Pressboard, bakelite or Permawood: which insulation material to use where
A plain comparison of the three main solid insulation materials used inside transformers and the parts each is best for.
Transformer makers use three main solid insulation materials: pressboard, phenolic laminate (bakelite) and densified wood (Permawood). Each has a place. Choosing the wrong one costs money or, worse, causes failures.
| Pressboard | Bakelite (phenolic) | Permawood | |
|---|---|---|---|
| Made from | Kraft pulp | Paper or fabric + phenolic resin | Wood veneer + resin |
| Strength | Moderate | High | Very high in compression |
| Machinability | Cut and punch | Drill, tap, mill | Drill, tap, mill |
| Oil impregnation | Excellent | Limited | Good after drying |
| Cost | Lowest | Medium | Highest |
| Best for | Strips, spacers, cylinders, angle rings | Terminal boards, support strips, wedges | Pressure rings, clamping blocks, cleats |
Use pressboard for
Anything inside the winding that needs to be oil-impregnated: strips, dovetail spacers, cylinders, angle rings, layer insulation. It is the cheapest and it dries and impregnates with the winding.
Use bakelite for
Parts that take a screw, a bolt or a mechanical load but do not need deep oil impregnation: terminal boards, tap-changer supports, support strips and wedges. F4 grade is the usual choice for electrical work.
Use Permawood for
The heavy-duty parts: pressure rings, clamping blocks, lead cleats and anything that must hold the winding tight under short-circuit force. It is the most expensive, so use it only where strength is needed.
Hardwood
Plain seasoned hardwood still has a place for simple rods, pegs and spacers in distribution transformers, where cost matters more than strength.
If you are unsure, send the drawing and tell us the duty of the part. We will suggest the material.