Design Chain: Rapid response – why the rules covering design are evolving
By Lou Reade
Posted 13 June 2012
Techniques such as selective laser sintering (SLS) have been helping to create prototypes for many years, helping to speed up the design process.
Some of these techniques are also being used to make finished products – which means that products must be designed in a different way to, say, those made by injection moulding. A large part of this progress is down to ongoing improvement in materials.
The Fraunhofer Additive Manufacturing Alliance in Germany, for example, has developed the first thermoplastic polyurethane (TPU) powder for SLS. It says this will allow the creation of a far wider range of products using the technique.
Frauhofer researchers have used SLS to make prototype soles for trainers
"SLS parts are currently made with polyamide, which is a much stiffer material," says Marcus Rechberger, who helped develop the new material at the Fraunhofer Umsicht Institute.
He says that the properties of TPU vary between hard and soft, creating opportunities for a range of potential products.
The new material combines the best properties of PU - toughness, elasticity and wear resistance – in a powder that can be processed by SLS. Parts can be made in a few hours, by curing the material with a laser.
The material has a tensile strength of 30MPa, elongation beyond 400%, Shore A hardness of 90 and a density of 1.2.
At the recent Euromold show, Fraunhofer used the material to make a prototype outer sole for a running shoe.
"This was just a sample to show the properties of the material," says Rechberger. "This is a long way from being a typical application."
Instead, he says, there are promising applications in areas where the properties of the material are combined with a need for short runs of intricately designed components. He cites orthopaedic shoes, automotive hoses (if produced in relatively small amounts) and grippers for robotic arms.
There are other advantages too. The temperature needed to process the TPU powder is around 80 degrees centigrade, rather than the 170 degrees needed for PA.
"One effect of this is that the PA powder is damaged by the high temperature, and cannot be re-used," Rechberger says. "Our TPU powder can be re-used again and again, so is 100% recyclable."
At the moment, the PU powder - which should be available imminently - is processed on the 'standard' machines used to make PA parts. But Fraunhofer is looking to develop a specific SLS machine that is optimised for use with PU.
And there is another breed of machine being developed, which could process more than one type of PU at the same time - opening up the possibility of SLS parts that combine hard and soft areas.
"If you had soft and hard TPU, you could combine them in a single step," Rechberger says.
This work is in its early stages, but he says that a way of laying down two separate powders, and curing them selectively, would allow the creation of these new 'hybrid' parts.
"SLS is a building process for complex parts," he adds. "You can produce every shape you can imagine - complete freedom of design."
But designers must change the way they think if they are to make the most out of these techniques.
"Many designers still think 'analogue' but then try to print 'digital'. It doesn't work. Printing parts is very different to injection moulding parts," he says.
As an example, Rechberger says that intricate structures can be designed into a product. A shoe, for example, could have tiny 'cage-like' structures incorporated into the sole, which would add cushioning. Devices such as piezoelectric sensors could then be incorporated into the sole.
And even though he says the 'demonstration' running shoe is not a typical application, he mentions the case of specialist running shoes - for professional athletes - which are one-off designs tailored exactly to the needs to the user.
"Once you start thinking this way, it opens up many new applications," he says.
Objet Design, the Israel-based 3D printing specialist that recently merged with Stratasys, says that these types of technique are crucial to the design process.
Objet's Connex technology allows various materials to be combined
"Our customers want to visualise and test a design, to make sure it feels and behaves how they intended," says Andy Middleton, Objet's general manager for Europe.
While many 3D printing companies have moved aggressively into rapid manufacturing - in which final parts are produced using the technique – Objet's main focus is still on prototyping.
Last year, it developed its Connex technology, which allows various material characteristics – such as stiffness, flexibility, transparency – to be combined in a single part. One of its largest customers, Adidas, has used Objet materials to mould prototype soles for training shoes.
"Adidas have very stringent rules," says Middleton. "They will produce a left-foot and right-foot sole for a new model, in a range of sizes."
Prototypes of various sizes and designs can then be assessed by marketeers to decide which should progress further. It is this 'design verification' for which rapid prototyping was first developed. But now, thanks to materials advances, it has moved further.
"Jaguar will use our materials to make a prototype air vent, to make sure that it fits perfectly into the console," says Middleton. "But they also want to test that it works - so they build it into a model car, connect hoses to the back, and blow hot air through it."
This functional testing puts much higher demands on the material, as it must now perform under 'real' conditions.
"A few years ago, prototyping materials would soften in your hand if you held them for long enough," he says. "Now, we are up to temperatures of around 120[degrees]C."
This, he says, makes the materials appropriate for many new applications. He cites kettles and toasters (for the likes of Braun), as well as working taps and shower heads as recent prototyping successes.
But what of the notion that 3D printing has 'no rules' - and it can achieve pretty much anything that a designer can throw at it?
"There's no limitation in design: we can print any geometry," he says. "But if the part is going to be manufactured, then the design may not be realistic."
However, he believes that this may not always be the case.
"If our techniques develop, so that the materials we print can be used for the end product, then there really will be no limitations," he says. "It will happen one day - just don't ask me when."
The whole issue of 'no limits' is one that irks Mike Ayre, managing director of Oxfordshire-based Crucible Industrial Design.
"This attitude has become so ingrained, that it sounds like heresy to say anything else," he says.
He believes that it has led to a misconception that additive layer manufacturing (ALM) is equivalent to a magic wand.
"Some ALM gurus have come out with glib phrases to the effect that there are no design rules," he says. "You simply have to press a button, and out comes the product."
The reality, Ayre argues, is more mundane and practical.
"You can do anything you like - up to a point - with these techniques, but the same is true of injection moulding. It depends if you have deep enough pockets. A multi-part injection tool allows you to do all sorts of complex things, but they cost a lot of money."
As with any technique, he says there are efficient and inefficient ways of using them. It's well known, for example, that there is no need to worry about undercuts and draft angles, when using techniques like SLS.
"This has led to the belief that there are no design rules," he says. "But it's not that simple. It doesn't mean that you can throw fundamental engineering practice out of the window."
A new set of rules for 'designing for ALM' are emerging, Ayre says. When put into practice properly, they help to maximise design efficiency - as should be the case with any design.
An example is in 'nesting'. Rather than building identical components in separate parts of the build chamber – the rough equivalent of making identical products in a family mould – they can be 'nested'. This means they are designed to fit inside one another – or stacked together, like drinking cups – and separated afterwards.
"Within the limited space of the build chamber, this improves efficiency and reduces the cost of each individual component."
He says that living hinges allow the creation of 'flat packed' parts - where, like Ikea furniture, they are created as 2D shapes and then 'assembled' at the end.
"If you design for the process, and work with it rather than against it, you could build tens or hundreds in a single build chamber," he says.
Ayre has also been involved in a project called Saving, which is looking to use ALM techniques to minimise energy usage. Much of it is focused on DMLS - which makes laser-sintered metal parts - but he says there are also lessons to be learned in making plastics products.
"An example is wall section thickness," he says. "If you put a thick wall section next to a thin one, then you will get warpage as they cool. But I would ask why you are designing thick sections in SLS anyway."
If he sounds like a traditionalist, railing against new fangled modern technology, nothing could be further from the truth.
"These are very powerful tools, but there are good and bad ways of using them," he says. "I want to make sure that we identify the good ways." l
This is the final extract from PRW’s Design Chain supplement. To see the digital version of the supplement, click on the Design Chain button on the PRW homepage.
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