The cartridge is made of stainless steel. To clean the cartridge, melt and extrude the remaining polymer in the cartridge, remove the plunger and the cartridge from the Printhead casing after it is cooled. Then fill or soak the cartridge in ethanol, acetone, chloroform or another compatible solvent, to dissolve the residual polymer. The solvent necessary is dependent on the polymer utilized within the cartridge. Perform this cleaning in a chemical hood and using the necessary PPE equipment. The nozzle can be cleaned in a similar way.
Full cleaning instructions can be found here.