Electro-photographic method and device with thermal fixing with reduced noise

An electro-photographic printing device has a development location where toner is transferred onto a drum in proportion to the amount of light charges on the drum and a transfer location where toner is transferred from the drum to a sheet of paper. A heater unit for fixing the toner by heating it to a predetermined temperature has a heater and a blower for heat removal. Responsive to a call for printing, data to be printed are stored in a memory. The heater is energized until a set temperature is obtained. Printing is initiated as soon as the set temperature is obtained. The blower is stopped after heating has been discontinued, after printing has been completed, until there is a new call.

BACKGROUND OF THE INVENTION 
The invention relates to methods and devices for electro-photographic 
writing. 
In widely used facsimile apparatuses, a light beam, typically delivered by 
a laser diode, is modulated by an input video signal and is focussed on a 
rotating photosensitive drum. The light beam generates, at each point of 
the drum, an electrostatic charge which represents the amount of received 
light. Dry toner in quantity proportioned to the deposited charges is 
transferred on the photosensitive drum. Then the dry toner is transferred 
on a paper sheet by an electrical field. The toner is fixed on the paper 
by pressing and heating it to a sufficient temperature. 
The invention is particularly suitable for use in a facsimile apparatus for 
printing on regular paper. However it is also applicable to other systems 
and for instance to printers for connection with a personal or home 
computer. 
In most electro-photographic devices using laser writing and thermal 
fixing, the thermal fixing means are continuously maintained at the 
required temperature for being immediately available at every instant. 
Heat generated by the fixing means should be exhausted to atmosphere for 
avoiding bringing other elements of the printer to an excessive 
temperature. For that purpose a blower is used and operates continuously, 
in stand-by conditions as well as in operating conditions. The sound level 
of the blower, which frequently exceeds 43 dB, is acceptable when the 
printer is in a noisy environment. On the other hand, it is not acceptable 
any longer for a desk top low-cost facsimile apparatus. Due to that sound 
level, facsimile apparatuses for that purpose have used thermal paper 
printers, in spite of the drawbacks of that substrate (high cost and 
difficulty of long term storage). A similar problem exists for a printer 
located close to a personal or home micro-computer. 
It has also been proposed (U.S. Pat. No. 5,063,459 to Nakatani et al) to 
maintain the fixing device at a reduced temperature when a facsimile 
apparatus is in stand-by conditions, for reducing power consumption. 
However, that does not reduce the noise level associated with continuous 
operation of a heat removal blower. 
SUMMARY OF THE INVENTION 
It is an object of the invention to reduce the noise level of an 
electro-photographic apparatus with thermal fixing when the latter is not 
in operation. For that purpose, there is provided a method comprising the 
steps of storing data to be printed following receipt of a call to receive 
data, initiating heating of the thermal fixing means if the temperature of 
the latter is lower than a predetermined required temperature, and 
beginning printing as soon as the required temperature is reached. 
There is also provided a printing device apt to implement the above-defined 
method. The device comprises: 
a light source delivering a light beam; a photosensitive rotative drum; 
means for modulating said light beam and causing said light beam to scan 
said photosensitive drum axially; 
means for transferring electrical charges onto said drum in proportion to 
an amount of light at each point of the drum; 
development means for transferring toner onto said drum in proportion to 
the amount of light charges on said drum; 
means for transferring said toner from said drum to a paper sheet; 
means for fixing said toner by pressing said paper and heating it to a 
predetermined temperature, said fixing means having heating means and a 
blower for heat removal; and 
means responsive to a printing request for storing data to be printed in a 
memory, energizing said heating means until said temperature of said 
fixing means have been obtained, initiating operation of said light beam 
and rotation of said drum as soon as said temperature is obtained and 
stopping said blower after said heating has been discontinued.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The printer of a facsimile apparatus whose general construction is 
schematized in FIG. 1 comprises a light source 10 (typically a laser 
diode) which delivers a beam which is focussed, then received by a polygon 
mirror 12 for line scanning and directed by a folding mirror 14 onto a 
photosensitive drum 16 which is rotated, typically continously rather than 
step-by-step, by an electric motor 18. The drum, typically having a 
diameter of some centimeters, is covered with a layer of photoconductive 
material (organic material or selenium). Upon radiation, each generatrix 
of the drum 16 in turn confronts a lamp 20 for removing residual negative 
charges, then a Corona generator 22 for uniformly depositing negative 
electrostatic charges on the drum 16 and then a location where exposure to 
the light beam takes place. The beam removes charges from the surface of 
the drum at each point in proportion with the level of radiation received 
by that point. The invention would also be applicable to a printer which 
makes use of positive charges and an appropriate dry toner. 
Each generatrix of the drum in turn then passes through a development 
location where the latent electrostatic image on the photosensitive drum 
is transformed into a visible image by transfer of toner. The toner has 
typically two components. It is carried by a magnetic roller which rotates 
within a sleeve 24. The toner adheres to the drum 16 on the zones which 
were exposed to the light beam, with a density which is in proportion to 
the degree of exposure. At a transfer location where a generator 26 
maintains a Corona discharge, the dry toner is transferred onto a paper 
sheet 28 moved by a paper feeding device 30. The paper sheet, from which 
the charges were possibly removed at a location 32, passes through a 
fixing device. The dry toner is fixed by pressing and heating it. For that 
purpose, the device comprises a pressing roller 34 driven by a motor 36 
(or more frequently by a linkage from the main motor 18) and a heating or 
thermal fixing roller 38. A central heater 40, typically a halogen lamp, 
maintains the heating roller at a regulated temperature, about 500.degree. 
C. with the usual toners. 
A sensor (not shown) detects the temperature of drum 38 and switches on the 
lamp as soon as the temperature is under a predetermined value 
.THETA..sub.0. The lamp is switched off when another temperature 
.THETA..sub.1, higher than .THETA..sub.0, is exceeded. 
For avoiding that heat from the drum 38 and lamp 40 affects other 
components of the facsimile apparatus, means should be provided for 
removing heat. On the present day facsimile apparatuses, they consist of 
an electric blower (not shown on FIG. 1) which continuously operates when 
the facsimile apparatus is energized, in stand-by condition as well as 
when printing or transmitting. The sound level of such a blower is 
typically between 43 and 48 dB and includes relatively high frequencies. 
It is consequently a nuisance in an office room. 
According to the invention, the blower is only energized during printing 
periods, which are particularly unfrequent for auxiliary office telefax 
devices. 
For that purpose, heating of the thermal fixing means is stopped except for 
printing. Consequently, the blower may also be stopped. The blower may be 
switched off, after printing and heating have ended, either after a 
predetermined period, or as soon as a thermal probe or sensor indicates 
that the temperature in the vicinity of the drum has decreased under a 
predetermined value .THETA..sub.2, lower than .THETA..sub.0. 
As a counterpart, that type of operation requires that, upon reception of a 
message, data be stored until energization of the heating means has 
increased the temperature of the thermal fixing roller up to the set 
temperature .THETA..sub.0. The thermal inertia of the drum of a low cost 
facsimile apparatus is low and the temperature of the drum may be 
increased from ambient temperature to .THETA..sub.0 in a time period which 
may vary from some tens of seconds to about one minute, when a halogen 
lamp of high power (several hundreds of Watts) is used. Heating may be 
initiated from the start of the connection protocole, which requires 4 to 
10 seconds for a facsimile apparatus of group 3. A static or dynamic RAM 
having a capacity of some hundreds of kbytes is consequently sufficient. 
More precisely, for a facsimile apparatus designed for printing a document 
received at a rate of 14.4 kbytes/s, with a maximum duration of 
pre-heating of 60 seconds, a minimum storage capacity of 100 k-bytes is 
necessary. Most facsimile apparatuses available on the market have a 
minimum size of 256 k-bytes. As a consequence, the need for a buffer 
memory is not truly of trouble. 
The method may be as schematized on FIG. 2, which may be easily implemented 
with software or hardware. It only requires adding some components to a 
conventional facsimile printer. Upon detection of an input call at 42, the 
electric motor of the blower is automatically energized, immediately or 
after a delay, by closure of a relay 44. At the same time, pre-heating of 
the thermal fixing roller begins. Pre-heating is controlled by a loop 
having an input unit 46 which is enabled by call detection, a test 48 for 
determining whether the temperature of the drum is lower than 
.THETA..sub.0 and forcing the temperature to increase to .THETA..sub.0 if 
necessary. The blower motor may as well be controlled by a temperature 
probe. 
As soon as the connection protocole has ended, the data are received and 
stored at 50. Printing 52 begins when test 48 indicates that temperature 
.THETA..sub.0 has been reached. Printing then occurs conventionally, from 
the stored data. 
When all data constituting a message have been received, a "writing 
finished" sensor disables input control 46, immediately or after a delay 
of some minutes. The "writing finished" detection signal also stops blower 
44, possibly after a delay of some minutes, as indicated at 56. Delaying 
may be replaced with a stop command delivered when a probe indicates that 
the temperature of the drum has decreased to a given value. The 
temperature of the fixing means may be maintained for a predetermined time 
after end of writing for immediate response if a new call occurs before 
the end of that predetermined time. 
As indicated above, the invention is also applicable to printers suitable 
for use with micro-computers or home computers, which operate 
unfrequently. 
In all cases, the invention decreases the length of time during which noise 
is emitted. It also increases reliability of the fixing means and the life 
of the heater (halogen lamp) since the heater is at the operating 
temperature only when necessary, rather than continuously. The invention 
only requires adding some components and possibly adding some instructions 
to the operating program of the printer.