How To Make A 3D Printer

Transcription

1 outerspace.co design & engineering resources Introduction to 3D Printing & Rapid Prototyping You may have heard references to 3D Printing, Rapid Prototyping or Additive Manufacturing before. If you have wondered what all the fuss is about, read on. Aaron Pooley Image source: cdn.nocamels.com

2 outerspace.co 2 I want it now! and Can you make it cheaper? These adages are more relevant now than ever. Consumers demand rapid evolution of products; the latest smart phones and computers are viewed as so yesterday the day after they hit the shelves. For product development companies, time to market is essential to success and fierce global competition dictates razor-sharp focus on cost. 3D printing is an essential tool in the fight to reduce development time and cost. 3D printers can produce parts from multiple materials at the same time, parts that were previously considered un-manufacturable and even multi-coloured assemblies of moving parts which require no assembly or painting activities post printing! A prototype of the new design concept, drawn on a computer, can be 3D printed in under a day and often for few hundred dollars or less. A printed prototype can be tested for function, durability and aesthetics and improvements can be integrated into the computerised design within days, ensuring the final manufactured product works flawlessly first time. Before 3D printing was around prototyping would take weeks, not hours and the labour intensive process would cost big dollars. The term rapid prototyping refers to a group of advancing methods used to produce prototype parts more quickly than has been traditionally feasible. Additive manufacturing refers to the method of building a part from ground up one brick at a time, as compared with traditional techniques like machining which start with a large block and cutaway (i.e. subtract) material to arrive at the desired shape. Three Dimensional (3D) printers form a subset of the rapid prototyping and additive manufacturing spheres and the three terms are often used interchangeably. 3D printers have been around for about 30 years, but were very expensive; now they are becoming common place in businesses and even at home. A conventional printer is a two dimensional (2D) printer. The printer head moves across the page and the roller scrolls the page up and down placing ink down on the paper as it moves to create a 2D image. A 3D printer effectively takes the same technology and extends it to the third dimension (i.e. in and out of the page). Instead of laying down ink, the print head lays down a paper-thin path of viscous liquid which quickly solidifies. The 3D shape is produced layer upon layer; in much the same way as a bricklayer would build the walls of a house. Like a household printer, a computer controls the process and a three dimensional shape, the size of kettle, can be produced in less than a day; smaller prints, like a cup, in a matter of hours. Before printing can commence, a computer based 3D virtual form needs to be created. The software used to do this is commonly referred to as CAD or Computer Aided Design. There are many different software companies that produce CAD software; Outerspace Design uses SolidWorks CAD software. In a similar way to drawing in Microsoft Paint or PowerPoint, a 3D shape of specific size is created in CAD, a file saved, and then print driver software is used to prepare

3 outerspace.co 3 3D Printers can be classified into 4 groups: Fused Deposition Modelling (FDM), Polyjet, Stereo Lithography (SLA) and Selective Laser Sintering (SLS). the file for printing. The print driver effectively slices the shape into numerous thin layers, between 0.03 & 0.25mm thick. Each layer can then be laid down or drawn in 2D by the printer with each subsequent layer being drawn on top of the previous one. Fused Deposition Modelling (FDM) Fused Deposition Modelling (FDM) is the entry level technology in the 3D Printer space. It works exactly as previously described. A moving print head is fed with filament from a reel (similar to fishing line), which is made from a polymer (plastic) material. The print head heats and extrudes the filament and it exits the print head through a nozzle as a viscous liquid. The liquid is laid onto a plate known as the build platform, where it cures or solidifies as it cools. Motorised mechanisms move the print head in 2 directions, forward-backward and side to side creating a thin layer of plastic patterned by the path travelled by the print head in much the same way as someone would pipe icing onto the top of a cake to decorate it. After each layer is created the build platform (the foundation surface for the build) is lowered by a layer thickness and the whole process repeats. Each layer is the same thickness for a given 3D print, with most FDM printers having a layer resolution of between 0.1 and 0.35mm thickness. The thinner the layers, the smoother the surfaces appear and feel, but longer the print takes to complete. An easy task for a 3D printer is to produce a cylindrical cup with vertical sides, the printer placing round layer upon round layer until the full height is reached. The greatest challenge for FDM machines is producing complicated shapes where layers cannot be directly supported by the previous layer formed; an example would be to print an archway. Without the support of the previous layer the newly formed layer will collapse to the foundation level and the 3D print fails. To prevent this from happening FDM printers produce a sacrificial support structure the purpose of which is simply to hold up unsupported print layers, until the whole structure solidifies. When the print is finished this support structure is removed. This works in much the same way as scaffolding works to support a lintel or archway while it is being constructed and is later removed when construction has finished. 3D print. The print driver software automatically generates the support structure for a given 3D CAD model, with a few simple inputs from the operator. Usually the support is made from the same material as the build or main body material. Removal of the support material is usually a manual process of breaking it away from the build material by hand and using tools like a paint scrapper to chip it away. Some new FDM machines print with two spools of materials: one build material and one support material, where the support material is dissolvable, making support removal much easier. FDM machines can be used to print in a number of plastic materials including: ABS, PLA, Nylon, High Impact Polystyrene (HIPS) and even some rubberised plastics. Build Material Spool Fused deposition modelling printing process Extruder Head Part Build Support Base Support Material Spool The support material is typically a lattice type structure, which is easy to separate from the main build layers of the Image source: techwire.lk

4 outerspace.co 4 Polyjet The next step up in technology from FDM is Polyjet. The Polyjet technology is very similar to household inkjet or bubble jet printers. Where FDM heats up and melts a solid filament/wire and extrudes it onto the build platform, Polyjets utilise a small nozzle that prays tiny droplets of resin onto the build platform and cures (hardens) it with UV light immediately after. The 3D form is built up layer by layer in as with an FDM machine, with resin being sprayed on top of the previously solidified layer and then cured. Polyjets also employ support material to enable complicated geometry to be build without collapsing and usually the print head has 2 jets - one for the main build material and the other for a support material; the support being easily dissolvable plastic or wax that can be melted away. Post processing Polyjet parts to remove the support involves either water blasting, or oven heating and ultrasonic bath to remove it. Stereo Lithography (SLA) Stereo lithography or SLA is at the high end of 3D printer technology and utilises a laser and a bath of ultra violet (UV) light curable fluid, otherwise known as photopolymer resin. Mechanised mirrors direct the laser beam onto the surface of the bath and move it forward-backward and side to side across the surface of the bath. When the beam contacts the resin surface it cures and solidifies. Imagine a layer of ice forming atop the surface of a lake. The printing software guides the lasers in a pattern such that the required 3D form is traced-out within the liquid and the solid is formed suspended in the viscous resin bath. The first layer is laid on the build platform which begins the process level with the top surface of the resin. As each layer is formed the build platform indexes downward (or upwards in inverted SLA printers), then a sweeper passes over washing a thin film across the previous layer allowing the next layer to be cured Jetting Head X Axis Laser Lenses Elevator X-Y Scanning Mirror Build Material Y Axis Laser Beam Vat Support Material Sweeper Liquid Photopolymer Z Axis Layered Part Polyjet printing process Stereo lithography printing process Image source: Image source: i.materialise.com

5 outerspace.co 5 atop it. Again unsupported areas are bolstered by support material which is formed from the same build material and shaped as small cylindrical columns, with a cross linking lattice. The support is cured less strongly than the build material, to make easy to remove. Selective Laser Sintering (SLS) Selective Laser Sintering, or SLS, also uses a laser to cure a material to form the 3D shape, but differs from SLA. The material starts out as a fine powder which is thinly spread across the build platform. The powder can be either a polymer (plastic) or metal and the size and appearance can be likened to plain flour in consistency. The laser beam heats small regions of powder (to near melting point) as it traces across it, fusing them together as a continuous solid. Software controls the path traced by the laser (forward-backward and side-to-side) and a thin, patterned layer of solidified material is formed. The machine then lowers the build platform slightly and distributes a fine layer of powder over the previously solidified layer, via a sweeping mechanism and the process of laser tracing over the powder repeats. These three steps are repeated over and over until the full shape has been produced. The regions of un-fused powder act as the support material, negating the need for a more elaborate support structure. When printing is complete an SLS part requires post processing to remove a large amount of un-fused powder, which is blasted away from the finished part by compressed air jet. Interestingly, the process of fusing (sintering) metal is conducted in a vacuum to prevent combustion of the metal; plastics are able to be sintered in air. The costs of the individual 3D print technologies differ significantly. FDM is the cheapest technology with machines prices starting from AU$1,500, which buys a machine that can print parts as large as 125x125x125mm, with the largest FDMs costing $200K. Polyjets are the next most affordable technology starting at $35K and ranging up to $200K. SLA and SLS machines are the most expensive, commanding prices upwards of $1M for machines that can print 1.5m x 0.75m x 0.55m, although a small SLA machine to print jewellery (think rings) can cost as little as $6,000. An important consideration to factor in is material cost, for most high end machines: FDM, Polyjet & SLA: the materials can cost b/w $200 & $400 per kilogram and the support material which commands a similar price is washed away at great expense. This is where low end FDMs have become popular, with reels of material (filament) selling for $30 per kilogram. Time is money, as they say and the differing technologies vary in speed. While print speeds vary from FDM to FDM and SLA to SLA, it can be said that an SLA printer will be significantly faster than the average FDM; up to 3 times faster in some cases. The applications of the four technologies also differ, based on the operational & aesthetic requirements of the part. FDMs produce a rougher surface finish, with SLA and Polyjet delivering a much smoother finish. The improved surface finish is a result of a combination of factors: the layer resolution (thickness), precision of print head/laser movement and curing technology itself. For example an FDM prints 0.1 to 0.35mm layers, where a Polyjet can print 0.03mm layers. There are however, methods to post process the FDM Lenses X-Y Scanning Mirror Laser Leveling Roller Laser Beam Sintered Part Powder Bed Powder Feed Supply Build Chamber Powder Feed Position Selective Laser printing process Image source: guideimg.alibaba.com, us-carmodels.com

6 outerspace.co 6 surface finish, using acetone to smooth out parts printed in ABS, to achieve an SLA type finish. FDM parts, made from ABS or Nylon, can be quite strong if printed in the correct orientation and can be used to produce functional parts that have moving parts or part that experience cyclic forces (think snap type clips). Likewise some of the proprietary SLA and Polyjet materials are designed to provide good mechanical properties to help parts behave as if they were injection moulded. However, if strength and stiffness for function is desired in a part it is best printed via SLS (sintering). SLS delivers parts which very closely approximate parts produced through conventional production means like injection moulding or blow moulding. Oxygen Permeable Window Projector UV Curable Resin Dead Zone Although 3D printing is incredibly fast compared with conventional methods for producing one-off prototypes, the technology has, until now, been considered too slow to be a viable method of high volume manufacture of production parts. As such, the holy grail in 3D printing has long been print speed; the key to unlocking the full potential of additive manufacturing. A new 3D print technology known as Continuous Liquid Interface Production or CLIP looks set to make great inroads toward this goal. CLIP is likely to hit the market in 2016 and its developer Carbon3D suggests it is 25 to 100 times faster than conventional 3D print technologies (i.e. SLA and FDM). Sharing similar fundamentals to the SLA process, CLIP uses projected light to cure a photopolymer resin and an inverted build platform which pulls the 3D form out of the resin bath; like a rabbit out of a hat. This is where the similarities end. The key to the speed increase is in how it controls the curing light to produce the desired shape. Instead of a focused laser beam, CLIP projects a 2D image (of light) onto the resin, curing the full 2D pattern instantaneously. This is much quicker than a mechanical drive system can articulate a laser beam to trace out an image. CLIP also uses oxygen as a resin curing inhibitor: a pool of resin sits atop an oxygen permeable window, through which both light and oxygen can pass. The CLIP system delivers controlled bursts of light and oxygen to form the shape. The oxygen saturated zones of resin, remain malleable and the areas subjected to light alone solidify. The other game changing feature of CLIP is that the bursts of light and oxygen are delivered so quickly that the build platform can move continuously, rather than indexing in a digital fashion like all other 3D print technologies. As such, its creators market it as the first true 3D printing technology, effectively referring to all previous designs as layered 2D printers. Early videos of this technology provide evidence of very fast printing speeds, but as yet there is no information on machine costs. Polyjet printing process Image source: carbon3d.com Another new frontier in the rapid prototyping landscape that is currently being explored is to rapid prototype tools or moulds for parts, rather than the parts themselves. Companies have started doing this in the last couple of years and it appears to be delivering significant cost and time savings. The mould tools, which are printed in plastic, are then used to produce between 10 and 75 injection moulded plastic parts before the tool is worn out and must be disposed. This process produces prototype parts that are almost identical to production parts in appearance and strength and has the added advantages of time and cost savings, if several parts are required.

7 outerspace.co 7 While 3D printing was originally conceived to produce prototype parts more quickly and accurately, there are now many commercial applications for the technology. The aerospace industry is leading the way in utilising 3D print technology to produce production parts that would have previously been considered near-impossible or too expensive to make. An example being a bracket made by an SLS 3D printer which is lighter and stronger than the conventional equivalent, by virtue of being hollow (see images, courtesy of General Electric). Such parts are printed and then fitted to aircraft where they will see decades of use. Customisation of products to individual users is another area where 3D printers are finding widespread application. Dentistry has found many applications for 3D printers, using them to produce tailor made crowns, bridges and partial denture frameworks. Instead of taking a physical mould of a persons teeth, a 3D computer scan (similar to a digital photo but in 3D) is taken of the inside of the mouth. The scan is inserted into computer aided drawing (CAD) software and a virtual model of the proposed dental prosthesis drawn around the scan; a 3D printer is then used to print it. This technique offers the patient a less intrusive procedure together with a more accurate and better fitting prosthesis produced in less time. Podiatrists are using similar processes to produce orthotics tailored to the exact contours of an individual s foot. Surgeons can now print replacement sections of bone for facial reconstruction. The uses for 3D printers appear limitless; like mobile phones, it may soon be hard to believe life existed without them. Conventional Bracket Source: Hollowed Bracket Source: winblog.blob.core.windows.net Organic Bone-Like Bracket Source netsolhost.com