are closely related to producing the desired performance the stiffness of the recording medium determines in large measure the useable range of values for the radius of curvature of the recording medium at the bottom of the 34 v 34 the reason for this is that the radius of curvature must be chosen small enough to ensure that the stress within the recording medium is sufficient to stabilize its shape and to provide firm contact with the platen 2 while at the same time the radius of curvature must not be so small as to result in creasing or permanent deformation of the recording medium from the continual bending beneath the pinch rollers 12 and 14 for example using a relatively stiff material like mylar instead of paper generally requires a larger radius of curvature to avoid permanent deformation having chosen an appropriate radius of curvature for the recording medium at the bottom of the 34 v 34 it is apparent from the geometry that the distance r and the angle u03b2 can be varied over a wide range as the angle u03b2 is increased so is the required distance r and vice versa for the chosen radius of curvature of the recording medium in this preferred embodiment r is typically of the order of 0 5 mm with u03b2 ranging from approximately 20 u00b0 to 30 u00b0 for commercially available recording media such as dry copying paper the radius of the drive roller 4 is not critical and can be chosen for convenience to correspond to available materials fabrication methods and desired gear ratio a typical radius for the drive roller 4 ranges from approximately.