G11 Glass Epoxy Sheet

G-11 is the high-temperature glass-epoxy, and the property that justifies it is hot flexural strength. Measured at 150C, one maker publishes 40,000 psi and another exactly 30,000 psi, which is the NEMA required minimum - so take 30,000 as what the grade guarantees and 40,000 as what good stock does. At room temperature expect 70,000 to 80,000 psi lengthwise typical against a NEMA minimum of 60,000, tensile strength 37,000 to 43,000 psi and compressive strength flatwise around 70,000 to 72,500 psi. It meets NEMA G-11 - now ANSI/NEMA IM 60000-2021 - along with MIL-I-24768/3 Type GEB and ASTM D709 Type IV Grade G-11.

Maximum continuous temperature is published two ways and the difference matters. The UL 746B temperature index is 170C electrical and 180C mechanical, which is a short-term index; makers separately describe 155C as the figure the material holds for decades. Treat 155C as the design figure for long continuous service and 180C as the index. Electrically it is close to G-10: dielectric constant 4.5 to 5.0 and dissipation factor 0.014 to 0.022 at 1 MHz, breakdown voltage parallel to the laminations 50 to 60 kV, dielectric strength perpendicular 485 to 900 V/mil depending on test thickness, tracking index 180 V, arc resistance around 180 seconds and water absorption 0.10 to 0.20%.

Flammability depends on the maker and the thickness rather than on the grade name, and this is worth getting right. One maker publishes UL 94 HB for its G-11 and describes it as not flame retardant; another publishes V-0 at 0.250 inch and thicker and HB below that. So do not assume either - tell us if you need a rating and we will confirm the classification on the lot. FR-5 is the grade with flame retardant designed into the resin from the start.

It goes into motor and generator slot wedges, coil supports and phase insulation, transformer spacers and blocking, high-temperature terminal boards, down-hole and aerospace components, and any insulator that has to carry load while hot. Natural stock is light green.

Machining and handling. The glass abrades tooling, so carbide is the floor and diamond-coated or PCD cutters are what hold tolerance in production; run high spindle speeds with secure workholding and design in generous internal radii, because small tools wear out fast against glass. Threaded holes are normally milled rather than tapped and external threads turned or thread-milled, since the plies can delaminate. The laminate has no hazards as shipped, but cutting it releases a fine dust, and the epoxy resin in that dust is classified as a skin and respiratory sensitiser - use local exhaust, a dust respirator, eye protection and gloves, and work to your own site's exposure limit, because published limits for glass dust differ by jurisdiction and by whether total or respirable dust is measured. It comes off as powder rather than chips, so vacuum it up rather than blowing it airborne. These are our shop notes, not a manufacturer specification.