XceL - 3D printed chair strength test
Design a chair for large size 3D printing
Large size 3D printing with FDM has been around for some years now and the use of this tool is increasing. FDM 3D printing applies its own rules for strength, form and design compared to any other type of manufacturing but the viability of up scaling, its rules has never been tested.
We used the XceL 3D Printer to print and test a large 3D print. The results of the design, print and test can be found below.
Design for 3D printing
It is wise to keep the method of production in mind when designing parts and products. Any production method will give better results when the method has been kept in mind during the design process. This is the same when it comes to 3D printing.
The basic rules are: preventing overhang, taking in account the wall thickness and the layer direction. Overhang just makes a print possible or not possible (without support). The wall thickness can become an issue when creating small, fine parts. And the layer direction can improve, or decrease the strength of a 3D printed part in a certain direction.
Then, of course, you have different parameters you can set when slicing your part and your choice of material. Together, these things can determine the quality of your print. But the layers stay roughly the same on a small or large print, so what can you expect in terms of strength of a life-size 3D print.
Creating a large, functional 3D print
Creating a 3D print is relatively easy if you know your way around a CAD modelling program. Now, there are many different types of software available and each one will allow you to model in a different way. Our engineer, Dorus, used Fusion 360, however most professional CAD programs will offer similar features.
The object to 3D print was going to be a bar stool because we want to build a 3D printed bar in the office (because why not) by using the build volume of the XceL. This would be an excellent test object because it could be directly used and it also provided plenty room for freedom. Form freedom was also important, because a 3D printed chair, has to look as if it is 3D printed. So the shape should be complex and only suited for 3D printing.
The base conditions
Two conditions were set while creating the bar stool. One was the height and width of the seat. This helped creating the design, since it gave clear boundaries to work in. The other condition was the static weight it should be able to hold. The regular weight of a Dutch male is around 86Kg, so to be on the safe side, we set this at 100Kg. Then you have the possibility that the 3D print will not be a 100% uniform or a homogeneous piece. And we knew that we wouldn’t be printing with 100% infill either. We multiplied the weight by a factor of 5, which gave us the condition of 500 kilograms of static load.
3D printed bar stool - Mark 1
Creating a complex shape can be a challenge because of obvious reasons. So our engineer wanted to use Shape Optimization of Fusion 360. This tool can optimise a model based on the amount and direction of the load your object should be able to hold. The optimization will be done by removing excess material of the model. This slimmed down version of your model then should still be able to hold the set load.
Dorus started of with a generic chair and a standard load. So four legs, the seat and a single load of 500Kg. This however gave a very symmetrical and generic design, not at all what we were looking for. So to force Shape Optimization to a more creative or more so organic design, two things were done. One: the base design was allowed more material to be cut away by Shape Optimization. Two: the load was divided over several points, placed asymmetrically on the seat.
The result was good. It got a unique texture and shape. But the complexity was not that high. The form itself was unique, but not ‘only-3D-print-possible’ unique.
New 3D design process - Mark 2
Dorus set out to use another tool: Pipe Tool. Here you can create a series of lines or vectors, which the program will convert to pipes. The pipe tool will then generate its own solution for intersections and crossing pipes.
A simulation was run to test the desired load for the bar stool. In this simulation, the amount of stress is shown with color and the deformity is shown by litery deforming the model. To decrease stress and deformity, vector had to be moved around for better force distribution. Dorus did this by trial and error and was able to create a decent structure within three iterations.
The model is now ready for printing.
3D printing a large model
The material used for 3D printing was regular PLA, to prevent warping. To get the print time below a couple of days, the infill was set to 3.5% and a layer height of 25 micron was given. This should provide a light and strong part. The inner structure should be able to absorb excessive force and we thought that the goal of a functional chair should be more than viable.
A large functional 3D print
The result was there and it looked good: A very organic shaped stool, made from a plastic and generated by technology! The first print was a bit scaled down to fit the print in between other projects, but it proved the printability of the print. All legs have an equal length, the seat is flat and sitting on it seemed no problem. However, we didn’t have a 500Kg load at hand and we were very curious how much of the original simulated load it could actually hold.
Luckily, one of our clients is specialised in testing furniture of any kind; tables, chairs and also stools. We asked if they would be so kind to help us out and they were willing to do so. So we went to VDS Culemborg to find the limits of the 3D printed stool.
Would you like a personal solution in large 3D printing for you business? We are more than happy to discuss your possibilities and options.
How strong is a 3D print
The strength of a 3D print is often determined by the material, the number of outer layers and amount of infill. The strength of a regular object is determined by the material and quality of the material.
When talking about the quality of an injection molded part, for example, when you are referring to the internal structure and surface of the part. A high quality part would have a very symmetrical internal structure and even surface and both these things can benefit the strength greatly.
Now if you would take the same part and 3D print it, then the ‘quality’ of that print, would be actually rather poor compared to an injection molded version of it. 3D printing is a less consistent way to form a material versus most conventional production methods. So the surface will be less even and the internal structure is prone to irregularities as well. However, the structure of infill can also provide additional strength for the amount of material used. So it’s not all bad.
That being said, the expectation of the strength of the 3D printed Bar stool were good enough for functioning, although not extremely impressive. The result however surprised us quite a bit.
Testing the 3D printed stool
The testing facility at VDS Culemborg is impressive. It looks like a lab and has multiple machines made to test, stress and break your product! We thought that we were going to leave with the 3D printed stool in pieces but that did not spoil the fun. All tests were done according the official parameters used for any piece of furniture with exception of the third test which will be explained here:
First test, endurance
The first test was a simulation of an average person sitting down and again and again on the bar stool. The jig was set up with a blue shape which resembled a pair of buttocks. These would go up and down on our 3D printed stool with a set force.
This test was a gradual appliance of force and would test any deformity within the stool. This would eventually even out, or break the stool. According to the test expert, nothing behaved out of the ordinary and the test was a success.
Second test, dynamic load
The second test was similar to the first test, but here the load was larger and the impact higher. Also here our 3D printed stool performed quite well. There was some deformity visible in a part of the pipe, but the structure held well and was able to absorb and distribute the force.
Third test, tip and fall
The fall test was a more spectacular and also a more punishing test to our 3D print. This was expected due to the way a 3D print is built up in layers. When force is applied perpendicular onto a stack of layers, the 3D print will be able to hold its own very well. This is similar to a wooden board of multiplex. However applying force in the length of the layers would result in a much lower absorption of impact. Additionally, the force is given in one impuls, which means that the impulse force is high.
This test was done on 80% of the norm which resulted already in a crack in the seat of the stool. However, the function of the stool remained fine so it was still able to go to the final test.
Final test, impulse load
The final test was basically dropping as much weight on the stool as possible. A static load of 500Kg was not possible, since that is never necessary, but a dropping load of 100kg or more, from 30 centimeters, would eventually provide an impulse load similar to 500Kg.
We didn’t expect the stool to be able to hold the full 500Kg, due to the lack of 100% infill and the crack in the seat. This is why we started at 80Kg and worked our way up from there.
In the end, we used up all the weight bags we could find, which was well above norm, and above 200Kg.
Conclusion: excellent weight vs strength ratio
Can a large 3D print become a functional and strong object? Yes! However, the direction of the layer needs to be taken into account. If the direction of force cannot be controlled, then a 3D print might not be suitable at all. This is because any force given in the length of the layers, can deform and destroy the 3D printed object quite easily. However, if the direction of force can be controlled, then 3D printing will be a very viable solution for production. Mind you that you have to take into account that 3D printing a large object will still take you up to several days to print. But then again, other costs, such as personnel and material, are relatively low.
That being said, the weight is fenomenal. The bar stool weighs only 4,5Kg, which is light given its size and strength. We can imagine that several industries could make good use reasonable strong object which are super light.