Abstract:
Provided is a method of stabilizing an elastomeric property of an elastomeric material. The method includes placing the elastomeric material in a pressurizing chamber and applying a suitable hydrostatic pressure to the elastomeric material within the pressurizing chamber to at least partially compress the elastomeric material. Application of the hydrostatic pressure is maintained for a period of time suitable to at least partially stabilize a restorative force exhibited by the elastomeric material in response to subsequent exposures of the elastomeric material to a compressive force.

Description:
BACKGROUND OF THE INVENTION 
     Compression set and compression stress relaxation are both physical properties of materials. These properties are discussed or disclosed for specific materials in elastomeric material data sheets. A measurement of compression set is the percent reduction in dimensional thickness of a material after the material is compressed for a fixed time and at a fixed temperature. A measurement of compression stress relaxation is the reduction in restoration force of a material as the material is being compressed at a constant strain for a fixed time and at a fixed temperature. 
     Compression set and compression stress relaxation cause negative side effects in platen rollers. For example, a platen roller used in a thermal printer that prints two different media widths experiences negative side effects due to compression set and compression stress relaxation. Specifically, if too many sheets of the smaller width media are printed, especially during an initial utilization, (the initial utilization being approximately the first 1000 prints or the first five percent of the printer life), the rubber in the center portion of the platen roller experiences compression stress relaxation. The rubber in the center portion of the platen roller refers to or corresponds to the location on the platen roller corresponding to the width of the small width media. As a result of the compression stress relaxation, when the larger width media is printed utilizing the platen roller, an image artifact of lighter optical density appears on the larger width media in the location that corresponds to the width of the small width media. 
     Optical density may be represented in a range from 0 (open air) to 4 (very black). The measurement of optical density is a logarithmic scale where the optical density (OD) value is a negative exponent of the log base 10 value of light transmission (T=10 −OD ). Light transmission is usually expressed in terms of a percentage, e.g., if OD=0, the light transmission is equal to 100% and all light is being transmitted: if OD=1, light transmission is 10%; if OD=2, light transmission is 1%; if OD=3, light transmission is 0.1%, and if OD=4, light transmission is 0.01%. If one part of a film has a different “background OD” than another part of the film, then the part of the film with the different background OD is considered to have an image quality artifact. 
     As noted above, this image quality artifact is a degradation of the image quality for the film because the used portion of the roller produces the different optical density on the film when compared to the unused portion. This lighter density is attributed to the reduction in reaction force by the relaxed rubber (in the center portion of the platen roller corresponding to the width of the small width media) which results in less thermal pressure/contact at the media and printbead nip. DIN 6868-56 is a regulatory standard for medical hard copy film imagers and requires that the images produce prints to meet certain image quality guidelines. If the film has a certain number or a certain percentage of image artifacts, the hard copy film imager generating the film may not meet regulatory standard DIN 6868-56, and this makes the medical image printer unusable in a medical imaging environment. 
     In addition, compression set and compression stress relaxation also cause negative side effects in gasket and O-ring materials. Gasket and O-ring materials tend to leak over time and can no longer provide a sufficient seal. In some cases, this is a result of the gasket or O-ring material relaxing. Initially, when a gasket is tightened, the force that the gasket exerts (pushes) against the mating parts is sufficient to seal properly. This force reduces over time as a result of the compression set and compression stress relaxation property of the gasket and O-ring materials, thus causing the leak. 
     A prior method to reduce the negative effects of the compression set and the compression stress relaxation is to select a different material for the platen roller, the O-ring, or the gasket. At the molecular level, compression set and compression stress relaxation are the result of the breaking (and subsequent reforming while compressed to try to achieve the lowest energy state) of cross-links in the molecular chains of the compound or material which is utilized to make the roller, O-ring, or gaskets. In order to reduce the breaking of these cross-links, a solution is to utilize a material that is a higher durometer material, i.e., a harder material. In other words, the durometer must be increased. This is a challenge in many applications because, for example, in an application utilizing platen rollers, this would cause small defects in the roller surface to be more likely to show up on the image (if it is a high durometer vs. a low one). In the case of an O-ring, temperature and pressure cycling causes movement of the seal and a high durometer O-ring may not be flexible enough to accommodate this variation and still provide an adequate seal. In other words, a lower durometer material may provide other benefits and these benefits may be essential or beneficial to the functioning of the apparatus that utilizes the roller, O-ring, or gasket. Also, selecting a different material (compound) may result in chemical compatibility issues with other materials being utilized in the apparatus.  FIG. 1(   a ) illustrates restorative force of two materials according to the prior art. As is illustrated in  FIG. 1(   a ), the sample # 2  material, which illustratively may be a preferable roller material, has decreasing restorative force which is sloping in a downward fashion even at 70 hours of utilization. 
     Accordingly, a need exists to have rollers made of a lower durometer material which is not as influenced by compression set and compression stress relaxation in a printing environment where a large format media width and a small format media width are utilized. A need also exists for O-rings, and gaskets to maintain sealing properties and to not leak due to the onset of compression set and compression stress relaxation. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
         FIG. 1(   a ) illustrates restorative force of two materials according to the prior art; 
         FIG. 1(   b ) is a graph illustrating a compression stress relaxation curve after pre-aging rollers according to an embodiment of the invention; 
         FIG. 2  illustrates a method for pre-aging rollers utilizing hydrostatic pressure according to an embodiment of the present invention; 
         FIG. 2(   a ) illustrates a change in optical density for a standard roller compared to a pre-aged roller (exposed to a hydrostatic chamber) according to an embodiment of the present invention; 
         FIG. 2(   b ) illustrates application of a pressure over a duration of 12, 24, and 48 hours in a hydrostatic pressure chamber pressurized at 90 pounds per square inch; 
         FIGS. 3(   a ) and  3 ( b ) illustrate an embodiment of the invention for dynamic loading of a platen roller according to embodiments of the present invention; 
         FIG. 3(   c ) illustrates a fixture for applying pressure according to an embodiment of the present invention; 
         FIG. 3(   d ) illustrates an exploded view of the fixture for applying pressure according to an embodiment of the present invention; 
         FIG. 3(   e ) illustrates a change in optical density in a roller that is placed in the pre-aging process fixture according to an embodiment of the invention; 
         FIGS. 4(   a ) and  4 ( b ) illustrate a pre-aging technique for flat or a complex-shaped structure or geometry according to an embodiment of the invention; 
         FIG. 4(   c ) illustrates a plurality of pressure apparatus and a complex geometry O-ring or gasket according to an embodiment of the present invention; and 
         FIG. 5  illustrates a flowchart of a method of extending platen roller life and increasing performance according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1(   b ) is a graph illustrating a compression stress relaxation curve after pre-aging rollers according to an embodiment of the invention. Material  2  is a lower durometer material which is more desirable to be utilized in an imaging printing environment. As shown in  FIG. 1(   b ), both material  1  and material  2  exhibit a logarithmic decay in restoration force. Most of the decay of restoration force occurs during an initialization or burn-in timeframe. This may be referred to as a pre-aging timeframe or pre-aging process. In the embodiment of the invention illustrated in  FIG. 1(   b ), a pre-aging timeframe of 25 hours is utilized. 
       FIG. 1(   b ) illustrates the effect of the pre-aging technique or method on rollers made of material I and material  2 . The pre-aging process is completed for a pre-aging timeframe or process. In comparing the restorative force for material  2  during the pre-aging process against the prior art ( FIG. 1(   a )), the decreasing slope-or decay in the restorative force is lessened due the application of the pre-aging process. For example, after 30 hours, the restorative force of material  2  is 27.65 pounds in the prior art, whereas during the application of the pre-aging process, the restorative force of material  2  is 27.25 pounds. After the pre-aging timeframe has been complete, the decay in the restoration force both of materials (i.e., materials  1  and  2 ) is better than if no pre-aging had occurred, especially for material  2 . Specifically, the slope of the material  2  compression stress relaxation curve in  FIG. 1(   b ) is lower than the slope of the second compression stress relaxation curve for material  2  in  FIG. 1(   a ) (where no pre-aging occurred). This increase in restoration force for material  2  allows the utilization of material  2  in rollers in an imaging printer that prints two different media widths because the material  2  rollers subjected to the pre-aging process do not experience the reduction in the reaction force that prior art “relaxed” rollers previously experienced. A roller of material  1  could be utilized in an imaging printer, but because a defect in the roller may show up more frequently or prevalently in the roller of material one, material one may not be as useful in a roller environment. 
       FIG. 2  illustrates a method for pre-aging rollers utilizing hydrostatic pressure according to an embodiment of the present invention.  FIG. 2  includes a roller  210  and a plurality of forces  212 ,  214 ,  216 ,  218 ,  220 ,  222 ,  224 ,  226 ,  228 ,  230 ,  232 , and  234  being applied against the roller  210 . In an embodiment of the invention of the invention illustrated in  FIG. 2 , the method of pre-aging of the roller  210  may take place prior to installation of the roller in a machine. In an embodiment of the invention, the roller  210  may be utilized in is an imaging printer, for example, in a medical imaging environment. Illustratively, the roller  210  may be a platen roller in a multi-media imaging printer, such as a Codonics Horizon™ Multi-Media Printer. 
     In an embodiment of the invention, the plurality of forces  212 ,  214 ,  216 ,  218 ,  220 ,  222 ,  224 ,  226 ,  228 ,  230 ,  232 , and  234  illustrated in  FIG. 2  illustrate that forces are applied across the outer surface of the roller  210 . The representative plurality of forces  212 ,  214 ,  216 ,  218 ,  220 ,  222 ,  224 ,  226 ,  228 ,  230 ,  232 , and  234  may be generated through hydrostatic pressure. In other words, the plurality of forces  212 ,  214 ,  216 ,  218 ,  220 ,  222 ,  224 ,  226 ,  228 ,  230 ,  232 , and  234  do not illustrate all of the forces generated by the hydrostatic pressure, but more represent that the hydrostatic pressure surrounds the roller  210  and applies forces to outer surfaces of the roller  210 . In an embodiment of the invention, the hydrostatic pressure may pre-compress the roller  210  in order for the roller to advance to a time where the restoration force of the roller is more stable. For example, the application of hydrostatic pressure for a pre-aging timeframe allows the restoration force to stabilize (and have a smaller slope of decay) in the timeframe after the pre-aging timeframe (or in the timeframe immediately following the pre-aging timeframe). 
     In an embodiment of the invention, the hydrostatic pressure is applied by surrounding the roller with a pressurized gas in a pressurized chamber. In an embodiment of the invention, the hydrostatic pressure may be applied by a pressurized liquid in a pressurized chamber. The pressurized chamber may be any shape having a volume that allows for one or for a plurality of platen rollers to be placed in the pressurized chamber. The pressurized chamber may include a shelf or a ledge where the platen rollers, O-rings, or gaskets may be placed when undergoing the application of hydrostatic pressure. For example, the pressurized chamber may be a two foot by three foot rectangular chamber with an inlet or input port having a diameter of ¼″. In an embodiment of the invention, the pressurized chamber may be a 2′ by 3′ cylindrical tank that receives a specified atmospheric pressure through a ¼″ port. 
     In an embodiment of the invention, the pressurized gas may be compressed air. In an embodiment of the invention, the pressurized gas may be compressed nitrogen. In certain environments, compressed nitrogen may be easier to use on a production basis because compressed nitrogen is more inert than air. Additional illustrative gases, (this list is not meant to be limiting), may include noble gases which are very inert at room temperature. These gases include helium, neon, argon, krypton, and xenon. Certain gases may not be used if they cause a chemical reaction with the particular material being utilized for either the platen roller, O-ring, or gasket. For example, fluorine gas likely produces a reaction with the silicone rubber platen roller of a Codonics Horizon™ Multi-Media Imager and should not be utilized in a pressurized chamber where pre-aging of platen rollers is performed. 
     In an embodiment of the invention, the liquid may be water introduced into the chamber via the input port. In an embodiment of the invention, the liquid may be vegetable oils, glycerol, and/or isopropyl alcohol. These examples are merely illustrative and are not meant to be limiting. Liquids may be utilized in environments where pressure may need to be relieved safely because gases may “explode” if they for some reason rapidly expand, for example, if there is a processing equipment malfunction. 
     In an embodiment of the invention, the applied pressure within the compression chamber may be 90 pounds per square inch. In an embodiment of the invention, the applied pressure within the compression chamber may be 200 pounds per square inch. Almost any range of pressures 0-500 pounds per square inch (psi) may be utilized if platen rollers are placed in the compression chamber. As the applied pressure is lowered, the effect of the pressure may take longer. In other words, at a pressure of 20 psi, it may take three or four times the duration to introduce the same compression set or compression stress relaxation as compared to an applied pressure of 90 psi. The upper range limited only by where the deflection in the material cause by the hydrostatic loading exceeds the elongation limits of the material being utilized in the platen roller, O-ring, or gasket. If the deflection in the material exceeds the elongation limits of the material being utilized in the platen roller, O-ring, or gasket, damage would be visible on the surface of the platen roller, O-ring, or gasket. For example, in an embodiment of the invention utilizing gaskets or O-rings, up to 2000 psi may be utilized, if 2000 psi does not result in deflection in the material exceeding the elongation limits of the material. 
       FIG. 2(   a ) illustrates a change in optical density for a standard roller compared to a pre-aged roller (exposed to a hydrostatic chamber) according to an embodiment of the present invention. Larger changes in optical density are not desired in imaging printers because these larger changes result in artifacts within the medical images.  FIG. 2(   a ) illustrates the improvement in the change of optical density for a hydrostatic chamber pre-aged roller. Illustratively, at the print count of 3000, the hydrostatic chamber pre-aged roller has only a change of 0.035 as compared to a change in optical density of 0.105 for a prior art platen roller. 
     In an embodiment of the invention, the duration that the pressure may be applied is 12 hours. In other embodiments of the invention, the pressure may be applied for 24, 36, or 48 hours.  FIG. 2(   b ) illustrates application of a pressure over a duration of 12, 24, and 48 hours in a hydrostatic pressure chamber pressurized at 90 pounds per square inch. As illustrated by  FIG. 2(   b ), 12 hours of 90 psi may produce results where the change or variation in optical density is reduced as compared to the prior art. 24 hours of 90 psi pressure results in a better reduction of the change in optical density. Further, 48 hours of 90 psi does not further reduce the reduction in the optical density variation and the results are very similar to the results after 12 hours of pressurization. 
     In an embodiment of the invention, the pressure may be applied at 90 psi for 12 hours, which results in a material of which the platen roller, O-ring, or gasket is constructed exhibiting the desired compression set and compression stress relaxation properties which do not change substantially over time. In an embodiment of the invention, the pressure may be applied at 200 psi for 5 hours which also results in a material exhibiting the desired stable compression set and compression stress relaxation properties. In an embodiment of the invention wherein a platen roller is exposed to 90 psi pressure for 12 hours and also longer durations, the compression set and compression stress relaxation within the first 12 hours does not change drastically if the pressure is applied for a longer period. In other words, the pre-aging may occur and most of the deflection or change of roller, gasket, and O-ring material may occur during this timeframe. After that, the curves tend to flatten out (as is illustrated in  FIG. 1(   a )). In an embodiment of the invention, a platen roller could be exposed to a hydrostatic pressure of 500 psi and be exposed to a pressure duration of two hours. A low limit of the time for the application of hydrostatic pressure is the time constant of the chemical reaction to break the chains and then reform the chains at the lower energy state. 
     In an embodiment of the invention, the hydrostatic pressure may be applied at a constant pressure which results in a static loading. In an embodiment of the invention, hydrostatic pressure may be applied at a variable pressure which may result in a dynamic loading. In an embodiment of the invention, the hydrostatic pressure may be applied for a first timeframe, no pressure (or a lessened pressure) may be applied for a second timeframe, and this may continue for a pre-established timeframe. This may be referred to as cycling of hydrostatic pressure. This cycling of high hydrostatic pressure and no or low hydrostatic pressure may be continued for the entire pre-aging timeframe. In an embodiment of the present invention, the cycling may occur with multiple pressure readings or pressurizations rather than a high pressure and no pressure. 
     The aspect ratio of the roller  210  may also have an impact on whether the pre-aging process is successful for platen rollers, O-rings, or gaskets. The aspect ratio may utilize the length, width, thickness, and height of a roller  210  and may impact on the efficiency of the roller  210 . If the length, width, and height of the material which makes up the roller or the roller jacket  205  is not compressible, the material may not deflect. For example, if the roller jacket is a rubber sphere, which has an aspect ratio of 1 (i.e., the cross section is the same in all planes, then the sphere will not deflect because the rubber is incompressible. Accordingly, no pre-aging occurs with a rubber sphere roller jacket. A thin jacket roller  205  having a better ratio of diameter to the jacket thickness is deflectable, which allows for better radial compression while under hydrostatic pressure. Because the thin jacket roller  205  is deflectable, the pre-aging method utilizing hydrostatic pressure may improve the imaging characteristics of the roller  210 . Accordingly, the larger the cross section of the jacket is relative to the circumferential area of the material being subjected to the pressure, the less effective the pre-aging process is. In an embodiment of the invention, the roller jacket thickness is 0.062″ thick on a 0.75″ diameter roller, where the roller is 14.5″ in length. In this embodiment of the invention, the aspect ratio (i.e., the circumferential area divided by the cross sectional area) is  81  and pre-aging is effective. 
       FIGS. 3(   a ) and  3 ( b ) illustrate an embodiment of the invention for dynamic loading of a platen roller according to embodiments of the present invention. In this embodiment of the invention, the platen roller is not stationary and mechanical forces press against the platen roller  310 . The mechanical forces may be applied via multiple roller sets. Although  FIGS. 3(   a ) and  3 ( b ) illustrate a three roller set and four roller set, respectively, any number of pressuring rollers may be utilized to dynamically load the platen roller, for example, two, six, ten, or twenty pressuring rollers. The pressuring roller sets may dynamically load and unload a nip area (e.g., the jacket area) of the platen roller. This dynamic loading of the roller sets may introduce compression set and compression stress relaxation to the platen roller  310  during the time of the dynamical loading. 
       FIG. 3(   a ) illustrates a platen roller  310  and three pressuring rollers  320 ,  330  and  340 .  FIG. 3(   b ) illustrates a platen roller  310  and four pressuring rollers  350 ,  360 ,  370  and  380 . In each embodiment (i.e.,  FIGS. 3(   a ) and  3 ( b ), the platen roller  310  includes a roller jacket  305 . In  FIG. 3(   a ), the three pressuring rollers  320 ,  330  and  340  apply a pressure force, illustrated by the arrows designated F in  FIG. 3(   a ), to the roller jacket  305  of the platen roller  310 . The pressuring rollers  320 ,  330  and  340  apply the pressuring force to an outer surface of the roller jacket as portions of the outer surface pass by the pressuring rollers  320 ,  330  and  340 . In the embodiment of the invention, the platen roller  310  is rotating and the pressuring rollers  320 ,  330 ′ and  340  are not rotating. In an embodiment of the invention, the platen roller  310  may be rotating and the pressuring rollers may be rotating slightly in an opposite direction. In an embodiment of the invention, the platen roller  310  may not be rotating and the pressuring rollers  320   330  and  340  may be rotating. The platen roller  310 , if rotating, may rotate in a clockwise direction, as illustrated in  FIGS. 3(   a ) and ( b ) or may rotate in a counter-clockwise direction. The pressuring rollers  320   330  and  340  may also rotate in a clockwise direction. In the embodiments of the invention illustrated in  FIGS. 3(   a ) and  3 ( b ), the rotation of the platen roller  310  allows the pressuring rollers  320   330  and  340  to place a force against an outer surface of the roller jacket  305  in order to pre-age a material that makes up the outer surface of the roller jacket  305  and allow for no image artifacts on prints produced by the platen roller  310 . 
     In an embodiment of the invention, illustrated in  FIG. 3(   a ), the pressuring rollers  320 ,  330  and  340  may be placed an equal distant apart from each other around the platen roller  310 . Illustratively, the pressuring rollers may be placed 120 degrees away from each other on the circumference of the platen roller  310 . In alternative embodiments of the invention, the pressuring rollers  320   330  and  340  may be placed at different locations on the circumference of the platen roller  310 . 
     In embodiments of the invention where the platen roller  310  is not rotating, one or more of the pressuring rollers may be rotating at a time. In other words, in these embodiments of the invention, one pressuring roller may be rotating for two seconds, a second pressuring roller may be rotating for a second set time, a third pressuring roller may be rotating for a third set time, and a fourth pressuring roller may be rotating for a fourth set time. Likewise, pairs of pressuring rollers or triples of pressuring rollers may be rotating at the same time while the other rollers of the roller set are not rotating. 
     In the embodiment of the invention illustrated in  FIG. 3(   b ), the four pressuring rollers  350 ,  360 ,  370  and  380  apply the force to the roller jacket  305  of the platen roller. The four pressuring rollers operate in a similar fashion to the three pressuring rollers  320 ,  330  and  340  of  FIG. 3(   a ). The four pressuring rollers  350 ,  360 ,  370  and  380  contact more of the outer surface of the roller jacket  305  than does the three pressuring roller set  320 ,  330  and  340 . This may result in a quicker dynamic loading time for the roller jacket  305  of the platen roller if the four pressuring rollers  350 ,  360   370  and  380  are utilized rather than the three pressuring rollers  320 ,  330  and  340 . This may result in a shorter time for the pre-aging process. 
       FIG. 3(   c ) illustrates a fixture for applying pressure according to an embodiment of the present invention.  FIG. 3(   d ) illustrates an exploded view of the fixture for applying pressure according to an embodiment of the present invention. The fixture includes a bottom plate  391 , a top plate  395 , a bottom right vertical support plate  392 , a top right vertical support plate  393 , a bottom left vertical support plate  396 , a top left vertical support plate  397 , and a gear  390  that is driven by a motor (not shown). The fixture also operates on and includes the pressuring rollers  320   330  and  340  and the platen roller  310 . The fixture may also include a plurality of bearings  398  and a washer  399 . 
     In the embodiment of the invention, the bottom right vertical support plate  392  and the bottom left vertical support plate  396  are connected to the bottom plate  391 . The top right vertical support plate  393  and the top left vertical support plate  397  are connected to the top plate  395 . A pressuring roller  320  is connected, in one embodiment, via bearings  398  to the top right vertical support plate  393  and the top left vertical support plate  397 . The pressuring rollers  340  and  330  are connected to the bottom right vertical support plate  392  and the bottom left vertical support plate  396  via bearings  398 . The platen roller  310  fits into an opening  394  in the bottom right vertical support plate  392  and the bottom left vertical support plate  396 . One end of the platen roller  310  includes a bearing  398 , a gear  390 , and a washer  399 . The other end of the platen roller  310  includes a bearing  398 , and a washer  399 . 
     In the embodiment of the invention illustrated in  FIGS. 3(   c ) and  3 ( d ), the platen roller  310  is driven by the motor and may rotate in a clockwise or counterclockwise direction. The pressuring rollers  320  and  330 , the bottom two pressuring rollers, passively rotate on their bearings  398 . In this embodiment of the invention, the pressuring rollers  320  and  330  are not driven by a motor. In an embodiment of the invention, the pressuring roller  340 , the top pressuring roller, also passively rotates on its bearing  398 . Alternatively, the top pressuring roller  340 , which is mounted the top plate, is effectively free to slide on the guide pins and holes found in the vertical support plates. In other words, the top pressuring roller is not rotationally constrained by the bearing  398  but it is constrained in a horizontal plane by the vertical support plates  397  and  393  because the guide pins and holes allow the top right and left vertical support plates to move up and down in response to any contact from the platen roller  310 . 
     In the embodiment of the invention illustrated in  FIGS. 3(   c ) and  3 ( d ), a deadweight load is applied to the top plate  395  in order to exert a pressure on the platen roller  310 . The application of the deadweight load presses the top pressuring roller  340  against the platen roller  310  and also brings the platen roller into stronger contact with the two bottom pressuring rollers  320  and  330 . 
     In an embodiment of the invention, the deadweight load may be 10, 15, 25, 30, or 40pounds. The mass of the deadweight load is determined by knowing the pressure to be applied to the roller in order to initiate the pre-aging process and start the compression set/compression stress relaxation process. For example, the application of 25 pounds on a platen roller equates to 3.1 pounds per linear inch of the platen roller length. In an embodiment of the invention, the deadweight load may be applied for one day, one week, two weeks, eighteen days, or one month. The duration of the application of the deadweight load is determined based on the characteristics of the material of the platen roller  310  and how much weight is applied (i.e., the deadweight load). 
       FIG. 3(   e ) illustrates a change in optical density in a roller that is placed in the pre-aging process fixture according to an embodiment of the invention. The higher line represents the change in optical density for a platen roller of the prior art. The lower line represents the change in optical density after a set number of prints for a platen roller exposed to the pre-aging process in the three-roller set utilizing a fixture. As is illustrated in  FIG. 3(   e ), the change in optical density is lessened (i.e., improved) with the roller subjected to the pre-aging process in the fixture. Illustratively, at 3000 prints, the pre-aged roller has a change in optical density of 0.035 while the prior art roller has a change in optical density of 0.045. At 4,500 prints, the pre-aged roller has a change in optical density of 0.05 while the prior art roller has a change in optical density of 0.07. 
       FIGS. 4(   a ) and  4 ( b ) illustrate a pre-aging technique for flat or a complex-shaped structure or geometry according to an embodiment of the invention. In embodiments of the invention, the flat or complex-shaped structure or geometry may be an O-ring or a gasket. The O-ring or gasket may be formed of a low durometer material. The low durometer material may be, for example, Neoprene, Silicone, Urethane, EPDM, or Buna-N.  FIG. 4(   a ) illustrates a structure  405 , a first pressure apparatus  410 , and a second pressure apparatus  415  according to an embodiment of the present invention.  FIG. 4(   b ) includes a complex geometry structure  420 , a first pressure apparatus  425 , and a second pressure apparatus  430  according to an embodiment of the present invention. The structure  405  or  420  may be referred to as a O-ring or gasket, but the invention is equally applicable to other structures made of low durometer materials. The pre-aging method of the invention can also improve higher durometer materials as well. The need for pre-aging is less for higher durometer elastomers or materials in general, however improvement may sometimes be obtained. For example, SBR rubber has poor compression set properties at higher durometers and better at lower durometers. Potentially, the pre-aging technique can be applied to SBR and improve its performance at high durometers as well. 
     In the embodiment of the invention illustrated in  FIG. 4(   a ), the pre-aging technique utilizes a first pressure apparatus  410  to press against a top surface of a O-ring or gasket  405 . Alternatively, or in addition to, the pre-aging technique utilizes a second pressure apparatus  415  to press against a bottom side of the O-ring or gasket  405 . In the embodiment of the invention, the first pressure apparatus  410  and the second pressure apparatus  415  may press against the O-ring or gasket  405  simultaneously. In another embodiment of the invention, the first pressure apparatus  410  may press against the O-ring or gasket  405  first and the second pressure apparatus  415  may press against the O-ring or gasket  405  immediately after or at a predetermined time after the first pressure apparatus  415 . In other words, the first pressure apparatus  410  and the second pressure apparatus  415  may operate sequentially. In an embodiment of the invention, another pressure apparatus may press against a side surface  408  or  409  of the O-ring or gasket  405  at a different time than the first pressure apparatus  410  and the second pressure apparatus  415  press against the O-ring or gasket  405 . The pressure applied by the first pressure apparatus  410  and the second pressure apparatus  415  is illustrated by the arrows and the symbol F in both  FIG. 4(   a ) and  FIG. 4(   b ). The pre-aging technique illustrated in  FIGS. 4(   a ) and  4 ( b ) causes compression set and compression stress relaxation and accelerates the movement of the O-ring or gasket to a more stable area of the compression stress relaxation curve. In other words, the material, in the case of O-rings and gaskets, is moved on an accelerated pace to where the material is in a stable state where minimal or no material relaxation occurs, i.e., the reactive force is predictable (either flat or at a linear slope). 
     In the embodiment of the invention illustrated in  FIG. 4(   b ), because the O-ring or gasket geometry  420  is complex, i.e., it is not flat and may have different heights for different surfaces, the first pressure apparatus  425  may be a die that is formed to conform with the surfaces of the O-ring or gasket. As illustrated in  FIG. 4(   b ), the first pressure apparatus  425  may include a cutout  450  that accommodates the raised surface  435  or the O-ring or gasket  420 . This allows the first pressure apparatus  425  to press in a top surface of the complex geometry O-ring or gasket  420  in a uniform fashion. Alternatively, or in addition to, the second pressure apparatus  430  may also press against the complex geometry O-ring or gasket  420 . As illustrated in  FIG. 4(   b ), the complex geometry O-ring or gasket  420  may include a raised portion  440  on its bottom surface, and the second pressure apparatus  430  may include a cutout  455  to allow for the second pressure apparatus  430  to press in a uniform fashion against most of the bottom surface of the complex geometry O-ring or gasket  420 . 
       FIG. 4(   c ) illustrates a plurality of pressure apparatus and a complex geometry O-ring or gasket according to an embodiment of the present invention.  FIG. 4(   c ) includes a pre-aging system including a first pressure mechanism  470 , a second pressure mechanism  472 , a third pressure mechanism  474 , a fourth pressure mechanism  476 , a fifth pressure mechanism  478 , a sixth pressure mechanism  480 , and an O-ring or gasket- 485 . In this embodiment of the invention, a plurality of pressure apparatus apply a force against a surface of the O-ring or gasket. As compared to  FIG. 4(   b ), it is different because each pressure apparatus is pressing against a flat surface of the O-ring or gasket. In addition, the first pressure mechanism  470 , second pressure mechanism  472 , and third pressure mechanism  474  may press against the O-ring or gasket  485  at different times. In an embodiment of the invention, the first pressure mechanism  470 , the second pressure mechanism  472 , and the third pressure mechanism  474  may press against the O-ring or gasket at the same time but at a different time than the fourth pressure mechanism  476 , the fifth pressure mechanism  478 , and the sixth pressure mechanism  480 . In an alternative embodiment of the invention, all six of the pressure mechanisms ( 470 ,  472 ,  474 ,  476 ,  478 , and  480 ) apply force against the O-ring or gasket at the same time. 
       FIG. 5  illustrates a flowchart of a method of extending platen roller life and increasing performance according to an embodiment of the invention. A number of sheets of small format media are loaded and a number of sheets of large format media are loaded  510  into an imaging printer. In an embodiment of the invention, like the embodiment illustrated in  FIG. 4 , a roller pre-aging software program is executed  520 . In other embodiments of the invention, no roller pre-aging software program may be executed and the following steps may be performed manually. The roller pre-aging software program automatically prints  530  out a number of small format media sheets. The roller pre-aging software program may then print  540  a number of large format media sheets. The roller pre-aging software program may determine if a pre-determined time  550  has elapsed. Alternatively, or in addition to, the roller pre-aging software program may determine if a threshold number of prints have been printed  555 . If the pre-determined time  550  has elapsed or the threshold number of prints have been printed, the roller pre-aging software program may stop or may stop executing  560 . If the pre-determined time has not elapsed, the roller pre-aging software program returns to step  530  and prints out a number of small format sheets. If the pre-determined number of prints has not elapsed, the roller pre-aging software program returns to step  530  and prints out a number of small format media sheets. The roller pre-aging software program keeps operating until the pre-determined time has elapsed or alternatively, the pre-determined number of prints has been reached. 
     In an embodiment of the invention, for each iteration of the pre-aging cycle, i.e., steps  530  and  540 —printing of small format media sheets and large format media sheets, a different number of media sheets may be printed out. Illustratively, under certain operating conditions, during the first iteration, ten small format media sheets may be printed and 30 large format media sheets may be printed while in the second iteration, twenty small format media sheets may be printed and 40 large format media sheets may be printed. In an embodiment of the invention, during each iteration of the pre-aging method, the same number of small format media sheets and a different number of large format media sheets may be printed (with the different number of large format media-sheets being the same for each iteration). For example, for each iteration of the pre-aging method, 2 small format media sheets may be printed and 10 large format media sheets may be printed. 
     In an embodiment of the invention, the roller pre-aging software program may keep the ratio of small format media sheets to large format media sheets the same during each iteration so the roller pre-aging software program may print out a corresponding (in terms of ratio) small format media sheets to large format media sheets. For example, in iteration one of the roller pre-aging software program, two small format media sheets may be printed and six large format media sheets may be printed. In the second iteration of the roller pre-aging software program five small format media sheets may be printed and fifteen large format media sheets may then be printed. In this example, the ratio remains three large format media sheets to one small format media sheet. 
     In an embodiment of the invention, the media may be film for printing images. Alternatively, the media may be small format and large format bond paper. The ratio of small format to large format is established to minimize the negative effects of the compression set and compression stress relaxation of the roller. The result of the this pre-aging software program is that there is a minimal differential in material relaxation and set between the portions of the roller that are compressed with just the large format media (edges of roller) and the portions of the roller that are compressed by both the large and small format media (center of the roller). 
     Illustratively, the roller pre-aging software program may print an image on one small format media sheet followed by 2 large format media sheets, followed by two small format media sheet followed by four large format media sheets. The software program continues this printing until either a time threshold is reached, e.g., 8 hours or 16 hours, or a print threshold is reached, e.g., 5,000, 10,000, or 20,000 prints). After either the time threshold or print threshold is reached the relaxation and the compression set of the media is relatively stable and minimal changes may occur.