The small-media mill is a ball mill. Ball mills with balls several inches in diameter show in the history of paint making, back when pigment particle size reduction was in the grinding foreman's bailiwick. At some time in this history, it was found that the big balls were not necessary, and small balls were more efficient. This progression continued until late ball mills used media in the range of 3/8" to 5/8" diameter. Little by little the pigment makers caught up even with these mills, but they ground away for years with little change.

Several people were at work in the area however, and, intrigued with the mathematics of it, reasoned that as the pigment was getting better, that less energy was needed to disperse it. In several momentous jumps, it was found that plain Ottawa sand, a true ball with many of the physical characteristics of the big balls, had all the mass necessary, assuming that sufficient velocity could be applied. Thus, instead of moving tons of balls, it was found that a few pounds of sand would do much the same work. An early paper by DuPont's Dave Bosse (1) pointed out that a 700 micron (25 mesh) particle of sand, moving at 2,000 feet per minute, could exert sufficient force to do most of the dispersing then (40 years ago) necessary to disperse the pigments then available, in a sensible vehicle system. This same size "ball" could be crowded 64,000 per cubic inch, each little ball doing work at six point, as against 25 such balls 3/8" in diameter in the same cubic inch! With much better pigment and better understanding of vehicle behavior, it is apparent that even better results should be possible today. All small-media mills work on this principle, that if only a small mass is needed, and that many points of contact can be provided in a small volume of media, all that is necessary is a mechanical device to impart velocity to it. Thus, whether they stir, spin, or shake, whether they have bars, discs, cylinders, or no apparent agitator at all, if they lie horizontally or stand vertically, if they give this velocity somehow, they will do the same work. The proof is there - all these machines work, each claiming superiority in some area, but all indeed doing the job.

Most of the original sand mills started as vertical cylinders. So indeed was the original - the bucket of sand - a vertical cylinder. Instead of pouring in a bucket of paste, stirring it, then dumping it out ground (this is the way a ball mill works) someone soldered a screen on the bottom and started pouring paste in the top. When coffee breaks became too demanding, another wise operator talked the maintenance crew into giving him a pump to feed it from a really big premixer. Others went off on the batch track, making big mills which could (and still do) make all their own premixes, and which could just be run until the grind was accomplished, then drained through the bottom. Someone began pumping paint into the bottom and screening it over the top, and the most popular mills operate on this principle now. The late bottom feed mills have a little further refinement of a "closed" top which contains the flashed off volatiles, and handle a wider viscosity range. They all came from the original bucket of sand, however, and they all are controlled by exactly the same principles of physics and mathematics. Certain space and mechanical advantages result from making the machine horizontal, and many, if not most, are now built that way.

These are the keys to successful small-media mill operation. They are all the variables available without making some change in the mill or media. There are other variables, indeed, which will be discussed, but if a plant had only one mill, one charge of media, and one operator, it could produce almost any product in any color in any vehicle common to the paint industry.

Dwell time, which we shall study first, has the same meaning as in a ball mill. Any operator can be taught to run the mill longer to get a grind. If this means stir a batch mill longer, or vibrate a shaking mill longer, or pour it in slower, or turn the pump down a little, it is still increasing dwell time. This is the first and frequently the only variable used in a carefully handled and set up mill.

If slowing the pump down doesn't do the job, drop to the second variable, media/base ratio. If the ball charge in a ball mill is too low, the move is to add more balls. If the media in a small-media mill is too low, try adding more beads. Conversely, if you don't need a full charge, don't use up all that horsepower moving dead weight. Changing the charge in a small-media mill is not all that hard, and there are easier ways to get very dirty in a paint factory.

If the first two variables don't produce the desired results, check the temperature. Heat is feared for good reason in the industry, but this can be a trap. The polarity effects of certain solvents prevent dispersion until some small-media mill, on a roller mill, in a disperser or what-have-you, this critical temperature must be reached or passed for speedy dispersion. Old mills smoked violently and were unnoticed, but a wisp off a sandmill screed is beaten back with ponds of cold water. Let it heat up! Small-media mills have run at several hundred degrees, and the formulator will know when the safety limit is reached.

A mill is correctly defined as a machine which manufactures by the continuous repetition of some simple action. In a small-media mill power is fed to a disc, or a bar, or some other power transmitting shape. This power is transmitted through a layer of fluid to a layer of media particles, causing them to rotate and to move in some other direction or directions, transmitting their power in turn through other layers of fluid to other layers of media until it is used up in heat or in mechanical action on the fluid or some component of the fluid, or the balls themselves. IT IS THE EFFICIENCY OF THIS FLUID AS A POWER-TRANSMITTER WHICH DETERMINES THE OVERALL EFFICIENCY OF THE MILL. Work is done either by the impingement of ball on ball with particle of pigment between them, or by shear on the fluid and the pigment particle as the balls move past each other at different velocities. If, as in one illustration, the fluid is truly viscous to some excess, the balls are held apart, so they cannot impinge on one another, nor can they rotate freely in the "sticky" fluid. This is typical of a vehicle too high in resin solids, too cold, or, in the case of a truly thixotropic vehicle, subject to too little mechanical shear.

While considering the mechanical effects of pigments, let us look at an area where some difficulty might be experienced in the use of small-media mills, the mixed-pigment system or the wide particle size range pigment.
 
 
 
 

Mill action on a pigment system having a narrow particle size RANGE.

Mill action on a pigment system having a wide particle size RANGE.
 

If we consider a typical TiO₂ pigment, the largest particle size may be in the order of the 3 ½ microns, while the smallest is about 1/3 micron, or a size RANGE of one to ten. Then consider an organic blue or green, where the largest particle might be 1/3 micron diameter, and the smallest less than 1/100 as large, or a size RANGE OF 100 to one. Then let us mix the blue and the white! The smallest particle, then, is about 1/1000 the size of the largest, and to the small balls in the mills, only the largest are apparent, the smallest appearing to be nothing more than part of the fluid. The largest must be dispersed first, to get at the smaller agglomerate groups and separate them. In a small-media mill, the media just does not have the mass to mash the larger particle before all its energy is expended, leaving no power to work on the smaller particles. This is why the much greater weight of the balls in a ball mill are at an advantage in mixed pigment systems.

This is not to imply that small-particle pigments cannot be handled by the small-media mill, but they do require careful formulation, and sometimes special techniques. One of these techniques is to grind the pigments separately into the letdown change tank and blend them. Another is to double-pass the most difficult, which may be done in a fast pass to smooth out the larger particles and slow following pass to obtain final grind.

These, then, are the variables which control the small-media mill. Many of your competitors are making good trade sales, industrial, automotives, colorants, water-dispersed dyes, inks, primers, flatting pastes, magnetic tape coatings, and many other fine dispersions, using just the standard machines and medias, and these variables! Let us suppose, however, that you have indeed some odd product outside the paint field which requires further variables to make the small-media mill economical. Let's return to the same list and work with the variables one by one.

If the feed rate cannot be made slow enough, then we must slow down the pump or whatever is used to control dwell time. Some mills designed for 300 GPH are run at 12 GPH, by simply using a small pump. Others may run at higher than normal range by speeding up the pump or enlarging it.

Vehicle and Pigment variables must be left to alert formulators or to product requirements, of course, but often a sample review of what is really needed or what can be used as alternates will uncover moves which help greatly.

Another area of possible change is in screen design or size. There are mills with larger medias using larger screen holes, and some fine enough to hold back very fine medias, in the order of over 100 mesh. Some mills use no screen, depending on magnetic separation, or closely spaced rotation members to hold back errant media particles. The screen hole size is not critical as long as it is smaller than the media, of course, but the open area of the screen is important. "Mesh size" of particles of media actually has a meaning, for most of these are graded by the standard U.S. Mesh system. "Mesh" of screens is generally meaningless, however, as it implies only the number of wires per inch, so any change in the screen must be made with definite knowledge of the terms involved.

This leaves media as the other available variable.

Many years ago we looked for something better or different from the natural sands, and found little. There were a few glass beads, low in strength and poorly graded. The smallest ceramics were about ¼ inch or larger. Steel shots were already in use in some mills making inks.

To this day, there are MORE MILLS ON SAND AND SODA-LIME GLASS THAN ALL OTHER MEDIA COMBINED. Looking at the math of it: in a ball mill the typical ¾ inch ball will squeeze into the tune of 3-1/3 per cubic inch. The 3/8 inch ball, just half the diameter, jumps to 25 per cubic inch. Now to the small-media mill, where 1/8 inch beads pack in 800 per cubic inch. At 18 mesh or about 1,000 microns, we have over 22,000 in the same space. 20-30 mesh sand, the common media, has 64,000 particles in each such space!! Go on up to 60 mesh as some mills use, and we find almost a million and a half busy little balls in each cubic inch!

We must stop somewhere, of course, for we hit two limits. One is that it becomes impossible to separate media and paste, and the other is that the small particles of media have too little mass to accomplish any work on the pigment particles. There are THREE moves in media, and ONLY three; size, density, and velocity. The velocity in most mills is fixed, so we have two; size and density. We may trade size for density or vice versa, to vary our effect. For a given size and density of media, the media may be ANY material, there is NO process difference. Thus, to claim that a ceramic bead disperses better than glass beads, which is better than sand, just because of its material, is silly. Surface hardness and smoothness will impact the durability of the mill and the media. It is the higher density of the ceramic bead that makes the difference. Remember, most small-media mills in most plants are fully successful with 20-30 mesh sand or glass beads, so there is little process reason for changing. If we must change, then our only moves are density and size.

If we make the same 700 micron bead twice as dense, for instance, we can obviously do more work. If we go on up to steel shot IN THE SAME SIZE, with over three times the density, there is very little in the way of pigment that could resist the forces applied. Then why don't we just use a dense bead and forget the sand, glasses, or ceramics in the same size range? Again, we run face on into simple physics. A media twice as dense is half as buoyant, and in anything other than very heavy pastes, a dense bead drops to the bottom and packs. A dense steel shot simply eats itself up, severely discoloring the product. A dense glass just wears itself away. A dense ceramic becomes very aggressive, and abrades itself and the machine quickly, sometimes going off with the product as discoloration, sometimes as a claylike wear product. If we could, indeed, run one of the mills constantly on 140 KU pastes, forgetting washing cycles, we might sensibly expect long bead life and little machine wear, and that is just what dense beads are for. There is, indeed, increasing need for the use of dense medias as the environmentally-dictated trend to higher solids gains volume. The caveat is that dense beads should only be used in viscous or heavy pastes, as they can sink and pack in multi-purpose mills which are asked to disperse light pastes also. Increasing density may not cure any more problems than it causes.

Then if density has such traps, let us change the size. There are several strong slags and glasses available now in densities slightly greater than sand, and they are well worth a try on primers, large inerts, wide-particle-size-range pigments, cheap pigments, etc., but we cannot overlook the mathematics of a couple of paragraphs back. Large beads lose efficiency on small-particles pigments simply because they lose points of contact, but they gain in handling coarser materials or mixed pigments. These beads are much lower in cost than the ceramics, and are not abrasive, being amorphous by nature. Minor bonuses of going the larger bead rather than the heavier bead route are easier wash-up, and far less screen problems.

While on media, and this goes back to ball mill practice, the closer the grading of media the longer the life. In a ball mill or in a small-media mill, the smaller balls abrade away the larger, until all are about the same size. When you buy media, you should select a supplier who offers the most uniformly sized media available. Low cost, poorly graded beads that have wide size ranges or that are not very spherical are no bargain.

Thus we aim for the least expensive, the smallest, the closest graded, and the lightest media that will do the job needed.

WE GET VARIABLE RESULTS ON EXACTLY THE SAME FORMULA AND MILL, ESPECIALLY IF WE AREN'T RUNNING THEM FREQUENTLY. SAME PIGMENT, SAME RESIN, EVEN THE SAME MANUFACTURER AND LOT NUMBER. ANY SENSIBLE REASON?

This can be a tough one, but you can localize your search to the materials themselves. USUALLY it is a pigment matter, and even more specifically, a pigment like a blue which is used in small quantities, and which by nature is made of almost colloidal-size particles and is highly hygroscopic. If you open a bag in March, and finish it off on a batch in May, you have gone through some violent changes in humidity, certainly enough to cause havoc in the condition of the pigment. Other pigments, as the pigment people will tell you, are extremely hard to hold to exact similarity unless they are kept under good moisture conditions. Look under the last bag or skid of pigment the next time you use up a pile, and you will probably see a moist spot in the concrete! If you have such a pigment, buy as sparingly as possible, and store carefully, favoring these materials in location in storage.
 

WHY DOES MY SCREEN PLUG SO RAPIDLY? MY OPERATOR HAS TO STAND WITH A BRUSH OR SPATULA AND SCRAPE IT OFF TO KEEP FROM FLOODING.

You could have a worn batch of media, or an excess of fines, but probably not. Medias just do not break up in a mill, except possibly some sands, unless there is something like a loose disc or a broken piece in the mill. There is just not enough force to break up a quality media, even sands, certainly not the glasses or shots, all of which are much stronger. One popular type of zirconium silicate bead that is made in Europe does commonly fracture in a mill. The solution is to consider a stronger zirconium silicate media such as Quackenbush's QBZ-58. Worn media usually comes right through the screen and is picked up in a filter or in the filling area in a strainer. More often screens skin over because of too high pigment loading, producing high pigment concentration as the volatiles hit the screen and flash off, right where they are hottest and most agitated in the presence of air. Sealed screens help, but the mill will still be operating at low rates. Check your PVC and raise your resin solids a little.
 

I HAVE TO RUN AT BOTH HIGH PIGMENT AND HIGH RESIN LOADINGS, ESPECIALLY IN SMALL PARTICLE PIGMENTS. I GET MORE THAN PUFFINESS - IT IS ACTUALLY LIKE A MARSHMALLOW TEXTURE. IS THERE ANY MOVE INDICATED HERE?

Marshmallow is a good word. Marshmallow is nothing more than corn syrup and air. Corn syrup flows. Marshmallows do not. The air is the only difference. Your high resin solids are just like corn syrup. At the top of the mill, if there is air available, it will be whipped into the mix and produce a tough, stable, air emulsion, for the exposed discs are excellent air pumps and whippers. You can eliminate these discs (at some small loss in output) or you can seal the mill against air. This is one of the real advantages of a sealed mill.
 

WHY CAN'T I PUMP SOLVENT UNLESS THE PUMP IS NEW?

Because the pump is worn already, and the clearances are opened up. Any rotary pump depends on close metal fits, and when they open up, the material pumped is merely bypassed inside the pump. This is no problem with paste or viscous material, but thin materials bypass readily. Some adjustment may be possible within the pump, and your pump peddler can show you how to do it. At best, it is a short-lived patch. Improve your pre-mixing, and keep the beads out of the pump. Quackenbush stocks all of the Viking pumps and parts common to many mills and can also offer technical assistance for your Viking Pump questions.
 

WHY DO WE HAVE SO MANY PUMP TROUBLES?

Because the pumping job you have to do is a tough one, and most pumps would not do the job at all! Centrifugal type pumps cannot handle the viscosities, and few can give the pressure at the low volumes required. Piston pumps would wear out very rapidly, and they have a pulsating discharge characteristic, as do diaphragm types, which must have teflon diaphragms for some of your solvents. It is possible to use a pulsation dampener with an air diaphragm pump and many users do this. A dampener adds to the area that can trap pigment and can extend wash up times.

We have found that a rotary type is usually the best, and even some of those are out entirely. The tube-type and the flexible impeller type are limited by the available elastomers. The progressing-cavity pump or the screw pump is good but must be metal-on-metal, again subject to wear. Thus we narrow down to the lobe or gear type, and even those must be oversized and run slowly to get any life at all. Then, unless your premixing is thorough, they are asked to handle dry, extremely abrasive pigments. If there is no strainer, they are fed nails, balls, and other material which they find difficult to digest. Worst of all, they are too often fed some of the media from the mill, and only a few revolutions with that in their craw will eat up the best of them. It is a tough pumping job, and the pump must be treated right if not pampered.
 

WHAT IS THE BEST PUMP FOR ME, THEN?

Either lobe type rotary or a gear type rotary. The gear type has a price advantage, while the lobe type will accept larger lumps. If your experience is such that a pump lasts a year or more, then consider going to the abrasion-resistant types of gear pumps, as they are more costly but much longer lived. Abrasion resistant pumps often use tungsten carbide or hardened cast iron wear surfaces and special hard-faced mechanical seals. If your pumps last only a few weeks or months or you cannot keep from feeding them media or steel balls or drum bungs, then buy the cheapest that will fit.
 

MY STRAINERS PLUG UP AND THE MEN JUST LEAVE THE SCREENS OUT, RATHER THAN CLEAN THEM. IS THERE ANY HELP FOR THIS?

Indeed. First, keep junk out of your premix, bag scraps, cigarettes, etc., which CAN be kept out. Next, see how long it takes on a typical batch to plug your strainer. If you get half a batch out before having to clean the strainer, then get a strainer with twice the open area of screen. Use a basket type or a so-called self cleaning type and make it reachable. Put a pan under it. You can also look at buying a duplex strainer, or pipe in two simplex strainers in parallel so that one strainer can be cleaned without shutting down the mill. If the man doesn't have to make a project out of it, he will clean it with less fuss.
 

WE RUN OUR SANDNMILLS AT HALF-MAST BECAUSE WE CAN'T GET THE GRIND. THE MILL PEOPLE TELL US WE SHOULD GET 80 GPH AND WE HIT 45 IF WE ARE LUCKY. WHAT'S WRONG?

Running a mill must be learned. Read the foregoing list of variables and work at them ONE BY ONE, your answer will probably be obvious. If this fails, go to the pigment people. Each of them can tell you how to handle their pigments in a sandmill, or give you formulation help. Sometimes the fault is as simple as an improperly cleaned screen, or an operator who is expected to gauge heat with his palm instead of a thermometer. Get someone form the other end of the plant and have HIM check out your variables. We find that firms who take the time to reformulate to take advantage of newer types of media and mill types are the companies who maximize production from their mills. When you change to a new mill or new media, it pays to re-optimize the mill base formula.
 

WHAT DOES A PUFFY OR BUTTERY PASTE HANGING ON A SCREEN INDICATE?

Almost invariably-too low resin solids. If the vehicle demand of the pigment is not satisfied, it will hang onto anything it can get, and the most available "fluid" is air. Just move up a few points at a time on resin solids, or down on pigment concentration. Take solvent out to compensate.
 

I GET GRIND MEASUREMENTS THAT ARE OFF THE SCALE FROM THE SCREEN, BUT SOMEHOW WE LOSE GRIND IN THE LETDOWN AREA. WHY? HOW DO I SOLVE THIS PROBLEM?

This is obvious. The mill is not at fault, for it has proven, at the screen, that it can do the dispersing, even though the product is unstable. Again, this is usually too much pigment or too little resin. Solvent will shock it out, or air, or even resin sometimes. If you can't reformulate, then try dropping the paste into some resin while mixing.

ON OUR CLOSED MILL, ESPECIALLY WHEN WE ARE STARTING A BATCH, THE MEDIA SEEMS TO JAM BETWEEN THE TOP DISC AND THE UNDER SIDE OF THE COVER. IT GETS HOT, SOMETIMES ENOUGH TO SMOKE OR EVEN CHAR. WHY?

Almost certainly you are pumping your paste in too fast behind the solvent. If the media bed is full of solvent, it is like damp beach sand, and has little flow. It is just extruded ahead of the paste-solvent interface, and it never becomes homogeneous with the more lubricant paste. When it gets into that dead area at the top, it becomes a relatively dry, non-fluid mass with low heat loss, so it just gets hotter and hotter. When you start such a mill on paste, start it dead-slowly allowing the media to mix with the paste rather than being pushed ahead of it. This takes a few seconds longer but is worth the time. This will help keep you from blowing out your screens also, because that dry media jams itself into the screen tightly, and before you catch it the pressure is up.
 

MY SCREENS BREAK FREQUENTLY, MORE SO THAN IN OTHER PLANTS. WHAT DO I LOOK FOR?

Careless handling, usually. If a screen is perfectly round, the media just rolls slowly around in it. If there is a dent, a crease, or some irregularity, it can't roll on, and will wear right into the obstruction. Wash your screens carefully, and put them in a safe place. If you must soak them, do it in a small bucket or tank where only the one will fit and another cannot be thrown on top of it. Another possible cause is running with too thin pastes and too much sand. If the top media is drained dry and is thrown about, it will wear into the screen.  An abrasive media or too large a media in thin pastes can also accelerate screen wear.
 

WE ARE BOTHERED BY VARYING AMOUNTS OF DISCOLORATION AND GRAY-OFF. WHAT SHOULD WE LOOK FOR?

"Varying amounts" gives us a hint. Discoloration is usually one of three things. First, poor or incomplete washup between batches; second, too long rinsing with solvent; or, third, an outright abrasive media. We can include steel shots with the last, not that they are abrasive, but, at best, they do discolor anything light or clear. So let's eliminate. If you were using an abrasive media, you would notice discoloration much of the time. Sand would not be this abrasive, and amorphous glass beads are not abrasive.

Incomplete washup can be isolated simply by seeing what color your contamination is. If you just finished a red and your discoloration is red, then your washing technique is at fault. If your discoloration is graying or hazing, then look at the rinsing angle.

If the operator rinses out the mill with solvent, and just keeps running the agitator, there will be some metallic discoloration from the vessel walls and the agitator. This is less noticeable with glass than with anything, but even smooth glass beads, especially dense ones, if they are just agitated in solvent and allowed to settle to the bottom of the mill, will grind away at each other and the mill. Higher density ceramic beads will be much worse. Since this is an operator error, the amount of discoloration will vary depending on just how long he rinses. Just jog the agitator, don't keep it running while rinsing. The media washes in a snap. The color is in the pump and piping.
 

MY OUTPUT RATE VARIES. THE MEDIA GOES UP AND DOWN. WHY?

This is a pump feeding problem. The pump will feed exactly the same amount constantly, but only if it can get to it. Your suction lines to the pump are too small, too long, or of crushed hose. Possibly you are forgetting to clean the strainer. Make sure you have a good gasket in your quick-couplings, and that there are no air leaks in the suction line of the pump, even tiny ones.
 

WE GET ALL KINDS OF FIGURES ON VISCOSITY FOR OUR MILLS. WHAT RANGE OF VISCOSITIES CAN WE HANDLE IN MOST SMALL MEDIA MILLS?

This is one of those things that can't be limited. Go back to the section on formulation. If a paste is too highly pigmented, it will fight the mill with dilatancy, even though it is apparently thin. If the resin solids are too high the grind will be slow, and the mill will flood at moderate rates, again even though the material is highly fluid. Some pastes work well at 50 KU and sandmills on sand are running successfully at well over 120 KU. The figures you hear of 70 or 80 KU up to 95 or 120 KU are not limits, they are just coincidentally where most successful formulations happen to fail. If in doubt, go to your pigment people; they can give you a good starting formula.
 

HOW HOT SHOULD I RUN MY MILL? I DON'T WANT IT BLOWING FUMES INTO THE ROOM.

Run it as hot as the material will allow. We touched on this earlier. If you don't, you won't get the rate up, and may not even get the grind. If you have a closed mill, fumes are no trouble. If you have an open mill, KEEP THE FUME GUARD ON! The fumes will condense rapidly and just fall back into the trough. Do not guess at the temperature. Put in a thermometer where the material flows over it all the time, and put the safe top temperature on the batch card for the operator.
 

I DON'T RECIRCULATE MY PASTE, YET I FIND BEADS IN MY PUMPS. HOW DO THEY GET THERE?

Simple things first. Watch for stray bouncing beads if you have an open mill. Clean out your premix cans carefully, and check them for beads before using. Most beads in pumps get there by backing up the feed line to the mill, however, and some means must be provided to absolutely eliminate this source. Beads can easily siphon back from the mill. If the change can is empty, and the mill is full of paste or solvent, as soon as the mill is turned off the fluid will rush back down the line. The pump will hold it back only to a limited extent, for a rotary pump is not a tight shutoff, especially against solvent. Don't count on a check valve to hold it, for there is no check valve made that will seat against junk, and there is always some foreign material in premixes. Don't count on a siphon loop. Such a loop would have to go up through the next floor to be high enough. The one sure way to keep media from siphoning back to the pump is to put a MANUAL valve in the line, and as the pump is shut off, the valve is closed by hand. This should, from experience, be a ball type 90 degree turn, teflon-seat valve with a long comfortable handle, and should be mounted just as close to the vessel inlet as possible. Finally, be SURE your operator uses it every time.
 

MY MILL FREEZES UP AND WON'T START. WHY? AND HOW DO WE GET IT GOING?

There are several reasons why a mill will freeze up when not in use, but the most common is that it was left with too dry a bed of media in it. This might be just that the operator left the mill running after the pump had nothing more to feed it, and the top discs act as a low-grade centrifugal pump, throwing fluid out and leaving the bed relatively dry. Another reason may have to do with the material itself, for some pastes body up when cooled. Others get actually sticky. For the "sticky" types, there should be some solvent or some different material put into the mill last. To get a frozen mill "unstuck" can be a problem, unless you realize that the nature of the media bed is just like quicksand, or "floating" sand. If you can get fluid coming in the bottom somehow, and loosen up the bed, the agitators will free themselves. This is most easily accomplished by pumping in solvent, SLOWLY, but some mills which freeze up frequently may be also equipped with an air inlet at the bottom for blowing loose. DO NOT try to pump up a mill with excessive pump pressures or with paste. The discs and the jammed sand are like a hydraulic ram, and unless they can be separated and freed so the fluid gets thru the bed, you can lift the shaft bearings right off the mill!
 

WHY DO I GET SKINNING ON MY SCREEN? IT VARIES WITH THE SAME MATERIALS.

Again, if you have your mill closed or the fume guards on, this is not a frequent complaint. Skinning is polymerization or it is solvent evaporation, and both are related to variations in the amount of vagrant air going by the screen. Keep your mill shielded from door drafts or window drafts if possible.
 

WHY DOES MY MILL SMASH (BREAK UP) BEADS?

The first step is to look at the "broken" beads under a microscope or magnifying glass. If the particles are well rounded and do not have angular faces like that of broken glass, then you are looking at worn out beads. The beads may not necessarily be spherical, as they can wear to a variety of shapes such as flattened plates or footballs, or they can be spherical, but very small. Bead wear is a normal process and does not indicate defective media.

Even though you may find broken beads in the sample from the mill and believe the breakage is due to defective beads, it almost never is. In a properly operating mill, the maximum force available is only a few psi. Compare this number to the crush strength of glass beads (70,000 - 80,000 psi), silica sand (12,000 psi), and zirconium silicate (>130,000 psi). It is easy to see the difference between the force available to break beads versus the force required. To put these numbers into perspective, a human is capable of generating much more than a few psi by simply pinching two fingers together tightly.

If force is applied to a single bead in a mill, the pressure wave of the liquid near the disc surface simply pushes the bead out of the way. If the viscosity is quite low, as during a solvent or water wash up, the pressure wave is not strong, and beads can contact the discs. This is one reason why we recommend short wash cycles. Beads can also wear upon other beads and wear upon mill parts during wash up with low viscosity liquids. For maximizing bead and mill life, use a resin for wash up and maintain adequate mill base viscosity during production. Contact Quackenbush Company for mill base viscosity recommendations for various types of mill media.

Most complaints regarding broken beads occur soon after dropping the old charge and recharging with new beads. We hear, "I just tore down my mill, cleaned it out, and recharged it with fresh beads. After my first batch, I found a lot of broken beads in the media separator and in my product. The only thing I did was drop the charge." Here are ideas and solutions to this type of problem.

1. MILL REASSEMBLED IMPROPERLY - Discs loose on the shaft, broken-off disc rims, foreign objects loose in the mill, chattering check valves, one disc has slipped on top of another, or a dropped stabilizer riding on bottom or very close to the bottom. These are all possible situations that can cause bead crushing.

2. WORN PARTS OR IMPROPERLY SET UP MILL - Gap separators and screens can wear, and screens can be installed crooked or backwards. If media passes the media retaining system and enters a rotary pump, the pump will crush the media before the pump jams or fails. Call Quackenbush for Viking pumps and parts that can be shipped immediately.

3. EXCESSIVE DISC TIP SPEED - This is most common in a horizontal mill. Very high tip speeds can damage media, especially if the viscosity is low.

4. BACK PRESSURE - The mill forces beads back into the pump when the pump is shut off. When the pump is started, it crushes the beads. Back pressure valves are not infallible and will sometimes permit beads to bypass before the valve shuts off.

5. DRY BEAD BED - This can occur if the beads settle to the bottom of the mill or excessive pump speed forces beads to one end of a horizontal mill. The solution is to jog the mill to help suspend the bead bed in the mill base before milling.

6. WORN DISCS - This can be very hard to spot. Where worn disc edges are scalloped, there may be extreme turbulence and the edge is where the speed is the greatest. Additionally, media tends to pack near the edges of the bottom discs in a vertical mill. This combination of factors can cause beads to actually fracture. We have seen this most often with automotives and primers where the pump rates are slow and dense media is used. Cure the problem by replacing the shaft assembly and refill the mill with beads from the same lot. You'll find the beads now run problem-free.

7. MIXING QBZ-58A WITH SEPR ZIRCONIUM SILICATE MILL MEDIA OR WITH QBZ-58A - There is enough difference in density between QBZ-58A and other brands of electrofused Zr-Si to generate a lot of bead trash. The solution is to NEVER mix different  brands or types of beads. Beads suspected of having been mixed should be thrown out as there is no way to sort them.

8. MILL INADVERTENTLY RECHARGED WITH USED BEADS - Under the magnifying glass, you will see the familiar signs of worn beads. The beads will not have angular fractures on the surface.

9. TRASH AND SMALL USED BEAD BUILD-UP - There are often trash and small used beads in the screen area the stabilizer, the disc hubs, and other corners of the mill. When the mill is recharged, the new beads will loosen the build-up and create what appears to be small bead fragments. They are actually worn beads, pigment lumps, skins, and other debris. They are not broken beads. The trash can usually be "conditioned out" of the mill over time if the problem is not too great. Otherwise, you must drop the charge and recharge with new, unused beads.

Much can be learned by looking at a sample of filtered trash. The photo above is typical of a sample from an actual user who claimed that his media was breaking up in his mill and coming through the screen. Broken media, hairs, wood chips, pigment and what looks like a few whole beads. This had been caught in a filling machine screen and decanted clean in solvent.
 
 
 
 

The photo above is of the same beads which had "gone through the screen" pictured beside an actual piece of sandmill screen. Obviously they did not go through the screen, but possibly around it or through a hole. If they had not been so smooth, or had been spalled all over the surface, we would suspect that they were being chipped by a worn disc or had been subjected to long dry running.
 
 

The picture above shows broken pieces from the same lot as shown earlier that do not have the sharp points and edges of freshly smashed shards which would be typical if they had been broken by a loose mill part, for instance. Instead, they are smoothed off by at least one long pass in the mill. We could guess that they were the same whole beads pictured above, which had been carried through the pump in a double-passed paste or left in a corner of an unclean change can.
 
 
 

An unusual occurrence is shown in the above photo. This illustrates the presence of iron particles which are easily separated from the mass in the first picture by a magnet. The iron is no doubt from the pump, as there is no other source. Such samples as this tend to take a pattern, and can be used in tracing faults in the mill or its operation.
 
 
 

Above are badly spalled beads from a mill which had apparently been run for some time with nothing in it to provide lubrication. Note that there is not a single broken bead, and especially note one bead at center resting on top of others.
 
 
 

Here are selected beads from spalled lot above. These are beads which had large air voids, and which might have been expected to break under unusual stresses. Instead, they have merely cratered. This is further indication that exceptional forces are needed to break Q-Beads, forces that are not available even in very carelessly run sandmills.