The idea.

I was looking for a safe, strong and flexible solution to attach something to a dog's collar. Soon I thought of the silicone rings on the ends of objects as they are often used. On my desk was a sample of TPU filament (I had never got around to printing with it) and playing with the filament I thought: "if I can attach this to the model"... After repeated attempts to come up with a clamping system, I soon found out that the TPU filament reacted like elastic. Every attempt to clamp failed as it thinned and came loose. Then I thought of a time when I put a rubber band through a small hole and then tied a knot in the elastic so she couldn't get out. That is how the system presented below came into being.

The design.

The TPU filament is pushed through a hole (square 1.8mm) and is guided by a curve in the chamber and comes out through the large hole. The intention is to tie a knot in the TPU filament (tighten it firmly!), cut of the excess filament 2-3mm behind the knot, and then pull the filament back, hiding the knot in the chamber. The chamber is square 3.4mm. This is just enough to pull the knot in (even a double knot will fit!) and the spring properties of the material also "locks" the knot in the chamber when the filament is withdrawn. The depth of the room makes the knot completely disappear from view. The only thing that remains visible are 2 holes of square 3.4mm in the surface of your model.

How strong is it?

I've done tests and tried to break a loop. This was not possible doing manual. In none of the tests did any of the knots pull through the 1.8 hole. Because I was curious, I investigated whether I could determine how strong the coupling was. According to online information, the tensile strength of TPU is 35N/mm2 (MPa). With a diameter of 1.75mm this gives a force of 84 N. Each loop consists of 2 strings, and taking into account variation in the diameter and a little margin, this comes to 150N per loop. This means that each loop can carry aprox. 15Kg of weight! If you want to go to the max, I suggest putting a double knot in the filament (ensure they are over each other and not after each other).

Free to use and share.
I added a step file of an anchorblock. This is 10x5x5mm and very universally applicable due to its small size. To create a loop, two anchor points are needed (see also the fidget example, added for testing). You can incorporate these into your own designs, or you can add them to an existing model placing the STL on top of the back of your model, or let is sink in by using negative volumes and merging the separate STL of the anchor block. I would really appreciate it if you credit me as the originator on your shared model when using this idea.

Where stands TULAS for?
TULAS stands for TPU - Universal - Loop - Anchor - System. It's just to give it a name..

Examples:
I made a few examples using this technique:

• Hanging Hook
• Towel Clamp with Loop
• Magnet with Loop
• Screw on Hanging Loops
• Apple Airtec Holder

NOTE: As a TPU filament I tested with Ninjatek Ninjaflex which has a hardness of 85A. If you use TPU with a higher hardness (e.g. 95A) you cannot pull such tight knots and the chambers for the knots may be too small. It is then best to scale up the model by ± 15%.