Patent Grant
7924300
The present invention relates generally to an apparatus and method for processing photothermographic film, and more specifically an apparatus and method for thermally developing an imaging material employing a cooling section with varying heat transfer characteristics.
Photothermographic film typically includes a base material, such as a polymer, coated on at least one side with an emulsion of heat sensitive materials. After the film has been imaged (i.e., subjected to photo-stimulation), the resulting latent image is developed through application of heat to the film so as to heat the film to a prescribed temperature for a prescribed time. This relationship between time and temperature is critical to achieving a high quality image.
As such, controlling heat transfer to the film during the development process is crucial. If heat transfer is not uniform during development, visual artifacts, such as non-uniform density and streaking, may occur. If heat is transferred too rapidly, the base material of some films may expand too quickly resulting in expansion wrinkles that can cause visual and physical artifacts in the developed film.
Likewise, once the film has been heated to make the latent image visible, it is important to cool the film in order to prevent overdevelopment of the image. In the same way it is critical to control the heating process, it is also important to control the cooling of the film. If the chemical reaction of the emulsion (i.e., image development) is not stopped in a uniform fashion, non-uniform density and streaking may occur. If the film is cooled too rapidly, the base material may contract too quickly resulting in contraction wrinkles that can cause visual and physical artifacts in the developed film.
Various cooling techniques have been developed and employed by thermal processors for cooling photothermographic film. One technique employs a cooling plate, wherein heat is transferred from the heated film to the cooling plate, which is cool relative to the film, by sliding the film across the plate. As “throughput” requirements of processors have increased, active cooling has been added by blowing air across the side of the plate opposite the side contacting the film to remove heat from the cooling plate to enable the film to be cooled more quickly.
While such a technique is effective at cooling the imaging media, sliding the film on the fixed cooling plate may scratch the emulsion, which is still soft from the elevated processing temperature. Additionally, a further increase in the throughput requires an increase in size (where space is typically limited) or an increase in the rate of cooling, which may result in wrinkling of the base material of the imaging media.
In light of the above, as the throughput requirements of processors continue to increase while the size of processors continue to decrease, it is evident that there is a need for a compact cooling section providing increased throughput while maintaining a high level of image quality.
In one embodiment, the present invention provides a thermal processor including an oven configured to heat an imaging media to a development temperature and a cooling section. The cooling section is configured to cool the imaging media from the development temperature to a desired exit temperature as the imaging media moves along a transport path from an entrance to an exit, wherein the cooling section is configured to provide a varying rate of heat transfer from the imaging media along the transport path so as to create a varying cooling temperature gradient in the imaging media substantially equal to and not exceeding a varying maximum cooling temperature gradient of the imaging media.
In an embodiment, a thermal conductivity of the cooling section increases along the transport path from the entrance to the exit to vary the heat transfer rate.
In an embodiment, the cooling section is configured to provide a temperature level which decreases along the transport path from the entrance to the exit to vary the heat transfer rate.
By varying the heat transfer rate along the transport path as the temperature of the imaging media decreases so as to substantially match the cooling temperature gradient of the imaging to a maximum cooling temperature gradient, the cooling section is able to substantially minimize a time necessary to cool the imaging media from a development temperature to a desired exit temperature without introducing visual and physical artifacts resulting from wrinkling.
Embodiments of the invention are better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.
Cooling section 36 includes an entrance 48, an exit 50, and a transport system 52. Cooling section 36 receives imaging 32 substantially at the development temperature at entrance 48 and cools the imaging media from the development temperature to a desired exit temperature as transport system 52 transports imaging media 32 from entrance 48 to exit 50 along transport path 46. Unless compensated for, a temperature difference between imaging media 32 and cooling section 36 will decrease as imaging media 32 moves along transport path 46, resulting in a decrease in a rate of cooling of imaging media 32 as it moves from entrance 48 to exit 50.
In accordance with one embodiment of the present invention, cooling section 36 is configured such that a heat transfer rate of heat from imaging media 32 to cooling section 36 varies along transport path 46 from entrance 48 to exit 50. In one embodiment, as will be described in greater detail below, a thermal conductivity of cooling section 36 increases from entrance 48 to exit 50 so as to vary the heat transfer rate of cooling section 36 along transport path 46. In one embodiment, as illustrated by
It is noted, as mentioned above, if imaging media 32 is cooled too rapidly as it moves along transport path 46, a base material of imaging media 32 may contract too quickly and cause wrinkling in the base material resulting in visual (e.g., density variations) and physical artifacts (e.g., wrinkles) in the developed media.
Polymer materials, including the polymer base material of some types of imaging media, such as imaging media 32, have a glass transition temperature, Tg. As generally known, the glass transition temperature represents the approximate midpoint of a typically narrow temperature range over which a rapid change in viscosity of the polymer occurs. Above its glass transition temperature, the polymer (e.g., the base material) is in an amorphous state where it is rubbery in nature, while below its glass transition temperature the polymer is in a more crystalline or glassy state where it is more rigid in nature. While in the glass transition temperature range, the polymer is transitioning from a more amorphous state to a more crystalline state.
As illustrated by
If first zone 54 transfers (i.e., absorbs) heat from imaging media 32 at too high of a rate such that cooling temperature gradient (TGRD) 70 is greater than a maximum cooling temperature gradient (TGRDmax) associated with the base material when the base material is above its glass transition temperature (i.e., T2>Tg), the base material may form wrinkles (as indicated by the “wrinkle” lines at 76) as it contracts from width W274 to width W172.
If temperature T1 of leading portion 62 remains above Tg of imaging media 32, and at a temperature where a chemical reaction in the emulsion is continuing at a substantial rate, such wrinkles may cause uneven cooling of the emulsion and produce visual artifacts in the developed image in the form of uneven image densities (e.g. streaking). If temperature T1 of leading portion 62 is below Tg of imaging media 32, in addition to the above described visual artifacts, physical artifacts may also be produced as the wrinkles may become “frozen” or fixed into the developed imaging media when the imaging media transitions from the amorphous to a more crystalline state.
To avoid causing such wrinkles, the rate of heat transfer of first zone 54 may be such that TGRD 70 does not exceed TGRDmax associated with the base material when the base material is above its Tg (i.e., T2>Tg). However, the further the level of TGRD 70 is below TGRDmax (see
It is noted that TGRDmax of imaging media 32 increases in a non-linear fashion as the temperature of imaging media 32 decreases. In other words, imaging media 32 can be cooled at an increasingly higher rate as its temperature drops.
In light of the above, in one embodiment, as will be described in further detail below, cooling section 36 is configured such that the heat transfer rate of cooling section 36 varies along transport path 46 so as to provide TGRD 70 in imaging media 32 at a level substantially equal to and not exceeding TGRDmax of imaging media 32. In one embodiment, cooling section 36 is configured to provide TGRD 70 in imaging media 32 substantially at TGRDmax at least until imaging media 32 cools to its associated Tg. It should be noted that a maximum heat transfer rate of cooling section 36, without exceeding TGRDmax depends on a transport rate of imaging media 32 by transport system 52. The faster the transport rate, the higher the rate of heat transfer of cooling section 36 can be without exceeding TGRDmax of imaging media 32.
By varying the heat transfer rate along transport path 46 as the temperature of imaging media 32 decreases so as to substantially match TGRD 70 to TGRDmax, cooling section 36 is able to substantially minimize a time necessary to cool imaging media 32 from a development temperature to a desired exit temperature without introducing visual and physical artifacts resulting from wrinkling of the base material. As a result, the “throughput” of thermal processor 30 is increased while maintaining a small physical footprint.
A portion of upper rollers 80, illustrated as rollers 80a through 80c, and a portion of lower rollers, illustrated as rollers 82a through 82c, form first zone 54 of cooling section 36. Rollers 80a through 80c and rollers 82a through 82c of first zone 54 include a cylindrical shaft 84 covered with sleeves of a first support material 86. A portion of upper rollers 80, illustrated as roller 80d, and a portion of lower rollers 82, illustrated as rollers 82d and 82e, form second zone 56 of cooling section 36. Rollers 80d, 82d, and 82e include cylindrical shaft 84 covered with sleeves of a second support material 88. A portion of upper rollers 80, illustrated as rollers 80e and 80f, and a portion of lower rollers 82, illustrated as rollers 82f and 82g, from third zone 58 of cooling section 36. Rollers 82f and 82g include cylindrical shaft 84 covered with sleeves of second support material 88, with rollers 80e and 80f including a cylindrical shaft 90 having no support material.
In one embodiment, first support material 86 has a first thermal conductivity, second support material 88 has a second thermal conductivity, and cylindrical shaft 90 has a third thermal conductivity. In one embodiment, the third thermal conductivity is greater than the second thermal conductivity, and the second thermal conductivity is greater than the first thermal conductivity. As such, in one embodiment, third zone 58 has a higher thermal conductivity than second zone 56, and second zone 56 has a higher thermal conductivity than first zone 54. In one embodiment, first support material 86 comprises foamed silicon rubber. In one embodiment, first support material 86 comprises foamed silicon having a density of 34±6 pounds per cubic foot and a hardness of 40 (Asker® Type C). In one embodiment, second support material 88 comprises solid silicon rubber having a hardness of 62±5 (Shore® “A”).
In one embodiment, cylindrical shafts 84 and 90 are metallic. In one embodiment, cylindrical shafts 84 and 90 comprise extruded aluminum. In one embodiment, as illustrated by roller 80a in
In one embodiment, as illustrated generally by a top view of portions of cooling section 36 in
In operation, with reference to
As described above, imaging media 32 has an associated glass transition temperature, Tg, and a maximum cooling temperature gradient TGRDmax which, if exceeded, may cause wrinkles in imaging media 32. In one embodiment, Tg is approximately 70° C. In one embodiment, Tg is approximately at the center of a glass transition temperature range. In one embodiment, the glass transition temperature range is from approximately 55° C. to 80° C. In one embodiment, the glass transition temperature is greater than the desired exit temperature, but below the development temperature (i.e., TE<Tg<TD).
As imaging media 32 moves along transport path 46, it is initially engaged by rollers 80a through 80c and 82a through 82c of first zone 54 which begin to absorb heat from and cool imaging media 32. The rate of heat transfer can be described by the following Equation I:
q=(ΔT)(k)(c) (Equation I)
where:
Because imaging media 32 enters first zone 54 substantially at TD, the heat differential, ΔT, between imaging media 32 and cooling section 36 is at its greatest in first zone 54.
As such, the thermal conductivity (k) of rollers 80a through 80c and 82a through 82c and, thus, the thermal conductivity of first support material 86, is selected so as to be smaller relative to rollers 80d, 82d, and 82e of second zone 56 and rollers 80e-80f and 82f-82g of third zone 58. In one embodiment, the thermal conductivity (k) of first support material is selected so that the rate of heat transfer (q) from imaging media 32 when moving at the desired transport rate is such TGRD 70 formed across imaging media 32 by first zone 54 is substantially equal to and not exceeding TGRDmax associated with imaging media 32. In one embodiment, as described above, first support material 86 comprises foamed silicon.
However, as imaging media 32 moves through and is cooled by first zone 54, the ΔT between imaging media 32 and first zone 56 begins to decrease. In one embodiment, as imaging media passes from roller 80c of first zone 54 to roller 82d of second zone 56, the temperature of imaging media 32 is below TD, but above Tg. As a result of the decreased temperature, a level of (TGRD) 70 formed across imaging media 32 begins to drop increasingly below TGRDmax (see
As such, to increase the rate of heat transfer (q) from imaging media 32 and thereby increase the level of TGRD 70 formed across imaging media 32 such that it is again substantially equal to but not exceeding TGRDmax, the thermal conductivity (k) of second support material 88 of rollers 80d and 82d-82e is selected so as to be greater than that of first support material 86. In one embodiment, as described above, second support material 86 comprises a solid silicon rubber.
However, as second zone 56 continues to cool imaging media 32, the ΔT between imaging media 32 cooling section 36 again begins to decrease. In one embodiment, as imaging media 32 passes from roller 82e of second zone 56 to rollers 80e and 82f of third zone 58, the temperature of imaging media 32 has cooled so as to be further below TD, but remains above Tg. As a result, a level of TGRD 70 across imaging media 32 again begins to fall increasingly below TGRDmax (see
As such, to again increase the rate of heat transfer (q) from imaging media 32, the thermal conductivity (k) of rollers 80e-80f and 82f-82g of third zone 58 is increased relative to that of rollers 80d and 82d-82e of second zone 56 and selected so as to increase TGRD 70 formed across imaging media 32 such that it is again substantially equal to but not exceeding TGRDmax. In one embodiment, as illustrated, lower rollers 82f-82g continue to employ second support material 88 while upper rollers 80e-80f comprise bare aluminum having a higher thermal conductivity (k) than second support material 88. As imaging media 32 passes through third zone 56, the temperature drops below Tg and continues to cool until exiting cooling section 36 at a temperature substantially equal to TE.
In one embodiment, as illustrated by
By offsetting upper and lower rollers 80a through 80d and 82a through 82e of first and second zones 54, 56 to form a corrugated transport path 46, cooling section 36 enables imaging media 32 (e.g. the polymer base material) to more freely contract, particularly when the temperature differential (ΔT) is greatest (e.g. in first and second zones 54 and 56), thereby reducing the potential for wrinkling. Additionally, by adding beam strength through the bending of imaging media 32 by corrugated transport path 46, TGRDmax of imaging media 32 is effectively increased, thereby enabling cooling section 36 to transfer heat at a higher rate without causing wrinkling of the polymer base material.
In one embodiment, as illustrated by
Graph 110 illustrates waveform 112 as having three segments 116, 118, and 120. Segment 116 illustrates the temperature of imaging media 32 as it travels through first zone 54, segment 118 as it travels through second zone 56, and segment 120 as it travels through third zone 58. With reference to segment 116, at time to, as indicated at 122, imaging media 32 enters first zone 54 of cooling section 36 at a temperature substantially equal to the development temperature (TD), as indicated at 124. Initially, as imaging media 32 travels through first zone 54 and begins to cool, the rate of temperature drop approximately follows the optimal temperature curve 114, as indicated at 126. However, as imaging media 32 continues to cool as it moves through first zone 54, the temperature differential (ΔT) between imaging media 32 and cooling section 36 begins to decrease causing the rate of temperature drop to decrease and temperature curve 112 to increasingly deviate from optimal temperature curve 114, as indicated at 128.
At time t1, as indicated at 130, imaging media 32 passes to second zone 56, which has a higher thermal conductivity than first zone 54. As a result, the rate of temperature drop of imaging media 32 increases, and temperature curve 114 again begins to approach optimal temperature curve 114, as indicated at 132. However, as imaging media 32 continues cool as it moves through second zone 56, the temperature differential (ΔT) between imaging media 32 and cooling section 36 begins to decrease causing the rate of temperature drop to decrease and temperature curve 112 to again increasingly deviate from optimal temperature curve 114, as indicated at 134. In the example of
At time t2, as indicated at 138, imaging media 32 passes to third zone 58, which has a higher thermal conductivity than second zone 56. As a result, the rate of temperature drop of imaging media 32 again increases, and temperature curve 114 again begins to approach optimal temperature curve 114, as indicated at 140. At time t3, as indicated at 142, the temperature of imaging media 32 reaches its glass transition temperature, as indicated at 144. After reaching its glass transition temperature, imaging media 32 continues to cool as it moves through third zone 58 until at time t4, as indicated at 146, it reaches the desired exit temperature (TE), as indicated at 148, which corresponds to exit 50 of cooling section 36 (see
Although described above primarily in terms of varying the thermal conductivity (k) of the rollers, in view of Equation I, the rate of heat transfer (q) is also based on the temperature differential (ΔT) between imaging media 32 and cooling section 36. As such, in one embodiment, a temperature of cooling section 36 is decreased along transport path 45 so as to adjust the temperature differential (ΔT) between imaging media 32 and cooling section 36 and maintain TGRD 70 at a level substantially equal to, but not exceeding, TGRDmax associated with imaging media 32.
In one embodiment, as illustrated by
Although illustrated by
In one embodiment, the temperatures of the air flows provided to upper and lower rollers 80, 82 (e.g. air flows 164, 166) decreases along transport path 46 from entrance 48 to exit 50 in order to adjust the temperature differential (ΔT) between cooling section 36 and imaging media 32 as it cools so as to achieve a desired rate of heat transfer (q) from imaging media 32 to cooling section 36. In one embodiment, the temperatures of the air flows and, thus, the temperatures of upper and lower rollers 80, 82, are decreased from entrance 48 to exit 50 so that heat is transferred (i.e. absorbed) from imaging media 32 as required to create and maintain TGRD 70 substantially equal to but not exceeding TGRDmax of imaging media 32.
As such, with reference to
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
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