Linear vibration welding of polyethylene pipes

TWI Bulletin, November/December 1998

Mike Troughton joined the Plastics Group at TWI in 1993. His work involves the development of techniques for both welding and determining the structural integrity of plastics welds.

Hirokazu Nomura is Senior Managing Director of Nippon Kokan Koji Corporation. He is Head of the Technical R&D Center and has responsibility for the Welding Department of Rail and Reinforcement Bar, the Welding Construction Department and Specialised Constructional Engineering Department.

Kiyoteru Hirabayashi is Head of the New Technical Incubation Section of the Engineering Department of the Technical R&D Center of Nippon Kokan Koji Corporation. He also has responsibility for the development of vibration welding technology of PE pipes.

Linear vibration welding is a technique which can significantly reduce the overall weld time for plastics pipes compared to methods currently used in industry. Mike Troughton, Hirokazu Nomura and Kiyoteru Hirabayashi report on recent developments in this exciting new technology.

In Japan, the demand for PE pipes for the transportation of gas and water has increased rapidly in recent years, due to their acknowledged performance in earthquakes. ^[1]

Although current methods of welding PE pipes, such as butt fusion welding and electrofusion welding, can produce joints of very high quality, they suffer from the fact that the overall time required to make the joint can be very long. Welding PE pipes using the linear vibration method can significantly reduce the time required to make the joint. ^[2,3] In addition, the linear vibration method has many other potential advantages over existing techniques, including:

• Less effect of contamination in the weld, since the scrubbing action during vibration welding should be more effective at moving any contamination away from the interface.
• Self-regulating weld temperature, making overheating impossible.
• Lower energy consumption compared with butt fusion welding.

Based on the results of an initial programme of work to examine the feasibility of joining PE pipe using the linear vibration welding technique, ^[2,3] a more detailed study has been conducted to assess the suitability of this technique for joining Japanese PE gas pipes. Some of the results of this study are given below.

Principles of linear 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. The main vibration welding parameters for plastics are; weld (vibration) time, weld force, frequency, amplitude (peak to peak reciprocating displacement) and hold time (time for which the force is maintained after welding). The majority of commercial equipment operates at frequencies of between 100 and 240Hz and amplitudes of between 1.5 and 4.0mm.

Equipment

The welding trials were carried out on a Bielomatik K3215 linear vibration welding machine. A schematic of the arrangement for butt welding is shown in Fig.1, where it can be seen that the pipes are welded vertically. The maximum length of pipes that could be butt welded with this equipment was 310mm.

For this particular machine the vibration mechanism is based upon a hydraulic drive system, which works against a set of flat plate springs attached to the top vibrating plate (see Fig.1). The springs have three functions: they are resonating members, they support the vibrating part against the vertical welding pressure and they ensure that the vibrating plate returns to its original position.

Welding procedure

Butt joint configuration - two lengths, which had previously had the ends machined flat in a lathe, were clamped into the linear vibration welding machine (see Fig.2). The lower pipe clamp was actuated pneumatically, whereas the upper pipe sample was clamped manually. Neither pipe was back-stopped, ie the weld force was resisted solely by the circumferential clamping.

The position of the lower pipe sample was adjusted, via the two plates under the lower clamp, to ensure that the pipes were parallel and coaxial. The lifting table was then raised and the pipes were brought together at the welding pressure. After a set delay (dwell time) of 3 seconds the vibration started, and continued until a set displacement was reached. The displacement, determined by the distance moved by the lifting table during vibration, was due to molten PE being pushed out to form a bead during welding, and was measured using an optical linear displacement transducer.

At the end of the hold time the lower clamp was automatically released and the lifting table descended, leaving the welded pipe hanging from the upper clamp.

Tee joint configuration- the branch pipe, which had been machined at one end with a saddle of diameter equal to the outside diameter of the main pipe, was clamped manually into the vibrating plate of the linear vibration welding machine, with the saddle positioned to fit over the main pipe (see Fig.3). The main pipe was positioned in the lifting table and was held in place using an adjustable end screw.

The lifting table was then raised and the position of the branch pipe was adjusted as necessary to ensure that the surfaces to be welded were flush with each other. The welding procedure was then as for the butt joint configuration.

Weld assessment

In order to assess the quality of the linear vibration butt welds, the following tests were performed:

• Tensile test, using a waisted specimen
• Tensile test, using a dumb-bell specimen ^[4]
• Charpy impact test ^[4]
• Full notch creep test (FNCT) at 80°C. ^[4]

The specimen geometries are shown in Fig.4. The results of the mechanical tests were compared with those for butt fusion welds made on the same size of pipe, using Japanese standard welding parameters as defined by the Japan Gas Association (JGA). ^[5]

In order to assess the quality of the tee joints, a JGA standard seismic evaluation test was used. This test consisted of pulling the branch pipe off the main pipe at a constant speed of 10mm/min until either fracture occurred inthe weld or the branch pipe yielded. The test arrangement is shown in Fig.5.

Results and discussion

The total weld/cool times for the vibration welds are given in Table 1. These times are, on average, around 10% of the times specified for either the butt fusion or electrofusion techniques in the field.

Table 1: Linear vibration weld times 

Visual examination of the welds revealed that, unlike butt fusion welds, there was only one bead, and that this bead was not uniform in shape around the pipe circumference. There was also a difference in shape between the external and internal beads (see Fig.6); on the outside of the pipe the weld bead was rounded, whereas on the inside it was flatter and protruded further. This suggests that, in service, it may well be necessary to specify the removal of all internal weld beads from linear vibration joints.

As can be seen in Table 2, removing the beads results in failure load values that are very similar for the two welding techniques.

Table 2: Results of waisted tensile tests on 165mm OD PE pipe welds

Results of the tensile tests on the dumb-bell specimens showed that for all specimens, both vibration and butt fusion welded, failure always occurred away from the weld.

The average Charpy impact values, for welds made in 165mm outside diameter (OD) pipe, were 20 ± 5kJ/m ² for the vibration welds and 18 ± 2kJ/m ² for the butt fusion welds. This suggests that there is very little difference in the impact performance of the two types of weld.

The FNCT test results are shown in Fig.7. Again, it can be seen that there appears to be very little difference in the long-term performance of these two different types of weld.

The results of seismic evaluation tests showed that the weld failure loads were consistently greater than the minimum load required to cause the branch pipe to yield. An example of a tested tee joint is given in Fig.8.

Conclusions

This study has shown that butt and tee joints can be made in PE pipes using the linear vibration technique in significantly shorter times than either the butt fusion or electrofusion techniques. The mechanical properties of the linear vibration welds are at least as good as for standard butt fusion welds and easily meet the requirements of the Japanese standard.

Future work

Based on these very promising results, a prototype linear vibration welding machine has been developed for PE pipeline construction in the field. This machine, which is shown in Fig.9, can butt weld plastics pipes of any length and of outside diameters up to 216mm.

Future work, using the above machine, will include:

• vibration welding in the field
• vibration welding of plastics pipes other than PE
• vibration welding of plastics pipes at low frequencies

with the overall aim of developing commercial portable equipment for welding plastics pipes in the field.

If any company is interested in collaborating with Nippon Kokan Koji KK in this development, please contact Mike Troughton at TWI.

References