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Cotton Phenolic Specifications — NEMA LI-1 Grade C & CE

Cotton phenolic sheet, rod, and tube stock is manufactured and tested to NEMA LI-1, the industry standard for industrial laminates in North America. Grades C (mechanical) and CE (mechanical/electrical) are the applicable classifications for cotton fabric–reinforced phenolic. This page covers sheet and rod standard sizes, thickness and diameter tolerances, flatness and bow requirements, NEMA LI-1 test methods, military cross-reference specifications, and what certifications to request on a purchase order.

TL;DR — Specifications at a Glance

  • Standard sheet: 36" × 48" and 48" × 96"; thickness 1/16" through 4"
  • Standard rod: 1/2" through 12" diameter in 4-ft lengths; some diameters available in 8-ft lengths
  • Tolerances tighten with thickness: ±10% below 1/4", ±0.015" at 1/4" and above
  • NEMA LI-1 governs minimum property values per grade; third-party certs available on request
  • MIL cross-reference: MIL-I-24768/11 (MFG-CE) for grade CE laminates
  • All NEMA grades require lot-level traceability; request material certification (C of C) on order

Governing Standard: NEMA LI-1

The National Electrical Manufacturers Association (NEMA) LI-1 standard, Industrial Laminated Thermosetting Products, defines:

  • Minimum mechanical, electrical, and physical properties per grade
  • Test methods for each property (referenced to ASTM standards)
  • Dimensional tolerances for sheet, rod, and tube
  • Sampling and acceptance criteria for manufacturing lots

NEMA LI-1 is a voluntary industry standard, but it serves as the de facto specification for commercial cotton phenolic procurement in North America. When purchasing, reference "NEMA LI-1 Grade C" or "NEMA LI-1 Grade CE" on the purchase order to ensure property compliance.

Key NEMA LI-1 Test Methods

Sheet Specifications

Standard Sheet Sizes

Sheet Thickness Increments

Standard mill stock increments (fractional inch): 1/16, 1/8, 3/16, 1/4, 5/16, 3/8, 1/2, 5/8, 3/4, 7/8, 1, 1-1/4, 1-1/2, 1-3/4, 2, 2-1/4, 2-1/2, 3, 3-1/2, 4 inches

Non-standard thicknesses can be machined from the next thicker standard size. Skiving (precision surface grinding) is available for thickness tolerances below ±0.010".

Sheet Thickness Tolerances (NEMA LI-1)

Thickness tolerances are on the as-manufactured laminate, measured at least 1" from any edge. Edge zones are excluded because resin pooling and laminate tension during cure create edge-thickening artifacts. If your application requires uniform thickness to the sheet edge, specify skived-and-ground stock.

Flatness and Bow

NEMA LI-1 permits a maximum bow of 1/4" per 48" length for sheets 1/4" and thicker. Thinner sheets (below 1/4") are allowed greater bow because they are flexible enough to flatten under fixturing. For precision jig plates requiring flatness better than 0.005"/ft, specify thermally stress-relieved and ground sheet.

Rod Specifications

Standard Rod Diameters and Lengths

Common stocked rod diameters: 1/2, 5/8, 3/4, 7/8, 1, 1-1/8, 1-1/4, 1-3/8, 1-1/2, 1-3/4, 2, 2-1/4, 2-1/2, 3, 3-1/2, 4, 4-1/2, 5, 6, 7, 8, 9, 10, 12 inches. Non-standard diameters above 12" may be molded to order.

Rod Diameter Tolerances (NEMA LI-1)

Rod tolerances are unilateral positive (oversize) to ensure sufficient material for finish machining to final dimension. When machining a rod to a precise OD or bore dimension, always plan a final pass at the correct feed rate with flood coolant — phenolic will spring slightly after roughing passes.

Rod Ovality

NEMA LI-1 permits ovality (maximum minus minimum diameter in a cross-section) of up to 1% of nominal diameter for rods through 4" diameter, and up to 0.75% for larger diameters. For bushing blanks where concentricity between OD and bore is critical, rough-turn the OD first to remove ovality before boring the ID.

Tube Specifications

Tube stock is less standardized than sheet or rod. Manufacturers produce tube by filament winding (for small diameters) or by wrapping flat laminates into a tubular mold. Standard wall thicknesses start at 1/4". Custom ID/OD combinations are available on a made-to-order basis with minimum quantity requirements.

Typical tube specifications:

  • OD range: 1" to 18"
  • Wall thickness: 1/4" minimum; 3" maximum for most configurations
  • Length: 4 ft standard
  • Tolerance: ±0.010" on wall thickness (within the tube wall, not diameter)

Military and Aerospace Cross-References

For defense procurement and aerospace-class applications, cotton phenolic is cross-referenced to:

Military-specification material requires additional documentation: material certification (C of C) referencing the applicable MIL spec, test report with actual property values per lot, and traceability to the resin and fabric raw material lots. Request these documents on the purchase order before shipment — retroactive certification on phenolic laminates is not possible once the material is cut.

Flatness and Bow Specifications in Practice

Why Flatness Matters for Jig Plates and Precision Parts

Cotton phenolic sheet develops stress during the cure press cycle as different plies experience slightly different cure temperatures and pressure distributions. The resulting residual stress can cause bow (curvature across the length or width) when the sheet is removed from the press. NEMA LI-1 limits this bow to 1/4" per 48" of length for sheet 1/4" and thicker, but some applications require better flatness than this.

For drill jig plates, precision fixture bases, and metrology surfaces, request thermally stress-relieved and ground sheet. The stress-relief cycle heats the sheet to 200–225°F and allows residual stresses to equalize over several hours, reducing bow dramatically. Post-relief surface grinding brings the sheet to a consistent thickness with flatness typically better than 0.005"/ft.

Storing Sheet Stock Flat

Cotton phenolic sheet stored vertically (on edge) can develop bow under its own weight over time, particularly in warm or humid storage environments where moisture softens the resin slightly. Store sheet stock flat, supported uniformly along its full length, to maintain flatness through the storage period. Standing sheets against a wall unsupported causes progressive bow that may require re-grinding before use in precision applications.

Certification and Documentation Requirements

Material Certifications

A standard commercial cotton phenolic purchase ships with a Certificate of Conformance (C of C) on request. The C of C should state:

  • Manufacturer name and lot/batch number
  • NEMA LI-1 Grade (C or CE) to which the material conforms
  • Form (sheet/rod/tube), nominal size, and quantity
  • Reference to the test standard (NEMA LI-1)

For applications requiring actual test data (measured tensile strength, measured dielectric strength), request a Test Report in addition to the C of C. Some manufacturers include test data by lot; others test representative samples per production campaign. Clarify the documentation requirement before order placement — retroactive testing on a cut piece is not equivalent to lot certification.

MIL-Spec Documentation

For MIL-I-24768/11 (Grade MFG-CE) requirements, the documentation set expands to include:

  • Manufacturer’s qualification to the military specification (QPL listing or First Article Test report)
  • Lot acceptance test report referencing specific MIL-I-24768 test methods
  • Certificate of conformance citing the MIL spec and contract number
  • Material traceability to the phenolic resin and fabric raw material lots

Not all commercial cotton phenolic manufacturers maintain MIL-spec qualifications. Confirm QPL or MIL qualification at the request-for-quote stage if the application requires MIL documentation.

What to Put on a Purchase Order

A complete purchase order for cotton phenolic should specify:

  1. NEMA LI-1 Grade C or Grade CE (not just "phenolic" or "cotton phenolic")
  2. Form: sheet / rod / tube / bushing blank
  3. Size: thickness × width × length (sheet) or diameter × length (rod)
  4. Quantity: pieces or weight
  5. Color: natural or black (if functionally relevant)
  6. Certifications required: C of C, test report, MIL-spec compliance (if applicable)
  7. Special processing: oil-impregnated, skived and ground, stress-relieved

For pricing on standard stock and custom sizes, request a quote through our line card.

Get a quote on NEMA LI-1 grade C or CE sheet, rod, and tube with full certification

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