Abstract:
An apparatus for heat staking utilizes an infrared lamp to direct radiant energy onto a plastic part to heat and so soften it prior to the staking punch impacting the part. The apparatus comprises an energy directing means for concentrating the infrared energy onto the part, and a moveable carriage for moving the punch toward and away from the part. One embodiment of the energy concentrating means is a reflector, wherein the reflector includes a central aperture for admission of the part, and wherein the reflector comprises different curved sections for concentrating the energy over the surface of the part. In an alternate embodiment, the energy directing means comprises fiber optic cables for directing the infrared energy to the surface of the part.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 09/531,543, filed Mar. 20, 2000 and now U.S. Pat. No. 6,296,470. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to heat staking machines for joining parts together, and more particularly to a device for use on such a machine and having a radiant heat source to heat and thereby soften the part to be deformed. 
     Heat staking is a process for permanently joining first and second parts at one or more discrete points marked by a plastic protrusion, hereinafter referred to as a stud, which extends upwardly from the first part and through an aperture in the second part when the second part is placed over the first part. The stud is sufficiently long to provide a volume of thermoplastic material which extends beyond the upper surface of the second part. Therefore, the plastic stud is heated until it is plastically deformable and then flattened and flared out with a metal punch to form a rivet-like head which locks the two parts together. 
     It is possible to accomplish the heating and the deforming of the stud simultaneously by heating the punch prior to bringing it into contact with the stud, the punch transferring its heat to the stud to soften it as it is being shaped. In such an operation, the punch is typically resistance heated by electrical current. Heat staking machines operating in this manner are disclosed in U.S. Pat. Nos. 4,767,298 and 5,227,173. 
     Another known heat staking technique is to heat the stud prior to it being contacted by the punch. In the past, this has been achieved by blowing hot air over the stud. U.S. Pat. No. 5,018,957 discloses a staking machine using electric heaters to generate the hot air and blowers to circulate the hot air over the stud. In some manufacturing operations, this pre-impact heating of the stud has been found to be advantageous in that it minimizes the amount of residual stress in the deformed stud after it has cooled. In the past, however, the apparatus necessary for the heating and circulation of air has resulted in a relatively large and mechanically complicated machine. Also, such a machine is relatively energy inefficient in that a large percentage of the heat generated is not transferred to the stud but rather is wasted. Moreover, the heat may be damaging to elements, such as printed circuits, on the parts being joined. 
     It is therefore desirable to provide a heat staking machine that is energy efficient and that is simple and compact in construction, and which overcomes the problems associated with prior devices. 
     SUMMARY OF THE INVENTION 
     The present invention addresses and solves the above-mentioned problems and meets the enumerated objects and advantages, as well as others not enumerated, by providing an apparatus for heat staking in which the stud is heated by precisely focused infrared energy. The apparatus comprises a housing for holding the apparatus and for defining a cavity which can be placed in such a position as to substantially surround the stud. An infrared energy source is affixed to the housing with an energy directing means for directing the energy to the stud for the purpose of softening the stud. A deforming tool, hereinafter referred to as a punch, is mounted on a moveable carriage and designed for movement relative to the energy source toward and away from the stud. 
     A preferred embodiment of the present invention hereinafter described utilizes at least one broadband incandescent lamp as the infrared energy source. This lamp is preferably a halogen lamp. The energy directing function is performed by one or more reflectors which are preferably gold plated to provide preferentially high reflectivity of infrared, thus increasing the percentage of total energy produced which reaches the stud. 
     In a specific embodiment comprising primary and secondary reflectors, the secondary reflector is segmented to direct infrared energy to different areas of the stud, thus distributing the infrared energy over a larger area of the stud and reducing the time required to produce the softened state. 
     In an alternate embodiment, the energy directing means includes a lens for focusing the energy into fiber optic cables. The fiber optic cables are arrayed around the stud, whereby the energy is directed from the cables onto the stud. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent by reference to the following detailed description and drawings, in which: 
     FIG. 1 is a side elevation view of a staking device according to a first embodiment of the invention with a staking punch in a retracted position; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a side-elevation of the heat staking device of FIG. 1 with the staking punch in the extended position to contact a workpiece; 
     FIG. 4 a  is a side elevation view of a secondary reflector in the heat staking device of FIGS. 1-3; 
     FIG. 4 b  is a side elevation view of a secondary reflector in the heat staking device of FIGS. 1-3 with a plurality of curved sections for directing the energy onto a stud; 
     FIG. 5 is a view of the primary reflector/punch assembly of the heat staking device of FIGS. 1-3; 
     FIG. 6 is a view of the body assembly portion of the heat staking device of FIGS. 1-3; 
     FIG. 7 is a partial side view of a second embodiment of a heat staking device according to the present invention; 
     FIG. 8 is a cross-section view taken along line  8 — 8  of FIG. 7; 
     FIG. 9 is a partial side view of a heat staking device according to another embodiment of the invention; 
     FIG. 10 is a bottom view of the heat staking device of FIG. 9; 
     FIG. 11 is a side view of another embodiment of the present invention during the stud heating cycle; 
     FIG. 12 is a side view of the heat staking device of FIG. 11 in a raised position; 
     FIG. 13 is a side view of the heat staking apparatus of FIGS. 11-12 during the staking stroke; and 
     FIG. 14 is a schematic view of a heat staking apparatus using fiber optic cables to focus the energy. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1-3, a heat staking device  10  according to the present invention is shown positioned above first and second workpieces  12 , 14  which are to be joined. As is well known in the heat staking art, a boss or stud  16  formed of a thermoplastic material such as ABS plastic projects upwardly from the first workpiece  12 , passes through a hole  18  formed in the second workpiece  14  and extends above the second workpiece  14  to provide a volume of deformable plastic material. The stud  16  is deformed into a fastener head utilizing heat staking device  10  in the manner to be described below to secure the first and second workpieces  12 ,  14  together. 
     The heat staking device  10  comprises a hollow cylindrical body  20 , an assembly  22  having a cylindrical portion which is received within the hollow interior of the body  20  and a parabolic portion which surrounds an incandescent lamp  40  and defines a primary parabolic reflector  38 . An end cap  24  attaches to the body  20  and defines a secondary reflector to receive radiation from the primary reflector  22  and directs it to the strut  16  where it is located within the end cap  24 . In essence, the primary reflector  38  captures radiation emitted radially from lamp  40  and directs it axially toward the end cap  24 . The secondary reflector defined by the end cap redirects the radiation radially inwardly toward a stud  16  protruding through the aperture in the end of cap  24  to heat and soften it. 
     As shown in FIGS. 2 and 4 a,  the body  20  and end cap  24  are circular in cross section. The lower end of end cap  24  has a polished interior surface  48  with an axis of symmetry  50  oriented vertically as shown in FIG. 4 a.  A central aperture  52  is formed at the vertex of the cap  24  and is sized to allow stud  16  to protrude upwardly therethrough as seen in FIG. 1. A cylindrical rim  54  extends upwardly from secondary reflector  48  and has an annular shoulder  56  immediately adjacent the upper edge of secondary reflector  48 . 
     As seen in FIGS. 1,  2  and  5 , the assembly  22  further comprises a punch  42   a  having legs  42   b  straddling the lamp  40  and connected to a carriage  42  for vertical sliding movement relative to the lamp  40  and the primary reflector  38 . As shown in FIG. 5, the carriage  42  moves between the retracted position, shown in solid lines, and the extended position shown in phantom lines. The punch head  42  is shaped in this case like an inverted cup to define the desired shape of the stud  16  after deformation. The punch head  42   a  and legs  42   b  are typically cast from a suitable metal and the contact surface may be cast, engraved or embossed to impart any desired design or logo to the finished plastic fastener formed from stud  16 . Punch legs  42   b  are connected at the top by plate  42   c.    
     The incandescent lamp  40  is preferably a 100 watt halogen lamp which produces substantial radiant energy in the infrared band, and is hereinafter referred to as an infrared lamp  40 . The adjustable carriage  42  is selectively driven toward and away from the stud  16  by a drive piston  28   a  of an air cylinder  28 . The infrared lamp  40  projects through a round opening  44  at the vertex of primary reflector  38  (see FIG.  2 ). 
     The body portion  20  which holds the reflector/punch assembly  22  and the end cap is shown in FIG.  6 . The body portion  20  comprises a generally cylindrical housing  26 , an air cylinder  28  mounted to an upper end of the housing and is supplied with air pressure through hoses  30 , a hollow receptacle  32  at a lower end of the housing, and electrical connectors  34  at an upper end of the receptacle  32 . Electrical power is supplied to connectors  34  through a power cord  36 . 
     In an alternative embodiment, the secondary reflector  48  comprises a plurality of connected curved sections  49  with a central aperture  52 , as shown in FIG. 4 b.  The secondary reflector  48  concentrates the light directed from a primary reflector  38  onto the stud  16 . The plurality of curved sections distributes the concentrated light over the portion of the stud  16  inserted into the cavity through the aperture  52 , rather than concentrating the light on a single smaller area of the stud  16 . This provides for a more rapid and more even distribution of energy to the portion of the stud  16  inserted into the cavity, and therefore a more rapid softening of the stud  16 . In another alternative, the secondary reflector  48  may comprise a single nonparabolic curved section for distributing the concentrated light over the portion of the stud  16  inserted into the cavity. 
     To assemble the heat staking device  10  from the three components shown in FIGS. 4-6, the primary reflector/punch assembly  22  is inserted upwardly into receptacle  32  in the bottom of the body portion  20  so that the infrared lamp  40  makes contact with electrical connectors  34  and carriage butt plate  42   c  contacts a drive piston  28   a  of air cylinder  28 . Air cylinder piston  28   a  preferably has a magnet  64  at its lower end which magnetically engages the butt plate  42   c  of the carriage  42  so that when the piston  28   a  returns to the retracted position it carries the carriage  42  along with it. This magnetic connection provides for superior field servicing of the heat staking device  10 , as there is no mechanical connection which must be disconnected before disassembling the heat staking device  10 . Although the magnetic connection is preferred, any means to create a detachable mechanical connection is contemplated to be within the scope of this invention. Alternatively, a spring (not shown) may be provided to return the punch  42  to the retracted position when air cylinder piston  28   a  is withdrawn. The end cap  24  is then fitted over the lower end of the body portion  20  such that the outer rim of the primary reflector  38  is seated on shoulder  56 . The end cap  24  and body portion  20  may be secured together by a friction fit with a detent at the fully seated position, or rim  54  of the end cap  24  may have female threads formed on its inner circumference which mate with male threads formed on the lower end of the body portion  20 . An O-ring  58  may be provided around the body portion  20  to achieve a moisture-tight seal with the end cap  24 . 
     As seen in FIG. 1, the workpieces  12 , 14  are supported on top of a lower platen  60  of a staking machine, and the heat staking device  10  is attached to an upper platen  62  of the staking machine. Upper and lower platens  60 , 62  are vertically movable relative to one another so that the heat staking device  10  is movable between a lowered position wherein stud  16  projects through aperture  52  in the end cap  24  (as shown in FIGS. 1 and 3) and a raised position (not shown) wherein the stud is withdrawn from the aperture  52 . 
     In operation, a staking cycle begins when the workpieces  12 ,  14  are positioned directly below the heat staking device  10  and the device is moved to a lowered position shown in FIG.  1 . The lamp  40  is energized and the radiation emitted thereby is directed downwardly by the primary reflector  38 , collected by the concave inner surface of the secondary reflector  48 , and focused radially inward onto the stud  16 . The lamp  40  is energized for a length of time sufficient to heat the stud  16  to a temperature at which it is plastically deformable. The required heating time depends upon the power output of the lamp  40  and the type and color of the plastic being heated. Using a 35 watt lamp  40  and white ABS plastic, for example, it has been found that it takes approximately 15 seconds to heat the stud  16  to 350-400° F., the temperature at which it may easily be formed. Darker colored plastic will heat up more quickly. In a preferred embodiment, the energy source is a 100 watt halogen lamp. The halogen lamp  40  produces energy across a broad band including the infrared, and heats the plastic to the desired temperature rapidly. 
     Once the stud  16  is sufficiently softened, the lamp  40  is de-energized and the air cylinder  28  is actuated so that the drive piston  28   a  is extended to drive the carriage  42  downwardly, urging the punch  42   a  into contact with the stud  16  and deforming the stud as shown in FIG.  3 . The stud  16  is deformed into a fastener head to secure the first and second workpieces together. Punch  42   a  preferably has a highly reflective surface finish so that it remains relatively cool. Accordingly, contact between the punch  42   a  and the stud  16  causes the stud  16  to quickly cool and resolidify so that it retains its deformed shape when the air cylinder drive piston  28   a  is retracted and the carriage  42  and punch  42   a  return to their raised position. 
     Rather than completely de-energizing the lamp  40  prior to actuation of the air cylinder  28 , it may be advantageous instead to reduce the electrical voltage supplied to the lamp  40  to a low level. This keeps the lamp  40  filament somewhat warm between heating cycles so that the lamp  40  can quickly return to the desired operating temperature when full power is reapplied. 
     It should be noted that lamp  40 , primary reflector  38 , and secondary reflector  48  are oriented so that nearly all of the output of the lamp  40  is collected by the secondary reflector  48  and is concentrated onto the stud  16 . Accordingly, there is very little undesirable and wasteful heating of the structure of the heat staking device  10  or the surface of the first workpiece  12  surrounding the stud  16 . 
     The concave inner surfaces of the primary reflector  38  and secondary reflector  48  are highly reflective of the wavelengths of infrared radiation emitted by lamp  40 . It has been found that a polished aluminum or stainless steel surface has desirable reflective properties. The secondary reflector  48  may be machined from a billet of aluminum or stainless steel, with the complex shape of the concave inner surface being formed by a computer numerically controlled milling machine. Preferably, a layer of gold is deposited on the surfaces of the primary reflector  38  and the secondary reflector  48 . The gold is deposited by dip-plating, electroplating, or by any means that deposits a thin layer of gold on the reflectors  38 ,  48  surfaces. Preferably, the gold is deposited only on the surfaces of the reflectors  38 ,  48 , but in an alternative, as an example, the entire end cap  24  may be dipped. Considerations for choosing the method of coating the reflectors  38 ,  48  include balancing the cost of the method of coating the reflectors  38 ,  48  with gold against the amount of gold used in the process of coating. Gold has the desirable property of reflecting virtually all of the energy in the infrared band thereby providing a very high efficiency for the transfer of infrared energy from the lamp  40  to the stud  16 . 
     After punch  42  is returned to the retracted position, workpieces  12 , 14  are lowered relative to the staking device  10  (this may be achieved by raising upper platen or by lowering lower platen) to withdraw stud  16  from central aperture  52 , and another pair of workpieces to be joined are placed in the position shown in FIG.  1 . The heat/punch staking cycle is then repeated. Although FIGS. 1-3 depict a single staking device  10 , it is well known in the art to construct heat staking machines having a plurality of staking devices which are driven simultaneously, sometimes by a single air cylinder, so that multiple heat staked joints may be formed with a single stroke of the machine. 
     In an alternative, rather than using a true parabolic primary reflector which is designed to direct its rays parallel to its central axis, it is possible to use a primary reflector having a convergent design. This type of reflector directs its rays inwardly toward a focal point, and this allows the secondary reflector  48  to be of smaller outer diameter than the primary reflector while still capturing all of the output of lamp  40 . 
     In another embodiment of the invention shown in FIGS. 7 and 8, a heat staking device  110  comprises two primary reflectors  138  and two lamps  140  disposed in a side-by-side relationship above a secondary reflector  148  generally similar to that described in relation to the embodiment of FIGS. 1-6. The adjustable carriage  142  is disposed between the two primary reflector/lamp combinations and is movable along the central axis of the secondary reflector  148  during the staking stroke. The adjustable carriage  142  is a cylindrical shaft, rather than having two legs for straddling the centrally located lamp  40  in the embodiment shown in FIGS. 1-6. 
     This multiple primary reflector configuration may be desirable in order to construct a staking press to meet certain space constraints, or where higher heat requirements require the use of two or more lamps. The interior surface of secondary reflector  148  may be specially designed to collect and focus the radiant energy from radiant heat sources located away from the main vertical axis of the secondary reflector. Any number of primary reflector/lamp assemblies may be disposed about the axis of adjustable carriage  142 , space permitting. When multiple lamps are used, and disposed around the axis of the adjustable carriage  142 , the lamps may include the primary reflector in the lamp unit. The use of a commercially available lamp and reflector unit provides for an energy source properly positioned within the reflector. This also provides for the convenient replacement of halogen lamps and reflectors. 
     In another embodiment of the invention shown in FIGS. 9 and 10, a heat staking device  210  has first and second lamps  240  disposed in a side-by-side relationship within the concave interior of single reflector  248 . Reflector  248  has a central aperture  252  for receiving stud  16 , just as in the previously described embodiments, and a significant portion of the output from lamps  240  is captured and focused onto the stud by the single reflector without the need for primary reflectors to initially direct their output downwardly. As in the embodiment of FIGS. 7 and 8, adjustable carriage  242  passes between the lamps  240  during the staking stroke. Any number of lamps  240  may be used in this embodiment and spaced around the central axis of reflector  248  and adjustable carriage  242 . 
     In another embodiment of the invention depicted in FIGS. 11-13, a heat staking device  310  has a primary reflector  338 , a radiant energy source  340  disposed within the primary reflector, and a secondary reflector  348  disposed below the primary reflector  338  to collect and focus energy from the source onto a stud  16 . A punch  342   a  is disposed on an arm  343  pivotingly mounted on the outside of the reflector assembly. An air cylinder  328  is connected to the reflector assembly and has a vertically oriented drive piston  328   a  which is connected to the arm  343 . During the heating cycle of the staking operation, staking device  310  is in a lowered position relative to the workpieces  12 , 14  and air cylinder drive piston  328   a  is retracted to rotate arm  343  and punch  342   a  to a raised position wherein it is pivoted outwardly and upwardly as shown in FIG.  11 . After the stud  16  has been heated for a sufficient length of the time to soften it, the entire heat staking device  310  is raised upwardly with respect to the workpieces as shown in FIG.  12 . The air cylinder piston  328   a  is then extended to rotate the arm  343  in a downward direction until punch  342   a  is located directly below the secondary reflector  348 , blocking its central aperture  352  as shown in FIG.  13 . The heat staking device  310  is then moved downwardly to urge punch  342   a  against the stud  16  and deform it, as shown in FIG.  13 . 
     In an another alternative embodiment of the invention shown in FIG. 14, an air cylinder  28  drives a selectively adjustable carriage  42  toward and away from stud  16  in a manner generally similar to the first embodiment disclosed hereinabove. In this embodiment, the energy source, such as an infrared lamp  40 , is mounted in a housing  26 , and adjacent to the adjustable carriage  42 . The movement of the adjustable carriage  42  being selectively driven by the air cylinder  28 . The energy from the lamp  40  is directed toward a convergent lens  70  by a reflector  38 . The energy is then focused by the convergent lens  70  into a fiber optic cable  64 . The fiber optic cable  64  extends from the lens  70  and splits into a plurality of sub-cables  66  which have distal ends  68 . The distal ends  68  are arrayed around and directed at a region into which the stud  16  is positioned. Preferably, the distal ends  68  are arrayed evenly around the region that encompasses the circumference of the stud  16 . The energy travels along the cable  64  and is split among a plurality of sub-cables  66 , and exits the sub-cable ends  68 . Fiber optic cables are thin glass or plastic filaments which conduct light by internal refraction, and are well know in the art. 
     The use of a heat lamp in a staking machine according to the present invention provides a heat source with nearly instant on/off control, thereby providing precise temperature control. The radiant heat source heats only the stud, thus achieving an overall efficiency of approximately 80%. Commercially available infrared lamps are relatively inexpensive and have lives on the order of 2000 hours, contributing further to the economic advantage of the invention over the prior art. The use of commercially available 100 watt lamps provide sufficient energy for most plastics, but when greater energy is needed larger wattage lamps can be used. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.