This article explains how to clean the Thermal Head on a TMT88 receipt printer

Epson recommends cleaning the thermal head periodically (generally every 3 months) to maintain receipt print quality. 

Turn off the printer, open the roll paper cover, and clean the thermal elements of the thermal head with a cotton swab moistened with an alcohol solvent (ethanol or IPA). 

• After printing, the thermal head can be very hot. Do not touch it and let it cool before cleaning it.
• Do not damage the thermal head by touching it with your fingers or any hard object.