1-800-906-7278

The Hockmeyer immersion mill is a unique, patented machine that does not operate under all of the same theories as traditional horizontal and vertical media mills. The immersion mill eliminates the need for transfer pumps, valves, hoes, mechanical seals and secondary holding tanks, therefore reducing process time, clean up time and waste. Processing time is further reduced by the ability of the immersion mill to produce the desired results in significantly less time, with a very narrow particle size distribution and a universal homogeneous batch. Due to the inherently high throughput of the immersion mill’s cooling dome and jacketed tanks, a high rate of heat exchange facilitates accurate temperature control. The immersion mill can also be completely sealed to eliminate or reduce emissions, as well as protect the product from contamination during processing. The immersion mill has a unique but limited feature that will allow the incorporation of dry materials and still produce a uniform batch. The similarity between the immersion mill and traditional horizontal and vertical mills is in their use of media to achieve fine grinds of pre-dispersed materials.

The selection and matching of the media to the mill base is still very critical to achieving optimum results. Because the immersion mill does not suffer from the same ills as traditional mills, such as hydraulic media packing, floating of the charge, and seal features, it will demonstrate more flexibility, greater performance, reduced mill times and very low maintenance. Media Loading Unlike traditional media mills, the immersion mill has a specific media load regardless of the product or media type. A specific volume has been determined for each size machine. This specific volume is pre-measured (see Installation). Over or under-filling of the basket will affect the performance, efficiency, and the final product. Under-filling the basket may allow greater throughput, but it will also allow the media to form an undesirably larger area of expanded beads. This will result in the dispersion of pigment through impact rather than shear, decreasing the cleanliness of the color development and the gloss of the product. Over-filling of the basket may reduce throughput, draw more amperage, increase milling times, and increase wear on both the mill and media.

Conditioning/Polishing of Media

Most media require preparation prior to use in a batch. New media typically includes fragile or undersized beads. The fragile beads may include poorly manufactured beads or beads with air pockets. These beads will fracture during initial use and can cause occasional clogging of the screens. In most cases the immersion mill will not allow the fractured beads to remain in the screens for long. The fractured beads will be worn smaller and eventually forced through the screens. The particles, along with the rest of the batch, will be cycled through the basket many times, ground as fine as all other particles, and become part of the final product. When polishing new media, operate the mill at the lowest possible speed for the shortest possible time in order to prevent premature wearing of both the mill and media. Polish new media using resin and solvent to remove ceramic or metal dust from the media.

Media Selection and Formula Optimization

The selection of the media is critical and should not be undertaken without examination of all pertinent factors: media density, media size, mill base viscosity, formulation, solids loading, type of pigments, raw particle/agglomerate size, and final grind or particle size required. When the batch viscosity has been matched correctly to the size and type of media, you will find that the immersion mill will out perform current methods and will probably produce results that far exceed current standards (color development, gloss, grind, particle size distribution, transparency, etc). The immersion mill is capable of producing maximum color development, thus reducing the amount of pigment required to produce a given color strength.

Media Size and Density in Relation to Mill Base Viscosity

Proper viscosity of the mill base must be maintained within a range for a given size and density of media. Normal milling operations work well with media in the 1 mm range. Larger sizes allow for greater throughput and will allow you to reach lesser grind requirements more quickly. Larger sizes (and higher densities) also work better with higher viscosities. Smaller sizes will take longer to “rough out” the grind but will get to sub-micron dispersion more rapidly. The optimum mill base viscosities for these medias will vary but as a frame of reference we would consider anything below 1000 cps to be low and any above 3000 cps to be high. The immersion mill has run viscosities as high as 20,000 cps using steel beads and a “pumper” style blade to ensure circulation. One should note that the mill base viscosities are relevant only during normal immersion mill operating conditions. Viscosities prior to, or after milling, are of no consequence to the dispersion process. The use of cooling or heating may be employed to adjust mill base viscosity to the optimum range. The development of color and gloss is largely related to the particle size and distribution of the finished dispersion. The immersion mill will maximize shear when the media/viscosity relationship is optimized. Viscosities below the optimum range will allow media to contact each other without a film of dispersion between them. This will result in rapid media wear. A grind will still be obtained, but mass tone color and undertone, especially in high oil absorbing organic pigments, will not be optimized. Alternative media include a variety of plastic and glass, all of which are designed for lower viscosity aqueous dispersions. Spherical steel media, both mild and stainless, may be employed to disperse high oil absorption pigments such as carbon black. The steel media will require more horsepower, impart maximum shear to the mill base, and generate maximum heat. The minimum recommended size for steel media is .8mm. Media of smaller size will require a basket screen opening, which may be too narrow to allow proper flow of a high viscosity mill base. Mill base viscosities of greater than 1000 cps are recommended for .8mm steel media. Steel media of greater size will require proportionately high viscosity.

Summary

  1. The viscosity/media size-density relationship is the most important parameter in the proper operation of the immersion mill.
  2. Mill base viscosities should be formulated within the media’s operational viscosity. Mill base viscosity measurements are at mill operating temperature and conditions only. All other viscosity measurements are not relevant.
  3. Cooling and heating may be used to adjust mill base viscosity.

Batch Size, Portable Tank Selection

The selection of a portable tank for use with the immersion mill can have an effect on the performance of the mill. The mill will perform best when the bottom prop is positioned 1 diameter off the bottom of the tank, and the top draft tube is 1 diameter below the surface of the batch. Setting the mill too close to the bottom of the tank will foul the prop, reduce throughput and possibly cause backpressure within the mill (see Operation Instructions). Although this positioning will not produce optimum results in regards to processing time, it will still produce the required results. Locating the top draft tube too closely to the surface of the batch can also reduce throughput by restricting the amount of fluid available to the mill. Processing time will again increase, but the required results will still be achieved. Over-vortexing can also occur, pulling air into the batch and possibly causing media loss. If a short batch must be run, locate the mill offcenter to reduce vortexing. As the tank diameters increase, batch size also increases, and therefore the influence of the mill on the product nearest the side walls is reduced. Providing the mill is positioned 1 diameter (bottom prop) off the tank bottom, 1 diameter (top draft tube) under the batch surface, and proper flow is obvious within the batch at operating temperature, the mill will continue to produce at its proper rate. If the tank diameter becomes too great for the mill to produce proper flow, it will either require more milling time or the result will include a greater particle size distribution. Maximum batch size will be determined by the operating viscosity of the batch, media size and screen slot opening.

Mill Wear and Media Life

The immersion mill is made of traditional materials, including stainless steel and hardened stainless steel. By the nature of the milling process, wear is a fact of life. Keeping the wear to a minimum should be of utmost concern to the owner/operator of the immersion mill. The most important factor in reducing wear and extending media life is to properly match the media with the mill base viscosity (see above). The second most important factor is clean up. Keeping clean up times and shaft speeds during clean up to a minimum will greatly reduce wear on the mill and extend media life.

Introduction of Dry Materials

Although it is recommended that only pre-dispersed materials be used with the immersion mill, it is possible to carefully incorporate small amounts of dry material directly into the mill. The restricting factor appears to be batch volume. If the fluid level is not sufficient to cover the mill properly (1 diameter under the surface) it is difficult to create the proper flow required to disperse dry materials into the batch. Most commercial dispersions require pigment loadings of 30%-50%, and therefore, the initial volume will not cover the mill. It is recommended that if you wish to experiment with this option that you be aware of the potential consequences. You may wish to refer to the earlier section concerning bead loss at this time. If dry pigments are loaded too quickly, the result may include complete or partial clogging (tar ball effect) of the screens resulting in the complete or partial loss of media into the batch. You will also have quite a mess on your hands.

Featured Products

Top