DE4112346A1 - Heated tool, esp. metal band, for welding of plastic, e.g. polyethylene@ - comprises flexible band which deflects to give reduced pressure at edges, improving weld strength and avoiding penetration and extrusion - Google Patents

Abstract

Process and tool for welding plastics film in which the heated tool (16, 22, 24) is applied against the film (10, 12) with progressively smaller pressure towards its edge(s) (26, 28). A welding band may be constructed of a lower narrow central support core (2) and a thin wide upper band (24) which bows elastically under pressure applied through bar (22). USE/ADVANTAGE - Hot tool lap welding of plastics films, e.g. PE, redn. of pressure at the edges avoids the usual depression and extrusion of melt at the edges of the weld and poor weld properties leading to splitting of seams, e.g. if massive goods are dropped into a pre-sealed bag.

Description

The invention relates to a method for welding art fabric films by means of a heatable welding element, which against a predetermined force during the welding process the film to be silenced is pressed on. The invention relates furthermore on a heatable welding organ for through conduct such a procedure.

In film processing packaging machines are used for ver welding the foils often used metal tapes heated a short pulse of current and healed with egg ner certain force against the film to be welded be pressed. It is immaterial with regard to the invention Lich whether the sweatband is heated first and then against the film pressed, or whether it is first pressed and then heated. Even with a continuously heated sweatband, the Invention find application.

The sweatbands are mounted and insulated on metal rails result in a welding device. The traditional way welds generated with it are indeed sufficiently strong, i. H. the foils are fused well, but they are on the seam edges more or less squeezed because the over the entire cross-section heal metal substantially uniformly tapes with their marginal edges into the material of the foils to press.

So far, attempts have been made to min change the sweatbands somewhat spherically and / or their edges are beveled. However, it has been shown that these measures to avoid edge crushing are not sufficient are sufficient because the parameters essential for the welding result ter pressure and temperature quite evenly over the surface of the sweatband are distributed and therefore on the side  Edges there is an abrupt drop in pressure and temperature, which results in the unwanted edge of the square.

The best welding results are currently with the so-called Sil Spur band achieved, which has a stronger temperature gradients th between the middle area and the edge zones of a Sweatband causes. Apart from the fact that the manufacture of Silver track bands is technically very complex, could also at These have not yet been overcome by the lack that the steep pressure increase on the lateral edges later damage by bursting the welded foil right next to the Seam.

The invention has for its object a method of type mentioned and an associated welding organ for To make available with their help in a very simple way a "softer '' pressure and temperature rise to the middle zone of the welding organ is maintained.

The above object is achieved in that the welding organ from its central area to at least one ner edge edge with increasingly less surface pressure is pressed against the film, preferably the surfaces pressure on the edge of the welding element is so low that during the welding process Level of the film surface.

The method according to the invention is particularly easy to load Execute way with a heatable welding element, which in the Cross section of at least one with a ge against the film directed force to the core and at least one supported, resilient flange part. At the Press against the film to be silenced in such a Welding element the maximum surface pressure only in the area of Kerns, d. H. with symmetrical construction of the welding electrode in reached the middle zone. The one supported only at the core, too cantilevered spring elastic on its lateral edge  cal flange part, however, is due to the pressure against the Film bent back in the manner of a spring tongue, which was used Success is achieved that the surface pressure between the film and the welding element from the middle area, i.e. over a relatively rough range stood, becomes increasingly smaller towards the edge.

It represents a particular advantage of the proposed sweat organs shows that not only the surface pressure, but the same timely, too, without there being any complex control measures or a complicated tool structure like the silver track belt a sufficiently large temperature gradient to the edge is achieved. The temperature drop is immediate in addition to the core of the welding element, relatively small, because the air volume in the cavity between the flange part and the lower or rear support of the welding member, which whose contact pressure generates or absorbs acts as an insulator. With the edge, however, the flange part is preferably up to bent back to touch the pad. The heat drain over the point of contact causes a relatively strong Temperature drop in the edge area of the flange part of the welding gate goose, so that overall the edge area a gradual, white Chen transition in pressure and temperature between the not welded condition next to the weld and the Represents welding conditions in the core zone. The invention can take advantage of the fact that a very thin Bend the flange part well on the one hand in the manner of a spring tongue sam is, on the other hand heat capacity and heat flow in the thin NEN flange part are limited so that automatically Edge edge over a relatively rough distance a nega tive temperature gradient results.

In practice, optimal welding conditions are achieved if the dimensions of the welding element and the rigidity of the Flange part are dimensioned so that the outer edge selects during the welding process straight up to the support the edition is bent back. It doesn’t quite come to the fore  tion between the edge of the flange part and the support, depending on the remaining distance, an in their harmful effects depending on their extent Pinch edge in the film. In addition, the temperature drop to the edge not so cheap because the heat flow over the contact point is missing. On the other hand, finds a strong support tongue of the outer edge of the flange part on the support instead, one keeps the advantage of the favorable temperature gradients ten, but must be strong depending on the support force at the edge Do not accept any pressure drop over a short distance.

In a preferred embodiment of the invention, the welding element heated by a current pulse for each welding process. Here the time factor can also be used as the third welding parameter be used part of the invention, because of the larger Heat capacity of the core of the proposed welding element in the In relation to the flange part, the latter cools down faster and leads to the edge of the weld during a welding process less heat.

The basic idea of the invention applies to all types of Welding organs, including those for creating welding spots and roll-shaped welding elements. Have in practice however sweatbands are of the greatest importance. An inventive ß sweatband can z. B. in one piece with a substantially T-shaped cross section, the middle Web part forms the core of the welding electrode. Simpler and it is cheaper to use two tapes on top of each other, namely a lower band supported on a support, which is relatively thick and narrow, and one arranged above it nice upper, relatively wide band. Both tapes best hen preferably made of steel and can by welding dots relatively loose a few centimeters apart be connected. It comes with this connection le diglich that the tapes are relative to each other Maintain in the long term, because they work during the welding process  the forces normal to the major surfaces of the ligaments, so that result in essentially no displacement forces.

It is understood that the times of the invention Align the sweatband according to the respective welding task. On the packaging sector thermoplastic films such. B. PE-, PP or polyamide films with and without reinforcement up to one Processed thickness of about 200 microns. For a simple weld to produce, can in a welding device according to the invention upper band z. B. have a width of 6 mm and 0.08 mm thick be. The width of the lower band is in the example only about 1/3 the width of the upper band, but is different partly three times as thick as the upper band. From the latter preferably the top, although it is already very thin Edges beveled or rounded to the pressure and tempera door transition at the edge to be even more even and to avoid any crushing effect.

Since only the top, i.e. H. with reference to the foil to be welded front band in cross section should form a spring tongue, weh rend the lower or rear band only a support of the represents upper band and a heat store, for both bands use different materials So especially for the upper band steel with a after Use case selected coefficient of elasticity while this does not play a decisive role in the lower volume. However, this can be found on the side of flat over oval to approximately circular rounded or to the Side edges beveled towards the curvature of the top Tape during the welding process. One can So if necessary use a round wire as the lower band.

In order to obtain particularly reliable sealed weld seams, he one creates them in the form of a double seam. Here too, a inventive sweatband can be used, which in the case from a thin, elastic, upper band part and two spaced apart in parallel  Cores e.g. B. in the form of narrower, thicker strips, round wires ten or the like. Alternatively, one would also be in this case one-piece design possible. In all cases the upper, thin, elastically flexible band part, which during the Welding process also in the area between the two cores is elastically bent back for a negative gradient all welding parameters with increasing distance from the core Zone.

The invention is also not limited to that of the core zone from an elastic flange part, for. B. in the form of egg thin upper band, extended on both sides. If the film to be silenced immediately next to the weld to be cut, you will be on one side of the core one knife edge and only an elastic on the other side Provide flange part. In a corresponding way, on the perforation may be applied to one side of the weld.

Embodiments of the invention are described below the invention are based on the drawing he he purifies.

Show it:

Figure 1 is a representation of the pressure curve when welding thermoplastic films with a substantially rigid sweatband in the transverse direction.

Fig. 2 is an illustration corresponding to FIG. 1 with a sweatband according to the invention and

Fig. 3 is a simplified perspective view of a sweatband according to the invention for producing a double seam.

Fig. 1 illustrates the situation when two foils 10 , 12 lying one on top of the other by means of a conventional sweatband 14 shown in cross section, which behaves transversely to its longitudinal direction practically like a rigid body when it is sealed during the welding process by a support 16 welding foils 10 , 12 is pressed on. Even if the upper edges are chamfered or rounded, the sharp rise in temperature and pressure, as indicated at 18 and 20 in FIG. 1, results in an edge notch effect at the edges of the weld seam. The main reason for this is the short transition between the warm and cold zones, which is made more difficult by the steep rise in pressure in the same area. This deficiency is particularly noticeable in the case of thermoplastic films with poor welding properties, which include PVC films in addition to the abovementioned, as well as when heavy, lumpy goods are to be packaged or when, such as. B. with liquids, increased reliability is required.

Fig. 2 shows, in comparison to Fig. 1, a welding member according to the invention during the welding process. It consists of a lower, relatively narrow but thick band 22 supported on the base 16 and a z. B. connected by spot welding, relatively thin upper band 24 , which in the example is about three times as wide as the lower band 22 , so that it projects laterally with about 1/3 of its width. The thickness of the upper band is only about 1/3 or 1/4 of the thickness of the lower band. Both bands 22 , 24 are made of steel.

During the welding process, only the central core area, ie a zone with the relatively narrow width of the lower band 22 , is pressed against the foils 10 , 12 to be welded with the usual surface pressure. Towards the edge edges, the thin upper band 24 can absorb less and less force, so that it is bent back in response to the pressure against the film 12 , as shown in FIG. 2, until it finally supports the support with its side edge edges 26 , 28 16 touches. The upper edges are preferably beveled.

With a welding strip according to FIG. 2 gives the there schematically indicated temperature and pressure profile with relatively weakly inclined lateral edges 30, 32, for the same width of the welding strips as in FIG. 1 and FIG. 2 are provided in the latter embodiment about 2 / 3 of the width for the gradual temperature and pressure increase from both sides.

If one looks at the welding process with a sweatband according to FIG. 2 over time, the following can be ascertained:
The heating of both strips 22 , 24 to the required welding temperature by means of a current pulse occurs essentially in the same time. The dissipation of the welding heat is, however, different. The lower, thick band 22 with higher heat capacity releases welding heat over the contact zone with the thin band 24 over a longer period of time. The two laterally over the band 22 protruding areas of the upper band 24 ge ben heat a little faster, but not too quickly, since there is a cavity 34 below each of these areas next to the thick, lower band 22 , the air volume insulates. In contrast, at the contact points 26 , 28, heat is dissipated from the upper band 24 into the base 16 , so that the outer edges of the upper band 24 cool down relatively quickly.

The described heat and pressure curve results in an optimal weld with a smooth transition without notch effect on the adjacent material areas.

In this context it should also be borne in mind that the pressing of the welding member into the Fo to be welded lien a cross-sectional reduction and thus a ver leads to a relatively strong weakening of the material. A reduction in cross-section in places can be done  zipiell not ver with the sweatband according to the invention avoid, but the gradual transition to the adjacent mate areas has decisive advantages in individual cases. This is particularly useful when packing heavy lumpy goods noticeable. Such a heavy piece falls into one before welded foil pouch, so its kinetic energy to split the already cooled and little ge weakened edge zone of the seam and is then at the Errei the endangered middle zone, which is already sufficient is consolidated so far that it is no longer used tear is sufficient.

In comparison with welding with a complex silver track band, the advantage of a sweat band according to FIG. 2 is that not only the temperature but also the pressure towards the edges of the weld seam decreases more evenly.

Fig. 3 shows a simplified, perspective front view of a severed sweatband with which a double seam can be produced. It consists of two lower, relatively narrow, but thick bands 36 and 38 which extend parallel at a distance from the two seams of the double seam. The bands 36 , 38 , which correspond to the band 22 of the embodiment according to FIG. 2, are connected to a common upper, substantially wider, thin band 40 , which corresponds to the band 24 of the embodiment according to FIG. 2. During the welding process, not only the lateral edge regions of the upper band 40 bend back until the support 16 is touched, as shown in FIG. 2, but also the region of the band 40 between the two bands 36 and 38 is shown in FIG. 3 bent back. In this way, there are also smooth transitions between the two welds from the respective healthy welding zone to the adjacent cold zone.

Both in the sweatband according to FIG. 2 and that according to FIG. 3, the upper sweatband 24 or 40 can have an uneven thickness over its cross section in order to influence the line of curvature during the welding process. In addition, the band can consist of several layers, which may not need to extend over the entire width of the band.

Other advantages of the new sweat that have not yet been mentioned procedure or the new one proposed for its implementation Welding organs are the simplified manufacture of one of the Border edges rounded shape - this results from curvature of the upper band or flange part by itself - and still reducing the consumption of polytetrafluoroethylene cover films, which so far due to the strong pressure and temperature differences de were quickly destroyed. Finally, on the Untersei te of the upper band a company print or other identification feature be printed. Incidentally, the inventor can also method and the new sweatbands the previous in the individual case necessary or considered conventional measures.

Claims (15)

1. A method for welding plastic films by means of egg nes heatable welding member, which is pressed during the welding process with a predetermined force against the film to be silenced, characterized in that the welding member from its central region to at least one edge edge with increasingly less Surface pressure is pressed against the film. 2. The method according to claim 1, characterized in that the Surface pressure at the edge of the welding element so ge ring is that this is in the welding process is located essentially in the plane of the film surface. 3. The method according to claim 1, characterized in that the Edge area of the welding member resiliently against the fo lie is pressed. 4. The method according to claim 1, characterized in that the temperature of the welding torch during the welding process goose in the edge area is much lower than in the middle area. 5. The method according to claim 4, characterized in that the Edge area of the welding element by heat dissipation on the the side opposite the film is cooled. 6. The method according to claim 1, characterized in that the Welding member is pulse heated in a conventional manner. 7. Heated welding element for carrying out the method according to one of claims 1 to 6, characterized in that it has a core ( 22 , 34 , 38 ) and at least one having at least one core which can be acted upon by a force directed against the film ( 10 , 12 ) and at least one supported, fe derelastic flange part ( 24 , 40 ). 8. welding element according to claim 7, characterized in that it has the shape of a one-part or multi-part metallic Ban des ( 22 , 24 ; 36 , 38 , 40 ). 9. welding element according to claim 8, characterized in that it consists of an upper, relatively wide, thin band ( 24 , 40 ) and at least one associated therewith, relatively narrow, thick band ( 22 , 36 , 38 ) and the Tapes with their ends can be connected to a voltage source. 10. Welding element according to claim 9, characterized in that the upper side edges of the upper band ( 24 , 40 ) are beveled or rounded. 11. Welding member according to claim 7, characterized in that the top of the lower band ( 22 , 36 , 38 ) is rounded or beveled towards its side edges. 12. Welding member according to claim 7, characterized in that the upper and the lower band ( 22 , 24 ; 36 , 38 , 40 ) are connected by welding points. 13. Welding element according to claim 7, characterized in that the upper band ( 24 ) is connected off-center to the lower band ( 22 ). 14. Welding element according to claim 7, characterized in that two lower bands ( 36 , 38 ) are arranged at a spacing and are connected to a common upper band ( 40 ) to produce a double seam. 15. Welding element according to claim 7, characterized in that the upper and the at least one lower band ( 22 , 24 ; 36 , 38 , 40 ) consist of different metals.