DE1114630B - Method and device for welding thermoplastic plastics or materials coated therewith - Google Patents

Description

Method and device for welding thermoplastic plastics or materials coated therewith. The invention described below is used the purpose of expanding the scope of the known welding devices for thermoplastic Expand plastics. The welding devices either use the high-frequency method or the current or heat pulse welding process.

Thermoplastics are plastics with poor heat conduction that with soften with increasing temperature. Their welding is possible if at the junction the melting temperature and the material brought to the melt flow for intimate connection flows into each other. The latter is achieved through pressure, which, among other things, is also in front of the welding is necessary to allow the cold material to be pressed onto one another without disturbing air inclusions.

Presses are used to generate the required pressure, which the welding tools in the previously usual welding process with a previously apply adjustable pressure. This pressure should increase with warming and parallel to this, the cross-section of the material to be welded is reduced If possible do not decrease, so that there is still a sufficient final pressure when the Warming is present. However, the initial pressure must not be too high, as the welding device penetrates further and further with increasing softening of the material pushes the material out of the work area before the melting temperature is reached, which can result in cross-section reductions that are no longer permissible. It is here namely to take into account that the required pressure of the stamp in relation the material thickness increases, since the punch of a larger amount of material and in particular has to counteract the gas occurring in it at the melting temperature. The Stamp as a result, it effectively penetrates the softening material too early.

It should also be noted in this connection that for goodness of a welded joint, only the strength of the joint is decisive is, but not the material thickness. However, the latter must not exceed a minimum value, d. that is, the reduction in cross-section at the welding point caused by the pressure do not assume arbitrarily small values.

To prevent material from being pressed out due to excessive initial pressure avoid, arrangements of heating and cooling stations connected in series are used which exert only a slight pressure on the welding zone when it is heated. practice, with increasing cooling, however, let the pressure increase progressively. Farther has been proposed between welding device (electrode) and pressure element To provide a spring or the like, which is tensioned at the beginning of the pressing process. This one and other devices is common that the tool on the to be softened Material floats and is not in a fixed position during softening. The invention is based on the idea that the pressure on the welding zone during the heating time must be as constant as possible, namely at one of the viscosity of the material correspondingly small value. Before and after the warming period is provide high pressure for the reasons mentioned above.

The invention now provides a method for welding thermoplastics Plastics or materials coated therewith, whereby on the to be welded Material a different pressure is exerted by means of a press This method is carried out according to the invention in such a way that the pressing in of the welding tool interrupted in the still cold material and only again after the end of the heating period is made possible.

From the above it can be seen that the tools during the softening of the material to be welded cannot penetrate deeper, than was possible with the form. The pressure on the welding zone increases accordingly the temperature-dependent viscosity of the material and adjusts itself automatically to the desired low value. Acts after the heating time then after unlocking, finally an increased holding pressure on the welding zone, whereby this can advantageously be cooled so far that only the welded The inside of the material has melting temperature.

According to what has been stated, the basic pressure setting results for the Press through the required size of the reprint. The latter is among other things determined by the slightly smaller tool spacing for welded material and has about the same size as the form, where the tool distance is about the corresponds to unwelded material thickness.

The invention has the following for the high frequency welding process Meaning: In general, the temperature distribution in the material cross-section is between Stamp and lower plate of the welding device approximately elliptical, as with With this method, the heat is applied immediately after exposure to the high-frequency energy every place arises. Heat is dissipated by the stamp that is pressed on, the temperature ellipse is flattened according to the stamp width.

There is therefore nothing worth mentioning immediately below the stamp Softening of the material.

The prerequisite for the welding process is that on the inside of the material to be welded the melting temperature is reached. So must the rate of heating in the material must be significantly greater than the rate of heat dissipation. If this is the case, the temperature ellipse becomes inevitable with increasing material thickness longer, d. H. the proportion of the softening material cross-section to the non-softening The proportion near the electrodes is greater because the heat is dissipated while the electrode dimensions remain the same has stayed the same. If the punch is not held, it flows with increasing Material thickness under the pressure of the stamp floating on the softening material more and more substance to the sides before the inner surfaces, d. H. the Joints that have reached the melting temperature. The reduction in cross section gets bigger and bigger.

If, on the other hand, the pressure stamp, as proposed according to the invention, - held during the heating time, the material can reach the melting temperature be heated without substantial fractions flowing away laterally. After the heating-up period the heat flows off to the pressure stamp, and the temperature ellipse becomes flatter. The then still stands for the welding that takes place under pressure full material cross-section available.

It enables this at the same time, material thicknesses that are stronger than usual to be welded together.

Conversely, the proposed delayed release of the pressure enables to choose the material cross-section to be pressed smaller than previously necessary, since the pressure necessary for the welding process, i. H. according to the invention the unlocking, only needs to act at the moment when only the necessary, for a flawless weld a sufficiently small cross-sectional proportion on the inside the welding point is at the melting temperature.

Finally, through this invention, a special property of most thermoplastics are exploited, namely the increasing power consumption with increasing temperature. After the warming can change the temperature of the material Cool down to below the melting temperature before pressing without any disadvantages, if you give a new heating impulse afterwards, which is a sufficiently small one Lets produce a proportion of material suitable for pressing. This measure has been special proved to be advantageous for acrylic glass injection-molded parts because of the considerable irregularities of the structure can lead to local overheating when heated for the first time.

While the high-frequency welding process has so far been based on welding thermoplastic films was absolutely limited, enables the application of the Invention, as can be seen from the explanations, now an extended use, z. B. for welding plastic injection molded articles, especially those made of hard plastic, z. B. acrylic glass, polyamide, etc., and of sheet material made of hard thermoplastics.

The invention also applies to the devices under the now conventional plastic welds are used, a number of advantages. the Locking of the electrode during the heating time acts electrically as diminished Change in capacitance, resulting in lower frequency shifts and mismatches occur in the high frequency generator. The frequency hikes are allowed according to the regulations of the high frequency law occur only in a narrow range, they must therefore either due to high reactive power in the generators, combined with corresponding losses, or compensated for by expensive automatic retuning. Mismatches cause considerable loss of performance. All of these through the welding process Disadvantages caused by changes in capacity are noticeably reduced by the invention.

After all, the power of the generator must be so large that it also meets the greatest field strength requirement with a reduced material cross-section can meet so that the material temperature continues to rise. It is important to consider that the sinking of the electrode into the material, the heat dissipation due the enlarged dissipation area of the electrode has become larger. This heat loss must be covered by an increased generator output.

This increased proportion does not apply if the electrode is held in place.

The reduction in cross-section during the heating time can be thin materials lead to material thicknesses that are so small that they lead to production the necessary amount of heat required electric field strength would have to be higher than it can tolerate the diluted material because of the risk of breakdown. To be in such In cases of avoidance of breakdowns, insulating materials are therefore used, have to fulfill the following task: 1. Reduction of the heat dissipation to the counter electrode. The field strength can thus be selected to be lower.

2. Avoidance of short circuits between the electrodes, a) if these do not sink in plane-parallel due to partially different pressure; because of the then existing different distances occur different field strength values on; b) as a result of field strength differences in the case of very extensive electrodes; c) as a result of cutting edges on the electrodes.

The disadvantages of these insulating material layers include: 1. They increase the distance between the electrodes and also cause a voltage division a reduction in the field strength in the material to be welded.

2. The foil lying on the intermediate layer is completely warmed through, which results in a considerable deterioration in strength.

Because the invention described, the strong cross-sectional reduction is avoided, the properties of the intermediate layer shift in favor of listed advantages.

For the current or heat pulse welding process, the invention allows Method following extensions: So far only thin, easily weldable Weld foils (i.e. foils with the lowest possible melting temperature) because a low heat capacity stamp is used, which is supplied with a brief surge of electricity is heated up and quickly cools down again. To weld the inside To achieve this, the heat generated on the outside must flow to the inside, whereby The melting temperature must be present at least on one outside. The heat gradient but between the outside and inside must not become too big, hence the one that has been used up to now the current impulse method is restricted to thin foils.

With the stamp held, as proposed according to the invention, can the temperature through thermal conduction over any cross-section in the practical context be transmitted. Then leave the stamp and material after the warming period cool, so results due to the preferred heat dissipation to the welding devices a temperature gradient in the opposite direction, namely from the inside to the outside. At choice at the right time to cool down, the pressure can then be applied over a large cross-section are transferred, primarily the inner sides of the material to be welded enter into an intimate connection.

At the same time, there is a further advantage that it can be used To displace product residues from the seam zone, e.g. B. when closing tubes or Cans, which was previously not possible with external heating.

The invention is based on an exemplary embodiment with the aid of the drawing to be discribed.

With 1 is an electrode for seam welding two plastic films 2 denotes which can be moved up and down on a pressing member 3 by means of a slide 4 is, in turn with an adjustable stop limiting its downward stroke 5 is provided.

In the stop area of the stop 5 on the pressing member 3 engages a bolt 6 which is rotatable about a hinge 6a and articulated to an articulated linkage 7 is, whose joint 7a by means of a magnet 8 via a rod 9 in the position 7a ' can be buckled.

At the beginning of the welding process, the electrode 1 still penetrates cold material 2 until the stop 5 hits the bolt 6 and another penetration the electrode is prevented by the articulation rod 7 in the position shown does not allow the bolt to tilt down. Only when the heating then begins the electrode 1 is finished, the magnet 8 switches, triggers the articulated linkage, which then takes the position shown in dashed lines and moves down of the bolt 6 permitted.

In summary, for this method according to the invention, the welding process To divide up into a heating and pressing process, it can be said that there are a number of offers considerable advantages.

It gets all of the material between the electrodes on the weld joint involved, which demonstrably achieves greater seam strength.

It is also now possible to use composite materials with different To weld melting temperatures. In addition, the welding of larger material thicknesses is possible possible.

In addition, when welding thin materials, the electrodes can also be used without intermediate layers. The requirements for the precision of the Electrodes and the presses can be downsized, which in turn reduces costs the production of these parts means. The required high frequency power is not so big.

In addition, the operation of such a device is less is complicated because the setting of the pressing pressure is no longer as critical as with the floating stamp. It affects this in a reduced reject rate the end.

PATENT CLAIM: 1. Method for welding thermoplastic Plastics or materials coated therewith, whereby on the to be welded Material a different pressure is exerted by means of a press, thereby characterized in that the pressing of the welding tool into the still cold material interrupted and only enabled again after the heating period has ended.

Claims (1)

2. Apparatus for performing the method according to claim 1, characterized characterized in that the press can be locked and immediately during the heating-up time can be unlocked again after the cooling period has started. 3. Apparatus according to claim 2, characterized in that in the Area of a stop (5) which limits the press stroke and which prematurely limits the stroke Latch (6) engages, which after heating is completed automatically, for. electromagnetic, can be brought out of effect. 4. Apparatus according to claim 2 with a high frequency generator, one Current pulse generator, characterized in that the press up to a predetermined Position closes and is only released after the generator or pulse generator has been switched off continues to a predetermined point. Documents considered: German Patent No. 896 101; German utility model No. 1 673 783, 1687453, 1759998.