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Product Line - MAGNACOATER® - Process Overview |
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 Process Overview |
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Unique Features
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Options
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The Wurster process is an effective means to coat powders, spheres, granules and tablets. A variety of coating formulations can be used including aqueous, organic solvents, hot saturated solutions, and hot melts. There are a number of processing parameters that control the end product produced when Wurster coating. Characteristics of the product and coating solutions are of major concern. However, as these variables are independent of the equipment used to coat the product, we will focus on the process parameters related to the equipment including airflow, distributor plate configuration, partition configuration and spray nozzle type.
In a FLUID AIR Wurster coating process the airflow through the system is controlled by an air-handling package which consists of the fan, fan damper, inlet air conditioning system and chamber filter system (fig. 1). Total airflow through the system needs to be accurately controlled and adjusted as particle size and weight changes during the process. |
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FACTROL, our process control software, provides the ability to precisely control airflow throughout the coating cycle. Once system airflow control is achieved, the ratio of air directed through the inner and outer zone of the coating bowl must be regulated. Traditional Wurster plates utilize round holes drilled in a distributor plate to create an airflow pattern which, when properly designed, works well for a given application. However, if you are running a multitude of products, as in research, you will need a number of plates to cover the range of products being processed. Different plates may also be needed just to change the amount of material in a given batch. A much more efficient and flexible approach is to use the new FLUID AIR MAGNACOATER ® distributor plate system and eliminate the need for multiple plates.
The MAGNACOATER plate system is comprised of two parts. The outer or down-bed portion of the plate is a standard perforated/drilled plate, or it can be a Turboflo screen. The inner or up-bed portion of the plate is a drop-in assembly comprised of two plates riding one on top of the other that function as an adjustable shutter. The shutter is designed to be adjusted to predefined open area percentages, and adjustments can even be made without having to remove product from the product container. The ultimate result is adjusting the ratio of airflow in the inner bed to airflow in the outer bed. This permits running a multitude of products with a single plate system.
In multi-partition Wurster coating, balancing the airflow to all of the coating zones is critical to maintaining a functioning system and assuring uniform production. In addition to the primary distributor plate described above, multi-partition FLUID AIR MAGNACOATER's come equipped with an airflow balancing system that provides a means to balance the pressure drop between the zones. The result produces identical airflow in each of the coating zones. It will also prevent the "loss" of a coating zone due to an upset in the system. |
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Recent years have seen a lot of attention focused on increasing throughput and spray rates. To do this effectively you need to ensure that product does not travel too close to the nozzle tip area and get overwetted. Mechanical means, such as collars, can be installed to do this. However, mechanically stopping the product can cause abrasion of the product. Thus, we prefer to keep the product away from the nozzle tip using the fluidizing air. This is done by using a flared partition. The flared partition is configured with a standard upper diameter and larger bottom diameter. The larger bottom diameter means the product, once inside the partition, has a longer distance to travel before it reaches the spray nozzle. The distributor plate is designed to make sure the product is adequately fluidized before it reaches the spray nozzle. |
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In conjunction with the flared partition we use a conical shaped spray nozzle. The top of the nozzle is smaller in diameter than the bottom, which helps ensure product does not travel too close to the tip area. There are three different nozzle types available for use with FLUID AIR MAGNACOATER's: standard, self-clearing and in-process removable. The standard nozzle is used in applications where the coating solution does not tend to build-up in the tip or on the cap of the nozzle. For solutions that have suspended solids which tend to build-up in the tip, we recommend the self-clearing nozzle. The self-clearing nozzle has a pin that is fired when a high liquid pressure is sensed. The pin clears the nozzle tip, then retracts to permit normal spraying. For solutions that tend to build-up on the cap, the in-process removable nozzle is the best type to use. The flow to the one nozzle can be stopped and the nozzle can be taken completely out of the process while other nozzles continue to run. The build-up on the cap can then be removed by hand and the nozzle reinstalled. |
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The FACTROL control system allows the programming of a progression of stages of coating to match the most complex layering formulations. Various solvent recovery or solvent destruction options are also available to match the particular process, EPA, and economic requirements. |
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