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Pellets might be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes a lot more important when considering the ever-increasing demands positioned on compounders. Irrespective of what equipment they now have, it never seems suited for the next challenge. Progressively more products may need additional capacity. A brand new polymer or additive may be too tough, soft, or corrosive for your existing equipment. Or perhaps the job demands a different pellet shape. In such cases, compounders need in-depth engineering know-how on processing, and close cooperation with their pelletizing equipment supplier.
The initial step in meeting such challenges starts with equipment selection. The most frequent classification of pelletizing processes involves two classes, differentiated by the state the plastic material at the time it’s cut:
•Melt pelletizing (hot cut): Melt originating from a die that is certainly almost immediately cut into pvc compound that happen to be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt coming from a die head is converted into strands that are cut into pellets after cooling and solidification.
Variations of those basic processes could be tailored towards the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps as well as other levels of automation might be incorporated at any stage of your process.
To find the best solution for the production requirements, begin with assessing the status quo, along with defining future needs. Establish a five-year projection of materials and required capacities. Short-term solutions fairly often prove to be more costly and less satisfactory after a period of time. Though virtually every pelletizing line with a compounder need to process a variety of products, any system can be optimized simply for a small range of the whole product portfolio.
Consequently, the rest of the products will have to be processed under compromise conditions.
The lot size, together with the nominal system capacity, will have got a strong impact on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibleness of your equipment is generally a serious problem. Factors include comfortable access to clean and service and the opportunity 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 working with a simple water bath for strand cooling often may be the first option for compounding plants. However, the average person layout may differ significantly, due to demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and so are transported by way of a water bath and cooled. Once the strands leave water bath, the residual water is wiped through the surface through a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled into the cutting chamber from the feed section in a constant line speed. In the pelletizer, strands are cut from a rotor and a bed knife into roughly cylindrical pellets. This can be put through post-treatment like classifying, additional cooling, and drying, plus conveying.
In case the requirement is perfect for continuous compounding, where fewer product changes are involved and capacities are relatively high, automation can be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may use a self-stranding variation of this kind of pelletizer. This is observed as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and give automatic transportation in the pelletizer.
Some polymer compounds are quite fragile and break easily. Other compounds, or a selection of their ingredients, could be very understanding of moisture. For such materials, the belt-conveyor strand pelletizer is the greatest answer. A perforated conveyor belt takes the strands from your die and conveys them smoothly towards the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-enable the best value of flexibility.
When the preferred pellet shape is much more spherical than cylindrical, the most effective alternative is definitely an underwater hot-face cutter. By using a capacity cover anything from from about 20 lb/hr to many tons/hr, this technique is applicable to any or all materials with thermoplastic behavior. Operational, the polymer melt is split in to a ring of strands that flow through an annular die in a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into rigid pvc compound, which can be immediately conveyed out of the cutting chamber. The pellets are transported as a slurry towards the centrifugal dryer, where they are separated from water by the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. Water is filtered, tempered, and recirculated back to the method.
The primary aspects of the device-cutting head with cutting chamber, die plate, and initiate-up valve, all on a common supporting frame-are one major assembly. All the other system components, like 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 selection of accessories and combined right into a job-specific system.
In just about every underwater pelletizing system, a fragile temperature equilibrium exists in the cutting chamber and die plate. The die plate is both continuously cooled from the process water and heated by die-head heaters and the hot melt flow. Lowering the energy loss from your die plate on the process water produces a much more stable processing condition and increased product quality. In order to reduce this heat loss, the processor may select a thermally insulating die plate or switch to a fluid-heated die.
Many compounds are usually abrasive, causing significant deterioration on contact parts such as the spinning blades and filter screens from the centrifugal dryer. Other compounds might be responsive to mechanical impact and generate excessive dust. For these two special materials, a whole new kind of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an aura knife, effectively suctioning from the water. Wear of machine parts and also damage to the pellets can be greatly reduced in contrast to a positive change dryer. Considering the short residence time on the belt, some kind of post-dewatering drying (like using a fluidized bed) or additional cooling is usually required. Great things about this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of all parts coming into connection with pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is needed.
Another pelletizing processes are rather unusual inside the compounding field. The easiest and cheapest strategy for reducing plastics to an appropriate size for more processing might be a simple grinding operation. However, the resulting particle size and shape are extremely inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease and the free-flow properties in the bulk will be poor. That’s why such material are only appropriate for inferior applications and should be marketed at rather inexpensive.
Dicing have been a common size-reduction process considering that the early twentieth century. The value of this method has steadily decreased for up to 3 decades and currently creates a negligible contribution to the current pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this procedure of production is used primarily in many virgin polymer production, including for polyesters, nylons, and styrenic polymers, and has no common application in today’s compounding.
Air-cooled die-face pelletizing is actually a process applicable simply for non-sticky products, especially PVC. But this product is a lot more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible levels of PVC compounds are transformed into pellets.
Water-ring pelletizing can also be an automatic operation. Yet it is also suitable simply for less sticky materials and finds its main application in polyolefin recycling and also in some minor applications in compounding.
Selecting the best pelletizing process involves consideration in excess of 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, including various kinds of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-inside a bulk of pellets.
In an underwater pelletizing system such agglomerates of sticky pellets could be generated in just two ways. First, immediately after the cut, the top temperature in the pellet is merely about 50° F higher than the process water temperature, as the core from the pellet remains to be molten, and also the average pellet temperature is only 35° to 40° F below the melt temperature. If two pellets come into contact, they deform slightly, making a contact surface in between the pellets which might be free of process water. In that contact zone, the solidified skin will remelt immediately because of heat transported from the molten core, along with the pellets will fuse to each other.
Second, after discharge of the clear pvc granule from your dryer, the pellets’ surface temperature increases on account of heat transport from your core for the surface. If soft TPE pellets are held 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-since the ratio of area to volume increases with smaller diameter.
Pellet agglomeration may be reduced with the help of some wax-like substance to the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing a number of pelletizing test runs at consistent throughput rate will provide you with a solid idea of the highest practical pellet temperature for that material type and pellet size. Anything dexrpky05 that temperature will raise the level of agglomerates, and anything below that temperature will increase residual moisture.
In certain cases, the pelletizing operation can be expendable. This really is only in applications where virgin polymers could be converted instantly to finished products-direct extrusion of PET sheet from your polymer reactor, by way of example. If compounding of additives and other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is necessary, it is always best to know the options.