Because of the numbers of different ingredients and the accuracy necessary for precise colour matching, a pre-mix is made where the individual ingredients are weighed into a blender and mixed together before extrusion.
There are two alternative methods, low speed blending or high speed mixing. In the low speed blending, the equipment can be very simple, like the traditional drum tumbling devices used in many industries, or larger trough mixers. The mixing process is low energy with long mixing cycles of 20 or 30 minutes, so the blender volumes are large. The alternative high-energy dispersive mixer uses high-speed imFeed kg/h Power kW pellors and short cycles, so mixer volumes can be smaller for the same throughput. The other advantage of high speed mixing is the more efficient dispersion of ingredients and breaking down of resin flakes, to give a more uniform pre-mix.
The blend of powders and resins is metered into the extruder, where it is melted and the different ingredients dispersed within the resin, using shear forces of the screws. Efficient mixing and narrow residence time distribution are key elements in this process. The melt mixing in an extruder is designed to operate as close to the melting point as possible, to minimise the risk of cross-linking reactions that will make the product unsuitable for Spraying later.
Once the product has been mixed it needs to be rapidly cooled and converted into a form that can be handled in downstream processes. Passing the melt through two cooled rolls to make a thin sheet and then via a conveyor belt into a flaking unit where the brittle sheet is broken into flakes usually does this