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Laser welding of PEEK for inflatable bag manufacture

Many pressurised air-filled structures demand high integrity joins and seals whether they’re joined using adhesives, welding, induction heating or even stitching, the requirements are that they must arrest the egress of air, whilst retaining the flexibility of the joined material.

TWI undertook a project to use laser welding process to develop a technique for making bags from Victrex® APTIV® polyether ether ketone (PEEK) film. Each finished bag was to be used as novel internal support in the consolidation and forming of a hollow three dimensional thermoplastic composite structure. The pre-pressurised bag was to be placed inside the pre-preg structure before thermal consolidation and would become an integral part of the internal surface of the composite as thermal processing was completed (Victrex patents pending).  

Laser welding uses near infra-red absorbing coatings or additives in the plastic, in combination with proven fibre or diode laser welding equipment to join two sheets of material, one of which needs to be clear to allow the transmission of laser light.

The finished component had to be able to withstand a pressure of two bar unsupported, and up to 11 bar with external support. The crucial welded seals were required around the edge of the bag and at the inflation adapter join.

Materials utilised included Victrex® APTIV® natural and black PEEK film in a variety of thicknesses between 25 and 200 microns.

The equipment

An infrared wavelength (940nm) diode laser was used. This provided up to 150W, typically delivered in a spot size of 5 x 5mm for the weld trials.A double-axis CNC controlled and programmed moving table was used, to follow the path of the join. Pressure was applied at the point of application of the laser beam using a 15mm diameter ring-shaped pressure pad with a PTFE sliding surface. A pressure of 50psi was supplied to the pneumatic system applying a load of 400N. See Fig.1.

Bag construction

Construction of a typical bag included natural film laid over a black film or a thin black film sandwiched between two thicker natural films. The black film acted as an absorber for the laser source. The APTIV films were positioned on the flat bed and overlaid with PMMA sheet. Pressure was applied and with the laser activated, the flat bed was manipulated below the laser following a path shown in Fig. 2. Typically a laser power of 75W was used, with a 5mm wide laser beam, at a welding speed of 3.0m/min.A bespoke gas attachment was connected to a pre-cut hole in one side of the bag, for inflation. Whilst inflated the neck of the bags were sealed, again using transmission laser welding, to provide a pre-pressurised sealed bag constructed wholly in PEEK.

An example of an inflated bag is shown in Figure 3. Bags such as these were used inside a composite pre-form, which was placed in a box providing external support during thermal consolidation in an oven. An example of a finished structure, manufactured by Victrex, is shown in Figure 4.

Fig.1. A dual wavelength diode laser, with associated spray dispenser, clamp, and 2-axis table.
Fig.1. A dual wavelength diode laser, with associated spray dispenser, clamp, and 2-axis table.
Fig. 2. Design for the bag
Fig. 2. Design for the bag
Fig 3. Inflated bag made from laser-welded APTIV PEEK film
Fig 3. Inflated bag made from laser-welded APTIV PEEK film
Fig. 4. Completed Victrex PEEK-carbon fibre composite structure
Fig. 4. Completed Victrex PEEK-carbon fibre composite structure

Conclusion and benefits

The process of internal bagging for thermoplastic composite consolidation, using the laser welding process to seal the bags, has been demonstrated and the technique has been shown to produce a completely closed hollow composite component.  This new composite processing method, currently being developed by Victrex, would provide more flexible manufacturing, responsive to new designs and scale-up without the need for large autoclave equipment. For further information read about Laser Processing at TWI or email contactus@twi.co.uk

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