Static Mixer Cleaning/Replacement 5

[YoungTurk]

I'm designing a project in which I intend to use a static mixer to mix a two part epoxy. The usage would be intermittent, and I'm worried about the compound curing in the mixer body. Of course, I've considered the use of a disposable mixer, but they all seem too small for my high viscosity/high volume operation- not to mention cost. The larger static mixers would work if it were a continuous operation...

I've also considered designing some sort of a flushing system, but this stuff is soluble only to paint thinners and other nasties. So there's environmental impact, cost, and associated design to worry about there.

So the question is, what can I do short of living with the limitations of disposable static mixers that are on the market?

 

[jmw]

Do they not have a removable static mixer for hygienic applications you could use? That would save having to flush the pipeline as it could be individually cleaned.
I am sure Chemineer, Vortab, Statiflo or Cleveland should be able to advise.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[btrueblood]

Youngturk,

I'm not quite clear what you are asking.

There are static mixers out there that are as small (fit into a 1/4" i.d. tube) as the disposable mixers, made from any alloy you want. If the stuff you are putting through it is cleanable (i.e. there is a solvent for it), then yes, you can make a static mixer work. Design the part so that you can disassemble the mixer and pull it out of the tube for cleaning. Dunk it in solvent and use ultrasonic cleaning. The trick would be to not let the user put the gun down for however long it takes for the epoxy to cure...

Ben T

 

[YoungTurk]

As always I appreciate the input. Seems the same people always pop up to answer my questions, but the answers are good!

You might not have gotten my drift entirely this time, though, so I'll explain more. My problem is that the disposable mixers are too small for my application. And cleaning a large permanent mixer adds time and solvent to the system, primary problems with the hand mixed system I'm attempting to replace. Not to mention it may be impossible to clean hardened epoxy from the internals. The idea of a removable mixer "blade" (for lack of a better term) is not a bad idea, but still doesn't eliminate the problems. I'm not sure about the ultrasonic cleansing, I'll have to look into cost, durability, and feasability of such cleaning systems for my application and location. Not a bad idea though.

To further complicate things, this isn't a gun application, or I'd just grab one off the shelf and go. I hesitate to present too many details, but the system will have to operate in a batch mode do the nature of the work. Batch time spacings are always longer than the cure time for the same reason. (Make a batch, apply a batch, tool a batch, repeat). This is also the reason I need a larger mixer, the batch needs to be uniform and fairly large.

So my requirement are large/fast flow at high viscosity and easily cleanable or disposable. I'd really prefer disposable, but haven't located an adequate vendor. Cleveland does make some with a removable blade, perhaps I have no choice but to go that route, but I thougt I'd air my problem here to benefit from the groups wisdom before making a key design decision. Any other ideas?

 

[btrueblood]

YoungTurk,

Can you change your epoxy formulation? There are epoxies that can be cleaned up (when uncured) with soap and water.

How large is large - 1" pipe, 2" pipe? Cleaning the outer tube and nozzle/outlet of a removable-type mixer should be easy, even with hardened epoxy (just fabricate a scraping or grinding tool to the proper dimensions). You could also look at using PFA tubing for the externals. As far as a disposable mixer, that depends on how cheap, and how many you'd go through (cost always drops with quantity). There was a company I talked with that had an automated machine for making the mixers from stainless strip. Sorry I can't remember who they were, but look for people selling the smaller diameter version in stainless steel. The machine worked by having a rolling punch/die that cut L-shaped slots, and a second roll that folded the resulting tabs to create the mixer vanes. The parts were longer than the molded plastic versions, but were much "cleaner" for my application (was trying to create "perfect" cappucino foam), because there was less flash/ejector pin indents/etc. Just nice, clean, sheared stainless edges. And cheaper in bulk than the plastic ones. My point for all this rambling is that it wouldn't take much effort to come up with a similar process in the material of your choice.

Ben T

 

[jmw]

How about a coated mixer?
Is there a material that the opoxy won't stick to that could be coated onto your mixer?

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[YoungTurk]

Well, at the risk of tipping my hand, let me fill in the information gap in hopes of some more constructive input. I'm designing a mixing system for an end user of a commercial fairing compound. The manufacturer will not alter the compound for the end user and the end user cannot alter the compound or its application conditions for warranty purposes. It's a fairing compound similar to Bondo, except its 1:1 and used in great quantity. It seems to stick to everything, at least everything I've seen it get on, including aluminum, steel, nylon, polypropylene, etc.

I'll be using either 1.5" supply lines or 2". I'm looking for a mixer of the same diameter. Reason for this is high volume spec along with viscosity in excess of 250,000 cP and appparent increase in viscosity with increase in shear rate. (thanks to jmw for the viscosity help in an earlier thread)

So assuming I go the route of a removable mixer with removable vanes, and come up with some sort of tool or process for the cleaning, I've still got one problem. How to empty the material from within the mixer before cleaning (if at all possible this would save some material cost)? Has anyone seen a mixer of this type that is not static, i.e. has a rotating vane assembly? I've seen these on the small scale. Am I right in assuming this would purge most of the material from within the mixer body if the supply was shut off?

 

[jmw]

Perhaps a different type of mixing?
You might find some clues here:

http://www.bakker.org/

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[YoungTurk]

Prop type mixers have been tried, they don't do the job, same for paddle type mixers. Static mixers, however, have been used abroad (outside north america) and there are claims that they work...

 

[btrueblood]

Well... I don't know of anything like a static mixer that would be capable of self-purging. And having small hold-up volumes doesn't seem to equate to high-volume, high-efficiency mixing in my mind. If an air purge won't clear the tube, I'm not sure how else you'd accomplish it.

A couple of random thoughts: could you "clean" the epoxy off using a very hot oven bake (e.g. 600 or 750 deg. F). Along the same line, but in the other direction: could you slow the cure time way down by chilling the mixer and nozzle, thus minimizing the cleaning cycles to once per day or once per shift or whatever... (or maybe by pumping only the resin through (or whichever is cheaper) in a purge cycle, thus diluting the product left in the nozzle...)

Ben T.

 

[jmw]

Hmmm.
The two part epoxy is mixing in the static mixing which should initiate the reaction.
I really like Btruebloods suggestion and would add to it by recomending that you keep the flow velocity as high as you can within the mixer as the flow will help to scour the surfaces of the mixer and purge it of any product that is at an advanced state of reaction.
However, I suspect that by their very nature, there will be some regions that will be extremely difficult to clean. It may be that some static mixers are perhaps less efficient as mixers but easier to clean or which are less suscepptible to trap product.
It may not be the price of the component that affects the choice of flush, but its properties.
One or other of them may be more susceptible to cleaning than another. Don't neglect mechanical cleaning if you have prevented a build up of reacted products i.e. rags and brushes and a neutral cleaning fluid.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[YoungTurk]

I really like the idea of a flush with one of the two components. I'll have to play around with that once I get the setup, and run it by the user, too (not sure whether they're up for uneven component use). Same thing goes for the bake to clean method- I'm wondering if that wouldn't make the stuff practically fall off the mixer due to accelerated cure. I will definitely look into that as well as the ultrasonic cleaning, but I guess those trials will have to happen with the setup operational. Unfortunately, the user isn't willing to consider temperature variation during mixing because of warranty issues- we're talking about a very expensive fairing service using hundreds or thousands of gallons of compound per project. Of course, if we run into problems we have the option of making some samples under those types of conditions and sending them to the manufacturer for chemical analysis.

I've contacted Cleveland, since they're line seems to match up to this application. They have a high viscosity element that looks fairly cleanable, have a look if you're interested.

http://www.emimixers.com/products/static_mixers/high_viscosity/

Thanks Ben and JMW for the valuable input.

Turk

 

[fastphantom]

some late input if your interested.

i have experience in 2 pack epoxy wrt kevlar carbon composites for boat hulls and have experienced cleaning problems on mixers also. A couple of ways to keep ontop of things:-

You can design the system to have throw away liners where possible. eg A mylar liner inside a pipe. The epoxy sticks to the mylar but the mylar does not stick tot he pipe. Once set the milar/epoxy can be broken off.

Use multiple mixers. While using a clean mixer, another "dirty" mixer is being cleaned. Once the epoxy product has set, it is broken off during the cleaning phase.

This may not be ideal because you have to be set up for cleaning and try to remove as much of the 'wet' epoxy as posible before you leave the equipment and epoxy to set.

Also pipe runs are set up with interchangable spools. Blocked spools removed and clean ones installed.

This may not be suitable for your application but is worth considering

Cheers

 

[YoungTurk]

Fastphantom,

Your comments are more on point than I dare say. I am curious as to how you got mylar liners to stay put in the pipe, though. Did you use flanged sections and just sandwich the liner ends in between flanges? Also, where did you source your liners and mixers?

Thanks

 

[btrueblood]

YoungTurk,

When you get done with this project, could you post back your results, and what worked or didn't? Mostly curiousity for me, but might help others.

Thanks,

Ben T

 

[YoungTurk]

I'll definitely post up some results... Timeframe and other specifics are still a bit loose now since this is not a project being done by my employer, more of a side project. It will probably pick up pace in winter, since there is little else to do here in WI if you're not an ice fisher or snowmobiler. Certainly the results of the cleaning methods for the static mixer will be one thing I'll be happy to share.

Still waiting to here back from Cleveland- what's with these sales reps? Gotta be the most frustrating thing about engineering- the sales reps. Oh well, that topic is for another section.