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I'm going to 3D print a part that needs to meet certain strength requirements, due to its usages. I know how strong a particular plastic (eg. comrpessive/tensile/shear strength) is when dealt with in a solid chunk, but not when it is 3D printed. What is a good way to estimate the change?

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  • $\begingroup$ I am voting to close this question as "unclear what you're asking" because it does not identify a specific part, specific requirements, a specific printing process, a specific material,... We can not possibly answer this question in a useful way without knowing exactly what you're trying to achieve. $\endgroup$ Jan 13 '16 at 6:44
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    $\begingroup$ @TomvanderZanden I felt this would be less useful if I mentioned specific technologies. I don't see how this is impossible to answer in general terms. $\endgroup$ Jan 13 '16 at 13:08
  • $\begingroup$ You need to do a DOE (design of experiments). $\endgroup$ Jan 29 '18 at 23:46
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It's hard to tell without actually testing the part. There are many ways you can 3D print a part, even on the same machine, that can yield different results.

Here are some tips to help uphold strength requirements:

  • Identify where your stresses are and avoid allowing the natural "grain" of the print (i.e. each layer) to coincide with the stresses of the part. For example, I just printed a part with hinges today. I set my part up to ensure that the circular profile of the hinge on each layer, instead of having the machine "bridge" the circular profile.
  • Make your part more solid by increasing the infill. Note that somewhere around 35% won't really provide much more strength than say 50% with a standard infill pattern (i.e. hexagonal, diamond, catfill). However, I would imagine that if a spherical infill pattern was ever designed, that would be the strongest.
  • An easy way to beef up small areas of a part is to increase your shell variable (how many profile layers for each layer). Again, referencing my hinge design, I made sure that my hinges were completely printed using shell instead of shell/infill.
  • Don't be afraid to do some post-processing such as adding epoxy/epoxies in high-stress areas.

If it's not an expensive part to print, I would suggest playing with some of these ideas yourself and conduct controlled tests to see what setups work best for your application(s).

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  • $\begingroup$ "Note that somewhere around 35% won't really provide much more strength than say 50% with a standard infill pattern" - Where did you get that number? I have always heard that limit is at around 60% and found it in written at least once... but I have not run independent tests to verify the info, so I'd be glad to be corrected if I'm wrong. $\endgroup$
    – mac
    Jan 29 '18 at 22:08
  • $\begingroup$ That's from some MakerBot reports. I suppose that statement is also dependent upon the size of the part as well. It usually boils down to the size of the "dead space" of each cell in the infill pattern. I believe my comment mostly to parts that fall within a 5"^3 space $\endgroup$
    – tbm0115
    Jan 29 '18 at 23:34
  • $\begingroup$ "dependent upon the size of the part as well" - Interesting... I have to think about it. Intuitively I would say "no" as the size only changes the number of the cells in the infill, not their dimension, so - for a sample unit of infill - the density of the material and the orientation of the tensile vectors should remain exactly the same... but I feel I may be well missing something obvious. Thank you for the quick response anyway! :) $\endgroup$
    – mac
    Jan 30 '18 at 0:23
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This is a good question, which hadn't received enough researchers' attention. People regularly print different objects, some of them with strength requirements and the need for a method of strength estimation is high.

Good experimental way to estimate the change would be to find a COTS cast plastic object, be it ABS or PLA or whatever, buy 3-5 pieces, then reverse engineer it, copy the design and print it 3-5 times in different orientations. Then you need to start destroying your objects in a manner that coincides with your needs. If your parts would be experiencing compression - crush them, if stretching - tear them apart, and measure the required strength. Then compare and get relative strength, that you can further use in your calculations. You'll notice that adhesion between FDM layers is much weaker than strength of bonds in the layer plane, so you'll have two coefficients - one for Z axis, one for XY (note, that printer settings can heavily affect the result, so every coefficient will be a function of printing parameters). Can't tell 100% reliably, but SLA may have just one coefficient - I hadn't noticed any difference between parts' strength in Z and XY directions.

If you're not a fan of thorough scientific approach, then you can just print your part and test it under your target conditions as much times as necessary. Or make a casting mold, then a solid object.

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  • $\begingroup$ we also have problems with determining all the effects of weather on prints - high humidity or temperature might have unexpected print strengths. $\endgroup$
    – Trish
    Sep 5 '18 at 12:13

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