ONE OF the appealing aspects of 3D printing is the freedom the designer has in his design. But even with 3D printing there are limitations to what a 3D printer can produce. These limitations are inherent to a specific 3D printing process. In this post, I go into one of these limitations which is structural strength.

Structural strength is one of the most important limitation to recognize when designing for 3D printing. It defines if a 3D printed part can survive the different steps involved in the manufacturing of the part – from printing to cleaning until shipping.

Structural strength is also almost impossible to assess. For that reason, the structural strength is translated into a minimum wall thickness requirement / design rule. The minimum wall thickness is an approximation of a structural strength analysis, but is far easier to assess by an operator, than the structural strength itself.

Since it is an approximation of structural strength, the minimal wall thickness requirements can differ from operator to operator. Also, the print orientation of the critical areas or features of a part can impact the minimal wall thickness.

Difference between wall thickness and feature size

A wall is a structural part of the design. A feature is a protrusion or extrusion on a part which is not a structural part of the design.

A feature is any part of the design where the cross-section of the feature is smaller than the required minimum wall thickness.

Wall thickness design requirements

When I think of wall thickness, there are three distinct design requirements. I call them:

1. Bridges
2. Wings
3. Spikes

A bridge connects two distinct larger features of part. A great example is the neck of a figurine. The neck bridges the head and torso.

A wing is a surface area with variable thickness. Some parts of the wall are adhering to the minimum wall thickness; other parts are not. An example is a surface area, which is thick enough on one end and too thin at the other end. Like the wing of an airplane.

Spikes are lengthy protrusions starting with a base, which adheres to the minimum wall thickness, and ends with a top not adhering to the minimum wall thickness. An example are nose hears on a mouse.

Two examples

In the image on the left, you can see that the arm of the robot has broken off. This is a bridge problem. The critical part of the design is the connection of the arm to the shoulder. The connection is too thin to provide enough strength for connecting the heavy the lower part of the arm. In this case, the part was printed, cleaned and post-finished without problems. The part broke during shipment.

The image of the right shows a dragon with wings. The wings are actually solid planes in the 3D design, but they can out of the printer looking like this. The solid planes of the wings were too thin to be printed correctly. This is an example of the wing problem.

Other considerations

3D printing processes with a fragile green-state are an exception to the wall thickness requirements. With those processes, the wall thickness is a dynamic variable based on the design of the part. Green-state processes use a binder during printing and have a second process step to strengthen the part. I will explore this topic in more detail in another post. Examples of green-state processes are 3DP (ZCorp, Voxeljet and derivatives) and ProMetal. Non green-state processes are SLS, DMLS, SLA, Polyjet Matrix (Objet) and FDM (Stratasys, RepRap and derivatives like Makerbot).

Another exception is a large, thin surface or plane. Even though, it adheres to the minimum wall thickness, and has the required structural strength, it can still pose a problem for the printing process. Most printing processes use heat during the production of the part. This heat is not perfectly distributed over the build-area. When a large, thin surface is unevenly heated, it will warp and bend. This is also a topic for another post.

Structural strength is an important consideration during the design of a part for 3D printing. When adhering to the minimum wall thickness requirement for a specific process or even over-engineering the critical areas, than any problems can be avoided. Since the minimum wall thickness requirement is only an approximation of the structural strength requirements, it is possible to defy the rules in certain cases. But requires export knowledge on the process and close cooperation with the operator to ensure the right location, orientation and handling of the part during the production process.

TODAY I was pleasantly surprised by another addition to the collection of open source 3D printer designs. Andreas Bastian – an engineering student at Swarthmore College – designed and build a SLS 3D printer, and made the design available as open source. It uses an infrared laser, and sinters a wax and carbon mixture for use in lost-wax casting.

As motivation for this project he writes:

The current leading open source 3D printers print only in ABS or PLA and are restricted both by the limitations inherent in these materials and by the limitations of the FDM process. Though these printers are entirely capable of self-replication and of producing parts for a variety of light-duty applications, they cannot produce pieces that can replace the cast aluminum parts found in power machinery, CNC equipment, or in numerous other high-strength applications.

For more information, check out his website or his page at the RepRap wiki.

Here is a video of the printer in action:

Here is a list of the other open source 3D printing designs:

• Fab@Home
• Fused Deposition Modelling (RepRap project)
• SLA

Still missing are the 3DP technologies developed by ZCorp, Voxeljet, etc. and the Multi-jet/Poly-jet technologies of 3D Systems and Objet.

Interesting enough the US/USPTO patent for SLS has not expired yet and will be in force for the foreseeable future. I hope the patent owners / licensees can resist a cease-and-desist since this is nowhere near their business cases.

It is great to see more 3D printer designs become available as open source. These open source designs have a tendency to kickstart commercial applications. The list of FDM-like hobby printers is getting long with MakerBot, Ultimaker, Up!, etc.

Now who is going to be the first to make a commercial SLA hobby printer or SLS hobby printer?

IT IS a strange coincidence when particular things just fall together in the matter of hours. I was tweeting yesterday with @jonemo and @rpes12 about the killer application for 3D printing. We arrived at the topic of DRM-ing of 3D models as a protection mechanism. At some point, I wrote:

@jonemo yeah and be prepared. This fight will be 10x bigger than media. The dragons have not awakened yet though.

— Robert Schouwenburg (@rdschouw) January 24, 2012

An hour later somebody sends me a link to a blog post on The Pirate Bay about sharing physical objects. They call them Physibles, and the category is already available on their site.

They write:

We’re always trying to foresee the future a bit here at TPB. One of the things that we really know is that we as a society will always share. Digital communication has made that a lot easier and will continue to do so. And after the internets evolutionized data to go from analog to digital, it’s time for the next step.

A few months ago I already wrote that pirated 3D models where available on The Pirate Bay. It is not surprising that the media attention around CES on 3D Printing did not pass by the people of The Pirate Bay. I just hope it is not seen as an incentive for the industry to come up with another SOPA / PIPA to increase the protection around physical objects. We are nowhere near understanding on what the impact of personal fabrication is on society. Let’s not kill it before it even got started.

IN MY post Reality Check for 3D Printing I wrote about a few critical blog posts and news items on 3D printing. The question on the table is: what is the killer application of 3D printing? It is an interesting question and I think there is none. Or at least no specific killer application. What is the killer application for an inkjet printer? What is the killer application for a computer? What is the killer application of the web?

In each of these cases, there is no killer application. When I first showed the web – using mosaic – to my mom and dad, they looked at me and had no clue what I was talking about. None. You could argue that the web is the killer application of the Internet. Or is it email? Or is it my Twitter mobile app? Or maybe Skype? Or all of the above?

Same applies for regular printers. Who can remember The Print Shop (version 23.1 – amazing!) from Broderbund? You could make endless banners with it or front pages for your own photo albums. All neatly printed on your dot matrix printer. Nobody is doing that anymore. The quality was low, to say the least, but still everybody made them. So why do you have a printer? I cannot think of a specific killer application for 2D printers. Though it is useful to have a printer.

When I think about the killer application of 3D printing, I think about personal fabrication. That in itself will mean different things for different people. It could be that an architect or designer want to print their professional designs. Or it can be the unique production of a cool dice for a Warhammer player. I cannot look into the future. But the prospect to design, customize and make your own things at home, design them exactly according to your own specifications while not going to a store, just sounds so appealing to me. What do you think?

Update: Fabricatis wrote down his thoughts on the killer application for 3D printing. Recommended reading!