Pump
The pump we use is from Schoonewil Techniek https://schoonewiltechniek.nl/product/piccolo/. The piccolo is normally used for concrete repair but can also be used for 3D concrete printing with a flow rate of 0.5 to 8 liters per minute. The biggest reason we have chosen this pump is because this pump is adjustable to 10 positions. Because the printing of concrete is very accurate, it is important to have a lot of control over the pump.
There are no concrete pumps available yet that are relatively inexpensive and robust and, on the other hand, can handle the thoxitropic behaviour of 3D concrete mortar.
Piccolo
Owing to the fact that we were not able to control the production of concrete from dry mix to wet automatically, we initially mixed concrete by hand and fed it into a large pump (the M-Tec Duomix). This was labour intensive and required a lot of cleaning. We switched to a smaller pump for our mobile printer and found that it pumped just as well, with lower through (which is good for smaller printing) and found that cleaning was much simpler.
M-Tec Duomix
After several years of printing with the small pump, we tried our hand again at printing with the Duomix. The aim was to feed the dry mix through the machine, adding a set amount of water to the material, creating a 'continuous mix'. This resulted in us not having to make concrete batched. This had the added benefit of removing one very labour intensive step and the possibility of human error. It should be noted that with the mix and with experience we did get a very consistent mix, but it was still a labour intensive undertaking.
With the Duomix we were able to print continuously, leaving much more time for printing and developement.
We adjusted many things on the original machine. To start, we had the Duomix and not the Duomix 2000, which meant there was no frequency drive on the mixing motor. This meant the production volume was too high. First this frequency drive was built in. This meant it had similar capabilities to the Duomix 2000 (as featured in the photo).
The Duomix 2000 is a standard pump now in concrete printing (2021). However, the Duomix 2000 has several drawbacks. To start it is not made for 3D concrete printing. The viscosity of the material is such that it is hard to pump. You want the material to be as 'dry' as possible to improve printability, but at the same time this makes it harder to pump. So an optimum must be found.
We learned that what is important in pumping 'dry' mixes is to add as much energy as possible. Therefore, we guessed that mixing smaller batched with high intensity would allow us to pump it. This turned out to be the key to succes to print with a continuous mix.
Another drawback of the Duomix system is the manual water flow valve. This valve sets the volume of water that is added when mixing. This manual valve is not particularly accurate and changes in incoming water pressure (from mains) can influence the print.
We replaced the manual water flow valve with a digital water flow sensor. This allowed precise addition of water. However, in order to run the machine based on this parameter, we had to write software that would run the Duomix components based on this parameter. After having replaced much of the electronics, we were able to run the motors from our software.
This also resulted in the ability to control the revolutions of the material supply motor (the vertical motor on the back of the machine). We could then measure the exact volume of our dry mix that a revolution would produce. With this information we could set the digital water meter and the dry mix motor to measure exact amounts of material and water creating a very consistent material mix which is crucial to 3D printing.