Patent Publication Number: US-2013243972-A1

Title: Needle coating and in-line curing of a coated workpiece

Description:
FIELD OF INVENTION 
     The present disclosure relates to needle coating methods for work pieces, more specifically, work pieces such as those used in creating microreplicated structures. 
     BACKGROUND 
     Roll tools and other types of work pieces are used in a variety of applications including, for example, replication and microreplication. Work pieces can be made from a variety of materials, including metals and polymers. After a work piece is formed, a desired pattern for replication, often with micron-scale features is imprinted on the surface of the work piece. Patterning can be achieved through several different methods including diamond turning, lithography, and laser ablation. For laser ablation to be successful, the surface must be an appropriate substrate for ablation. While metals and ceramics can be ablated, polymers often ablate at a more rapid pace. Polymeric substrates appropriate for ablation can be coated over metal work pieces. A need exists for additional ways to coat polymeric substrates on a work piece to create a smooth finish with even thickness. 
     SUMMARY 
     A first method, in accordance with the present disclosure, is for coating a work piece with resin. The method includes providing a generally cylindrically shaped work piece and rotating the work piece about a longitudinal axis. The method further includes providing an applicator and applying a controlled volume of liquid resin to the work piece with the applicator as a lateral location of the applicator along the surface of the work piece is shifted such that the resin is deposited on the work piece along a helical path. The resin is deposited such that consecutive rings of resin are allowed to meld together to form a self-leveling surface. The method further includes actively or passively curing at least a portion of the resin during the applying step. 
     A second method, consistent with the present disclosure, is for coating a work piece with resin. The method includes providing a generally planar work piece and providing an applicator. The method next includes applying a controlled volume of liquid resin to the work piece with the applicator as a lateral location of the applicator along the surface of the work piece is shifted such as resin is deposited on the work piece. The resin is deposited such that consecutive streams of resin are allowed to meld together to form a self-leveling surface. The method further includes actively or passively curing at least a portion of the resin during the applying step. 
     A cylindrical work piece for use in microreplication, consistent with the present disclosure, is prepared by a process as detailed below. The process first includes rotating the work piece about a longitudinal axis and providing an applicator. The process then includes applying a controlled volume of liquid resin to the work piece with the applicator as a lateral location of the applicator along the surface of the work piece is shifted such that the resin is deposited on the work piece along a helical path. The resin is deposited such that consecutive rings of resin are allowed to meld together to form a self-leveling surface. Finally, the method includes actively or passively curing at least a portion of the resin during the applying step. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system for coating a cylindrical work piece with resin. 
         FIG. 2  is a flow chart of a method for coating a work piece with resin. 
         FIG. 3  shows an applicator for applying resin to a cylindrical work piece. 
         FIG. 4A  is a top view of a curing element housing and applicator for a cylindrical work piece. 
         FIG. 4B  is a side view of a curing element, curing element housing, and applicator for a cylindrical work piece. 
         FIG. 4C  is a perspective view of a curing element housing and applicator for a cylindrical work piece. 
         FIG. 5  is an exemplary application pattern for coating a cylindrical work piece with resin. 
         FIG. 6  is a block diagram of a system for coating a planar work piece with resin. 
         FIG. 7A  is a top view of a curing element housing and applicator for a planar work piece. 
         FIG. 7B  is a side view of a curing element, curing element housing and applicator for a planar work piece. 
         FIG. 7C  is a perspective view of a curing element housing and applicator for a planar work piece. 
         FIG. 8A  is an exemplary application pattern for coating a planar work piece with resin. 
         FIG. 8B  is an exemplary application pattern for coating a planar work piece with resin. 
     
    
    
     DETAILED DESCRIPTION 
     The system and method for coating a work piece detailed in the present disclosure can, in some embodiments, create a smooth, substantially seamless surface with uniform resin thickness. In some embodiments, a metal substrate of the work piece can enhance the ablation process by acting as an etch stop. Such a work piece can be ablated rapidly and precisely. Some patterns that may be ablated onto the work piece include lenses, text, cylindrical holes, posts and any other desired feature. An exemplary system for ablating such a work piece is disclosed in United States Published Patent Application No. 2009/0127238, entitled “Seamless Laser Ablated Roll Tooling,” incorporated herein by reference as if fully set forth. Achieving precise laser ablation can enhance the performance of a work piece in processes such as microreplication for a variety of applications. 
       FIG. 1  shows a block diagram of an exemplary system  10  for coating a cylindrical work piece  42  with resin. System  10  is controlled by a computer  12 . Computer  12  has, for example, the following components: a memory  14  storing one or more applications  16 ; a secondary storage  18  providing for non-volatile storage of information; an input device  20  for receiving information or commands; a processor  22  for executing applications stored in memory  16  or secondary storage  18 , or received from another source; a display device  24  for outputting a visual display of information; and an output device  26  for outputting information in other forms such as speakers for audio information or a printer for a hardcopy of information. 
     Work piece  42  can be implemented with a metal roll or a roll made of another appropriate material. For example, work piece can include aluminum, nickel, brass, copper, steel, chrome, glass, ceramic rubber, silicones, polyolefins, pure acrylics and fluoropolymers. Work piece  42  can be formed from a substrate made of one or more materials, for example, aluminum, which is then coated with a second material, for example, nickel. In one embodiment, work piece can have a hollow center. In another embodiment, work piece  42  can be a hollow cylinder with one closed and one open end. The closed end can be used to mount work piece  42  to mount  44 . In such an embodiment, mount  44  may have only one upwardly extending arm. 
     Applicator  36  performs the coating of work piece  42 . Applicator  36  may be a flat-tipped needle or non-flat tipped needle with any appropriately shaped cross section, for example, rectangular, square, or round. In one embodiment, applicator  36  can be positioned relative to the work piece  42  by clamping it to a magnetic base and positioning it horizontally and normally to a tangent point of the work piece  42 . In another embodiment, applicator  36  may be vertical or at any other angle relative to work piece  42 . 
     Material source  30  serves as a source of resin or any other material being applied to the surface of work piece  42 . Material source  30  may be, for example, a syringe with a pump that compresses the plunger of the syringe at a specific velocity to produce a controlled volumetric flow of material through applicator  36  and on to work piece  42 . The compression speed and resulting flow rate from material source  30  can be controlled by computer  12 . 
     Material contained in material source  30  and applied to work piece  42  can be a variety of resins. Examples of polymeric materials for machining are described in U.S. patent application Ser. Nos. 11/278,278 and 11/278,290, both of which were filed Mar. 31, 2006 and are incorporated herein by reference as if fully set forth. A diamond-like-glass (DLG) coating can be used to make a durable tool from a laser ablated polymer roll. DLG is described in U.S. patent application Ser. No. 11/185,078, filed Jul. 20, 2005, which is incorporated herein by reference as if fully set forth. After a roll is patterned by any desired method, for example, laser ablation, a fluoropolymer coating can also be used to improve the durability of an ablated roll. 
     The temperature of the material or resin applied to work piece  42  can be controlled by computer  12  through temp control  32 . Temp control  32  can include, for example, a heater coupled to material source  30 . A feedback device between material source  30  and temp control  32  can allow computer  12  to control the temperature of material in material source  30  so as to maintain it at a constant level. In some embodiments, maintaining a constant temperature can result in consistent flow of material, effective melding of consecutive streams of material or resin on work piece  42 , and appropriate curing of resin after application to work piece  42 . 
     To apply resin to the surface of work piece  42 , applicator  36  can be held stationary as work piece  42  is translated and rotated with respect to applicator  36 . Drive unit and encoder  40  can control translation and rotation of work piece  42  through multi-axis stage  46 . In particular, drive unit  40 , under the control of computer  12 , rotates work piece  32  in a direction shown by arrow  48  or a reverse direction. The work piece  42 , supported on mounts  44 , can be moved in a translational direction along the z-axis or in a vertical direction along the x-axis, through control of multi-axis stage  46  by computer  12 . Work piece  42  can be gradually moved in the direction of the z-axis through the resin application process such that the resin is deposited on the work piece along a helical path. The velocity of the movement of work piece  42  in the direction of the z-axis is dependent upon a number of factors, such as the physical characteristics of the resin, the size and shape of applicator  36 , the desired thickness of the resin coating and the distance between the applicator tip and the work piece. In one exemplary embodiment, work piece  42  may be translated at a rate of one-half of the needle diameter per revolution of work piece  42 . In such an embodiment, a stream of resin can physically contact the previous stream of resin as it is being applied. Factors including flow rate of the resin and translational speed of the applicator relative to the work piece can impact the uniformity of the coating thickness. 
     Work piece  42  can be moved in the direction of the x-axis to adjust the position of the applicator along the curvature of the outer surface of the work piece  42 . For example, when the applicator is intended to be horizontal and tangential to the outer surface of the work piece, it may be necessary to occasionally adjust the work piece along x-axis. Computer  12  can determine the necessity of adjustment along the x-axis based on feedback provided by sensor  38 . Sensor  38  can be coupled to both the applicator  36  and the surface of work piece  42  to determine the relative positions and distance between applicator  36  and work piece  42 . Sensor  38  can be, for example, a capacitance sensor. Capacitance sensors can be convenient because they can be non-contact and high resolution. Other sensors that can be used include non contact optical probes, and eddy current sensors or contact probes like an air bearing LVDT, sold by Colorado Precision Instruments, Inc., of Boulder, Colo. 
     Sensor  38  can provide input to computer  12  regarding the distance between work piece  42  and applicator  36 . Computer  12  can then control the multi-axis stage  46  movement of work piece  42  until position and distance of the work piece  42  and applicator  36  are properly calibrated. Alternatively, work piece  42  can be held stationary, except for rotation of work piece  42  as illustrated by arrow  48 , while the applicator  36  translates along the work piece. In one embodiment, the calibration can be completed prior to coating work piece  42 . In another embodiment, calibration can occur during the coating process, but this requires sensor  38  to assess the relative positions of applicator  36  and work piece  42  through any coated material already applied to the work piece  42 . 
     The position and movement of applicator  36  and work piece  42  relative to each other during the application process can contribute to the evenness of the surface of work piece  42  after resin is applied. This heightens the importance of controlling the distance between applicator tip and the surface of work piece  42  and the work piece&#39;s movement relative to the applicator tip. 
     Curing unit  34  can be used to actively cure resin applied to the work piece. Curing unit  34  can be mounted so that it is horizontal and tangential to an outer surface of the work piece  42  similar to the orientation of applicator  36 . Alternatively, curing unit  34  could be oriented vertically with respect to the work piece or at any other location. Curing unit  34  can include a shield and a light or thermal source, for example, a UV LED. The specific components in curing  34  may depend upon the type of resin applied to work piece  42  and corresponding appropriate methods for curing that resin. In one embodiment, curing unit  34  can be mounted to the same base as applicator  36 , but positioned any desired distance behind it. This allows curing unit  34  to actively cure resin already applied to work piece  42  while leaving a constant margin of resin already applied, but not yet cured between the applicator  36  and curing unit  34 . 
     In an embodiment where a urethane acrylate is used as a coating resin, the resin can be cured using a UV LED, or, for example, a fluorescent lamp. A UV LED or other curing source can have uniform intensity or can have graduated intensity. For example, the curing source could contain multiple UV LEDs with graduated intensity such that LEDs located nearer the applicator  36  have lower intensity than LEDs located further from the applicator  36 . In some embodiments, such an arrangement could decrease visual artifacts of the coating process. 
     Because the curing process could involve radical polymerization, which is inhibited by oxygen, curing unit can also flood the targeted resin-coated surface of work piece  42  with a gas substantially free of oxygen and water vapor such as nitrogen from gas source  28  to purge oxygen from the surface of the resin to aid in curing. The curing unit  34  shield can substantially contain the application of gas and UV to a resin-coated area of work piece  42  surrounded by the curing unit  34  shield. Computer  12  can control the rate of nitrogen supplied to curing unit  34  and applied to work piece  42  surface along with controlling the irradiance of the LED or temperature of a thermal source in curing unit  34 . In an alternate embodiment, with resin such as a urethane or an epoxy thermosetting resin, a thermal source can be used to cure the resin or to accelerate the cure rate of the resin. 
       FIG. 2  shows a flow chart of a method for coating a work piece with resin. The method can begin with step  50 , priming the substrate, or the uncoated work piece. The surface of the work piece  42  can be cleaned, for example, with a non-shedding wiper. Primer can be applied to the work piece to increase adhesion of the resin to the work piece  42 . For example, primer sold under the trade name of ScotchPrime 389, sold by the 3M Company of Maplewood, Minn. could be used. After applying primer, appropriate finishing steps such as baking the work piece to promote adhesion of the primer to the work piece, or cleaning any excess primer off of the surface of work piece  42  can additionally be taken. 
     The details of step  52 , mounting the work piece  42 , will depend upon the actual work piece, mount, and multi-stage axis used. In one embodiment, work piece  42  may be a hollow cylinder with one closed and one open end, as described above. Such a work piece  42  can be mounted to a spindle adaptor connected to mount  44  using pins, bolts or any other appropriate fastening method. 
     Step  54 , preparing the resin and applicator, can be completed in any order relative to steps  50  and  52 . The resin can be a variety of polymeric materials as described above. Steps included in preparing the resin will depend on the specific types of resin used. Several common steps include mixing the resin, heating the resin, and allowing the resin to sit to allow air to escape the mixture. The resin can be loaded into a material source  30 , such as described with respect to  FIG. 1 . The material source can then be heated such that the resin achieves a constant stable temperature. Typically, this temperature will be above the temperature of the work piece  42 . This allows the work piece  42  to act as a heat sink when the resin is applied. In another embodiment, the temperature of the resin and the work piece  42  can be similar or approximately the same. The material source can be connected to the applicator using a feed hose with a valve or any other appropriate connection method. Prior to beginning application of the resin to the work piece, the resin can be purged by beginning resin flow. This eliminates air that may be present in the material source or applicator, and increases the temperature of the applicator or needle to operating temperature so that thermal expansion of the needle and resin viscosity can stabilize prior to initializing application to the work piece  42 . 
     Applying resin to the work piece, step  56 , involves ensuring that the applicator  36  is in a proper location relative to work piece. The gap between the two components can be set using a capacitance sensor or by any other method. The distance between the applicator and the work piece can be related to the desired coating thickness. For instance, the distance may be 10%, 50%, 100%, 500% or 1,000%, or any other desired relationship to the desired resin coating thickness. 
     Step  56  also includes setting the speeds of various system components. For example, the computer  12  may rotate the work piece, through control of the drive unit, at a speed of one rpm, two rpm, five rpm, or more or any speed in between. The translational speed of the work piece can be related to the needle diameter, for example, one-half needle diameter per revolution. Additionally, the speed with which the material source supplies resin to the applicator must also be set. This speed will also impact the thickness of the coating. For example, a material source such as a syringe pump may deliver material at a rate of about 10 cc/hour, 15 cc/hour, 20 cc/hour or any other appropriate rate. While the actual resulting resin thickness will depend on numerous factors, exemplary thicknesses can be about 5 μm, 50 μm, 100 μm, 150 μm, 500 μm or 1 mm, or any thickness in between. 
     Step  58  includes allowing resin applied to a work piece to meld, or self level. When consecutive streams or rings of resin are deposited such that they are adjacent to each other, when undisturbed, they will meld together to form a self-leveling surface. In some embodiments, it may take a single rotation of the work piece for rings of resin to meld together. In embodiments where the temperature of the resin is greater than the temperature of the work piece when the resin is applied to the work piece, the work piece can act as a heat sink. This can increase the viscosity of the resin to allow it to self-level, but not to flow to destroy the uniformity of the coating surface. For example, in some embodiments, the viscosity of the resin may be below about 2,000 centipoise (cP) when applied to a work piece, but above about 10,000 cP shortly after application. 
     Step  60  includes pre-curing the resin after consecutive streams of resin have been allowed to meld together. As discussed above, a curing unit can photolytically or thermally cure resin already applied to the work piece as the work piece is concurrently being coated. Pre-curing can be an active process. For example, a curing unit may expose leveled resin to a UV LED. The resin can also be purged with a substantially oxygen and water vapor free gas, such as nitrogen. In an alternate embodiment, pre-curing may be a passive process. In an embodiment where the resin is blended like an epoxy upon application, the resin may self-cure. The pre-cure step can cure the resin so that it will not deform with movement or light touches and is less likely to collect debris. As a result, some embodiments achieve higher levels of cleanliness for the resin and can be excimer ablated more accurately. 
     Applying  56 , melding  58 , and pre-curing  60  can happen simultaneously to different portions of a work piece. Additionally, a single work piece can be coated and cured multiple times to achieve a desired coating thickness. Once each of these steps has been performed on the coated surface of the work piece, the entire work piece can optionally be cured a second time if necessary or desired in the post cure step  62 . Post cure  62  can harden the coating of the work piece so that the surface can be successfully ablated and can provide a more robust coating for handling. Post cure step  62  may include a second UV or thermal cure, or gas purge, with the same or different sources as the pre-cure, for example, a fusion lamp or baking in an oven. Step  62  can include any other appropriate method, such as air cure with a conventional fluorescent tube type lamp system. The resin coating may shrink during the curing processes so that the final thickness is slightly less than the thickness of the resin when first applied to the work piece. 
       FIG. 3  shows an applicator  36  for applying resin to a work piece  66 . As work piece  66  is rotated in the direction of arrow  48 , or in an opposite direction, applicator  36  can apply resin to the surface of the work piece such that it is deposited along a helical path. The distance d 1  between the tip of applicator  36  and work piece  66  can be maintained substantially constant by use of a sensor and control of the movement of work piece  66  as discussed above. In one embodiment, d 1  may vary by as little as 2 μm or less during the coating process. The desired distance d 1  can be set in relation to the desired coating thickness. For example, the distance may be 10%, 50%, 100%, 500% or 1,000%, or any other desired relationship to the desired resin coating thickness. 
       FIGS. 4A-4C  are views of a curing element housing and applicator for a cylindrical work piece.  FIG. 4A  is a top view showing the relationships between the curing element housing  68 , applicator  36  and work piece  66 . The distance d 2  between the curing element housing  68  and applicator  36  can be sufficiently large to time for allow consecutive rings or streams of resin to self-level by melding together. In one embodiment, d 2  may be about 1, 2, 2.5 or 3 centimeters or any other desired distance. 
       FIG. 4B  is a side view of a curing element  70 , curing element housing  68 , and applicator  36  for a cylindrical work piece  66 . In one embodiment, the lower perimeter of the curing element housing  68  can conform to the surface of work piece  66  such that any gas contained by the curing element housing or shield that escapes negligibly impacts the curing of resin not surrounded by the curing element housing  68 . Curing element  70  can be in one embodiment, a UV LED, or a thermal source. The distance d 1  between curing element  70  and the surface of work piece  66  can depend on the intensity of the UV or thermal source in curing element  70 , desired level of curing, desired curing speed, type of resin applied to work piece  66  and along with other factors.  FIG. 4C  is a perspective view of curing element housing  68 , applicator  36  and cylindrical work piece  66 , further illustrating the relationship between these three components and distances d 1  and d 2 . 
       FIG. 5  is an exemplary application pattern for resin coating a cylindrical work piece. A portion of a work piece  72  is shown with consecutive streams or rings of resin  74  deposited on its surface. When work piece is rotated along a longitudinal axis and shifted laterally, a single continuous stream of resin deposited by an applicator can form a helical path. 
       FIG. 6  is a block diagram of a system  75  for coating a planar work piece  78 . A planar work piece  78  can be any desired shape such as a square, rectangle, circle, triangle or any irregular shape. A planar work piece  78  can be formed from any desired substrate, such as any substrate discussed above with respect to cylindrical work pieces. 
     System  75  is controlled by a computer  12 . Gas source  28 , curing unit  34 , material source  30 , applicator  36 , temperature control  32 , and sensor  38  can function in generally the same was as the same components shown in  FIG. 1 . In system  75 , instead of being mounted horizontally, applicator  36  can be mounted vertically so as to deposit resin on a relatively horizontal surface of planar work piece  78 . 
     Planar work piece  78  is mounted on multi-axis stage  76  and moved in relation to applicator  36 . Stage control unit  77  is controlled by computer  12  and directs the movement of multi-axis stage  76 . Any appropriate multi-axis stage  76  can be used in system  75 . Multi-axis stages are known in the art and include any device having multiple axes for moving a work piece in multiple translational directions with respect to a tool, in multiple rotational directions with respect to the tool, or in both multiple translational directions and multiple rotational directions with respect to the tool. A six-axis stage is possible for providing movement of a work piece in three translational directions and three rotational directions with respect to a tool. Five-axis stages are more commonly used, and five-axis stages provide for movement of a work piece in three translational directions and two rotational directions with respect to a tool. Examples of multi-axis stages, including five-axis stages, are currently commercially available from the following companies: ONA America, Inc. (US); Agie Charmilles (UK); Sodie (FR); and Mitsubishi (JP). The process of depositing the resin can involve moving the work piece  78  via the stage  76 , moving the applicator  36 , or moving both. 
       FIG. 7A  is a top view of a curing element housing  82  and applicator  36  for a planar work piece. While work piece  80  shown in  FIG. 7A  is rectangular, work piece  80  can be any appropriate or desired shape. Curing element housing  82  can be positioned above planar work piece  80  such that the perimeter of curing element housing  82  surrounds an area on the surface of planar work piece  80 . In some embodiments, this area can be cured, as discussed above, through a UV or thermal cure and by flooding the surrounded area with gas such as nitrogen. Applicator  36  can be positioned a distance of d 4  from curing element housing  82  to allow for sufficient melding of consecutive streams of resin, as discussed above with respect to d 2 . 
       FIG. 7B  is a side view of a curing element housing  82 , curing element  84  and applicator  36  for a planar work piece  80 . The distance d 3  between the tip of applicator  36  and the surface of planar work piece  80  can be determined based on factors such as those discussed with respect to d 1  above. The shape of the lower perimeter of curing element housing  82  can be substantially planar to allow minimal distance between the lower perimeter of curing element housing  82  and the surface of planar work piece  80 . This contains nitrogen gas or any other gas with which the curing element is flooded to the surrounded surface area. Curing element  84  can be any desired thermal or UV source, for example, a UV LED.  FIG. 7C  is a perspective view of a curing element housing  82 , applicator  36  and planar work piece  80 , showing an exemplary spatial relationship between each of the above three elements. 
       FIGS. 8A and 8B  are exemplary application patterns for coating a planar work piece with resin. In the embodiment illustrated in  FIG. 8A , resin  86  is deposited on work piece  80  in a spiral pattern. Such a pattern can be achieved by movement of the work piece  80 , movement of an applicator or both.  FIG. 8B  shows a back-and-forth pattern for coating a planar work piece  88  with resin  90 . In this embodiment, as in the configuration shown in  FIG. 8A , or in any other desired configuration, the resin coating pattern can be created by moving the work piece  88 , an applicator, or both. While only two patterns for coating a work piece are shown here, any desired pattern can be achieved. 
     While the present invention has been described in connection with several exemplary embodiments, it will be understood that many modifications will be readily apparent to those skilled in the art, and this application is intended to cover any adaptations or variations thereof. For example, various types of materials can be used for the work piece or for the resin, and various configurations of the illustrated systems may be used without departing from the scope of the invention. This invention should be limited only by the claims and equivalents thereof.