Abstract:
A machine and a method of applying a non-Newtonian liquid composition onto a surface in a controlled manner. The composition is held in a chamber at a controlled variable pressure and is dispensed through a slit die nozzle as controlled by a valve. Characteristics of the composition are empirically developed and provided to a logic control circuit to assure that the composition is dispensed on either the entire surface or in one or more precise locations.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional Application No. 61/319,960, filed Apr. 1, 2010. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to precision fluid application systems. 
     BACKGROUND 
     Applicator systems are used in a wide variety of manufacturing operations to apply compositions such as glue. For example, glue application systems are used in bottle labeling, can labeling, corrugated box making, manufacturing disposable diapers, and manufacturing other products that involve the application of glue. 
     Hot melt glue is one type of glue that is particularly well suited for applications where it is desired to apply glue at a high rate of speed. Applicants&#39; assignee, B &amp; H Manufacturing Company, Inc., provides labeling machines with a glue application system in which a glue wheel is provided with hot melt glue that is applied to a knurled roller and scraped off with a doctor blade in a process that is comparable to a gravure printing process. 
     While this system generally provides a reliable system for applying glue to labels at a high rate of speed, issues remain that relate to precise control of glue quantity, consistency of glue properties and minimizing the volume of glue required to secure a label. In addition, if the system is not properly set-up and maintained, glue splatter may result in application of glue to the vacuum drum that supports labels and the surrounding area. The removal of glue splatter may require periodic cleaning with solvents and may result in machine downtime. Re-circulation of large volumes of hot melt glue from the glue wheel type applicator results in repeatedly reheating a substantial quantity of the glue which may adversely affect glue properties. In addition, exposure of the glue to air on a glue wheel may result in degradation of the adhesive properties of the glue. 
     One alternative approach is disclosed in US2008/0014344A1 that suggests spraying a container with hot melt glue to adhere a leading end of a label from a cut and stack label magazine or a roll fed labeler to the container. Hot melt glue was proposed to be wiped onto the trailing edge of the label by a slit die nozzle that directly contacts the label to apply glue to the trailing edge that is then attached to the container or the label. A problem with this approach is that glue is extruded from the nozzle and “waits” on the head until the glue is smeared onto the trailing edge of the label material. A lack of control over the quantity of glue laid down on the label, or lay down weight, results in variation in the glue lay down weight as labeling speeds increase. This system fails to provide a method of controlling the lay down weight at variable speeds and is believed to have been limited to systems that apply labels at a rate of no more than about 300 containers per minute. When the slit die nozzle for hot melt glue application directly contacts the label, particularly with thinner labels, the label may be wrinkled or otherwise distorted. Another problem with this proposed approach is that the spring that is used to contact the slot gun with the trailing edge pad does not compensate for increases in centrifugal forces as labeling speeds increase which results in an increase in the contact force that may damage the trailing edge pad. Another problem is that may arise from variation in lay down weight is that the label may become jammed in the machine or otherwise misapplied to the container. 
     In other approaches, glue is provided to the slit die nozzle at a pressure that is controlled with the objective of maintaining a constant pressure level. However, as a valve is opened and closed to apply the glue an uneven distribution of glue is applied to the label. Another strategy for applying glue is to control the volume of glue applied by changing the speed of the glue pump in an effort to maintain a constant pressure. One problem with this approach is that it may result in an intermittent pattern of pressure spikes and thickness variation in the adhesive deposits. Another problem with this approach is that the quantity of glue applied is subject to variation. 
     Continuous application of an adhesive to a web of a plastic substrate in the manufacture of adhesive tape or pressure sensitive label material may be performed with a slit die coating nozzle that is controlled by controlling the volumetric flow of adhesive to a web that is continuous and moving at a constant rate of speed. However, volumetric control does not yield consistent and reliable application of glue with a controlled thickness when the speed of application changes. The glue is a compressible non-Newtonian liquid which renders volumetric control unreliable because it does not compensate for internal friction, the Reynolds number for the passages, changing viscosity and changes in shear strength caused by the speed of flow of the glue. 
     Others have proposed various solutions to improve upon the performance of the glue wheel approach. It should be understood that all alternative prior art approaches are not attempted to be described above. Applicants&#39; development addresses the above issues and other issues relating to applying hot melt glue or otherwise extruding a non-Newtonian fluid onto a substrate through a nozzle. Applicants&#39; development may be adapted to a wide variety of applications that are not limited to container labeling applications or the application of hot melt glue. 
     Some aspects of the developments are summarized below in greater detail. 
     SUMMARY 
     According to the present invention, an applicator is disclosed that is capable of accurately applying a consistent volume of a non-Newtonian liquid composition with consistent thickness to a substrate at a widely varying rate of speed of the extrusion. A non-Newtonian liquid composition is a liquid that does not flow in the same way as a Newtonian fluid, such as water. More particularly, the viscosity of non-Newtonian fluids is not independent of shear rate. As a result, viscosity is not constant which complicates controlling flow volumetrically. The disclosed application system variably controls pressure based upon the temperature, speed of extrusion and the flow characteristics of the composition. Pressure is modulated at a given temperature of the composition as a function of the speed of the extrusion to obtain desired target deposit thickness and consistent lay down weights. 
     Examples of non-Newtonian liquid compositions include liquid plastic compositions such as polymer solutions, or molten polymers like hot melt glue. A selected liquid plastic composition is applied under pressure onto a surface by way of a nozzle. The liquid plastic composition may be applied without touching the surface with the nozzle. The adhesive is extruded in a controlled volume and at a consistent thickness in at least one predefined region on the surface. 
     Another aspect of this development relates to the concept of controlling the pressure of the liquid plastic composition supplied to an extrusion nozzle. Pressure is controlled based upon empirical data relating the volume of a specified composition applied over time. A controlled volume of the composition is applied to a surface through a nozzle. The pressure of the composition within the nozzle is closely controlled based upon the rate at which the composition is dispensed from the nozzle. 
     Application of the adhesive to a label, or segmented substrate, is based upon controlling the sensed pressure of the adhesive in the applicator, and label segment position data. The sensed pressure in the applicator and the output of a positional encoder are inputs for a servo motor controller for a fluid pump. A valve controls the timing of the deposit of adhesive onto the surface. Data from a look up table, or data matrix, may be provided to the processor to vary the quantity of adhesive based upon known or estimated values for the rate of application and pressure in the applicator. According to the method, a consistent volume of adhesive is applied to the label, or segmented substrate, surface in precise locations with consistent thickness. 
     The pressure of the liquid plastic composition may be modulated at the inlet of the slit die nozzle. The system may be adjusted by referencing a data table for a particular liquid plastic composition having certain flow properties. A self-adjusting system may be provided for applying a deposit of liquid plastic composition with a uniform thickness to a surface. According to the method, changes in the speed of intermittent extrusion do not cause variation in the thickness of the deposits. 
     The slit die nozzle in the disclosed system may be oriented with the slit of the nozzle being elongated in the vertical, horizontal or in any other orientation. 
     Another aspect of the disclosure is related to providing a labeling machine that has a rigid all metal vacuum drum that has rigid metal pads at selected locations on a vertically oriented surface of the vacuum drum. The rigid metal pads are radially adjustable to provide precise spacing relative to a vertically oriented slit nozzle adhesive applicator. 
     Another aspect of the disclosure is to provide a roll fed labeler that is capable of reliably applying a hot melt adhesive having a viscosity of more than 1,300 centipoises per second (cps). Hot melt adhesives having a viscosity of more than 10,000 centipoises have been successfully applied that offer favorable adhesive properties but were previously not considered to be usable in high speed labeling operations. The applicator facilitates the use of more economical glues that may have improved adhesion and strength properties. 
     Other aspects of applicants&#39; development as disclosed herein will be apparent to one of ordinary skill in the art of labeling containers or manufacturing products that require precise application of adhesives. More generally, the applicants&#39; developments may permit more effective and efficient application of a liquid plastic composition by providing a controlled volume of the composition having the desired strength characteristics for a specific application. 
     One aspect of this invention is to provide a machine that applies a non-Newtonian liquid composition in a controlled pattern of deposits to a surface. The composition may be hot melt glue or may be another type of liquid plastic composition that must be carefully applied in a predetermined pattern or in a full coating to a surface. The machine in the illustrated embodiment is a labeling machine but the technology may also be used in other machines that apply a non-Newtonian liquid composition to a surface. 
     In one example of a machine for applying a non-Newtonian liquid composition, hot melt glue may be applied to a label segment. The composition is received through an inlet and is dispensed through an outlet of a nozzle that deposits a controlled layer of the composition on the surface as the surface moves relative to the nozzle. A valve has an inlet that receives the composition from a pressurized source and is provided to control dispensing the composition through the nozzle outlet. A pressure transducer measures the pressure of the composition at the valve inlet and generates a signal representative of the pressure. A temperature sensor measures the temperature of the composition and provides a temperature signal. The machine has a logic circuit that controls the operation of the valve and an output that controls the pressure. The logic circuit uses inputs such as the signal representative of pressure, temperature of the composition, the location of the surface, the speed that the surface is moving relative to the nozzle to apply the composition in a controlled manner, and a fluid flow profile that provides for a given temperature and number of surfaces receiving a layer or the containers per minute determines the output that controls the pressure. 
     In another example of the machine for applying a composition, the machine, as described above, utilizes inputs that relate to the characteristics of the composition that include correlation data that correlates the speed of movement of the surface with the pressure of the composition. 
     Another aspect of the machine for applying a liquid plastic composition to a surface, the chamber may have a return conduit that provides a return flow path from the chamber to a reservoir. The return conduit may have a fixed metering orifice that partially restricts the flow of the composition through the return conduit. The metering orifice facilitates control of the pressure of the liquid plastic composition. A limited volume of adhesive is returned to the adhesive reservoir through the orifice, which is located downstream of the nozzle. 
     The invention may also be characterized as a method of dispensing a liquid plastic composition onto a surface. The method includes the steps of selecting the composition to be dispensed through a nozzle while the surface moves relative to the nozzle. The speed of the surface is determined, correlation data is referenced and variable pressure levels are used to control the rate of displacement of the composition from the chamber. The pressure applied to the composition is measured and compared to the variable pressure value level to adjust the rate of displacement of a supply pump for the composition. A deposit of the composition is intermittently released from the nozzle by a valve that is timed based in part upon the speed with which the surface moves relative to the nozzle, a data matrix of the fluid flow characteristics including the rate of fluid flow within a range of temperatures and a range of pressure levels. 
     The method of the present invention may also include empirically testing the composition to develop a pressure look-up table that includes correlation data for a plurality of speeds and a plurality of pressure set-points. Alternatively, correlation data may be provided from a pressure look up table that is generated by an algorithm. The algorithm for controlling pressure may use a direct feedback system that measures the actual thickness of the deposits. The direct feedback system may incorporate a laser measurement apparatus. 
     These and other features, objects and aspects of the invention will be better understood in view of the attached drawings and the following detailed description of the illustrated embodiments of the invention provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a container labeling machine that is provided with a liquid plastic composition applicator; 
         FIG. 2  is a fragmentary enlarged view of the portion of the labeling machine of  FIG. 1  taken at the circle  2  in  FIG. 1 ; 
         FIG. 3  is a fragmentary enlarged view of the portion of the labeling machine of  FIG. 1  taken at the circle  3  in  FIG. 2 ; 
         FIG. 4  is a perspective view of a glue applicator and a vacuum drum; 
         FIG. 5  is an elevation view of the glue applicator and the vacuum drum; 
         FIG. 6  is a fragmentary enlarged view of the portion of the glue applicator and the vacuum drum of  FIG. 5  taken at the circle  6  in  FIG. 5 ; 
         FIG. 7  is a high-level system flowchart depicting the controls and processes by which liquid plastic composition is applied to discrete labels by way of a slit die glue applicator; 
         FIG. 8  is a flowchart depicting an empirical data acquisition process whereby data can be collected for use in controlling the application of liquid plastic composition to labels by way of a slit die glue applicator; 
         FIG. 9  is a front view of a discrete label with a first glue deposit and a second glue deposit applied thereto; 
         FIG. 10  illustrates a chart generated using empirical data acquired by way of a data acquisition process similar to that described in the flowchart of  FIG. 8 . 
         FIG. 11  is a perspective view of a subassembly of a further embodiment of a vacuum drum, depicting radially adjustable leading edge and trailing edge pads; 
         FIG. 12  is a top view of the subassembly shown in  FIG. 11 ; 
         FIG. 13  is a fragmentary enlarged view similar to that of  FIG. 3 , but showing a portion of the vacuum drum subassembly shown in  FIG. 12 , and adhesive being applied near the leading edge of a label; and 
         FIG. 14  is a fragmentary enlarged view, similar to that of  FIG. 13 , but with adhesive being applied near the trailing edge of a label. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed below with reference to the drawings. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring to  FIG. 1 , a labeling machine  10  is illustrated to show one example of a manufacturing process that may benefit from Applicants&#39; developments. It should be understood that other manufacturing or packaging systems that include non-Newtonian liquid composition application operations may also benefit from the method and apparatus disclosed in this patent application. 
     In the illustrated labeling machine  10 , a web of label material  12  is provided in the form of a roll  16  to labeling machine  10 . A stream of containers  18  is provided to the labeling machine  10  on an in-feed conveyor  20 . The containers  18  are fed to a star wheel  22  that picks up the containers  18  and spaces the containers  18  from each other for labeling. An idler wheel  26  cooperates with the star wheel  22  to ensure proper positioning of the containers  18  in the star wheel  22  as the containers  18  are moved by the star wheel  22  toward the vacuum drum  28 . A liquid plastic composition, for example, hot melt glue or another adhesive is applied to the label  30 . 
     The star wheel  22  feeds the containers  18  to a location adjacent to a vacuum drum  28 . The vacuum drum  28  holds one or more discrete labels  30  after they are cut off from the web of label material  12  by a cutter assembly  32 . The vacuum drum  28  holds the labels  30  in position as the labels are transferred by the vacuum drum  28  past a hot melt glue applicator  34 . Turning now to  FIG. 7  for illustration, the glue applicator  34  is supplied with hot melt glue from a glue pump  36 . The glue pump  36  is precisely controlled by a servo amplifier  38  that is connected by a servo motor  76  through a gear reducer  40  to the glue pump  36 . Depending upon the requirements of the manufacturing or labeling system, glue is applied to the label  30  near the leading edge  54  and trailing edge  56  (see, for example,  FIG. 9 ), on the entire surface of the label  30 , or in discrete deposits. Application of glue to the entire surface, as would be used to make a pressure sensitive label, is not illustrated because it would cover the entire surface and would appear the same as the label  30  shown in  FIG. 4 . 
     Returning to  FIG. 1 , the illustrated embodiment is a roll fed labeling machine  10 . However, it should be understood that Applicants&#39; developments are potentially applicable to labeling machines that use cut and stack labels, pressure sensitive labels, or other types of labels. In addition, Applicants&#39; developments may be applicable to general manufacturing operations or other applications such as corrugated box manufacture where glue is applied to a surface. 
     Referring to  FIGS. 2 and 3 , an interface between the glue applicator  34  and vacuum drum  28  will be explained in greater detail. The glue applicator  34  dispenses glue from a slit die nozzle  44 . The slit die nozzle  44  has an elongated slit  45  that extrudes the composition to be dispensed. The slit  45 , as illustrated, is vertically elongated to apply a layer of the composition to a label  30  while the label  30  is held on the vertical, cylindrical side of the vacuum drum  28 . Precise control of the pressure of the composition results in consistent application of the composition regardless of the orientation of the slit die nozzle  44 . 
     Glue is provided to the slit die nozzle  44  through a glue chamber  46  defined within the glue applicator  34 . Alternatively, the chamber  46  could be eliminated and the glue could be supplied to a valve, such as a solenoid valve  50 . A pressure transducer  48  is provided to measure the pressure of the glue in the glue chamber  46  or upstream from the valve  50 . The glue chamber  46  may be in the nature of a rigid manifold. The pressure transducer  48  is a high temperature pressure transducer that is capable of measuring the pressure within the glue chamber  46  or upstream from the valve  50 . The glue applicator  34  includes a plurality of solenoid valves  50  that function as on/off control gates for the hot melt glue. The solenoids  50  are controlled by a programmable logic controller (PLC)  66  that receives position data from the encoder  74  and pressure inputs from the pressure transducer  48 . The encoder may be, for example, a linear or rotary encoder which allows a processor to determine the position of a label as the label is moved relative to the nozzle  44  of a glue applicator  34 . The label is preferably maintained in a spaced relationship relative to the nozzle  44 . Control of the glue application operation will be described more specifically below. 
     Referring to  FIG. 3 , the glue applicator  34  and the slit die nozzle  44  are shown in conjunction with the vacuum drum  28 . Two labels  30  are partially shown as they are supported on the vacuum drum  28 . An adhesive deposit  52  is shown near the trailing edge  56  of one label  30 . In phantom lines, a glue deposit  52  is shown where it will be placed near the leading edge  54  of a second label segment  30 . A bead of glue  58  is extruded from the slit die nozzle  44 . As the label  30  with the glue deposit  52  shown in phantom lines passes by the slit die nozzle  44 , the bead of glue  58  contacts the label  30  near the leading edge  54  and the bead of glue is applied as an extruded film on the label  30 . 
     Referring to  FIG. 4 , the glue applicator  34  and vacuum drum  28  are shown in isolation. The spacing between the nozzle  34  and the vacuum drum  28  is held constant. Generally, the spacing between the nozzles  34  and the label  30  should be between 0.0005 inches 0.002 inches. 
     Pressure is maintained within the glue chamber  46 , in part, by providing a fixed metering outlet orifice  96  (see, for example,  FIG. 7 ) in association with an outlet port of the glue chamber  46  through which a small portion of the glue is returned to the glue reservoir  100 . The fixed metering outlet orifice in one embodiment is 0.015 inches in diameter. However, it should be understood that the size of the fixed orifice may be changed. The glue applicator  34  is mounted to a positioning slide  60  that is used to set the spacing between the glue applicator  34  and the vacuum drum  28 . A positioning lock  62  is provided as part of the positioning slide  60  that functions to lock the glue applicator  34  in a desired location on the positioning slide  60 . 
     Referring to  FIGS. 5 and 6 , the glue applicator  34  is precisely located relative to the vacuum drum  28  so that a gap  64  is maintained between vacuum drum  28  and the slit die nozzle  44 . 
     Referring now to  FIG. 7 , a high-level system flowchart is depicted including the processes by which hot melt glue is applied to discrete labels  30  by way of glue applicator  34 . A human-machine interface (HMI)  68  provides the operator the ability to input, for example, the selected label length  82 , the selected positions and widths  84  of the glue deposits on the label  30 , and the selected glue recipe  80 . The HMI  68  may include a label/glue image  78  as part of a graphical user interface (GUI). The appropriate glue application pressure and timing are then controlled by the applicants&#39; method without further operator input, regardless of the line speed (containers per minute) selected by the operator. 
       FIG. 9  provides a depiction of a processed label  114 , which includes a first glue deposit  110  and a second glue deposit  112 , each having been applied to label  30  at positions along the longitudinal direction  108 . Label  30  is not shown to scale. Label  30  has a length  102 , a leading edge  54  and a trailing edge  56 . The first glue deposit has a first glue deposit width  104  and the second deposit has a second glue deposit width  106 . If desired, the complete label can be covered by a glue deposit. 
     Returning now to  FIG. 7 , a rotary encoder  74  reads the position of the cutter assembly  32  while the labeling machine  10  is in operation. A single 360-degree rotation of the axis of the cutter assembly  32  translates into the length of a single label  30 . The position data from the rotary encoder  74 , and the selected label length  82  and selected glue positions and widths  84 , are fed into a glue extrusion timing  88  portion of a programmable logic controller (PLC)  66 . The glue extrusion timing  88  determines and sends the appropriate on/off commands to the glue valves  50 . 
     The pressure within the glue chamber  46  is tightly controlled by way of a feedback controller  92  established in the PLC or other computing device. Feedback controller  92  operates within the PLC  66  or other computing device, and runs a control loop in which the pressure set point  94  is determined by matching an RPM calculation  90  with the corresponding pressure listed in RPM versus pressure tables  86 . The RPM calculation  90  relies on the rotary position data from the rotary encoder  74 , and is therefore dependent upon the operating speed of the labeling machine (in containers per minute). The RPM value with a rotary cutter assembly  32  that cuts one label per rotation is the same as a value of the number of substrates to which the glue is applied per minute. The particular RPM versus pressure table from which the pressure set point  94  is selected is generally unique to the selected glue recipe  80 . The RPM versus pressure tables  86  are typically generated prior to the operational use of the labeling machine by way of an empirical data acquisition process  72 , and generally uploaded into the HMI  68 . 
     The pressure transducer  48  reads the pressure in the glue chamber  46 . In systems that do not have a chamber, the pressure transducer  48  would read the pressure at the inlet of the valves  50 . The feedback controller  92  compares the pressure set point  94  to the pressure measured by the pressure transducer  48 . If the measured pressure is below the pressure set point  94 , the feedback controller  92  will send commands to the servo amplifier  38  to increase the speed of the servo motor  76  in the hot melt unit  70 . Increasing the speed of the servo motor  76  increases the speed of the glue pump  36  and, consequentially, increases glue pressure downstream in the glue chamber  46 . Conversely, if the measured pressure is above the pressure set point  94 , the feedback controller  92  will send commands to the servo amplifier  38  to decrease the speed of the servo motor  76 . Decreasing the speed of the servo motor  76  reduces the speed of the glue pump  36  and, consequentially, decreases the glue pressure downstream in the glue chamber  46 . 
     Turning now to  FIG. 8 , an example of an empirical data acquisition process  72  is depicted in greater detail. The empirical data acquisition process  72  may be run for one or more different glue recipes. The glue recipe is selected, and the glue nozzle temperature and spacing of the gap  64  are set. The labeling machine  10  is then initially set to a lowest line speed, typically sixty containers per minute (CPM), and the glue chamber  46  is set to an initial pressure. With the labeling machine running, a test deposit of glue is extruded onto a label  30 . The label with the glue deposit is then weighed, and the known weight of the bare label  30  is subtracted to determine the weight of the applied glue deposit. Using the known density of the glue recipe, and the weight, width and length of the glue deposit, the applied glue thickness is determined. 
     If the applied glue is determined to be below desired thickness, the pressure in the glue chamber  46  is increased, and the process returns to the extrusion of another test deposit of glue onto a label. If the applied glue is determined to be above desired thickness, the pressure in the glue chamber is decreased, and the process also returns to the extrusion of another test deposit of glue onto a label. 
     An algorithm may be used with a CCD laser measurement apparatus to either dynamically set the pressure in the chamber or develop a table of pressure values based upon available inputs. The laser measurement apparatus may measure the thickness of the deposit layer that is then used in a direct feedback system. 
     If the applied glue is neither below nor above the desire glue thickness, a determination is then made as to whether the longitudinal position of the glue deposit (for example, along longitudinal direction  108  in  FIG. 9 ) has deviated compared to the position of the deposit applied with the previous line speed setting. If such a deviation has occurred, an extrusion timing versus RPM compensation adjustment is made to the glue extrusion timing  88  portion of the PLC  66  (see  FIG. 7 ), and the process  72  returns to the extrusion of another test deposit onto a label  30  without increasing the machine speed, in order to verify the accuracy of the compensation adjustment. 
     Once glue deposit positional deviation is no longer occurring as a result of line speed change, the RPM (derived from rotary encoder  74  data) versus the glue chamber  46  pressure is recorded. If a single label segment is cut per RPM, there is a direct relationship of the number of cutter RPM to the number of substrates that are provided with glue per minute. If maximum desired line speed has not been reached, the line speed is increased by one increment, and the process returns to the extrusion of a test deposit of glue onto a label. If maximum line speed has been reached, the empirical data acquisition process  72  is generally terminated with respect to the selected glue recipe. The records of RPM versus pressure are then generally arranged in look-up tables corresponding to each glue recipe tested, and input into the HMI  68  (see  FIG. 7 ) as RPM versus pressure tables  86 . 
     In further embodiments, the empirical data acquisition process  72  for each glue recipe is attempted at lower glue temperatures. Applicants have determined that applying glue to labels at lower temperatures preserves the adhesive properties of the glue by, for example, minimizing temperature-related breakdown of the adhesive chemistry. Since lowering the glue temperature at the slit die nozzle  44  is likely to affect the flow properties of the glue, lowering the glue nozzle temperature also tend to change the results of the empirical data acquisition process  72  for a given glue recipe. As a result, the RPM versus pressure tables  86  for each glue recipe may depend on the operating temperature of the slit die nozzle  44 . 
     Applicants have found that the plotting of the RPM versus pressure data for a given glue recipe at a constant temperature and gap distance commonly results in a curved graph similar to that shown in  FIG. 10 . The curve tends to vary primarily depending upon the glue recipe and slit die nozzle temperature. Further, some of the applicants&#39; tests have shown that, in some instances with particular glues, such curves can be approximated as a straight line with a constant slope without significantly impacting the results of the glue application method at varying line speeds. For example, applicants have performed tests using one particular glue, Henkel  132 A, in which they were able to calculate appropriate glue chamber pressure setpoints for the control loop using the formula: T=(A−80)*0.6+p, where “T” was the pressure setpoint, “A” was the target line speed (containers per minute, starting at 80 CPM), and “p” was the initial pressure (50 psi). 
     As illustrated in  FIGS. 11 and 12 , certain embodiments may include a vacuum drum  28  with an alternative subassembly  116  which includes leading edge pads  118  and trailing edge pads  120  that are each independently radially adjustable to ensure the consistency of the gap  64  spacing between the slit die nozzle  44  and the vacuum drum  28  including the leading and trailing edge pads  118  and  120 . The version of the subassembly  116  illustrated in  FIGS. 11 and 12  provides for a three-station vacuum drum in which the three label stations  122  are generally defined by the three cylindrical arcs each beginning on a leading edge pad  118  and ending on a trailing edge pad  120 . 
       FIG. 13  depicts the application of a first glue deposit  110  near the leading edge  54  of a label  30 . The leading edge pad  118  is shown exposed from the top to illustrate a typical position of the leading edge pad  118  relative to the label  30 . Similarly,  FIG. 14  depicts the application of a second glue deposit  112  near the trailing edge  56  of a label  30 . The trailing edge pad  120  is also shown exposed from the top to illustrate a typical position of the trailing edge pad  120  relative to the label  30 . The precise control of the system enables application of layers of a non-Newtonian composition at any selected location or over an entire surface. The thickness of the glue deposits are substantially constant at any speed that the surface is moved by the machine. As used herein, the term “substantially constant” should be understood to be subject to a degree of variability but to be within about 10% of the thickness of the glue deposit. 
     While exemplary embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of the various illustrated embodiments and other described variations may be combined to form further embodiments of the invention.