Update!! The alternator, many of which's parts are shown in this instructable, I made using this technique with PLA, hard plaster, wire, and PVA glue, just went past 650 rpm! This was in testing my new wind turbine, or I should say at this stage, testing the test system for my turbine. If you want to see some unusual applications of hybrid 3D prints, come see my turbine testing page here https://www.instructables.com/id/How-to-Test-a-Wind-Turbine-an-Instructable-in-Prog/
And so, back to 3D printed parts, Faster Stronger Cheaper! with concrete and wire, or plaster and kevlar, or expanding foam and carbon fibre!!!
I've had a large format FDM printer for a few years. It's been incredible to have in so many ways. It does have it's limitations though, which got me thinking.
Thinking has taken me a lot of interesting places, and since January or so, I've been working on a number of concepts to do with making my VAWT design more accessible.
Here's my instructable on the VAWT blade Painted Canvas over plywood VAWT
And here's my instructable on the Axial Flux Alternator, and Dynamometer I built for the project, it's also got parts using this technique. Axial Flux Alternator/Dynamometer
In my experience, FDM printed parts can be quite strong, if the wall thickness is high, and infill rate is high, however setting those rates high, slows the print, and raises the price of materials. Even at very high infils and thick walls, the part is limited by the strength of the PLA, or other deposited filament. Also, in general, parts end up stronger, with the "grain", rather then across the "grain", unless your layer fusion is perfect. Even with perfect layer fusion, the printed surface ends up with tiny notches, which are perfect for originating stress cracks.
What I've been playing with are 3 different idea's that can be used separately, or combined.
- Stone your part! First, if you print a thin walled, but water tight shell of the part you want, pour it full of Concrete and additives like PVA, or pour it with polyester / epoxy resin loaded with chopped fibreglass, or high hardness plaster, or cast stone. The concrete can be mixed with conventional aggregate like sand, or other aggregates like perlite, or styrene beads. It can be aerated concrete for slightly reduced strength, but much lighter parts. Remember that the whole part may not need to be filled, on larger parts create a hollow shell. Also keep in mind there are lots of other fill materials that can be used, like cast-able urethane, expanding foam, epoxy and additives. There's lots of options here!
- Wire it up! It's generally fairly obvious where stress moves through a part, from the mounting bolts to the bearing retainer, for example in this part. Using small diameter high steel wire, these area's can be tied together, and wire can be pre tensioned, making the parts stiffer and stronger. As well, on failure, the wire may help to keep the parts together, and instead of complete catastrophic failure that you might expect from the FDM part, as the printed material yields to the stress, the wire will not, transferring stress to the stronger material, and potentially making part failure less dramatic.
- Plan for stress concentrations, and Paint ! In general structures fail at their weakest points, so structures can often be made much stronger, not by strengthening the whole object, but instead identifying places where the stress on the structure is passing through weak points, and strengthening them. Think of the sharp inside corners, use the fillet tool and give them substantial radius's. Remember that a rectangle isn't as stiff as a triangle, and so build more in triangles. The easiest stress concentrations points to identify in a well designed FDM part, are the layer join points. Cracks need to start somewhere, and any little notch in the structure is the most likely. The simplest solution is to paint the parts. Paint doesn't add much overall strength to an object, however FDM printed parts, with their layered structure, present myriad stress concentration points. The simplest fix to strengthening and stiffening your parts, is to paint them with strong finishes, like epoxy, PVA glue, or others. While the thin coat of epoxy doesn't add a lot of overall strength, it can make a great deal of difference in stiffness, as the epoxy layer, on the outside of the structure, must yield before the inner structure starts to flex.
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Step 1: Stone Your Part
Perhaps you want a really bomb proof CNC router, you've got some space, and time, but not a lot of money. If you print FDM parts, then you are printing with a thermoplastic like PLA, which over time in any kind of warm environment, will suffer from plastic deformation. It's not cheap, and it's slow if you want high stiffness solid parts.
Instead, if your part were made from polymer reinforced concrete it would be heavier, but orders of magnitude stiffer, and much less expensive.
However making moulds that you can release and use again are not easy, and in so many projects, it's only one part you need, so the moulds represent materials that you could have used elsewhere.
My solution is to print non removable moulds, that can be filled with a variety of materials to "program" the structure you need.
- Concrete- is one possibility, however it's high carbon footprint, and non recyclable nature, are downsides. Some upsides are that there are a great deal of well known mixtures and additives that can be used to modify the mix, including things like perlite for lightening, PVA for strength and waterproofness, polypropylene fibres for toughness and crack resistance
- Plaster- isn't used as much as concrete, but it should be! It's recyclable (in theory if not often in practice). It's carbon footprint is a fraction of concrete, and it's available in the well known "plaster of Paris", and the less often known Hard Plaster. Hard plaster can have a tensile strength of 3000 psi! Better than most hardwoods! and as good as badly mixed concrete! It's also fast! I've been using a product by CertianTeed called Easy Fill Heavy Repair 90. In an hour and a half your part is ready. It will get lighter over a few days as most of the water vents, but it's usable right away.
- Resin-there are so many kinds these days! I'll say off the top that polyester resin, the cheap one, is nasty stuff! Don't use it unless you have too, the fumes it gives off while you mix or work with it are very dangerous, and it will continue to offgass for months. Epoxy is much kinder to work with, generally very low VOC, so not as much fumes, BUT! most epoxy is about 1/3 BPA, the plastic that is known as a hormone disrupter, and some people can be come allergically sensitized to epoxy, which then means not only that you can't use it, but you might not even be able to be around it. There's a bunch of good casting resins, often urethane or acrylic, these are all candidates for filling up voids you've designed into your parts. Check out the "smooth on" family or products for more options.
- Foam- expanding foam is available in many forms. Spray cans of insulating foam, to 2 part mixes for hard structural urethane foams, or even soft pillow like foams. Urathane foams can be very strong, and wrapped in a shell of 3D printed skin (PLA,ABS or Carbon) the foam can form the core of a stressed skin panel, making the part an order or magnitude stronger, without adding much weight. Many RC planes could make good use of foam filled wings or fuselages.
Step 2: Wire It Up!
There's many inexpensive and very strong tension elements available to us today, from armide fibres like kevlar and spectra, to steel or stainless wire. These are available in small diameter fibre or wire and are very easy to work with. If you design your part so that you can thread wire, or other element through it, connecting the stress points, like mounting flanges and bolt holes, to bearing seats, then if the part begins to flex under load, the very stiff and strong tension element will start to take up the load, reducing the stress faced by the printed part. This has been part of engineering, since the beginning of engineering. I think of the carefully wired and trussed early airplanes. Think about designing your part to have voids connecting the stress points, and how you can thread tension elements through and around the part. For example I try to have wire wrap around bolt holes, and pass continuously to bearing sockets and back, all inside a void that I'm going to eventually fill with something like plaster or concrete. In the 4th photo if you look closely you can see the galvanized steel wire shadow through the pink 3d printed part.
Step 3: Plan, and Paint!
Plan for stress concentrations, and Paint !
In general structures fail at their weakest points, so structures can often be made much stronger by identifying places where the stress on the structure is passing through weak points, and focusing on strengthening those points. Sharp inside corners force loads to concentrate, so use rounded fillets and give them substantial radius's where ever possible. Remember that a rectangle isn't as stiff as a triangle, and so build more in triangles.
If you've planned well and your part has nicely filleted corners and joints then there's one other place to work on, and it might take a microscope!
The easiest stress concentration points to identify in a well designed FDM part, are the layer join points. Often layer fusion isn't as good as we would like, and a weak point is created in the joint. In a sharp corner, the stress on these weak points can really add up. Cracks need to start somewhere, and any little notch in the structure is the most likely. The simplest solution is to paint the parts. Paint doesn't add much overall strength to an object, however FDM printed parts, with their layered structure, present myriad stress concentration points. The simplest fix to strengthening and stiffening your parts, is to paint them with strong finishes, like epoxy, PVA glue, or others. While the thin coat of epoxy doesn't add a lot of overall strength, it can make a great deal of difference in stiffness, as the epoxy layer, on the outside of the structure, must yield before the inner structure starts to flex
Step 4: Samples
Participated in the
Stone Concrete and Cement Contest