Batch or continuous process
Mixing may be either a batch or a continuous process.
Batch process means discontinuous, the simplified production sequence as follows: venting spigot must always be open to vent air in a filter, preferably slightly under pressure; filling through the product inlet, with the unloading door closed: mixing, with unloading door closed, product inlet closed or open only if connected with a closed feeding hopper; emptying of the mixed product through the unloading door. Batch mixing can be done on an open flat surface with shovels or in containers shaped as cylinders, half-cylinders, cones or twin-cones with fixed baffles or moving augers, spirals, or paddles.
In the continuous process, the machine is categorised as an open system. The production sequence consists in a flow of product that enters from the loading inlet, than it is mixed and exits from the unloading outlet. Both inlet and outlet doors are always open. Therefore, it’s important to remember that the final product quality depends on the permanence inside the mixing chamber and on the volume of product in the processing phase.
Continuous mixing proportions by weight or volume, is a technique best suited for formula feeds with few ingredients and minimal changes and no (or minimum) liquids addition.
Depending on the different products to be mixed, you can use one of the following mixing tools:
Ploughshare: are composed by two symmetrical plough-shaped faces. The space between the two faces is closed with a specially shaped metal sheet to prevent the wedging of the product. This tool works on the principle of mechanical fluidisation obtains excellent mixing quality in a very short time.
Mix paddle: require a longer mixing time than ploughshare, but are easier to clean. Paddles are normally used with pastes and moist or sticky products.
Screw: the fourfold screw ribbon is used for non intensive and gentle mixing or to keep the product agitated to avoid lumps. It’s used also to homogenise different batches obtained with ploughshare or paddle intensive mixers in a single batch.