How to Calculate the Strength of 3D Prints

 

“Congratulations, you’ve printed your first 3D product design,” is such a lovely thing to hear, but it can’t end there. The testing phase, by all means, is just as important as the design and printing phases, especially if your product is meant to have any practical usage. Tom and Tracy Hazzard discuss product testing and calculating the strength of the products you’ve created. Tom and Tracy emphasize the importance of this phase to your career in 3D printing. Learn how to go about this important but often neglected portion of the development process.

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How to Calculate the Strength of 3D Prints

We got a question written in on Facebook from a listener over Sweden about how to calculate the strength of 3D prints. It seems that we have reached all over the world. This is coming from Lars in Sweden. He says that he is consistently finding ways to use his 3D printer in everyday use, which isn’t that easy. It isn’t as easy as people think it is. He is looking for really good designs. He read some news recently about a special custom bicycle printed over by the folks in ColorFabb.

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ColorFabb is a filament manufacturer, and they have FFF printed the joints of a bicycle frame. It is a bike frame made of tube metal material. The difference is that instead of welding those tubes together at the joints, they have printed five different FFF plastic 3D printed joints. We have seen that before when we were looking at the 3D print furniture store. There was a table that had a bunch of joints. That was Russel Singer’s table. What they were saying is that they were making a customizable size of a bicycle for an individual by not having pre set sizes. These joints can remain consistent. You can change the length of the tubes to change the bike’s main frame.

Lars is telling us that he wants to print a bicycle like this, and participate in a 300 kilometer long race around one of Sweden’s largest lakes. It is a fun thing to do. He is concerned with the strength of the material. He says that it would be great to invite someone onto our show who can explain about how to calculate strength and that can be applied in projects like this or any other project that challenges material strength. Thank you for that question, Lars.

We are not going to have a guest on this particular episode to talk about that as me and Tracy are going to talk about testing strength of materials from our own experience. However, we will seek to have a guest on that. Let’s talk about the strength of materials and how you are going to prove out something that can meet the load requirements, therefore being able to calculate the strength of 3D prints.

We call it product testing and sometimes destructive testing. You test to failure on any given thing. We test our chairs, casters, bases, and arms are tested. Everything is pulled out every which way possible. They get cycle tested until the point of failure or until you have met or exceeded a standard which you know to be an extreme amount of hours of use. Casters are usually calculated to have a life span of 30 years. If they meet that and they haven’t failed yet, then they just stop testing it at that point. We have even been involved in a lot of different products in our 24 year career. We have designed products that have been installed inside of automobiles. Car manufacturers do this high level UV test and rub test to make sure that the color is not coming off on you.

Everything that you imagine that you wear, buy, or use has set standards that have to meet somewhere, especially if it is something that your body sits in or rides on, whether that is in the car or on a chair within a car. I have been involved in specialized seating within automobiles. Those things go through serious testing of simulating crashes in cars. We have done other accessories to go to get installed in special automobiles in vans. You will be amazed because when you are sitting in a chair, you will think that it is just gravity that you have to worry about. Well, there are so many different test standards that that thing has undergone. They have a pressure test and pull testing where they are yanking the arms off. It is a pretty intensive testing to make sure your misuse of products still keep you safe at the end of the day.

We only think of automotive testing with the crashing to see what would happen when you get in an actual crash. Your products go through a lot of different testing before they even get to that kind of stage of testing before they get damaged from an outside force. As consumers you should be pleased that this happens because this is for your safety. Every different retailer has his standards. Walmart has a slightly lower standard in the industry as it has 15% to 20% lower costs on products, while Costco has 30% higher price on its products. When we design and develop something, we put in our standards for what our expectations are. We specify that on behalf of our clients and make sure that the system is in place to test that properly. We review test standards. We also review product over time because sometimes products break down.

Moisture can get into the products over the water as they are travelling in which is why they deteriorate. A lot of components in the office chair industry are made out of molded nylon. Nylon will actually absorb water and it changes is structural properties. There are lots of good CAD programs out there. This one is not only a CAD program but an engineering program. You can do an FEA which stands for Finite Element Analysis of the strength of the structure that you are going to propose to print or to mold, so you can calculate strength of 3D prints that way. You have metal parts and a plastic tube that goes with it. You have what the material that is made of.

The computer can calculate the strength of materials and therefore the strength of 3D prints. You can put some stresses and loads on it in different cases such as twisting forces, pull forces, load forces, and all sorts of different things. You can get a good idea if your structure has a good chance of meeting the requirements. After a couple of decades of experience, we can tell you that there is no substitute for testing it till it breaks. The only way that you are going to know how strong the object is, is by testing it to failure. It is more of an equalized analysis of the whole part and not necessarily a stress point. This is a thing that happens with certain types of products like flooring. They test flooring with stiletto high heels because that kind of tiny fine impact with all that stress on one tiny part can break and damage the floor, whereas if you just did  an analysis that is spread out in plenty of surface strength, then that would be a different scenario. You need to understand how it is used. The computer does not calculate that.  The computer does not know how you move position while you are moving with the bike.

Maybe you will lean up, forward, or maybe you will stand up a little bit to give your muscles a break. When you do that that is too much stress on an area that it averages into its calculation. I think it takes physical testing to do testing. The thing that it is very good at is pointing out where the weakest point in your area is or where it is most likely to fail first. You can then make calculated decisions. There are a lot of  good books that I might recommend that covers this. There are many great books on this written by Dr. Petroski like To Forgive Design. It was a book that I have read a couple of times in my career because there are lots of good examples out. There was a bridge that fell down in the mid west. This was an architectural failure, but what this did was to help to educate and train engineers for generations to come.

That was also an environmental issue. It had to do with snow load on some of the buildings. They also have failure on those type of environmental things that you don’t always have a system that predicts. There are many examples in the past couple of hundred years in architecture and engineering. The title of one of the books that I like is To Engineer is Human. Another one is Success Through Failure. The lesson is really valuable to understand. What it tells you is that there is no substitute for actual testing.

Lars, if you want to make this bicycle or download the parts and make this one version of yours, please take a test ride first. Don’t go on a 180 mile ride and discover it then that the strength of 3D prints is not what you thought it was. Don’t go on that ride and think that you’re okay. Test it first. You have to build it and do some significant real testing. We had a phone call here at Hazz Design last week from Underwriters Laboratories. They are the biggest testing and compliance worldwide company. They don’t just do electronic testing. If any of you have bought a TV stand in the last ten or twenty years and have assembled it yourself and you then put your TV on top of it, there is a UL standard for testing TV stands to be safe from tipping over. UL sets different standards for all sorts of different products.

UL called us last week and said that they have now a service where they are able to test 3D printed products and 3D printed parts and calculate strength of 3D prints. They have established some strength standards and criteria from which they are testing out this thing. It was great to hear. This is a much needed thing for consumer safety.

Part of Lars’ question about calculating the strength of 3D prints is about material strength. That is one of the things that a lot of the engineering packages lets you change the material type in their FEA analysis, but it is not typically a 3D printed version of that. You need to ask if that will perform in the same way if that is 3D printed. It is a little bit different process. You are not pressing plastic, you are laying it. You are not molding it. Every FEA portion of each different program is a little different, but yes, you do choose actual materials. In the case of metal, was it extruded or fabricated or is it bent to its shape? That makes a difference n the properties. There are also different compositions of metal. Is it injection plastic? Is it thermoform plastic? Is it blow molded plastic? I think they address some of those things. Is 3D printing one of them? I personally did not see that. It would surprise me if they haven’t caught up. There are so many issues especially with FFF 3D printing. If you don’t have your machine calibrated properly, you may not get good bond from layer to layer. That can weaken your part significantly and decrease the strength of 3D prints.

We had that problem here, I have a knot and bolt that is holding the scissor arm that my microphone is on. We have lived out actual use testing here and proved that it doesn’t make it after we use it. Color also makes a difference in that material composition because some of them have more fillers that others. You cannot always be sure that you are getting an absolute consistent material out as well. This is why you need to really test it to see what the strength of 3D prints is. You also need to confirm your material. This is where 3D printing in production is going to be different than 3D printing something for your own purposes and trying it out for yourself and see if it is going well. When you are trying to think consistently, you need to have a very consistent model of material in and output out. Everything has to stay very consistent for the strength of 3D prints to be high enough and exact.

There are opportunities to fail there. When we talked with the people in the aerospace industry who are 3D printing metal for aerospace aircraft parts or boat parts, we are finding that they are telling us that parts that used to be made of different pieces of aluminium that are molded together in a jig in a certain way, that those welds are only as good as the skill of the welder. There are robotic welds that you can now do too. But when you introduce heat to welding, you change the chemical composition of it and the temper of the metal. You can have some factors there. Additive manufacturing now provides more consistency thanks to the lasers. It can also be more economical in the material. So it not only produces a much stronger part but also a less expensive part in terms of material use.

3D printing offers a lot of opportunities for better parts as it makes manufacturing to be more consistent. Here, we are talking of making bicycle parts out of fused filament fabrication. I would do it. I think it would be a great bike.

There is also a question here on how many rolls of filament do they need? They need 4 spools for one bike. We have used 1 spool for the head tube sleeve, 2 for the bottom bracket sleeve, and then they recommend an extra spool in case something goes wrong, which makes it 4 spools in total. That is a lot of material. Having fun with different colors on the different parts could be problematic for you because they might have different qualities and might decrease the strength of 3D prints. This is where you air more on the cautious side. You want to remove as many variables as possible from the process for yourself so that you can get some feedback. Have a little more freedom.

People who do UL testing are usually big corporations which are why it needs lots of money. This is not a cheap enterprise. Lars, you could print parts yourself and do some less than scientific testing yourself. You can also try to overbuild them, as this is what engineers would do in the past. You can see buildings like the Empire State Building that has so much more material in it than any modern building would use today. It is over built because they did not have a good understanding of strength of materials at the time. They made it thicker, heavier, and stronger than it needed to be. During World War II, there were 15 or 17 bomber planes in the fog flying over New York City. It flew in to the Empire State Building and incidentally the bomb didn’t explode and the building was fine. The plane just went into it while the tail is sticking out of it. They ended up repairing the building then. They overbuilt that building.

If you are going to make your bike for yourself and you want to make sure that it is going to last your 300 kilometer bike ride, then just over build it. Make those wall thicknesses thicker. Unfortunately, the only way to know if that bike can meet the requirements is if you test it to failure and see that breaking point and find the strength of 3D prints. There is no way around it in my opinion. Simulate as best as you can some of those testing whether you are simulating those on a computer or physically testing models.

Just because somebody says that you should build it out of nylon or PLA or any of those materials, which is not consistent from company to company. Not all PLAs are made the same. We have discovered that here. Lots of them have much lower breaking points and are much more fragile and it easily cracks. Ask them what brand they used, if they tried it before. ColorFabb is really a premium filament provider. You also have to question the quality control consistency of the material that you are using. There is no substitute for testing when it comes to calculating strength of 3D prints. I hope it helped you out, Lars. Feel free to comment back and anybody else who are involved to have more knowledge about the subject. Calculating strength of 3D prints is a big complicated subject.

We don’t claim to know it all, but we know a lot as we have a lot of experience. There is a lot of room for a lot of debate on this issue, so just keep commenting and share it with everybody. This is just a good segue into our coffee sleeve. This is a fragile print. This is a print in which strength of 3D prints is not tested. It has a lot of fragile pieces, pieces that move. It was part of the point that we wanted to show. These are things that you can’t know until you start using your color. That is one of the things that we discovered on this file.

We have had materials that were strong while others were a lot weaker. We do recommend that when you print it, you use fill because there are a lot of thin parts. There is not enough space there to go  and have a lot of fill. You want to make sure that you got the full strength of it. It is not the easiest file to print. We have printed it on all of the printers that we have been testing in our Battle of the Minis. Different materials have different properties and there are a lot of things that you should consider. It is also a delicate item because you don’t want this big stinky thing on your coffee. You want it to be collapsible because I stick it to my back pocket on the way to Starbucks while you put it on your purse. Things like that happen, which is why you should be aware of that.

In this type of product, the worst case scenario is when your coffee will slip and fall. This is the US and everything is liability required insurance here. Thank you so much for being with us up until this 277^th episode. This is because of you guys, because you keep us going. You can also interact with us on social media and leave comments below!

Important Links

• ColorFabb XT-CF20 bike project release
• To Forgive Design by Petroski
• Success Through Failure – The Paradox of Design by Petroski
• To Engineer Is Human: The Role of Failure in Successful Design by Petroski
• Underwriters Laboratories
• Collapsible Coffee Sleeve 3D File Download on Thingiverse

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