Rubber is used to produce insulated cables. It is produced by heating Sap in a Furnace (an Industrial Blast Furnace or Smoker will not work).
A more efficient way to produce Rubber is with an Extractor; it will produce three times the amount of Rubber per Sap. The most efficient method is to use the Industrial Centrifuge; four Sap makes 14 Rubber.
Used as an ingredient in the Crafting Table to produce: