Hot water or oil flows through the tubes to heat the tool. The spacing of the tubes must be uniform to ensure uniform heating and cooling. This method gives much more control than the platen method. The whole tool is placed in an oven. While this saves machining, the cycle time is very slow because the system relies on convection to heat the tool, which can take a long time.
Integral heating is the most popular method.
Three main variations are used, each with their own advantages and weaknesses.