The top of the plunger must be completely below the lip within the cartridge to ensure a tight seal. It is recommended to fill the plunger to a maximum about 2 cm below the lip so the plunger can tightly fit within the cartridge.  

Since the thermoplastic head only heats at the bottom of the cartridge, it is recommended to only fill the cartridge with enough granulate for your experiments to ensure even melting and minimization of residual material in the cartridge.