Apr 30, 2016 | By Alec

Designing your own multi-component 3D printing projects, such as a cool DIY robot, can be both rewarding and frustrating. If your design calls for various load-bearing features, it can take a couple of iterations to get good, strong part – and even then the results can look thicker and bulkier than you imagined. But there is design solution. Adding ribs and gussets can be a good way to trim down a part without diminishing structural integrity, and fortunately Fictiv’s Sean Thomas has just shared some excellent tips for placing them on parts and avoiding those pitfalls that make a part even weaker.

Fictiv is a community-driven 3D printing service provider that bundles the strength of designers and engineers in the San Francisco area. Their Sean Thomas is a mechanical engineer and 3D printing veteran, who has often found that ribs and gussets are absolutely crucial in making a design work. Most importantly, they provide a means for reducing material volume and printing time and could thus make your prototyping phase far more efficient.

Furthermore, ribs and gussets are also a great way to create a plastic part that closely resemble injection molded parts – making it a perfect way to see what the final injection molded results of a prototype will look like. After all, ribs and gussets are often a necessity in injection molding. “Ribs and gussets are commonly used in injection molding because the parts are very sensitive to thick wall sections and variations in wall thickness. A variety of defects, such as shrinkage and warping, can occur if the features on injection molded parts are too thick,” Thomas explains.

But it is also indisputable that small features like ribs and gussets are very important from a structural standpoint. Thomas illustrates this with a series of tests that demonstrate the amount of stress and deflection a 3D printed ABS part can take. As you can see in the images above, parts with gussets can be exposed to a lot more stress (measured in psi) than parts without them. “It’s immediately apparent that the feature becomes much weaker without the gussets. In fact, [without a gusset the part] is now very near the yield strength of the ABS we have selected (2,900 psi),” he says. As you can see for yourself, parts with gussets are also subjected to far less deflection than parts without them.

The same goes for ribs. As Thomas shows, just a few ribs will add strength and rigidity to any part, even with very thin features. Showing this in another FEA study, an ABS shelf with just three thin horizontal ribs is subjected to a maximum stress of 825.1 psi – far less than the ABS yield strength of 2,900 psi. Without the ribs, stress levels shoot up to 1,288 psi. Still within the safe range, but far less so. The same can be said for deflection. “Without ribs the center of the shelf is sagging like a bag,” Thomas concludes after another test.

In the case of this shelf, horizonal ribs are far more effective than vertical ones.

So how do you add them to your own components? The crucial thing to understand is that ribs with wrong orientation are often completely useless. It’s not a case of just randomly adding extrusions and lines and hope they improve your part. “Mathematically, it’s best to add height in the direction of loading, so that means if we’re looking at the part from the front, we want the part to look taller from the addition of ribs,” he explains. “No draft angle is required, so wall thickness can be held constant where necessary.”

But looking at your filament’s properties is also crucial when designing ribs and gussets. “Ensure that wall thickness is within the recommended guidelines of the machine and/or process you will be using,” Thomas advises. Rib thickness should be in the 60 to 80 percent range of the wall thickness. It’s also almost always better to increase the number of ribs than simply their height. Whenever possible, always be sure to limit the rib height to three times the wall thickness.

For more tips, check out Thomas’s complete article here. With these simple guidelines, you’ll find that efficiently improving a part’s structural strength is certainly possible without wasting too much material. It’s another reminder that so much is possible when you make intelligent design choices, even without access to a top-level 3D printer.

 

 

Posted in 3D Printing Technology

 

 

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