The Sound of PLA (Gopichand / Ektara)

How cool would it be to make a stringed instrument entirely from PLA? 

Well now you can!

For the backstory and more technical details see the second half of this description.

After devising a way to make very strong thin strings from PLA filament that were suitable for stringed instruments, I started looking for a one-string instrument. It turned out there are a lot of these, way more than I expected! 

To name a few:

• Ichigenkin (Japan)
• Dahu / Dihu / Duxianqin / Tuntina (all from China)
• đàn bầu (Vietnam)
• Diddley bow (America)
• Endingidi (Africa)
• Tumbi / Gopichand (India)

I chose the Gopichand, a one-stringed instrument originating in India that falls under the group of instruments called Ektara. You can read more about this on Wikipedia: Ektara 

The Gopichand is considered more of a rhythmic instrument and is played by plucking the strings as the legs of the instrument are squeezed together creating a bending drone sound. This sound is instantly recognizable and is heard throughout Indian folk and traditional music.

Image: VaibhavMehtaMangoMonkeys, CC0, via Wikimedia Commons

I chose this one because it seemed relatively easy to design and print and I liked the way it looked. Also, no bow or electric amplifier is needed to play this instrument.

The instrument can be played in 3 ways [1]:

1. By vibrating the string with plucking and changing the pitch by moving the two sticks towards or from each other on the outside (the string is then tightened or loosened more).
2. By drumming on the bottom (additional use as a small drum). The string also works along when drumming!
3. Muting the string partialy while drumming achieves yet another effect.

Although it is mainly a drone instrument, I found someone who really masters this instrument and knows how to use it stand alone. [2]

A very nice ambient sound can also be created with the use of a guitar amplifier (you do however need original guitar strings then).[3]

Paul Simon used the instrument as intro for his song “The Werewolf” [4]

Printing:

All parts can be printed without support. I advise to use a brim (i used 2mm) for the Tuning Peg. All parts are positioned the right way in the STL's. The Neck Parts needs to be printed 2x.

I used 30% infill for all parts exept the Tuning Peg which I printed with 100% infill for strength. All is printed with nozzel 0.4 and layer height 0.2.

Assembly:

Make sure all parts are clean and smooth before assembly!  

Push both Neck Parts in the Drum Body. The holes and the Neck Parts are tapered so they will secure themselves. Secure them good! If they are partialy moving they will prevent proper operation of the instrument! If the press fit is not enough, glue them. Orientation is with the hollow side internal and the curve should point slightly inwards towards each other. 

Place the Peg Box on top of the 2 Neck Parts the same way.

Guide the self made string (see blockquote marked (*) in the technical details on how to make it, or watch the video!) through the hole in the drumhead. Now secure the end by gluing it in the String Drum Mount. Stick the string through the hole (you may need to bore the hole with a bore of 1mm first because printing such small holes is difficult), bend the end so that it does not slide back and add a drop of instant glue to it and let it dry. Cut of the protruding part of the string.

Feed the other end of the string trough the central hole in between the legs of the Peg Box. Now feed the string trough the centerhole in the String Post (wormwheel) and wind it counterclockwise a few times around. The end may not stick out, otherwise the windings can not be winded flat. You can make it easier if you glue the end of the string with a drop of instant glue (inside the square hole) before winding. Don't use too much glue, otherwise it will be hard to remove if you want to replace the string! Keep tension on the string and place the String Post in the round recess in the Peg Box. This part is a little tricky.

Now take the Tuning Peg (lubricate it, I used vaseline) and screw it in while blocking the String Post for turning until it can not be screwed in any further. Now you can tighten the string. While the string tightens, the Turning Peg is blocked due to the tension which will hold him in without any other form of securing. Thighten the string for tuning the way you like it. There is no basic tuning prescription, normaly they are tuned fitting the type of playing/singing. For proper functioning check that the String Post lays flat with the surface of the Peg Box.

The drum head bends inward by tensioning the string. Make sure you don't stretch the string extremely super tight (you can tighten it enormously with the worm gear) to avoid tearing the drum head. I did not test till breaking, but the string can be stretched tight enough to hit very high notes without breaking.

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The backstory and technical details.

When you think of 3d printed instruments, there are different types. Flutes, Drums and Stringed Instruments. For the 1st two types, fully printed versions have been made that do not require any additional parts to function properly. For the string instruments, however, it was the case that you simply cannot print strings. Filament strings (not even the very flexible TPU variants) are simply not suitable for it due to the poor vibration properties and once through the nozzle printed strings are just too weak and break easy. Inspired by the Musical Instruments contest, I challenged myself to investigate the possibility of making a 3d printed string instrument without the addition of separately purchased/added strings or other hardware.

The most important part: the string.

Most of us will know that you can deform PLA by heating it above 60°C / 140°F. This can be easily done by immersing the PLA in hot (boiled) water. After you remove the printed model or the loose string filament from the water, you can deform it for a short time. This can be done until the temperature has dropped so far that hardening occurs again. The first idea was to dip a piece of PLA filament using 2 pliers and then stretch it to see if the resulting string could be deformed with a constant diameter and good strength. However, it soon became apparent that the hardening (after lifting out of the hot water) went so fast that either the strings broke or the diameter was very variable. 

So I needed a constant (hot) temperature to keep deforming and stretching. The idea was to do it completely under water, but because I had already noticed that the stretching went extremely far, I suspected I would need a container of more than 2m for this. This was simply impossible to achieve. Also the area by the end of the pliers needed to be out of the water, otherwise the string would break due to micro scratches made by the (sharp) edge of the pliers. 

The solution was actualy very simple (*):

I took a bowl (suitable for the temperature of the boiled water), bored 2 holes of Ø2 in it, attached one end of the PLA filament string to a table using a clamp, fed the filament through both holes and took the other end with a plier. Then I poored freshly boiled water in the bowl and applied a steady force. After a few seconds the PLA becomes soft and stretching starts. When there is no more stretch, I moved the bowl to the next part to repeat until all was stretched (actualy my son did). I started with one loop (diameter filament roll) which is about 50cm and ended up with 2-2.5m! The stretch thus obtained was a factor of 4.5-5! After checking the diameter, this turned out to be correct. I started with Ø1.75mm and ended up with Ø0.8mm. The area of the cross section was therefore reduced by a factor 4.8. When stretching, you must exert a constant pulling force. It doesn't have to be extremely large. Keep the pressure on during the whole proces! When stretching you will automatically notice that the stretching stops at a certain point (all stretch is gone) after which you move to the next part. If done correctly, the diameter of 0.8mm is almost constant (I have tried several brands of PLA). The deviation in my case was a maximum of ±2%. 

* Alternatively, you can also use a heat gun. However, there is also a risk of firing too hot and burning the PLA or heating the string not evenly on all sides! (and second, I don't own one myself 😏).

* Use good PLA, preferably new, because with older (moist) PLA there is a chance that fragile sections will occur. And cut of the section which have been fed to the extruder with marking of the gear on it!

The resulting string is harder, stronger and has superior vibrating properties compared to untreated PLA filament strings. I printed a basic test with the string attached to a printed drumhead to amplify the sound, this confirmed for me the good vibration properties of the created string.

Why do the properties of stretched PLA change so much?

Plastics have long polymer chains. After the PLA filament rolls have been manufactured, these chains are laying criss-crossed in the material. By heating and (slowly) stretching, these chains are neatly aligned next to each other until the maximum length is reached. Because the chains lie next to each other after cooling down, they can absorb/resist much more force. A good explanation of this with a rubber band as an example explains this even better: Structure and property relationships for polymers

I found some other papers regarding stretching PLA. They mentioned stretching PLA films to 400%.

I would love to see, or better HEAR your make! I'm not a good musician myself and have a terrible sense of rhythm, but for real music makers this might be a way to discover/explore new sounds.

[1] The 3 ways of playing can be seen in this video.

[2] Johnny Herno masters this instrument. Videos he made can be found here, here and here (in this last one he uses a guitar amp).

[3] Nice example used as ambient sound (with guitar amp)

[4] Paul Simon - The Werewolf (you can hear a short section used as intro to the song)

Other examples: here or here