Printing apparatus and method

A printing apparatus and method can maintain an appropriate spacing between a print head and the overall printing surface of printing media of a wide range of thicknesses, with a platen mechanism that allows the printing medium to move in parallel displacement, namely both in a direction downstream along the advance of the printing medium and in a direction away from the print head.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a printing apparatus and method for 
performing printing with an optimum spacing maintained between the surface 
of a printing medium and a print head. 
2. Description of the Related Art 
Printing apparatuses print information such as characters and images on a 
printing medium such as a sheet of paper, fabric or a plastic sheet. An 
ink jet printing method, as a nonimpact type printing method, projects ink 
droplets onto the surface of a printing medium through ink nozzles, 
permitting high-density and high-speed printing. For these advantages, the 
ink jet printing method is widely used in printing apparatus of a 
diversity of pieces of office equipment including printers, photocopying 
machines, facsimile machines, and wordprocessors. 
The-printing apparatus that uses such a ink-jet printing method needs to 
keep constant the spacing between the printing surface of a printing 
medium and an ink jet print hand to form an optimum image on the printing 
surface. With an ink-jet head held to the printing surface of a printing 
medium with too narrow spacing therebetween, the ink-jet head may contact 
the printing surface of the medium, smearing the printing surface, or the 
head itself is possibly damaged. When the spacing between the ink-jet 
print head and the printing surface of the medium is too wide, the image 
quality may be degraded. 
The printing media used in the ink-jet printing apparatuses include not 
only particular types of paper, but a diversity of media including 
envelopes, postcards, overhead projector sheets, and fabrics. As the types 
of printing media vary, their thicknesses vary accordingly, and the 
spacings between the printing surfaces of the printing media and the 
ink-jet print head also vary. As a result, the printing surface of the 
medium is smeared, the ink-jet head is damaged, and the resulting image 
quality suffers degradation. 
In an attempt to preclude these problems, the ink-jet print head is shifted 
in accordance with the thickness of the printing medium using a lever to 
maintain an appropriate spacing between the printing surface of the 
printing medium and the ink-jet print head. The handling of the lever is a 
clumsy and delicate operation, and is not a satisfactory solution. 
One method of keeping constant the spacing between the printing surface of 
a medium and an ink-jet head regardless of the thickness of the medium has 
been proposed in Japanese Unexamined Patent Publication No. 7-81047. 
According to this disclosure, a pair of driven rollers for advancing a 
printing medium is disposed on the side of an ink-jet print head that 
faces a platen with the printing medium being interposed between the 
platen and the ink-jet print head. The platen is urged toward the rollers 
so that the position of the printing surface of the printing medium 
remains fixed relative to the ink-jet print head regardless of the 
thickness of the medium. 
FIG. 14 shows the construction of the major portion of such a known 
printing apparatus. As shown, an ink-jet print head 103 faces a platen 102 
with a printing medium 101 interposed therebetween. The ink-jet print head 
103 is mounted on a carriage 105 that slidably reciprocates along a guide 
rail 104, and thus prints a desired image onto the printing surface of the 
printing medium 101. A pair of transport rollers 107, 108 are arranged 
upstream of and downstream of the ink-jet print head 103 along the advance 
of the printing medium 101 that is moved from left to right in FIG. 14. 
The transport rollers 107, 108 are pressed into contact with the printing 
surface 106 of the printing medium 101 in a manner that the transport 
rollers 107, 108 are rotatable in their driving direction. A pair of pinch 
rollers 109, 110 that are rotatable supported are pressed into contact 
with the printing medium 101 against the transport rollers 107, 108. The 
pinch rollers 109, 110 along with the platen 102 are pressed toward the 
ink-jet print head 103 with the urging of pressure springs 111. As the 
transport rollers 107, 108 rotate to advance the printing medium 101, the 
pinch rollers 109, 110 run freely along therewith. The pinch rollers 109, 
110 along with the platen 102 are displaced in the direction toward the 
transport rollers 107, 108 to keep constant the spacing between the 
ink-jet print head 103 and the printing surface 106 of the printing medium 
101. 
U.S. Pat. No. 4,620,807 discloses a wire printer 5 for printing on 
envelopes 12 of a wide range of thicknesses. In the disclosure, a platen 
10 is displaced vertically downwardly by a link mechanism 36, 37 and 40 
and a slide mechanism 51, 58 in accordance with the thickness of an 
envelope that is transported in a horizontal direction. 
In the known ink-jet printing apparatus shown in FIG. 14, there are times 
when the printing medium 101 is caught between the upstream transport 
roller 107 and the pinch roller 109 with the forward edge of the printing 
medium 101 yet to reach the nip between the downstream transport roller 
108 and the pinch roller 110. There are also times when the printing 
medium 101 is caught between the downstream transport roller 108 and the 
pinch roller 110 with the back edge of the printing medium 101 already 
parted from the nip between the upstream transport roller 107 and the 
pinch roller 109. In such conditions, depending on its thickness, the 
printing medium 101 is slightly tilted with respect to the ink-jet print 
head 103, and thus the spacing between the ink-jet print head 103 and the 
printing surface 106 cannot be kept constant. 
The downstream transport roller 108 has on its circumference a 
spur-gear-like thin sheet with serrations to minimize contact with freshly 
printed ink on the printing medium 106. The pinch roller 110 displaces 
against the urging of the pressure spring 111, and therefore, the 
downstream transport roller 108 results in no sufficient friction with the 
printing surface 106. In the region where the printing medium 101 is 
advanced only by the nip between the downstream transport roller 108 and 
the pinch roller 110, the advance accuracy of the printing medium 101 may 
be degraded. 
In the recording apparatus disclosed in U.S. Pat. No. 4,620,807, when a 
recording medium, a thin one in particular, comes into a recording area, 
the downstream side of the recording medium fails to shift in a vertical 
direction and the platen is subject to a tilt, because of looseness in 
mounts of links in the link mechanism. Furthermore, the use of the link 
mechanism makes area inevitably bulky the structure of the recording area. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a printing apparatus 
and method that permit excellent quality printing by maintaining an 
appropriate spacing between the entire printing surface of a printing 
medium and a print head and by maintaining the advance accuracy of the 
printing medium. 
It is another object of the present invention to provide a printing 
apparatus and method that maintain an appropriate spacing between the 
printing surface and the print head by displacing the entire printing 
surface of the printing medium in parallel displacement, namely downstream 
along the advance of the printing medium and also in the direction away 
from the print head in accordance with the thickness of the printing 
medium with parallelism kept to the print head. 
To achieve the above objects, the printing apparatus for printing on the 
printing surface of a printing medium placed on a printing position with a 
print head, according to the present invention, comprises a platen mounted 
in a position facing the print head, for supporting the printing medium 
from the back surface of the printing medium opposite the printing 
surface, a transport mechanism for advancing the printing medium to the 
position where the platen faces the print head and for delivering the 
printing medium out of the position where the platen faces the print head, 
a medium positioning member disposed upstream of the printing medium 
placed on the printing position along the advance of the printing medium 
in a manner that the medium positioning member contacts the printing 
surface of the printing medium, a mechanism for supporting the platen so 
that the platen is displaced in parallel displacement, namely downstream 
along the advance of the printing medium and in the direction that the 
platen parts from the print head, and an urging for urging in the 
direction opposite to the direction of displacement of the platen 
displaced by the support mechanism in order to press the platen against 
the medium positioning member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, the embodiments of the present invention are 
discussed. In each of the embodiments, the printing apparatus of the 
present invention is incorporated in an ink-jet printer. The present 
invention can be incorporated not only in the ink-jet printer but also in 
a wide range of devices that prints on a printing medium with ink. In each 
of the figures, P represents the direction of advance of the printing 
medium, and R represents the area of an array of ink nozzles, namely a 
printing area. 
FIGS. 1 and 2 are the diagrammatic cross-sectional view and the fragmentary 
perspective view shoving the construction of the major portion of the 
embodiment 1 of the present invention. A guide rail 14 extends in the 
direction of width of a printing medium 13. The guide rail 14 passes 
through a carriage 12 to slidably support it in its front portion 
(downstream along the advance of the printing medium). An ink-jet print 
head 11 is detachably mounted on the carriage 12. On its rear portion 
(upstream along the advance of the printing medium), the carriage 12 
rotatably supports a guide roller 16 that rolls on a support rail 15 
extending in parallel with the guide rail 14. The carriage 12 is, on its 
front portion, engaged with a timing belt 17 that is entrained about a 
pair of right and left cog pulleys (unshown) that are rotatable in both 
normal and reverse directions by an unshown carriages driving motor. As 
the timing belt 17 moves, the carriage 12 along with the ink-jet print 
head 11 scans along the guide rail 14 in a direction different from the 
direction of advance of the printing medium, for example, in the direction 
of width of the printing medium 13. 
The ink-jet print head 11 ha unshown nozzles for projecting droplets of 
ink, and unshown electrothermal converter elements disposed inside pipes 
communicating with the respective nozzles, for generating thermal energy 
for propelling ink out. A platen 18 faces the ink-jet print head 11 and 
extends in the direction of scan of the ink-jet print head 11. A plurality 
of ribs 19 are projected from the surface of the platen 18 facing the 
ink-jet print head 11. The plurality of ribs 19 are spaced apart at 
predetermined intervals over across the width of the printing medium 13 
and run in the direction of the advance of the printing medium 13 (namely, 
in the direction from left to right in FIG. 1). The platen 18 on its 
underside has integrally a movable guide plate 21 having an inclined guide 
surface 20. The inclined guide surface 20 is inclined so that it parts 
farther from the ink-jet print head 11 as the printing medium 13 runs 
downstream in its direction of advance. Disposed right below the platen 18 
is a platen support plate 23 having an inclined guide surface 22 that 
coincides, in inclination, with the inclined guide surface 20 of the 
movable guide plate 21. The angle of inclination of the inclined guide 
surface 20 is 15 degrees or greater but 60 degrees or smaller with respect 
to the support surface of the platen 18 for the printing medium 13. 
Preferably, the angle of inclination of the inclined guide surface 20 is 
25 degrees or greater but 40 degrees or smaller, and is 30 degrees in this 
embodiment. 
In this embodiment, the plurality of ribs 19 are formed on the surface of 
the platen 18 to reduce friction between the platen 18 and the printing 
medium 13. If such friction is negligibly small, the platen 18 may be flat 
without ribs 19. 
The platen 18 is connected to the platen support plate 23 so that the 
movable guide plate 21 attached to the underside of the platen 18 slides 
on its inclined guide surface 20 against the inclined guide surface 22 to 
vary the spacing to the ink-jet print head 11. A compression spring 24 is 
interposed between the movable guide plate 21 and the platen support plate 
23 to urge the platen 18 toward the ink-jet print head 11. When the 
printing medium 13 is a typical recording paper such as a roll of paper 
and cut sheet, or a resin sheet, the compression spring 24 urges the 
movable guide plate 21 along the inclined guide surfaces 20, 22, 
preferably at a force of 3 grams or greater but 20 grams or smaller, and 
more preferably at a force of 5 grams or greater but 9 grams or smaller. 
The magnitude of urging, particularly, its upper limit, is subject to 
modification, depending on the weight of the printing medium to be 
printed. 
Disposed right above the back edge (upstream along the advance of the 
printing medium 13) of the platen 18 is an L-shaped medium positioning 
plate 25 that extends in parallel with the guide rail 14, facing the ribs 
19 on the backward edge of the platen 18. The underside of the positioning 
plate 25 as the medium positioning means placed upstream along the advance 
of the printing medium is pressed onto the printing surface 28 of the 
printing medium 13 by the platen 18. The platen 18 is urged toward the 
ink-jet print head 11 by the compression spring 24 as an elastic member, 
along the inclined guide surfaces 20, 22. Regardless of the thickness of 
the printing medium 13, an appropriate spacing is maintained between the 
printing surface 28 of the printing medium 13 and the ink-jet print head 
11. 
The platen 18 is constructed of a material such as polyphenylene oxide 
characteristic of a good ink-resistivity or polyacetal characteristic of 
low friction. The platen support plate 23 is constructed of a zinc-coated 
steel sheet, and the medium positioning plate 25 is of a stainless steel 
sheet. 
A pair of transport rollers 29, 30 are arranged along the advance of the 
printing medium 13, respectively, upstream of the medium positioning plate 
25 and downstream of the forward edge of the platen 8 (namely, downstream 
along the advance of the printing medium 13). The pair of transport 
rollers 29, 30 in parallel with the guide rail 14 and resin in contact 
with the back surface of the printing medium 13 opposite its printing 
surface 28. The pair of transport rollers 29, 30 are driven by an unshown 
driving motor. A pair of pinch rollers 31, 32 facing the pair of transport 
rollers 29, 30 with the printing medium 13 interposed therebetween are 
urged, respectively, toward the transport rollers 29, 30 by pressure 
springs 33, 34. The pinch roller 32 downstream of the printing medium 13 
has a spur-gear like outer circumference with a plurality of projections. 
The printing surface 28 of the printing medium 13 is pressed between the 
medium positioning plate 25 and the platen 18 and the spacing between the 
printing surface 28 of the printing medium 13 and the ink-jet print head 
11 is kept to a predetermined distance. When a printing medium 13 of a 
different thickness is introduced, the platen 18 moves downstream along 
the advance of the printing medium while parting away from the ink-jet 
print head 11 to accommodate a change in thickness of the printing medium 
13. The inclined guide surfaces 20, 22 are configured such that they part 
away from the print head as the printing medium 13 runs downstream in the 
direction of advance. The printing medium 13 slides along the platen 18 
with a predetermined advance accuracy maintained and without affecting 
printing quality, while the platen 18 is slides downward along the 
inclined guide surfaces 20, 22 against the urging of the compression 
spring 24 (in the direction away from the print head). Thus, an 
appropriate spacing between the printing surface 28 and the ink-jet print 
head 11 is maintained. 
As shown in FIG. 1, an edge of the ink nozzle array of the print head is 
placed in the vicinity of the downstream edge of the medium positioning 
plate 25. A position where ink droplets projected from ink nozzles are 
deposited is defined as the printing position. The ink nozzle array faces 
the printing position. As shown in FIG. 1, image printing is continuously 
performed on the printing medium 13 immediately before the backward edge 
of the printing medium 13 is just clear of the medium positioning plate 
25. Since the axes of rotation of the upstream transport roller 29 and 
downstream transport roller 30 are fixed, advance accuracy of the printing 
medium 13 is well maintained, when the printing medium 13 is advanced by 
the upstream transport roller 29 and the pinch roller 31 or when the 
printing medium 13 is advanced by the downstream transport roller 30 and 
the pinch roller 32. 
A medium position roller 26 to be described in connection with the 
embodiment 3 may be arranged facing the downstream side of the platen 18 
along the advance of the printing medium 13 with respect to the ink-jet 
print head 11. The medium positioning roller 26 may be designed to rotate 
in synchronism with the transport rollers 29, 30. In this way, the 
advantage of this embodiment is even more enhanced. 
The embodiment 2 of the present invention is now discussed. FIG. 3 shows 
the fragmentary respective view showing the appearance of the major 
portion of the embodiment 2. In FIG. 3, components equivalent or identical 
to those described in connection with the embodiment 1 are designated with 
the same reference numerals, and their description is not repeated. 
In the embodiment 2, the platen 18 in the embodiment 1 is split into a 
plurality of units along the direction different from the direction of 
advance of the printing medium 13, for example, a plurality of units 
extending in the direction of the advance of the printing medium 13 are 
spaced across the width of the printing medium 13. 
The platen 18 in this embodiment is made up of a number of platen units 
18U, having a projected rib 19. The platen units 18U are equally spaced 
across the width of the printing medium 13. Each platen unit 18U has a 
similar construction to that of the platen 18 in the preceding embodiment, 
namely each platen unit 18U has a movable guide plate 21 and a compression 
spring 24 (FIG. 1). 
Since each platen unit 18U is pressed by the medium positioning plate 25, 
the printing surface 28 of the printing medium 13 is reliably put into 
contact with the medium positioning plate 25 regardless of the width of 
the printing medium 13. Since each platen unit 18U is urged toward the 
ink-jet print head 11 by the respective compression spring 24, the 
printing medium 13 is uniformly pressed against the medium positioning 
plate 25 across its width, and the printing medium 13 is prevented from 
skewing in its advance. 
In this embodiment, each platen unit 18U has its own compression spring 24. 
The same advantage is expected even if all platen units 18U share commonly 
a single compression spring 24. 
Furthermore, a medium position roller 26 to be described in connection with 
the embodiment 3 may be arranged facing the downstream side of the platen 
18. The medium positioning roller 26 may be designed to rotate in 
synchronism with the transport rollers 29, 30. In ths way, the advantage 
of this embodiment is even more enhanced. In this embodiment, the printing 
medium 13 is prevented from curling or wrinkling when its ford edge 
reaches the nip between the downstream transport roller 30 and the pinch 
roller 32. The spacing bet the printing surface 28 of the printing medium 
13 and the ink-jet print head 11 is reliably maintained at an appropriate 
distance. If a cylindrical cleaning member that rotates with its 
circumference in contact with the medium positioning roller 26 and the 
downstream pinch roller 32 is supported in a pivotally Arable fashion, and 
if the circumferential speeds of the medium positioning roller 26 and the 
downstream pinch roller 32 are equalized, the printing medium 13 is 
prevented from curling or wrinkling when its forward edge enters the nip 
between the downstream transport roller 30 and the pinch roller 32. The 
cleaning member removes ink sticking to the circumferences of the medium 
positioning roller 36 and the downstream pinch roller 32, thereby 
preventing a printed image on the printing surface 28 of the printing 
medium 13 from smearing. 
The embodiment 3 of the present invention is now discussed. 
In the two preceding embodiments, the inclined guide surfaces 20, 22 allow 
the platen 18 to move in parallel displacement, namely both downstream 
along the advance of the printing medium and downward apart from the print 
head. Alternatively, a link mechanism may be used to move the platen 18 in 
parallel displacement. FIG. 4 is the diagruatic cross-sectional view 
showing the construction of the major portion of the embodiment 3 of the 
present invention. In FIG. 4, components equivalent or identical to those 
described with ref e to the preceding embodiments are designated with the 
same reference numerals, and their discussion is not repeated. 
Backward and forward lateral pairs of link brackets 37, 38 are projected 
from the underside of the platen 18 from its upstream portion and 
downstream portion, respectively. The link brackets 37, 38 pivotally 
support respective pairs of links 35, 36 on their top ends. Disposed below 
the platen 18 is a platen support plate 32 having a tray-like 
configuration in its vertically longitudinal cross section. Two lateral 
pairs of link brackets 39, 40, one lateral pair on its upstream portion 
and the other lateral pair on its downstream portion, are projected from 
the inner bottom of the platen support plate 32. The link brackets 39, 40 
pivotally support the bottom ends of the forward and backward pairs of 
links 35, 36. The forward and backward links 35, 36 are equal in length 
The separation between-the forward and backward link brackets 37, 38 on 
the platen 18 and the separation between the forward and backward link 
brackets 39, 40 on the platen support plate 23 are set to be equal. A 
parallelogram or pantograph link mechanism is thus formed by the platen 
18, platen support plate 23, and links 35, 36. 
A medium position roller 26.is arranged facing the downstream side of the 
platen 18 along the advance of the printing medium 13 and is rotatably 
supported by the shaft 27. The medium positioning roller 26 is designed to 
rotate in synchronism with the transport rollers 29, 30. 
The compression spring 24 is interposed between the platen 18 and the 
platen support plate 23 to urge the platen 18 toward the ink-jet print 
head 11. The introduction of the printing medium 13 causes the platen 18 
to move in parallel displacement, namely vertically away from the ink-jet 
print head 11 and downstream along the advance of the printing medium. 
The embodiment 3 requires components for platen support means compared with 
the embodiments in FIGS. 1 and 2, and also requires machining accuracy and 
assembly accuracy of parts of the link mechanism sufficient to allow the 
platen 18 to move in parallel displacement. The embodiment 3, however, Is 
free from resistance that takes place between the inclined guide surfaces 
20, 22 when the platen 18 and the platen support plate 23 slide against 
each other, and thus works with the compression spring 24 of a smaller 
force than those in the preceding embodiments. 
In each of the preceding embodiments, transport means (the transport 
rollers 29, 30 and pinch rollers 31, 32) for advancing the printing medium 
13 and positioning means (the Bus positioning plate 25, the medium 
positioning roller 26) for positioning the printing surface 28 of the 
printing medium 13 are separately constructed. The medium positioning 
roller 26 and its shaft 27 downstream of the ink-jet print head 11 may be 
dispensed with. The transport deans an the positioning means may be 
integrated in function. Such an ink-jet printing apparatus is shown in the 
diagrammatic cross-sectional view in FIG. 5 as the embodiment 4 of the 
present invention. In FIG. 5, like components are identified with like 
refer numerals, and their description is not repeated. 
Disposed right above the backward edge of the platen 18 (upstream along the 
advance of the printing medium) is a transport roller 41 in alignment with 
the rear ends of the ribs 19 on the platen 18, with the printing medium 13 
interposed therebetween. The transport roller 41 is rotatably supported 
about the axis of rotation of a driving shaft 42 that extends an parallel 
with the guide rail 14. Disposed right above the forward edge of the 
platen 18 (downstream along the advance of the printing medium) is a 
transport roller 43 in alignment with the forward ends of the ribs 19 on 
the platen 18, with the prong medium 13 therebtween. The transport roller 
43 is rotatably supported about the axis of rotation of a driving shaft 44 
that ends in parallel with the guide rail 14. The platen 18 urged by the 
compression spring 24 presses the printing surface 28 of the printing 
medium 13 against the pair of rollers 41, 43 from below. Regardless of the 
thickness of the printing medium 13, the spacing between the printing 
surface 28 of the printing medium 13 and the ink-jet print head 11 is 
constantly maintained appropriate. 
The pair of rollers 41, 43 arranged downstream of and upstream of the 
ink-jet print head 11 along the advance of the printing medium 13 also 
function as the already-described medium positioning plate 25 and the 
medium positioning roller 26. Since the downstream transport roller 43 is 
pressed into contact with the image printed on the printing surface 28 of 
the printing medium 13, a quick-drying characteristic is required of the 
ink projected-out of the ink-jet print head 11. 
This embodiment dispenses with the medium positioning plate 25, medium 
positioning roller 26, pinch rollers 31, 32 and springs 33, 34, and 
requires a fewer components than the preceding embodiments, and permits a 
compact design. Since the layout of associated comports does not pelt 
larger diameter transport rollers 41, 43, this embodiment particularly 
suits ink-jet printing apparatuses for small-size printing media 13 such 
as postcards. 
The embodiment 5 of the present invention is now discussed. 
According to embodiments shown in FIGS. 1 through 5, the platen 18 is 
pushed away from the mum positioning plate 25 against the urging the 
compression spring 24 as the forward of the printing medium 13 enters 
between the platen 18 and the medium positioning plate 25. Although the 
force of the compression spring 24 against the platen 18 is a few grams as 
already described, the printing medium 13, if thin and limp, is subject to 
jamming. Thus, it may be advantageous to retract initially the platen 18 
in the direction that it parts from the medium positioning plate 25 when 
the forward edge of the printing medium 13 is enters between the platen 18 
and the medium positioning plate 25. 
Such an ink-jet printing apparatus is shown in the diagrammatic 
cross-sectional view in FIG. 6 as the embodiment 5 of the present 
invention. In FIG. 6, components equivalent or identical to those 
described in connection with the embodiment 1 in FIGS. 1 and 2 are 
designated with the same reference numerals, and their description is not 
repeated. 
he inclined guide surface 22 of the platen support plate 23 has a slot 45 
that runs in the direction of sliding of the movable guide plate 21. A 
locking pin 46 that slidably passes through the slot 45 is projected from 
the inclined guide surface 20 of the movable guide plate 21. A catch 47 
facing the locking pin 46 for engaging with the looking pin 46 is conned 
to an unshown actuator in a Or that allows the catch 47 to reciprocate in 
the direction of sliding of the movable guide plate 21 along the inclined 
guide surfaces 20, 21. 
In the inactive state of the actuator as on in FIG. 6, the platen 18 under 
the urging of the compression spring 24 is pressed against the medium 
positioning plate 25 and the medium positioning roller 26. When the 
forward edge of the printing medium 13 is introduced into between the 
platen 18 and the medium positioning plate 25, the actuator is activated 
to move the catch 47 downwardly rightward in FIG. 6. The locking pin 46 
along with the movable guide plate 21 moves against the urging of the 
compression spring 24, and the platen 18 is retracted away from the medium 
positioning plate 25. In this case, the stroke of the actuator is 
preferably set such that the clearance formed between the medium 
positioning plate 25 and the platen 18 in greater than the thickness of 
the printing medium 13. 
As a result, the forward of the printing medium 13 is introduced into 
between the transport roller 30 and the pinch roller 32 without a 
resistance. The actuator turns inactive again at the moment the forward 
edge of the printing medium 13 advances past the medium positioning roller 
26. Under the urging of the compression spring 24, the printing medium 13 
is held between the platen 18 and the medium positioning plate 25 or the 
medium positioning roller 26. Thus, even a thin and limp printing medium 
13 is free from jamming. 
The embodiment 6 of the present invention is now discussed. 
The arrangement in the embodiment 5 may be incorporated in the embodiment 3 
shown in FIG. 4. 
Such an ink-jet printing apparatus is shown in the diagrammatic 
cross-sectional view in FIG. 7 as the embodiment 6 of the present 
invention. In FIG. 7, components equivalent or identical to those 
described in connection with the preceding embodiments are designated with 
the same reference numerals, and their description is not repeated. 
A locking member 48 is projected from the underside of the platen 18. A 
catch 47 facing the locking member 48 for engaging with the locking member 
48 is connected to an unshown actuator in a manner that allows the catch 
47 to reciprocate in the direction of movement of the platen 18 that moves 
integrally with the links 35, 36. In the inactive state of the actuator 
shown in FIG. 7, the platen 18 under the urging of the compression spring 
24 is pressurized against the medium positioning plate 25 and the medium 
positioning roller 26. The actuator is activated to move the catch 47 
downward in FIG. 7 when the forward edge of the printing medium 13 is 
introduced into between the platen I8 and the medium positioning plate 25. 
In this way, the locking member 48 along with the platen 18 is retracted 
away from the medium positioning plate 25 against the urging of the 
compression spring 24. 
AS a result, the forward edge of the printing medium 13 is introduced into 
between the transport roller 30 and the pinch roller 32 without any 
resistance and even a thin and limp printing medium 13 is free from 
jamming. 
The embodiment 7 of the present invention is now discussed. 
The retraction mechanism shown in FIG. 6 may be incorporated in the 
embodiment 4 shown in FIG. 5. Such an ink-Jet printing apparatus is shown 
in the diagrammatic cross-sectional view in FIG. 8 as the embodiment 7 of 
the present invention. In FIG. 8, components equivalent or identical to 
those described in connection with the preceding embodiments are 
designated with the same reference numerals, and their description is not 
repeated. 
In this embodiment, when the forward edge of the printing medium 13 is 
brought into between the platen 18 and the transport roller 41 by 
transport rollers (unshown) a e upstream thereof, the actuator is 
activated to mm the catch 47 downwardly rightward in FIG. 8, and thus the 
locking pin 46 along with the movable guide plate 21 is against the urging 
of the compression spring 24. The platen 18 is thus retracted away from 
the transport roller 41. 
As a result, the forward edge of the printing medium 13 is introduced into 
between the transport roller 41 and the platen 18 without any resistance 
and even a thin and limp printing medium 13 is free from jamming. 
FIGS. 9 and 10 are diagrammatic cross-sectional views showing the 
construction of the major portion of an embodiment 8 of the present 
invention. FIG. 11 is the fragmentary external perspective view of the 
major portion of the embodiment 8, and FIG. 12 is the diagrammatic 
perspective view showing the construction of the major portion of the 
embodiment 8 with parts shown in phantom. In these figures, components 
equivalent or identical to those described in connection with the 
preceding embodiments are designated with the same reference numerals, and 
their description is not repeated. 
The inclined guide surface 22 of the platen support plate 23 has a slot 49 
that in the direction of sliding of the movable guide plate 21. A pair of 
projections 50 that are slidably engaged with the slot 49 are projected 
from the inclined surface 20 of the movable guide plate 21. Consideration 
in given to reducing play of the movable guide plate 21 in the direction 
of width of the printing medium 13 with respect to the platen support 
plate 23 and enabling smooth sliding of the able guide plate 21 along the 
platen support plate 23. The inclined guide surface 20 is provided with a 
recess 51 between the projections 50. Accommodated in the recess 51 is a 
locking pin 52 integrally formed of the platen support plate 23 and 
extending in the direction of width of the printing medium 13. 
Disposed below the platen support plate 23 is a rotatable pivot shaft 53 
extending in the direction of width of the printing medium 13. The pivot 
shaft 53 is integrally connected to a pivot lever 54 having a bell-crank 
configuration one and of the pivot lever 54 terminates in a bifurcated 
fork portion 55 that is inserted into the recess 51 and engaged with the 
locking pin 52. The other end of the pivot lever 54 terminates in an arm 
portion 57 that runs generally vertically in FIG. 9) along a semicircle 
cantered on the pivot shaft 53. The arm portion 57 has a number of 
serrated teeth 56. 
When the forward edge of the printing medium 13 is brought into bete the 
platen 18 and the medium positioning plate 25, the plate 18 is pressed 
downward against the urging of the compression spring 24 in FIG. 9. The 
platen 18 and the lo pin 52 integrated with the movable guide plate 21 
move downwardly rightward. As a result, the fork portion 55 of the pivot 
lever 54 engaged with the lock pin 52 is pivoted clockwise in FIG. 9 about 
the pivot shaft 53 along with the arm portion 57. 
A latch lever 59 has, on its one end, a pawl 58 that is engaged with the 
teeth 56 of the pivot leer 54. The base of the latch lever 59 is pivotally 
supported about a lever shaft 60 that extends in parallel with the pivot 
shaft 53. The base of the latch lever 59 has a locking member 61 and a 
release lo 62, both diagonally oppositely projected thereacross. The 
locking member 61 is connected to a pulling spring 63 that urges the pawl 
58 to engage with the teeth 56. In this embodiment, the teeth 56 generate, 
between their engagement face and the pawl 58, a force component that 
causes the latch lever 59 to pivot clockwise about the lever shaft 60 in 
FIG. 9 against the urging of the pulling spring 63, when the pivot lever 
54 pivots clockwise about the pivot shaft 53 in FIG. 9. Furthermore, the 
teeth 56 are oriented such that the counterclockwise pivoting of the pivot 
lever 54 about the pivot shaft 53 is blocked in FIG. 9. 
When the platen 18 is displaced away from the medium positioning plate 25, 
the engagement position of the teeth 56 with the pawl 58 is shifted 
accordingly. The shifted position of the platen 18 is thus automatically 
locked. To cause a slight displacement of the platen 18 to move the 
engagement position of the teeth 56 with the pawl 58, the arm portion 57 
may be set to be 10 ti as long as the fork portion 55, for example. 
Assuming that a displacement of the movable guide plate 21 is 0.1 mm, the 
displacement of the teeth 56 relative to the pawl 58 is 1 mm. With the 
pitch of the teeth 56 being 1 mm or smaller, the displacement of the 
movable guide plate 21 as small as 0.1 mm is sufficient to shift the 
engagement position of the teeth 56 with the pawl 58. 
Disposed beside the release lever 62 is a lever handling arm 65 attached to 
a release shaft 64. The release shaft 64 is connected to an unshown 
driving unit that drives the release shaft 64 intermittently. With the 
lever arm 65 faking a counterclockwise turn in FIG. 9, the release lever 
62 is engaged with the lever handling arm 65, pivoting clockwise the latch 
lever 59 about the lever shaft 60. When the release lover 62 is disengaged 
with the lover handling arm 65, the latch lever 59 returns to the state 
shown in FIG. 9 by the urging of the pulling spring 63. 
A full turn of the lever handling arm 65 pivots clockwise the latch lever 
59 about the lever shaft 60 in FIG. 9. The pawl 58 is temporarily 
disengaged with the teeth 56. With no printing medium 13 interposed 
between the platen 18 and the mum positioning plate 25, the platen 18 is 
pushed upward by the urging of the compression spring 24 until it contacts 
the medium positioning plate 25. 
The release shaft 64 may be allowed to rotate each time a sheet of printing 
medium 13 is printed. However, it in advantageous to rotate the release 
shaft 64 each time the thickness of the printing medium 13 is changed or 
each time a single session of printing is completed. 
When the forward edge of the printing medium 13 is brought into between the 
platen 18 and the medium positioning plate 25, and when the platen 18 and 
the movable guide plate 21 are retracted away from the medium positioning 
plate 25 and in the direction of the advance of the printing medium 13, in 
accordance with the thickness of the printing medium 13, the engagement 
position of the teeth 56 with the pawl 58 of the latch lever 59 is 
automatically shifted in accordance with the amount of retraction, and 
thus the spacing between the platen 18 and the medium positioning plate 25 
is kept to the clearance corresponding to the thickness of the printing 
medium 13. As shown in FIG. 10, therefore, even when the backward edge of 
the printing medium 13 advances past the platen 18 and the medium 
positioning plate 25, the platen 18 remains in a retracted position unless 
the release shaft 64 is driven. The spacing between the ink-Jet print head 
11 and the printing surface 28 of the printing medium 13 is maintained 
appropriate. Thus, if the printing area of the printing medium 13 is 
widened, a good printing quality is maintained. 
The embodiment 9 of the present invention is now discussed. 
In the embodiment 8, the pivot lever 54 may be employed. Alternatively, the 
pivot lever 54 is dispensed with. 
Such an ink-jet printing apparatus is shown in the diagrammatic 
cross-sectional view in FIG. 13 as the embodiment 9 of the present 
invention. In FIG. 13, components equivalent or identical to those 
described in connection with the preceding embodiments are designated with 
the same reference numerals, and their description is not repeated. 
The inclined guide surface 20 of the movable guide plate 21 has a number of 
serrated teeth 66 along the direction of move of the movable guide plate 
21. The inclined guide surface 22 of the platen support plate 23 has an op 
67 facing the teeth 66. A latch lever 69 has on its one end a pawl 68 that 
is engaged with the teeth 66 through the opening 67. The base of the latch 
lever 69 is pivotally supported about a lever shaft 70. A locking Doer 71 
is projected from the base of the latch 1 69. The locking m r 71 is loaded 
with a pulling spring 72 that urge the pawl 68 of the latch lever 69 to 
the teeth 66 of the movable guide plate 21. 
The teeth 66 generate, between their engagement face and the pawl 68, a 
force component that causes the latch lever 69 to pivot counterclockwise 
about the lever shaft 70 in FIG. 13 against the urging of the pulling 
spring 72, when the platen 18 and the movable guide plate 21 move 
downwardly rightward along the platen support plate 23 in FIG. 13. 
Furthermore, the teeth 66 are oriented such that the upwardly leftward 
movement of the platen 18 and the movable guide plate 21 along the 
inclined guide surface 22 of the platen support plate 23 is blocked. 
When the platen 18 Is displaced away from the medium positioning plate 25, 
engagement position of the teeth 66 with the pawl 68 is shifted 
accordingly. The shifted position of the platen 18 is thus automatically 
locked. This arrangement offers the am advantage as the preceding 
embodiment shown in FIGS. 9 through 12. 
In the above embodiment, the Unix for releasing the pawl 68 of the latch 
lever 69 from the teeth 66, namely the mechanism corresponding to the 
release lever 62, the release shaft 64, and the lever handling arm 65 in 
preceding embodiments, is not shown. A mechanism similar to these may be 
employed. In embodiment 7 and 8, teeth 56, 66 and pawls 58, 68 are 
employed. Another alternative clutch mechanism may be used. 
Instead of the print head 11, all above embodiments may use an optical 
pickup head mounted on the carriage 12 for reading a document rather than 
printing a printing medium. In this case, the apparatus works as a 
document reader with the same advantage described above retained. 
According to each of the above embodiments, the spacing between the 
printing surface of the printing medium of an arbitrary thickness and the 
print head is maintained appropriate. 
When a pair of transport rollers for advancing a printing medium are 
designed to work as a medium positioning member&lt;as well, the component 
count of the apparatus is reduced with a compact design implemented. 
When a transport mechanism is constructed of a pair of transport rollers in 
contact with the back surface of the printing medium opposite the printing 
Surface for advancing the printing medium, and a pair of pinch rollers in 
contact with the printing surface of the printing medium, with the 
transport rollers engage with the-pinch rollers with the printing medium 
therebetween, the transport rollers are put into contact with the printing 
medium over its entire width. Thus, the apparatus enjoys an improved 
advance accuracy of the printing medium. 
When the forward edge of the printing medium is brought into between the 
platen and the medium positioning member, the platen is temporarily 
retracted from the medium positioning member by the platen retraction 
means. A printing medium, if thin and limp, is free from jamming. 
When the apparatus further comprises platen engaging means for holding the 
platen to a medium positioning means with the spacing kept therebetween 
corresponding to the thickness of a printing medium against the urging of 
a platen urging member and engagement releasing means for releasing the 
platen out of the platen engaging means, the spacing between the medium 
positioning member and the platen is automatically kept in accordance with 
the thickness of the printing medium. Even after the backward edge of the 
printing medium advance past the nip between the medium positioning member 
the platen, the spacing bet the print head and the printing surface of the 
printing medium remains unchanged. Thus, high quality printing is perform 
over a wide ax on the printing surface of the printing medium.