Quicker and cheaper - plastics pipe joining, the linear way

TWI Bulletin, January - February 1996

Mike Troughton reports the results of a recent investigation, suggesting that linear vibration welding can significantly reduce the overall weld time for polyethylene pipes compared to existing methods used in industry.

At present there are two main methods of welding polyethylene (PE) pipes: butt fusion (a form of hot plate welding) and electrofusion (a form of implant welding). Although both of these methods can produce very high quality joints, they both have one major drawback: the overall time required to make the joint (weld time plus cool time) can be very long. Indeed, for large diameter pipes the overall weld time can exceed 30 minutes.

For this reason a programme of work was performed at TWI to identify a welding technique for plastics pipes which could potentially reduce the current welding/cooling time by an order of magnitude, and therefore reduce the cost of installing a pipeline. The results of this work have highlighted a technique known as linear vibration welding.

Linear vibration welding has been used to join thermoplastic materials since the mid 1970s. It has found most use in the automotive and domestic appliance industries, with applications which include front and rear light assemblies, instrument panels, bumpers, manifolds, jug kettles and dish washer conduits.

The reasons why linear vibration welding was chosen in preference to either hot plate welding or implant welding for such applications include shorter cycle times, simpler equipment and lower energy consumption. [1-3]

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.

Weld times are typically 1-10sec, the frequency of vibration is in the range 100 to 240Hz with an amplitude (peak to peak reciprocating displacement) of up to 4mm. The pressures used are typically in the range 0.5-4.0MPa ( ie of the order of ten times greater than for hot plate welding).

Experimental

Equipment

The welding trials were carried out on a Bielomatik K3215 linear vibration welding machine which, like all commercial linear vibration welding machines, operates vertically. The lower part to be welded is held fixed while the upper part is vibrated horizontally.

The vibrating mechanism was based upon a hydraulic drive which works against a spring system attached to the top vibrating plate (Fig.1). The springs have three functions in that they are resonating members, they support the vibrating element against the vertical welding pressure and make sure that the vibrating plate returns to its original position.

The tooling was modified to clamp short lengths (~100mm) of pipe on to both the lower lifting table and the upper vibrating plate. The lower pipe clamp was actuated pneumatically whereas the upper pipe was clamped manually.

Fig.1: Hydraulic drive system

Welding trials

Most of the work was performed on two types of PE pipes: 125mm SDR11 blue MDPE and 180mm SDR17.6 blue MRS100 Excel, although some trials were also performed on 125mm SDR11 yellow MDPE.

Before welding, the pipes were clamped in a commercial butt fusion welding machine and the ends planed using a standard planing tool. The lengths of pipe were then clamped into the linear vibration welding machine (Fig.2) and the position of the lower clamp was adjusted to ensure that the pipes were aligned correctly.

The lifting table was raised until the pipes were forced together at the welding pressure (Fig.3). After a set delay of three seconds the vibration started for a preset time. After the vibration was complete, the weld was allowed to cool while still under pressure. The lower clamp was then automatically released and the lifting table descended, leaving the welded pipe hanging from the upper clamp.

Fig. 2. Pipe clamping arrangement.

Fig. 3. Arrangement of linear vibration
welding equipment immediately prior
to welding.

The range of welding parameters examined is given in the Table below.

Table: Range of welding parameters examined.

Testing

Tensile test pieces incorporating side notches as shown in Fig.4 were cut from two positions around the welded pipe assembly, one where the vibration direction was tangential to the pipe wall and the other, at 90 to the first, where the vibration direction was radial. Identical test pieces were also cut from the parent pipe. Tensile tests were performed on an Avery Denison 20kN universal testing machine at a crosshead speed of 5 mm/min. The weld beads were left on. The maximum load before fracture was recorded for the welded samples and compared to that of the parent material in order to determine a welding factor ( ie strength of weld divided by strength of parent material). Fracture surfaces were also examined in order to determine the mode of failure.

Fig. 4: Side notched tensile test specimen

(dimensions in mm).

Fig. 5: Appearance of linear vibration welds in
MDPE pipes.

Results

Examples of pipe samples welded using the linear vibration process can be seen in Fig.5 which shows that, unlike butt fusion welds, there is only one bead. This bead also tends to be more uneven than butt fusion weld beads.

Results showed that, within the welding conditions studied, providing the vibration time was greater than 10-15sec a continuous weld bead was produced around the circumference on both the inside and outside of the pipe and good quality welds resulted. Such welds failed in a ductile manner (see Fig.6) and had welding factors greater than 0.98 for MDPE and 0.92 for PE100.

Fig. 6: Fracture surface appearance of tensile test
specimen in 125mm PE80 pipe weld.

Conclusions

The initial work described here has shown that PE pipe samples can be successfully welded using the linear vibration technique in approximately 1/50th of the time required for butt fusion joining.

Potential benefits

Linear vibration welding has a number of potential advantages over conventional welding methods for PE pipes, including:

• Significant reduction in weld/cool time
  • this work has suggested that reductions of the order of 98% are possible.
• Simple production of branches
  • by machining the end of a branch pipe to fit over the main pipe it should be possible to produce branches without the need for expensive moulded fittings.
• Less effect of contamination
  • problems with dirty hot plates are eliminated and the scrubbing action at the interface during linear vibration welding should also be more effective at displacing contamination into the weld beads.
• Lower energy consumption
  • results from work in the automotive industry has shown that compared to hot plate welding the overall energy consumption is less for linear vibration welding.

Future work

Based on the results of the initial findings described here TWI is currently carrying out a major research programme, sponsored by Nippon Kokan Koji KK, Japan, to develop a vibration welding machine for on-site joining of plasticspipelines. As part of this programme we will also be looking at:

• branch production
• other vibration modes
• effect of contamination
• extensive testing of welded joints
• displacement controlled welding

References