In the context of scale modelling, there are three types of 3D printing technologies a modeller can choose from: FDM (Fused Deposition Modelling), SLA (Stereo lithography), and SLS (Selective Laser Sintering).
FUSED DEPOSITION MODELLING or FUSED FILAMENT FABRICATION
In essence this is a three-dimensional version of your old-fashioned paper printer. Imagine if your old ink-jet printer went back and forth a zillion times over a piece of paper, you would start to feel the surface effect of the printing. Think of some high-quality business cards (what are those, right?).
Filament printing is an extension of this concept, with the added notion of three-dimensional movement and resultant part construction. The software slices an object into flat layers and the nozzle follows the path that each layer defines, dropping material along that path. Each time the nozzle completes one layer, it goes up by the thickness of a layer and then drops the next layer. When the next layer is expected to overhang the first one, a support structure is also formed so the higher layers don’t drop into thin air.
The quality of the part printed will depend on several factors, including the quality of the nozzle, the type of material used, and the rigidity of the machine. In general, FDM printing is excellent where parts may be large, they do not require excellent surface finish, and they are not structural.
STEREO LITHOGRAPHY
Stereolithography (SLA) is an additive manufacturing process that creates three-dimensional objects layer by layer using a photopolymer resin cured by a UV laser. Instead of the nozzle going up layer by layer, a plate slowly sinks down through the resin.
The original digital model is sliced into thin layers, typically between 25 to 100 microns thick, depending on the desired level of detail. These slices serve as a guide for the SLA machine. This is our preferred 3D printing technology as it allows for greater detail than the FDM technology. For most modelling projects, the quality of printing that can be achieved with this technology is better than what can be achieved with FDM.
After printing, the object is typically cleaned to remove excess resin and undergoes a post-curing process to fully solidify the material. Also, the process creates support structures where needed, as in the FDM production, and these will need to be removed and the part finished to complete the assembly and insertion into a model.
SLA can utilize a variety of resin materials with different properties, such as transparency, flexibility, or strength, allowing for versatility in application.
The biggest drawback is that the process can be relatively slow compared to other 3D printing methods, and the resin materials used can be expensive. Additionally, SLA parts may be prone to warping or distortion during printing if proper supports are not used. It may take several iterations for a modeller to figure out the best orientation of the part in the print, the best positioning of support materials, and the best material to use for the part itself.
Despite these limitations, we feel this is the best technology to use for typical parts for scale models.
SELECTIVE LASER SINTERING
Selective Laser Sintering (SLS) builds objects layer by layer using a laser to selectively fuse powdered material. This method is really interesting because the powder itself is the support material, and the resulting parts need little post-printing processing.
The SLS process begins with a 3D CAD model, which is sliced into thin layers, as with the other two methods. A bed of powdered material, typically nylon, polyamide, or other thermoplastics, is spread evenly over the build platform. A high-powered laser then scans the first layer of the powder, selectively sintering (or fusing) the particles together based on the digital cross-section of the object.
Once the first layer is complete, the build platform lowers by one layer thickness, and a new layer of powder is spread over the previous one. The laser scans the new layer, fusing it to the layer below, and this process repeats until the entire object is formed.
After printing, the part undergoes a cooling process to solidify before being removed from the build chamber. Excess powder is brushed off, and the part is ready for detailing by the modeller.
SLS offers several advantages over the other two methods, including the ability to produce complex parts without the need for support structures. SLS is also capable of producing parts with excellent structural properties, meaning a modeller may be able to use these parts instead of milling them out of steel or wood.
However, SLS machines typically run ten to twenty times the cost of machines using FDM or even SLA technologies. These parts are best left to outsource companies. Additionally, the surface finish of SLS parts may not be as smooth as parts produced by the other additive manufacturing methods. Considering the substrate looks like sand, the parts will be porous and this needs to be dealt with. Sometimes the look will be just what is needed, but sometimes it may be preferable to cook the parts slightly to create a smooth surface.
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