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horizontal bead mill
time2012/04/22
Five variables control all dispersions in small-media mills. They are:
DWELL TIME
MEDIA-BASE RATIO
TEMPERATURE
VEHICLE COMPOSITION
PVC
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.
If these three variables leave something to be desired, then we must drop to the premises of the formulator. Here he must do a little studying-for the chemist is not usually concerned with the mechanics, but if he realizes how the mechanical conditions of his raw materials affect the action of the mill, he can see (we hope) what he must do to allow the mill to give him a salable product. Let us look at the mechanical action in typical simple small-media mill.
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Conditions typical of too high PVC, too low vehicle solids; similar to effect of too low sand/base ratio or too high true viscosity.
Condition typical of too low viscosity or too high sand/base ratio, also of too dense a media.
Preferred operation condition. Reasonable PVC, sufficient vehicle body to transmit power, proper sand/base ratio.
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.
DWELL TIME
MEDIA-BASE RATIO
TEMPERATURE
VEHICLE COMPOSITION
PVC
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.
If these three variables leave something to be desired, then we must drop to the premises of the formulator. Here he must do a little studying-for the chemist is not usually concerned with the mechanics, but if he realizes how the mechanical conditions of his raw materials affect the action of the mill, he can see (we hope) what he must do to allow the mill to give him a salable product. Let us look at the mechanical action in typical simple small-media mill.
点击打开链接
Conditions typical of too high PVC, too low vehicle solids; similar to effect of too low sand/base ratio or too high true viscosity.
Condition typical of too low viscosity or too high sand/base ratio, also of too dense a media.
Preferred operation condition. Reasonable PVC, sufficient vehicle body to transmit power, proper sand/base ratio.
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.