The mechanical strength member of polymer insulator is a fiberglass rod. The resin system can be either polyester or epoxy and the rod either cast or pultruded. Rod is the center of insulator.

Insulator rod

The rod, normally referred to as the core of the insulator, consists of between 70 and 75% by weight of axially aligned glass fibers bonded by an organic resin.
Today’s core is pultruded in various diameters with electrical grade E-type glass fibers and polyester resin. Two critical process parameters in the pultrusion of fiberglass rod for insulators are pulling speed and temperature of the forming die. An axial crack develops when the outside of the rod cures more quickly than the center of the rod. This occurs when either the die temperature is too high for the pulling speed or outside of the rod sets, shrinkage during curing of the bulk of the rod and produces an axial crack in the center of the rod. Bonding of glass fibers to the polyester resin is affected by process parameters as well. In pulling glass fiber, the fiber is sized or treated chemically for protection against mechanical damage during handling. Optimum bonding requires compatibility of the chemical systems; otherwise, the rod may exhibit porosity and wick moisture by capillary.

The rod may still be acceptable for mechanical applications but not for polymer insulators.

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