The material world of RP

4 min read

The proliferation of rapid prototyping and manufacturing methods relies on a range of materials to provide solutions for a number of engineering and manufacturing challenges. Matt Bailey reports.

As projects like the European aerospace and defence group's Airbike demonstrate, it is the development of new materials in association with processes like additive layer manufacturing (ALM), fused deposition modelling (FDM) and 3D printing that are driving rapid prototyping and rapid manufacturing. The Airbike is made out of a Nylon polymer which is strong enough to replace steel or aluminium and requires no conventional maintenance or assembly. It is built as one piece; the wheels, bearings and axle being incorporated within the additive layering process and built at the same time. In theory 3D printing can use any material that can be obtained as a powder. In practise a range of metals and plastic polymers are used both in powder and liquid form, the liquid being 'jetted' in layers to build up the part and then, generally UV-cured, or extruded in molten form. Powders an applied in thin layers and bonded, sintered or melted by lasers or electron beams. Through combinations of materials and processes a range of surface finishes and properties can be created. Israel-based Objet Geometries' thin-layer, high-resolution three-dimensional printing systems use PolyJet polymer jetting technology to print in 16-micron layers. The company's Connex machines can jet multiple materials simultaneously and create composite items on the fly using its own FullCure materials to create accurate, clean, smooth, and highly detailed 3D parts. Objet's marketing manager Europe, Eric Bredin, explains, "Our machines are able to print multiple resins at the same time. For example, we could use a VeroWhite resin along with a softer Tango Black resin. We can digitally mix these resins on the fly so we can build parts with different properties, we could for example make a hand razor prototype with softer grippable areas. In the CAD system we can specify different Shore values on the prototype and our system will automatically interpret this data and jet the correct material mix in the right area to give the desired texture. For more sophisticated applications where you need fit and form and assembly checking, the combined materials capabilities of the Objet Connex machines are ideal." Objet claims to be the only company currently offering this simultaneous multi-material printing capability. The technology can be scaled up or down and while it is currently only in Objet's Connex machines, the aim is to have multi-material jetting in a desktop machine in future. "There is potential to use this in many vertical segments," says Bredin. "It has many different applications. A prototype sporting shoe with the sole made from soft material which can be tested for flexibility before you manufacture is a good example. Every new material opens up a new range of potential applications in what we call the 'third F' – the first F being 'fit' the second F being 'form' and the third being 'function'. Functional testing is linked through what you can achieve through material development and most of our research and development investment is in materials properties. The more we can develop new materials, the more applications we will be able to address." Tim Heller, managing director of Stratasys says the importance of materials in the RP process cannot be overstated."When we changed from wax to ABS on our early fused deposition modelling (FDM) machines in 1994, it changed the perception of our technology because the quality and functionality of the parts got better." In the FDM process the machine dispenses two materials – one for the model and one for a disposable support structure - which are liquefied thermoplastics deposited by an extrusion head which follows a tool-path defined by the CAD file. The materials are deposited in layers as fine as 0.125 mm (0.005") thick, and the part is built from the bottom up. "There are a lot of known thermoplastics that we can use today including ABS and polycarbonates in a lot of potential variants," says Heller. "Our most recent success was the introduction of Ultem which is well known in the aerospace industry due to its high-strength to low-weight ratio, flame-retardant and high-performance properties, and its FST (flame, smoke, and toxicity) rating." Most approaches to additive manufacturing have to have a support mechanism. Stratasys' FDM process uses two sets of filament for the materials; one is for the model and the other for the support. Usually the support plastic is designed to snap off. "Some of our materials use this break away support system but after a few years using ABS we came up with the interesting idea of a plastic support material that would dissolve in water," explains Heller. "While this can't be used with high temperature plastics like Ultem or even polycarbonate, it is fine with ABS and can be washed off easily using a hot water cleaning solution. This allows you to produce highly complex internal structures because the support material will wash out in a solution. "When we looked a bit more closely at this water-soluble material it started looking interesting in its own right because, due to its water soluble nature, you can use it as the primary model material to build soluble cores for composite lay-ups, so the secondary material has now, in a couple of cases, become the primary material," says Heller. Tim Heller stresses that knowledge is a very important part of the picture with RP. "The mechanical properties of any new materials are crucial for design engineers to understand where and how they can be used," he says. "We have an interesting group of materials that we can do a range of different things with, but there are many more potential users of this technology who need a proper understanding of how the materials will perform." Colour also plays a major role in how we perceive and respond to objects. Massachusetts-based ZCorporation's 3D printing technique uses a four colour (plus clear) CMYK powder printing and binding process to build work pieces that can use up to 390,000 measurably different colours on the fly made out of its ZP150 material. Initially the system was mainly used for labelling parts and adding logos, but as the knowledge of the system's capabilities spread users started to use it to add more lifelike colour. "You can now look at some parts and be hard pressed to tell that they are printed parts because they look so real," says ZCorp's VP of product management, Joe Titlow. "We now have users striving for that realism with colour and able to do some amazing things." The footwear companies are once again at the forefront using the technology. "They are producing full mock-ups of their shoes in full colour in a way that is so detailed and realistic that you can put three shoes on a table and people will find it hard to point out the printed one," says Titlow. "You can get people to react to all the visual cues of that shoe without having to manufacture the finished product." In future, Titlow predicts improved performance. "I see even better, more realistic colours, better surface finish and improved materials properties," he says. "If you can get the colour, the performance and the material properties right, I think there is tremendous potential."