Ultem and polycarbonate CNC machining share one theme: both are amorphous plastics that cut well but are sensitive to stress and to the chemicals they touch during and after machining. Ultem is the brand name for polyetherimide (PEI), an amber, high-heat plastic with continuous service near 340 F, inherent flame resistance and strong dielectric properties. Ultem 1000 is unfilled, and Ultem 2300 is 30 percent glass-filled for stiffness. Polycarbonate is clear, very impact resistant and low in cost. Wexmar machines both at our shop in Canastota, New York.
Machinability of Ultem and polycarbonate
Plastics are not rated on the AISI 1212 steel scale. Both materials cut freely, and the limits are heat, stress, clamping and dimensional stability. Ultem is stiff and dimensionally steady, with low moisture absorption, which makes it a favorite for precise electrical and fluid parts. Polycarbonate is softer and more prone to gumming at high spindle speeds. The biggest risk with both is stress cracking, also called crazing, where fine cracks appear hours or days after machining because residual stress met an incompatible fluid.
Tolerances and surface finish
On Ultem, plus or minus 0.001 in. is typical on small turned diameters and plus or minus 0.002 in. on milled features. Polycarbonate typically holds plus or minus 0.002 to 0.003 in. depending on size and wall thickness. Surface finish of 32 to 63 Ra is typical as machined. Machined polycarbonate is translucent, not optically clear; faces that must be see-through require polishing after machining. Dimensional and flatness checks are done on our CMMs and reported through our inspection and quality program.
Stress cracking, tooling and chip control
- Coolant choice. Many petroleum-based coolants, cutting oils and alcohol cleaners attack stressed polycarbonate and Ultem. We use compatible water-based coolant or air blast and clean parts with compatible solutions only.
- Sharp tools, light cuts. Polished carbide with positive rake and generous clearance limits heat. Glass-filled Ultem 2300 is abrasive, so diamond-coated tools are used on production runs.
- Annealing. Stress-relieved stock is the starting point for critical parts, and a second anneal after roughing reduces the risk of crazing and movement.
- Drilling and tapping. Sharp drills, pecking and moderate feeds prevent cracking around holes. Tapped threads in polycarbonate are kept conservative, and thread inserts are considered for repeated assembly.
- Clamping. Soft jaws and even support prevent chipping on Ultem and marring on polycarbonate. Protective film stays on polycarbonate sheet as long as practical.
- Burrs. Ultem edges can chip if tools are dull. Programmed chamfers and fine finishing passes keep edges clean.
Annealing and finish options
Annealing of Ultem and polycarbonate, when specified, is performed through qualified outside processors. Vapor polishing and optical polishing for clear polycarbonate faces are also arranged through qualified outside processors. Most Ultem parts ship as machined, since the material needs no coating for electrical or thermal service.
Common Ultem and polycarbonate parts
- Fluid manifolds, valve bodies and analytical instrument blocks in Ultem (horizontal machining centers)
- Electrical insulators, terminal boards and connector housings in Ultem (horizontal mills and Swiss-type to 32 mm diameter)
- Test sockets, burn-in fixtures and electrical test hardware in Ultem (horizontal milling)
- Machine guards, sight windows and covers in polycarbonate (horizontal machining centers)
- Clear flow cells, fixture plates and inspection gauges in polycarbonate
- Round insulators, spacers and bushings in either material (CNC lathes)
Most parts in these materials are block or plate work, which runs on our pallet-changing horizontals in the CNC milling department.
Cost index vs 6061 aluminum
| Material | Approx. machinability (1212 = 100) | Cost index vs 6061 | Notes |
|---|---|---|---|
| 6061-T6 aluminum | ~180 | 1.0 | Baseline |
| Delrin / acetal | Not rated; cuts freely | 1.0 | Opaque, lower heat limit |
| Polycarbonate | Not rated; cuts freely | 1.3 | Clear, impact resistant |
| PTFE | Not rated; cuts freely | 1.4 | Chemical resistant, soft |
| Ultem (PEI) | Not rated; cuts freely | 3.0 | High heat, flame resistant, stable |
| PEEK | Not rated; cuts freely | 4.5 | Higher heat and chemical resistance |
These values are typical and illustrative for a comparable part; actual quotes depend on geometry, quantity, tolerances and mill pricing. When chemical exposure or temperature exceeds what Ultem handles, see PEEK machining.
Applications and industries
Ultem machining services support electrical and connector manufacturers, semiconductor test and instrumentation companies, aerospace hardware programs and medical hardware makers who need a sterilizable, high-heat plastic. Polycarbonate parts serve machine builders, industrial OEMs and laboratory equipment makers that need clear, tough guards and fixtures. Parts ship nationwide from Central New York.
Getting a quote
To price Ultem or polycarbonate parts, upload a drawing for an instant estimate or call (315) 400-2654. A human estimator confirms every quote, including grade, color and any polishing requirement.