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3D Printed Parts Bending After Cure


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Hi all,

 

I've made and sent out a few kits of my Fairey IIIF and I've noticed some of the wings bend/warp slightly. This isn't an issue, as most resin conversions/kits I've ever made are slightly bent, a quick dip in hot water straightens them out.

 

I'm just wondering if there are certain things which exacerbate the warping and if there is a way to mitigate it?

 

I assume the warping is caused by residual stresses left in the resin after printing, bending the resin over time - if this is the case, it could in theory be resolved by putting the parts in hot water immediately after curing (heat treat stress relief, this is exactly what we do in the aerospace industry on metallic components to prevent distortion due to residual internal stresses post machining).

 

Or is it some other process that is leading to the warping?

 

Cheers,

Ben

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I know nothing of 3d printing but perhaps what I do can be done

In certain resin parts which are thin and weak or liable to bend I embed a length of thin brass rod

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do you remove the supports before or after curing?

 

A lot depends upon how you define the structure. Certain shapes are more prone to warping than other.  One of the major offenses is difference in wall thickness.

 

Are you printing the wings as one solid?  or as two (upper & lower) halves?   If one solid, I can see why that would be prone to warp.  You've got thick and thin sections all over the place. Small parts are generally okay but the larger you go, things are more prone to warp

The best you could do there is perhaps core it out, or clamp it in position while curing under sunlight. I find the UV lamps generate too much heat and that also causes warping.  Plastic (though this is resin) will always warp towards the hot side. For large pieces I always try and clamp them somehow to prevent warp. The geometry and wall thicknesses need to be carefully thought out to minimize warp issues

I normally do a partial cure with supports in place, then remove the supports for final cure. I'll try and do that in sunlight and rotate the parts as often as possible so one side doesn't get too warm.

 

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Where warping looks likely (usually for thin things like gun barrels) I’ll leave one or two supports in place as bracing during curing.

 

For curing I put the model in a jar of war and put that on a cheap solar powered jewellery display stand. That works really well in getting an even cure - and the stand is activated by either the sun or a uv lamp so it works for winter too!

 

Have fun 

Finn

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40 minutes ago, wellsprop said:

The warping is happening post cure regardless of if I leave the supports in place or not.

 

The wings are solid parts, they are too thin to make hollow 😕



Not sure I have an answer, but perhaps another way to deal with it.  As I haven't tried it myself, there's still a bit of experimentation to be had.  And it might be a crap idea anyway!

Regarding thin, long structures, I've often considered designing in a hole that runs the length of the part to accept a small diameter steel rod: something readily available at the local hobby store or readily purchasable.  Of course this depends on...the model being "thick" enough to accept a say 2 mm diameter rod, the printer being able to print accurately long, small diameter extrusions that is clear (ie, doesn't end up full of resin and closed shut post-post-sure), and the 2 mm rod's ability to even be useful.  An alternative might be thin section brass angle that you run the length/span of the offending part and bond in.  Once upon a time I used brass angle for various front and rear wing gurney's on 50% Indy Car wind tunnel models (this is the brand, https://www.ksmetals.com/brass, note the brass angle at .014", .3mm wall thickness, by 1/8" x 1/8" legs).  Though naturally the height of the legs on the brass angle might immediately preclude it's use if the wing section in question won't accept it (i.e., the leg is taller than the part is thick)...the point of my idea was simply to work around the limitations of the material and to find something that is drastically stiffer that augments the overall stiffness of the part after the fact.

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On 10/12/2020 at 3:31 PM, wellsprop said:

I think I'll try adding a "spar" it should strengthen the wing/fuselage join too!

Yes, guess that's what I was describing!  A spar out of material that is stiffer than what the wing section was printed in.

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1 hour ago, Woodstock74 said:

Yes, guess that's what I was describing!  A spar out of material that is stiffer than what the wing section was printed in.

I think leaving a hole for a spar rather than  putting one in yourself might be best as  different people might have different preferences for sizes and materials that they  might like to use for the spar. Some might  prefer brass, or piano wire or Stainless steel.

 

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4 hours ago, matti64 said:

I think leaving a hole for a spar rather than  putting one in yourself might be best as  different people might have different preferences for sizes and materials that they  might like to use for the spar. Some might  prefer brass, or piano wire or Stainless steel.

 

Yes, and telling them the design intent for the hole size so they can spec the material themselves (and so they can open it up, should it print slightly smaller).  The only thing I'm unsure of is how useful a 2 mm thick piece of piano wire (or whatever) might be to actually stiffen the part up?  

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On 10/12/2020 at 3:31 PM, wellsprop said:

I think I'll try adding a "spar" it should strengthen the wing/fuselage join too!

 

what orientation are you printing the wing in?  That will also have an effect on warp.  Flat? from root to tip? on leading/trailing edge? or?

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8 minutes ago, hendie said:

 

what orientation are you printing the wing in?  That will also have an effect on warp.  Flat? from root to tip? on leading/trailing edge? or?

Printing from leading edge first to trailing edge. I.e. wing vertical pointing down.

 

I printed another set vertical but with the wing root as the base, printing from root to tip.

 

In both cases, the wing warped, with the wing tips warping to point down (I.e. the opposite way to which wings flex when planes take off etc).

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  • 1 month later...
On 18/10/2020 at 10:59, FinnN said:

Have you tried printing with a raft?

 

Hi Finn,

 

No, but the issue is once I have cured and removed the parts from the supports, the wings bend after a while.

 

I'm having similar issues with my Chipmunk now. I redesigned the wing so that it's hollow, I'm not sure if that will help long term.

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Hi Ben,

 

With the hollow wing, is the hollow sealed i.e. no outlet from within?  I am just wondering if there might be uncured resin inside the wing and that is keeping the inner area uncured and soft?

 

Mike

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1 minute ago, bootneck said:

Hi Ben,

 

With the hollow wing, is the hollow sealed i.e. no outlet from within?  I am just wondering if there might be uncured resin inside the wing and that is keeping the inner area uncured and soft?

 

Mike

Hi Mike,

 

I put holes in so the uncured resin drains out. I only printed the first one yesterday so it hasn't had time to bend (if it will bend). One of the earlier "thick" wings did bend whilst the other was fine. The main issue with the hollow wing is it was far to weak with a 1mm wall thickness.

 

Ben

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That's fine then, and I'm assuming by that the innards get a wash through as well.  Do you have a UV pen, or suchlike, that you could aim up in to the openings, just to ensure complete curing inside?   I have a Bluefixx resin glue pen  that comes with a little UV light, similar to this Bondic version, and I find it ideal for pointing the UV light into the holes.

 

The only other thing I can think of is to store the wings upright, on their leading or trailing edge, to minimise the weight/gravity on the flat areas until completely cured.  That might take a couple of days in full sunlight.

 

Mike

Edited by bootneck
added info on UV resin pen and light
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I've not thought about a UV pen like that, it would be helpful for doing the insides of the fusealge too.

 

I've changed the Chipmunk wings back to "thick" and I'll test print them, cure them and leave them for a while to see if they bend again. It's possible that the original Chipmunk wing only bent as I may have not properly cured it (I then left the assembled model on the windowsill for a week).

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  • 2 months later...

Just reviving this thread as this is a recent issue I have suffered. I am currently making a Piper Malibu, which has a long, thin wing, and at least 50% turned out like this:

2021-02-23_03-58-38

 

These wings are thin and only about 7cm in length, so no real possibility of splitting or hollowing.

 

@hendie, I was thoroughly swayed by your analysis as I have been curing parts under a uv lamp. Although I always do both sides, with a wing I guess once you've done the first side the damage is done, even if it hasn't actually bent yet (these deformations appeared over the course of a month). 

 

So for my second attempt, I tried curing in sunlight with this setup, clamping between clear acrylic:

2021-02-24_01-35-19

So far it looks like a good cure, and straight as I dare hope for. That said, I'll keep you posted as any issues are likely to appear in a week or so...

 

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It also happened to me with thin and long pieces.

 

I think that they need to be painted quickly to insulate them from UVs.

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A quick spray of primer might help?  Another idea, although not tested, might be to stand them along their leading edge.

 

Mike

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