GPO-3 HM (High-mechanical grade)
Grade
ASTM D709 Grade GPO-3
Use for load-bearing barriers and machined structural parts where standard GPO-3 stiffness is marginal.
Values for the HM condition
Other GPO-3 grades and fills
GPO-3 is listed in 5 conditions; see the grade overview at GPO-3.
Compare all 5 grades and fillsHide
| Grade · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness |
|---|---|---|---|---|---|
UL94 5VA rated high-flame-resistance grade | Use where enclosure-level 5VA burn performance is required (typically ≥3 mm sheet). | — | 65 | 1 | 108 HRM |
UL94 V-0 flame-rated stock (thin sections from ~1.5 mm) | Pick when the UL Yellow Card V-0 rating must be held down to thin barrier thicknesses. | — | 68 | 1.2 | 106 HRM |
Enhanced track/arc-resistant grade | Pick for medium-voltage phase barriers where surface tracking and arc erosion dominate the design. | — | 68 | 1.2 | 108 HRM |
High-temperature grade | Choose for arc chutes and coil insulation running continuously above 130 °C. | — | 70 | 1.1 | 108 HRM |
HMThis page High-mechanical grade | Use for load-bearing barriers and machined structural parts where standard GPO-3 stiffness is marginal. | — | 95 | 1.5 | 100 HRM |
What is GPO-3 HM?
GPO-3 HM is GPO-3 in the HM grade — use for load-bearing barriers and machined structural parts where standard GPO-3 stiffness is marginal. GPO-3 HM has a tensile strength of 95 MPa (13.8 ksi), weaker than 69% of composite grades. Elongation at break is 1.5% and the elastic modulus is 11.5 GPa (1.67 Msi). GPO-3 HM has a density of 1.8 g/cm³ (0.065 lb/in³), heavier than 59% of composite grades.
Advantages
- Low embodied carbon — 4.2 kg CO₂/kg, better than 88% of composite grades
Limitations
- Hard to polish — 20/100, worse than 90% of composite grades
GPO-3 HM properties
Typical room-temperature values for GPO-3 HM; 16 are specific to this condition. Other listed values are GPO-3 reference data and may not represent this condition. Bars are the position among composite grades.
Physical3
GPO-3 HM has a density of 1.8 g/cm³ (0.065 lb/in³), heavier than 59% of composite grades.
Mechanical11
GPO-3 HM has a tensile strength of 95 MPa (13.8 ksi), weaker than 69% of composite grades. Elongation at break is 1.5% and the elastic modulus is 11.5 GPa (1.67 Msi).
Thermal8
GPO-3 HM is rated for continuous service to 130°C (266 °F). It conducts heat at 0.3 W/m·K (0.173 BTU/hr·ft·°F), worse than 63% of composite grades. Thermal expansion is 22 µm/m·K (12.2 µin/in·°F).
Electrical7
GPO-3 HM is an electrical insulator with a dielectric strength of 14 kV/mm (355.6 V/mil) and a resistivity of 1×10¹¹ Ω·m.
Chemical & Environmental3
Corrosion resistance is very good (72/100), worse than 58% of composite grades. UV resistance is good.
Sustainability4
Producing a kilogram of GPO-3 HM takes about 70 MJ of energy and emits 4.2 kg of CO₂ — less than 93% of composite grades. Typical recycled content is 0%.
Manufacturability2
GPO-3 HM's machinability is fair (40/100, better than 57% of composite grades).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with GPO-3 HM
Is GPO-3 easy to machine?
Very abrasive (ATH + glass): use carbide or PCD tooling, high speed / moderate feed, sharp positive rake and dust extraction — the dust is a respiratory irritant.
Can GPO-3 be welded?
Thermoset — cannot be welded or thermoformed; join by bolting, riveting or epoxy/polyurethane bonding after abrading and cleaning.
Can GPO-3 be formed, bent or molded?
Supplied as flat compression-molded sheet, rod or tube; net shapes are made by matched-die molding of the polyester/glass mat charge at roughly 140–160 °C, not by reheating stock.
What surface finishes work on GPO-3?
Machined edges should be deburred and sealed with polyurethane or epoxy varnish for humid/tracking-critical service; the sheet takes no plating and needs an adhesion primer before painting.
GPO-3 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| unsaturated polyester resin (matrix)styrene-crosslinked UP; some grades use vinyl ester or DAP-modified resin | 20 – 32 | 26 |
| glass fiber (chopped strand mat)random mat, gives quasi-isotropic in-plane properties | 20 – 35 | 28 |
| pigment / process additivestypically red, orange or natural sheet color | 0.5 – 3 | 1 |
| aluminum trihydrate / mineral fillerATH provides flame retardance, arc and track resistance | ≥ 35 (balance) | — |
GPO-3 HM — frequently asked
What is GPO-3 HM used for?
GPO-3 HM is typically used for switchgear arc chutes, high-voltage insulation barriers, circuit breaker components and bus supports. In short: arc- and track-resistant; switchgear standard laminate; V-0 and 5VA conditions.
What is the tensile strength of GPO-3 HM?
GPO-3 HM has a typical tensile strength of 95 MPa (13.8 ksi) — weaker than 69% of composite grades. Strength varies by condition: see the 5 listed tempers.
Is GPO-3 HM easy to machine?
Not especially — machinability is rated fair (40/100, better than 57% of composite grades). Very abrasive (ATH + glass): use carbide or PCD tooling, high speed / moderate feed, sharp positive rake and dust extraction — the dust is a respiratory irritant.
Can GPO-3 HM be welded?
Thermoset — cannot be welded or thermoformed; join by bolting, riveting or epoxy/polyurethane bonding after abrading and cleaning.
What surface finishes work on GPO-3 HM?
Machined edges should be deburred and sealed with polyurethane or epoxy varnish for humid/tracking-critical service; the sheet takes no plating and needs an adhesion primer before painting.
Can GPO-3 HM be formed, bent or molded?
Supplied as flat compression-molded sheet, rod or tube; net shapes are made by matched-die molding of the polyester/glass mat charge at roughly 140–160 °C, not by reheating stock.
What is the maximum service temperature of GPO-3 HM?
GPO-3 HM is rated for continuous use to about 130°C (266 °F), and briefly to 180°C. Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
Sources
- ASTM D709 — Standard Specification for Laminated Thermosetting Materials (Grade GPO-3) — ASTM International (2023)
- EN 60893-3-2 — Industrial rigid laminated sheets, thermosetting resins, type UP GM 203 — CENELEC/IEC (2013)
- IEC 60112 — Method for determining proof and comparative tracking indices — IEC (2020)
- GPO-3 polyester glass mat laminate sheet datasheets — Von Roll / Glastic / Norplex-Micarta (2023)
- Engineered Materials Handbook Vol. 1 — Composites — ASM International (1987)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.