A clean and constant wear occurring on the inner grinding bowl surface is a result of the material quantity and size (fine or coarse), the number and size of grinding balls, the adjusted rotations of the planetary system, the grinding duration, and the grinding condition, for example dry or wet grinding.
Working with the Highest Rotational Speeds is Ideal
- The best possible grinding effect will be obtained when there is an even distribution of the grinding balls over the entire grinding bowl surface. Extremely low rotational speeds will result in the grinding balls being restricted to the lower area of the bowl, or only moving across the bottom to the inner radius of the bowl.
- The necessary grinding duration will be the shortest possible, and therefore unavoidable contamination and abrasion will be at the lowest level. A low rotational speed leads to a reduction in the grinding energy/powers. The grinding duration will be infinitely too long and the wear is greatly increased as a result.
- Due to the high grinding energy levels, the grinding system rapidly becomes heated, and with wet grinding after 30-45 minutes, it is necessary to cool down the hot grinding bowl so that the evolved steam pressure inside the grinding bowl does not release the suspension through the grinding lid seal. The “pause” button can be programmed with a cooling duration prior to starting, and the built-in ventilator then uses air to rapidly cool the clamped-down and resting grinding bowl.
If all these parameters are kept constant, the same amount of attrition will be obtained in a grinding bowl type (kind), so that the grinding bowl surface is not altered, for example, the inner grinding bowl surface will be roughened through contact with sharp, coarse or hard material.
The inner surface of the different grinding bowls may become worn-out, depending on the properties of materials subjected to the grinding process.
In the example provided in this article, a mixing and deagglomeration test with low grinding effects of a combination of 200 g of black and white fine ceramic was conducted without mixing powder samples, using a 500 ml grinding bowl made of polyamide and 100 x 10 mm - ZrO2 grinding balls.
A sintered corundum grinding bowl and 100 x 10 mm - Al2O3 grinding balls were also used. The feed size is approximately < 5 µm. The materials have a specific weight of approximately 4-5 g/ml.
The homogenization was conducted wet with 80 ml alcohol (ethyl alcohol) with the Planetary Mono Mill PULVERISETTE 6 classic line at 250 rpm in one hour. The added suspension has a highly viscous fluidity, similar to motor oil. This prevents the separation of the slightly different particle sizes of the white and black sample due to sedimentation after mixing.
A lower rotational speed could not be selected, based on the specific weight of the ceramic sample and the grinding balls (also between 4 - 5.7 g/ ml), as the grinding balls would only move along the bottom of the bowl, which would result in a bad dispersion/mixing.
It is essential for the grinding balls to strike and roll audibly. This indicates the complete utilization of the entire inner grinding bowl wall, where the grinding balls strike and roll off on different levels.
Grinding Paths Inside Polyamide Grinding Bowls
These actions are clearly recognizable in the photos of the polyamide grinding bowls, where 7-8 different levels of horizontal roll-off paths, which have already developed at the bottom radius and end below the grinding bowl lid, are clearly visible.
The deepest grinding path is also found here, as the grinding balls striking the grinding bowl lid always strike the upper area of the grinding bowl walls and roll off, due to the high centrifugal force involved.
Therefore, a greater number of grinding balls can be expected in the uppermost path than on the rest of the grinding bowl wall.
Grinding Paths Inside the Sintered Corundum Grinding Bowls
The slightly stained, upper grinding path is also clearly visible in the sintered corundum grinding bowl. This upper path is round and smooth in all grinding bowls, whereas the lower grinding paths may look different, depending on the material the grinding bowls are made of.
Based on the grinding ball size, the paths are developed at different distances to each other, for example with 10 mm grinding balls 7-8 grinding paths are obtained on average; with 20 mm grinding balls 3-4 grinding paths result, and with 30 mm grinding balls 2 paths are created.
The grinding paths vary in depth, based on the hardness and abrasion resistance of the grinding bowls. In hard materials, they may not be perceptible, or even occur at all. This is because when harder materials are used, there is no deformation of the bowl, therefore only very slight abrasion occurs when the grinding balls strike the wall of the grinding bowl; the grinding balls strike only split seconds later.
As a consequence, the grinding balls will not form an impression as they would with a polyamide grinding bowl.
In this method, with the exception of the upper most grinding path, the inner wall of the grinding bowl is hit by the grinding balls at all points, and therefore the grinding bowl is utilized ideally, without the development of grooves, and with grinding paths which have a very low level of abrasion.
The polyamide grinding bowl is considered to be the softest. During the comminution process, when the first grinding ball strikes the inner wall, a minimal impression will occur at the edge of the impression path. The additional grinding balls will then slide into this as they strike and shape the grinding path further.
In this manner, impressions will develop next to one another in a grinding path (see the dark areas stained with grinding samples), alternating with light areas where the grinding balls do not strike.
This deformation becomes extreme and the abrasion will be correspondingly high if the grinding bowl is always filled with grinding balls which have the same diameter. For this reason, working with polyamide grinding bowls is avoided. These bowls are mostly used only to homogenize pharmaceuticals, where low rotation speeds of the planetary mills and extremely small grinding balls 10 mm or smaller are utilized.
In cases where the use of the polyamide grinding bowl is necessary, larger grinding balls with a 20 mm or 30 mm diameter are recommended for occasional use for grinding the deformed areas to a smooth level.
Grinding bowls with the next grade of hardness, like sintered, corundum, or agate, show slight grinding grooves after a longer grinding duration, and these grooves run uniformly though the bowl, without wave-like impressions in the grinding paths.
This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.
For more information on this source, please visit FRITSCH GMBH - Milling and Sizing.