66; consider glass-filled printed grades (PA12-GF) for stiffness. Avoid nylon in hot, wet acid environments — hydrolysis degrades the polymer above ~60 °C in moisture.

4. Selection guidance

Molded/CNC: PA66 for strength and temperature (to ~90 °C continuous), PA6 for toughness and cost, PA12 for dimensional stability and low moisture. Printed: PA12 for functional prototypes and end-use brackets; PA12-GF for stiffness; MJF gives finer detail than SLS on small features.

Design clearances for moisture growth in molded nylon, specify conditioning (water absorption to equilibrium) for precision fits, and add a shrinkage allowance if the part will be post-annealed.

Quick reference

Grade	Density g/cm³	UTS MPa	Water abs. 24h	HDT °C	Notes
PA6	1.13	75–85	1.3–1.8%	80 (1.8MPa)	Tough, cost-effective
PA66	1.14	83	1.0–1.5%	95	Strongest molded nylon
PA12	1.01	48 (printed)	~0.3%	95	SLS/MJF standard
PA12-GF	1.25	70 (printed)	~0.3%	130	Stiff, low creep

Sources & references

• ISO 1874 — Polyamide moulding materials
• ASTM D638 — Tensile
• ASTM D570 — Water absorption
• Xometry Learning Center — Nylon PA12 Guide (public engineering reference)