DE966402C - Arrangement for high-frequency treatment in a capacitor field, in particular for welding plastic films - Google Patents

Description

Arrangement for high frequency treatment in a capacitor field, in particular for welding plastic films It is known to use a wide variety of dielectric materials to treat in the high frequency field, d. H. they a high-frequency capacitor field suspend. One makes use of this above all to bring these substances together welding or to make similar high frequency connections. It has however, it has so far proven difficult to apply plastic sheets or films, e.g. B. such based on polyvinyl chloride, to be welded together or for similar purposes to be heated in the high-frequency capacitor field.

If, for example, plastic films are to be welded together be connected, as is the case with the side wall and the folded bottom of plastic bags is the case, one uses narrow capacitor electrodes, between which the one another Plastic parts to be connected are introduced and under during the welding process be held light pressure. There is a changing strength of the item to be treated inevitable. However, if there are air gaps between the material and the electrode, so occurs for known reasons because of the difference in dielectric constant the air on the one hand and that of the material to be treated on the other hand at the air gap high voltage, which can easily lead to breakdown. To this risk of breakdown it has been proposed to eliminate an electrode, preferably the lower one, to be flexible so that they fit into the bumps at all points of The item to be treated fits snugly and prevents the formation of air gaps.

For reasons of tolerance, such devices can be used Do not handle protruding electrodes, and there is thus the risk that the Treatment or welding process, because of the flexible electrode, an edge short circuit occurs.

Attempts have been made to avoid this edge short circuit in that a dielectric intermediate layer is inserted between the electrodes, or at least one of the electrodes is covered with a dielectric, for example a ceramic one Coating. But here too the phenomenon already described occurs again, that in the small air gaps that exist between the electrode and the dielectric Interlayer occur at the edges of the material to be treated, field concentrations that lead to flashovers. This could in and of itself be by belittling the welding or treatment tension can be avoided, but this increases the duration of the treatment elevated.

According to the invention there is therefore an arrangement for high-frequency treatment Proposed in a capacitor field in which the small thickness of the to be treated Good increased risk of overturning on those protruding laterally over the good Field electrodes is reduced by special means, which are characterized is that of the partial capacities lying in series that are covered by at least one of the electrodes protruding above the material with a dielectric layer in the Edge zone are obtained and partly air, partly the dielectric as an intermediate layer contain, the capacitance at the dielectric as small as possible, that at the air gap as possible is large.

The invention is explained below using a few examples; In the drawing, FIG. I shows a schematic for better illustration Representation of a device for high-frequency treatment, FIG. 2 a known example to avoid edge punctures, FIG. 3 shows a known electrode arrangement dielectric intermediate layer, FIGS. 4 and 5 show an exemplary embodiment of the electrode arrangement according to the invention, FIG. 6 shows a further exemplary embodiment with a widened edge Ground electrode in partial top view.

The embodiment shown in Fig. I is particularly for the welding of plastic sheets or foils in the high-frequency capacitor field thought. It is therefore in the following for the sake of a clearer explanation on this Application referenced alone. In the case of the plastic films to be welded or -Scrolling can be lengths of clothing, e.g. B. of raincoats, of Bags or packaging bags and the like act. This should be in the chosen Embodiment the webs on their superimposed edges by welding seams be connected to each other.

In Fig. I, the electrode I is with the help of insulators 2 on the Base attached. Above this is the raised and lowered second electrode 3 of the treatment capacitor arranged. The electrodes I and 3 are connected to the high-frequency generator 4.

The plastic sheets to be welded together are between the electrodes are introduced, which are only narrow according to the weld seam to be formed are executed.

In Fig. 2 the electrodes are shown on an enlarged scale. The lower electrode I is designed to be flexible and for this purpose in the form of a Metal foil stretched over a flexible cushion 5. The item to be treated 6 consists for example from two or more plastic sheets to be welded together. When the treatment pressure is applied during the welding process, the Electrode I because of the flexibility of its underlying pad 5 in the sense of a Avoidance of air gaps close to the unevenness of the item to be treated. It can Therefore, the electrodes are attached to the edge zones protruding beyond the material to be treated 6 the flexibility of the electrode I and cause an edge short circuit. At least there would be such a strong reduction in the air gap in these edge zones occur that thereby the risk of rollover through in this air gap occurring increase in field strength would be given. To reduce this risk, are in a manner known per se in the named edge zones of the capacitor field dielectric dummy body 7 introduced, which the breakdown endangering air gap almost fill in. Apart from the fact that in this arrangement on both sides of the dielectric dummy body, weak air interlayers favoring flashover are formed, it is particularly due to the mass production of objects disadvantageous for the dummy bodies to be introduced and removed with each work step. One has already tried to eliminate this drawback in that according to FIG. 3 at least one of the electrodes with one extending over the entire length of the electrode dielectric layer 8 occupied constant loss angle or such Layer is placed between the electrode and the item to be treated. Snuggles during the welding process but the lower electrode I on the item 6 and it can be do not avoid creating again small air gaps between the electrode I and the dielectric Intermediate layer, in particular at the edges of the item 6 to be treated, are present, and that is the capacitance that is formed with the dielectric, with the Air gap corresponding capacity in series. With a series connection of Capacitors behave inversely to the voltages across these capacitors their capacities.

We denote the capacitance of two separated by the dielectric Deposits with C1 and the associated voltage, which is applied to the dielectric, with U1 and analogously, the capacity with Ca corresponding to the air layer and the voltage across it Air layer with U2, the following applies: C1: C2 = U2: U1.

But U1 + U2 is equal to the voltage U between the electrodes lies and those for treatment of the good is necessary. By Insertion results in: U2 = U C1 t + C2 U2, i.e. the voltage at the air gap, should be made as small as possible according to the invention in order to avoid a rollover in the Avoid edge zones.

U can be viewed as constant. U2 can be done according to the formula Reduction of C1, i.e. the partial capacitance at the dielectric, and / or by enlarging it of C2, i.e. the partial capacity at the air gap, can be reduced.

The following examples show some embodiments to reduce the size the partial capacitance C1 at the intermediate layer and to increase the partial capacitance C2 at the air gap.

To this end it should first be remembered that a level capacity is can be calculated according to the formula C = A F, where d is a quantity that results from absolute and relative dielectric constant, F is the area of the electrodes and d is the distance between the electrodes.

The reduction of the partial capacitance C1 in the dielectric interlayer is therefore possible in various ways according to this formula. For example, can by reducing the dielectric constant of the intermediate layer in the Edge zones are achieved, that is, in that the coating 8 on the electrode 3 on Edge has a smaller dielectric constant than above the good.

4 shows an exemplary embodiment according to the invention for this purpose. The dielectric constant of the dielectric intermediate layer 8 is symbolic here represented by the density of the hatched lines. The dielectric constant decreases towards the edge zones.

A second possibility to reduce the partial capacity is to use the To increase the thickness of the dielectric. An embodiment for this is shown in Fig. 5 shown. The electrode 3 jumps back here on both sides of the item to be treated, and the intermediate layer is reinforced accordingly at these points. Here can it can be advantageous - as shown - to create an inclined transition.

The partial capacity C2 at the air gap can be increased by widening of the bare electrode I at the edge zones. Fig. 6 shows an embodiment for this purpose in a plan view of the electrode arrangement. Between those shown in section Electrode 3 and the lower electrode I are those to be connected to each other Plastic sheeting 6.

As can be seen from the drawing, the lower electrode I is to both Pages of the item to be treated 6 widened. This becomes the partial capacity C2 of the air gap increased, and since the partial capacitance C1 remained the same at the dielectric intermediate layer is, the tension of the unchanged height of the air gap becomes smaller.

PATENT CLAIM I. Arrangement for high frequency treatment in one Capacitor field, especially for welding plastic films, in which the Increased risk of flashovers due to the low thickness of the material to be treated reduced by special means on the field electrodes protruding laterally over the material is, characterized in that of the partial capacities lying in series, the by covering at least one of the electrodes protruding above the material with a dielectric Layer in the edge zone are obtained and partly air, partly the dielectric as Contain intermediate layer, the capacitance at the dielectric as small as possible, the at the The air gap is as large as possible.

Claims (1)

2. Arrangement according to claim I, characterized in that the dielectric constant the intermediate layer decreases at the edge zones of the capacitor field. 3. Arrangement according to claim I, characterized in that the intermediate layer is reinforced at the edge zones of the capacitor field in the field direction. 4. Arrangement according to claim 1 to 3, characterized in that the Working surface of an electrode covered with a dielectric intermediate layer and the other electrode is widened at the edge zones of the capacitor field. Documents considered: German Patent No. 731 272; British Patent Nos. 577 474, 577 720; "Theory and Application of Radio-frequency heatsingi; von Brown, Hoyler and Bierwirth, p. 262, Fig. 201, and P. 338, fig. 259; Austrian patent specification No. I58 6o7; U.S. Patents No. 2,117,456; 2,370,624; Funkechnischer Vorwärts>, 7th year, issue 22, p. 650, fig. 3.