Principle
High-frequency (HF) welding of plastics is very often applied whenever PVC and PU products have to be welded. Well-known products whereby HF welding is applied are car mats, baby cushions/pillows, screen canvas, waterbeds, inflatable boats, drip and blood bags, deck canvases, conveyor belts, rain clothing, writing materials, tents, air filters, advertising canvases, etc.
Principle and advantages
The principle of HF welding is based on dielectric heating of the material to be welded. The material to be welded is put between two metal plates (the electrodes), whereupon a high-frequency voltage is connected to the plates. As a result, the molecules in the material start vibrating, which will result in heating up to the melting (fusing) temperature. By bringing two layers of material into the HF field at the same time and pressing them together, the layers will melt together (fuse) and a strong welded joint will be formed.
In a high-frequency field some materials generate more heat than others, and are therefore more suitable for HF welding. This has to do with the extent to which the molecules of the material allow themselves to vibrate as a result of the HF field. This is known as the loss factor of the material. The greater the loss factor of the material, the easier it is to weld the material using high-frequency equipment. Particularly easy to weld are thermoplastics, such as PVC and PU; these materials are specifically welded using high-frequency welding machines. Much more difficult to weld with HF equipment are, for example, PE and various hard plastics.
The great advantage of HF-welding is the speed: because the material is heated from the inside out, the fusion takes place very quickly, often already within a few seconds. With all other kinds of heating, like with filaments, hot air or infrared radiation, the heat has to be added from the outside, in which case the heat must first penetrate the material.
The HF welding machine
The HF welding machine consists of two components: the HF generator and the welding press.
The HF generator
The high-frequency generator develops the HF field at a frequency of 27.12 MHz and supplies a power, depending on the application, of 500 Watt to 60 kW. Up to 2 kW one uses semiconductors in the generators, but for higher powers one still uses an oscillator tube.
The HF welding press
The material to be welded is laid on or in the welding press. In the welding press there are two electrodes present with a double function: transfer the HF energy to the material and press the two layers of material together.
Usually the welding press table serves as bottom electrode. This table is earthed and can be touched without any problem. Under the pressure head of the welding press the top electrode is applied in the shape of the required seal/weld. The HF generator supplies the HF energy to this top electrode (simply referred to as 'electrode').
There are many different HF welding machines available, which have been optimised for various applications.
High-frequency welding
Before you are able to make a good weld, you have to make various adjustments to the HF welding machine.
Levelling the electrode
Levelling the electrode is very important in order to be able to apply an even pressure to the material. If the electrode has not been levelled correctly, the pressure will not be evenly distributed, as a result of which the quality of the weld will not be the same everywhere. A deviation of the electrode that is so great that an air gap is created could result in burn-outs.
Pressure and depth adjustment
Because the welding is done under pressure, the electrode will sink into the material as soon as the material starts to melt. If the pressure is too high, it costs too much power in the case of foot-operated machines, but if the depth setting is correct, it will cause no further problems. If the pressure is too low, the layers will not be pressed together well enough which will result in a poor joint, or an air gap will be created which could result in burn-out.
The depth to which the electrode sinks into the material can be limited using the depth adjustment of the HF welding machine. If the electrode is unable to sink deep enough into the material, you will not obtain a good joint. If the electrode sinks too deeply into the material, the material becomes too thin, which reduces the strength in situ of the weld/seal.
Setting the HF power, welding time and cooling time
The necessary HF power is mainly dependent on and proportional to the welding surface area of the electrode. Applicable as a rule of thumb is a power of 1 kW per 20-30 cm2. The variation is caused by the type of material, the thickness of the material and the number of layers.
To determine the appropriate power in case of a new application (new material or a new electrode), start with a low HF power and a long welding time. Switch the HF welding machine on and increase the power slowly until the potentiometer starts to increase and the required power (you can read it on the potentiometer) has been achieved. If you set the power too low, the welding process goes too slow or not at all. Setting the power too high results in burn-outs.
Then you set the welding time so that you get a good weld/seal. Setting the welding time too short results in poor fusion and therefore a poor weld/seal. Setting the welding time too long can deform the material.
The cooling time is the time the material is kept under pressure after the welding in order to cool down a bit. The cooling time is dependent on the material; if this cooling time is not long enough, you do not get a tight weld because the material deforms.
During the welding cycle the power will go down a bit, because during said welding cycle the electrode sinks into the material. You can correct this manually. Larger welding presses usually have a built-in automatic power control, which automatically corrects this.
If the HF welding machine is used continuously, the heated material will slowly heat up the electrode until - after a certain time - a balanced temperature is achieved. This heated electrode ensures that new material is pre-heated to a certain extent, as a result of which a bit less HF power is needed to obtain a good weld.
In actual practice this means that when a machine is "cold", for example in the morning, you set the power optimally and then slowly reduce it somewhat in the first hour as the electrode heats up. Once the electrode has reached the balanced temperature, the power can remain constant.
Choosing and applying back-up material
On the table, the bottom electrode, a back-up material is virtually always used. This back-up material is not heated by the HF field and it has the following functions:
thermal insulation: prevents heat from "leaking out" to the bottom table;
electrical insulation: reduces the chance of burn-out;
mechanical: prevents the material from sticking to the bottom table.
Radiation Measurement
More people are increasingly being exposed to electromagnetic fields in their daily lives without being aware of it. Known examples of sources are radio, telephony and television transmitters, mobile phones and radar installations. The electromagnetic fields from these sources are also called non-ionizing fields, because the frequency (and therefore the energy content) of the fields is too low for atoms to be able to ionize and thus become harmful.
The last few years at both national level (Health Council) and European level (International Commission on Non-Ionizing Radiation Protection, ICNIRP), standards have been published in which the field strengths to which the human body may be exposed (exposure levels) are laid down. These standards also apply to industrial high frequency equipment. Fortunately, modern and well-maintained high frequency equipment is able to meet these standards.
We advise all users of industrial high frequency equipment to carry out radiation measurements at least once a year to check if the respective employee(s) has not been exposed to high radiation levels. These radiation measurements can, for example, be combined with the annual preventive maintenance activities.