Flexi Rex with stronger links

by DrLex, published

Flexi Rex with stronger links by DrLex Dec 30, 2017
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This is another remix of Kirbs' Flexi Rex, based on airfish's remix. That model in turn was most likely inspired by the Flexy Rex by zheng3.

My print of Kirbs' model was not very kid-proof: one of the links broke after some rough handling. Hence I made all links more robust. This means this model can probably be printed even smaller than the original ones before the links become too weak. Of course you can also scale it up at will.

If you want to make a dual extrusion print like this one from Adafruit, check out the remix by AndrewSink.

If you want to create your own flexible model, make sure to check the Hinge Design section below.

Print Settings

Printer Brand:








0.2 mm




Printed with 3 perimeters for more strength. No supports needed.
This should move freely right after you take it off the build plate. If you print with hairspray on a glass bed, the Rex should spontaneously jump into a random pose all by itself when it pops off the plate after cooling down. If you need to use force to free the hinges, you still have some optimization work to do on your printing technique.

Hinge Design

If you want to create your own flexible model, by all means copy the hinge design from this one, or at least use the below image as a guideline. I see way too many ‘flexi’ models that still use the weak hinge design from the original model, or other shapes that have way too little material in the most crucial locations, often the end of the shackle or ‘ring’ is too thin. Make sure to provide as much material around the axle hole as possible, instead of cutting away as much as possible, as some designers tend to do. You won't gain any flexibility by providing more clearance than needed, the model will only become weaker.

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With my new Prusa i3 pro B, at scale 100%, all the pieces are fuse together.
I still need to adjust the flow and few other things of my printer, but my advice would be to use a higher scale.

Despite all, the t-rex is really nice!

People (including me) have printed this at 75% scale without problems. You need to tune your print settings. If you already have problems with this model that has quite forgiving tolerances, it will be a lot worse on finer models…

I know, indeed I have specified that my printer is new and that I have to adjust some settings.
For people like me, I suggest to use an higher scale. The t-rex is nice and it worth the time required.

We made these as prizes for a charity event. The kids were thrilled. By far their favorite choice of prizes.

This is a great design! I made it for one of my young cousins. I was worried that it would not be able to withstand rough play, but It seems to do so very well thus far! How hard is it to apply such hinges to an otherwise unhinged model?

It will probably depend a bit on the model. The basic idea isn't difficult: just cut slots at the desired locations, and add the hinges. I have added some hints and the dimensions of the hinges used in this model to the description (because have seen some new models that still use weak hinge designs).

Love this. Just sent you a tip. Hopefully you get it all, instead of only a 50% cut from Makerbot.

My wife runs a therapy clinic. Young kids get to pick a toy from toybox as a reward for good behavior. They've bypassed all the other toys and keep coming back to this one. And this one lasts because of the bigger hinges. Well done.

How hard would it be to apply bigger hinges to another model, like the new articulated butterfly that is now featured?

Thanks for the tip! It's mostly PayPal that takes a share, MakerBot bundles the tips in a monthly pay-out to reduce the share lost per transaction.

The butterfly could be rather easily improved by adding some extra material to the outer ring of each hinge, which is the weakest part. There is no reason to flatten this part, because it has to rotate around the axle anyway. I had a quick go at this for one hinge, but doing it for the entire model will require a few hours of tweaking and fiddling. I might do it someday if I find the time.

I shrank yours to about 80%, which for some reason all the women and kids think is much cuter. No idea why! The same treatment for the butterfly makes the rod the weakest point.

I'd love to learn modelling well enough to modify the butterfly. If you ever decide to do it, let me know and I'd love to follow along!

Printed on my MP Select Mini at 120% scale, printed beautifully. No grinding or fusing issues at all. Niece and nephew love it - thanks for the upload!

Do we need to print this with support?

See the ‘Print settings’ section. The answer is: no.

Any problems with 200%? Will try it this evening, when it should work.

200% turned out great!

No, the bigger your print this, the easier the print.

What are the tolerances for this?

I'm not sure what's the purpose of your question, but the most important dimensions and clearances of the hinges can be found below, in my reply to burt_23 from Feb 11 2018.

This is my favorite print so far. Printed beautifully at 150%, I will be making more. Thanks!!

Thank you for the great model. Printed perfectly!

Thanx Dr.
I printed two very nice ones on a Monoprice Select Mini at orig size with Hatchbox green filament.
They both flex just fine.
Layer height = 0.02
Top/bottom & shell thickness = 0.80
Print speed = 60
Nozzle = 210 deg
Bed temp = 60 deg

Comments deleted.

I printed two very nice ones at orig scale with Hatchbox green filament. Layer = 0.02, Top/bottom & shell thickness = 0.8, Print speed = 60mm, Nozzle temp = 210, Bed temp = 60 deg. It flexed fine right off the build plate and seems strong enough for a five year old.

I've printed a couple of these but end up breaking them trying to flex it! Has anybody got any tips on how to separate the links?

See the reply I made below on the question from @Aurora_Borealis.

Try scaling the model up! I printed a big one (160%) and the hinges wrok perfectly :)

Did you print them with 3 perimeters?
I printed one and the links turned out stiff ... I am printing a bigger version at the moment...

Thanks. I tried scaling to 140% and it worked perfectly!

Hey, very nice model.
I printed one and the hinges are pretty stiff, the first one the head doesn't move. What settings could I change?

This really should move freely the moment you take it off the build plate. You shouldn't even hear a crack when you try to move the parts for the first time. Some things that might be causing parts to become fused:

  • Stringing: can cause connections between the gaps. Try one of the many stringing tests available on Thingiverse, many of them also contain some hints to avoid stringing.
  • Over-extrusion: would need to be pretty severe though, but doing a calibration test would be a good idea nevertheless.
  • Curling up of overhangs: try an overhang test (again, many can be found on this site). You may need a cooling fan to combat this.
  • Poor bed adhesion: if one of the parts detaches from the bed mid-print and is pushed around by the nozzle, the hinge will be messed up and become solid. If necessary, print with a brim or raft to get better adhesion.

Hmm, the only thing I cannot exclude is stringing. But I only see tiny bits ...
The bigger version I printed works perfectly :)

Well, stringing is usually the least of these problems, and I also have problems eliminating it with certain types of PLA in an all-metal hot-end. If you wiggle the parts around, and poke in between the gaps with a needle, you can usually break the strings easily. A quick pass with a hot air gun can also help to remove the strings (but be careful not to melt everything!)

I've been watching my MPSelect Mini v2 for the last while. I sliced it using the Cura for Mac v3.2.1 and I've paused it a couple of times to try and get some of the excess material out. I can see that there are sections that are going to need dremel love - might end up fusing - something to do with the back ridges. I've verified the STL file is great. IT IS NOT the model at all - something the printer or Cura is doing, so I'm going to experiment a bit. Will post photos when it's done.

It seems that Cura had the support setting turned on. It popped off the bed and was a prefect flexy rexy just needing a bit of cleanup. Ok, a lot. I have sliced it again making sure that the supports are turned off, and I'll see how it turns out. I'm also going to try some other software and see how it behaves.

Great design and the kids loved it. Stands well and moves. So fun!

Comments deleted.

This looks great. I'm going to eventually try to mod it so it can hold a desk cable or pen or something.

Hi thank you for this model.
sorry for my english, i speak french.
I printed a couple for my kids and they love it. Now they ask me for another dinosaure!
I can try to create it with Catia V5 but I need some information. What is the clearance you give arround the mecanism?
You design it with what kind of software?
Thank you again and have a nice day

I did not design the original model, I only modified it, and I did this in Blender (which is not the easiest program to work with). I cut up the model and reassembled it with improved hinges. I attached a diagram with the most important dimensions if you want to create something similar.

Ok nice thank you, i will try something :)

Noob question, I'm starting on 3d printer with a new matter mod-t, how do I set perimeters on Cura 3.xx ?

You can enable ‘wall line count’ in Preferences -> Settings to be able to specify this. Otherwise, you could set wall thickness to the nozzle diameter times the number of perimeters (although I'm not sure this will always do what you expect).

Comments deleted.

Good job. I printed 10. I just couldn't stop.....

Really great job! after printed, pulled out from hotbed outstanding flexible and with stronger joints.

Tks for this remix!

Printed perfectly on my Monoprice Mini. Great design. Thanks!

I am not sure if it is just me, but it does not print the tail until 4-5 layers up hence ruining the tail. any tips?

Judging from the 49 successful makes so far, it might be just you. What software and settings are you using? Are you printing this at normal scale? If you get a preview of the print, are the layers also missing there, or is it just your printer that skips them?

I use cura, and I print on a monopolize select mini v2. So update: I tried printing again and this time it only skipped the first layer of the tail so I quickly put a piece of painters tape down and you can't even tell it missed a layer. I'll have to keep looking in to why it skips. Thanks for the file.

It may be that the model is not sitting flat on the bed in Cura.
I just printed it at 100% and 200% scale and both finished perfectly.

Printed perfectly! Awesome design, I would love to see more Flexi Creatures.

Does this scale up well?

Question to the upscalers.
Is a 7,5 hour print duration ok for 200%, or have i some settings disguised?

Yes. I made a 285%, the biggest that would fit on my print bed. Works perfectly. He also prints great down 40%.

I printed at 100% and 200% too (will post a make shortly) and both turned out great.

Yes, someone just made a 200% size print of this.

Flexi Rex with stronger links
by Ciruelo

Well that I bust out laughing - riding a roomba. Good one. Now I will HAVE to try one at 200% JUST to ride mine.