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Flexure in 3D printed devices

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#1 Schneebäuelli

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Posted 07 April 2021 - 04:08 AM

At the moment I am mounting my Evoguide 50ED in a little mounting shoe on my C8 Edge HD, which is not optimal as flexure doens't allow me to take subs longer than 3 minutes or they will be slightly elongated.

 

So at 2032 mm FL  flexure plays an important role and should be minimized, best with a OAG. But I dont have the financial resourcces for that, so I would like to 3D print a dovetail, where I could mount my guidescope and fasten it at multiple places.

 

My question is if that project is pointless from the start, since 3D materials are not as rigid as they have to be, thus introduce more flexure than my inital setup with the guidescope in the mounting shoe. Or do 3D materials exist that have the properties I need to reduce my flexure?

 

 



#2 bbasiaga

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Posted 07 April 2021 - 07:17 AM

A lot of it is about the design of the part, the thickness of the walls you print and the infill setting you use.  I think you'll have to do some trial and error. You could also consider a printed design with a tunnel in it where you can slip in some aluminum rods to add to the rigidity.  

 

If your printer can go hot enough on the nozzle and bed, and you have a a way to keep your filament dried during printing, you can print in nylon or poly carbonate.  Both are very strong.  PETG and ABS can also be quite strong if the parts are printed properly and PETG espcially can be printed on most printers with a heated bed without much other fuss or need for enclosures.  

 

 

-Brian



#3 PEterW

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Posted 07 April 2021 - 04:54 PM

For stiffness you need one of the carbon fibre loaded filaments, but you’ll need hardened nozzle of you’ll destroy your nozzle... I know as I did this twice before i realised!
As noted the stiffness scales with the cube of the thickness,
So you can improve things with good design as well.

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#4 megachriops95

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Posted 07 April 2021 - 05:04 PM

Do not use PETG for parts that require stiffness. PETG has a high youngs modudus, it wild elongate and bend at the yield point instead of breaking. This gives it non-stiff properties compared to something like PLA. Dont listen to people saying it isnt strong enough, strong is a relative term and the application, construction, and dozens of other controllable variables influence strength. Not the Yield/Elongation Chart of a material.

 

Try increasing extrustion width with a .4mm nozzle, this will increase the cross sectional area of solid plastic when it interfaces with another layer line.

 

More here: https://www.youtube....h?v=9YaJ0wSKKHA

 

 

Hope that helps

 

-Sasha



#5 bbasiaga

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Posted 07 April 2021 - 06:19 PM

Do not use PETG for parts that require stiffness. PETG has a high youngs modudus, it wild elongate and bend at the yield point instead of breaking. This gives it non-stiff properties compared to something like PLA. Dont listen to people saying it isnt strong enough, strong is a relative term and the application, construction, and dozens of other controllable variables influence strength. Not the Yield/Elongation Chart of a material.

 

Try increasing extrustion width with a .4mm nozzle, this will increase the cross sectional area of solid plastic when it interfaces with another layer line.

 

More here: https://www.youtube....h?v=9YaJ0wSKKHA

 

 

Hope that helps

 

-Sasha

If you try PLA, use one of the HTPLAs that you post-treat in an oven.  PLA has a low glass transition temp, meaning it gets soft at low temps.  Depending on your climate, this could be a big concern.  Especially if left in a car or closed space where it can get very warm in the sunlight during the day.  PLA is hard, and brittle which does mean it is stiff, but I refuse to use it for anything carrying a load that may get warm.  HTPLA is more expensive (I use Protopasta's version, and PLA+ is NOT the same thing) and the parts I did with it have been good.  They do change size a bit after heat treating, so be aware of that.  My electronic tollbooth holder and telrad cover did not make it in PLA, but have been good in HTPLA (at least until I lost the telrad cover in the dark).  

 

 

CF is a good suggestion as well, but part orientation comes in to play there.  The fibers are good at making a single layer stronger, but the amount of cross-support between the layers isn't high, given how 3D printers lay down material there just isn't much way for the fibers to stick in to the layer below and add strength that way.  If you can print the bracket as one part in the right orientation it could be good though.


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#6 Schneebäuelli

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Posted 08 April 2021 - 03:17 AM

Thank you very much for your advice. Never heard of HTPLA . Sounds very intresting. Is the size of change in the mm regime?  If so do you just scale your print accordingly like 1.05 x. Or how do you plan in that size change?

So where I live in winter it can get below -20° C and in Sommernights it reaches at max + 20° C. Also the humidity can get quite high at any point in the year 90 % +. Can Humidity and/or cold temperatures also be problematic for PLA?



#7 bbasiaga

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Posted 08 April 2021 - 09:31 AM

Thank you very much for your advice. Never heard of HTPLA . Sounds very intresting. Is the size of change in the mm regime?  If so do you just scale your print accordingly like 1.05 x. Or how do you plan in that size change?

So where I live in winter it can get below -20° C and in Sommernights it reaches at max + 20° C. Also the humidity can get quite high at any point in the year 90 % +. Can Humidity and/or cold temperatures also be problematic for PLA?

I don't think humidity is a big concern, but honestly I don't know.  Most problems with PLA that are reported are due to heat and UV exposure.  Remember no matter how cold it is outside, it can still get very warm in a sun baked car, or outdoor shed/observatory.  So even at 20C you might find high temps in those places.  Thats what you have to be careful of with PLA. 

 

I would go over to Protopasta's website.  They have some info on HTPLA and its shrinkage properties.  I can't recall what they are right off the top of my head, but I think mm is correct.  You could potentially build a jig to help keep it from warping - you want to keep your rail straight afterall.  Warping during heat treating is probably a bigger issue than shrinking for a rail

 

-Brian



#8 BoriSpider

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Posted 08 April 2021 - 12:19 PM

I've heard in the past that you could put tiny holes throughout your model, in the right orientation, to help strengthen the model.

 

I'm trying to find something w/google to support my claim but can't find it now. I guess it's been that long already.



#9 chantepierre

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Posted 08 April 2021 - 04:20 PM

I've heard in the past that you could put tiny holes throughout your model, in the right orientation, to help strengthen the model.

I'm trying to find something w/google to support my claim but can't find it now. I guess it's been that long already.


That’s how prusa printers printed parts are reinforced, by subtracting thin planes to the model to force the slicer to add perimeters inside the part.

From the few tries of this technique I have done it worked great.
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#10 harbinjer

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Posted 09 April 2021 - 10:16 AM

That’s how prusa printers printed parts are reinforced, by subtracting thin planes to the model to force the slicer to add perimeters inside the part.

From the few tries of this technique I have done it worked great.

But isn't more infill just as good in general?  This could work great if you need differential strength: one large part with lower infill, but one section needs to be much stronger.



#11 bbasiaga

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Posted 09 April 2021 - 12:00 PM

But isn't more infill just as good in general?  This could work great if you need differential strength: one large part with lower infill, but one section needs to be much stronger.

There have been some tests, and I suppose the answer is 'it depends'.  

 

Most infill is a single line width (matching whatever your nozzle size is.  it can be patterned in lines or hexes or all kinds of interesting things - all of which have something to do with the resulting strength.  Walls are typically 2-5 lines thick, and as a result much stronger.  If you put those in the right place in the model, they will add strength compared to single line infill.  They have to be in the right place of course, to be strong in the locations and directions you want them to be strong. 

 

To resist bending in a rail like this, you need and I-beam - so extra walls running the length, and spread across the width, with extra top and bottom layers is where I would start.  Now you also want to reduce sideways flex, which means you need a way to keep those I-beams from curving along their length.  The top and bottoms (flanges of the I-beam) do a lot for that already, so its possible that infill in that direction may be enough of a boost to get the desired performance.  

 

-Brian


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