Abstract:
The invention disclosed hereunder is a process and a device for the preparation and formation of a plastic weld connection between a workpiece ( 13 ) and a second component to be attached thereto by heating by means of a heating device ( 12 ). If force is applied, the heating element ( 12 ) tends to deform the contact surface especially in the case of hollow, thin-walled workpieces such as plastic containers. So far, methods were disclosed for the measuring of the degree of deformation by means of a sensor, subsequently readjusting the position of the heating device according to the determined deformation. The invention disclosed here under dispenses with any measurement of deformation, instead using an elastic element which automatically readjusts the position of the heating element ( 12 ) in the case of deformation of the workpiece ( 13 ) to effect a pre-load between the heating device ( 12 ) positioned at the contact surface of the work-piece ( 13 ) and the workpiece ( 13 ).

Description:
SCOPE OF THE INVENTION 
     The present invention relates to a method and apparatus used to prepare and form weld connections between thin walled articles which may in themselves be subject to distortion or deformation when subjected to critical pressures or loading. The method more preferably involves the softening of the contact surface of the workpiece by means of a heat applying application device which is moved into engagement with the contact surface of the workpiece. 
     BACKGROUND OF THE INVENTION 
     When two components are connected by means of bonding, gluing or welding, the elasticity of the components, namely their ability to bend or yield to pressure, often constitutes a significant problem in terms of the quality of the connection. The quality and integrity of the connection can be reduced, particularly in the case of extrusion-blown hollow plastic bodies, due to the inconstant positioning of one of the components during the melting and pressing process. Leaky weld connections can lead to serious consequences in the context of applications where weld connections of a high quality are required, as for example in the manufacture of plastic fuel tanks which require high tolerance limits as far as the location of the connecting surfaces is concerned. In the context of such applications, the compensation of tolerance limits for the positioning of the contact surfaces with help of a welding system is required. 
     German Patent No. DE 35 37 670 C2 to Oxenfarth, which issued on Mar. 3, 1988, discloses a conventional process and corresponding device for use in forming a plastic weld connection. A minimum-force sensor mechanically determines the position of the contact surface on a workpiece such as a hollow plastic container (e.g. a fuel tank of a vehicle). Once the workpiece position is determined, a welding head or other heat applying device is moved by the sensed distance into contact with the workpiece to soften and melt a portion to which a second component is to be welded. This process assures safe welding of the second component, which for example may be a vent connection or filler neck onto the workpiece in spite of high tolerance limits as far as the dimensions of the workpiece are concerned. Therefore, the use of thicker materials to ensure complete welds, and which is almost impossible in the case of thin-walled hollow bodies, is not required to effect the connection. This procedure, however, neglects the forces that melting head apply to the workpiece and which can cause distortion or bending of workpiece surfaces, leading to incomplete welds. 
     DE-PS 39 22 066 discloses a further development of the state of the art used to achieve improved welds. This reference discloses a method to compensate for position changes and deformation of the part of a workpiece to which a second component is to be connected by readjusting the position and contact pressure of a heating element, as well as the position and contact pressure of the second component to be connected to the workpiece. For this purpose, the position of the contact surface on the workpiece is also determined by a minimum-force mechanical sensor. The result of the measuring process is then used to readjust a piston which presses the component to be added against the heating device and the heating device against the workpiece. 
     Published Japanese Patent Abstract No. 04 234633 to Yokoyama Takahito (published Aug. 24, 1992) discloses a holder used to position a polyethylene joint on a tank, following melting and heating of contact surfaces. The holder includes a compression spring used to bias a melted surface of the joint against a melted surface of the tank. 
     Published Japanese Patent Abstract No. 08 112861 to Imagawa Kanji (published May 7, 1996) discloses a heating device used to heat weld together two sheets. The heating device includes a heating part which is biased by means of a spring to project downwardly through an opening, into contact with the sheets which are to be welded. 
     The determination of the position of the workpiece surface using an additional sensing device constitutes a disadvantage in that it is not sufficiently accurate, as the method requires the establishment of additional reference surfaces on a potentially elastically deformable surface of the workpiece or component. In addition, the reference surfaces are often not feasible for manufacturing related reasons, or are located too far from the relevant connecting position, thus distorting the result. 
     Plastic welding units that are exclusively equipped with load-sensitive sensors such as load cells can compensate for the disadvantages of external sensing equipment but are often expensive and are not sufficiently sensitive for design-related reasons. Additionally, the impression depth achieved by the partial melting of the component or workpiece surface is not adequately determined. 
     SUMMARY OF THE INVENTION 
     The invention is therefore based on the need to develop an improved device and a process for the formation of weld connections of constant quality as between a workpiece and a second component which is to be connected thereto. More preferably, the present invention seeks to achieve the formation of high quality plastic weld connections where either the workpiece and/or the component may be elastically deformable upon the application of a predetermined minimum force thereto. 
     The inventor has appreciated an improved method of forming a plastic weld connection between a workpiece which can be shaped or deformed by application of mechanical force and a second component to be connected thereto. The method preferably involves the softening of the contact surface of the workpiece by application of heat, however, other means of joining components to the workpiece, such as gluing, sonic welding or the like are also possible. The process broadly comprises the steps of positioning of an application device adapted to melt, glue or weld the contact surface of the workpiece, activating the application device to prepare the contact portion of the workpiece while repositioning of the application device in relation to the workpiece during preparation of the contact portion. The method is further characterized by the fact that the repositioning comprises a defined displacement of the application device in relation to the contact surface of the workpiece, and which is independent of a determination of the position by means of measuring. 
     The present invention also overcomes the disadvantages of the prior art by providing a device for the preparation and formation of a weld connection between a workpiece which can be distorted or deformed upon the application of predetermined mechanical force and a second component which is to be connected thereto. The apparatus preferably includes a hot plate used in the softening or partial melting of the contact surface of the workpiece when applied thereto. The apparatus is characterized by the fact that it is adapted to reposition the hot plate during melting without measuring of the position of the contact surface, and most preferably during displacement of the hot plate while it is applied to the contact surface of the workpiece as deformation and melting of the contact surface occurs. 
     The invention preferably relates to a process for the preparation and formation of a plastic weld connection between a deformable thermoplastic workpiece which may be distorted by the application of a mechanical force and a second thermoplastic component to be connected thereto. In a preferred embodiment, the method includes a step of softening or partial melting of the contact surface of the workpiece by application of heat. More preferably, the process includes the following steps: positioning a heat applying device at the contact surface, activating the heat applying device, and continuously repositioning the heat applying device in relation to the workpiece during the softening of the workpiece. 
     Additionally, the present invention relates generally to a plastic welding device for the preparation and formation of a weld connection between a workpiece and a second component to be connected thereto. 
     The process and the device disclosed hereafter ensure that the load forces of the element used in preparing or performing the welding process is directly applied to the elastic component to be connected to the workpiece, and applies a defined load and maintains a defined path during welding operations. 
     The basic principle of the present invention and process disclosed also compensates for different degrees of bending or flexure due to production-related differences in workpiece wall thickness, aging or variations in material quality. Basically, the principle of the device disclosed hereunder is suitable for all processes where a workpiece having an elastically deformable surface must be prepared under path-dependent and load-dependent conditions. 
     The preferred field of application, however, is for use in formation of plastic weld connections where the contact surfaces of the two components to be joined are softened by heating and pressed together in a defined fashion while still soft and partially melted. The pressing phase is followed by a cooling phase that allows the softened material to harden to form a finished weld. By way of non-limiting examples, the present invention may be used to weld components such as thermoplastic vent nipples, filler necks or the like to blow moulded thermoplastic workpieces, such as automotive gas tanks, fluid reservoirs or any other plastic parts which may require welding. 
     According to the present invention, a pneumatically, hydraulically or electrically operated advancing system is preferably used to press a heating element, which is controlled via a load-sensitive element attached to the advancing device into contact with a partially elastic contact surface. The mobility of the load-sensitive element should preferably be limited to a translational one-dimensional motion. with the preferred design allowing for adjustment of an admissible maximum path. More preferably, the heating element is also equipped with a load-sensitive sensor. 
     Through the load-sensitive sensor, the heating element can be pressed onto the elastic contact surface until a predefined load value is reached. As soon as the predefined contact pressure is reached, the load-sensitive sensor operates to cease operation of the advancing system and the further movement of the heating element. 
     To achieve production-related even and homogeneous softening in the area of the contact surface, the heating element is preferably advanced a minimum impression depth into the melting material at the contact surface. The minimum impression depth of the heating element being defined as the maximum local shape or thickness at the contact surface and position tolerance required to compensate for any irregularities of the contact surface. 
     The predefined distance of movement of the load-sensitive element determines the impression depth. As soon as the maximum impression depth is reached, the heating element engages the contact surface without any further force being applied so as to ensure sufficient radiation and penetration of heat and, thus a sufficient amount of softened material for the subsequent connecting process. Depending on the requirements of the application, the maximum impression depth can be monitored by a signal transmitter. 
     The load-sensitive mechanism ensures that reproducible impression depth and heating of the contact surface are achieved by simple mechanism, regardless of the elastic bending of the contact surface. An accurate relation between the heating element and the contact surface is given at any moment, as the heating element directly performs the function of a position sensor. The system disclosed hereunder is free of the disadvantages which are characteristic of systems that use position sensors and reference surfaces which, for production-related reasons, must be located at a certain distance from the contact surface. 
     The process disclosed hereunder does not in principle require the contact surface to be additionally pre-loaded or biased against the heating element. This may be done, however, for economic reasons, because higher contact pressures applied to compensate for irregularities, while the heating element advances into the material, allow a significant reduction of production cycle times. 
     The load-sensitive sensor should preferably be an adjustable device, thus ensuring that the contact pressure required for different production processes and workpieces can be defined. The load-sensitive sensor may also be designed in the form of a load cell, thus ensuring that the documentation and recordal of the load parameters during the process is possible. 
     Depending on the counter forces originating from the deformation of the contact surface, the load-sensitive element can be provided with a path limitation feature such as a stop. Thus it is possible to ensure that the heating element advances into the material exerting a relatively great force and within a correspondingly short time. As the contact surface approaches the path limitation point, the external forces decrease towards zero and the heat can penetrate from the heating element into the material, substantially without any further forces being applied. In contrast to other prior art hot welding systems, it is not necessary to remove the load of the heating element from the deformed contact surface during the heating process. 
     The separate pressing or advancing mechanism required to connect the components to each other can be designed as desired, however, pneumatically, hydraulically or electrically operated systems are preferred and will become readily apparent. 
     Although not essential, the softening of the component to be attached to the workpiece should preferably be affected by means of the same heating element used to soften the elastic workpiece proper, using the surface of the heating element opposite from the elastic workpiece. 
     Path limitation and load measuring systems can be used to compensate for tolerances regarding the contact surfaces or for irregularities of the component to be attached to the original workpiece. Considering the fact that the system disclosed hereafter can also be used for components with deflection-resistant surfaces, it is recommended to use a load-sensitive mechanism according to the present invention to support the component to be attached to the workpiece. 
     The present invention compensates for manufacturing irregularities and height tolerances in the area of the contact surface, at the same time ensuring that the heating element advances to sufficiently deep level into an irregular contact surface, without requiring external path measuring or continuous control devices and without additional reference surfaces in the vicinity of the elastic contact surface. 
     In a process for the preparation or heating of an elastic contact surface, heating, is effected by a heating element that is connected to an advancing mechanism via a sensing element. The advancing mechanism preferably consists of a self-locking threaded spindle with an adequate driving mechanism. 
     A particular advantage of the present invention resides in the fact that the adjustment of the loads exerted during the warming and the heat diffusion stage, the registration of the location of the elastic contact surface, and the determination as well as the limitation of the path traveled by the heating element during the warming stage can be effected by one single device. Additionally, this device performs all required movements. 
     Accordingly, in one aspect the present invention resides in a device for the preparation and formation of a weld connection between a workpiece and a second component to be connected thereto, and including means for the softening of the contact surface of the workpiece by applying an application device to the contact surface of the workpiece, characterized in that 
     the device includes means for repositioning the application device without measuring of the position of the contact surface, and means for displacing the application device applied to the contact surface of the workpiece in relation to the workpiece as soon as deformation of the contact surface occurs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference may now be had to the following detailed description taken together with the accompanying drawings in which: 
     FIG.  1 : is a side view of a first embodiment of the present invention; 
     FIG. 2 is a side view of a modified form of the invention of FIG.  1 . 
     FIG. 3 is a side view of a second embodiment of the present invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The device shown in FIG. 1 comprises a base unit  1 . Two articulating arms or rods  2  and  3  are pivotally attached at one end to base unit  1 , with the other ends of the arms  2 , 3  articulated with an adapter plate  7 , and the arms  2 , 3  together forming a parallelogram. Due to this design, the adapter plate  7  can practically move in only one direction (in FIG. 1 along the vertical axis). The second (horizontal) component of movement present in this system is insignificantly small, provided that the components are arranged accordingly, and can thus be neglected. It is to be appreciated, however, that instead of arranging the two arms  2 , 3  to form a parallelogram, it would also be possible to attach the adapter plate  7  to only one arm that glides up and down in a guiding device on the base unit  1 . This design requires a greater effort to mount the arm in a guiding device on the base unit  1  but offers the advantage of more complete elimination of the horizontal component of movement. 
     The base unit  1  comprises two fork-like extensions  21  and  22  between which the arms  2  and  3  are located. A spring  5  which acts on arm  3  is attached to extension  21  , pre-loading a arm  3  at a distance from the other extension  22  of the base unit. A mechanical path limitation device  6  is attached to the second extension  22  of the base unit Thus the maximum movement of arm  3 , which can be triggered by the force of spring  5  acting on arm  2 , which in turn is coupled with arm  3  via the base unit I and the adapter plate  7 , is limited. 
     The mounting positions of arms  2  and  3  on the base unit  1  and the position of the multi-purpose spring  5  create a lever system, the advantages of which will be discussed below. The adapter plate  7  holds a heating device  12 . This heating device  12  can be designed as required on the desired application 
     The base unit  1  is connected to an adjusting element, e.g. a feed bar  11 . Feed bar  11  is connected to a drive unit  10 , e.g. a motor, via a converter unit  8  and a threaded spindle  9 . 
     The motor  10  as well as the converter unit  8  is attached to one side of an angular supporting element  15 . The other side of the supporting unit  15  carries an advancing unit  16  as for example is of the type disclosed in U.S. Pat. No. 5,614,118 to Weber, which issued Mar. 25, 1997. The end of the advancing unit  16  is provided with a component holding device  17  which carries a second thermoplastic component  18  to be attached to the contact surface  24  of a thermoplastic workpiece  13 . 
     The converter unit  8  converts the rotational movement transmitted via the threaded spindle  9  into a translational motion, at times allowing for a superimposed rotational movement 
     When the drive  10  is operated and rotates in the correct direction, the feed bar  11  performs a combined translational movement (e.g. in FIG. 1 a sliding movement in the vertical direction) and swiveling movement (e.g. 90°). The base unit  1  is moved from its basic position spaced above the workpiece  13  into the working position shown in FIG.  1 . In this working position, the centers of a contact surface  24  on the workpiece  13  and the center of the heating element  12  are aligned. The converting unit  8  is appropriately designed to ensure, as soon as the centers of the heating element and the contact surface are aligned, that the base unit  1  performs only a translational movement (vertical downward movement) transmitted by the drive shaft 
     As the base unit  1  is lowered relative the workpiece, as soon as the heating element  12  touches the elastic contact surface  24 , the multi-purpose spring  5  is compressed until arm  3  engages extension  21  of the base unit  1 , thus triggering a sensor  4  mounted on the end of arm  3 . 
     The spring force of the multi-purpose spring  5  is selected depending on the elasticity of the contact surface  24 . The multi-purpose spring  5  should preferably be a spring with adjustable spring force and combined with a load sensor or a load cell. In addition to regulating the sensitivity of sensor  4 , the multi-purpose spring  5  serves to compensate for the dead weight of the lever system depending on the position in which the entire system is installed. 
     Triggering of sensor  4  immediately stops the drive  10 . The self-locking effect of the threaded spindle  9  helps to stabilize the position of the base unit  1 , maintaining it in an accurate spaced relation to the contact surface  24 . Thus it is ensured that the heating element  12  always stays in contact with the contact surface  24  independently of the height tolerance of the contact surface  24 . 
     The force applied by the compressed multi-purpose spring  5  and the restoring force of the contact surface  24  press the heating element into the contact surface  24 . The adjustable mechanical path limitation device  6  allows the impression depth of the heating element  12  to be limited to a threshold which can be defined depending on the material of the workpiece, and in a preferred embodiment used in gas tank manufacture should amount to approximately 2 mm. The heating element  12  now rests on the contact surface  24  substantially without exerting any further force, and the heat of the heating element  12  can now penetrate into the supporting surface  24  practically without displacing any further material. 
     The mechanical path limitation device  6  can also be equipped with a sensor. In this case, compliance with the defined impression depth upon the return of the arm  2  into contact with the device  6  can be verified and documented. 
     FIG. 2 shows a modified form of device shown in FIG. 1 used to effect welding operations while maintaining the workpiece  13  in a deformed state. In FIG. 2, like reference numerals are used to identify like components. If the elastic return of the contact surface  24  from a deformed state as a result of applied downward force of the heating element  12  during the warming phase is not desired for production-related reasons, the return of the contact surface  24  to an undeformed configuration may be prevented by the use of a mechanical path limitation system  14  mounted on the base unit  1 . The mechanical path limitation system  14  should be mounted in the vicinity of the contact surface  24  at a point where the plastic will not be softened and where it will be triggered prior to the path limitation system  6 . 
     In operation of either the device shown in FIG. I or the modified version of FIG. 2, during the heating of the contact surface  24 , the contact surface of the second component  18  should for practical reasons be simultaneously heated by the end of the heating element  12  opposite the contact surface  24 . This process can be effected by any desired component holding device  17  and an advancing unit  16  connected thereto, as for example is disclosed in U.S. Pat. No. 5,614,118, and which is connected with the device of the present invention via a supporting element  15 . 
     After arrival at the mechanical stop limitation  6  (shown in FIG. 2) and a subsequent waiting period required to let the heat of the heating element  12  penetrate the contact surface  24 , the drive  10  is next operated in the opposite direction to raise the heating element  12  from the workpiece  13 . It is to be appreciated that the drive  10  must not be activated before the heating process of the second component  18  is completed and the advancing unit  16  has returned to its basic position retracted above the contact surface  24 . 
     With the activation of the drive  10  in the opposite direction, the heating element  12  is raised from the contact surface  24  (lifted off vertically) and swiveled out of the working zone by a superimposed rotational movement. Then the advancing device  16  lowers and presses the melted portion of the second component  18  onto the melted contact surface  24  of the workpiece  13 . 
     FIG. 3 shows a side view of the second embodiment of the invention. Identical or similar features in FIG. 3 are allocated the same numbering as a FIG. 1, each number being augmented by a value of 100. For a description and explanation of the functions of identical or similar components please refer to the description of the first design according to FIG.  1 . 
     In the device designed as illustrated in FIG. 3, an angular holding device  130  is mounted at the end of the feed bar  111 . The free end of this holding device is provided with a holding ring  131 . In the central opening of the holding ring  131 , one leg of an angular supporting rod  132  slides up and down on a pair of bearing bushes  136 . The angular rod  132  is provided with a collar  135  which supports a compressible spiral spring  105  which coils around the leg of the angular rod  132  and exerts its pressure against a bearing bush  136 . The end of the angular rod opposite from the spring  105  is provided with a stop  133 . The free leg of the angular rod  132 , forming an angle of 90° with the first leg, carries an adapter plate  107  at its free end. The holding ring  131  is provided with a holding element  134  that suspends the sensor  104  in the vicinity of the free leg of the angular rod  132  in the area where the latter performs its translational movement. 
     This second design works similarly to the first design. When the motor  110  is activated, the feed bar  111  rotates horizontally through an angle of 90° from the basic position (not shown in the drawing) into the working position shown in FIG.  3 . In this position, the heating device  112  is located between and vertically aligned with the contact surface  124  and the second component  118 . As the motor continues to work, the feed bar  111  performs a translational movement lowering vertically with the heating element  112  being pressed against the contact surface  124 . As the drive  110  continues to work, the restoring force of the workpiece  113  at the contact surface  124  acts against the heating element  112  and compresses the spring  105 . Finally the drive  110  is stopped as the horizontal arm of the angular rod  132  engages and triggers the sensor  104 . The position of the heating device  112  is now stabilized due to the self-locking effect of the spindle  109 . The workpiece  113  is deformed at the contact surface  124  due to its contact with element  112  and the elastic force of the compressed spring  105 . Due to the force of the compressed spring  105  and the restoring force of the deformed contact surface  124 , the heating device  112  moves inwardly relative to the contact surface  124  displacing the material of the workpiece  113  as it begins to melt. The melting depth can be limited by the adjustable stop  133 , e.g. to a value of 2 mm. As soon as the stop  133  is in contact with the bearing bush  136 , heat from the heating device  112  penetrates the material of the workpiece  113  at the contact surface  124  without further force being exerted or further melted material being displaced. 
     While the contact surface  124  is heated, the second component  118  is positioned by the advancing unit  116  on the side of the heating device  112  opposite from the contact surface  124  and simultaneously heated. As soon as a sufficient amount of time for the heating and melting of the contact surface  124  and the second component  118  has passed, the advancing unit  116  raises the second component  118 , and the drive is reactivated in the opposite direction to lift the heating element  112  off the contact surface  124  and to swivel it, at the end of its path of translational (e.g. vertical) movement, by an angle of 90° out of the working position back into the basic position. Then the advancing unit  116  alone lowers and presses the still hot and softened surface of the second component  118  against the still hot and softened contact surface  124  of the workpiece  113  to weld the component  118  to the workpiece  113 . After cooling of the plastic weld connection, a durable, strong and excellent leak-proof connection is achieved. 
     Both designs illustrated in FIG.  1  and FIG. 3 would allow the workpiece to be elastically or resiliently supported (e.g. on a spring device) and to be moved so as to achieve an initial tension between the heating element and the workpiece. 
     For the designs described above, only the creation of a plastic weld connection by heating and softening the material of both the workpiece and the component to be attached thereto has been assumed. The expert, however, is aware that the invention disclosed hereunder can also be used for applications where the softening of the workpiece is not achieved by heating but through chemical treatment, glue, and/or sonic welding either with or without pressing the second component supported by an elastic device against the workpiece (or vice versa). 
     While the invention describes and illustrates preferred embodiments of the invention, it is to be appreciated that the invention is not so limited. Other modifications and variations will now become apparent to a person skilled in the art. For a definition of the invention, reference may be had to the appended claims.