Constructional Features of Compounding Machinery for Filled Polymers
Combining fillers with thermoplastics is normally undertaken when the polymer is molten. Depending on the physical form of both filler and polymer and the amount of filler present, premixing operations involving these materials in solid form are often undertaken prior to this stage, which creates an initial distribution of the components. Alternatively dosing the filler directly to the melt compounding machine is possible, which, in the case of continuous compounding plant, relies on accurate metering of both filler and polymer. Due to the wide variety of filled compound types with differing mixing requirements, the constructional design of compounding plant must be sufficiently flexible to accommodate differences in the necessary heating and shear requirements, as well as throughput rate. Whether the machinery is batch or continuous in operation, most commonly with filled thermoplastics, the resulting product is a uniform pellet containing well-dispersed filler, suitable for subsequent extrusion die-forming or injection molding secondary processing stages.
Although the melt mixing unit is central to the production of filled plastic compounds, various forms of ancillary equipment are required to ensure consistent commercial production. As mentioned previously, this includes specialized feeding equipment for accurate metering filler and polymer, together with cooling and pelletization operations after mixing has been achieved. Increasingly, means for monitoring the quality and consistency of filled compound are also incorporated. A summary of considerations required for the compounding of fillers into thermoplastics is given in Table 1, with further explanation of these requirements discussed below.