Rapid prototyping turns to rapid tooling

Contributing editor Rex Narraway reviews all of the the rapid prototyping techniques that are being put to use in the manufacturing industry

Because it is now possible to produce parts straight from computer, without the need of workshop involvement, it is easy to see why rapid prototyping (RP) is generating so much interest from the whole manufacturing industry.

To produce a rapid prototype from a 3D computer-generated conceptual model, it is first necessary to break up the component into a series of slices which depict the profile and contours of the surface in an STL format. The data is stored in the computer software as an STL file and used to create toolpaths for the various scanning or cutting heads that generate the solid prototype models.

In the case of Fused Deposition Modelling (FDM), which was developed in the USA by Stratasys, the process involves the use of QuickSlice software which operates on the STL file to produce toolpaths `at the touch of a button’. Results are downloaded to the FDM machine.

FDM generates a model by means of two heated extrusion heads which deposit hot layers of thermoplastic material on top of one another. The most common materials employed are ABS and investment casting wax, but elastomers similar to polyethylene and polypropylene are also coming into use.

The extrusion heads move in X, Y and Z axes and feed fine threads of hot, semi-liquid thermoplastic to deposit a succession of layers of between 0.05mm and 0.762mm thickness onto a fixtureless base. One head deposits the material from which the actual model is made up while the other deposits support material. When cooled, a solid part is formed.

Stratasys FDM machines are available as small benchtop units for producing models fitting into a 254mm cube and, since the middle of 1997, as larger floor standing units for models measuring up to 457mm x 457mm x 610mm. Accuracies of +/-0.127mm are achievable and it is possible to produce snap-fit features on the components. The company recently introduced a benchtop model incorporating the new 2000 head which gives sharper details and speeds-up what is a comparatively slow process.

In spite of the widespread use of FDM, Stereolithography (SL) is probably the most common form of rapid prototyping technology used in the UK. In the SLA process, the contours of the surface on the computer model are broken up in the form of small triangular facets based on a list of X, Y and Z co-ordinates. These are stored in the STL file and the contours of each slice are fed, one at a time, to the rapid prototyping machine.

The process was developed in the USA by 3D Systems and the stereolithographical apparatus (SLA) relies on the fact that certain photo-sensitive resins become solid when exposed to high-intensity ultraviolet light. Consequently, when a flat plinth that can be raised by an elevator is retained just beneath the liquid level of one of these photopolymers – usually based on ABS – it is possible to produce a solid layer on the top face, simply by scanning the resin surface with a HeNe-laser beam.

In practice, two laser beams are used: one to scan the slices of the model and another to detect the level of the surface of the liquid resin. In addition, the SLA process uses a sweeper to overcome the problems of surface tension and maintain a flat surface on the resin.

When producing a prototype, the plinth is initially set below the surface by a distance equal to the thickness of the bottom-most layer of the model. The laser scans across the surface following the contours of the first slice causing it to become solidified. When one complete pass has been made, the plinth is lowered by the elevator to the position of the next slice and the process is repeated.

As the elevator moves downwards, successive layers are formed and are bound together by the inherent properties of the resin to form a homogenous prototype of the part stored in the computer modeller. After the part has been removed from the vat of resin, it is normal to oven cure any liquid resin still trapped inside. Parts produced by SLA tend to be comparatively brittle and snap-fits are normally unsuccessful.

Whereas the FDM and SLA processes are based on liquid resin, Laminated Object Manufacturing (LOM) developed by Helisys, uses a spool-fed sheet material of about 0.125mm thick and coated with a suitable heat-curing adhesive. The medium is normally paper but, recently, polyester film has been used to improve moisture resistance.

To generate a LOM prototype, the slices of the 3D computer model are defined as laminates which are pressed together to form the solid part. By its nature, the process lends itself to larger components and has been used very successfully in the automotive industry for producing prototypes of complex engine manifolds and bumpers. The results resemble wooden models constructed made by traditional pattern makers and, if necessary, a manual finish can be easily applied.

In the process, the material is rolled-up on a spool and fed across a platform to a collecting spool whilst a heated roller is driven over the sheet to bond successive portions of the laminate to each other. Each slice is depicted within a rectangle and a CO2 laser beam traces out vertical and horizontal paths across the rectangular section of the laminate, cutting out small square sections of the area surrounding the profile.

The traced paths do not cut across the slice of the model itself and the laser generates the actual contours amid cubes of waste material. When the construction has been completed, the cubes are broken away to reveal the true shape of the prototype.

Another RP system using a CO2 laser is Selective Laser Sintering (SLS), originally developed by the University of Texas at Austin. The process is similar to SLA in the way that the model is built up but a heat-fusible powder is used instead of liquid polymer. The apparatus uses two powder cartridges – one to feed the process and the other to collect any unused powder – which are bridged by a thin web on to which surface the powder is deposited. A cylindrical platform on which the prototype is constructed is located between the cartridges and pokes through the web.

The feeding cartridge delivers powder to the surface of the mesh where it is spread over the platform by a levelling roller. The CO2 beam is directed via a system of optics and scanning mirrors to the surface of the powder deposition where it traces out the appropriate profile of a slice from the STL file.

Heat from the laser causes the powder to undergo two phase transitions: from powder to liquid, then from liquid to solid. When one slice has been solidified, the platform is lowered by the thickness of one slice and the process is repeated for each successive slice until the solid model has been built. When complete, it is removed from the cylinder of powder.

The SLS process is now being commercialised by the DTM Corporation. The machines that DTM offers for the production of SLS prototypes are known as Sinterstation Systems. The most recently introduced model is the 2500 System which is capable of building prototypes within a 330mm x 380mm x 420mm chamber – more than twice the capacity of the earlier machines.

To date, a wide range of resins can be worked with SLS, including standard nylon, glass-filled nylon and polycarbonate, plus a powdered metal material. The last-mentioned can be used to produce mould inserts using DTM’s RapidTool process which involves coating the SLS prototype with a polymer solution prior to heating in a furnace. When heat-treated, the metal mould insert can be directly mounted in a standard injection machine and is capable of producing thousands of plastic parts. Normally, the overall process from design to production takes between five and 10 days according to complexity.

Last February, DTM announced two new materials for the SLS process, one of which was an elastomeric polymer from Dupont, called Somos 201. This will allow the process to be used for functional prototypes such as gaskets, seals, mouldings and athletic shoes. The other material, VeriForm polymer, offers benefits over the current nylon family in that it provides better surface finish, fine feature definition and recyclability.

Techniques originally developed for the printing industry are now being exploited for many RP processes in what are loosely referred to as 3D printing. In Solid Ground Curing (SGC), developed and commercialised in Israel by Cubital, a transparent image of the working slice is first printed on a glass photo-mask by adopting an electrostatic process similar to that used in laser printing. This is then used to project an ultraviolet floodlight on to a thin layer of liquid photopolymer that has been spread on top of a platform. The exposed resin hardens while the uncured resin remains liquid and is vacuumed off.

A film of liquid wax is then spread across the whole of the working area and is hardened by a chilling plate. The whole area is milled to the correct thickness and the process is repeated for the next slice. Successive layers are built up in the same way, each layer adhering to the previous one, until the part is completed. The whole cube is removed from the platform and the wax is melted and rinsed away.

To a large extent, the process is rather like a combination of SLA and LOM but, instead of lasers, it incorporates a UV-lamp and a shutter arrangement.

In addition, the platform moves horizontally and vertically while the use of a milling cutter allows complete removal of the layer that was last deposited, if necessary.

SGC claims 10 to 15 times the throughput of other RP methods based on photopolymers and virtually any shape can be generated in any orientation. The largest machine available from Cubital is the Solider 5600 which is capable of producing prototypes within a working volume of 500mm x 350mm x 500mm and a resolution down to 0.1mm.

For desktop operation, Sanders Prototype Incorporated (SPI) offers a range of Model Maker 3D plotting systems based on printing processes. MM-6PRO plotting system, uses a dual inkjet system which rides on an X/Y drive carriage under programmable control and deposits both thermoplastic ProtoBuild material used for the final part geometry and ProtoSupport wax to support overhanging geometry.

The profile is traced out onto a platform in a series of uniformly-space micro-droplets which are placed with an accuracy of 7 micro m. The droplets adhere to one another during a drying process which is fast enough to allow a milling cutter, operating in the Z-axis, to remove the excess height of each layer before the next deposition cycle. The height of each layer is machined to an accuracy of 3 micro m and produces a surface of known reference onto which each successive layer is built.

At one time, Sanders was the only company to market desktop RP systems based on printing processes, but now other companies have followed suit. 3D Systems’ Actua 2100 uses a 96-jet linear array modelling technology to generate prototypes up to 254mm x 203mm x 203mm, while the Stratasys Genysis 3D printer creates models within a 200mm cube using a thermal extrusion process similar to that used in FDM.

Another desktop machine can create models with the strength of balsa wood from layers of starch or cellulose powders, bound with printed adhesive. This machine, designated the Z402, has been built by the Z Corporation under licence to the Massachusetts Institute of Technology. It is capable of building parts twenty times faster than any commercially available RP system.

At present, the nature of the materials used means that most parts manufactured by rapid prototype processes are normally restricted to purposes involving concept models. However, some are looking at ways to produce fully functional components using similar techniques from ceramics and metallic powders.

At the University of Michigan, Professor Halloran is investigating the use of a suspension of ceramic particles in a UV-curable monomer liquid to create ceramic resin. This could be used on an SLA machine to build prototypes in the same way as conventional resins. Systems could be based on either Al2O3 and SiO2 and could be used for fabricating shells and cores for investment casting. Functional prototypes could be produced directly in alumina or silica.

By using a combination of RP and powder injection moulding techniques, the amount of time and money needed in the manufacture of ceramic parts could be reduced. A major difficulty exists in the prediction of and compensation for the shrinkage that occurs when using powder processing.

At the Rapid Prototyping and Manufacturing Institute (RPMI) at Georgia Tech, researchers are developing a model based on aluminium oxide that would predict the dimensions of a fired component to within 0.5% of the intended size. Designers could then create moulds that would compensate for shrinkage. The technique should work with metal as well as ceramic parts.

Other researchers at RPMI are looking at ways of using RP to develop injection moulding tools to produce a small quantity of parts. One technique is to build shells, then fill them with epoxy or metal. It normally takes about 35 hours to build solid models on an SLA machine but a shell could be constructed in about 15 hours and at one-tenth of the cost. Inside three years it may be possible to go from CAD model to moulded part in a matter of four days.

{{Stratasys (Laser Lines)Tel: Banbury (01295) 267755Enter 500}}

{{3D SystemsTel: Hemel Hempstead (01442) 282600Enter 501}}

{{Helisys (UMAK)Tel: 0121-766 8844Enter 502}}

{{DTMTel: Germany +49 2103 95770Enter 503}}

{{CubitalTel: Germany +49 671 61071Enter 504}}

{{Sanders Prototype (SPI)Tel: US +1 603 654 5100Enter 505}}

{{Georgia TechTel: US +1 404 894 5676Enter 506}}

{{Morris Ashby CastingsTel: Witham (01379) 500700Enter 507}}