Rapid moves

Rapid prototyping covers a range of technologies that create physical 3D objects from 3D data sources output by solid modelling CAD systems, 3D laser scanners, and CT/MRI scanners.

The technology is currently dominated by ‘additive’ processes developed by companies such as 3D Systems, Stratasys, EOS, Z-Corp and Object Geometries. These build models by joining a range of materials — including liquid plastics, resins and sheet paper/metal — layer by layer using thin, horizontal cross-sections of the computer model to drive the RP equipment.

However, with the recent emergence of high-speed/high-performance machining, the process now includes the traditional subtractive technologies. These use high quality CAM-generated five-axis toolpaths to create 3D objects efficiently on a machining centre or CNC router and have the advantage of a fine finish.

The chief use of RP objects is as functional models, followed closely by visual aids used to test assemblies for fit prior to production, and as patterns for castings. For example, stereolithography (SLA) can be used to provide accurate master parts for design verification and soft tooling prior to injection moulding. Vacuum casting can be used to manufacture production-quality piece-parts from low-cost silicone rubber tooling generated from the SLA masters.

RP has also proved useful for prototype tooling patterns, quote requests/proposals and to actually make products.

This last category is leading to a new area called ‘rapid manufacturing’, where the finished product is made entirely by using RP techniques — a practice that is destined to grow once materials technology has advanced sufficiently.

The popularity of RP as a functional and visual aid is a direct product of the intangibles in designs that cannot be conveyed via a computer screen. While the ‘ray-traced’ Open-GL style renderings seen on most of today’s 3D CAD systems are excellent, a 2D monitor cannot convey how, for instance, an ergonomic-handgrip design really feels.

If it is a large model, and you are only interested in one small area, a window can be drawn around the area of interest, copied from the digital model, and an RP model built of just that area. This alone will greatly improve on the time it would take to determine if a design is all right. On the other hand, if the required part is huge and an RP machine is available with only a limited working envelope, the model can be constructed from several pieces, by joining them at logical split-lines determined by the CAD software as if it were a mould tool.

RP is also proving to be useful in determining new and better ways of manufacturing a product. Today’s drive towards lean manufacturing and concurrent engineering practices has led designers, toolmakers and machinists to look at ways of optimising a design for production by ironing out machining problems early on in the cycle.

Subtle changes that may seem insignificant to the designer can often make a huge difference to overall production costs. These will be picked up way before any real time or money is spent on tooling, setting or programming, provided the communication channels are effective and there is a physical model close to hand.

Wall thickness on plastic components is a good example. If walls are too thin for their size they will be difficult to fill. If they are too thick they will form sinks. These potentially costly errors will show up clearly on a physical model. Naturally, there are a few areas where a CAD model might need to be tweaked a little to build an assembly from RP objects.

None of the available technologies are as accurate as a precision-machined component, hence the need to machine prototypes for some applications. For example, model makers might enlarge a hole and glue in a threaded fastener because most RP materials are not strong enough to pressfit mating components.

Spotlight on ‘additive technologies’

Z-Corp of Burlington, US, has developed a system that uses ink-jet heads to deposit a liquid binder that fuses powder according to each slice of the CAD model. A mechanism lowers a build platform, ready for the next layer, and the process repeats. Materials include a ceramic powder and a starch-based powder that is not as strong, but can be burned out for investment-casting applications. Models have a slightly grainy surface but are produced very quickly.

Fused-deposition modelling from Stratasys builds models from the bottom up, one layer at a time, with Acrylnitrile Butadiene-Styrene (ABS) plastic. Accompanying software positions parts and creates any necessary supporting structures. The software also plots a deposition path for the machine. ABS (in filament form and auto-loading cartridges) is fed into an extrusion head, heated to a semi-liquid state, and deposited in layers. The company makes several FDM machines ranging from fast-concept modellers to slower, higher precision machines. Materials also include an elastomer (96 Durometer), polycarbonate, polyphenolsulfone, and investmentcasting wax.

Multi-jet modelling, from Object Geometries of Israel builds models in layers down to 16 microns via eight heads that slide back and forth along the X-axis, depositing a single-layer of photopolymer on to a build tray. UV light cures and hardens each layer and models are supported using a gel-like photopolymer.

Stereolithography from 3D Systems is perhaps the best-known technology, having been around the longest. This builds models using a combination of laser, photochemistry, and software. A UV laser is directed on to the surface of a vat of liquid photopolymer, turning a thin layer of liquid plastic to solid wherever the beam is focused. The cross section is then lowered and the process is repeated for the next slice.

Laminated Object Manufacturing (LOM) from Helisys, constructs a model from layers of adhesive-backed paper, plastic or metal. Layer fabrication starts with sheet being adhered to a substrate with the heated roller. The profile of the slice is then cut with a laser. Once the cutting is complete, the platform moves down and out of the way so that fresh sheet material can be rolled into position ready for the next layer to be cut. The process is repeated until the model is finished.

Paper models, like wood, can allow moisture to enter and cause dimensional instability. Therefore, most models are sealed with paint or lacquer.

Selective layer sintering, also from 3D Systems, uses a laser beam to selectively fuse-powdered materials, into solid objects. Parts are built on a platform that sits just below the surface in a bin of the heat-fused powder. This technology is often used for rapid tooling as it is able to fuse powered metals.

The current trend is to combine aspects of two or more existing technologies to address limitations in areas such as materials, accuracy, and speed. But to entice more users to adopt RP, the overall cost of system ownership must be reduced.

New machines and applications must also be developed that support the production of finished manufactured parts. In the not-too-distant future production components will simply be ‘grown’. However, for now keep supporting existing technologies — but don’t ignore advantages of subtractive RP methods such as carving Styrofoam, shaping cardboard, and using HSM machining centres.

Martello towers above the rest

Last October, Martello was approached by product development consultant Absolute Product Design regarding an optical scanner project it was designing. This needed production quality piece-parts for working systems to be used in field trials across Europe.

The customer had commissioned the design to replace a large collection of metal parts with a small number of complex plastic mouldings.

Absolute knew of Martello and its specialist ThinRim polyurethane resins including its UV stable system. The company felt this technology was ideal for manufacturing production-quality piece-parts, accurate and robust enough to be used in field trials.

The project was complex in detail, and accuracy was paramount. Lead times were critical with over 100 sets being required in a matter of weeks.

Martello built the stereolithography (SLA) masters in-house using watershed resin, and cast the parts using vacuum casting technology. Minor modifications to the design were incorporated into the process to ensure the highest possible quality.

The production schedule was tight and the parts complex to produce — but Martello achieved the target date.

Mark Brown, purchasing manager for the scanner manufacturer, said: ‘Martello gave us excellent service and we are very happy with the work they did. The SLAs and vacuum castings were invaluable. The parts proved out a complex design prior to injection moulding and enabled us to supply production-quality field trial units to customers in a short space of time.’