# How to best mix a viscous fluid, multiple mixers or a single mixer?

I am designing a mixer for some 3D printer resin vats and am stuck on the best way to approach the problem. At the beginning I thought I should use three mixers in a line, all spinning in opposite directions so as to compliment each other (like an egg beater but with an extra mixer). But the more I research the less sure I am that this would be better than one, large mixer. Without doing a complex CFD I have no way of knowing which would be better.

I am hoping that someone with a bit more knowledge on fluid mechanics can inform me on the pro's and con's of each, and which approach would be better to mix fluid?

EDIT: I appreciate all the answers but they seem to avoid the question. I would just like to know the advantages of using a single mixer vs multiple mixers with a slightly viscous fluid.

• Do you have any information on the expected flow rate and how fast the mixture sets when it is properly mixed. I imagine the biggest trade-off is between the risk of the material being incompletely mixed and the material hardening while still in the mixing chamber. Jul 18, 2016 at 15:43
• The material won't harden. It is a photopolymer, the mixing process is carried out when the machine is off. It is purely to mix the fluid when it has been sitting for a while and has separated. Jul 23, 2016 at 9:38
• I don't understand "three mixers in a line". Can you elaborate (in your question rather than in the comments)? What does the mix handle like? Powders, liquid, dough, cement? Jul 24, 2016 at 12:55
• You might want to update the body of the question with the information in the comment Jul 24, 2016 at 14:04
• Three mixers in a line. Imagine a line, and place three mixers along it. Like an egg beater with an extra mixer. Each mixer turns in the opposite direction to the next one, so the direction they spin compliments each other. It is mixing a photopolymer resin, it is a fluid with a viscosity between 200 and 600cps. Jul 25, 2016 at 8:00

Consider ultrasonic (or acoustic) mixing for this. It may be the simple solution you're after. This can also be used to de-gas the liquid. If you do some googling, you will find plenty of videos, photos, equipment and scholarly papers about it.

In my experience the three main issues with mixing viscous resins are :

1) 'dead' spaces where you don't get adequate mixing, especially at the sides and corners of the mixing vessel. Here having some sort of scraper which closely conforms to the shape of the container with blades which redirect the fluid into the main flow can help a lot.

2) Introducing air bubbles into the mixture which can be transferred into the casting or extrusion. Vacuum treatment can be effective in getting rid of these but this is not always convenient and it is better to avoid them if possible.

3) Ease of cleaning. This is a major issue for time sensitive mixes, less so for photo-setting resins but even so you may well want to clean or just dispose of parts in contact with the resin between batches or in the case of contamination.

One thing I would suggest is to look closely at the dispensers used for chemical masonry fixings. These are often two part, very viscous resin systems (eg filled polyester) which are dispensed by extruding the two parts through a disposable nozzles with an internal spiral channel which is very effective at mixing the two parts for direct injection into a cavity. They are usually dispensed from a cylindrical cartridge containing both parts in separate compartments but the principal could be adapted to continuous or batch extrusion.

It may even be that the existing nozzles (which you can buy separately) will work as they are for your application and you just need to design the extruder.

Generally a one rotor mixer can be designed to mix as well as a multiple rotor mixer. You also generally employ some form of stator to keep the volume from rotating as a single mass without turbulence. This can be done by simply making the volume not round or not putting the mixing rotor in the center of the volume.

If particulate is settling out like mud, that will not reintroduces into the stream by the motion of fluid alone, a larger rotor for scraping may be required.

As Chris Johns mentioned, bubbles creation can be an issue. To avoid this you need to lower the mixer rotor rpm and increase the distance below the surface level.

A mixer design I recommend considering is a magnetic stirring plate. It can be placed under any thin container and has no seals that can fail. They are typically high rpm, but a lower rpm one could be construed. The magnetic coupling design also allows for experimenting with different locations in the mixing chamber.