Abstract:
For joining two faces ( 41, 42 ) of thermoplastics in a penetration beam welding method the covered face ( 42 ) is multiply scanned and thus incremently heated by a laser beam ( 10 ) directed through the covering face ( 41 ). This preheating is repeated until the melting temperature is attained. In the region of the covered face ( 42 ) a melt thus materializes simultaneously over the full seam length which wets the covering face ( 41 ) and thus also transfers this face into melt by heat conduction. On completion of material softening the faces ( 41, 42 ) are mutually moved until abutting. Thus the two faces ( 41, 42 ) weld in principle as if they had been plasticized simultaneously over the full seam length which significantly enhances the seal and strength of the seam, whilst the expenditure the apparatus is low.

Description:
TECHNICAL FIELD AND BACKGROUND OF TEE INVENTION 
     The invention relates to a method and to an apparatus for welding a joint, particularly for joining thermoplastics, with a beam of energy whose wavelength may be in or near the infrared range. The short-wave radiation may be in the range of 0.7 to 2.0 μm. The two joint zones or joint faces are held coincidental or in contact with each other during plastification. In this or some other way first one joint zone is plasticized before then the other joint zone is plasticized. Thereby the second joint zone is wetted with the plasticized first joint zone and is thus likewise plasticized by heat conduction. 
     In butt welds both joint zones are plasticized separate from each other, but simultaneously, and then pressed against each other in the plastic state to become welded. Due to the contact pressure plasticized material may break out from the joint and then form a bead which is usually undesirable. The heating effect may also be done by vibration, namely by friction at the joint face or by friction of both joint faces. This results in abrasion particles which interfere and are difficult to eliminate from many products. In penetration beam welding, by contrast, the welding energy is applied to the coincident joint faces, for example through the exposed cross-section and its joint face onto the joint zone of the covered cross-section. As a result of this the joint zone adjoining the joint face is plasticized as a partial or flux layer of the cross-section. Thereafter the plasticized material mass moistens by flux, mutual transverse motion or contact pressure the other joint face and plasticizes also the joint zone thereof, resulting in the weld. When this weld is made along the joint or seam progressively, then field sections of the seam are already solidly welded whilst others are still plastic or in need of being plasticized. This makes homogenous contact pressure of the joint faces difficult in the various sections of the seam like also levelling of the joint faces by melting down. Melting down enables to equalize differences in tolerance of the shape and location of each joint face. 
     It is conceivable to beam the complete joint zone simultaneously with energy, for example by an array of diode lasers or beam focal points. This needs a complicated apparatus especially when the run of the weld is not straight. Apart from this the beam output is not suitable for workpieces differing in shape. 
     OBJECTS OF THE INVENTION 
     An object of the invention is to provide a method respectively an apparatus with which the disadvantages of known configurations or the aforementioned kind can be obviated. Another object is to make it possible to soften or maintain plasticized each of the two joint faces over the full seam length substantially simultaneously. Thus in this state the two cross-sections or components to be joined should still be mutually positionable into that predetermined orientation in which they are to be finally joined by the solid weld. A further object is to achieve a seam which remains sealed and has tensile strength even when exposed to high loads or pressures. Namely the same strength as of parts of the cross-sections adjoining the seam or spacedly juxtaposed to the seam is intended. Still another object is to make the joint with little consumption, irrespective of the seam course. 
     SUMMARY OF THE INVENTION 
     According to the invention only one or both of the joint faces is/are entirely preheated to an intermediate temperature slightly below the working or melting temperature. At the latest then the joint faces are brought into mutual contact and the entire preheated joint face is heated to the final melting temperature. The melt or melting mass then heats the other joint face simultaneously over the entire seam length and up to the melting temperature. This results in the melt mass of the two joint zones intermingling homogeneously and solidifying on cooling to a solid weld. This prevents the melts from solidifying in some seam area before both joint zones of all other seam areas have attained the melting temperature, namely are fused together. All seam areas consequent melt near to simultaneously or each seam area fuses as long as all remaining seam areas are still soft enough to allow the components to be mutually moved so that the softened material is able to yield or be displaced. The seam melt solidifies over the full length and width of the seam simultaneously or practically at the same time thus also avoiding strains to rise. 
     Preheating may be done in several steps or cumulative from a first intermediate temperature to a next higher intermediate temperature. For this purpose the beam focus or field is moved along the joint zone so quickly that each section of the joint zone between two sequential heatings by the beam field cannot cool down to the last initial or minimum temperature of the previous heating. For example, the heatings may be done in one, two or three seconds once, ten or more times, whereby also every whole number between one and twelve is possible. This incremental or gradual heating of the joint zone occurs whilst the joint faces are mutually urged in contact. As soon as the one joint face has become soft or plasticized simultaneously over the full length of the seam, the associated component can be moved by the contact pressure over a desired degree relative to the other component. This enables tolerance differences in the dimensions of the components or in the superficial shape of the joint faces to be equalized. This motion is limited either by stops on the components themselves or by stops on the pressing/gripping device with which fixture the components are held or pressed together during production of the joint. 
     During preheating and/or melting the energy beam penetrates the one joint face, which it does not heat or only slightly so due to its physical properties. The beam impinges the joint face directly juxtaposed which due to its physical properties absorbs the radiation in heating up until it enters into the melting phase which heats the first joint face likewise up to plastification. Thus even long welds can be produced in less than 15, 10 or 5 seconds including preheating. The method is particularly suitable for plastics or for circumferentially continous seams as used in joining container components. It is also suitable for other purposes, e.g. for complementary preadaptation of joint faces where these need to be non-distructively separable. 
     The method is implementable with known apparatuses, namely with a writing or scanner head having a beam output for an energy beam such that the beam can also be deflected continuously in differing directions with the head stationary. However, the beam output may also be secured to a robotic arm to be thereby moved articulatedly and powered in all three dimensional directions so that the beam is moved along the seam by the beam output which is held at a constant spacing from the seam. According to the invention the apparatus has control means which guide the beam field multiply in sequence over each of the sections of the joint zone or seam, particularly in timed intervals of maximally one to six or four seconds. The beam output may be formed by a mirror or a focussing lens. Where for beam steering two sequential mirrors are used, each is swivable about a separate axis independently of the other. Thus the beam can be moved simultaneously in two spatial axes which are mutually perpendicular. With the focussing optics or a Z-axis module the beam field can also be powered in the third spatial axis whilst maintaining its field area constant. Thus varying distances between beam output and joint zone are compensated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Example embodiments of the invention are explained in more detail in the following and illustrated in the drawings in which: 
     FIG. 1 is a simplified illustration in perspective of an apparatus according to the invention; 
     FIG. 1 a  is an illustration on a magnified scale of a detail shown in FIG. 1, and 
     FIGS. 2 to  6  are illustrations of various example embodiments of joint configurations. showing their joint faces and cross-sections. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates apparatus  1  including scanner head  2  with base  4  and means  3  for stationarily holding the workpiece to be worked. Stationary base  5  of fixture  3  is fixedly connected to base  4 . Each of units  2 ,  3  is exchangeably mounted on a base frame independently of the other. 
     Head  2  includes beam guiding means comprising a light mains  6 , with output  7 , means  8  for steering beam  10 , one two or more mirrors  9 , objective lens  11 , beam output  12  and a motor  13  or  14  as advancing means for powering each mirror  9  arranged on base  4 . Lightguide  6 , for instance a glass-fiber optic cable, guides the polarized laser light from a light source to intermediate output  7  from which beam  10  impinges directly on mirror  9  directly guiding beam  10  through lens  11 . Mirror  9  or lens  11  forms beam output  12  from which beam  10  is directed over a free distance directly onto the working zone of the workpiece. Motor  13  carries mirror  9  on a shaft so that the pivot axis  15  or  16  is located in the plane of this mirror and at an angle to the center axis of output  12 . Mirror  9  can also be pivoted about two mutually perpenticularr axes, its pivoting angle being e.g. 20° to 30°. FIG. 1 a  illustrates a further mirror  9 ′ having a separate motor  14  and located upstream of mirror  9 . Pivot axis  16  of mirror  9 ′ is angular with axis  15  of mirror  9  and with the axis of lens  11 . 
     Unit  3  has a device or gripping fixture  17  with two opposite jaws or cheeks  18 ,  19  for tensioning the components to be joined against each other. Jaw  18  is fixedly connected to brackets  4 ,  5 . Jaw  19  is movable relative to jaw  18  with an actuator  20 , such as a pneumatic cylinder. Exchangeably secured to each jaw  18  and  19  is a tensioning jaw  21  and  22  adapted to the corresponding workpiece and in direct contact therewith for secure clamping. Jaw  19  is reversingly mounted on base  5  by guide  23 . A sensor or reveiver  24  continuously senses the travel of jaw  19  relative to jaw  18 . The force which stresses jaw  19  against jaw  18  or workpiece is continuously sensed by a receiver  25 . 
     Control means  30  for actuating steering means  8  and for controlling characteristics like the check path or travel and the check force comprise means  26  for entering the corresponding data to which an electronic data storage with the corresponding programs for differing workpieces is assiged. Via the input or keyboard  26  these programs can also be entered, altered and retrieved so that means  3 ,  8  are sequenced as provided for. The data may be displayed on monitor  27  which is connected to input  26  via a signal cable  28  and to means  13 ,  24 ,  25  as well as to actuator  20  via separate signal and control leads  29 . 
     FIG. 1 a  illustrates a flat working zone  34 , representative for a workpiece, in the plane of which two spatial axes  31 ,  32  at right angles to each other are shown. A third spatial axis perpendicular to axes  31 ,  32  is perpenticular to working zone  34 . Means  8  continously move beam  10  over the zone  34  by superimposing the beams motions which are oriented parallel to axes  31 ,  32 . Zone  34  is impinged by beam  10  with beam field  35 , the size of which is varied by optics  11  or its distance between output  12  and zone  34 . When zone  34  is moved nearer to output  12  the beam field  35  becomes larger and vice-versa. If zone  34  extends not only in one plane but also in direction  33 , field  35  is maintained constant in size while moving over the entire zone  34 . 
     FIG. 2 illustrates workpiece  36  comprising two components  37 ,  38  which are here a container  38  closed off by a cover  37 , made from thermoplastics. The cross-sections  39 ,  40 , namely annular rim  39  and container shell  40  have to be annularly or continuously sealingly welded to each other at annular joint faces  41 ,  42 . For assembly part  37  is put on part  38  in direction  45  or opposite to direction  46 . Parts  37 ,  38  are mutually clamped in direction  45 ,  46  by means  17 . The spacing between faces  41 ,  42  then increases slightly in direction  45 , for example by a few angular degrees. The leading end of part  38  or of cross-section  40  respectively as related to direction  46  forms a cross face  44  oriented transverse or perpendicular to faces  41 ,  42  and adjoins face  42  at an acute angle. A corresponding face  43  adjoins likewise face  41  and with a spacing opposes face  44  as an abutment. Thus the sharp transition edge of faces  42 ,  44  contacts the inclined face  41  with tension and by this spacing from face  43 . The spacing between end  47  of face  41  remote from face  43  and face  43  defines the width of the weld seam, the length extension of which is perpendicular to the drawing plane. The centrally symmetrical beam field  35  has a width or diameter equalling this seam width and is thus multiply smaller than the seam length. 
     Cross-section  39  is permeable to beam  10  without any substantial energy absorption, i.e. transparent to beam  10 . Relative thereto cross-section  40  absorbs the energy of beam  10  considerably more in the vicinity of face  35  and is thus heated by beam  10 . This may be achieved by incorporating additives or absorption substances such as pigments, carbon black, talcum or the like. 
     Means  8  guide beam  10  totally through cross-section  39 , the size of field  35  on face  42  always being kept constant. Beam  10  is simultaneously so deflected by means  8  that beam field  35  travels in a sole direction at high speed continually over area  42  along the full length of the seam. This results in the temperature of cross-section  40  increasing continuously around the joint zone  42  from room or first initial temperature, namely from one intermediate temperature to the next higher intermediate temperature whilst cross-section  39  in the region of joint zone  41  is initially not heated or much less at the most. Between two passes in which beam field  35  falls sequentially on the seam portion as shown in FIG. 3, the joint zone  42  cools only unsubstantially or not at all. 
     After a plurality of more than five or ten passes the temperature of the joint zone  42  has increased to the melting temperature and the melt or flux layer expands over the seam width up to contact with face  41 . This results in the heat of joint zone  42  being directed through face  41  into cross-section  39 . Thus joint zone  41  likewise attains melting temperature simultaneously over the full length of the seam to provide a further flux layer on face  41 . Previously, or simultaneously, parts  37 ,  38  move mutually due to clamping force  45 ,  46  until the non-plasticized faces  43 ,  44  about mutually. Control means  30  then prevent any further positioning travel by controlling actuator  20 . The weld melt does not “weep” from the gap between zones  41 ,  42 . Instead the melt is drawn into this gap so that no seam bead forms at end  47 . Initially the gap spacing between faces  43 ,  44  corresponds to the mean gap width between faces  41 ,  42 . This gap width is less than one or half a millimeter. Rim  39  surrounds the outer circumference of container shell  40  so that penetration beam welding may be done from the outside of the container. Once the gap has been filled the melt cools and the seam becomes solid and sealed throughout. After this, workpiece  36  is released and removed from means  17 . 
     As evident from FIG. 2 face  42  is oriented parallel to directions  45 ,  46  and face  41  is at an angle thereto. 
     FIG. 3 illustrates how inversely face  41  is parallel to clamping direction  45 ,  46  and face  42  is at an angle of less than 2° or 3° thereto. Face  43  is an annular end face of wall  39  and not, as shown in FIG. 2, a recessed face. Face  44  is a shoulder face set back relative to the free end of face  42  so that the gap between faces  43 ,  44  is exposed to the outside of the workpiece. There is likewise no expulsion of melt from between faces  43 ,  44  since melt is prevented from entering this gap. 
     FIG. 4 illustrates cross-section  40  as an annular web. This web freely protrudes cross-sectionally beyond the thicker associated wall of part  38  so that each of its flanks adjoins a shoulder face of this wall. These shoulder faces form faces  43 ,  44 . Both web flanks may be joint faces  42  to be welded with joint faces  41  of part  37 . These joint faces  41  are formed by flanks of a groove in cross-section  39 . The groove flanks converge at an acute angle to the groove bottom. The latter and the longitudinal edge of web  40  too may form the abutting faces. Cross-section  40  is sufficiently thin for being entirely transferred into melt until this fully fills the groove and welds both web flanks to the two groove flanks. 
     FIG. 5 illustrates cross-section  40  as a container wall and the outer face thereof is the joint face  42 . Cross-section  39  is a web or a sleeve, the longitudinal or end face of which forms joint face  41 . Web  39  is the freely protruding rim of a nipple  37  whose trough duct is to be connected to an opening in the container wall so that the seam sealingly surrounds this opening. Beam  30  is here directed through rim  39  parallel to the axis thereof, namely also through that end wall of nipple  37  which adjoines rim  39 . This end wall is spaced from face  42 . Parts  37 ,  38  are mutually clamped in directions  45 ,  46  perpendicularly or transversely to faces  41 ,  42 . Simultaneously with plasticizing, parts  37 ,  38  are mutually approached in directions  45 ,  46  until control means  30 ,  24  limit this travel. 
     FIG. 6 illustrates how two parts or tubes  38  are mutually longitudinally or coaxially positioned and joined to part  37 . Tubes  38  are inserted from opposite directions into sleeve  37  prior to plastication and abut by their ends  44  against mutually remote faces  43 . Faces  43  are formed by a collar protruding beyond the inner circumference of part  37 . Joint faces  41 ,  42  are spaced from faces  44 . Tubes  38  are not moved forward each other parallel to faces  41 ,  42  during welding. Instead the melt is able to flow without tension from joint zone  42  to joint zone  41 . Faces  41 ,  42  may, however, also be moved transverse to each other e.g, by radially stressing cross-section  39  against cross-section  40 . 
     Instead of a single beam  10  several beams  10  may be provided simultaneously so that their beam fields  35  are distributed over the full length of the seam. These beam fields are interspaced and scan along the seam at the same speed or differing speeds. Thus the welding energy can also be entered even quicker into cross-sections  39 ,  40  so that each zonal section of face  42  heated by field  35  cools even less until it is reheated by the next pass. 
     It will be appreciated that the cited features and effects may be precisely as described, or merely substantially or approximately so and may also greatly deviate therefrom, depending on the particular requirements.