TWI Knowledge Summary

Welding of thermoplastic pipes

by Felicity Chipperfield

Butt Fusion Welding

The butt fusion welding technique is used for welding pipes made from polyethylene (PE), for the water and gas industries, and from polypropylene (PP) and polyvinylidene fluoride (PVDF) for the chemical industries.

The pipes are mounted in the clamps of the butt fusion equipment and checked for initial alignment. The pipe ends are then planed to ensure that they are flat and square. The final alignment of the pipes is then checked.

The welding sequence begins when a flat hot plate, at a preset temperature, is positioned between the two pipe ends. The pipes are pushed towards each other until the pipe ends come into contact with the hot plate and the pressure is increased to give good thermal contact. The pipe ends melt and the interface pressure forces the molten material outwards to form 'weld beads' at the outside and inside pipe surfaces; hence the term 'bead-up' stage. At the end of this stage, the pressure is reduced to a value sufficient only to maintain the pipe in contact with the hot plate. This allows the melt depth to increase without increasing the size of the weld beads. At the end of this 'heat soak' stage, the pipe ends are pulled away from the hot plate. The hot plate is removed, and the two molten pipe ends are pushed together at the same pressure as used during the initial bead-up stage. This causes further growth of the weld bead and is called the 'bead roll over' stage. The pressure is maintained until the weld is fully cooled.

Butt fusion welding of thermoplastic pipes can be carried out on a wide range of pipe sizes, typically between 63 and 1500mm outside diameter. Manual, automatic and semi-automatic butt fusion welding machines are available commercially from a number of suppliers.

Electrofusion Welding

The electrofusion technique is mainly used for welding pipes made from PE, for the water and gas utilities although PP and PVDF pipes can also be electrofusion welded.

This technique permits joining, of pre-assembled pipes and fittings, to be carried out with minimum equipment. It also offers a number of practical advantages to the installer. It is easy to use for repairs and where the available space and pipe movement is limited.

The electrofusion welding process involves the use of a fitting. This is basically an outer sleeve which the two pipe ends slide into. An internal stop at the centre of the fitting prevents the pipe ends from meeting. Fusion indicators are commonly designed into the fitting, such that when sufficient melt pressure has been generated the indicators will protrude - giving the operator a visual indication that the welding process has been carried out successfully. If the indicators fail to protrude, then the welded fitting should be cut out from the pipeline, and a new fitting should be welded in place.

It is widely acknowledged that, in order to establish a consistent and structurally sound joint, it is necessary to follow a strict preparation procedure. If the appropriate procedures are followed, contamination and disturbance effects that might inhibit the fusion mechanism will be minimised.

For successful joining of pipes, at least three important pipe preparation stages must be followed. Firstly the pipe ends must have finished squared ends. This ensures that the central cold zones function to contain the melt.

Secondly, the pipe surfaces to be joined must be properly scraped to reveal uncontaminated material. With the electrofusion joining process, there is little or no relative movement between the pipe and the coupler. Therefore, any contamination on the pipe surface is retained at the joint interface, which can significantly reduce the strength of the joint.

Finally, the pipe and fitting should be clamped during welding to eliminate relative movement. This ensures that the molten polymer is contained at the fusion interface, allowing the development of a strong joint.

The joining process during electrofusion welding can be divided into three stages: I - initial heating and fitting expansion, II - heat soaking to create the joint and finally III - joint cooling. The duration of stages I and II is commonly termed 'fusion time'.

Electrofusion fittings are typically available in sizes from 16mm to 500mm. However, sizes up to 800mm are now available.

Socket Fusion Welding

The socket fusion technique is mainly used for welding pipes made from PE, PP and PVDF for fabricating process and chemical pipework.

As with electrofusion, socket fusion welding involves the use of a fitting. The process operation is generally manual. The welding cycle consists of a heating phase and a cooling phase. In socket fusion welding, a socket mounted on a hot plate is used to heat the outside surface of the pipe being welded. On the opposite side of the hot plate, a spigot is used to heat the inside surface of the injection moulded fitting.

Both the fitting and the pipe are heated for a set period, known as the heating time. When the heating time is complete, the heated pipe and fitting are removed from the socket and spigot, and the pipe is pushed inside the fitting, producing the weld. Depending upon the size of the pipe, this process can either be carried out by hand (for pipe sizes up to 50mm OD) or on a manual machine, similar to a manual butt fusion welding machine, for pipe sizes typically between 63mm and 150mm OD.

Infrared Welding

The infrared (IR) technique is used primarily for welding pipes made from PP and PVDF for semiconductor, pharmaceutical and chemical process industries.

The system uses an electrically heated metal plate, which can be coated with a ceramic. The plate is typically heated to a temperature between 320 and 530°C, depending on the thermoplastic to be welded and the size of the welding machine. The pipes to be welded are brought into close proximity to the hot plate - typically 1.5-2.0mm but without touching it - and heat up due to radiation and convection. When the pipe ends become molten, the plate is withdrawn and the pipes are forced together to form a weld.

The resulting joints have smaller weld beads compared with butt fusion joints because there is no 'bead-up' stage.

Manual IR welding machines are available commercially from a number of suppliers, and come in the following typical sizes: 20-63mm and 63-225mm.

Bead and Crevice Free (BCF) Welding

The bead and crevice free (BCF) welding technique is used for joining PVDF piping systems for the semiconductor, biotechnology, pharmaceutical, foodstuff and beverage industries.

The BCF welding technique is based on the use of a rubber inflatable bladder, which is placed at the joint line inside the pipes, before welding commences. The pipes are clamped remote from the joint, and a heated metal collar surrounds the pipes at the joint line. As the polymer around the joint melts, it cannot deform outwards because it is constrained by the collar nor inwards because it is constrained by the bladder. In addition, the thermal expansion caused by the heating of the joint area cannot be accommodated by movement of the pipes, because they are clamped.

After a predetermined time period, the heat supply to the collar is switched off and the joint cools. Welds produced in this way exhibit no weld bead, which means that fluid can pass through the pipe unheeded by the joint. It also means that there are no crevices inside the pipe in which bacteria might grow.

BCF welding machines are available commercially for pipe sizes between 20 and 63mm.

Vibration Welding

A new welding technique for thermoplastic pipes, which is curently under development at TWI, is vibration welding.

The mechanism for generating heat during a vibration weld is by the interaction of two rubbing surfaces. This is produced by the linear motion of one of the parts relative to the other whilst a force is applied between them. Once molten material has been generated at the joint interface, the vibration is stopped and the parts are aligned. The force is maintained as the weld cools and consolidates.

Vibration welding has a number of potential advantages over conventional welding methods for thermoplastics pipes including:

Over 95% reduction in weld/cool time
Less effect of contamination, as scrubbing action during vibration welding should tend to move any contamination away from the interface
Self-regulating weld temperature, making overheating impossible
Simple production of branches
Lower energy consumption compared to butt fusion welding

You can use the Weldasearch literature database to supplement what you find in JoinIT.