Plastic Square Bar Stock: Sizes, Materials & Applications

Plastic square bar is solid square-cross-section engineered plastic supplied from 1/4" to 6" per side, most commonly in Acetal (Delrin) and Nylon 6/6. The square profile reduces setup time when machining keys, slides, guide rails, gibs, and flat-sided structural members — the flat faces clamp directly in a milling vise without the V-block setup required for round rod. Square bar is also blanked and sawn into flat pieces that function as precision spacers, wear pads, and electrical bus supports.

TL;DR

• Materials: Acetal (natural and black) and Nylon 6/6 primarily; PEEK, HDPE, and UHMW available on order
• Size range: 1/4" × 1/4" to 6" × 6"; standard increments match fractional inch sizes
• Standard lengths: 4 ft for most sizes; 2 ft for sizes above 3"
• Side tolerance: ±0.010"–±0.030" on extruded square bar; ±0.003"–±0.005" on precision-milled
• Acetal is preferred for dimensional stability; Nylon for toughness and impact resistance
• Common applications: keys, gibs, guide slides, wear pads, electrical standoffs, and busbar supports

Size Table — Plastic Square Bar

Rectangular bar (non-square cross-section, e.g., 1" × 2" or 1/2" × 1-1/2") is available in Acetal and Nylon on a cut-from-sheet basis; contact for pricing.

Materials Available in Square Bar

Acetal Square Bar

Acetal square bar is the engineering benchmark for precision square stock. Delrin and Celcon Acetal have near-zero moisture absorption (0.25% saturation), which means dimensions machined from Acetal square bar stay accurate after installation — unlike Nylon, which swells with absorbed moisture. Natural white Acetal is FDA-compliant for food and pharmaceutical contact. Black Acetal is used for industrial and aesthetic applications where white is undesirable.

Acetal's compressive strength (18,000 psi) supports gib and wedge applications where the square bar is loaded in compression against a mating surface. Its low coefficient of friction (0.2–0.35 against steel, unlubricated) reduces breakaway force in sliding-contact applications. Hardness (Rockwell M90) resists surface scoring under light abrasion.

For keys and keystock applications — where a square bar is inserted into a shaft keyway to transmit torque — Acetal square bar in 1/4" to 1" sizes is a common replacement for metal keys in low-torque, chemical-process, and food-processing equipment. Acetal keys are lighter, resist corrosion, do not score the shaft, and can be replaced without metalworking tools.

Nylon Square Bar

Nylon 6/6 square bar delivers higher impact toughness than Acetal (Izod impact: 1.0–2.0 ft-lb/in notched vs. 1.3–1.4 ft-lb/in for Acetal) at the cost of greater moisture sensitivity. Nylon square bar is the choice for guide rails, ways, and slide liners where a degree of toughness is needed to absorb shock loads without fracturing. It is also used for structural machine members in chemical-process equipment where corrosion would destroy metal alternatives.

MoS₂-filled Nylon square bar (gray color) is stocked in common sizes for dry-running slide applications where the bar is a moving surface. The MoS₂ filler reduces the coefficient of friction to 0.15–0.20 against steel and lowers the rate of abrasive wear.

Moisture swell must be accounted for: Nylon square bar can grow 0.006"–0.010" per inch of cross-section from dry to saturated conditions. Specify and machine Nylon square bar at or near its expected service moisture content; for outdoor or wash-down applications, size fits generously to accommodate swell.

PEEK, HDPE, and UHMW-PE Square Bar

PEEK square bar is produced to order for semiconductor carrier guides and aerospace structural sections. HDPE square bar provides low-cost chemical resistance for outdoor structural members and tank supports. UHMW-PE square bar suits wear pads and guide liners where abrasion resistance outweighs the need for tight dimensional tolerances.

How Plastic Square Bar Is Made

Extrusion Through Square Die

Acetal and Nylon square bar is produced by screw extrusion through a square-profile die. The die maintains the four flat faces, but because the corners of a square profile cool faster than the flat surfaces, a small corner radius (R ≈ 0.015"–0.030") results. This corner break is normal in extruded square bar. The extrudate is pulled through a vacuum calibrator that constrains the dimensions while cooling.

Residual stresses in extruded square bar are higher near the corners and surface than at the centerline — for precision machined parts requiring flat faces within 0.002", specify stress-relieved (annealed) bar or plan for a light face-milling pass after sawing.

Flat-Milling from Round or Plate

For materials not available in extruded square (PEEK, PVDF, PPS), square bar is milled from round rod or plate on a CNC milling center. Each of the four sides is faced in sequence, producing sharper corners (R < 0.005") and tighter side tolerance (±0.003"–±0.005") than extrusion. This is the standard path for custom cross-sections in specialty materials.

Specifications & Tolerances

Side (Width) Tolerance

Squareness and Length

Extruded square bar holds 90° squareness within 0.5°–1.0°; precision-milled within 0.1°–0.3°. Face-mill two reference surfaces for precision slides. Standard lengths: 4 ft. Saw-cut tolerance: ±0.125"; CNC cutoff: ±0.030".

Applications in Detail

Keys and Keystock

Plastic keystock in 1/4" × 1/4" through 1" × 1" is cut to length and used in shaft-hub assemblies for low-torque, corrosion-sensitive power transmission. Acetal is preferred over Nylon for keystock because its lower moisture swell maintains the key-to-keyway fit. Keys longer than 2" should be milled from sheet rather than square bar to maintain flatness.

Slides and Linear Guide Rails

Square bar mounted in milled slots serves as a low-cost linear guide or gib for machine tables, valve body guides, and conveyor diverter rails. Nylon square bar in MoS₂-filled grade provides the best sliding performance. Mill the slot to a close sliding fit (+0.002" / –0.000" over bar size) and allow 0.010"–0.020" clearance for thermal expansion in heated environments.

Wear Pads and Liners

Large square sections (2"–6") are sawn into rectangular pads and bonded or fastened to machine surfaces subject to sliding wear. Acetal wear pads are used on conveyor guide rails; UHMW-PE and HDPE pads for heavier-duty dock bumpers and loading-dock guide liners. The flat faces of square bar stock self-align during installation without a secondary facing operation.

Cutting & Finishing

Sawing

Square bar clamps cleanly in a standard milling vise for band-saw or cold-saw cutoff — a significant setup advantage over round bar, which requires V-blocks or strap clamps. Use 14–18 TPI blades for sizes under 1-1/2"; 10–14 TPI for larger sections. Coolant or compressed air keeps the cut face cool and prevents gumming on Nylon.

Milling

Face milling establishes flat, parallel reference surfaces. Use carbide face mills at 600–900 SFM for Acetal and Nylon. Depth of cut 0.010"–0.050"; feed per tooth 0.002"–0.005". Flood coolant prevents surface melting on Nylon; Acetal tolerates dry milling with compressed air chip evacuation.

Drilling and Tapping

Square bar drills and taps readily with sharp HSS tooling. Plastic swarf packs into tap flutes aggressively — use thread-forming taps in Acetal and Nylon for cleaner threads, longer tool life, and no chip management.

More related guides

Common materials and applications stocked in this form:

Top materials

• PEEK
• UHMW
• Delrin Polycarbonate / Lexan
• G10 and FR4

Applications

• Bushings & bearings
• Wear strips & guide rails
• Electrical insulation

Industries

• Aerospace plastics
• Food & beverage plastics
• General industrial