FEATURE
Issue: 

Think laser!

Jessica Rowbury looks at the design considerations that need to be made when turning to laser processing

The laser is a highly effective cutting or welding tool, but those using a laser – from engineers at the pre-prototype stage to manufacturers running high-volume production – have to keep in mind the characteristics of laser processing when designing a product or a processing step.

For welding applications, engineers are turning to lasers as opposed to more conventional welding tools as they can provide a more efficient production process. ‘A single laser process can replace multiple traditional processes, which might involve cutting, then cleaning, then polishing − whereas the laser process, because it’s so clean with a small heat affected area, the rest of the [surface] area can pass quality measures immediately,’ said Andy May, managing director of Rofin-Baasel UK. ‘So, sometimes you can use a laser and eliminate multiple stages in the production process.’

Companies might also look to lasers because they can enable the production of parts using fewer materials – which, apart from saving cost across many applications, is highly important for reducing weight in the aerospace industry. ‘Aircraft panels that have to be very accurate, you can laser weld over a much bigger area with no distortion, but with TIG welding you just couldn’t do that, so they traditionally would have had to rivet those panels together,’ Tony Jones, managing director of Tec Systems, pointed out. ‘If [airline companies] can reduce a component part by half of its weight by laser welding rather than conventional welding then that it is a massive saving.’ High power fibre lasers in the range of 4 to 20kW are being used more for these types of applications, Jones added, as they have become more competitively priced.

A higher level of flexibility is also a factor that is drawing companies, such as car manufacturers, towards lasers, as it gives them more freedom to design complex parts. ‘Quite often, plastic housings in the automotive industry are used to house sensors. They might hold some sensitive electronics, and the engineer may find that… the method that they originally had in mind, for example ultrasonic welding, introduces too many vibrations and causes damage in the electronics inside,’ explained May from Rofin-Baasel. ‘So companies with applications like these come to laser manufacturers.’

Details in design

It is often necessary to make changes in design of the part when moving from a traditional welding technique to a laser one. ‘Sometimes, people have come to us and we have to advise them to go away and relook at their part. It is usually when people want to weld using a laser, and their part has really not been designed to be able to get an accurate fit-up − so if two parts just won’t fit together close enough to allow you to weld it with a laser,’ Jones from Tec Systems pointed out. ‘That usually tends to be companies that have been welding components with traditional MIG or TIG processes that are a lot more forgiving.’

Traditional mechanical welding methods often require a means of alignment, such as an aligning rib, which isn’t needed for laser welding. This makes the part more lightweight, but it also means that the part design must ensure that they will be welded in the correct position, explained Martin Griebel, manager of the Laser Customer Application Centre at Jenoptik. ‘For laser welding, we apply the energy by radiation exactly at the connecting zone; there is no rib needed,’ he said. ‘So the customer will have to change the design of the connecting zone... and have a smooth contact between the parts.’

With many industries progressing from metal towards plastic − such as the aerospace and automotive sectors in order to reduce weight, or the medical industry turning to single-use plastic instruments as part of infection control − laser welding of plastics has become more popular, according to Griebel: ‘There is a substitution of metal by plastic, which increased the need for plastic welding methods.’

Particularly for transmission welding, the materials used must meet strict criteria otherwise it would simply not be possible to weld them. ‘Transmission welding requires that the top piece of plastic is transparent [to radiation] and the bottom piece is absorbing, and then if you press them together and apply the laser as a heat source from the top, you heat the interface and cause the weld to take place in the middle,’ described May, Rofin Baasel. ‘The laser should pass through the top piece of plastic and get absorbed by the second plastic.’

‘Customers must ensure that they have this special property of being transparent to laser radiation in one part and absorbent in the other one,’ added Jenoptik’s Griebel.

Working with the customer

As the design of the part is an important factor when it comes to laser welding, laser vendors work closely with customers to analyse first if the part can be welded, and if so, how to design their part to maximise production efficiency. ‘It will normally be at the prototype stage where the customer approaches, where maybe they’ve made a few parts which perform… and then the engineer will have to think about how to produce the product at high volumes,’ explained May. ‘Sometimes, if they had come to us earlier, they could have made a few small changes to save money. For example, if it’s a plastic part − if they’ve already had their injection mould tooling manufactured at a cost of £100,000, it’s going to be difficult to change. If it’s not too late, they can accommodate design changes.’

Or, sometimes, engineers may come to a laser manufacturer without a design, but with an idea or concept. In these cases, there is much more flexibility from the start, as the customer can optimise the part to achieve the most efficient production. ‘One of the more exciting areas that we’re doing a lot of work on at the moment is ultrashort pulse lasers, so picosecond and femtosecond lasers, which can do cold processing without significant heat input. So, you can do cutting or micro-structuring on products,’ explained May. ‘In those types of applications that are highly specialised, engineers tend to come to us without a fully designed product, but more the open principle of the material and idea of the end product, but they haven’t started off with the design.’

For customers already carrying out high-volume production using conventional welding methods, however, it is a challenge to convince them to switch processes, according to Jenoptik’s Griebel: ‘That is one of the hardest ways to get new business − if the customer decides to go with a laser, typically there is no way back.’

Therefore, the laser machine must offer significant advantages to the customer for them to consider making an investment, Griebel added: ‘Typically the prices for our customised machines are at least the same as or sometimes a little bit higher [than conventional machines].

‘That is why the technology must offer specific features that are ahead of the traditional methods. It needs to offer advantages, for example, a cleaner process − it does not generate any particles or dust on the part. Furthermore, the laser joint has a higher strength; laser welding is more rigid than traditional methods. These are a few points why customers decide to go for laser welding.

‘That’s the reason why we have our application centre. There, we can show potential customers, under serial conditions and with a real production machine, that we are capable of providing a stable process.’

Cutting edge

According to Griebel, there is a growing trend of customers moving from traditional punching tools to laser-based cutting machines, which can offer more flexibility; one factor that is becoming increasingly important for the automotive industry.

He explained: ‘Punching is a traditional application, and is a perfect process if you have high numbers that is more than 100,000 pieces per year. But, now that there are a lot of different car models with parts that have a volume of about 30,000 to 40,000, it is very expensive to have an individual tool.

‘Manufacturers are looking for flexible solutions. That can be provided by laser cutting. You can program the system, adjust the parameters from one model to the next. You can have a mixed-step production. Higher flexibility is one of the driving forces for laser machines.’

Laser technology also allows for the integration of functional properties, which cannot usually be created using conventional methods. The Jenoptik-Votan laser perforation system, for example, is used to create invisible predefined weakening points for airbags in passenger car instrument panels and other interior parts.

Although a complete redesign of the airbag was required to allow it to be integrated in such a way, it created many benefits in both the production and the function of the airbag, according to Griebel. ‘In the past, you had a separate piece mounted into the dashboard. A separate piece meant that you needed extra machinery to produce it, sampling work, man power and so on,’ he explained.

‘But, with the laser technology, we integrate all of this work into one production step. It meant a complete re-design of the airbag part, but it led to a lot of cost savings by reducing production steps. I call it function integration.’

Laser design

For laser cutting processes, there are many fewer factors to take into consideration when compared to welding. However, the design of the laser still needs to be optimised depending on the application. ‘If you design a cutting system for certain market segments, you have to look through the sheet thickness you have to cover, and to the type of customer − what sort of productivity are they able to handle?’ explained Wolfram Rath, product manager for laser sources at Rofin-Sinar. ‘You have to adapt the total productivity of the system to your total production. It makes no sense to cut the parts in one second and to need ten seconds to handle them,’ he added. ‘From the system’s mechanical design you need to decide what should be the highest cutting speed, and what acceleration you need, and then you find out the interaction times you need to communicate with the laser.’

Although in general laser cutting is well-known, laser vendors also have application labs to help integrate lasers into production equipment and to ensure that customers can fulfil their application requirements. ‘Our speciality is also to be a consultant for those system manufacturers, and to share our knowhow so that they are able to integrate our lasers in their systems easily,’ Rath noted. ‘We are running an application lab in Hamburg where we develop new processes together with customers.’ 

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