There are many different types of 3D printers and many processes, but at London 3D Printing we are specialists, among other things, in the use of SLS 3D printing. SLS stands for Selective Laser Sintering, and the machines that we use for this are very expensive and high-end, and they require a considerable degree of technical know-how.
SLS 3D printing is an additive manufacturing process and in this process a laser selectively sinters the particles of a polymer powder, fusing them together and building up the object layer by layer. The powder that we use is a standard polyamide powder known as PA12 or Nylon 12 and costs approximately £45 – £50 per kilogram. SLS printing is ideal for the formation of prototypes and small production runs, providing a high degree of accuracy, and it can handle extremely complex designs producing objects with great strength.
The sintering process works by first of all heating the powder bin and the build platform in the machine to just below the melting point of the nylon powder. The recoating blade then spreads a fine layer of powder over the top of the build platform. The CO2 laser then scans the contour of the next layer and selectively sinters the particles of the powder which fuses the together. The complete cross-section of the part is scanned so that it is built solid.
When this has been completed, the build platform moves down one level and the recoating blade now moves forward and recoats the surface with another layer of powder. The process continues like this until the whole part has been constructed.
Once the part or parts have been completed, they are completely enclosed within the unsintered powder and the whole bin must be left to cool down before the parts can be unpacked and removed. The cooling process can take up to 12 hours which can be as long as the building process. When the parts have been removed, they can then be cleaned with compressed air, or a blasting material such as sand, while the unsintered powder will be collected and can be reused.
A big advantage of SLS printing is that no support structures are required because the unsintered powder provides all the support needed, and this is why the process can produce intricate geometries which cannot be replicated by any other method.
When you are printing small production runs of parts, it is important to use the whole of the bin. A bin of any given height will take the same time to print, regardless of how many parts there are. This is because the recoating process determines the production time, and the machine will have to use the same number of cycles whether there is one part or a hundred.
Typical SLS printed parts are around 30% porous, but they can be vacuum impregnated with a resin in order to ensure that they are watertight. Because they are porous they can also be dyed in a warm colour bath in a choice of different colours, and this will penetrate to a depth of about 0.5mm and completely cover all surfaces. The parts can also be spray painted if required. Another finish that can be achieved is a matte or satin look by bead blasting.
You can also achieve a polished finish by placing the parts in a tumbler containing small ceramic chips to smooth them down. If you want a metallic look, they can be electroplated in nickel, gold, or copper.
Since SLS printed parts need no support structures they can be produced with hollow structures. A couple of small escape holes will need to be added into the design so that the unsintered powder can be removed. This means that the part uses less material, thus reducing both the weight and the cost.
However, if a high degree of stiffness is required, the part can be printed solid. Alternatively, it can be printed without any escape holes and the unsintered powder will be trapped within the part increasing its’ mass without having any effect on the build time. Another way of adding support would be to use an internal lattice structure inside a hollowed part which will also increase its’ stiffness and resistance to mechanical loads.