Heated fuser roll

DETAILED DESCRIPTION It is therefore an object of the present invention to overcome many of the disadvantages of the hot roll fusers described in the prior art, and to provide a hot roll fuser (i) that is temperature-self regulating, (ii) will permit relatively simple control of the temperature of the roll in the critical area where fusing occurs, and (iii) will be able to limit the degree of the initial switch-on current surge.
It is a further object of this invention to provide a hot roll fuser which will provide a relatively even temperature gradient along its surfaces, avoid large temperature fluctuations, and eliminate center to edge temperature differentials.
It is a further object of the invention to avoid the use of high temperature heating elements for heating a fuser roll, thereby avoiding deterioration of the fuser roll.
It is still a further object of the invention to provide in addition to all of the objects listed above, a hot roll fuser that employs positive temperature coefficient heating elements in a manner which maintains constant electrical and thermal contact throughout differential expansion cycles.
The foregoing objects and others are accomplished in accordance with the present invention by providing a heated fuser roll for use in a fusing apparatus for fixing toner images to a support surface, the fuser roll comprising a circular sleeve member having at least one heating unit positioned within the sleeve member.
Each heating unit includes (i) a plurality of axially disposed thermally conductive members each formed of a chordal section having a curved surface positioned in contiguous relation to the inside curved surface of the sleeve member and a substantially flat surface, (ii) a plurality of wafer shaped heating elements formed of a semiconducting ceramic material having a positive temperature coefficient of resistivity and exhibiting a Curie temperature transition point at which the resistance of the material increases with increasing temperature, the elements being positioned along the flat surface of each of the conductive members, and (iii) resilient means positioned in contiguous relation to the heating elements for exerting a substantially continuous pressure on the heating elements to keep them in contact with the conductive members