Abstract:
A heat stake head assembly includes a heating module including a heatable head surface, and a module head which includes a housing having a plurality of through receptacles formed between a first surface and a second surface. The module head is mountable onto the heating module such that the first surface is in contact with the heatable head surface. A set of stake tip elements are provided, each having a head end and a tip, each for fitting into one of the plurality of through receptacles such that the tips of the stake tip elements protrude from the second surface. A plurality of biasing structures bias a respective stake tip element into contact with the heatable head surface. A method for forming a stake dot pattern in a bag includes positioning a set of removable stake tip elements in corresponding receptacles in a module head in an arrangement to define the stake dot pattern;attaching the module head to a heating module with head ends of the stake tip elements biased into contact with a heated surface of the heating module, and tips of the tip elements protrude from a tip surface of the module head; positioning the bag in a flattened state on a fixture; and providing relative motion between the fixture and the head module to bring the tips of the tip elements into contact with the bag without contacting the bag with the tip surface to apply heat and pressure to form the stake dot pattern between opposed sides of the bag.

Description:
BACKGROUND 
   Fluid containment structures which generate back-pressure are used in applications such as ink-jet fluid supplies and print cartridges. A back-pressure, i.e. a negative fluid pressure at a fluid outlet, is employed to provide proper system pressures and prevent fluid from drooling from fluid outlets or fluid nozzles. There is a need for back-pressure generating mechanisms that are reliable and are cost-effective to produce. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features and advantages of the disclosure will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein: 
       FIG. 1  is an exploded view of an exemplary embodiment of a fluid supply employing a staked bag for maintaining a negative fluid pressure within the fluid reservoir. 
       FIG. 2  is an isometric view of the bag of  FIG. 1 , showing a stake dot pattern. 
       FIG. 2A  is an exploded isometric view of an exemplary bag film and fitment. 
       FIG. 2B  is a partial cross-sectional view of the bag of  FIG. 2 , taken along line  2 B— 2 B of  FIG. 2 . 
       FIG. 3  is an exploded isometric view of an alternate embodiment of a fluid supply with a bag employing an internal adhesive to create negative pressure within the fluid reservoir. 
       FIG. 4A  is an isometric view of the bag and fitment of the embodiment of  FIG. 3 . 
       FIG. 4B  is an isometric view similar to  FIG. 3 , with a side of the bag cut away to show the internal adhesive layer. 
       FIG. 5  is an isometric view of another embodiment of a bag suitable for use in a fluid supply or print cartridge, employing a solid stake pattern to create negative pressure. 
       FIG. 6  is an isometric view of a further embodiment of a bag suitable for use in a fluid supply or print cartridge, employing an adhesive dot pattern to create negative pressure. 
       FIG. 7  is a simplified isometric view of an exemplary three-chamber inkjet printhead using an expandable bag to create negative pressure in each chamber. 
       FIG. 8  is a cross-sectional view taken along line  8 — 8  of  FIG. 7 , showing the bags in an initial state after ink fill, prior to initiating printing. 
       FIG. 9  is a cross-sectional view taken along line  9 — 9  of  FIG. 7 , in the initial state and showing an exemplary stake pattern. 
       FIG. 10  is a cross-sectional view similar to  FIG. 8 , but showing the bags in partially expanded states after some printing, with the respective ink reservoirs half-empty. 
       FIG. 11  is a cross-sectional view similar to  FIG. 9 , but showing an exemplary bag in side view in a partially expanded state. 
       FIG. 12  is a cross-sectional view similar to  FIG. 8 , but showing the bags in fully expanded states at end of life for the print cartridge. 
       FIG. 13  is a cross-sectional view similar to  FIG. 9 , but showing the bag in a fully expanded state. 
       FIG. 14  is a partially-exploded isometric view of a print cartridge with a single reservoir, employing a pleated bag to create negative pressure. 
       FIG. 14A  is an isometric view of the cartridge body and lid and bag assembly of the print cartridge of  FIG. 14 , with the body separated from the lid and bag assembly. 
       FIG. 15  is a partially-exploded isometric view of an ink supply for a printhead, using a bag to create negative pressure. 
       FIG. 16  is a simplified isometric view of a plurality of ink supplies using bags to create negative pressure and a printhead structure to which the supplies are connectable. 
       FIG. 17  is a simplified isometric view of an exemplary embodiment of a modular stake dot heat assembly for fabricating negative pressure bags. 
       FIG. 18  is a reverse isometric view of the assembly of  FIG. 17 , showing an exemplary stake dot tip. 
       FIG. 19  is a cut-away side view of the assembly of  FIG. 17 . 
       FIG. 20  is an isometric view of an exemplary staking system for fabricating sacrificial bond structures for a fluid supply bag. 
   

   DETAILED DESCRIPTION 
   In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. 
   An exemplary embodiment of a fluid containment structure is for a backpressure-generating, free ink based replaceable fluid supply. In an exemplary application, the supply is used to store and supply ink for an inkjet printing system. An exemplary embodiment of a fluid supply  20  is illustrated in  FIGS. 1–2 , and includes a containment vessel  22  defining an interior fluid chamber  24 . A thin membrane bag  30  is positioned in the interior of the vessel, and is vented to the outside atmosphere through a vent hole  32 A in a plastic fitment  32  which is sealed to the bag. The periphery of the fitment  32  is sealed to a hole in the vessel wall, so that only the exterior of the bag is exposed to the interior chamber  24  of the vessel. A fluid interconnect (FI)  40 , e.g. an open foam/screen, or septum for a needle septum interface system, with a bubble screen  42 , provides fluid communication between the outside of the housing and the fluid chamber  24 . A cover  44  attaches to the vessel body  22  to seal the fluid chamber  24 . 
   The bag  30  is shown in the isometric view of  FIG. 2 . In an exemplary embodiment, backpressure for the fluid supply is generated by the bag, which in an exemplary embodiment is constructed from a single, or multilayer non-elastic film with a form factor and volume that closely match the internal volume of the fluid chamber  24 . To aid in material handling, assembly and pressure testing, the bag is constructed using the plastic fitment  32  with a through hole  32 A, which provides air communication from the external atmosphere through the hole into the interior of the bag. Then the bag  30  is substantially evacuated and fixtured, so that two of the sides are flattened together and a sacrificial stake dot pattern  36  that has been tuned to the acceptable back pressure range for the system is applied to stake the two sides together. The stake pattern bonds only the adjacent internal sides of the bag together. In one exemplary application, the stake pattern  36  comprises a pattern of dots  38  having a typical diameter of 1.0 mm to 2.0 mm, arranged on center-to-center dot spacing ranging from 3 mm to 9 mm. The stake time is on the order of one second or less, at a temperature of 175 to 210° C. These parameters are for a bag fabricated from single-layer or multi-layer polyolefin type film with low WVTR (water vapor transmission rate). An exemplary film thickness is typically 2.5 mils (0.064 mm) or less. Depending on the supply and bag geometry, this operation may be repeated on more sides. 
     FIG. 2A  shows in exploded isometric view an exemplary bag film  30 -A and fitment  32 . The bag film has a hole  30 -B punched through it, and is ready for fitment staking. In this example, the top of the fitment is to be staked to the inside-top surface of the bag film. Alternatively, the size of the hole  30 -B can be reduced, and the bottom surface of the fitment staked to the outside-top surface of the bag film. The choice may depend on the film compatibility for staking to the fitment. Some films may be balanced, i.e. the same on both sides, or unbalanced, i.e. different because of layers added for WVTR/air barrier properties, for example. 
     FIG. 2B  is a partial cross-sectional view of the bag  30 , taken along line  2 B— 2 B of  FIG. 2 , and showing bag films  33 A,  33 B comprising the bag  30 , and an exemplary stake dot  38  formed between the inner surfaces  33 A- 1 ,  33 B- 1  of the bag films. The stake dot  38  is formed to provide a relatively weak bond between the inner surfaces, which will break after a force threshold has been exceeded. 
   The fitment  32  is sealed to an interior wall of the vessel body  22 , or the cover  44 , and the remaining assembly steps are completed, including attachment of the cover  44  to the vessel body  22 , so the supply is ready for fluid fill. A fill port  26  is provided in the vessel body, through which fluid is released into the fluid chamber  24 . In an exemplary embodiment, in order to maximize the fill volume, the bag is substantially evacuated again through the fitment during the ink fill process. When the supply is full, the fill port is sealed with a seal element  28 . Initial back pressure is created by priming the supply through the FI. Since very little air is left inside the supply initially and the majority of the bag volume is restrained by the stake dot pattern, only a minor volume of fluid is extracted to create an initial backpressure in an exemplary 1–2.5 in. H 2 O range, i.e. between 248.8 Pascal (Pa) and 622.1 Pa. 
   There will inevitably be some open volume within in the bag after it is assembled to the vessel body and substantially evacuated, for example between the layers of the bag, as illustrated as volume or space  35  ( FIG. 2B ), or adjacent the fitment. To improve robustness against damage caused by dropping the supply after filling the supply and before insertion into a printing system, which might tend to break one or more of the sacrificial bonds due to the shock, e.g. during shipping, the open volume within the bag can be filled with a liquid or gel having a density similar to the fluid which fills the reservoir. For example, if the fluid reservoir holds a supply of water-based ink, the fluid filled into the bag open volume can be water. This filling can be done by a syringe through the fitment. To prevent or reduce leakage or evaporation, a labyrinth vent can be used as the vent  32 A. 
   Consider the case in which the fluid supply  20  is used as an ink supply for a printer, and the fluid is liquid ink. When the supply  20  is inserted into a printer and ink is consumed, the negative pressure inside the supply fluid chamber increases until the pressure on the bag  30  breaks one or more of the stake dots  38  restraining the bag. When this occurs, fractional volume from the bag is released, air enters this fractional volume through the vent  32 A, and the pressure drops to a lower level. Thus, volume is exchanged between the extracted fluid and the expanding bag. The restraining force on the bag due to the stake dots creates the supply backpressure. As the sacrificial stake dot bonds break, the rising backpressure is reduced. This process repeats throughout the life of the supply to keep the backpressure within an acceptable range until the bag volume is maximized. At both the beginning and end of life the supply is robust during altitude, or temperature excursions because of the fixed minimal volume of air inside the supply. 
   For an exemplary backpressure range of interest of 1–12 in. H 2 O, i.e. between 248.8 Pa and 2986.1 Pa, stakes  38  applied to the exterior of the bag only create a light bond between the inside surfaces of the bag. This is beneficial because when the stake dot bonds are broken the bag film integrity is maintained to prevent leakage. 
   In the embodiment of  FIG. 1 , backpressure in the fluid supply is generated by a sacrificial stake dot pattern applied to the outside of a bag structure comprising a bag formed from a film material and a plastic fitment. The plastic fitment serves only to seal the bag to an interior wall of the supply vessel, or the cover or lid of the supply, and to port the bag directly to atmosphere. In order to maximize supply efficiency, the fitment volume can be minimized. In other embodiments, the fitment can be eliminated altogether by attaching the bag directly to the containment vessel lid or vessel wall. 
   The embodiment of  FIGS. 1–2B  employs a negative pressure structure comprising a bag with a sacrificial stake dot pattern. Three additional sacrificial bond embodiments are shown in  FIGS. 3–6 , and respectively utilize a solid adhesive pattern applied to the inside walls of the bag, a solid stake pattern applied to the outside of the bag, and an adhesive dot pattern applied to the inside walls of the bag, respectively. 
   FIGS.  3  and  4 A– 4 B illustrate an embodiment of a fluid supply  50  employing a negative pressure bag structure  60  including bag  60 A. The supply includes a fluid vessel body  52  and a cover lid  54  which encloses an interior fluid chamber  56 . An FI  58  with a filter screen  58 A provides for fluid extraction from the fluid chamber. To provide negative pressure for the fluid supply, a bag structure  60  is disposed within the fluid chamber as in the embodiment of  FIGS. 1–2 . The bag  60 A is vented to the outside environment through a vent hole  62  formed in the vessel body, and is otherwise sealed. A sacrificial bond structure provides a relatively weak bond between opposed sides of the bag, which in this embodiment is a solid adhesive layer  66  applied to the inside walls of the sides of the bag. 
   Referring now to  FIG. 4A , the bag  60 A is sealed to a plastic fitment  64  with a through hole, which in turn is attached to the wall of the vessel body. A tubing  68  is positioned in the through hole between an opening of the bag and the vent hole formed in the vessel body to provide an open passageway between the bag opening and the external atmosphere. 
     FIG. 4B  is a simplified isometric view of the bag structure  60 , with a facing bag side cutaway to show the solid adhesive layer  66  which forms a sacrificial bond structure between the bag sides. The filling and usage of the fluid supply are as described above regarding the embodiment of  FIGS. 1–2 . Exemplary adhesives suitable for the purpose include silicone, cross-linked silicon, and acrylic based adhesives, all of which have good creep resistant properties, i.e., the ability to hold under a constant force load (below the threshold at which the sacrificial bond is to break). 
     FIG. 5  shows an alternate embodiment of a bag structure  70  which can be used as the negative pressure generating structure in the fluid supply  50  of  FIG. 3 . The bag structure includes a fitment  64  as with structure  60  ( FIG. 4A ). In this case, the sides of the bag have a solid sacrificial stake applied to the bag sides to form a sacrificial bond structure. This embodiment is similar to that of FIGS.  3  and  4 A– 4 B, except that the solid bond structure is formed by a heat stake bond instead of a layer of adhesive. In use, as fluid is drawn from the fluid chamber of the fluid supply, the bag sides will be drawn apart by the negative pressure, and the solid stake bond structure will incrementally break apart, allowing the bag sides to separate and relieve increasing negative pressure. in region  72 . In other respects, the bag structure  70  is similar to bag structure  60 . 
     FIG. 6  shows yet another alternate embodiment of a bag structure  80  which can be used as the negative pressure generating structure in the fluid supply of  FIG. 3 . The bag structure includes a fitment  64  as with structure  60  ( FIG. 4A ). In this case, the sacrificial bond structure holding the sides  82 ,  84  together is an adhesive dot pattern comprising adhesive dots  86  between the adjacent surfaces of the bag sides  82 ,  84 . In use, as fluid is drawn from the fluid chamber of the fluid supply, the bag sides will be drawn apart by the negative pressure, and the adhesive dots will incrementally break apart, allowing air to enter the bag and relieve the increasing negative pressure. In other respects, the bag structure  80  is similar to bag structure  60 . In an exemplary embodiment, the adhesive dot pattern comprises a pattern of dots  86  having a typical diameter of 1.0 mm to 4.0 mm and center-to-center dot spacing ranging from 2 mm to 9 mm. Exemplary adhesives suitable for the purpose include silicone, cross-linked silicon and acrylic based adhesives with good creep resistant properties. 
   For an exemplary backpressure range of interest on the order of 1–12 inches of water, or from 248.8 Pa to 2986.1 Pa, stakes applied to the exterior of the bag only create a light bond between the two inside surfaces of the bag, so that when they release, bag film integrity is maintained. This is beneficial because the cycle time for this stake process is minimized, requirements for the material set are reduced since additional components do not require attachment and the risk associated with ink compatibility is also reduced since the exterior of the film is not affected. Likewise, in other embodiments described above, adhesive is only applied to the inside of the bag, so similar advantages are again realized. 
   The exemplary fluid supplies described above are relatively inexpensive free-ink designs that are more efficient than foam based, or partial-foam-partial free-fluid designs. Free fluid systems also offer greater flexibility because, the physical size can be reduced due to their greater flexibility. At the time of manufacture, the supply is filled with ink so very little air is left inside the supply and the initial backpressure is created by priming the supply through the FI. This minimizes any air expansion during shipping when the supply could be subjected to altitude/temperature excursions and eliminates supplying the printheads with large volumes of air upon start-up. Since the majority of the bag volume is restrained by the stake dot pattern (tuned for a higher operating pressure range), only a minor volume of fluid must be extracted to create an initial backpressure in the 1–2.5 inches of water range, or 248.8 Pa to 622.1 Pa, dependent upon supply height. Since additional air does not accumulate in the supply throughout life, altitude/temperature robustness is maintained. 
   Exemplary embodiments provide simple, adjustable, high efficiency free-ink systems. Backpressure generation is accomplished using a simple, low cost bag assembly with one, or two components. Since the bag operates in a backpressure range suitable for most ink jet products and the form factor is easily changed, it offers extensibility to new platforms. Volumetrical efficiency of exemplary embodiments for ink supplies decreases the number of supply interventions by the customer. 
   Backpressure-generating structures described above also apply to a replaceable inkjet cartridge instead of a fluid supply. In the case of an inkjet cartridge, a printhead structure, e.g., a THA (TAB head assembly), substitutes for the FI. An exemplary embodiment of a tri-chamber inkjet cartridge  100  with a backpressure generating bag structure for each chamber is illustrated in  FIGS. 7–13 .  FIG. 7  shows the cartridge  100  in isometric view. The cartridge includes a cartridge body  110 , to which is assembled a lid structure  120 . A THA  102  is attached to surfaces of the body, and carries the printhead nozzle arrays which are fired to eject ink drops during operation. The body  110  includes interior walls  122 A,  122 B ( FIG. 8 ) which divide the interior of the body into three ink chambers  124 A,  124 B,  124 C. A feed channel with filter screen (not shown) for each chamber leads from the chamber to a printhead plenum (not shown) for delivery to a nozzle array. 
   As shown in  FIG. 8 , backpressure-generating means are provided in each ink chamber of the print cartridge. These means include, for chamber  124 A, a bag structure  130  attached to a fitment  132 , in turn attached to the lid  120 , and vented to the atmosphere through vent  136  formed in the lid and through the fitment  132 . Similarly for chamber  124 B, a bag structure  138  is attached to a fitment  140 , in turn attached to the lid  120 , and vented to the atmosphere through vent  142  formed in the lid and through the fitment  140 . For chamber  124 C, a bag structure  144  is attached to a fitment  146 , in turn attached to the lid  120 , and vented to the atmosphere through vent  148  formed in the lid and through the fitment  146 . 
   Each of the bags includes a sacrificial bond pattern, e.g. a stake pattern, between opposed sides which opposes bag opening to create negative pressure, yet incrementally releases to maintain the negative pressure in a desired range until the free ink within the chamber is substantially exhausted.  FIG. 9  is a cross-section taken through line  9 — 9  of  FIG. 7 , and shows an exemplary stake dot pattern  150  comprising stake dots  152  formed in bag structure  144 . 
     FIGS. 8 and 9  illustrate the full fluid state wherein each chamber  124 A,  124 B,  124 C is filled with fluid, and the bags are in their fully collapsed state with the stake dots intact.  FIGS. 10–11  are similar to  FIGS. 8–9 , but show the state in which the ink in each chamber has been partially depleted. Here the stake dots in an expanded portion  160  of the bags adjacent the vent have released, allowing the bag sides to open apart and for air to enter through the vent into the bag into the opened portion. The stake-dots in portion  162  of the bags have not released.  FIGS. 12–13  show the state in which the bags are fully opened. Here, all the stake dots have released, and the bag has opened to its capacity with air drawn through the vent. The ink is substantially exhausted from the chambers. Of course, it will be appreciated that the chamber depletion rates will typically vary, and the chambers may not all be depleted at the same time, for embodiments in which each compartment holds a different color. 
   Another embodiment is shown in  FIGS. 14–14A . Here, the print cartridge  170  has a single interior fluid chamber, instead of multiple chambers as in the embodiment of  FIGS. 8–13 . To provide a form factor and volume that closely match the internal volume of the single fluid chamber, a segmented, “saddle-like” bag  180  is employed. The cartridge  170  includes a body  172  which defines the chamber  174 . A lid  176  has assembled to it the back-pressure generating bag structure  180 . This bag has a generally U shape as folded into the body  172 , with a bridge portion  182 A extending along the lid, and two leg portions  182 B,  182 C connected by the bridge portion. The bag is gusseted to create the shape, with interior passageways connecting the bridge portion to each leg portion. The bag sides forming the bridge portion have a set of sacrificial stake dots, or other sacrificial bonding means, formed therein. Similarly, the bag sides forming each leg portion each have a set of sacrificial stake dots or other sacrificial bonding means formed therein. In use in a printer, with the bag in a collapsed state and the print cartridge filled with ink, the sacrificial bond patterns are all intact. As ink is ejected by the printhead on the print cartridge, ink is drawn from the ink chamber  174 , increasing the backpressure in the chamber. Eventually, the backpressure increases to a point at which sacrificial bonds are broken. This typically will first occur in the bridge portion of the bag. Air enters the bridge portion through the vent  184  formed through the lid and fitment  182 , relieving the increase in backpressure. As ink continues to be drawn from the chamber as a result of printing or printhead maintenance operations, backpressure will increase again, and the sacrificial bond structures will incrementally be broken, allowing additional air to enter the bag  180  and the leg portions while maintaining a negative pressure within a desired range, until all the bonds have been broken, and the bag has assumed its fully inflated state within the body  172 . 
   A backpressure generating structure as described above can be employed in a variety of fluid supplies and printhead arrangements.  FIGS. 15–16  illustrate a fluid supply  200  suitable for use in a “snapper” type of fluid supply/printhead system, i.e. a system which utilizes a fluid supply and printhead which reside in a carriage, i.e. “on-axis,” with the fluid supply separable from the printhead. The fluid supply  200  is shown in exploded isometric view in  FIG. 15 , and comprises a fluid vessel body  210  which defines a fluid chamber  212 . A lid  220  is attached to the body  210  to enclose the fluid chamber. A fluid interconnect (FI)  204  provides a means to pass fluid through the body from the fluid chamber. The FI in this exemplary embodiment comprises a septum which has a slit through which a hollow needle can be passed to allow fluid communication. A backpressure generating structure  230  is attached to the lid in this exemplary embodiment, and includes a bag structure  232  having an open end attached to a fitment  234 . The fitment is attached to the lid, and includes a vent  236  which passes through the lid  220  to allow communication between the external environment and the interior of the bag. A sacrificial stake pattern  238  is formed in the bag as described above, and includes a plurality of stake dots  240 , which weakly bond interior side surfaces of the bag together. 
     FIG. 16  shows a printhead structure  250  which includes mounting stalls  260 A– 260 D for a plurality of replaceable fluid supplies  200 A– 200 D. The fluid supplies may, for example, hold cyan, magenta, yellow and black inks, respectively. Fluid interconnects  262 A– 262 D respectively provide fluid communication to the fluid supplies to feed ink to printhead arrays (not shown) on the printhead structure  250 . Each of the fluid supplies  200 A– 200 D includes a backpressure generating structure as shown in  FIG. 15 . 
   Referring now to  FIGS. 17–18 , an exemplary embodiment of a modular stake head  300  is illustrated, which can be employed to create a sacrificial stake-dot pattern for a backpressure generating bag assembly, as illustrated above in  FIGS. 1–2 , for example, for a free-ink fluid supply or print cartridge. Depending on the product form factor, different bag geometries may be utilized to maximize the delivered volume. With each new bag geometry, the stake-dot position relative to the fitment and bag folds, the stake-dot spacing and the bond diameter will all affect the pressure required to break the sacrificial bonds. By using a modular stake head with removable stake-dot tip elements, pressure characterization for different bag geometries, stake-dot bond diameters and individual dot positions can all be accomplished quickly and cost effectively, compared to making multiple dedicated geometry stake heads. 
   Exemplary embodiments of a modular stake head enable the use of replaceable stake-dot tip elements while maintaining planarity across them when the head is fully populated. A problem associated with using a modular stake head is how to eliminate the tolerance stack-up between the retaining feature of each tip element, and the corresponding surfaces in the modular stake head. This variation causes two problems which alone, or combined, affect accurate pressure characterization of the stake-dots created on the bag. First, each tip element is preferably constantly biased against the heated surface to create uniform heat transfer and a consistent temperature. Secondly, inconsistent tip element height produces inconsistent heat transfer to the bag. By utilizing compression springs in an exemplary embodiment to bias each tip element against the heated stake head surface  312 , the tolerance stack-up is eliminated, and the planarity across all stake-dot tip elements is directly related to the overall length tolerance specified for each of them. 
   The modular stake head assembly  300  includes a generic stake head heating module  310 , which houses standard electrical resistance heater elements and thermocouple control circuits (not shown in  FIG. 17 ). The assembly  310  is connected to a source of electrical power, for powering the heater elements and control circuits. The heating module  310  includes a planar mounting face surface  312 . The heating module  310  thus provides a surface  312  and a means for heating the surface. 
   The assembly  300  also includes a stake-dot module head  320 , which includes a grid  322  of through hole openings or receptacles  324  formed therethrough for receiving stake-dot tip elements and corresponding bias springs. For clarity, only a single stake-dot tip element  326  with its spring  328  is shown in exploded fashion in  FIG. 18 . Some of the receptacles of the grid may be vacant for a particular application, depending on the shape and size of a particular bag, although all openings may receive a tip element in many applications. This embodiment of the module head  320  includes a planar mating surface  330  and an oppositely facing tip surface  332 . 
   After loading the desired stake-dot tip elements to produce a given stake-dot pattern, and their corresponding springs, into the appropriate through hole openings  324 , the modular stake-dot head  320  is attached to the heating module  310 , e.g. using threaded fasteners. The respective mating surfaces  312 ,  330  of the generic head module  310  and the module head  320  are ground flat when manufactured to maintain planarity and provide effective heat transfer between the heated surface  312  of the heating module and the module  320 . In an exemplary embodiment, the face  330  of the module head  320  is equipped with two recessed areas  334 ,  336  where each column and row of stake-dot positions are marked with a letter and number, respectively. As stake-dot tip elements are loaded, this facilitates recording which positions are being used for an experiment, or which ones are needed for different types/sizes of bags. 
     FIG. 19  is a partially-broken-away side view of an exemplary embodiment of the module head  320 . As shown therein, each stake-dot tip element  326  with its spring  328  is fitted into a through hole or receptacle  324  formed through the head housing  320 A. The receptacle diameter is stepped to form two shoulders  324 A,  324 B. Shoulder  324 A provides a stop surface for the spring. The shoulder  324 B is defined by a counterbore to provide clearance for the spring  328  and the head  326 B of the stake-dot tip element within the housing  320 A. The tip end  326 A of each tip element protrudes from surface  332  of the housing  320 A, and comes into contact with the material to be staked during a staking procedure. The tip end  326 A is sized to provide a tip surface diameter to define a stake dot of a desired dimension. The head portion  326 B in this exemplary embodiment has a diameter larger than the tip end, and is biased against the heated surface  312  of the heating module  310  when the module head  320  is assembled to the module  310 . (In  FIG. 19 , the spring  328  is shown in its compressed state, and the tip element  326  in position as though the module head  320  were assembled to the heating module  310 .) The tip elements  326  have a length greater than the depth of the head housing  320 A, so that, with the head portions  326 B in contact with the heated surface  312 , the tips  326 A of the respective tip elements protrude from the surface  332 , and serve as stand-off elements, spacing the surface  332  away from the material to be staked. Thus, only the tips  326 A of the tip elements are brought into contact with the material to be staked during a heat staking operation, so that the heat staked areas are defined by the tip elements. 
   In order to easily align the stake-dot pattern to the bag, the module head  320  is equipped with two alignment holes  342 ,  344 . Referring now to  FIG. 20 , these holes  342 ,  344  mate to precision dowel pins  352 ,  354  extending from an alignment fixture  350 . The alignment fixture has a lower set of dowel pins, including pin  356 , which in turn mate to alignment holes  362 A,  362 B in a lower tooling plate  360  that fixtures the bag. The lower tooling plate is in turn fastened to a vacuum plate  370  by a set of fasteners  372 . The vacuum plate is mounted on a horizontal slide assembly  380  which can move the lower tooling plate in a horizontal plane or axis. The lower tooling plate and vacuum plate are mounted through four clearance holes  374  with fasteners (not shown) so the fasteners can be loosened, the fixture  350  inserted into both plates and the fasteners re-tightened. Thus, to accurately position the stake head  320  to the lower tooling plate, the head  320  is lowered by hand and the tooling plate assembly is floated into position so that the lower dowel pins  356  engage holes  362 A,  362 B in the tooling plate. The fasteners  374  are then secured, and the alignment fixture  350  is removed. 
   A bag/fitment assembly is placed on the lower tooling plate  360  and vacuum is applied through the vacuum plate  370 , which secures the bag in place for subsequent operations. An opening  376  is formed in the tooling plate  360  to provide a relief recess for the bag fitment, so that the top portion of the bag will lie flat when vacuum is applied. The fitment may also be connected to a vacuum line to evacuate the bag, so that it will lie flat during the stake process. Evacuating the bag during the stake process may be omitted, e.g. when the bag is not pleated. Evacuating a pleated bag may be used to assist in holding the bag flat during the stake process. The horizontal slide brings the bag assembly forward in line with the head  320 , at which time the vertical slide brings the stake head  320  down, bringing the tip elements into contact with the bag, to stake the bag at the desired force/pressure. After the staking operation, the vertical slide is retracted, followed by the horizontal slide to allow for removal of the finished bag and subsequent staking of a new one. 
   In an exemplary embodiment, the stake-dot tip element length is controlled to within a tolerance of ±0.001 inch (0.0254 mm) which translates into overall planarity when all tips are inserted equal to ±0.001 inch (0.0254 mm), which are standard machined tolerances that still provide sufficient precision without adding significant cost. 
   To ensure uniform heat transfer and expansion, the housings of the heating module  310  and module head  320 , and the stake-dot tip elements are all fabricated from the same material. Exemplary materials with good heat transfer properties such as aluminum and copper are suitable for these structures. 
   Exemplary embodiments of the modular heat staking system allow cost-effective, rapid-prototyping and pressure characterization for different bag designs and stake-dot patterns. The modular approach enables the user to quickly characterize individual stake-dot positions, groups of stake-dots, or produce a complete pattern on multiple bag geometries. If a different stake-dot size is desired, new sets of tips are easily produced with different end diameters. Otherwise, dedicated one-piece stake-dot heads would have to be fabricated to test each different combination, adding significant development time and cost. The modular approach is also extensible to long-term manufacturing, since the replaceable stake-dot tip elements can easily be replaced as they wear out. 
   Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims.