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I am designing an enclosure, and top and bottom should be linked thanks to snap fit. I have to use 3D-printing to make both parts. The material is ABS. I made some trials, but they keep breaking. I am using the design in the following picture. enter image description here Do you have any idea about how I can strengthen them or something else to prevent them from breaking ? Thank you for your help !

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3D prints are weak in the direction of each layers plane. You could try to make the part in a different orientation.

Second thing to try is to use a coarser print. My friend tested mechanical properties of different 3D prints and concluded that the printers were worse the smaller the layer size was. Even materials that technically better didnt do well in a real test since the failure mode is not in the material itself but rather the bond between layers. Finer layers having more seeds for the error to start from.

You could also strengthen the print with some other material by sewing or taping something on the part.

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In addition to joojaa's excellent advice, you might try reducing the height of the interference and/or increasing the length of the flexure to which it is mounted — such changes would reduce the internal stress in the plastic to manageable levels.

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Also in addition to joojaa's advice, you want to print in a direction such that the cross section of the hook is parallel to your bed, ie, it prints up lots of L shapes, and there's no weak point (in the two other directions either the connection between the case and the L will break, or the curve of the L). Having three snaps makes it so that one of the snap's cross section will always have to be perpendicular to the bed base, and so this one will always be weaker. I would fix in one direction by a slide / overlap / pin, and clip in the other. Undertolerance and test to find your correct fit.

Your design also uses features based on injection moulding techniques ie: single sided mould release by having the material to hold your screws and (pins?) going all the way down to the Base of your container. In 3d printing, you're not constrained by this, and you can print on top of nothing using cheaper support material. But chamfer the edges underneath or that will be weak.

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