Patent Publication Number: US-6702195-B2

Title: Multi-layer slot coating die with selective ultrasonic assist

Description:
FIELD OF THE INVENTION 
     The present invention relates to the printing and/or copying arts. It finds particular application in conjunction with the production of photoreceptor belts for electrophotographic copiers, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications where it is desired to apply a plurality of coatings with even thicknesses to a web material. 
     BACKGROUND OF THE INVENTION 
     In the case of electrophotographic devices such as, e.g., copiers, it is known to employ a photoreceptive belt to form latent electrostatic images within the device. Belt type photoreceptors typically include a photoreceptive material applied to a polymer or other like continuous web which is moved about an arrangement of rollers. Belt type photoreceptors generally have larger photoreceptive surfaces as compared to drum type photoreceptors, and accordingly, can hold more latent images per cycle. Hence, belt type photoreceptors are often employed in higher-end electrophotographic devices or like applications where high speed is desired. 
     The photoreceptive material applied to the web may include as many as four separate layers. The four layers include: a first layer (nearest to the web) known as the undercoat layer; a second layer known as the charge generation or binder generator layer, i.e., where the charge is actually generated by converting photons into electrostatic charge; a third layer known as the small molecule transport layer; and a optional fourth or top layer (farthest from the web) known as the overcoat layer. Coating techniques suitable for applying the layers are known in the art. However, many of the previously developed techniques suffer insomuch as they only apply a single layer at a time. This is disadvantageous to the extent that the manufacturing of the photoreceptive belt then involves as many coating operations as there are layers. When coating one layer at a time, photoreceptor belt production can be undesirably time consuming. 
     Generally, uneven thickness in the layers of the photoreceptive material results in performance degradation of the belt. Accordingly, it is desired that each layer have a substantially uniform thickness across the web. For example, manufacturing specifications for the small molecule transport layer, which is typically the thickest layer, may have a tolerance of plus or minus one-half of a micron over a web that is a thousand feet long by forty inches wide. Unassisted coating techniques suffer to the extent that they cannot provide the uniformity of thickness desired. Many unassisted techniques have a limited coating thickness uniformity, e.g., in the neighborhood of plus or minus two percent. Consequently, ultrasonic assisted coating techniques have been developed which aid in achieving a uniform thickness for a coating layer. However, to date, the developed ultrasonic assisted coating techniques have been limited to applying a single layer at a time with the ultrasonic energy being introduced through the entire die or from behind the web. For multi-layer applications, such an introduction of the ultrasonic energy can have undesired effects. For example, the locationally generalized application of ultrasonic energy through the entire die may cause the layers to become undesirably intermixed, or insomuch as the ultrasonic energy is introduce from the back side of the web and has to travel through the layers, the effects may be significantly different in the various layers due to the relatively different acoustic impedances thereof. 
     The present invention contemplates a new and improved multi-layer slot coating die with ultrasonic assist and/or associated method which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, a multi-layer slot coating die is provided. The die includes a housing having a cavity therein, and a divider arranged within the cavity of the housing such that a plurality of separate channels are defined therein. The channels have elongated openings on an output side of the die from which layers of coating material are extruded, and an ultrasonic transducer is mechanically coupled to the divider. 
     In accordance with another aspect of the present invention, a method is provided for coating a web with a plurality of layers of coating material. The method includes: advancing the web in a first direction; in a second direction transverse to the first direction, extruding the plurality of layers of coating material onto the advancing web such that there is a contact interface between two of the layers; and, applying ultrasonic energy to the contact interface. 
     In accordance with yet another aspect of the present invention, a multi-layer slot coating die includes a housing having a cavity therein. The housing includes first and second portions. The first portion contacts an upstream meniscus which is formed as layers of coating material are extruded from an output side of the die onto an advancing web, and the second portion contacts a downstream meniscus which is formed as layers of coating material are extruded from the output side of the die onto the advancing web. The die also includes a divider arranged within the cavity of the housing such that a plurality of separate channels are defined therein. The channels have, on the output side of the die, elongated openings from which layers of coating material are extruded, and an ultrasonic transducer mechanically coupled to at least one of the divider, the first portion of the housing and/or the second portion of the housing. 
     One advantage of the present invention is that it provides for multi-layer coating. 
     Another advantage of the present invention is that it provides for even layer thickness via ultrasonic assistance. 
     Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. Further, it is to be appreciated that the drawings are not to scale. 
     FIG. 1 is a diagrammatic illustration showing in cross-section an exemplary multi-layer slot coating die with ultrasonic assist in accordance with aspects of the present invention. 
     FIG. 2 is a diagrammatic illustration showing in cross-section an alternate embodiment of the exemplary multi-layer slot coating die of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, a multi-layer slot coating die A with ultrasonic assist is shown applying two layers  10  and  12  of, e.g., fluid photoreceptor material or other like fluid material, to a web  14  being advanced in the direction of arrow  16  in front of the output side of the die A. As the layers  10  and  12  are being extruded or otherwise output from the die A to coat the advancing web  14 , an upstream meniscus  20  is formed, a downstream meniscus  22  is formed, and a fluid-to-fluid contact line  24  results between the layers  10  and  12 . 
     The die A preferably includes an outer housing  30  and an internal divider  32 . Together the housing  30  and divider  32  define a pair of separate cavities or channels, i.e., one for dispensing each of the layers  10  and  12 . The fluid materials which form the layers  10  and  12  are simultaneously pumped or pushed (from left to right as shown in FIG. 1) through the separate cavities or channels of the die A which are defined by and formed between the housing  30  and the divider  32 . The die&#39;s housing  30  and divider  32  are shaped and/or arranged to provide narrow (e.g., on the order of five thousandths of an inch) elongated openings for each of the cavities or channels on the output side of the die A, i.e., the right side as shown in FIG.  1 . Preferably, the elongated dimension of the openings extend along substantially the entire width of the web  14 . Accordingly, as the layers  10  and  12  are extruded from the openings, they will form thin coatings that span substantially the entire width of the web  14 . Alternately, the elongated openings may be some fraction of the width of the web  14 . However, multiple pass would then be involved if the entire width of the web  14  were to be coated with the layers  10  and  12 . Note that, in any event, the width dimension as shown in FIG. 1 is taken as the dimension normal to the x-y plane, and the narrow opening dimension as shown in FIG. 1 is taken in the y direction. Optionally, the housing  30  is made out of stainless steel or some similar metal or other suitable material. 
     Preferably, a frequency generator  40  or other like electrical oscillator generates a signal which is optionally amplified by amplifier  42 . The signal is applied to and/or drives an ultrasonic transducer  44  which is mechanically coupled to the divider  32 . The driven ultrasonic transducer  44  generates ultrasonic energy which is introduced through the mechanical coupling  46  into the divider  32  on a backside of the die A, i.e., opposite the output side of the die A. 
     The divider  32  preferably includes an inner core  32   a  to which the ultrasonic transducer  44  is mechanically coupled, and an outer covering  32   b  in between which the core  32   a  is sandwiched. As shown, the divider  32  as a whole is tapered to form a wedge shape having a relatively thick backside, and a thin output side lip which is arranged between and separates the narrow elongated openings of the cavities or channels. Preferably, the core  32   a  itself also similarly tapers from a thicker backside to a thinner output side. Likewise, preferably, each portion (i.e., the top and bottom as shown in FIG. 1) of the covering  32   b  similarly tapers from a thicker backside to a thinner output side. The core  32   a  is preferably made from a material having an acoustic speed which is faster than that of the material from which the covering  32   b  is made, and the core material and covering material also preferably have mismatched acoustic impedances. In a preferred embodiment, the core  32   a  is stainless steel which has an approximate acoustic speed of 5.5 km/sec and an acoustic impedance of about 4.5 g/(cm 2  sec), and the covering  32   b  is Teflon® (i.e., polytetrafluoroethylene) which has an approximate acoustic speed of 1.4 km/sec and an acoustic impedance of about 0.3 g/(cm 2  sec). Of course, other similar alternative materials as are suitable may be substituted. 
     The tapered shapes and faster acoustic speed of the core  32   a  relative to the covering  32   b  guide and/or focus the ultrasonic energy introduced at the backside of the divider  32  to the thin lip at the output side thereof. Additionally, the acoustic impedance mismatch tends to cause the ultrasonic energy to be reflect at the core-covering interface so that it travels through and is maintained in the core  32   a  rather than being transmitted through the covering  32   b  to the rest of the die A. 
     Without the ultrasonic energy at the output side lip of the divider  32   b , the fluid-to-fluid contact line  24  may move or drift back and forth across the face of the lip thereby creating a hydrodynamically unstable condition and potentially limiting the range of web speeds and/or fluid flow rates for which an adequate coating is obtainable. That is to say, the desired uniformity of thickness in the layers  10  and/or  12  may not be achieved, and/or acceptable operating conditions or parameters for the production of photoreceptor belts may be undesirably limited. However, guiding to and/or focusing sufficient ultrasonic energy at the lip of the divider  32   b  effectively pins the fluid-to-fluid contact line  24  to a substantially fixed position thereby stabilizing the same such that uniform thickness for the layers  10  and/or  12  is achievable. 
     With reference to FIG. 2, the upper portion  30 ′ and lower portion  30 ″ corresponding to the housing  30  are constructed and operate or function like the divider  32 . That is to say, the upper portion  30 ′ includes a core  30   a ′ and covering  30   b ′ which correspond to and function like the core  32   a  and covering  32   b , respectively. Similarly, the lower portion  30 ″ includes a core  30   a ″ and covering  30   b ″ which also correspond to and function like the core  32   a  and covering  32   b , respectively. Ultrasonic energy generated by ultrasonic transducers  44 ′ and  44 ″ is applied to the backsides of the upper and lower portions  30 ′ and  30 ″ of the housing via mechanical couplings  46 ′ and  46 ″ preferably connected to the cores  30   a ′ and  30   a ″, respectively. Optionally, as shown each transducer  44 ,  44 ′ and  44 ″ is driven by its own frequency generator  40 ,  40 ′ and  40 ″ and optional amplifier  42 ,  42 ′ and  42 ″. Alternately, a single frequency generator and/or single amplifier is used to drive all or a plurality of the transducers  44 ,  44 ′ and  44 ″. Note that, in the case of a single frequency generator or a single amplifier, optionally the device is a multi-channel device so that different signals may by be supplied to and/or drive the respective transducers  42 ,  42 ′ and  42 ″ as desired. In this alternate embodiment, ultrasonic energy is guided down the upper portion  30 ′ of the housing and/or focused at the output side thereof such that the downstream meniscus  22  is stabilized in similar fashion to the fluid-to-fluid contact line  24 . Likewise, ultrasonic energy is guided down the lower portion  30 ″ of the housing and/or focused at the output side thereof such that the upstream meniscus  20  is also stabilized in similar fashion to the fluid-to-fluid contact line  24 . Importantly, the particularities (e.g., frequency, amplitude, phase, etc.) of the ultrasonic energy applied to each of the upstream meniscus  20 , the downstream meniscus  22  and the fluid-to-fluid contact line  24  can be individually tailored as desired to best stabilize the respective surface or interface. 
     While the slot coating die A shown in the illustrated examples of FIGS. 1 and 2 is only a two layer die, it is to be appreciated that more layers are contemplated. For example, by including another divider within the housing  30 , a three layer slot coating die results. Additionally, if the additional divider is constructed and operated (i.e., has ultrasonic energy applied thereto) like the divider  32 , then the fluid-to-fluid contact line resulting between the second layer and the additional third layer would be stabilized in similar fashion to the fluid-to-fluid contact line  24 . In this manner, each additional divider provides for an additional layer such that the number of layers is equal to the number of dividers plus one. A four layer slot coating die is particularly advantageous for the production of photoreceptor belts. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.