HF welding

In order to make a first-class weld, various adjustments have to be carried out on the HF-welding machine.

Setting the electrode level

Setting the electrode level is important in order to obtain a uniform pressure on the material. If the electrode is not levelled properly, the weld seam will not obtain the same overall quality. If the tolerance of the electrode is so large that an air gap is created, this may result in ‘burn-throughs’.

Pressure and depth setting

Because the welding is done under pressure, when the material it begins to melt, the electrode sinks into it. If the pressure is too high, this costs a lot effort with foot-operated machinery. If the depth setting is good, then no problems should arise. However, if the pressure is too low, the layers will not be properly compressed together. In that case, a good bond is not obtained or an air gap may be created, with a ‘burn-through’ as a consequence.

The depth to which the electrode sinks into the material can be set precisely. If the electrode does not sink deeply enough into the material, a good bond will not be achieved. If the electrode sinks too deeply into the material, then the material becomes too thin, such that the weld strength decreases.

Setting the HF power, welding time and cooling time

The required HF power is particularly dependent on, and proportional to, the welding surface of the electrode. As a rule of thumb, a power of 1 kW per 20-30 cm2 is applicable. The variation is caused by the type of material, the thickness of the material and the number of layers.

In order to determine the appropriate power with a new application (new material or a new electrode) it is best to start with a low HF power and a long welding time. Switch the HF welding machine on and slowly increase the power until the power meter reaches the desired power (to be read from the power meter). If the power is set too low, the welding process will run too slowly or even not at all. If the power is set too high, then this will result in ‘burn-throughs’.

Next, the welding time is set so as to create a good weld. If the welding time is set too short, then a good fusion and welding seam will not be obtained. If the welding time is set too long, the material may deform.

The cooling time is the time that the material is kept under pressure in order to cool off after welding. The cooling time is dependent on the material used. If this cooling time is not long enough, a tight weld will not be obtained because of material deformation.

The power will drop slightly during the welding cycle because the electrode sinks into the material during the welding cycle. This can be manually corrected. With larger welding presses, an automatic power control feature is usually built in, which automatically corrects this.

If the HF welding machine is used continuously, the heated material will slowly heat the electrode up until, after a certain time, this reaches an equilibrium temperature. This heated electrode ensures that new material is preheated to some extent, so that less HF power is needed for a good weld.

In practice therefore, the power is optimally set with a 'cold' machine, after which it is slowly turned down during the first hour of operation, commensurate with the electrode heating up. When the electrode has reached the equilibrium temperature, then the power can remain constant.

Selection and application of barrier material

A barrier material is almost always used on the table (the lower electrode). This barrier material is not heated up by the HF field and has the following functions: