Butt fusion welding is the name given to hot plate welding of thermoplastic pipes. It is one of two main techniques for the fusion joining of polyethylene gas and water pipe.
The heating phase, sometimes referred to as 'bead up', is where the pipe ends are pressed against a heated plate for a period of time. This is followed by the 'heat soak' phase where the pressure is reduced to just hold the pipe ends on the hot plate. This allows time for the heat to soak into the material at the pipe ends.
After the heat soak phase, the hot plate is removed and the pipe ends brought together. The time taken to do this is called 'dwell time' and needs to be as short as possible. The final phase is the welding/cooling time, predetermined subject to pipe diameter and wall thickness.
Machine set up
Before making any pipe welds, the butt fusion welding machine needs to be checked for smooth running and set up for the pipe materials to be welded.
- Selection of the correct clamps or inserts, ensuring all fixings are tight, to reduce the possibility of misalignment due to axial movement.
- Correct temperature of the hot plate, for the material being welded; this should be checked with a surface temperature probe and digital thermometer in several positions after a stabilisation period of at least 20 minutes. Between welds the hot plate should be covered by a heatproof bag to protect it from surface contamination and to prevent heat loss.
- Check the planer blades used to trim and square the pipe ends; they need to be sharp, undamaged and firmly fixed to the planer surface to avoid slippage of the planer during rotation.
- Check all moving parts for smooth operation and, if using a hydraulic machine, check the hoses and fittings for signs of leakage.
Preparing the pipe
Prior to welding, correct preparation of the pipes is necessary. When measuring pipe lengths, allowance should be made for the trimming and melting sequences to guarantee correct lengths after welding.
Before clamping the pipes into the machine, the ends should be checked for irregular shape, damage, or embedded grit. The maximum allowable depth of this must be less than 10% of the wall thickness. Damaged or deeply scored pipe should be discarded. Any loose contamination can be removed by wiping the pipe ends with a lint-free cloth on both inner and outer surfaces.
Once cleaned, the pipes are clamped into the machine. To help alignment, it is good practice to clamp the pipes in such a way that their stamped markings are in line. This also helps with identification at a later date if required.
Once securely fitted in the clamps, the pipe ends should be brought into contact with the rotating planer tool until continuous shavings are cut from each end. The planing process ensures the pipe ends are smooth and square ready for the welding phase. Loose shavings should be removed from the machine and inside the pipes taking care not to touch the planed ends. This ensures that no grease or dirt is transferred from hands to pipe ends. The pipes should then be checked for alignment and adjustments made to the clamps where necessary to ensure there is minimal mismatch in diameter.
Before the welding sequence, heating and cooling times and fusion pressures should be noted for the specific pipe diameter and written down for quick reference during the welding cycle. Some machines have all the relevant tables on them for convenience. A timer or stopwatch should be available for accurate timing.
The heatproof bag should be removed from the hotplate, and the temperature should be checked using a digital thermometer and surface probe.
It is good practice to complete a dummy weld before undertaking actual welding. This is to ensure the surface area of the hotplate in contact with the pipe ends is totally clean of any dust particles or other contaminants.
Place the hotplate between the pipe ends, ensuring that it is properly located and square to the pipe faces. Move the pipes into contact with the surface applying an axial force. The force should be applied smoothly making sure that the required pressure is not exceeded. The force needs to be held securely, allowing the formation of a bead of molten material around the pipe.
The bead needs to be even around the pipe circumference, on both sides of the hotplate. This is the 'bead up' phase of the process.
The means of applying the force will vary with the type of equipment. On certain types of machine the force will be applied by mechanical means using a spring loaded mechanism with the force being maintained by a locking screw. On other types of equipment, hydraulic rams are used with the pressure maintained by switching valves in the hydraulic power pack.
When the required bead has been achieved, the pressure is reduced for the heat soak phase. The pipes rest on the hot plate which allows the heat to permeate the material, reducing the possibility of cold welds.
This time will vary subject to pipe diameter and wall thickness, therefore manufacturers recommended times should be used.
When this phase is completed, the pipe faces are moved away from the hotplate as smoothly as possible to ensure that none of the molten bead sticks to the surface and the hotplate is removed. The pipes are then brought together as smoothly and quickly as possible to minimise the possibility of temperature drop, taking care not to exceed the required force.
The welding/cooling phase begins when the required force has been achieved. The weld force should be maintained throughout this phase, to ensure maximum weld strength
On completion of the cooling time, the pressure can be reduced to zero, and the pipe removed from the clamps. The finished weld can now be visually inspected for uniformity and alignment.
More information on TWI's work on plastic pipe can be found here.
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