Design Considerations for SLA

Stereolithography is a process whereby a component is built up in a series of layers, from a UV-curable resin. For a detailed description on SLA, please see our article ‘What is Stereolithography’.

Stereolithography has a number of advantages for rapid prototyping & rapid manufacture, one of the main ones being that difficult shapes including hollow areas are now feasible to manufacture. Unlike in other methods of manufacture, Stereolithography has no tooling requirement, thus reducing the cost & time that a part takes to build.

On occasion, parts are designed with a point or knife-edge built into them, due to current SLA technology, we would not recommend points or knife-edges being designed into the parts. Depending upon the accuracy of the SLA machine you are using, 0.5-0.7mm should be achievable on an edge – the same goes for radii on the ends of shafts. Thread design is one area where knife-edging is most prominent, due to this we would advise not to model the thread into the part, unless it is quite coarse. Cured resin is an easy material to tap; we would suggest sizing holes accordingly & tapping threads by hand. Hole sizes in SLA are relatively accurate, although we would recommend running a reamer through any holes to ensure they are accurate.

One big advantage of the stereolithography process is that parts can be made hollow to save weight, or to run channels through the inside of the part. Usually, if the part is being hollowed out, a drainage hole should be added somewhere on the part so that excess resin can be drained out once built. Parts can also have lightweight structures applied to the hollow inside to add strength when hollowing out. It is also possible to run channels and tubes through the part for air or other parts such as tie-wraps to run through, in these instances, it is important not to size the channels too small or to place them too close to the side walls of the part.

Due to the nature of the process, some faceting or stepping will always be present on SLA parts. The amount of faceting is dependent upon the orientation of the part when it is built, and the layer thickness that the machine builds at. Thankfully, this faceting and stepping is easily removable from the part, due to a number of finishing techniques that are available. Please see our article on Finishing Rapid Prototyped Parts for more information.

Within most rapid prototype CAD packages, it is possible to add text or logos to parts before they are built. This is very useful for numbering parts for identification, or for adding a brand name to a prototype. These logos or text can either be stamped into the surface of the part, or raised above the surface.

Following on from Thread design, if the part requires threads or fixings in it with any strength, we would always recommend designed bosses into the part that can then accept a threaded insert. This will allow much greater strength within the threads, and means the part can be cleaned out a lot easier. We recommend using a keensert non-locking insert with appropriate adhesive – please see http://www.keensert.com for more information.

Whilst stereolithography is a fantastic technology, it does have its limitations. Whilst the parts do have quite high strength, they can be brittle, and would be unsuitable in an environment where the prototype or final component may be handled roughly. It also can have quite a high weight, especially if the component is large or has a high wall thickness, some of this can be negated by hollowing the part out. With very small components, production volume isn’t too much of an issue, as many parts can fit on one machine. However, if quite a lot of post processing is involved, or the part are quite sizeable, then the process could end up taking a very long time – meaning that for high volume manufacture, other avenues should be explored.

What is an STL file?

Simply put, an STL file is a file used by sterolithography software that can be ready by additive manufacturing machines. The part is broken up into triangles, with each triangle having its normal & vertices defined. An STL file only describes the surface geometry for a three-dimensional part – no common CAD model attributes, such as colour or texture are represented.

STL files come in 2 different formats; Binary and ASCII. An ASCII STL file is readable by a human, and can be modified by a text reader, if necessary. Because of this, ASCII files have much larger file sizes than Binary & it is usually impractical to use an ASCII STL for anything other than debugging purposes. A Binary STL is not human readable, and is made up of sets of numbers to define the component.

The quality of an STL can vary a lot, depending on a number of variables. Usually, the triangles orientation must be defined correctly, and all triangles must be joined together. Specialist software for modifying STL files and small gaps & inconsistencies in the part can be fixed. However, more fundamental issues may require modification of the original CAD model. One of the variables that affects the quality (and file size) of an STL is the number of triangles. If too few triangles are defined, the part becomes very facetted and loses its original shape. If too many triangles are defined, the STL file size can become very large & may end up being unmanageable within the machine and processing software.

CAD models can normally be converted to STL within most CAD packages, by way of an automated process. As long as you have a well-surfaced 3D model, the user may be given several options to adjust the resolution of the output model – this will affect the quality of the STL file as well as the file size. Smaller or fine detailed parts may require a higher triangle count that large, simpler parts.

If you do not have access to STL conversion software, Design For 3D can provide a conversion for you as part of our CAD Design Service. For any STL conversion enquiries, please contact us on: info@designfor3d.co.uk

What is DMLS?

What is DMLS?

DMLS is a process whereby a laser is used to sinter a powdered material (usually metal) together to create a 3D structure or part. It is similar to SLS (selective laser sintering), but has different technical details.

The process begins with a STL file, as per other additive manufacturing processes, and an operator orients the part and adds any required support structures. The file, along with its support structure, is then sliced into layers and is sent to the DMLS machine. These layer slices are equal to the thickness that the machine will build each layer at. The powdered material is dispensed on the build platform, and a high power laser fuses the metal powder into a solid part. The build platform then lowers by the thickness of one layer, more powder is dispensed, and the process repeats until the part is complete.

DMLS allows very complex geometries, that would normally be thought impossible, to be built very quickly. It does not require any special tooling to be manufactured before hand, and internal geometries such as passageways and channels can be built inside the parts. This makes the process ideal for prototyping, but also for short production runs of final components. It has the added benefit that provided the build envelope is large enough; multiple parts can be built at once. DMLS is used across aerospace, motorsport, automotive, dental & medical industries, as it gives the ability to manufacture highly complex, small to medium parts very quickly.

DMLS does have some limitations – the build envelopes on current machines are still relatively small, so large DMLS parts cannot be manufactured in one piece, and feature details and surface finish are not quite up there with CNC milled or turned parts – to achieve a mirror or extremely smooth finish, the surfaces will have to be polished. Also, the support removal method is much more involved than in SLA & FDM, as the support structures have to be removed by way of machining, EDM and/or grinding – a very time consuming process that requires a high level of accuracy. The surface finish of completed parts is comparable to cast parts, whilst the mechanical properties are comparable to that of cast or machined parts.

Materials currently in use in DMLS include Aluminium, Stainless steel, Maraging steel, Cobalt Chrome, Inconel & titanium.

What is FDM?

What is FDM?

FDM (Fused Deposition Modelling) is an additive manufacturing technology that uses a thermoplastic extruded through a nozzle to build up a part layer by layer. The process begins with an STL file that is sliced into layers and then sent to the FDM machine. A plastic filament or metal wire unwinds & supplies material to the extrusion nozzle, typically being pushed by a worm drive. The nozzle is heated to melt the thermoplastic, which is then deposited on the build platform. The nozzle can be moved both horizontally and vertically, controlled by servos or stepper motors, and follows a tool path created by the software (similar to a CNC mill). The extruded material then cools soon after it is deposited – creating the solid part.

Some FDM parts will require a support structure, which can either be made from the same material as the part itself, or from a soluble material. Although FDM is capable of dealing with small, unsupported overhangs, there are restrictions of the angle of the overhang – this is where using the support materials can be very useful. Some machines are also capable of laying down different colours of the same part material.

Once the machine has finished building, the part can be removed and any support structures either removed by soaking in a water and detergent solution – in the case of soluble supports, or broken off by hand when using thermoplastic supports. The parts can then be finished by hand to improve the surface finish, milled, painting or electroplated to improve function.

There are a vast number of materials that can be used in an FDM machine, including ABS (Acrylonitrile Butadiene Styrene), PLA (Polylactic Acid), PC (Polycarbonate), PA (Polyamide) & rubber. These materials all have differing temperature & strength properties.

FDM has a wide number of uses, and is used across the motorsport, aerospace, automotive & manufacturing industries. FDM is also commonly used by engineers to build prototype parts that can withstand being fitted to assemblies, whilst remaining dimensionally accurate. Thermoplastics are quite resilient to heat, chemicals and mechanical stress – making FDM an ideal process for prototyping and small volume manufacture.

The drawbacks of FDM are that, in comparison to other 3d printing processes, the build times can be comparably longer, and hand finishing the parts to a high standard will take longer than a similar SLA or SLS part.

What is SLS?

What is SLS?

Selective laser sintering (SLS) is a process whereby a powdered material is sintered by a high power laser; this process binds the powder together to create a 3d shape. The process begins with an STL file being sliced into layers that the SLS machine can build. Once this data is transferred to the machine, it puts down a layer of powder which the laser then scans the first layer onto. After each layer, the powder bed drops by the thickness of one layer and another layer of powder is laid on top – the process then repeats until all the parts are complete.

Unlike in other additive manufacturing processes, such as SLA and FDM, a support structure is not required with SLS, as the parts are always surrounded by un-sintered powder. This means that every part in a build is kept stable and geometries that would otherwise be thought of as impossible can be manufactured. This also means that once a build is complete, the parts are left buried inside an amount of un-sintered powder – it is then the job of the operator to remove the parts from within this powder.

Once the parts are built there can be some layer lines & faceting left on the surface of the parts, these can usually be removed by a number of different finishing methods. SLS materials can be finished and painted to a high standard; however, the finish can vary depending on the material used. There is a vast range of materials that can be used in an SLS machine, the most common being nylon – available with or without a number of filler materials designed to strengthen the finished parts.

What is SLA?

What is SLA?

Stereolithography is a process where a vat of UV curable photopolymer resin is cured by a laser to build parts one layer at a time. It begins with the generation of the final 3D part as an STL file, which is then sliced into layers at the same thickness that the machine builds – this is typically 0.05 to 0.15mm. These slice files are then transferred to the SLA machine where the parts will be built layer by layer. Components can be converted to STL from other CAD file types, or can be created using a CAD design service.

The laser then scans across the elevator platform, drawing the cross section of the first layer. Once the laser has finished drawing this cross section, the elevator platform lowers by a distance equal to that of one part layer & a recoater blade then sweeps across the vat, re-coating it with fresh resin. The process is then repeated, layer-by-layer, until all the parts have built to their full height.

SLA requires the use of support structures for a number of reasons: Firstly, they are needed to anchor the parts to the elevator platform to prevent them from floating away in the vat of resin. Secondly, they are required to prevent any deformation in the part whilst it is building. Especially with tall parts, they could be susceptible to movement within the vat – the support structures help prevent them from moving about. Once the part has finished building, these support structures must be removed (usually by hand) before the component can be finished & used.

Once the parts are built, the excess resin must be washed off – they are then ready for the finishing process. In order to be ready for end use, the parts require post-curing in a UV oven. Prior to this, any finishing of the parts can be undertaken. The finished parts may have some faceting & layer lines left over from the build process – these can normally be removed quite easily by hand. Once post-cured, there are a number of painted & metal coated finishes that can be applied to SLA parts to provide a multitude of finishes.

3D Printing Processes

3D Printing Processes

3D printing, also known as additive manufacturing, is a prototyping process where a real object is created from a 3D model. The 3D CAD data is saved as an STL file and then sent to the 3D printer. The printer then prints the model layer by layer to form the complete model.

There are a number of technologies within 3D printing that use different ways of building up a part. SLA (stereolithography), SLS (selective laser sintering) and FDM (fused deposition modelling) are the 3 most common used technologies in 3D printing.

 

SLA (Stereolithography) is a process whereby a vat of UV curable photopolymer resin is cured by a laser to build parts one layer at a time. The STL file is sliced into single layers (typically 0.05mm to 0.15mm thick) and the laser draws the cross section of each layer. Once the layer has been drawn, the elevator platform descends by a distance equal to one layer. A resin-filled blade then sweeps across the vat, re-coating it with fresh resin. On the surface of this fresh resin, the next layer is then drawn. After being built, the excess resin is then washed off the parts, which are then cured in a UV oven.

SLA requires the use of support structures to attach the parts to the elevator platform and to hold them in place whilst building. Without these support structures, it would be almost impossible to build the complex shapes that SLA is renowned for. Once the build is complete, these supports are removed from the parts manually; usually they are broken away from the parts.

 

In SLS (Selective Laser sintering), a high power laser is used to sinter powdered material, binding the powder together to create a 3d shape. Similarly to SLA, in an SLS machine the laser scans cross-sections generated from a sliced STL model of the final part on the powder bed. After each layer has completed scanning, the powder bed is lowered by the thickness of one layer – the process is then repeated until the full part has been built.

Unlike in SLA and FDM, SLS does not require support structures in order to keep the parts stable within the build. This is due to the fact that the parts are always surrounded by powder – allowing for near-impossible geometries to be created.

 

Fused deposition modelling (FDM) is a process whereby thermoplastic material is extruded through a nozzle to build up a part layer by layer. The plastic filament is unwound from a coil and heated as is passes through the nozzle, which can be moved in both the X and Y axis. On some models, the nozzle can move up vertically, on others the bed can move downwards. Like SLA, FDM requires support structures, sometimes these are manufactured from the same material as the part & need to be broken away, and other machines have water soluble support structures that can be washed away once the part is built.