Pelletizing: Established solutions and challenging trends

Plastic pellets facilitate handling and enhance processibility, thus pelletizing companies-raw material producers, compounders, and recyclers-offer a real benefit to plastics processors.

Pelletizing equipment is available with throughputs from just a few kg/hr in lab applications up to more than 70 tons/hr in polyolefin production. When designing a new pelletizing operation, the first step is selecting the most suitable of the various process options:

Granulating or grinding, although a kind of pelletizing process, shall not be covered here since it gives no defined or consistent product shape and serves primarily to reduce the bulk density of off-spec production.Dicers have been used for some thermoplastics (PVC, ABS, PUR, etc.) since the early 1950s. Extruding a sheet (web) and processing it with dicers has the advantage of imparting the lowest stress level to the plastic melt. Still, dicer use is declining. Water-ring pelletizing (hot die-face cutting) is widely used for plastics recycling and to some extent in compounding and color concentrate production. Water-ring pelletizer designs have a higher sensitivity to adhesion of plastic materials on the knives and to each other if contact is made before the surface is adequately cooled. However, start-up is quite easy; the process is not sensitive to non-consistent polymer flow. Where the water ring performs well, no other system performs as cost effectively.Strand pelletizing must be considered the first choice in terms of flexibility and quick product changes for compounding operations. Conventional strand pelletizing systems with water bath are inexpensive and easy to operate. They require almost permanent operator involvement. If large numbers of strands must be handled and production lots are much above average-e.g., when pelletizing virgin PET or large compound lots-automatic strand pelletizing should be considered. Such systems use water slides for cooling and for forwarding the strands. Strand pelletizing systems can achieve throughputs of up to 20 tons/hr. Underwater pelletizers have been in use for more than 30 years. They surpassed strand lines to become predominant in reactor, polymer finishing, compounding, and recycling operations. In the field of large underwater die-face pelletizers, throughputs exceeding 70 tons/hr have been achieved in the production of polyolefins. Small- and medium-sized underwater pelletizers are used primarily in compounding and in the production of engineering thermoplastics. Their throughput ranges from just a few kg/hr up to approximately 20 tons/hr. Market and machinery development trendsThe economic development of the plastics industry as well as the geographic orientation of many suppliers and processors to the rapidly growing Far Eastern markets have had their impact on machinery and processing innovation. These include options to reduce production costs, simplified equipment controls, and a target-oriented combination of process steps:Quick-cutting head exchange (in less than 5 minutes) has become a "must" for large-sized strand pelletizing equipment; standby equipment and its handling are too expensive, require too many staff, and include too many potential sources of error.The controls of conventional compounding machinery became more informative and reliable; digital line speed displays and pellet length adjustment "on the fly" are standards today.Process water pumps and heat exchangers have been integrated into the base frame of the cooling trough, while classifiers are often attached to the frame of the strand pelletizer.Some years ago a newcomer in the field of underwater pelletizing proposed the suspension of all the system components on an over-head rail in a support frame. Since then other suppliers have adopted that philosophy or plan to do so. The components of small underwater pelletizing systems have been combined on a flexible common support carriage.In many components of pelletizing systems, fast, easy and accurate cleaning possibilities, as well as means for simplifying adjustment operations, have been provided. These actions aim to reduce operator involvement and equipment downtime.Various options of material selection and surface coating, designed to reduce wear and increase product life, have been tested and introduced into pelletizing system components.Direct crystallization of PET, using the specific internal heat of the pellets, has been combined with pelletizing-both underwater and strand-pelletizing-to simplify and reduce failures during PET production.Pelletizing equipment has been adapted to process new compounds and product shapes in high-demand growth markets such as:Long-fiber-reinforced thermoplastics with pellet lengths of 7-50 mmMicropellets, e.g., for color concentrate useWood-plastic composites, for many applications in building and constructionNatural-fiber reinforcements, e.g., for automotive applicationsNanocomposites, e.g., nanoclays in high-barrier resins for food packaging.The pelletizing operation completes raw material production, compounding, and recycling. The basics have been established for many years, but innovation remains dynamic and offers a bright future with a great deal of promising opportunity. Horst Mueller, product manager, Rieter Automatik GmbH