October 26, 2026
Traction auxiliary transformers, reactors, and electrical equipment for signaling and measurement installed in potentially explosive atmospheres on rail vehicles place demanding requirements on insulating encapsulation materials. The material must maintain electrical insulation integrity under long-term thermal cycling, comply with railway fire safety standards, and effectively dissipate heat generated by windings or electronic modules within constrained spaces. As a pre-filled, solvent-free epoxy casting system, Araldite® CW 1302 Resin / Aradur® HY 1300 Hardener offers a property profile closely aligned with these demands.
The system was shortlisted primarily on the basis of several key properties verified by standard test methods.
Thermally, the glass transition temperature by DSC is 74–76 °C, the system temperature class is UL 1446 Class 180 (H), and the temperature index is 181 °C (IEC 60216). This indicates that castings possess a certified long-term thermal ageing margin within the design temperature rise of a Class H insulation system. For traction auxiliary transformers whose winding temperatures may continuously approach or exceed 120–130 °C, this classification provides a fundamental safety boundary. The thermal conductivity of 0.87–0.89 W/mK sits at the upper-middle range for filled epoxy systems and assists in conducting heat from the winding interior to surface dissipation paths, retarding the development of local hot spots.
In fire safety, the system holds the railway-specific certification EN 45545-2 R22 HL2 / R23 HL3, achieves UL 94 V0 at 3.0 mm thickness, and has an oxygen index (LOI) of 36.1%. For dry-type transformers mounted beneath passenger compartments or within car bodies, R22 and R23 correspond to different material usage and location hazard levels, while HL2/HL3 sets acceptable limits for smoke density and smoke toxicity. These data indicate that the material is assessed not only for ignitability but also for secondary fire hazards under fire conditions.
Electrically and against moisture, the volume resistivity is 1×10¹⁴ Ω·cm at 25 °C, CTI exceeds 600, and water absorption after 1 day at 23 °C is only 0.03–0.05%. Low water absorption is particularly critical for railway equipment exposed to outdoor or fluctuating temperature-humidity environments: moisture uptake simultaneously lowers volume resistivity and increases dielectric loss, and the low absorption of this system underpins long-term insulation reliability. Furthermore, the mixed system has a gel time of 50–75 minutes at 40 °C and a pot life (time to 15,000 mPas) of 36–38 minutes, providing a reasonable process window for vacuum casting of medium-sized windings without excessive open time that could lead to uncontrolled filler settling.
In production, the system is processed by vacuum casting. The following parameters are set in accordance with the TDS processing guidelines and general engineering practice for vacuum casting; they do not introduce requirements beyond the TDS but operationalize its logic.
Resin preheating and homogenization. Araldite CW 1302 contains inorganic fillers that inevitably settle during storage. Before use, the resin is heated in its original container to 40–50 °C (e.g., overnight) to reduce viscosity to a level suitable for stirring and transfer, and to redisperse settled filler. Preheating should not exceed 50 °C to avoid adverse effects on storage stability. Stirring must span the full depth of the container to ensure bottom filler is re-entrained into the liquid phase. In equipment storage tanks, intermittent gentle agitation is maintained during casting to prevent re-separation of filler, which would distort the metering ratio.
Mixing and degassing. Metering is by weight at 100 : 11 (resin : hardener) or by volume at 100 : 19. After thorough mixing, brief degassing is carried out under 5–10 mbar vacuum. The purpose is not only to remove macro-bubbles introduced by mechanical mixing but also to eliminate micro-voids entrained at filler particle surfaces and at the resin–hardener interface. For dry-type transformer winding encapsulation, residual voids in regions of concentrated electric field act as initiation sites for partial discharge; the low bubble content after degassing provides a material-level foundation for the long-term partial-discharge resistance of the winding insulation.
Casting and curing. The degassed mix is injected into a preheated mold/winding cavity under vacuum or pressure assistance. The TDS-recommended cure schedule is 24 hours at room temperature (approximately 25 °C) followed by 6 hours at 60 °C. The initial room-temperature gelation stage allows the network to form under relatively low internal stress, while the subsequent 60 °C post-cure drives crosslinking closer to completion, yielding low shrinkage and a good surface finish. The minimum cure condition is 48 hours at 25 °C, applicable to shops without heating capability or to large thin-walled parts highly sensitive to warpage. Adequacy of cure should be released on the basis of glass transition temperature measurement on the actual casting rather than nominal adherence to a time–temperature profile alone, because different gelation and cure histories lead to different crosslink densities and final Tg values.
In potentially explosive atmospheres, the encapsulation layer of electrical equipment serves to physically isolate circuits that may produce sparks or elevated temperatures from the explosive atmosphere. The system's suitability in this scenario arises from the intersection of three properties. First, the cured volume resistivity and dielectric strength (26–28 kV/mm, 2 mm plate) provide an adequate insulating barrier through the encapsulation. Second, low water absorption makes the encapsulated part resistant to declines in insulation resistance or surface leakage caused by moisture uptake over long-term service. Third, the UL 94 V0 flammability rating and high oxygen index reduce the risk of the encapsulation itself acting as an ignition source or flame propagation path. The TDS explicitly lists "electrical devices working in potentially explosive environment" as a core application, and in industrial practice the system is frequently evaluated as an epoxy candidate for explosion-proof encapsulation (Ex m) designs.
The value of Araldite CW 1302 / Aradur HY 1300 in railway dry-type transformer and explosion-proof electrical encapsulation lies not in an extreme value of any single property, but in the balance it strikes among thermal class, fire compliance, electrical insulation, low moisture absorption, and a workable process window. UL 1446 Class 180 (H) and EN 45545-2 HL2/HL3 establish third-party-verified performance baselines from the perspectives of long-term thermal ageing and railway fire safety, respectively. The vacuum casting process translates the system's low-void potential and filler dispersion stability into practical partial-discharge resistance of winding insulation. Low water absorption and high CTI provide material-level redundancy for long-term insulation reliability under humid, polluted, or explosive atmospheres. For small-to-medium dry-type transformers and explosion-proof electrical modules that must simultaneously satisfy thermal management, fire safety, and electrical isolation within constrained volumes, this system offers an encapsulation choice supported by standard-based data.
