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.

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