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Pellets could be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.

This becomes even more important when thinking about the ever-increasing demands positioned on compounders. No matter what equipment they now have, it never seems suited for the upcoming challenge. Progressively more products may require additional capacity. A whole new polymer or additive can be too tough, soft, or corrosive to the existing equipment. Or maybe the job takes a different pellet shape. In these cases, compounders need in-depth engineering know-how on processing, and close cooperation because of their pelletizing equipment supplier.

The first task in meeting such challenges starts with equipment selection. The most typical classification of pelletizing processes involves two classes, differentiated by the condition of the plastic material at that time it’s cut:

•Melt pelletizing (hot cut): Melt from a die that is almost immediately cut into pvc compound that are conveyed and cooled by liquid or gas;

•Strand pelletizing (cold cut): Melt coming from a die head is transformed into strands that happen to be cut into pellets after cooling and solidification.

Variations of those basic processes can be tailored for the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps as well as other degrees of automation may be incorporated at any stage in the process.

To find the best solution for your production requirements, start out with assessing the status quo, along with defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions often end up being more pricey and less satisfactory after a period of time. Though virtually every pelletizing line with a compounder will need to process many different products, any given system can be optimized just for a little selection of the complete product portfolio.

Consequently, the rest of the products will need to be processed under compromise conditions.

The lot size, in conjunction with the nominal system capacity, will have got a strong influence on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibility in the equipment is often a serious problem. Factors include easy access for cleaning and repair and the ability to simply and quickly move from a product to another. Start-up and shutdown of your pelletizing system should involve minimum waste of material.

A line utilizing a simple water bath for strand cooling often will be the first selection for compounding plants. However, the person layout can vary significantly, as a result of demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and they are transported via a water bath and cooled. Once the strands leave the water bath, the residual water is wiped in the surface by means of a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled in to the cutting chamber with the feed section at a constant line speed. In the pelletizer, strands are cut from a rotor and a bed knife into roughly cylindrical pellets. These can be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.

If the requirement is designed for continuous compounding, where fewer product changes are participating and capacities are relatively high, automation might be advantageous for reducing costs while increasing quality. This type of automatic strand pelletizing line may use a self-stranding variation of this type of pelletizer. This really is characterized by a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and provide automatic transportation in the pelletizer.

Some polymer compounds are quite fragile and break easily. Other compounds, or some of their ingredients, could be very understanding of moisture. For such materials, the belt-conveyor strand pelletizer is the best answer. A perforated conveyor belt takes the strands through the die and conveys them smoothly towards the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-allow for the best value of flexibility.

Once the preferred pellet shape is much more spherical than cylindrical, the very best alternative is surely an underwater hot-face cutter. With a capacity range between from about 20 lb/hr to a number of tons/hr, this product is relevant to all of materials with thermoplastic behavior. In operation, the polymer melt is split in a ring of strands that flow via an annular die into a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into upvc compound, that happen to be immediately conveyed out of the cutting chamber. The pellets are transported as being a slurry on 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 straight back to the procedure.

The key components of the machine-cutting head with cutting chamber, die plate, and start-up valve, all on the common supporting frame-is one major assembly. The rest of the system components, including process-water circuit with bypass, cutting chamber discharge, sight glass, centrifugal dryer, belt filter, water pump, heat exchanger, and transport system can be selected from your comprehensive variety of accessories and combined in to 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 also the hot melt flow. Lowering the energy loss from your die plate towards the process water generates a a lot more stable processing condition and increased product quality. As a way to reduce this heat loss, the processor may pick a thermally insulating die plate or change to a fluid-heated die.

Many compounds are usually abrasive, contributing to significant wear on contact parts including the spinning blades and filter screens from the centrifugal dryer. Other compounds may be sensitive to mechanical impact and generate excessive dust. For both of these special materials, a 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 problems for the pellets may be cut down tremendously in contrast to an effect dryer. Given the short residence time on the belt, some kind of post-dewatering drying (for example having a fluidized bed) or additional cooling is normally required. Benefits associated with this new non-impact pellet-drying solution are:

•Lower production costs as a result of long lifetime of parts coming 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.

Some other pelletizing processes are rather unusual inside the compounding field. The best and cheapest way of reducing plastics with an appropriate size for further processing might be a simple grinding operation. However, the resulting particle size and shape are incredibly inconsistent. Some important product properties may also suffer negative influence: The bulk density will drastically decrease as well as the free-flow properties of the bulk would be very poor. That’s why such material are only suitable for inferior applications and should be marketed at rather low cost.

Dicing ended up being a frequent size-reduction process since the early twentieth century. The importance of this method has steadily decreased for nearly three decades and currently makes a negligible contribution to the present pellet markets.

Underwater strand pelletizing is a sophisticated automatic process. But this procedure of production is commonly used primarily in certain virgin polymer production, like for polyesters, nylons, and styrenic polymers, and contains no common application in today’s compounding.

Air-cooled die-face pelletizing is really a process applicable only for non-sticky products, especially PVC. But this product is a lot more commonly compounded in batch mixers with heating and air conditioning and discharged as dry-blends. Only negligible amounts of PVC compounds are turned into pellets.

Water-ring pelletizing is likewise an automatic operation. But it is also suitable only 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 of more than pellet shape and throughput volume. By way of example, pellet temperature and residual moisture are inversely proportional; that may be, the better the product temperature, the less the residual moisture. Some compounds, like many types of TPE, are sticky, especially at elevated temperatures. This effect may be measured by counting the agglomerates-twins and multiples-in the bulk of pellets.

Inside an underwater pelletizing system such agglomerates of sticky pellets may be generated in two ways. First, soon after the cut, the top temperature in the pellet is only about 50° F above the process water temperature, while the core of your pellet remains molten, and also the average pellet temperature is merely 35° to 40° F underneath the melt temperature. If two pellets enter in to contact, they deform slightly, creating a contact surface between your pellets which might be clear of process water. Because contact zone, the solidified skin will remelt immediately on account of heat transported through the molten core, as well as the pellets will fuse to one another.

Second, after discharge in the clear pvc granule in the dryer, the pellets’ surface temperature increases due to heat transport from the core for the surface. If soft TPE pellets are kept in a container, the pellets can deform, warm contact surfaces between individual pellets become larger, and adhesion increases, leading again to agglomerates. This phenomenon might 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 soon after the pellet dryer.

Performing a variety of pelletizing test runs at consistent throughput rate will provide you with a concept of the utmost practical pellet temperature for this material type and pellet size. Anything dexrpky05 that temperature will raise the level of agglomerates, and anything below that temperature improves residual moisture.

In a few cases, the pelletizing operation could be expendable. This is correct only in applications where virgin polymers can be converted straight to finished products-direct extrusion of PET sheet from a polymer reactor, for example. If compounding of additives and other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is needed, it will always be better to know your options.