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Plastic fantastic

Gemma Church investigates the explosion of cross-industry plastic welding techniques

A microfluidic device with an internal channel filled with liquid (on the left) and in an empty state. This example shows the capability of laser welding with an appropriate laser wavelength without absorbing additives.

Plastics are an essential part of industrial production. But, as the demands placed on this material grow, so do the requirements for the related production and processing technologies.

The appropriate laser is needed depending on the processing technique. CO2 lasers are generally used for cutting, diode-pumped solid-state lasers and fibre lasers for marking and diode lasers for welding. 

There have been significant advances in the laser welding of plastics, which has now been used in industry for two decades, thanks to increases in the precision and process stability of these systems, coupled with their increased affordability to manufacturers.

Diode lasers provide the best foundation for welding plastic workpieces of any colour, providing wavelengths ranging from 808 to 1,470nm. For quasi-simultaneous welding applications, whereby the laser bean effectively heats and plasticises the entire welding seam at the same time, a range of welding contours can also be realised with the help of a scanner head.

The usage of closed loop welding temperature controls also allows for greater process safety and control, which is a technique developed by optics specialist Limo in its laser systems.

Limo uses micro-optics to distribute a well-defined laser energy in a range of geometries. Its laser systems use industrial process controls, including pyrometric temperature measurement, to maintain a constant temperature during the production process.

A wide range of different plastic components can be processed thanks to the high-level of process stability and precision these temperature-controlled systems enable. Dirk Hauschild, CMO at Limo, explained: ‘Polymers processing can be improved where we can control the energy and the temperature at the interface of the two materials we need to weld. With this closed-loop process, we can weld thick polymer parts and thin foils, because we do not burn the polymer and the temperature does not drop below the required range.’

These systems can be used for contour welding, mask welding, simultaneous welding or quasi-simultaneous welding. Limo’s laser systems can also be used for high-quality non-contact welds involving various plastic-based materials, such as ABS, PMMA, PA, PC, POM and PP.

This opens up a wealth of applications and industries. For example, laser plastics processing is a well-established tool in the automotive industry, as the high level of precision allows for the laser welding of expensive, high-end parts. ‘Lasers are a superior tool for polymer welding in the automotive industry, compared to mechanical clamping, which is not reliable and gluing, which takes excessive amounts of time,’ Hauschild commented.

Gluing is also an unsuitable technique for customers buying high-end furniture, as it leaves behind a visible joint. Laser manufacturer Laserline worked with IMA Klessmann, an international manufacturer of production plants for the furniture industry, to develop jointless edge banding using a diode laser. The joining technique uses a polymer edging tape with an absorbent functional layer, which is melted on one side directly with the laser before meeting the narrow face of the wood panel, which is then compressed by rollers. The surface heating with the laser is followed by rapid cooling, so the joint is strong enough for immediate post-processing.

Processing of various edging tape heights was implemented by means of a zoom optic with an adjustable linear focus of 12 to 54mm, which is suitable for almost all commonly used edging tapes.

The glueless and seamless joint is invisible to the naked eye, so it looks like the piece has been crafted from a single piece. The furniture component is also steam-resistant, which is a major advantage for kitchen and bathroom furniture. Furthermore, the laser systems eliminate the need for time- and labour-consuming glue changes to process different edge tape colours.

Outside of the high-end automotive and furniture industries, this increased reliability of plastics processing using diode lasers, coupled with the reduced cost of these laser systems, has opened the floodgates to the production of packaging and consumer electronic parts, such as the components used in smart watches.

As the parts used in consumer devices get smaller and more sensitive to vibrations, traditional processing techniques, such as ultrasonic welding, are also no longer suitable to join them together.

However, mechanical and thermal load is low during laser processing, so the sensitive electronics in these devices are not damaged. This capability, coupled with reduced costs and increased precision and stability of laser systems, will further drive their adoption in the consumer device market. 

Hello to hybrids

Plastics technologies are under constant development, with new polymers and composite materials regularly introduced to the market. Consequently, laser processing techniques using hybrid structures of polymer and metal are currently under ongoing development. Dr Alexander Olowinsky, group manager of micro joining at Fraunhofer ILT, explained: ‘A combination of polymer (even fibre-reinforced polymers (FRP)) with metallic components, opens the way to function-based lightweight design, where each material is chosen for its specific properties: FRP for low weight and metal for stiffness and connectivity to metallic structures.’

Here, the metallic part is structured with the laser to create undercuts in the surface where, in the subsequent joining step, the matrix of the polymer part flows into these undercuts. After the cooling phase and the re-solidification of the polymer, a tight and durable connection is created. The resulting lightweight designs can be used for automotive applications (such as roof stiffeners or reinforcement bars in the door of a vehicle) as well as packaging applications. ‘Joining of polymers to metals without adhesives avoids the curing time which is crucial in series production,’ Olowinsky added.

However, there are still challenges to overcome for hybrid polymer and metallic structures. Olowinsky said: ‘The main challenge for the hybrid combinations is the productivity: as each and every metallic part has to be structured by lasers, the area rate has to be increased. New approaches in the structuring process, such as multiple beams, have to be investigated.’ 

Carbon fibre reinforced plastic (CFRP) is a very dynamic field of application, which has gained ‘a lot of momentum’ in recent months, according to Markus Rütering, sales manager for Asia and Germany at Laserline, who said: ‘The lightweight applications paired with very high strength are the absolute highlight [for CFRPs].’

Consequently, CFRPs are finding a place in items such as aircraft parts, pressure vessels, seals and pipes, which are traditionally made of steel or aluminium. CFRPs improve the mechanical properties of the product and can create weight savings of up to 70 per cent.

 

A demonstator part from the automotive industry. The black roof stifener consists of a fibre-reinforced polymer part with three additional metallic components: two clamps at both ends and one central bracket to attach the internal lighting of a car. The sample has been produced in the frame of the FlexHyjoin project.

During CFRP processing, the material is supplied as a tape – with the carbon fibres in a thermoplastics matrix – to the workpiece. The laser melts the matrix and, by putting layer-on-layer of the materials at pressure after the melting point is achieved, a joint is formed between the two top layers. Laserline’s systems are used to remelt the plastic in this plastic metal joint.

Together with system integrators and its partners, Laserline also recently put more lasers into the market for CFRP processing. Rütering explained: ‘The CFRP tapes can be used to reinforce standard PE and PA plastic containers to withstand much higher pressures, which can get them certifications in hydrogen tanks, to name one example.

‘The material also allows the lightweight design of tubes in the oil and gas industry to transport from off-shore to on-shore. They are also used in the drilling equipment itself, to offer high rigidity paired with low weight. Not all of the possible ideas can be made public, but CFRP tapes with lasers in either tape winding for tubes or tape laying for 3D applications are gaining more and more interest.’

For example, laser-assisted tape placement supplier AFPT worked with Laserline. It built a tape-placement-head mounted on a robot to place CFRP tape on a 3D preform. Working in close cooperation with AFPT, Laserline developed special homogenising optics to heat the CFRP strips as they were placed.

These optics produce a rectangular laser focus with a very uniform energy distribution. A coaxial multi-point temperature control regulates the laser power to ensure safe heating of the material, and keeps it below the decomposition temperature. This allowed the customer to build a robust, compact, class-1 welding cell, suitable for transport on roads.

Laserline expects demand for CFRP processing techniques to grow. Rütering added: ‘There is not a single case we can recall at Laserline, where the CFRP tapes with laser did not finally pass the strength tests or product specifications required by the final customer.’

Transparent processing

Absorption rates must be carefully managed to optimise the laser processing of plastics and unlock further application areas. This is usually achieved by adding different materials to the plastics in question, to adjust their absorption rates accordingly. 

This can cause issues from an aesthetic and environmental perspective – additional materials can affect the finish and colour of the plastic, and make them difficult to recycle. However, increased collaboration between plastics and absorption material producers has had a positive consequence for the environment. Hauschild explained: ‘Now, we know what is inside each polymer, which means we can recycle and repurpose more plastic parts than ever before.’

Transparent polymers are also starting to be welded without additional absorbers. All polymers show distinct absorption spectra according to their molecular chain structure, with most absorption bands in the near and mid IR range from 1,400nm upwards.

Olowinsky said: ‘Choosing the appropriate laser wavelength enables an energy deposition in the material. Combining that wavelength with dedicated focusing, allows us to position the melt zone at the interface between two parts to be welded.’

The main application area for such transparent materials is microfluidics in medical applications, and the removal of additional absorbers is important, due to the regulations in materials choice.

However, challenges remain, as Olowinsky explained: ‘Concerning the transparent materials, the thermal management of the energy input has to be adapted to the optical properties of the polymer. New laser sources, with tuneable wavelength at a sufficient power level are needed.’ 

As new laser sources are developed, coupled with the decreasing cost and increasing process stability and precision of processing techniques, the manufacturing sector looks set to increasingly rely on laser systems to produce a range of plastic-based goods and components. 

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Gemma Church investigates the explosion of cross-industry plastic welding techniques