Method and apparatus for preventing attraction of a work sheet to a toner image forming surface

A method for xerographic printing, comprises the steps of, holding a part of a work sheet at a position other than a leading end of the work sheet, before the leading end contacts with a toner image forming surface on which a toner image is formed by an electrostatic, charge of toner, holding both surfaces of the leading end of the work sheet and applying a frictional force to at least one of the surfaces of the leading end so that a tension is applied to the work sheet between the held part and the leading end, before the leading end contacts with the toner image forming surface, and making the work sheet contact with the toner image forming surface, and transferring the toner image from the toner image forming surface to the surface of the work sheet by generating an electric field for attracting the charged toner to the surface of the work sheet.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
The present invention relates to a method and apparatus for xerographic 
printing and more particularly, to a method and apparatus for preventing a 
work sheet from being attracted by a toner image forming surface from 
which a toner image is transferred to a surface of the work sheet. 
As disclosed in Japanese Patent Unexamined Publication No. 2-95668, a work 
sheet is plastically bent between a high-rigidity roller and a 
low-rigidity roller before the work sheet contacts a toner image forming 
surface, in a manner that a leading end of the work sheet proceeds away 
from the toner image forming surface. In the prior art xerographic 
printing apparatus, the prevention of the work sheet being attracted by 
the toner image forming surface after a toner image is transferred to the 
work sheet cannot be prevented. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and apparatus for 
preventing the work sheet from being attracted by the toner image forming 
surface after the toner image is transferred from the toner image forming 
surface to the surface of the work sheet. 
According to the present invention, a method for xerographic printing, 
comprises the steps of: 
holding a part of a work sheet at a position other than a leading end of 
the work sheet, before the leading end contacts with a toner image forming 
surface on which a toner image is formed by an electro static charge of 
toner, 
holding both surfaces of the leading end of the work sheet and applying a 
frictional force to at least one of the surfaces of the leading end so 
that a tension is applied to the work sheet between the held part and the 
leading end, before the leading end contacts with the toner image forming 
surface, and 
making the work sheet contact with the toner image forming surface, and 
transferring the toner image from the toner image forming surface to the 
surface of the work sheet by generating an electric field for attracting 
the charged toner to the surface of the work sheet. 
According to the present invention, an apparatus for xerographic printing, 
comprises: 
a toner image forming surface on which a toner image is formed by an 
electro static charge of toner, 
first holding means for holding a part of a work sheet at a position other 
than a leading end of the work sheet, before the leading end contacts with 
the toner image forming surface, 
second means for holding both surfaces of the leading end of the work sheet 
and applying a frictional force to at least one of the surfaces of the 
leading end so that a tension is applied to the work sheet between the 
held part and the leading end, before the leading end contacts with the 
toner image forming surface, and 
transferring means for making the work sheet contact with the toner image 
forming surface, and for transferring the toner image from the toner image 
forming surface to the surface of the work sheet by generating an electric 
field for attracting the charged toner to the surface of the work sheet. 
In the present invention, since a part of the work sheet is held at the 
position other than the leading end of the work sheet, both of the 
surfaces of the leading end of the work sheet are held and the frictional 
force is applied to at least one of the surfaces of the leading end so 
that the tension is applied to the work sheet between the held part and 
the leading end, an undesirable curvature of the leading end is corrected 
to be changed to a desirable substantially straight shape, before the 
leading end contacts with the toner image forming surface When the leading 
end has the substantially straight shape, the leading end is not attracted 
by the toner image forming surface after the toner image is transferred 
from the toner image forming surface to the surface of the work sheet. 
Therefore, the work sheet is prevented from being attracted by the toner 
image forming surface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In a copy machine as shown in FIG. 5, a work sheet 4 supplied from a hopper 
13 is transferred by two pairs of register rollers 8 onto a feed belt 2 on 
which the work sheet is electrically charged to be held by the feed belt 2 
through a coulombic force. Thereafter, the work sheet 4 is fed into a 
transferring portion between a toner image forming drum 1 on whose outer 
peripheral surface a toner image is formed by electro static charge of 
toner and an electrical field generating device 3. The work sheet 4 is 
electrically charged through the feed belt 2 by a corona charging of the 
electrical field generating device 3 so that the image of the charged 
toner on the toner image forming drum 1 is attracted onto the work sheet 4 
by the coulombic force. After the toner image is transferred onto the work 
sheet 4, the work sheet 4, held by the feed belt 2 through the coulombic 
forced, is transferred to a pair of toner image fixing rollers 6 so that 
the toner image is fixed to the work sheet 4. Thereafter, the work sheet 4 
is discharged from the copy machine by a pair of discharging rollers 10. A 
guide apparatus 5 is provided for guiding the work sheet, fed by the feed 
belt 2, to the fixing apparatus, with a cleaner 9 being provided for 
removing unnecessary toner from the toner image forming drum 1. A toner 
image forming apparatus 11 for forming the tone image on the toner image 
forming drum 1 and a tray 7 for enabling manual feeding of work sheets 4 
are also provided in the copying machine. 
The corona charging of the electrical field generating device 3 applies a 
voltage opposite to a voltage of the charged toner on the toner image 
forming drum 1 to the feed belt 2 and the charged feed belt 2 electrically 
polarizes the work sheet 4 so that the work sheet 4 is attracted to the 
feed belt 2 by the coulombic force. As shown in FIG. 6, when a clearance 
4b exists between the work sheet 4 and the feed belt 2 and the charged 
toner does not exist between the work sheet 4 and the toner image forming 
drum 1, there is a large possibility that a coulombic force between the 
work sheet 4 and the feed belt 2 is less than a coulombic force between 
the work sheet 4 and the toner image forming drum 1. When the work sheet 4 
has an undesirable shaped end 4a to form the clearance 4b between the work 
sheet 4 and the feed belt 2 toner image forming drum 1 more strongly 
attracts the end 4a of the work sheet 4 in comparison with the feed belt 
2. As shown in FIG. 7, after the end 4a of the work sheet 4 is attracted 
by the toner image forming drum 1 so as to be separated from the feed belt 
2, the clearance increases continuously so that the coulombic force 
between the work sheet 4 and the feed belt 2 decreases continuously and a 
difference between the coulombic force between the work sheet 4 and the 
feed belt 2 and the coulombic force between the work sheet 4 and the toner 
image forming drum 1 increases continuously. As shown in FIG. 8, the 
coulombic force between the work sheet 4 and the feed belt 2 at the end of 
the work sheet 4 is significantly small in comparison with the coulombic 
force therebetween at an intermediate position thereof. Therefore, the end 
of the work sheet 4 can be easily removed from the feed belt 2. 
In order to prevent the attraction of the work sheet 4 to the toner image 
forming drum 1, it is necessary to correct the undesirable shape of the 
end 4a of the work sheet 4 to be changed to a substantially straight 
shape, preferably without stopping the feed of the work sheet 4. A device 
for correcting the undesirable shape of the end 4a may be arranged between 
the pair of register rollers which correct an attitude or direction of the 
end 4a and the transferring portion between the toner image forming drum 1 
and the electrical field generating device 3. Alternatively a pair of 
register rollers 21 which correct an attitude or direction of the end 4a 
may be arranged between the device for correcting the undesirable shape of 
the end 4a and the transferring portion, as shown in FIG. 10. The 
direction of the end 4a is corrected by a wedge shaped clearance along a 
contact line formed between the register rollers 21 when a rotation of the 
pair of register rollers 21 is stopped, and then the register rollers 21 
rotate to feed the work sheet 4 toward the transferring portion 
Before the work sheet 4 reaches the transferring portion, the undesirable 
shape of the leading end 4a of the work sheet 4 fed by the pair of feed 
rollers 8b is ,corrected by the pair of frictional movement rollers or 
members 8a. A force for pressing the work sheet 4 between the feed rollers 
8b is generated by a spring 12, and a force for pressing the work sheet 4 
between the frictional movement rollers 8a (preferably, force/work sheet 
width is 500 g/254 mm,) is generated by a weight of the frictional 
movement rollers 8a and/or a spring (not shown). The force for pressing 
the work sheet 4 between the feed rollers 8b is preferably larger than the 
force for pressing the work sheet 4 between the frictional movement 
rollers 8a and/or a frictional coefficient of an outer peripheral surface 
of the feed rollers 8b is preferably larger than that of the frictional 
movement rollers 8a so that the feed speed of the work sheet 4 is 
determined by the feed rollers 8b. A feed speed Vb of the work sheet 4 by 
the feed rollers 8b is less than a circumferential speed or frictional 
movement speed Va thereof by the frictional movement rollers 8a so that 
the frictional movement rollers 8a slide on the leading end 4a of the work 
sheet 4 held securely by the feed rollers 8b to apply a frictional force 
to the leading end 4a to generate a tension of the work sheet 4 between 
the pair of the frictional movement rollers 8a and the pair of the feed 
rollers 8b when the leading end 4a reaches the pair of the frictional 
movement rollers 8a. Since the leading end 4a is pressed between the 
frictional movement rollers 8a and the frictional force is applied to the 
leading end 4a to generate the tension of the work sheet 4, the 
undesirable shape of the end 4a can be corrected to be changed to the 
substantially straight shape without stopping the feed of the work sheet 
4. 
When the circumferential speed or frictional movement speed Va of one of 
the frictional movement rollers 8a is larger than the feed speed Vb of the 
work sheet 4 by the feed rollers 8b, the undesirable shape of the end 4a 
can be corrected. In that case, the circumferential speed of another one 
of the frictional movement rollers 8a is make substantially equal to the 
feed speed Vb of the work sheet 4 by the feed rollers 8b. 
A distance between the pair of frictional movement rollers 8a and the pair 
of feed rollers 8b is less than a length of the work sheet 4 so that the 
part of the work sheet 4 is held by the pair of feed rollers 8b when the 
undesirable shape of the end 4a is corrected by the pair of frictional 
movement rollers 8a. 
As shown in FIG. 4, when the sheet feed speed Vb by the pair of sheet feed 
rollers 8b is 254 mm/sec, a ratio of the frictional movement speed or 
sheet urging speed Va of the pair of frictional movement rollers 8a to the 
sheet feed speed Vb by the, pair of sheet feed rollers 8b is greater than 
2, and, with the work sheet 4 having a thickness of 90 .mu.m, the 
probability of undesirable attraction of the work sheet 4 to the toner 
image forming drum 1 is substantially zero. 
As shown in FIGS. 2 and 3, if the frictional movement rollers 8a are 
separated from each other not to apply the friction to the work sheet 4 
after a leading end detector 20 detects the leading end 4a which has 
passed the pair of frictional movement rollers 8a, the friction is applied 
only to the leading end 4a and a frictional damage of a part other than 
the leading end 4a of the work sheet 4 is prevented. In order to achieve 
the same effect, the circumferential speed Va of the frictional movement 
rollers 8a may be made equal to the sheet feed speed Vb by the pair of 
sheet feed rollers 8b after the leading end detector 20 detects the 
leading end 4a. 
The outer peripheral surfaces of the pair of frictional movement rollers 8a 
for applying the friction to the work sheet 4 is preferably made of a 
stainless steel. 
As shown in FIG. 9, the frictional movement rollers 8a may be replaced by 
frictional movement pads 8a including plane surfaces for applying the 
friction to the work sheet 4. The feed speed Vb of the work sheet 4 by the 
feed rollers 8b is smaller than the frictional movement speed Va thereof 
by the frictional movement pads 8a so that the frictional movement pads 8a 
slide on the leading end 4a of the work sheet 4 held securely by the feed 
rollers 8b to apply the frictional force to the leading end 4a to generate 
the tension of the work sheet 4 between the pair of the frictional 
movement pads 8a and the pair of the feed rollers 8b when the leading end 
4a reaches the pair of the frictional movement pads or members 8a'. Since 
the leading end 4a is pressed between the frictional movement pads 8a' and 
the frictional force is applied to the leading end 4a to generate the 
tension of the work sheet 4, the undesirable shape of the end 4a can be 
corrected to be changed to the substantially straight shape without 
stopping the feed of the work sheet 4. When the movement speed Va of one 
of the frictional movement pads 8a' is greater than the feed speed Vb of 
the work sheet 4 by the feed rollers 8b, the undesirable shape of the end 
4a can be also corrected. In that case, the speed of another one of the 
frictional movement pads 8a' is made substantially equal to the feed speed 
Vb of the work sheet 4 by the feed rollers 8b.