Abstract:
Precision embossing press. The press includes a rigid symmetric box frame and upper and lower parallel embossing platens mounted within the box frame for movement towards one another to provide an embossing compressive force on a work piece between the platens while maintaining parallelism between the platens. A thermal system controls platen temperature to heat the work piece for embossing and to cool the work piece for de-molding. A pneumatic actuator moves the lower platen toward the upper platen to emboss the work piece. A closed loop control system employing feedback and feed forward control loops controls platen temperatures and embossing force.

Description:
[0001]    This application claims priority to provisional application Ser. No. 61/659,665 filed on Jun. 14, 2012, the contents of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to an embossing press and more particularly to a precision embossing press that can be constructed from common an inexpensive mechanical elements, but that provides high resolution motion, loading and sensing with high bandwidth thermal actuation. 
         [0003]    Polymer embossing can be used to form surface geometry in polymer work pieces. However, producing microscale parts by hot embossing is a method currently not commonly employed on an industrial scale. Achieving quality formation of microscale features at a sufficient rate has been a major difficulty. In microfluidic applications, however, embossing has been a viable manufacturing process for moderate product volumes in comparison to injection molding. 
         [0004]    A hot embossing process utilizes three primary steps: the work piece is heating, compressed between two optionally patterned platens to form a desired surface geometry and cooled and de-molded from the platen. The embossing process is sensitive to temperature and load parameters, requiring careful control of temperature and pressure. 
         [0005]    In many embossing applications, including microfluidic device manufacture, the registration of surface geometry to the work piece is a critical manufacturing feature. Thus, an embossing press must provide means for locating work pieces and provide for repeatable motion. 
       SUMMARY OF THE INVENTION 
       [0006]    The precision embossing press of the invention includes a rigid symmetric box frame and upper and lower parallel embossing platens mounted within the box frame for movement towards one another to provide an embossing compressive force on a work piece between the platens while maintaining parallelism between the platens. A thermal system controls platen temperature to heat the work piece for embossing and to cool the work piece for de-molding. An air bushing supports the lower platen for motion toward the upper platen and includes a flexural bearing to fix rotary position of the lower platen. A pneumatic actuator moves the lower platen toward the upper platen to emboss the work piece and a flexural bearing is provided having three degrees of freedom for adjusting position of the upper platen. A load cell measures embossing force and a closed loop centered system employing feedback and feed forward control loops controls platen temperature and embossing force. 
         [0007]    In a preferred embodiment the box frame deflects less hem 5 micrometers per 1,000 newtons of loading force. In this embodiment, the box frame is constructed of aluminum plate and includes individual elements connected by kinematic structures and removable fasteners. In a preferred embodiment, the thermal system heats the platens with resistive heaters and cools the platens with liquid cooling. It is preferred that platen temperature be monitored by a thermocouple. 
         [0008]    In yet another embodiment, each platen includes an insulating block, a conductive cooling block, a conductive heating block and an embossing tool bearing a desired embossing pattern. The insulating block may be made of fiberglass reinforced ceramic and the conductive blocks may be made of aluminum. The embossing tool may be made of an amorphous metal. It is preferred that the pneumatic actuator be a rubber bladder or bellows. 
         [0009]    The embossing press design disclosed herein offers several advantages over existing methods. The embossing press of the invention is designed to maintain micron-level precision during the embossing process that is simple in design and significantly lower in cost than competing systems. The press according to the invention is highly compact and presents a much smaller form factor than competing devices while still offering more than sufficient force and temperature capabilities necessary to emboss, for example, a 25 mm by 25 mm polymer substrate. The invention is also capable of micron-level repeatability. Utilizing passive kinematic alignment features and precise, completely linear motion, the embossing press of the invention achieves precision and repeatability of forming on the micron level using relatively simple structures. The system is capable of rapid heating and cooling cycles due to low thermal mass of its forming platens. The rapid heating and cooling enables low cycle times for producing single parts. 
         [0010]    An important aspect of the present design is that it can be scaled for different sized products. For example, the design can be scaled up to emboss a 55 mm by 85 mm polymer substrate (credit card size) or larger using the design disclosed herein but with suitably adjusted dimensions to allow for economical “right sizing” of the embossing press. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]      FIG. 1  is a cross-sectional view of the embossing press according to an embodiment of the invention. 
           [0012]      FIG. 2  is an exploded view of a platen stack according to an embodiment of the invention. 
           [0013]      FIG. 3  is a perspective view of a planar flex oral bearing providing three degrees of freedom according to an embodiment of the invention. 
           [0014]      FIG. 4  is a block diagram of a representative controller architecture to control embossing force and temperature. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    With reference first to  FIG. 1  an embossing press  10  includes a box frame structure  12 . The box frame  12  provides superior deflection and thermal performance while being smaller and lighter than other comparable designs. 
         [0016]    During embossing, a load most be applied to material compressed between two platens while maintaining parallelism between the platens. In a preferred embodiment, the box frame  12  is designed to deflect less than 5 microns per 1,000 newtons of loading force. Because of the symmetry of the box frame  12 , any deformation has no parasitic angular component that will affect platen alignment. 
         [0017]    The embossing press  10  must also sustain large changes in temperature during embossing and de-molding cycles. The symmetry of the box frame  12  ensures that any deformation due to thermal gradients is manifest as a linear translation between the platens and not a change in parallelism. In a preferred embodiment, the box frame  12  is constructed of aluminum plates and may be comprised of individual elements that are connected by a series of kinematic elements and removable fasteners, thereby allowing precise reassembly of the frame  12  after servicing. 
         [0018]    With reference still to  FIG. 1 , an upper platen  14  engages a planar flexural bearing  16 . A lower platen  18  is mounted on a shaft  20  and includes an LVDT  22  for measuring platen displacements. The shaft  20  is supported in an air bearing  24 . A load cell  26  measures the load force and pneumatic bellows  28  provides a force urging the upper and lower platens together to emboss a work piece therebetween. Because temperature control of the embossing process is important the thermal system disclosed herein is an important component. The thermal system includes the platens and components adjacent to the embossing work piece that are designed to beat the work piece for embossing and to cool the work piece for de-molding. Precision embossing requires that the temperature of each platen be carefully controlled while short and economical processing cycles require a fast thermal response time. 
         [0019]    In a preferred embodiment of the invention, thermal actuation at each platen is provided by resistive heaters and liquid cooling, with separate control circuits and valves (not shown) for each component. The temperature at each platen is monitored by, for example, a thermocouple. With reference now to  FIG. 2 , each platen is comprised of an insulating block  30 , a conductive cooling block  32 , a conductive heating block  34  and an embossing tool  36  with a desired embossing pattern. In a preferred embodiment, insulating materials in the insulating block  30  comprises fiberglass reinforced ceramic. The conductive blocks  32  and  34  may be aluminum and the embossing tool  36  may be made from an amorphous metal. In a representative embodiment, the insulating layer and heating blocks are each about 6 mm in thickness. In an alternative embodiment, the heating and cooling blocks  34  and  32  can be combined as one element. In yet another embodiment, the cooling and healing blocks could be interchanged or combined without changing the utility of the design. 
         [0020]    The insulating block  30  isolates the heating block  34 , cooling block  32  and embossing tool  36  from the substantial thermal mass of the press disclosed herein. This isolation permits faster heating and cooling times with smaller heaters and cooling equipment. 
         [0021]    It should be noted that a valve may be provided to purge the liquid cooling circuit with air during die heating cycle. The equivalent thermal mass of the press is much smaller when liquid is purged from the cooling block  32 , providing a faster heater response time with smaller heating elements. In a preferred embodiment, the embossing press  10  is able to heat from about 60° C. to about 150° C. in 60 seconds, and cool from 150° C. to about 60° C. in 20 seconds. With these heating and cooling times, one work piece can be processed in about 3 minutes. 
         [0022]    In a preferred embodiment, the upper and lower platen thermal systems are constructed identically for symmetric heating and cooling of the work piece during embossing and de-molding. 
         [0023]    Returning to  FIG. 1 , die top platen  14  is fixed to the top of die box frame  12 . The lower platen  18  translates vertically beneath the upper platen  14 . The hearing  24 , actuator  28  and sensor system for this translation has been designed to utilize frictionless machine elements. Frictionless design permits nominally infinite motion resolution, good heatability, and observation of very small embossing or de-molding forces. 
         [0024]    The lower platen  18  is fixed to the rigid circular shaft  20  that is guided by the air hushing  24 , The bushing  24  is rigidly fixed to the box frame structure  12 . The use of the air bushing  24  provides precise motion, high stiffness in the constrained directions, and a compact form (relative to rolling element bearings or flexural bearings). To fix the rotary position of the shaft  20  a flexural bearing is used to constrain the shaft relative to the box frame. In a preferred embodiment, the flexural bearing is a single flexure blade element. Alternative, the rotary position of the shaft  20  may be constrained by a second parallel guide shaft that travels in a second parallel air bushing (not shown). 
         [0025]    In a preferred embodiment, embossing force is provided by the pneumatic actuator  28  mounted between the circular shaft  20  and the box frame  12 . The pneumatic actuator may be a rubber bladder or bellows. For example, a rubber bladder about 100 mm in diameter can provide large forces at moderate pneumatic pressure but eliminates stick-slip friction associated with pneumatic cylinders. In an alternate embodiment, a pneumatic cylinder (not shown) may be used in place of the rubber bladder  28 . In yet another embodiment, pressurize fluid can be used to drive the actuator. Measurements conducted on a prototype embossing press according to the invention show that the rubber bladder  28  provides exceptionally linear pneumatic pressure-applied force behavior (R 2 =0.999). 
         [0026]    As mentioned earlier, the upper and lower platens need to remain parallel during the embossing process. Thus alignment is an important issue. To register the upper platen  14 , a flexural bearing  16  ( FIG. 13 ) provides three degrees of freedom, two orthogonal translations and one rotation for adjustment. Screw stops allow fine adjustment of the platen position. The flexural bearing  16  remains rigid in compression against the top of the box frame structure  12  during embossing. It is preferred that each platen  14  and  18  be fit with a thermocouple for high bandwidth, high resolution measurement of temperature. 
         [0027]    A linear variable differential transformer (LVDT)  22  is mounted between the two platens  14  and  18 . This analog sensor provides sub-micron resolution and can be installed in a contact-free configuration to eliminate friction. In an alternative embodiment, a linear encoder (not shown) may be used as a non-contact measurement technique to measure the displacement between the upper and lower platens. 
         [0028]    Force is measured using the load cell  26  mounted between the actuator  28  and guide shaft  20 . The load cell  26  is positioned far from the thermal actuators to minimize thermal drift and maintain sensor accuracy. These sensors permit accurate, high resolution measurement of processing conditions. The combination of load and displacement sensors allows monitoring the mechanical embossing and de-molding energies for the purposes of process studies (i.e., experimentation), process monitoring, and process feedback control. 
         [0029]    With reference now to  FIG. 4 , in a preferred embodiment the temperature of each platen  14  and  18  is controlled using feedback and feed forward control loops. These loops ensure reference tracking and precise processing temperatures within the accuracy and resolution of the temperature sensors. In this embodiment, the embossing force is also controlled using feedback and feed forward control loops to ensure good reference tracking and precise processing forces within the accuracy and resolution of the load cell  26 . 
         [0030]    It is recognized that modifications and variation of the present invention will occur to those of ordinary skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims such modifications and variations be included within the scope of the appended claims.