A printing apparatus comprising a printing plate drum (2) and a pressing drum (13) that is disposed so as to face said printing plate drum, wherein cardboard sheets are passed between said printing plate drum (2) and pressing drum (13) which rotate in mutually opposite directions, thus causing specified printing to be performed on said sheets, said printing apparatus further comprising:
- an ink transfer roll (4) which rotates in contact with a printing plate of said printing plate roll (2) at a time of printing;
- an adjustment means (5) which makes contact with said ink transfer roll (4) during printing and adjusts an amount of ink by wringing;
- a pair of regulating members (44, 44) which are disposed at both ends of said ink transfer roll and adjustment means with respect to an axial direction thereof and are used to demarcate an ink collecting area between said ink transfer roll (4) and said adjustment means (5);
- an ink supply source (45) which is disposed near an upper end of said ink collecting area, a specified amount of ink being stored in said ink supply source (45);
- a first tubular body (48) and second tubular body (49), opening part of one of said first tubular body (48) and second tubular body (49) is inserted into said ink supply source, and an opening part of another of said first tubular body (48) and second tubular body (49) is caused to face the ink collecting area, an ink feeding pumps (47, 50) being respectively connected to said first tubular body (48) and second tubular body (49);
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- an ink viscosity measuring instrument installed in said first tubular body (48) so as to measure a viscosity variation of ink that is supplied to said ink collecting area that is demarcated between said ink transfer roll (4) and adjustment means (5), said ink viscosity measuring instrument being comprised of:
- a rotating body (33) which is disposed inside said first tubular body (48) so that said rotating body (33) can freely rotate;
- an electrical rotation-imparting means (22) which is disposed outside said first tubular body (48), magnetically coupled to said rotating body (33), and imparts rotation to said rotating body (33);
- a load current value detection means (23) which detects changes in a load current value that accompany changes in a viscosity of ink that contacts said rotating body (33) when rotation is imparted to said rotating body (33) by passing an electric current through said rotation-imparting means (22);
- a memory means (151) which stores said load current values that correspond to respective changes in a viscosity value of said ink;
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- a calculating means (152) which compares respective load current values stored in said memory means (151) with said load current value detected by said load current value detection means (23) and calculates an ink viscosity value at a current point in time.