Best way to reinforce PETG for snap on type application

[Oliveros 11]

I have an application where I'm trying to get a PETG object to snap onto rails, I've printed one so far and it just snapped the piece being snapped in. 

 

What do you guys recommend for making something like that stronger? I made it 2MM thicker but I'm thinking it might not be enough. It was with 20% infill and just 2 walls.

[owen 19]

You may need to print it in a different orientation so the weakness of layer adhesion isn't causing the problem. Check out This Thread.

Also you might want to show us the model.

[Oliveros 11]

Hey Owen,

 

I attached a OBJ file of the part I'm trying to do.

 

Ideally I can print it standing up so that I can print more than 2 or 3 at a time and just let it run overnight.

Belt Peg Collared Version 1.2.obj

[geert_2 558]

I guess the snap-areas are the two "keyhole" openings?

 

First I would make the splits a bit wider, so the clamps would deform less while snapping over the rails. That should still give enough retention.

 

Depending on the load it has to see, I would also use a higher infill, maybe 50...70% or so? And the layer-lines should definitely go in such a way that they cause no weaknesses. Just like when you cut wood along the grain. You also need to print hot enough, and slow, to get a good layer-bonding and no underextrusion.

 

Personally, I have no problems with PET snap-fits, like these chains. In PLA they crack after some time, due to the PLA being too brittle and stiff. But PET is flexible enough.

 

The cream ones are PLA, and they begin to crack and deform after repeated use. The green one is PET: no cracks, no deformation.

 

For reference: text caps-height is 3.5mm (=hollow watermark in the model). I printed these solid 100% infill, at 215°C, and slow at 25mm/s, 0.1mm layers.

 

[Oliveros 11]

Yes, you are correct on the snap opening.

 

I thought about making the keyholes larger but I fear that if I do that, it will unsnap from the rails too easily. Perhaps I can try it and see how it works.

 

Good to see that PETG can survive it better than PLA, this application won't be repetitive snapping and unsnapping, just snap on once and ideally stay there for a long time.

[owen 19]

Hey Oliveros

You could try something like this.

Make the walls 2 line widths thick.

Shaft_Clip.obj

[Oliveros 11]

Hey Owen,

 

That;s not a bad idea, I could give that a shot.

 

I tried making the wall thickness 7 MM, 2MM, 1MM and 0.5MM and the one that seemed the most promising in terms of snapping on was the 2MM thick one.

 

I'm going to keep experimenting today so any advice is helpful and than k you so far.

[Oliveros 11]

Owen,

 

Your idea was great, it lead me onto the right path, after making one of those shapes, I decided to print it standing up to avoid having to brace the sections and it turns out PETG is WAY more flexible when printed across and not lengthwise, like day and night difference. 

 

So I tried 3 or 4 different versions of your idea and they are almost too flexible to work, I don't know if they'll stay on the rail but I'm still experimenting.

[owen 19]

Great work Oliveros. Thanks for the updates.

[Oliveros 11]

Thank you. 

 

Got any advice for making PETG stick to the bed? It's driving me nuts when it gets partway through the print and then starts sliding around.

 

On a seperate note, your idea may just be the key, I tried printing a partial object with the full solid piece like it was on the original and it couldn't quite flex far enough without snapping, so now I'm printing one with your idea. 

 

One of which just came off the bed so now it's round 2.

[owen 19]

I haven't printed with PETG but I think it's similar to NEXT GEN. I use UHU glue and a Brim and make spharp corners on the base base round where possible.

[geert_2 558]

I print PET on clean bare glass. Sometimes I use the "salt method", however this does not increase bonding for PET (contrary to for PLA), but rather reduces it slightly, but it makes it a lot easier to remove parts. To prevent warping, I use no cooling fans, or the absolute minimum. But no fans makes it more difficult to bridge gaps, so this is not suitable for every design. Most of my designs are long flat models without bridges.

 

I tried dilluted white wood glue too a couple of times (ca. 10% glue in water), but this bonded way too strong: it tended to chip the glass while cooling, even before I started pulling the model off. So if you need a strong bond, this should work.  :-)

 

But the bed has to be hot enough: 60 or 70°C won't do for me. I need to set it to ca. 80...85°C.

 

The photo below is what the bottom of my PET parts looks like: no corners lifting, and no problems making tiny holes. These are small models (see ruler behing in mm and cm), but it looks the same for my bigger models. Printed with the salt method to facilitate removal: you can see the tiny pits caused by the salt (looks like corrosion pits).

 

[Oliveros 11]

Interesting.

 

I was running my bed at 80-95*C and it still wouldn't stick well, now I'm running the bed cold with glue and it seems to stick really well.

 

It ran a 3 hour print yesterday and it stuck really well, print also came out really nice so I think i'll go with a cold bed for now. 

 

I got blue masking tape on it, nozzle temp is 230-235*C and running the bed at 0*C but it's around 30*C while printing, probably just ambient temp in the box. Put some glue stick on it before it prints and it seems to work good.

 

Better than with a hot bed and the print isn't getting messed up so that's good.

 

You are printing directly onto the glass, with no form of adhesion on the glass?

 

I couldn't get PETG to stick to the blue tape with a 95*C bed and 235*C nozzle, just ran right over it.

[geert_2 558]

20 hours ago, Oliveros said:

...

You are printing directly onto the glass, with no form of adhesion on the glass?

...

 

Yes indeed.

My models typically take 2...3 hours to print; I rarely have longer printing times. So I don't know how it would hold up for long prints that take a day.

[Oliveros 11]

Interesting, perhaps I should try that.

 

It works great for me to start with a cold bed and glue and once it gets the first layer down, crank the heat up.

 

But I like the idea of a no hassle type printing.

 

Now it seems I got this problem somewhat cracked (pun intended) due to the diameter of the cylinders snapping on, the inside would have a hollow spot and due to how small or slim that hollow spot was, it wouldn't infill it very well SO I increased the wall count from 4 to 6 which then maxes out the walls in that cylinder so it's "solid" which increased the strength of that piece by a chunk, to the point where I need channel locks to snap it onto a rail now. 

 

For some reason, when I print the complete part, one side snaps, I've printed 2 different ones now and both had 1 side which snapped, so I tried increasing the I.D of the cylinders to give it a little more room, only by 0.2 MM so hopefully that does something.

 

But it seems like it's getting there, got a whole box filling up of failed prints, lol. 

 

One thing I would like to solve is how do I get the same look and feel of the top when it's ironed (Enabled iron in Cura) on the bottom where the brim is? Is there a way to do that? I tried increasing bottom layers and changing the pattern but it doesen't do much, it's still open which I don't like.

 

Thank you for helping me here Geert, I appreciate it.

[geert_2 558]

I have no solution for a smooth bottom plate on top of supports, for a single-nozzle printer (like my UM2). The underside where the support was, is always a bit rough.

 

Except design changes: cutting the model in half, printing both halves on their flat cut side, and glueing both parts together afterwards. But you will see the seam. Or making the model asymmetrical, so that one side can be laid flat on the glass without need of support (if the design or function allows this).

 

I once made testpieces for supports to try to minimize this roughness, and still be able to get the support off. A custom support with ribs gave the best result, but some roughness is still there. (See pic below: the numbers indicate the vertical gap in mm between the support ribs and the underside of the top plate. The ribs are 0.5mm, separated 1mm.)

 

For testing, I recommend making a design with only the features under test, for example when testing the clamps, only print these clamps without the rest. Until you get this perfect. Idem for the supports: make a small test plate with just that. So you don't waste too much material and time.

 

[Oliveros 11]

Okay, I guess that'll have to be figured out then.

 

It's mainly the 2 cylinders, on their side that are really ugly when they come out, it's essentially not a closed print so water and stuff could get inside.

 

Those 2 cylinders are touching the bottom of the plate, currently blue tape, perhaps I'll try to do it with a 90*C bed just bare glass and see what happens.

 

I like the idea of doing my own supports as well.

 

Thank you.

[Oliveros 11]

Owen,

 

I wanted you to know your idea actually ended up being the one that worked. The solid cylinder was simply too unreliable. But your idea worked every time.

 

PETG won't work for me though, it warps due to the temperature so I won't be doing them in PETG, I tried Kodak's Nylon 6 which absorbed a lot of water and so the peg became very flexible, it was pretty weird.

 

Now I'm looking at using this, a food safe ABS https://www.innofil3d.com/product/abs-natural-white-1-75mm-750g/

[owen 19]

Thanks for the update Oliveros. That ABS has a glass transition temperature of 105C compared to PETG (the one I just googled ) of 75C, so it should be much better. You'll have to post a pic when you make your final design.

[Oliveros 11]

Will do. 

 

I also tried Kodak's Nylon 6 which was a very interesting material, incredibly flexible I was very impressed. 

 

For PETG your design was the solver but it couldn't hold up to the temp. With the Nylon, doing it like the original with the cylinders complete, it works but that's due to Nylon's extreme flexibility.

 

I don't know how ABS will act though.

[BiMNrd 11]

Great thread, so are you guys modeling your down reinforcement inside the 3d part?

for example, how can I thicken the walls around the holes?  I know other mfr. slivers allow for this, bit its proprietary to their product. 

 

BTW, I want to know about the internal watermark. Doesnt that type of think make the STL exporting more difficult and cause issues?  I always though we wanted clean watertight geometry.

[geert_2 558]

Watermarks do reduce strength of the model, obviously, since there is no material there, and the gaps might concentrate stresses. Apart from that, I never had issues with internal watermarks. Except if the hollows are too big, and their "roof" doesn't close very well: then you see some spaghetti or grape-like blobs sagging inside. And if they are too deep into the model, they become invisible or blurred, due to the layer lines acting as a blur filter. But this is just cosmetic. In my keychains above, it hasn't caused any mechanical or software-trouble.

 

Originally I made watermarks by subtracting the watermark text from the main model. But this proved unnecessary. It is sufficient to just move the text inside the model, without any further operations. Then, upon export to STL it is automatically subtracted correctly, since STL are just triangles without any meaning.

 

As far as I understood, "watertight" has nothing to do with hollow design features like watermarks, but with the edges of the model touching each other correctly. If you fold a paper model incorrectly, it has gaps in the seams, so it is not watertight. The same in STL: edges of triangles not touching each other.

 

I made these keychains in DesignSpark Mechanical, and exported to STL. Don't use SketchUp: this causes problems with watertightness indeed: its vectors don't match, and they don't close gaps in the seams.

 

Concerning the reinforcement of holes, I think this is possible by defining a different infill percentage around the holes. But I don't know the details, so I will leave that to others.

 

[Oliveros 11]

Up your wall count and the hole walls will get thicker.

[BiMNrd 11]

yeah, but would be great just do this on holes.

[Oliveros 11]

Yeah, it won't allow you to do just the holes though, sorry buddy.