Basket mills as the key to short production periods

The author: Martin Getzmann Managing Director, VMA-Getzmann

From the attractive shiny pearl effect on a car or the scratch-resistant varnish on a room door through modern house paint or extra-hard dental implants to trendy sparkling lipstick, slow-release vitamin pills or super glue from a DIY shop, all these products have something in common – they were dispersed in the course of their production process. Solids which are not soluble in fluids, like pigments for paint production, are generally dispersed with a dissolver in the first production step. Put simply, the dissolver is a fast-rotating toothed disc which crushes solid particles (agglomerates) mechanically in a liquid under defined conditions.

However, very often the dispersion capacity of the dissolver is not sufficient. In this case, horizontal bead mills are used to crush the agglomerates further. They do this by fine dispersion or fine grinding. The agglomerates are deagglomerated due to the shearing and impact forces that occur between the grinding beads of a bead mill.

Horizontal bead mills are used in various processes, such as passing, yo-yo or circulation. These processes each have different advantages and disadvantages depending on the product to be manufactured. Horizontal bead mills are technically demanding and an expensive investment. On the other hand, they allow virtually any product quantity to be manufactured. Many machines are only used for a single product because cleaning is so time-consuming. The other peripherals such as pumps, hoses, stirrers and containers also have to be cleaned apart from the actual machine.

As the world becomes increasingly globalised, demand for economical manufacturing processes that are simultaneously more stable and reproducible is rising. The growing customisation of product ranges (irrespective of individual industries) is accompanied by a clear trend towards small to medium-sized, fast and flexible batches. As already mentioned, horizontal bead mills are only limitedly suitable for process and design reasons.

The Torusmill SK and Torusmill TM basket mills from VMA-Getzmann can be used to compensate for the drawbacks of horizontal bead mills. They are an increasingly popular choice for the production of small to medium product batches – and hence for flexible production up to 1500 l – due to their compact design. The milling basket filled with milling beads is low-ered into the product, which has been pre-dispersed by a dissolver.

In the same way as with horizontal bead mills, fine dispersion is achieved by the rotation of a milling disc, which circulates the beads in a high-energy movement. The container used for pre-dispersion is simply used again here. The dispersed product is extracted back into the container via a sieve at the bottom of the mill-ing basket. A dissolver disc located below the basket supports the extraction process with its pumping effect. The result is a very homogeneous, circulating product stream throughout the milling container.

Torusmill basket mills have a particularly innovative and efficient grinding system. The product to be dispersed, often referred to in this step as the mill base, is sucked up by a patented pump wheel integrated in the top of the mill-ing basket. The circulation rate of the product through the basket is thus significantly increased, facilitating much faster production times and even allowing fine dispersion of high-viscosity or thixotropic products.

The compact, optimised milling basket needs fewer milling beads and therefore offers many advantages for both the production process and cleaning. Identical, if not better, dispersion results are achieved with fewer beads. A narrow particle size distribution is possible owing to the effective circulation of the product through the milling basket. The proportion of particles that deviate from the average is smaller and the product manufactured thus has more stable properties.

As soon as the desired product fineness is obtained, the milling basket is simply lifted out of the mill base and emptied by spinning it briefly. A very high mill base yield is ensured in this way. The milling basket and the milling disc are then lowered into a separate, smaller container filled with solvent for cleaning. The disc rotates inside the container for a short time and the Torusmill is soon ready for its next assignment. The basket mills are also available with special scraper systems for the container rim and outer basket wall. As a result, the basket is still cooled efficiently even if high-viscosity or thixotropic products need to be finely dispersed. Basket mills for vacuum operation can likewise be supplied, of course.

The Torusmill TM is the star of the product range. It is a patented dispersing system that combines a dissolver and a basket mill in a single, smart machine. It is used as a classic dissolver for the pre-dispersion step. The milling basket, which is parked at the top in the cover, is then simply lowered, transforming the machine into a basket mill with numerous advantages for the system.

An extensive portfolio of modular laboratory dispersion systems is available. To enable easy scale-up of newly developed formulations to the batch mode of the basket mills, compact milling basket systems for laboratory dissolvers are offered for fine dispersion of mill base quantities as small as 250 ml. Powerful Dispermat laboratory and pilot plant dissolvers additionally allow larger, adaptable milling systems in the TML series to be used for fine dispersion of up to 50 l for pilot operation or small-scale production.

Thanks to the comprehensive process control of the Dispermat laboratory dissolver as the basic instrument, modern formulations can be developed more easily and in a scientifically sound way. The results can then be transferred to the production scale of the Torusmill basket mills without any problems. Thanks to the optional “Nano” kits, very small beads can now be used for demanding applications both with Torusmill basket mills and with Dispermat TML milling systems.

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