Abstract:
A system for transferring a marking material from a ribbon to a substrate is provided. The ribbon has a marking material and a tie layer. The system includes a ribbon take-up device; a ribbon supply source that supplies the ribbon to the ribbon take-up device such that the ribbon is moved in a process direction; a first laser beam source configured to project a laser beam onto the ribbon to define an edge outline to a pattern of a marking portion of the marking material of the ribbon, the marking portion being a portion of the marking material of the ribbon that is to be transferred to the substrate, the first laser beam source being configured to create a weakening of the edge outline; and a heating unit configured to melt the tie layer of the ribbon at the marking portion, the melting taking place at a location where the ribbon is in contact with the substrate, the heating unit being configured to melt the tie layer such that the marking portion transfers to the substrate.

Description:
BACKGROUND 
     Disclosed herein are systems and methods for transferring a marking material from a ribbon to a substrate. 
     Embodiments of the disclosure are well suited for transferring thick metallic foils and other materials from a ribbon to a substrate using a digitally addressable laser. 
     SUMMARY 
     Transferring a portion of a metallic (or other) ribbon to a substrate to form a patterned metal (or other) layer of marking material is useful in various products and materials. 
     Thin continuous metal films can be transferred to substrates via thermal transfer over (TTO) printing using a thermal resist head array. An example of such a device is shown in  FIG. 1 . The head  110  is brought into contact with a thermal transfer ribbon  130 . Ribbon  130  has a geometric structure that when heated, promotes the transfer of the metal film via adhesion layers and release layers. As shown in  FIG. 1 , after head  110  selectively heats portions of ribbon  130 , a marking portion (for example, a metalized layer)  120  of ribbon  130  separates from a film  100  and adheres to a substrate  150 . The ribbon can have a structure as shown in  FIG. 2 .  FIG. 2  shows a ribbon  200  having a heat resistant coating  210 , a PET substrate  220 , a conductive release layer  230 , a vacuum metalized layer  240  and a thermoplastic tie layer  250 . 
     To obtain a metal pattern, a thermal head melts the thermoplastic tie layer creating pattern-wise adhesion to the substrate. The adhesion strength of the tie layer must be high enough to allow the metal layer to shear when the ribbon is released from the substrate. During this time, shear stresses at an adhesion edge break the edge of the metal film without compromising adhesion. This means thin metals such as aluminum are easier to use because it is very difficult to break thick metal films cleanly without compromising the adhesion. More ductile metals such as copper must be laid down still thinner than aluminum as such metals tend to not break as easily as aluminum. Typical thicknesses of the metalized layer are limited to about 0.5 microns or thinner even for the case of aluminum. Thus thin submicron continuous metal structures can be patterned over a variety of flat substrates without the need for evaporating a solvent or sintering of metal particles which is advantageous in terms of process control, thin film conductivity, and substrate compatibility. Additionally, the surface quality of these metal films is free of voids and surface roughness typically associated with inkjets and sintering. Such defects often lead to enhanced Ohmic losses due to skin conduction losses at high frequencies such as UHF RFID (300 MHz). These losses are often not detected when doing simple 4-point probe measurements. Finally, this process lends itself to very fine control over conductor linewidths at or below 100 microns. 
     Unfortunately there are many other high volume applications where thicker metal must be used for adequate performance. For example, printed circuit boards (PCB) typically use much thicker layers in order to handle high power and current requirements. Copper metal thicknesses of 25 microns are very common in PCBs to allow for adequate current supply to the whole board. Solid metal films this thick cannot be printed with TTO because there is not enough adhesion of the tie layer to shear the metal apart. 
     For many, it would be very desirable to be able to create complex printed circuit boards without needing to employ an outside board maker, but instead, quickly print and prototype single boards in-house, thus lowering the cost and turn-around time for PCB fabrication. TTO is attractive because no toxic or complex chemistry is needed to form metal patterns. However, because it requires thin metal lines (especially in the case of copper), this put limits on the design of the board as power and ground lines using TTO are too thin for most applications. 
     In order for this sector to be addressed with TTO technology, a new ribbon structure is needed that allows thicker layers of metal yet provides some of the various advantages of TTO, such as the surface smoothness quality for low resistive skin losses in the GHz frequency range. 
     Single pass imaging using rapidly addressable laser lamination is a way to perform hot transfer stamping or hot foil printing in a digital fashion at much higher speeds than can currently be done with resistive thermal heads. 
     Besides PCBs, an example of a product that can take advantage of embodiments of the disclosure is chipless RFID labels. One of the problems with the adoption of chipless RFID is that the RFID labels need to be applied and encoded (with variable data antenna structures) at high speed while the labels are being printed. Other applications of high speed foil transfer include security printing and decorative short run variable data printing at speeds greater than 0.5 m/s. 
     Embodiments of the disclosure provide a solution to the above problems. Embodiments of the disclosure apply pressure to a ribbon by pressing it between a pressure roller and a substrate and then directing a digitally addressable laser to the pressure location. Metallic foil ribbons require more energy to properly transfer to a substrate for at least the reason that metal spreads heat more quickly than many other materials, such as plastics, do. This is especially true for relatively thick metallic foils. The laser heating sources of embodiments of the disclosure provide a higher energy that provides a higher quality transfer. 
     An example of an appropriate laser for use in embodiments of the disclosure is an imaging (e.g., lithographic) apparatus including two or more spatial light modulators and associated anamorphic optical systems that modulate homogenous light and form anamorphically imaged in the process and cross-process directions, and concentrated (converged or linearly-focused) light fields in a substantially one-dimensional imaging region on a targeted scan structure (e.g., a drum roller). Each spatial light modulator (e.g., digital micromirror (DMD) devices, electro-optic diffractive modulator arrays, or arrays of thermo-optic absorber elements) includes individually addressable elements having light modulating structures that modulate (e.g., either passes or impedes/redirects) associated portions of the homogenous light according to predetermined image data. Each anamorphic optical system images and concentrates the modulated homogenous light received from an associated spatial light modulator to form an associated scan line portion, and the scan line portions formed by each anamorphic optical system collectively form the elongated scan line in the imaging region of the scan structure. Here the term anamorphic optical system refers to any system of optical lens, mirrors, or other elements that project the light from an object plane such as a pattern of light formed by a spatial light modulator, to a final imaging plane with a differing amount of magnification along orthogonal directions. Thus, for example, a square-shaped imaging pattern formed by a 2D spatial light modulator could be anamorphically projected so as to magnify its width and at same time demagnify (or bring to a concentrated focus) its height thereby transforming square shape into an image of an extremely thin elongated rectangular shape at the final image plane. By utilizing the anamorphic optical system to concentrate the modulated homogenous light, high total optical intensity (flux density) (i.e., on the order of hundreds of Watts/cm.sup.2) can be generated on any point of the scan line image without requiring a high intensity light source pass through a spatial light modulator, thereby facilitating a reliable yet high power imaging system. Furthermore, it should be clarified that the homogenous light generator, may include multiple optical elements such as light pipes or lens arrays, that reshape the light from one or more non-uniform sources of light so as to provide substantially uniform light intensity across at least one dimension of a two-dimensional light field. 
     Embodiments of the disclosure provide systems and methods of transferring at a high speed marking material from a ribbon to a substrate using digitally addressable lasers. 
     Embodiments of the disclosure address the problem associated with thick metallic layers of marking material by weakening an edge outline of the marking material before the marking material is applied to the substrate. By weakening the edge outline, i.e. creating etch edge cracks or voids that predefine a perforated outline in the continuous marking material, the portion that is to adhere to the substrate more easily separates from the remaining portion of the ribbon. 
     An embodiment of the disclosure may include a system for transferring a marking material from a ribbon to a substrate, the ribbon having a marking material and a tie layer. The system includes a ribbon take-up device; a ribbon supply source that supplies the ribbon to the ribbon take-up device such that the ribbon is moved in a process direction; a first laser beam source configured to project a laser beam onto the ribbon to define an edge outline to a pattern of a marking portion of the marking material of the ribbon, the marking portion being a portion of the marking material of the ribbon that is to be transferred to the substrate, the first laser beam source being configured to create a weakening of the edge outline; and a heating unit configured to melt the tie layer of the ribbon at the marking portion, the melting taking place at a location where the ribbon is in contact with the substrate, the heating unit being configured to melt the tie layer such that the marking portion transfers to the substrate. 
     Another embodiment of the disclosure may include a method of transferring a marking material from a ribbon to a substrate, the ribbon having a marking material and a tie layer. The method includes supplying a ribbon from a supply source to a ribbon take-up device such that the ribbon is moved in a process direction; projecting a laser beam from a first laser beam source onto the ribbon to define an edge outline to a pattern of a marking portion of the marking material of the ribbon, the marking portion being a portion of the marking material of the ribbon that is to be transferred to the substrate, the first laser beam source creating a mechanical weakening of the edge outline; and a heating unit for melting the tie layer of the ribbon at the marking portion, the melting taking place at a location where the ribbon is in contact with the substrate, the heating unit melting the tie layer such that the marking portion transfers to the substrate. 
     Some embodiments also include the heating unit and the first laser beam source being located at different locations in the process direction. 
     Some embodiments may include a pressure roll located between the ribbon supply source and the ribbon take-up device in the process direction, the pressure roll being configured to apply pressure to the ribbon at a pressure location when the ribbon is positioned between the pressure roll and the substrate. 
     Some embodiments may include the heating unit being a second laser beam source that directs a plurality of laser beams through the pressure roll and onto the ribbon at the pressure location. 
     Some embodiments may include the second laser beam source being a digitally addressable laser beam source that is stationary in a direction perpendicular to the process direction. 
     Some embodiments may include the marking material of the ribbon being a metal film that is under tensile stress when the ribbon passes by the first laser beam source, and the first laser beam source being configured to rapidly shock the metal film along the edge outline such that the marking portion separates via edge cracking of the material from a remaining portion of the marking material, the remaining portion of the marking material being that portion of the marking material that is not the marking portion transferred to the substrate. 
     Some embodiments include the metal film being greater than or equal to 0.3 micrometers thick. 
     Some embodiments include the marking material of the ribbon being a metallic paste matrix having metallic particles dispersed within a binder, the binder having a melting temperature that is higher than or equal to a melting temperature of the tie layer, and the first laser beam source being configured to melt the binder along the edge outline such that the marking portion separates from a remaining portion of the marking material, the remaining portion of the marking material being that portion of the marking material that is not the marking portion transferred to the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of process by which a transfer is made from a ribbon; 
         FIG. 2  is a schematic sectional view of an example of a ribbon in accordance with embodiments of the disclosure; 
         FIG. 3  is a perspective view of an example of a system in accordance with embodiments of the disclosure; 
         FIG. 4  is a schematic view of an example of a system in accordance with embodiments of the disclosure; 
         FIG. 5  is a schematic view of an example of a system in accordance with embodiments of the disclosure; 
         FIG. 6  is a schematic view of marking material cracking in accordance with exemplary embodiments of the disclosure; 
         FIG. 7  is a schematic view of marking material melting in accordance with exemplary embodiments of the disclosure; and 
         FIG. 8  is a diagram of an example of a method in accordance with embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Two exemplary embodiments of the disclosure will be described below. Both of the two embodiments share a common thread as they both pre-define the edge definition of the metal marking material (film) and the film is mechanically weakened at these edges before metal film transfer to the substrate occurs. 
     A base ribbon supporting material can be made from an optically clear polyethylene terephthalate PET or polyester mylar film. To this film an absorption layer can be constructed having an infrared absorbing dye or pigment. Examples include carbon black or NIR absorbing dyes that are commercially available which are predispersed in a polymer resin and coated as a thin layer on top of the top supporting film. 
     An optional release layer is one that de-bonds upon the thermal absorption of the layer and could be made from gas-producing polymers including, but not limited to, nitrocellulose materials or thermally decomposable polycarbonates deposited using various coating methods including, but not limited to, gravure. 
     An example of a thick metal ribbon material is one with CuO nano particles at approximately 80% by weight that is held together with a binding matrix of Cyclododecane wax. Such nanoparticles have reduced melting temperatures and can be engineered in formulations to be photonically cured by the pulsed forge method developed by the company Novacentrix. Such particles can be engineered to be dispersible in aqueous or solvent based solutions and supplied as pasted flexo or screen printable formulations that make them easily miscible with a variety of binding agents and capable of being deposited uniformly onto a thermal transfer ribbon. 
     The final tie (adhesion) layer which yields increased adhesion to a substrate based upon the laser exposure can be made from a hot melt adhesive or thermoplastic resin with improved adhesion upon heating. This layer can be made thin enough that it is porous and permeable and allows sublimated cyclododecane to easily permeate through it in a gaseous phase. 
     During edge weakening formation with a pulsed laser, the laser power is presented before the metal transfer ribbon is in contact with the substrate and therefore by pulsing the laser and raising its temperature along the edges of the pattern to be transferred, the cyclododecane sublimates at around 65-100 C and escapes through the porous adhesion layer, greatly weakening the edges of the metallic layer to be transferred and allowing for easier separation from the carrier ribbon. 
     Another exemplary construction would be to use a standard metallic layer on a thermal transfer ribbon such as provided by IIMAK located in Amherst N.Y. known as Metallograph® Conductive Thermal Transfer Ribbon. 
     By pulsing a laser at the edge of the metal film, a pattern can be defined before transfer by using a pulsed laser to pre-fractured or pre-shear the metal along grain boundaries according to patterning techniques using pulsed laser induced thermal elastic shock. 
     The film deposition parameters can be tuned empirically to provide built in tensile stress and grain sizes that help reduce the necessary laser power to pre-weaken the transfer pattern edges. 
     An example of the laser shock absorption method is discussed in Parallelized Laser-direct Patterning of Nanocrystalline Metal Thin Films by Use of a Pulsed Laser-induced Thermo-elastic Force ( Nanotechnology.  2009 Jun. 17; Yoo H, Shin H, Sim B, Kim S, Lee M.). Thin film patterning by the conventional lithographic technique requires a number of steps including the deposition, development, and removal of the photoresist layer. Here it is demonstrated that metal thin films evaporated on glass can be directly patterned by a spatially modulated pulsed Nd-YAG laser beam (wavelength=1064 nm, pulse width=6 ns) incident from the backside of the substrate. This method utilizes a pulsed laser-induced thermo-elastic force exerted on the film which plays a role in detaching it from the substrate. High-fidelity patterns at the micrometer scale have been fabricated over a few square centimeters by a single pulse with pulse energy of 850 mJ. This is attributed to the fact that deposited metal films are polycrystalline with nano-sized grains, and thus localized etching of the material is possible with shearing along the weakly bonded grain boundary regions. The authors have also developed a nano-block model to simulate the laser-direct patterning of nanocrystalline thin films. The patterning process presented here provides a simple photoresist-free route to fabricate metal thin film patterns on transparent substrates. 
     In a first embodiment of the disclosure, a thick tensile stressed metal film is used as the marking portion of the ribbon. This approach uses the same standard structure of a continuous metal ribbon film. However the film is put down in a tensile stress state when the ribbon is manufactured. When controlling the deposition parameters during evaporation or electroplating metallization, it is possible to control the mechanical stress of thin films. Cracked metal from electroplating problems can be a problem. By precisely controlling the metal stresses during deposition, a tensile film can be applied that while highly stressed is still below the stress threshold necessary for spontaneous cracking. 
     In order to facilitate pattern-wise edge definition of the patterned transferred film through its thickness, rapid thermal shock is applied to define patterned edges of the metal film. Rapid thermal shock can be obtained using a laser induced thermal transfer system. Typical resistive element thermal heads are limited in the amount of power they can generate over a given area without compromising the head life. Also, this thermal energy must first pass its heat through a thick carrier layer such as PET, thus diluting the strength and time-compression of the electrical pulse. 
     By pulsing lasers (for example a galvo-based fiber, solid state YAG, or CO2 laser), a rapid thermal shock can be used to pattern-wise crack a thick metal pattern along pre-defined edges where one would like the metal film to break apart. In addition, a system implementing single pass imaging using rapidly addressable laser lamination (described below) allows for power density levels at least 10× times higher than regular thermal heads and approximately 10 times lower diffusion times. 
     Once these crack edge propagation points are formed, a lower amount of laser power in a continuous wave (CW) mode can be used to activate and melt the tie layer in the middle area between these edges while the metal film is in contact with a receiving substrate. This CW mode can also activate an optional temperature sensitive release layer. Heat is confined laterally inside the thicker metal provided the mechanical cracks at the edges reduce thermal conduction laterally. Because the edges of the trace are pre-defined by higher power laser edge pulses, a thick layer of metal of a desired shape can then be transferred. Here too, a single pass imaging using rapidly addressable laser lamination approach can provide the thermal energy necessary for thicker films. 
     In a second embodiment of the disclosure, instead of a thick stressed metal film a metal (such as, for example, copper) paste matrix with metal nano-particles dispersed inside a wax or other binder is used as the marking material in the ribbon. The binder has a sublimation point that is higher than or equal to the melting temperature of the tie layer such that the matrix can be transferred onto a substrate without first fusing this layer. In particular embodiments, the middle metal layer can be structurally held in place between a top vacuum evaporated metal layer if desired to give one side higher smoothness. A waxy based binder such as cyclododecane could be used as the matrix binding agent with clean low temperature sublimation properties. 
     One approach of this second embodiment is to melt the tie layer and metal paste simultaneously such that the metal is both sintered and transferred simultaneously. Because the binder matrix has little structural integrity once melted, and re-solidified, thicker metal layers can be transferred if they fuse preferentially to the substrate and themselves. However, the waxy (or high MW solvent) binder has no path to escape under this scenario unless predetermined escape paths are patterned as this approach may require CW laser power during pressure contact with the substrate. 
     Due to the complexity of the simultaneous melting of the tie layer and the metal paste, a more controlled 2-3 step approach may be used. 
     A second approach is to first melt the binder at edges of the defined pattern with laser pulses, thus weakening the metal matrix binder along those edges. This could be done before the film is in contact with the substrate so that a path for the binder to sublimate and escape is available. These pulses would raise the temperature to melt the binder but be well below the actual sintering temperature. While these laser pulses may compromise the tie and release layers at the edges, the thermal energy is pulsed at a short enough time scale (10 s of nanoseconds) that thermal diffusion is still on the 10 micron scale so it does not spread far laterally (less than 100 microns) and compromise these layers at the center of the trace desired to transfer. 
     Next, a lower temperature CW laser step is applied to heat the bulk pattern of the defined layer for transfer in contact with the substrate. This step activates the tie and release layers. With the edge of the metal film already weakened, the thick metal matrix film will transfer completely in its desired shape. 
     The last step is to then to raise the temperature of the already transferred matrix to a high enough level that the binder matrix sublimates and the metallic particles fuse. This could be accomplished, for example, by laser sintering or using UV pulse forge techniques. 
     While this second embodiment, as compared to the first embodiment, will likely produce a metal film less than bulk density and therefore electrical conductivity, it allows more materials freedom during the manufacturing of the metal film on a carrier substrate. The film can be applied using screen printing or doctoring processes at low temperatures without any residual mechanical stress. The metallic matrix is likely to have much higher metal loading than could be obtained by inkjet, for example, and is therefore advantageous towards the creation of dense metal films. 
     As stated above, both of the two embodiments share a common thread as they pre-define the edge definition of the metal marking material (film) and the film is mechanically weakened at these edges before metal film transfer to the substrate occurs. 
     This edge weakening process utilizes laser pulses to pre-define the edges. While the tie and release layers may be compromised during this edge definition process at these edges, the thermal energy is confined laterally due to the short thermal duration of a laser pulse and its direct absorption into the metal instead of the need to be transferred thermally through the carrier substrate. 
     For each of these approaches a small amount of absorber such as carbon black can be added into the thermal release layer to assist in more efficient laser absorption at a specific wavelength. The materials and techniques envisioned in this disclosure are not be limited in any respect to specific materials or metals. 
       FIG. 3  shows an example of a system  300  utilizing single pass imaging using rapidly addressable laser lamination. System  300  transports ribbon  200  from a supply roll  310  to a take-up roll  320 . Between supply roll  310  and take-up roll  320 , ribbon  200  passes between a pressure roll  330  and a substrate  350 . Substrate  350  is the product or surface onto which the marking material is transferred. As mentioned above, an example of substrate  350  is a chipless RFID label. 
     Ribbon  200  is subjected to pressure between pressure roll  330  and substrate  350  at nip or pressure location  380 . A laser array  360  is positioned above pressure roll  330  such that laser beams  370  are projected through pressure roll  330  and onto ribbon  200  at nip  380 . In order for laser beams  370  to reach nip  380 , pressure roll  330  must be laser clear. In some embodiments, pressure roll  330  is a clear optical glass cylinder with a clear silicone outer layer. An example of appropriate lasers are arrayed DLP lasers with a resolution of 1200 dpi, a power of approximately 160 W, and wavelengths of approximately 400 nm, 975 nm or 1064 nm. Line speeds of approximately 1 m/s to 5 m/s are possible with embodiments of the disclosure. 
       FIG. 4  shows an example  400  of the first embodiment discussed above. Similarly to  FIG. 3 , the system in  FIG. 4  has a ribbon supply roll  410  that supplies a ribbon  404 ,  406  to a ribbon take-up roll  420 . The ribbon is pinched between a pressure roll  430  and the substrate  450  onto which the marking material is deposited. A laser array  460  is positioned above pressure roll  430  such that laser beams  470  are projected through pressure roll  430  and onto ribbon  404  at a nip. In order for laser beams  470  to reach the nip, pressure roll  430  must be laser clear. In some embodiments, pressure roll  430  is a clear optical glass cylinder with a clear silicone outer layer. 
     A pulsed laser  480  projects laser pulses  490  onto ribbon  404  to crack the edges of the pattern around the portion of marking material that is to be deposited on substrate  450  (“marking portion”). 
     Laser array  460  then heats the tie layer and the marking portion  408  adheres to substrate  450 . Reference number  406  represents the ribbon after marking portion  408  is removed from the ribbon. 
       FIG. 5  shows an example  500  of the second embodiment discussed above. Similarly to the system in  FIG. 4 , the system in  FIG. 5  has a ribbon supply roll  510  that supplies a ribbon  502 ,  504 ,  506  to a ribbon take-up roll  520 . The ribbon is pinched between a pressure roll  530  and the substrate  550  onto which the marking material is deposited. A laser array  560  is positioned above pressure roll  530  such that laser beams  570  are projected through pressure roll  530  and onto ribbon  504  at a nip. In order for laser beams  570  to reach the nip, pressure roll  530  must be laser clear. In some embodiments, pressure roll  530  is a clear optical glass cylinder with a clear silicone outer layer. 
     A pulsed laser  580  projects laser pulses  590  onto ribbon  502  to melt the edges of the pattern around the portion of marking material that is to be deposited on substrate  550  (“marking portion”). 
     Laser array  560  then heats the tie layer and the marking portion  507  adheres to substrate  550 . Reference number  506  represents the ribbon after marking portion  507  is removed from the ribbon. 
     Marking portion  507  is then subjected to IR laser sintering techniques or UV PulseForge® techniques as disclosed by Novacentrix of Austin Tex., for example, to raise the temperature of the already transferred matrix to a high enough level that the binder matrix sublimates and the metallic particles fuse to form the final marking portion  509 . In this example, a laser  600  projects a laser beam  610  on marking portion  507  to produce final marking portion  509 . 
       FIG. 6  shows the cracking of the edges of a metal foil in accordance with the first embodiment shown in  FIG. 4 . When ribbon  404  is subjected to laser pulses  490 , metallic layer  620  breaks at areas  640  that represent the edges of the marking portion  622 . In this example, ribbon  404  has a laser clear layer  630  and a tie layer  610  that border metallic layer  620 . 
       FIG. 7  shows the melting of the edges of a metallic matrix in accordance with the second embodiment shown in  FIG. 5 . When ribbon  502  is subjected to laser pulses  590 , metallic matrix  720  melts at areas  740  that represent the edges of the marking portion  722 . In this example, ribbon  502  has a laser clear layer  730  and a tie layer  710  that border metallic matrix  720 . 
       FIG. 8  shows an example of a method in accordance with embodiments of the disclosure. At  810  a ribbon is supplied from a supply source. At  820  a laser beam is projected onto the ribbon to define an edge outline to a pattern of a marking portion of the marking material and weaken the edge outline. In the first embodiment, this weakening is the cracking of the metal layer. In the second embodiment, this weakening is the melting of the metallic matrix. At  830  the tie layer is melted below the marking portion. At  840  the marking portion of the marking material is transferred to the substrate. Action  850  applies to the second embodiment. At  850  the temperature of the marking portion on the substrate is raised such that the binders sublimate and the metallic particles fuse together. 
     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.