manufacturing and testing of cast resin transformer
http://www.transformersource.com/products/product_epoxy.html
"Epoxy Encapsulated transformers are usually a mixture of silica sand resin forming a solid mass completely encloses and protects the core and coil, also significantly reduces audible noise. The Enclosure is constructed from heavy gauge steel."
http://www.powertransformer.us/epoxyencapsulated.htm
"Epoxy encapsulated transformers are designed to perform in extreme environmental conditions"
http://www.regionalmfg.com/epoxy-encapsulated-transformers.html
"Special electrical grade epoxy and silica in heavy gauge steel enclosures facilitates rapid heat tranfer."
http://www.marcustransformer.com/uploads/marcus_product.pdf1/Type%20RES0001.pdf