Skip to 0 minutes and 10 seconds So material selection is a critical component, a critical step in designing a 3D printing project. Steve and I will talk to you about what types of materials can be used to create a finger prototype which, of course, could eventually be assembled into a prosthetic hand. We need to have structural support within any sort of prosthetic. We need to have lightweight as possible. We need to have robustness. And we need to have long life. ABS is a potential material. So, Steve, is ABS the only type of material that could be used to create this prosthetic hand? No, absolutely not. But it is a very good example where we can access low-cost equipment and it produces high-resolution components using fused deposition modelling.
Skip to 0 minutes and 54 seconds So it’s an existing technology that we can use rapidly to create high-detail components such as this. And what would be an alternative to the ABS? Well, we could use other accessible materials like PLA. And ideally, we will use a material that’s going to provide the highest dimensional accuracy as well as long life stability. Why would you pick fused deposition then? They’re highly accessible. The most commonly used printers globally today are fused deposition modelling printers. People can access them for a very low cost, and achieve high-definition and high-resolution components. How do we actually make it move? How do we make it functional for the patient? Well, the parts that we produce are just the structural components.
Skip to 1 minute and 37 seconds We have to couple them with electromechanical components such as a motor, a drive gear, and a tendon-mimicking wire. And this is a component that aims to mimic a finger. It does not have the same degrees of movement as a finger would. Obviously, in the future, we need to work towards a greater range of materials to allow us to mimic the actual motion of a finger. And is that sort of resolution suitable for what we’re trying to create in a prosthetic hand? It’s ideal for a prototype prosthetic hand. It would be ideal for skeletal type features.
Skip to 2 minutes and 12 seconds And whether or not that’s going to give us the fine definition or material compatibility that we need for a finely finished prosthetic hand is questionable. So the final structure then, does have some limitations in terms of mechanical properties and resolution. But you’re saying it would be ideal still for doing prototypes, in designing and creating those prototypes. Absolutely. It is the key step in generating the first prototype.
Skip to 2 minutes and 42 seconds See the different elements of the finger, even incorporating something as trivial as a fingernail. The components got recesses that allow the incorporation of tendon-mimicking wires. And we’re able to assemble each of these components together to allow us to mimic the movement of the finger. But to get to a real 3D-printable hand, we have to be able to mimic the fingers, have four fingers and a thumb, and base them around a palm. And the palm is where we’d mount the motors and couple in each of the tendon wires. So what we can then do is take each of these individual elements and print them on a 3D printer.
Skip to 3 minutes and 27 seconds Clean them up, finish them, and then assemble into the finished component. So here, we have an element of that finger. As you can see, it’s a downloaded STL, so there’s a tessellated surface. We can highlight that. So that’s each of the different elements that make up the STL file. You can also see here where there’s hexagonal recesses to mount nuts. You can also see the pass-throughs for the wire that would run over this contoured surface to allow it to pivot around the joint. What we now do is we’ll save this file as an STL, which we can then transfer to a 3D printer to allow us to reproduce.
Skip to 4 minutes and 13 seconds So now that we have the STL file saved, we import it into slicing and printing software, which is coupled to our FDM printer. So we can position the joint wherever we like to on the build tray. For the moment, we’ll just position it right in the centre. We have the opportunity to change it between draft settings and fine settings, which produces a higher resolution finish on fine and a lower resolution on draft, given that this particular FDM printer only has one material associated with it. The build material also functions as the support material. So in this particular section here, a thinner version of the material is printed as a support structure for these areas which are overhanging.
Skip to 5 minutes and 1 second So prior to producing any part on FDM, a raft is initially laid down. This creates a structure for the part to stick to and makes it easier to remove the part without getting any bending or warping.
Skip to 5 minutes and 20 seconds So the part has finished printing. And now we can scrape it off.
Skip to 5 minutes and 32 seconds So we just part printed. We now need to clean the part. So we need to remove the raft, and we need to remove the support material we mentioned earlier.
The prosthetic hand: Material selection and printing process
The 3D printing process
Fused deposition modelling
Fused deposition modelling (FDM) uses extrusion to lay down thin lines of thermoplastic material in the shape of the object being manufactured.
Only polymers with appropriate melting points and melt-flow characteristics can be used. Commercial FDM printers have commonly relied on materials such as ABS (acrylonitrile-butadiene-styrene) and PLA (polylactic acid), two common engineering plastics, as structural building materials.
One of the great benefits of this method is its simplicity. FDM constructs the model on a base plate that can move in the vertical direction. During manufacture, a filament of the feedstock polymer is fed into a heated melt chamber and heated to melting point. The heat-softened filament is extruded through a nozzle with a diameter of approximately 0.4 mm. The hot plastic is then laid down on the base plate, much like piping icing on a cake, by moving the extrusion head along a computer driven pathway over the build surface.
As the thermoplastic and the environment are hot, the material bonds with the build surface before cooling and hardening. Additional layers are built up in the same way, depositing more hot plastic on the layer previously put down, fusing the layers together and cooling to a solid state.
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