Electrical insulation epoxy casting compound

01

Plan mixing, degassing and casting

Huntsman's data sheet for Araldite CW 229 / Aradur HW 229-1, a hot-curing two-component epoxy casting system with mechanically reinforcing fillers, recommends it for electrically insulating components and indoor medium- and high-voltage insulation, including SF6-insulated types. It mixes 100:100 by weight or 100:90 by volume; the mixed system has a specific gravity of 1.80–1.85 and viscosity of 25,000–35,000 cPs at 25 °C, 2,500–3,500 at 50 °C and 700–1,200 at 70 °C. [1]

For large vacuum castings it describes preheating resin, hardener and mould separately to 60–80 °C, mixing for about 30 minutes under 2–6 torr, and filling the preheated mould inside a vacuum chamber at 4–8 torr to avoid trapped air. For automatic pressure gelation, components are kept degassed at 50–60 °C and the mould is preheated to 140–160 °C, filled in 2–5 minutes and gelled under 1–3 atm for 7–15 minutes. [1]

Atul's Lapox technical data sheets describe two hot-curing anhydride systems for APG and conventional vacuum casting of post insulators, switchgear parts, bushings, instrument transformers and dry-type transformers: ARC-16/AH-118 (resin 100, pre-accelerated hardener 80, plasticiser up to 8, silica 320–350 parts by weight; 8-hour pot life for a 5 kg mix at 25 °C) and the Class-F system C-17/K-12 (resin 100, hardener 100, accelerator 1, plasticiser up to 20, silica up to 410 parts; pot life one day at 25 °C, 14 hours at 40 °C and 1.5 hours at 80 °C). Both call for blending at 60 °C under 0.5–5 mbar vacuum, pre-drying silica at 150 °C for 24 hours, and thorough degassing for high-voltage castings, warning that filler impurities and moisture can cause a strong exotherm, shrinkage and cracking. [2] [3]

Editorial calculation: at the C-17 system's stated mix of 100 : 100 : 1 : 10 : 400, silica is about 65% of the mix by weight (400 ÷ 611), which is why filler drying and wetting dominate process quality. [3]

It warns that the resin should not be stored long above 30 °C or exposed above 100 °C for more than 2 hours, and that the hardener is moisture-sensitive, so part-used containers must be closed at once. [1]

Give the supplier the part type and size (bushing, insulator, instrument transformer, coil), process (vacuum casting, APG, potting), mixing and dispensing equipment, mould and component temperatures, required cured properties and test standards, colour, UL or other recognition, packaging and storage conditions.

02

Set the gel and cure schedule and check cured properties

The data sheet gives gel times (10 g) of 13–15 minutes at 130 °C and 8–9 minutes at 150 °C. For small, thin castings it suggests gelling for 4–6 hours at 80 °C or 2 hours at 100 °C, then post-curing for 2 hours at 140 °C or 10 hours at 130 °C; for very large parts it recommends a step or linear ramp from the mould temperature to 140 °C over 16–24 hours, a 4-hour post-cure at 140 °C and slow cooling to 100 °C or below before demoulding, to limit internal stress. [1]

The data sheet's cured properties (typ.) include tensile strength of 11,000–12,500 psi, flexural strength of 18,000–19,000 psi, Shore D 92–96, glass transition temperature 110–120 °C, 24-hour water absorption 0.020–0.025%, thermal conductivity 0.7–0.8 W/m·K and dielectric strength of 420–480 V/mil at 3 mm. The system is UL-recognised with a thermal index of 200 °C. [1]

Cured and filled, ARC-16 (10 hours at 140 °C) lists breakdown strength of 17–21 kV/mm (IEC 60243), loss factor 2–3%, dielectric constant 4, volume resistivity above 10¹⁵ Ω·cm, tracking resistance above 600 V (IEC 60112), tensile strength 75–85 MPa, flexural strength 130–150 MPa and Tg 100–110 °C. C-17 (6 hours at 80 °C plus 10 hours at 130 °C) lists 18–22 kV/mm, loss factor 1.5–2.0%, dielectric constant 3.8–4.2, above 10¹⁵ Ω·cm, above 600 V tracking, thermal conductivity 0.8–0.9 W/m·K, expansion of 31–36 × 10⁻⁶ K⁻¹, flexural strength 125–135 MPa, Tg 90–100 °C and 0.1–0.2% water absorption over 10 days. Atul notes that more plasticiser lowers the glass transition temperature. [2] [3]

Editorial calculation: 420–480 V/mil equals about 16.5–18.9 kV/mm (1 V/mil ≈ 0.0394 kV/mm). Ask for Tg measured on parts cured with your own schedule. [1]

A Tianjin University review of large epoxy/Al₂O₃ insulators for gas-insulated switchgear (GIS) and lines (GIL) shows the size of the stresses a cure schedule must control. In a cited curing simulation of a tri-post insulator, stress at gelation (90 min) was low and concentrated near the insulator–sleeve bond, but by the end of curing (880 min) cure shrinkage and cooling raised the maximum first principal stress to 58.9 MPa; because the tensile strength of epoxy/Al₂O₃ is well below its compressive strength, the first principal stress is used against a maximum-tensile-stress criterion. Measured residual stresses it collects include tensile stress up to 13.2 MPa near the central cylinder and inserts of a 550 kV GIL tri-post insulator, 0.38 MPa and 0.12 MPa compressive at two checkpoints on a 252 kV GIS basin insulator, up to 5.8 MPa tensile near the flange of a 550 kV basin insulator, and up to 18.10 MPa tensile on a basin insulator where the pattern followed the pouring direction, attributed to alumina sedimentation, curing and cooling shrinkage. [4]

Editorial calculation: the cited simulation runs from gelation at 90 min to the end of curing at 880 min, 790 min or about 13 hours, shorter than the 16–24 hour ramp Huntsman suggests for very large castings; ask the supplier for a cure profile validated on a part of your size, not only gel times on a 10 g sample. [4] [1]

03

A reinforced 100:100 system and two silica-filled anhydride systems, plus potting compounds

The reviewed data sheets cover a reinforced 100:100 system for indoor and SF6-insulated medium- and high-voltage parts (Araldite CW 229 / Aradur HW 229-1) and two silica-filled anhydride systems for APG and vacuum casting of insulators and transformers (Lapox ARC-16/AH-118 and Class-F C-17/K-12). [1] [2] [3]

Listings include casting epoxy resins, epoxy potting compounds for transformers and electronics, insulating paints and release agents.

04

Epoxy resin, hardener, filler, accelerator, pigment and release agent

Filler dominates the mix by weight: in Atul's C-17 system silica is 400 of 611 parts, about 65% (editorial calculation), so its drying and wetting matter as much as the resin grade. [3]

05

Dielectric strength, resistivity, Tg, pot life, CTI and cure time for a casting system order

Tg and cure time depend on your own schedule rather than the data sheet: Huntsman gives 110–120 °C (typ.) for its recommended cures, so ask for Tg measured on parts cured the way you will cure them. [1]

06

Send the part, process and property data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Part: [type; size; voltage class]
Process: [vacuum casting; APG; potting; equipment]
Temperatures: [components; mould; cure schedule]
Properties: [dielectric; Tg; CTI; thermal class]
Recognition: [UL; other approvals]
Supply: [packaging; storage; shelf life]

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