G11/FR5 Glass Epoxy Sheet

FR-5 is the flame-retardant version of G-11: high-temperature glass-epoxy with a flame retardant designed into the resin, meeting NEMA FR-5 - now ANSI/NEMA IM 60000-2021 - along with MIL-I-24768/28 Type GEB-F and ASTM D709 Type IV Grade FR-5. It is the grade to specify when a part has to hold load at temperature and carry a flame rating. The published properties are measured at 0.500 inch: flexural strength 61,000 psi lengthwise and 52,000 crosswise, compressive strength flatwise 70,000 psi, tensile strength 41,000 and 32,000 psi, notched Izod impact 14.8 and 11.2 ft-lb/in, shear strength perpendicular 39,000 psi and hardness Rockwell M115. Specific gravity is 1.95 and the glass transition temperature is 190C, against about 185C for G-11.

Hot strength is the point: 51,000 psi lengthwise and 46,000 crosswise measured at 150C, which is better than half the room-temperature figure retained. The temperature index is published as 170C electrical and 180C mechanical, footnoted by its maker as a NEMA LI-6 recommendation rather than a UL 746B index; a second FR-5 grade from the same maker carries 130C and 140C to UL 746B instead, so check which one you are being quoted. Electrically, the dielectric constant is 5.02, the dissipation factor 0.018, moisture absorption 0.05% and breakdown voltage parallel to the laminations 70 kV. The dielectric strength perpendicular reads 200 V/mil, which looks low next to G-10 and is not - it was measured at 0.500 inch, and every grade in this family reads low at that thickness.

Two honest cautions. The FR in the name does not settle the rating: one maker publishes two FR-5 grades, one at UL 94 V-1 and one at V-0, so tell us which rating you need and we will confirm the grade and the lot. And FR-5 is made thick - one grade is available 0.250 inch and up, the other 0.500 and up - so thin stock sold as FR-5 is usually something else, and it is worth asking. As with FR-4, heating this grade above 300C decomposes the flame-retardant package and releases hydrogen bromide.

It is used for motor and generator insulation running hot, transformer components, high-temperature terminal boards and barriers, aerospace and defence insulation, and any flame-rated structural insulator with a thermal duty.

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.