Measuring of Torque During Mixing

"So far we have tried straight blade impellers and different pitched blades, but none worked adequately."

What constitutes 'working adequately?' You've already a means to measure torque, so measuring torque doesn't seem to be the actual question, but one of mixer blades or possibly one of technique.

What is your actual question?

If I understand your question, you could use your motor amps that you draw during the mixing period you require (after, of course, establishing your base line ampere and then following through with any reducers efficiencies).

I do have an old excel program that calculates the torque requirements given a certain amount of variable entered, such as number of paddles, viscosity, and such. Let me know.

Well, the fancy "Overhead Stirrer" we have bought does exactly that: on one hand it keeps RPM constant (independently from viscosity) while an integrated computer reads the torque applied on the impeller, in order to display a value expressed in Ncm.

Job done, then.

I agree we need you to flesh out exactly what the purpose or goal is....is it mixing efficiency vs time energy....

http://www.dynamixinc.com/mixing-101-the-basic-principles-of-mixing-and-impellers

Have to keep in mind that stirring a polymer during a given phase of polymerization (???) may have non-newtonian effects.

Need to know the cps range....

Knowing the data of the actual phase of polymerization helps.

Perhaps a collapsing impeller...

I'm speculating, but there's so much to this.

It's been such a long time since I had to design blenders and mixers

As one of the questions already ask, what do you want to create? Rather, what is the goal?

• turbulent flow, or not if this reactor and just want thermal mixing
• Or maybe you have to mix, but yet avoid and type of shear to the product

And then there are (4) types of shear btw,

• Axial
• Rotational
• Telescopic
• Linear

Nothing like limited information and then having everyone make it more complicated that it needs to be.

A friend of mine used to say, "information must be brief and confusing!" I will reply to my OP for all to follow-up.

Keep it simple and we'll write a novel about it.

Please go to reply # 31

Just thinking aloud: could you put a disposable vessel on a recording weighing scale and allow the stuff to flow out of a hole of known dimensions?

Well, what you mention is a sort of Ford viscosimeter, which will not work under these circumstances because the viscosity is continuously increasing during the stirring.

I realise we are probably way beyond this now, but, that's why I said a "recording" scale.

In the OP you did say 3 to 5 minutes after mixing.

If you turn on the recorder at 3 minutes then off at 5 minutes and save the data; you only have to differentiate to get the rate of flow.

Polymers we manufacture are tested at the QA lab before further processing. These tests are hand made and consist in pouring both components into a 50 ml mixing cup at specific temperatures, while recording their physical properties while being stirred. Our goal is not to depend any more in the criteria or "hand" of the person/s that analyze the product, but to search for accurate data provided by a measuring device.

The stirrer we have bought can be set to different stirring speeds and the most important feature is that (within it´s operating ranges), speed is kept constant independently from viscosity of the polymer.

Ideally, the impeller should stir all (or most) of the polymer contained in the jar.

So far, the different impellers we tested tend to form a film of polymer between the bottom of the impeller and the cup, while the rest gets accumulated on top of the impeller. The measured values are therefore not useful.

We also "sandwiched" polymer between a flat surface and a flat impeller, but centrifugal force pushed it out to the edge.

"So far, the different impellers we tested tend to form a film of polymer between the bottom of the impeller and the cup..."

If I understand, that's more than likely a dead area where the eddies cancel each other out.

Speculating, you may have to add an addition to the end of the impeller to disperse and/or lessen this dead zone.

Have you tried impellers for axial-pumping action?

Also mixing then transferring the suitably-mixed portion to a separate test vessel, though your time constraints may not permit this.

That's a rather small sample....I was thinking larger for some reason....A powder and a liquid....beat it like an egg and squish it like bondo.....lol

Thanks for pointing that out. I think this situation needs to be referred to a standard test procedure, maybe ASTM, but I would not know the reference number for it.

Our polymers are manufactured (and comply) with ISO 5833, which requires a lot of tests to be performed during production, as well as before releasing the final product/s.

The specific test we are discussing here will be additional to those specified by the norm.

Right! this is a small scale test we perform many times a day at our lab during the process of formulating different blends of polymers.

Maybe the impeller just needs an angle adjustment....

Great link! we are also working on a new reactor and some of the videos related to this one are really very interesting

See anything here that appeals to you....

Seems to me you need more of a stirrer component to your impeller....

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400057

What we need is to run simulations.....such as in the link provided, with proper data plugged in....

https://www.cfd-online.com/Forums/openfoam/90835-tank-mixing-simulations.html

Interesting link SE

The only impeller we have not tried is the last one (g): we´ll try to buy it or have it made at pur plant.

Doc: IF the polymer is fluid enough during entire reaction window, could you simply pump the binary (or ternary) components together into a static mixer, and then apply various pressure gauges downstream of the mixing point? If this is only a batch process, you could consider actually pumping the polymer in a loop, and watch the pressure at constant flow rate, or maintain constant pressure and monitor flow rate.

There are also online viscometers that simply measure viscosity directly, and this can be logged, recorded, or hand recorded.

Depending on the complexity of the reaction, you may even opt for separate injection points in the pumping loop for various component chemicals, and also one for the terminator. Pumped loops are perfectly valid means of obtaining delays in chemical reaction timing.

Hi James, the entire polymerization takes about 15 minutes. The window we need to analyze is of about 90 seconds after mixing both components. During this time span the initially fluid polymer turns into a a sticky dough. We perform other tests like "intrusion" (meaning that a rod is pushed into a certain volume of polymer) and behavior is recorded on another gadget.

Yeah forget the fancy gadgets, and listen to Lyn and me about using mixing tubes, static mixers, etc. You will get much better results.

Surely you can engineer a means to have a delay loop that is 90 seconds in length (coil), although I pity the fool that has to clean it out each time after the batch.

It almost sounds like you should be using an industrial bread mixing hook.

How do you actually discharge the reactor volume each time? Dig it out by hand?

Hi! the tests we want to improve are done on polymers from different sources, mainly our warehouse and our polymerization plant.

We have a series of 1.000 liters (about 250 gallons) reactors which are above the floor level. The polymer is discharged into centrifuges by gravity. From there on it is transported by means f Archimedes screws and / or conveyor belts.

You can't get there from here.

Bear with me.

Unless you mix all the two components, completely, you will never achieve a stoichiometric mixture, so your tests will be invalid/inconsistent, unless you take an average of a number of tests and accept a certain degree of variability.

Your customers will NEVER duplicate your stirring methods exactly so their results will always be different. Think stirring cup and tongue depressor.

You must use a mixing tube/nozzle like this

to assure complete, consistent mixing.

Vary the extrusion rate, or the nozzle length until the desired results are obtained and then specify that this method is the only method that will always produce the desired results.

I don't think, the OP is willing for adding a new process, but still a good possibility.

We have a similar mixing tube for tar/water mix.

on a R&D project we did about 2 years ago, we used a continuous mixer for tar/charcoal dust mixing.

it was a Canadian company, I have the info at work. I'll post a link tomorrow.

My thinking is that the reduction in viscosity is the result of exothermic reaction heating and that increases the odds of inexact mixing if you have large variations in viscosity in the mix. With a static mixer that isn't a problem.

The lab results can almost never be exactly duplicated in the customer's facility.

That was a constant problem in specifying/qualifying materials for aerospace/military applications. With satellites you don't get a second chance.

I had an argument about this today with a colleague, cause she would like the stirrer to mix the polymer and later on do the measurement, while i considered that we should mix it by hand (30 seconds), and start the measuring from there on.

We mix 2 components: one is a fluid (monomer), while the other one is a fine powder: therefore using a mixing cannula is not possible.

I agree with you about the fact that our customers will NEVER duplicate our stirring methods (we notice that on workshops), but we need to have certain reference values, cause certain types of polymers have to be blended / combined with other in the search of specific characteristics.

My initial response was deleted because your responses to my, and James, questions have been answered.

I will add that I once had to mix two liquids and three powders together to achieve a certain dielectric constant and loss factor for a waveguide filling compound.

As much black art as science went into that one.

So the customer is expected to mix a fine powder into a thickening liquid? I doubt you are the only ones that will spending time trying to figure out how to achieve good mixing.

Doesn't sound very friendly. Any way the powder could be held in a liquid that is miscible with the monomer?

The customer (end user) is usually an orthopedic surgeon, and our polymer is what people call a "bone cement". This cement is sold sterile as a two components product: monomer (ampoule) + polymer (bag) Ratio is usually 20 ml / 40 gr.

The scrub nurse prepares the cement some minutes before use: either directly by hand or using a mixing / delivery device (similar to caulking guns).

Although it may not seem very friendly, there are some small companies around (like Johnson & Johnson) that also do it the same way...

Um, let me see if I have this correct.

It may be a difficult process that is hard to replicate precisely, but that's not a big concern because;

It has to be done scrubbed in under sterile conditions;

Addressing any problems is very costly and somewhat risky as it requires reopening the patient;

and,

The competition doesn't do it differently.

.

Those don't really sound like mitigating factors. Those sound like reasons for looking hard for an easier more precisely repeatable process to disrupt the market. I don't know enough in this field, so perhaps there is an obvious reason this isn't a good solution, but it seems like there are many two part cement products that don't require mixing a powder,

That is not the question. The question is this: does the polymer cure well enough to hold bones together to knit, and does it or does it not leach any harmful chemicals to tissues surrounding the bone, or present toxicity to the bone marrow...

Hi James, all our products are thoroughly tested before release to the market, and MUST comply to a series of standards like ISO 13485, and these, specifically to ISO 5833. Your statement about "holding bones together" is almost correct: bone cements do not act as a glue, but hold the prosthesis fixed to the interior of the bone (bone marrow canal) by means of friction. Once the PMMA is completely cured (it happens during surgery, about 10 minutes after being mixed), it does not leach harmful chemicals into the bones, nor soft tissues surrounding the bone. This is established by stringent tests performed on a regular basis, and controlled by inspectors from health organizations that visit our facility many times a year.

These tests are not only performed in third parties specialized laboratories, but also in vivo as in vitro, and are part of the quality controls needed to sell such medical devices.

In the future there will certainly be better bone cements based on PMMA and / or some other chemicals, but the research all companies in this field do is very slow, and (should there be an alternative product), getting the approvals from FDA / CE, etc is even slower.

I could not agree more! Sounds like an interesting product line. It does help many get on a path to recovery!

All I know about medicine is that the patient bone is connected to the doctor bone, and the doctor bone is connected to the money bone. Now hear the word of the Lord.

Unless one has worked with this particular class of acrylic materials it may be difficult to envision the crystalline nature of the "powder" used as the catalyst.

Small batch, hand mixing is almost universal in many industries, not only the medical and aerospace industries but others, as well.

Very early on in my aerospace career, I worked in a lab that ran dozens of adhesive tests on various types of polymers, including some PMMA every day. These were all hand mixed small batches.

I see nothing wrong with your current methods as long as experienced technicians/operators mix the samples and perform the tests.

You are right about the nature of PMMA, and the way this material is used in many different industries.

Our company has been manufacturing these products for the last 60 years and most of the testing of the "properties" (like adhesiveness, setting time, curing temperature, etc) are -mainly made by hand by our technicians. These guys are so experienced that they can predict the outcome of the tests just by looking at the physical properties of the polymer... but in order to maintain and improve quality we need to record these findings by means of increasingly sophisticated measuring instruments.

My OP was to share an "issue" (I cannot call it a "problem") we have at our lab during the QC of our product before manufacturing the "final" product (bone cement), and just wanted to see if someone at CR4 had an idea on how to to it easier.

It may be a difficult process that is hard to replicate precisely, but that's not a big concern because:

It has to be done scrubbed in under sterile conditions;

Using the product we manufacture is quite easy: user just has to mix A + B with affordable elements at any OP room ... or he also can use the sterile plastic elements we also sell (mixing bowl + spatula or a mixing device similar to caulking cartridges)

Addressing any problems is very costly and somewhat risky as it requires reopening the patient;

We manufacture about 500,000 units of this product / year and sell it worldwide under different trade marks. Complaints about quality or difficulties in preparing the cement are almost nonexistent because bone cements have been around for more than 50 years, all are more or less similar, and because our company (as well as our competitors) teach scrub nurses and surgeons on how to correctly use / deliver the cement in the surgical field. The possibility of a malfunction of the cement that requires it´ removal in a second surgery is very low (2 - 3%), and usually related to infections due to inadequate surgical techniques, and / or intra-hospital infections. These complications are not very easy to diagnose, and patients usually present symptoms like the loosening of the implanted metal prosthesis / pain / signs of infection many months after the surgery.

and,

The competition doesn't do it differently.

Those don't really sound like mitigating factors. Those sound like reasons for looking hard for an easier more precisely repeatable process to disrupt the market.

The reasons I mentioned before will certainly make you change your mind about this statement

I don't know enough in this field, so perhaps there is an obvious reason this isn't a good solution, but it seems like there are many two part cement products that don't require mixing a powder,

well, all known / approved bone cements are made of PMMA (Acrylic), and so far there are very few medical devices as reliable as these cements, as hundreds of millions have been used since British surgeon used them for the firs time in the early 50´s.

Although there are other kind of cements that could be used for the same purpose, most do not comply with the required biocompatibility and mechanical performance such products must have.

Here is a link to a interesting paper on this issue: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3257425/

I will be glad to reply to any inquiry you may have about this subject (I mean it!)

I stand corrected. My misunderstanding. Carry on.

As someone said: eternal vigilance is the price of knowledge!

No,... that's not it.

It is always this:

The statement that "Eternal vigilance is the price of liberty" has been attributed to Thomas Jefferson and others, but the statement's truth is unquestioned. Samuel Adams said, "If ye love wealth greater than liberty, the tranquility of servitude greater than the animating contest for freedom, go home from us in peace."

A little knowledge is a dangerous thing, and a wealth of knowledge is a gift to posterity!

I call upon all freedom loving people in this world to stand up united, work together for the common good, and let no one lag behind or shirk in his duty. The watchman stands in vigil on the wall, and waits to sound his horn. We wept when we were taken away to Babylon, but now we have returned to Zion, and we will rebuild the wall.

https://www.comsol.com/model/laminar-static-particle-mixer-designer-23921

Have you looked into sonification - using ultrasound to mix the components?

Use the smallest vessel possible and high shear rates to turn the process from a batch one to a continuous one. The mixing process takes place in a special mixer called a 'homogeniser'.

Actually a homogenizer mixer (homo) isn't really used for mixing as much as to change the physical properties of the product. Such as creating a uniform globule or particle size.

but hey, I could be at my limits of knowledge.

This is a batch process as the tests are performed on samples obtained from polymers we have either in stock in the warehouse or that come from our production facility.

These tests are the first step for formulating the final product/s (which are in most cases a blend of different polymers)

And the reason for wanting to know the torque in particular is what, please?

Our goal is to measure the dynamic viscosity (lesser than 500 Pa s = 500,000 cp) while homogeneously mixing the polymer, and to record the torque during this time span.

Please respond to #35⇑.

Well, we gave up our original idea of mixing (stirring) both components of the polymer and to immediately read the viscosity on the machine described above.

The fact is that (as we need a perfect mixing) and a short time span to perform the test, we decided to hand mix the polymer and to immediately pour it into the measuring device... which is a procedure that seems to work and (so far) values displayed by the machine seem to be quite reasonable.

We will repeat this test on all the countersamples on our laboratory (it will probably take half a year) and see what conclusions we arrive to.

So, what is the torque involved in that little operation, then?

Torque was 8 Ncm 3 minutes after start of mixing.

This isolated figure has no special meaning, but after analizing some hundreds of samples, we will certainly be able to draw some conclusions.

"....but after analizing some hundreds of samples...."

.

...uh, wait just a moment. What exactly is this polymer for, again? When you say doctors or nurses handle this stuff, you mean real medical doctors...not role play, right? Are we talking orthopedics or proctology?

I see you also caught that slight Freudian slip?

Analyze the situation always before analizing one self.

I clearly explained a couple of times that the determination I want to make is during the control of some constituents of the final product. The slight deviations these particular polymer we analyze may have, have no impact at all in the properties of our final product both in the orthopedic nor in the proctologic fields.

Righto. You have confirmed freedom from impact in all properties as well as freedom from humor in any sense.

Now if we could work on the spelling....

No offense, but your sense of humor may not be as clever as you think you are.

About my spelling: Spanish is my mother tongue, but I also speak, read and write 6 other languages, and YES sometimes my spelling in English is not as good as if i were born in the UK or the US.

I hope you get to enjoy some humor in at least one of those other languages.

Well, it was sort of funny in English, but only for sick engineering minds.

Kudos on having more languages. I have, I swear, many languages. I meant that I swear in many languages.

Simplistically, couldn't you just run the stirrer at specific power settings, and count the revolutions during specific time intervals, in air (i.e.: running empty of the polymer).

Then, with specific quantities of polymer, run the stirrer again, at the same power settings for the same time intervals, and counting the lesser numbers of revolutions, to then then make your own empirical viscosity curve?...

Hi, thanks for the input, but the device we have bought is a bit more sophisticated than a stirrer.

We are currently testing a lot of samples from different batches, and doing our curve. It will take some time to have data we can rely on.

NOW you tell me that (ha ha)...

In any case, good luck with your efforts, and do tell us about your final results...

I actually did in posts # 46 and 62

Thank you anyway! (I mean it!)