The maintenance of the preheaters can for some types of thermoplastic play an essential role for both the melting capacity and lifetime of the preheaters.
Some thermoplastic materials can have components that settle at the bottom of the preheater. Besides decreasing the melting capacity, the settlement in a preheater with direct heating can cause the destruction of the preheater.
When you are working with 2 or more preheaters:
When filling the machine, take half of the material from one of your preheaters and refill the preheaters right away for quicker melting.
Next time you are filling the machine take half of the material from the other preheater and follow the same steps. As the preheaters are only emptied by "half" when filling the machine, the added thermoplastic will melt faster.