Pellets might be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when contemplating the ever-increasing demands positioned on compounders. Regardless of what equipment they now have, it never seems suited for the next challenge. Progressively more products might require additional capacity. A brand new polymer or additive may be too tough, soft, or corrosive for the existing equipment. Or maybe the job needs a different pellet shape. In such instances, compounders need in-depth engineering know-how on processing, and close cooperation using their pelletizing equipment supplier.
Step one in meeting such challenges starts with equipment selection. The most typical classification of pelletizing processes involves two classes, differentiated by the state of the plastic material back then it’s cut:
•Melt pelletizing (hot cut): Melt originating from a die that is certainly quickly cut into pvc compound that are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt coming from a die head is changed into strands which are cut into pellets after cooling and solidification.
Variations of such basic processes may be tailored on the specific input material and product properties in sophisticated compound production. Within both cases, intermediate process steps as well as other levels of automation can be incorporated at any stage from the process.
To get the best solution to your production requirements, start out with assessing the status quo, and also defining future needs. Build a five-year projection of materials and required capacities. Short-term solutions very often turn out to be more costly and fewer satisfactory after a period of time. Though just about every pelletizing line in a compounder will need to process various products, any system could be optimized only for a small array of the whole product portfolio.
Consequently, the rest of the products will need to be processed under compromise conditions.
The lot size, together with the nominal system capacity, will have a very strong affect on the pelletizing process and machinery selection. Since compounding production lots are generally rather small, the flexibleness in the equipment is often a serious problem. Factors include easy access to clean and repair and the ability to simply and quickly move from a single product to another. Start-up and shutdown of the pelletizing system should involve minimum waste of material.
A line utilizing a simple water bath for strand cooling often may be the first selection for compounding plants. However, the person layout may differ significantly, because of the demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and therefore are transported using a water bath and cooled. Once the strands leave this type of water bath, the residual water is wiped in the surface through a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled in to the cutting chamber through the feed section with a constant line speed. Within the pelletizer, strands are cut from a rotor along with a bed knife into roughly cylindrical pellets. This can be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
When the requirement is designed for continuous compounding, where fewer product changes are participating and capacities are relatively high, automation may be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may use a self-stranding variation of this sort of pelletizer. This is certainly observed as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and give automatic transportation to the pelletizer.
Some polymer compounds can be fragile and break easily. Other compounds, or some of their ingredients, may be very sensitive to moisture. For such materials, the belt-conveyor strand pelletizer is the perfect answer. A perforated conveyor belt takes the strands in the die and conveys them smoothly to the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-permit a good price of flexibility.
When the preferred pellet shape is a lot more spherical than cylindrical, the most effective alternative is definitely an underwater hot-face cutter. With a capacity range from from about 20 lb/hr to a few tons/hr, this method is relevant for all materials with thermoplastic behavior. Operational, the polymer melt is split right into a ring of strands that flow via an annular die in a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into rigid pvc compound, which are immediately conveyed out from the cutting chamber. The pellets are transported as being a slurry to the centrifugal dryer, where they may be separated from water from the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The liquid is filtered, tempered, and recirculated to this process.
The main elements of the program-cutting head with cutting chamber, die plate, and start-up valve, all on a common supporting frame-are one major assembly. All the other system components, including process-water circuit with bypass, cutting chamber discharge, sight glass, centrifugal dryer, belt filter, water pump, heat exchanger, and transport system may be selected from the comprehensive variety of accessories and combined in a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists inside the cutting chamber and die plate. The die plate is both continuously cooled from the process water and heated by die-head heaters and also the hot melt flow. Lowering the energy loss from the die plate to the process water generates a much more stable processing condition and increased product quality. As a way to reduce this heat loss, the processor may choose a thermally insulating die plate or change to a fluid-heated die.
Many compounds are quite abrasive, resulting in significant damage on contact parts for example the spinning blades and filter screens inside the centrifugal dryer. Other compounds might be sensitive to mechanical impact and generate excessive dust. For the two of these special materials, a fresh type of pellet dryer deposits the wet pellets on a perforated conveyor belt that travels across an aura knife, effectively suctioning off of the water. Wear of machine parts along with injury to the pellets might be greatly reduced in comparison with a positive change dryer. Due to the short residence time in the belt, some sort of post-dewatering drying (such as by using a fluidized bed) or additional cooling is normally required. Great things about this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of parts getting into connection with pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is necessary.
Various other pelletizing processes are rather unusual within the compounding field. The most convenient and cheapest way of reducing plastics with an appropriate size for further processing might be a simple grinding operation. However, the resulting particle shape and size are extremely inconsistent. Some important product properties will also suffer negative influence: The bulk density will drastically decrease as well as the free-flow properties in the bulk could be lousy. That’s why such material will only be acceptable for inferior applications and must be marketed at rather low cost.
Dicing was a common size-reduction process ever since the early twentieth century. The necessity of this procedure has steadily decreased for pretty much three decades and currently constitutes a negligible contribution to the present pellet markets.
Underwater strand pelletizing is really a sophisticated automatic process. But this technique of production can be used primarily in many virgin polymer production, for example for polyesters, nylons, and styrenic polymers, and contains no common application in today’s compounding.
Air-cooled die-face pelletizing can be a process applicable simply for non-sticky products, especially PVC. But this product is far more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible numbers of PVC compounds are transformed into pellets.
Water-ring pelletizing can also be an automated operation. Yet it is also suitable just for less sticky materials and finds its main application in polyolefin recycling and also in some minor applications in compounding.
Picking the right pelletizing process involves consideration greater than pellet shape and throughput volume. For example, pellet temperature and residual moisture are inversely proportional; that is certainly, the better the product temperature, the less the residual moisture. Some compounds, like various types of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-in a majority of pellets.
In an underwater pelletizing system such agglomerates of sticky pellets may be generated in just two ways. First, soon after the cut, the top temperature in the pellet is simply about 50° F above the process water temperature, while the core of your pellet continues to be molten, and the average pellet temperature is merely 35° to 40° F beneath the melt temperature. If two pellets enter into contact, they deform slightly, making a contact surface involving the pellets that may be free of process water. In this contact zone, the solidified skin will remelt immediately on account of heat transported from the molten core, along with the pellets will fuse to one another.
Second, after discharge of the pvc compound from the dryer, the pellets’ surface temperature increases because of heat transport from your core towards the surface. If soft TPE pellets are stored in a container, the pellets can deform, warm contact surfaces between individual pellets become larger, and adhesion increases, leading again to agglomerates. This phenomenon may well be intensified with smaller pellet size-e.g., micro-pellets-ever since the ratio of surface to volume increases with smaller diameter.
Pellet agglomeration could be reduced with the addition of some wax-like substance towards the process water or by powdering the pellet surfaces just after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate gives you an idea of the highest practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will increase the amount of agglomerates, and anything below that temperature boosts residual moisture.
In certain cases, the pelletizing operation might be expendable. This is correct only in applications where virgin polymers can be converted straight to finished products-direct extrusion of PET sheet coming from a polymer reactor, by way of example. If compounding of additives along with other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is important, it will always be wise to know your options.