DE2855051C2 - - Google Patents

Description

The invention relates to a method for connecting Plastic parts according to the preamble of claim 1 and a device for welding two parts thermoplastic material for carrying out the procedure rens according to the preamble of claim 3.

Known techniques of this type are for example in the Publication "Welding of Plastics" by Neumann and Bockhoff, published by Reinhold Publishing Co. im 1959, known and essentially comprise the Use heated plates and friction for one Weld. With each of these techniques, the edges the plastic parts to be connected are heated, to bring them up to their fusion welding temperature. Like this soon the edges are sufficiently softened by heat, they are quickly pressed together until they melt the edges have softened sufficiently, to make a strong seam. During the Welding process should be the pressure between the two softened edges of the plastic parts so strong that Air bubbles are expelled and the entire peripheral surface be brought into close contact. The  resulting pressure on the softened edges when merging leads to the formation of an abge rounded beads along the connecting seam between the two plastic parts. After cooling the weld ten edges, the rounded seam is known Ver drive away, for which the connection area between the both plastic parts sandblasted or the bead cutting is removed. This was followed by a Regrinding the seam area.

For many applications, however, a flawless, reliable and permanent welding or connection point le of critical importance. For example, if art fabric tubes by means of hot plate welding of the connected, the achievement of the required strength and durability of the weld extremely important to fluids under changing temperatu to be able to promote and press in the environment, in who experience significant shocks and vibrations As a further example, some cell sets Containers for batteries or accumulators through hot plates weld made. These cell vessels contain the Electrolyte fluid and store the heavy electrodes sentences. In operation, the cell vessels are vibrating and exposed to occasional impulsive forces, so that the welds have considerable strength and Must have permanence to last for long periods of time to remain intact.

To ensure flawless and reliable training in such a way Checking welds is a number of techniques has been developed. According to a known method a very strong electromagnetic field across the Welded away and determined whether a dielectric breakdown occurs. If small pores and / or there are cracks in the weld seam, so leads this leads to a reduction in dielectric strength and there will be a sparkover if one corresponds  strong electromagnetic field is applied.

Another method to examine the flawlessly welds are in a reliable condition therein, a mechanical impulse force on the weld let it act to make it break resistant average. As part of the production of cell vessels for Batteries or the like. This is accomplished by that an arrow-like drop weight with a tip fall onto the weld from a predetermined height is left to an extremely high point load generate a strong pressure drop across the weld Of course, other Beaufschla techniques depending on the intended requirements nisse of the end product can be used. This sub search techniques for reliability and strength of welds have been used in many applications proven useful in which the proper procurement weld seam is of critical importance.

Using this and other known studies methods, it has been shown that a warm plate welding, in which simply the edges together the thermoplastic parts to be connected heated and then against each other to form the weld be depressed, to a reduced tensile strength of the material in the area of the weld seam. The train The strength of the material on the weld seam is there e.g. B. at 85% of the tensile strength of the undisturbed mate rials of the body of the plastic part, and even represents under. In addition, it has been shown that the Applying a strong electromagnetic field across one hundred times higher failure over the weld dielectric test rate, i.e. a breakdown, results than with the undisturbed material. The shock resistance such welds when examined by a Fallge weight with tip is clearly below that of the undisturbed material and is also subject to  different places along the weld and from weld to weld not inconsiderable Fluctuations, so that the overall reliability liquidity of the weld is reduced. After all It has been shown that the flexural strength in particular the Bendability of the weld seam around the weld axis as the bending axis is significantly reduced.

From "Fundamentals of plastics processing", Carl Hanser Verlag, Munich 1970, pages 4 to 10 and 200 to 201 known that residual stresses inevitably with everyone Weld caused by cooling, which in unun other cases to lower the strength and Can cause stress cracking. So sometimes it can be useful to anneal the weld. An annealing below the melting point leads to an increase in Degree of crystallization especially in the zones in follow faster cooling a lower crystalli have degree of station. Although by tempering overall increasing degree of crystallization probably the local fluctuations of this size are reduced, the However, residual stresses cannot be removed. Orientation and internal tension can only be achieved then undo when the framework of the crystalline Be rich is destroyed by melting.

From the generic DE-OS 23 48 609 is a thermo Welding process for two plastic walls known, wherein at least one edge of each of the two parts on at least Softening and melting temperature of the thermoplastic Material is heated, the heated edges to form a weld are pressed together, the Pressure on at least one side of the weld Bead of thermoplastic material and that melted or softened material to a temperature abge below the melting or softening temperature  is cooled. Through a stop or stamp arrangement there is a lateral pressure on the weld seam exercised to stop the formation of a bead to prevent.

This stamp arrangement is applied as long as long the plastic is warm to prevent the formation of an external to suppress the outer bulge in a simple manner. In which The method according to DE-OS 23 48 607 is however also disadvantageous that residual stresses arise in the material remain, which in turn weakens sweat seam leads.

The object of the invention is therefore a method of type mentioned, in which an improved connection strength and reliability between thermoplastic parts is enabled, as well a device for performing the method create.

This task is characterized by the characteristics of the Claim 1 and claim 3 solved.

Because the weld seam and that adjacent to it thermoplastic material under an elevated temperature Exposed to pressure, which are sufficient for the temperature of the thermoplastic material at least approximately raise its melting or softening temperature and flatten the at least one bead against the weld squeeze, and that the heated areas quickly under the melting or softening temperature of the thermopla be cooled material and that for cooling Ambient air in the area of the squeezed thermo plastic material is sucked, it is achieved that the resulting residual stresses are significantly reduced become, resulting in a significantly increased strength  and reliability of the weld. Farther the physical properties are advantageous ten of the plastic material in the immediate neighbor shaft of the weld with regard to impact resistance, di electrical strength and bendability improved.

A simple device for performing the method is created in that a device for quick len heating of the weld seam and the bead to a little very close to its softening or melting temperature and for cooling the heated bead to a Temperature below the softening or melting temperature rature is provided that the facility for fast Warm up a band of electrically conductive material has high electrical resistance and the tape on a power source is connected, one with the opening in the middle provided device for supporting the Has band, the band is above the opening is organized and define a closed space the frame with an opening on its surface points, which the outside of the frame with the abge closed space connects, the band and the Ab support are attached to the frame such that the Opening in the support with the opening in the surface che of the frame coincides, and that one with the enclosed space related suction device for drawing in ambient air from the outside an opening in the support and further through the opening is provided in the enclosed space.

In operation of the device after training Weld the tape against the weld formed pressed and to about the softening or melting temperature rature of the plastic material heated. Then the heated weld area and band cooled quickly, which creates ambient air at the connection zone and the belt  is guided through the network of cooling grooves. If the plastic material has cooled sufficiently the tape can be lifted off the plastic.

The respective subclaims have advantageous further formations of the invention to the content.

The invention is based on the following description and the drawings explained in more detail. It shows

Fig. 1 shows a schematically simplified representation of a weld bead with a rounded bead on each side of the seam,

Fig. 2 is a simplified perspective view of one embodiment of apparatus for carrying out an improved Warmplattenschweißung,

Fig. 3 is a perspective view of a preferred embodiment of the device according to Fig. 2,

Fig. 4 is a cross enlarged view lying to the cooling grooves partial section through the, on direction of FIG. 3

Fig. 5 a simplified schematic representation a cross section through a Herge with the method presented weld,

Fig. 6 is a sectional view of a simplified device in training accelerator as Fig. 3,

Fig. 7 is a perspective view of two end portions which fäßes by welding to form a cell housing can be connected together,

FIG. 8 is a side view of the cell vessel formed and welded from the halves according to FIG. 7 and

Fig. 9 is a plan view of the cell container of FIG. 8.

In Fig. 1, a cross section through a welded connection is shown schematically simplified, which has been generated by heating the respective edges 10 and 12 of two thermoplastic plastic parts. After warming the edges to their softening temperature, the edges were pressed against each other to form the welded joint. The pressure of the melted or melted plastic edges of the thermoplastic material due to the compression of the edges creates a rounded bead 11 on each side of the weld. The dashed lines 13 and 13 ' in the vicinity of the respective edge 10 and 12 illustrate simplified that area 15 of the thermoplastic material which has been reheated in the course of the welding. Weld fractures or cracks when examined with case weight with tip in the manner described in the introduction often occur between and along the respective boundary lines 13 or 13 ' between the reheated sections 15 and the unheated sections 17 of the thermoplastic material.

Show weld fractures during drop weight analysis that the thermoplastic material in the range of The weld seam is more brittle and less ductile than the uneven one disturbed material. While the tensile strength of the Material at the weld is well known the higher brittleness compared to the undisturbed material Ductility and lower ductility of the material in the area the weld even more serious side effects of the Welding process. In addition, the mate stands out rial in the weld area due to a significantly higher Failure rate in the dielectric investigation ent speaking of the dielectric requirements of the industry out.

There are several reasons for the changes in shock weld seams:

• 1. The Mon molecular weight distribution of the thermoplastic material as if the impact resistance of the weld could be affected sen and be responsible for the differences;
• 2. that Material was oxidized during welding and was thereby more brittle; and
• 3. the crystalline structure of Ma terials on and in the vicinity of the weld seam coarsened by the heat of welding.

However, it could last until it has not been conclusively proven that any one of the reasons above, or a combination thereof the drop in the impact resistance of the thermopla material in the seam area is responsible.

It was found that the Shock resistance of the material at the weld seam as a result increased and the failure rate for dielectric tests material in the area of the weld seam in the we could be considerably eliminated by the fact that

• 1. that Material in the welding area to a temperature at least corresponding approximately to the softening temperature of the thermo plastic material was heated using the increased pressure is exempt, and
• 2. the warmed up Connection area quickly to a temperature at least  is quenched below the softening temperature.

When welding two plastic parts made of ther plastic material occurs in the introduction a bead at least along one side of the Weld on. The bead can be before the Er warming and deterrence will be removed, however preferably not removed.

The exact heating temperature depends on the specific welded thermoplastic materials and can for example for branched polyethylene at only about 150 ° C, but also around 480 ° C, when it comes to thermoplastic polyethylene you high acts. The heating temperature therefore depends on the Softening or melting temperature of the thermoplastic Material, i.e. from the temperature at which the thermoplastic material is melted. In the Practice the exact heating temperature for one special thermoplastics, for example be that the temperature is chosen at which sufficient melting or softening over time span of up to about 25 seconds occurs.

Pressure is applied to the weld joint during or after heating. The pressure must be sufficient to substantially flatten the material at the weld joint. In practice, the pressure can fluctuate greatly, depending on the device used and the selected temperature, the fluctuation range being between approximately 1.4 and 6.4 kg / cm ² . If the bead on the weld is not removed, the pressure must be high enough to squeeze the bead against the weld until it is substantially flat or flat. In a preferred procedure, the pressure is applied during the heating, the combined effect of the heating and the pressurization being sufficient to squeeze the bead flat.

After the pressurization, the heated sweat is immediately put off. In the wake of deterrence rapid cooling of the heated, welded seam to a temperature at least below the softening point point of the thermoplastic material, preferably on a temperature at which the thermo plastic material is more present. By the abre The material will then be rewrapped at the weld increases.

The deterrent must affect the pressure or heat Follow me pressure treatment of the welded seam. The deterrence must therefore rule out that the pressure or pressure and heat treated weld under ambient conditions cools down. Surprisingly, it has been shown that a Deterrence after joining the softened edges and pressing these edges together under pressure, so immediately after the formation of the weld in which Practice in improved properties of the weld regarding the dielectric properties, the shock resistance strength and the ability to bend around the axis of the sweat near no precipitation. The deterrent can for example by immersing the treated Weld in water.

In order to eliminate the problem of reduced tensile strength, low shock resistance and the high failure rate in dielectric tests, as well as the problems which arise from the formation of a round bead along the weld seam in the longitudinal direction, a device was developed which in its simplest form tion form is illustrated in Fig. 2. In Fig. 2, an insulating tape 19 is illustrated, which is in the preferred embodiment of ceramic material or a high temperature resistant plastic such as Torlon, a poly amide. A tape 21 , preferably made of a titanium alloy, with a thickness of 0.30 mm and a width of 25 mm is placed over the insulating tape 19 . As can also be seen from the drawing, the insulating tape 19 has the dual function of electrical and thermal insulation on the one hand and rapid cooling un inter alia also the tape 21 on the other.

After an annealing treatment, the strip 21 has an electrical resistance of approximately 180 microohm-cm, excellent corrosion resistance and a yield strength under tensile stress of approximately 9150 kp / cm ² at room temperature. The high strength of the alloy is important for the resistance to the local pressure forces that occur when there is initial contact with the weld bead. The high strength is also important due to the forces acting on the titanium strip when it is exposed to high temperatures. For example, when the titanium tape is heated to 230 ° C to 260 ° C, thermal expansion occurs. On the other hand, the plastic outside the welding area 15 prevents the surface area of the bead on the heated band 21 from expanding or contracting along the weld seam during the squeezing of the bead and the cooling of the squeezed bead.

As a result, the expansion and contraction of the heated band 21 in its longitudinal direction leads to shear stresses between the band 21 and the weld bead. These shear stresses could lead to defects in the surface of the plastic and to possible warping of the welded plastic part. Therefore, the band 21 made of titanium alloy is subjected to tensile stress of such an elongation which is slightly larger than the maximum change in length due to the action of heat during heating and cooling. This elongation is kept constant by suitable clamping or the like. During the heating of the belt 21 , for which purpose the screws 55 and 56 according to FIG. 6 can serve, for example. In this way each point remains ent long the strip 21 during heating and cooling relatively constant as the projection is substantially at rest, and the length of he heated portion of the belt 21 remains substantially. Since at room temperature a mechanical tension of about 2460 kp / cm ² is required for the necessary elongation of the tape, the strength of the tape 21 must be relatively high, even if this mechanical bias is largely reduced by the thermal elongation at high temperatures. However, it is obvious that a material with a yield strength at a tensile load of 9150 kp / cm ^{2 has} a sufficient strength for the prestress to be applied to the belt 21 .

As can be seen from FIG. 1, the heated band 21 together with the insulating support band 19 is pressed against the bead 11 in order to soften it. The bead is squeezed in the vicinity of the weld seam. During this process, melted or softened thermoplastic material adheres to the belt 21 . When the tape 21 has cooled below the melting point of the thermoplastic material, the adhesive effect between the tape and the thermoplastic material ceases and the tape can be removed from the material.

It has been found that the heated weld must be quenched quickly to achieve the benefits of improved tensile strength, shock resistance and improved dielectric properties of the material at the weld. Accordingly, a plurality of holes 23 are provided in the band 21 , each of the holes 23 communicating with the other holes 23 via a trough-like depression 25 in the insulating support band 19 . In one embodiment, cool air is blown out through the trough 25 and through the holes 23 on the heated thermoplastic material, as illustrated by arrows in FIG. 2. As a result, the thermoplastic material and the tape are rapidly cooled to give the desired crystalline structure, the crushed thermoplastic material showing a smooth, closed surface with a very low failure rate in the dielectric test. The formed weld bead forms an additional flat material layer, which is layered on the base material. This also greatly reduces the possibility of small defects in the weld leading to leakage.

A preferred embodiment of the device is illustrated in FIGS. 3 and 4. An insulating tape 29 , which consists, for example, of ceramic material or high-temperature-resistant plastic, has a groove or trough 35 running in its center line and a plurality of transverse grooves 33 of comparatively small size, which are arranged along the length of the insulating tape 29 . A heating tape 31 is arranged above the insulating tape 29 , which preferably consists of the titanium alloy 6AL4V and has a thickness of 0.30 mm and a width of 25 mm. The band 31 is, as already explained above in connexion in Fig. 2, stretched so much in the idle state at room temperature by tensile stress that the theoretical maximum expansion due to the heating by this mechanical pre-stretching is slightly exceeded to the position to keep the tape unchanged with respect to the thermoplastic bead during the Warmquetschvor.

In the embodiment according to FIG. 3, ambient air is drawn in through grooves 33 by means of a suction pump (not shown in more detail), which generates a negative pressure of approximately 0.2 at. Characterized in that cool ambient air is sucked through the Nu th 33 , there is a more uniform distribution of the air on the belt 31 and the pinched bead and thus a more uniform cooling of the belt 31 and the pinched thermoplastic material.

The trough 35 should have a comparatively small width, so that a clean support of the band 31 is not jeopardized, so that the trough 35 must be made relatively deep for it to have a sufficient cross-section to direct the air flowing from the individual grooves 33 to the suction pump receive. In addition, the grooves 33 should have a sufficient width to give a large cooling surface for the belt 31 , but on the other hand should not be so wide that a clean support of the belt 31 is jeopardized.

In the preferred embodiment, the grooves 33 have a width of 1.7 mm and a depth of only 0.17 mm, each groove being separated from the adjacent grooves by a 0.5 mm wide web. This groove arrangement and training serves to keep bending stresses in the band 31 low and at the same time expose a comparatively large area of the band 31 to the cooling air. At the same time, the grooves serve for insulation during the heating and avoid a strong heat transfer from the band 31 to the support band 29 . The central groove or trough 25 is deep and narrow, so that it rests on the band 31 with a comparatively small area and therefore supports it comparatively rigid, while at the same time achieving a sufficient cross section to guide the air from the grooves 33 to the suction pump.

If, in the embodiments according to FIG. 2 or according to FIGS. 3 and 4, current is passed through the band 21 or 31 , this becomes sufficiently warm to bring the bead 11 according to FIG. 1 to about its softening or melting temperature or a temperature bring higher temperature. Support brackets 20 (see FIG. 1) for the back of the insulating tape 19 or 29 and the tape 21 or 31 press the warm tape 21 or 31 against the bead 11 to squeeze it flat against the welding area. The Rückerwärm te material of the bead 11 is thereby connected to the plastic in the welding area, as shown in Fig. 5, and thus improves the weld connection overall. The welded connection is not drawn to scale in FIG. 5 in order to be able to clearly illustrate how the crimped bead forms a thin additional layer of adhesive plastic material at the weld point.

In Fig. 6, a simplified longitudinal section through the device for producing cell vessels for batteries and accumulators is illustrated, which uses the training illustrated in Fig. 3 in detail. The band 31 is arranged above the insulating support band 29 , which in turn is supported on a steel frame 60 which surrounds a closed space S. The space S is on the one hand with a Saugpum pe P and on the other hand with the trough 35 in connection, which in turn is in communication with the grooves 33 . When the vacuum pump P is started up, it sucks air under pressure drop along the band 31 and the welded joint and cools both. A copper coating 57 , which is shown exaggeratedly to improve the clarity, is located on those loading areas of the tape 31 above the tape 31 , which do not come into contact with the plastic material, and avoid overheating of the tape 31 in these areas.

The band 31 , as explained in detail above in connection with FIG. 2, is held for elongation under tensile load. The screws 55 and 56 illustrate schematically a set of devices to achieve this purpose, but any other designed holding devices can be used just as well ver. When the screw 56 is rotated, one end of the band 31 is pulled around the upper end of the housing 60 in the example and thus subjected to the desired tensile load. As can be seen from FIG. 6, the support band 29 on which the band 31 is mounted can be seen on a flat surface. However, the support surface for the support band 29 does not necessarily have to be flat or flat, but can also have a surface which is adapted to the surface of the thermoplastic workpiece in the connecting region. Thus, if two thermoplastic pipes are to be welded together, this support surface can be annular or cylindrical. A piston and cylinder arrangement 61 serves to actuate the frame 60 in the sense of contacting the welding point 41 .

A second device 58 is anschaulicht in Fig. 6 schematically ver that the plastic part 14 is arranged on the opposite side of the weld 41, and serves to heat the 41 formed bead on the opposite side of the welding point in a corresponding manner and to swage.

example

The device according to FIGS. 3 and 4 in connection with the further training according to FIG. 6 was used for the manufacture of cell containers for batteries from a propylene-ethylene copolymer mixture, the plastic parts for the cell container and the cell container according to FIG . 7 to 9 were formed.

The elongated cell vessel according to FIGS . 8 and 9 has two end halves 37 and 39 , which are illustrated in FIG. 7 and are each open at the top and have an elongated open side and three elongated closed sides. The distance between the surface defining the elongated open side of each half and the opposite closed side is at least several times less than the distance between the upper side and the bottom side. As a rule, the distance between the open longitudinal side and the closed back wall about _^1/4 to ¹ / _10th of the distance between the top and bottom wall of each half 37 and. 39 Each half 37 and 39 has a wall thickness which remains essentially the same from the top to the bottom side, so there is no taper or draft from the top to the bottom side, and each half is mirror-inverted to the associated other half. Halves 37 and 39 are heat welded at the respective elongated ends to form the cell vessel illustrated in Figs. 8 and 9.

With regard to the production of the halves 37 and 39 , for the sake of simplicity, reference is made to US Pat. No. 4,118,265, to which full reference is made for further details.

The main requirements for a cell vessel are that it has to be resistant to battery acid must be liquid-tight, a certain dimension must have bility, have a high impact resistance to avoid accidental bumps in the battery manufacturer resisting and their use, a equal Width and length from top to bottom must have, so no taper or bevel may sit, have straight side walls that neither are still bent inwards, and a certain ses bending and / or deformability must have Avoid breaks during handling.

As already explained, the softened plastic material 11 forms beads on the inside and outside of the cell vessel of the type shown in FIGS . 1 and 8 and 9, if the corresponding edges of the halves 37 and 39 , as illustrated in FIG. 7 , heated to the melting or softening temperature and then pressed together to form a weld. After the weld seam has cooled, the insulating support band 29 and the band 31 according to FIG. 3 are aligned both along the inside and the outside of the welding area against the beads formed during welding, for which purpose a device of the type shown in FIG. 6 is used becomes.

The titanium alloy band 31 is then heated over a period of time ranging from 2.5 seconds to over 20 seconds while under pressure on the beads. As a result, the beads are softened and squeezed flat against the heated welding zone 13 according to FIG. 1, so as to form the flattened weld seam according to FIG. 5. The tape 31 and the crushed beads are then cooled by sucking air at room temperature through the grooves and the trough in the insulating support tape 29 ge. When the squeezed beads have cooled down sufficiently so that they no longer th the band 31 anhaf, the tape 31 and the insulating support belt 29 are removed so that the final result cell box.

It has been shown that using longer heating the strength and stability of the drop test means of a drop weight provided with a tip. However, it has also been shown that with prolonged heating The cell vessels show faults, namely into the special at the top near the open en of the cell vessel, so that the cell vessel in to an extent that is no longer tolerable, internally or externally is bent. This warping with prolonged warming are obviously due to shrinkage in plastic material after heating to the melting point. The material in the area over which the bead ver is squeezed, is heated up to the melting point and shrinks accordingly as it cools down  the surrounding material, which is not in such Extent was reheated, not shrinking.

One way to avoid such problems due to warping is to preheat the welded cell vessels to a temperature of 82 ° C to 93 ° C before squeezing the beads. As a result, the entire cell vessel will shrink somewhat as it cools, so that the difference in shrinkage between the material in the vicinity of the weld seam and the rest of the cell vessel can be significantly reduced. Such a procedure is not desirable in large-scale production, since the extended cooling time, which is required for preheated cell vessels, noticeably increases the total manufacturing time for the cell vessels. It has been shown that an improved weld with high impact resistance without significant warping can be achieved by heating for only a very short time of about three to four seconds, and the bands made of titanium alloy are heated to higher temperatures. The extent of faults was further reduced by the fact that relatively cool ambient air is sucked in under the band 31 , so that the material of the cell vessel cools in the immediate vicinity of the band. This leads to a narrower range of heating of the plastic material, which is exposed to shrinkage phenomena, which in turn warps or twists in the walls of the cell vessel are reduced as a result of the shrinkage. So that a relatively short heating time is used and air is sucked in from the surroundings of the heated plastic material, the total treatment time for the weld is less than 30 seconds.

Claims (7)

1. A method for connecting two parts made of thermoplastic material with each other, at least one edge of each of the two parts being heated to at least softening and melting temperature of the thermoplastic material, the heated edges being pressed together to form a weld seam, the pressure being at least one side of the weld forms a bead of the thermoplastic material and the melted or softened material is cooled to a temperature below the melting or softening temperature, characterized in that the weld and the adjacent thermoplastic material are exposed to an elevated temperature under pressure, which are sufficient to raise the temperature of the thermoplastic material at least approximately to its melting or softening temperature and to squeeze the least least a bead against the weld seam that the heated areas quickly below the melting or softening temperature temperature of the thermoplastic material are cooled, and that ambient air is sucked into the area of the squeezed thermoplastic material for cooling. 2. The method according to claim 1, characterized in that that each side of the weld seam of the elevated tempera structure and exposed to pressure.   3. Device for welding two parts made of thermoplastic material for performing the method according to one of claims 1 or 2, with a device for heating at least one edge of each of the parts to at least the softening or melting temperature of the thermoplastic material and a device for pressing together he warmed the edges to form a weld, characterized in that a device for rapid heating of the weld and the bead ( 11 ) to at least approximately its softening or melting temperature and for cooling the heated bead ( 11 ) to a temperature below the Softening or melting temperature is provided that the device for rapid heating has a band ( 21, 31 ) made of electrically conductive material with high electrical resistance and the band ( 21, 31 ) is connected to a power source, one with the opening (trough 25 ; 35 ) in the middle of the facility (volume 19; 29) for supporting the belt (21; comprising 31) wherein the tape is placed over the opening and a closed space (S) bounding (comprising 60) frame having an opening on its surface which the outer side of the frame (60) connects to the enclosed space (S) , the band and the support being fastened to the frame such that the opening (trough 25; 35 ) in the support (band 19; 29 ) coincides with the opening in the surface of the frame, and that one with the enclosed space (S) in connection with the suction device (suction pump P) for sucking in ambient air from the outside through an opening in the support and further through the opening into the enclosed space (S) is easily seen. 4. The device according to claim 2, characterized in that the band ( 21; 31 ) has a titanium-containing mate rial. 5. Device according to one of claims 3 or 4, characterized in that the support device (band 19; 29 ) has insulating material and the opening is designed as a trough ( 25; 35 ) which with grooves cut into the material ( 33 ) From the support device is connected. 6. Device according to one of claims 3 to 5, characterized in that the opening in the upper surface of the support device (band 19; 29 ) is designed as a trough-shaped trough ( 25; 35 ) with grooves ( 33 ) in the surface the support device (band 19; 29 ) is connected, the grooves leading ambient air along the band ( 21; 31 ) to cool it. 7. Device according to one of claims 3 to 6, characterized in that a device for generating a bias of the band ( 21; 31 ) is provided such that in the electrical heating of the band ( 21; 31 ) substantially no movement occur between the belt ( 21; 31 ) and the thermoplastic material.