Heating element butt welding comprises several aspects of time that have to be considered.

The most important are:

Heating-up time

Preparation time of jointing pressure

Cooling-off time

Heating-up time

The heating-up time is the time required for the heating of the material to be welded up to the right welding temperature and to the sufficient depth (plasticising).

Residual welding stresses are inevitably caused by the following factors:

• Flow and diversion current during the welding procedure and
• Shrink stresses during the cooling process

Residual welding stresses are considerably increased by non-observance of the right welding parameters or environmental effects.

Excessively high residual welding stresses may considerably reduce the strength and service life of a welding seam and therefore lead to premature failure. Experience shows that residual welding stresses in welding procedures according to DVS-guidelines are within the uncritical range.

• Have the pipe ends been cut off at right angles?
• Has the oxide layer been removed?
• Have the welding temperature, welding pressure and welding times been calculated correctly?
• Have environmental effects (e.g. moisture, wind, temperature, etc.) been considered in this connection?
• Has the roundness of the pipe ends been secured (oval pipe ends have to be adjusted by adequate redeformation measures)?
• Have the semi-finished products …

Heating element butt welding

The joint faces of the pipes to be welded are heated at the heating element under pressure and connected after the removal of the heated element.

Electrofusion welding

Electrofusion fittings are equipped with integrated resistance wires, which are stressed by electric current during the welding process.

• Checking the roundness of the pipes and adjusting if necessary
• Clamping the pipe ends axially into the welding slide
• Processing the pipe ends by squaring-off and cleaning them, if necessary
• Checking the plane parallelism and displaced joints and adjusting, if necessary
• Cleaning the heating element
• Checking the temperature of the heating element

• The pipe ends should be axially aligned before clamping in the welding machine

• Cleaning of the …

In order to get heated, the joining areas have to lie plane against the heating element under low pressure. For this purpose, the pressure is reduced to almost zero (≤ 0.02 N/mm²). During the heating process, the heat intrudes into the areas to be welded and takes them to the required welding temperature.

After heating-up, the joining areas are to be removed from the heating element. The heating element is to be taken out without damaging or soiling the heated joining areas. The joining areas are to be brought together quickly until they almost meet. The relocating time is to be kept as short as possible as otherwise the plastified areas cool down. This would have negative effects on the quality of the welding seam.

The areas to be welded are supposed to connect with a velocity of near zero. The necessary jointing pressure is preferably applied in linear increase and is, for instance, 0.15 ± 0.01 N/mm² for PE. Sudden use of full pressure would press the hot plastic material out of the welding zone which would result in an inadequate weld factor [Simona11].

The jointing pressure has to be maintained during the cooling-off period.

Devices and materials required for heating element butt welding:

• welding slide
• insertion shoulders for different pipe diameters
• facer
• heating element with control unit
• cleaning tissues and ethyl alcohol
• pressures and times are to be taken from indications of manufacturer

According to [GSTT2], only trained personnel in possession of welder's certificates may be used …

Before the welding procedure starts, the temperature of the heating element that is required for welding has to be controlled. This is carried out e.g. by means of a temperature measuring device with a fast display used for surface measurements. The control measurement has to be carried out within the area of the heating element that corresponds with the semi-finished product. In order to create a thermal balance, the heating …