Patent ID: 7675533

Claim:
A thermal transfer printer for film, comprising: conveyance means for conveying a plastic film along a predetermined path; an ink ribbon moving mechanism which includes holding means for holding a plurality of ink ribbons, moves an ink ribbon arbitrarily selected from the plurality of ink ribbons to a print position arranged on the predetermined path; a print head which prints on the plastic film by heating the ink ribbon moved to the print position; a platen roller which supports the ink ribbon and the plastic film from an opposite side of the print head; a displacement preventing mechanism which brings the platen roller and the plastic film into close contact with each other such that the platen roller and the plastic film are not displaced from each other; wherein a pinch roller which presses the plastic film against the platen roller is provided as the displacement preventing mechanism; and a contact angle θ between the plastic film and the platen roller, backward tension T 1 generated in the plastic film in an opposite direction of a conveyance direction of the conveyance means, forward tension T 2 generated in the plastic film in the conveyance direction of the conveyance means, average tension T (=(T 1 +T 2 )/2) of the background tension T 1 and the forward tension T 2 , a difference in tension ΔT (=|T 1 −T 2 |/T) between the backward tension T 1 and the forward tension T 2 , force P with which the pinch roller presses the plastic film against the platen roller, and a dynamic friction coefficient μ between the platen roller and the plastic film satisfy the following formula (1): |2μT sin(θ/2)|+μP>Tx(1+ΔT) (1) (where θ=0 deg. to 180 deg.).