Printing mechanism comprising: - a frame; - a thermal printing head arranged on said frame; - a printing roller having an operating arrangement wherein it is rotatably coupled in a removable manner with said frame and faces said printing head to achieve in use a printing and advancement step wherein a printing support advances interposed between said roller and printing head; - engaging means rotatably coupled with said frame with the possibility of rotating around a rotation axis adopting at least one active position wherein it engages said printing roller with said frame in said operating arrangement and at least one inactive position wherein it leaves said printing roller free to be removed from said frame; - a motor arranged on said frame to rotate said printing roller at least in one rotation direction, said motor having a drive shaft with reversible rotation motion in a first rotation direction and in a second rotation direction opposite the first; - transmission means for transmitting the motion of said drive shaft at least in said first rotation direction to said printing roller to drag the printing support interposed between said roller and printing head; charaterised by - unidirectional joint means arranged for transmitting the motion of said drive shaft in said second rotation direction to said engaging means to rotate said engaging means from said active position to said inactive position and enable said printing roller to be removed, and in order not to transmit the motion of said drive shaft in said first rotation direction to said engaging means to maintain said printing roller in said operating arrangement. - a frame; - a thermal printing head arranged on said frame; - a printing roller having an operating arrangement wherein it is rotatably coupled in a removable manner with said frame and faces said printing head to achieve in use a printing and advancement step wherein a printing support advances interposed between said roller and printing head; - engaging means rotatably coupled with said frame with the possibility of rotating around a rotation axis adopting at least one active position wherein it engages said printing roller with said frame in said operating arrangement and at least one inactive position wherein it leaves said printing roller free to be removed from said frame; - a motor arranged on said frame to rotate said printing roller at least in one rotation direction, said motor having a drive shaft with reversible rotation motion in a first rotation direction and in a second rotation direction opposite the first; - transmission means for transmitting the motion of said drive shaft at least in said first rotation direction to said printing roller to drag the printing support interposed between said roller and printing head; charaterised by - unidirectional joint means arranged for transmitting the motion of said drive shaft in said second rotation direction to said engaging means to rotate said engaging means from said active position to said inactive position and enable said printing roller to be removed, and in order not to transmit the motion of said drive shaft in said first rotation direction to said engaging means to maintain said printing roller in said operating arrangement.