Pellets can be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes a lot more important when it comes to the ever-increasing demands placed on compounders. Whatever equipment they now have, it never seems suited for the next challenge. An increasing number of products might need additional capacity. A new polymer or additive may be too tough, soft, or corrosive for that 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 because of their pelletizing equipment supplier.
The initial step in meeting such challenges begins with equipment selection. The most typical classification of pelletizing processes involves two categories, differentiated by the state of the plastic material back then it’s cut:
•Melt pelletizing (hot cut): Melt coming from a die that is very quickly cut into pvc compound which can be 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 could be tailored to the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps as well as other degrees of automation could be incorporated at any stage in the process.
To find the best solution for the production requirements, get started with assessing the status quo, as well as defining future needs. Establish a five-year projection of materials and required capacities. Short-term solutions frequently end up being more costly and much less satisfactory after a period of time. Though virtually every pelletizing line with a compounder need to process many different products, virtually any system may be optimized exclusively for a compact selection of the whole product portfolio.
Consequently, all the other products will need to be processed under compromise conditions.
The lot size, in combination with the nominal system capacity, will possess a strong effect on the pelletizing process and machinery selection. Since compounding production lots tend to be rather small, the flexibility from the equipment is often a serious problem. Factors include easy access for cleaning and repair and the cabability to simply and quickly move in one product to the next. Start-up and shutdown of your pelletizing system should involve minimum waste of material.
A line by using a simple water bath for strand cooling often will be the first choice for compounding plants. However, the person layout can differ significantly, due to the demands of throughput, flexibility, and degree of system integration. In strand pelletizing, polymer strands exit the die head and are transported using a water bath and cooled. Right after the strands leave the liquid bath, the residual water is wiped in the surface through a suction air knife. The dried and solidified strands are transported towards the pelletizer, being pulled into the cutting chamber with the feed section with a constant line speed. Within the pelletizer, strands are cut between a rotor as well as a bed knife into roughly cylindrical pellets. These could be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
In case the requirement is perfect for continuous compounding, where fewer product changes come to mind 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 seen as a a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and offer automatic transportation to the pelletizer.
Some polymer compounds are quite fragile and break easily. Other compounds, or a selection of their ingredients, could be very sensitive to moisture. For such materials, the belt-conveyor strand pelletizer is the perfect answer. A perforated conveyor belt takes the strands from 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-enable a good deal of flexibility.
When the preferred pellet shape is much more spherical than cylindrical, the most effective alternative is an underwater hot-face cutter. Having a capacity range from from about 20 lb/hr to a few tons/hr, this method is relevant to any or all materials with thermoplastic behavior. Functioning, 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 rigid pvc compound, that happen to be immediately conveyed out from the cutting chamber. The pellets are transported like a slurry towards the centrifugal dryer, where they are separated from water from the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. Water is filtered, tempered, and recirculated straight back to the procedure.
The key parts of the program-cutting head with cutting chamber, die plate, and initiate-up valve, all on a common supporting frame-is 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 could be selected from the comprehensive array of accessories and combined right into a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists in the cutting chamber and die plate. The die plate is both continuously cooled by the process water and heated by die-head heaters as well as the hot melt flow. Reducing the energy loss in the die plate to the process water produces a considerably more stable processing condition and increased product quality. To be able to reduce this heat loss, the processor may choose a thermally insulating die plate or switch to a fluid-heated die.
Many compounds are usually abrasive, contributing to significant damage on contact parts for example the spinning blades and filter screens inside the centrifugal dryer. Other compounds can be responsive to mechanical impact and generate excessive dust. For these two special materials, a whole new form of pellet dryer deposits the wet pellets over a perforated conveyor belt that travels across an air knife, effectively suctioning from the water. Wear of machine parts in addition to injury to the pellets might be greatly reduced compared to a positive change dryer. Considering the short residence time on the belt, some type of post-dewatering drying (for example by using a fluidized bed) or additional cooling is normally required. Advantages of this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of most parts getting into exposure to pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is necessary.
Various other pelletizing processes are rather unusual inside the compounding field. The easiest and cheapest strategy for reducing plastics with an appropriate size for more processing may well be a simple grinding operation. However, the resulting particle shape and size are incredibly inconsistent. Some important product properties will also suffer negative influence: The bulk density will drastically decrease as well as the free-flow properties of your bulk could be lousy. That’s why such material are only suitable for inferior applications and should be marketed at rather low priced.
Dicing was a frequent size-reduction process since the early twentieth century. The necessity of this technique has steadily decreased for up to three decades and currently constitutes a negligible contribution to the present pellet markets.
Underwater strand pelletizing is really a sophisticated automatic process. But this procedure of production can be used primarily in a few virgin polymer production, including for polyesters, nylons, and styrenic polymers, and contains no common application in today’s compounding.
Air-cooled die-face pelletizing is actually a process applicable only for non-sticky products, especially PVC. But this material is more commonly compounded in batch mixers with cooling and heating and discharged as dry-blends. Only negligible levels of PVC compounds are transformed into pellets.
Water-ring pelletizing is likewise an automated operation. Yet it is also suitable exclusively for less sticky materials and finds its main application in polyolefin recycling as well as in some minor applications in compounding.
Deciding on the best pelletizing process involves consideration greater than pellet shape and throughput volume. For instance, pellet temperature and residual moisture are inversely proportional; which is, the larger the product temperature, the reduced the residual moisture. Some compounds, for example various kinds of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-in the 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 surface temperature from the pellet is simply about 50° F on top of the process water temperature, even though the core of the pellet is still molten, as well as the average pellet temperature is merely 35° to 40° F beneath the melt temperature. If two pellets come into contact, they deform slightly, developing a contact surface between the pellets that may be free of process water. Because contact zone, the solidified skin will remelt immediately on account of heat transported from your molten core, along with the pellets will fuse to one another.
Second, after discharge of your pvc compound in the dryer, the pellets’ surface temperature increases because of heat transport from your core on the surface. If soft TPE pellets are saved 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-because the ratio of area to volume increases with smaller diameter.
Pellet agglomeration may be reduced with the addition of some wax-like substance on 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 gives you an idea of the highest practical pellet temperature for that material type and pellet size. Anything dexrpky05 that temperature will raise the quantity of agglomerates, and anything below that temperature increases residual moisture.
In certain cases, the pelletizing operation can be expendable. This is correct only in applications where virgin polymers can be converted instantly to finished products-direct extrusion of PET sheet from the polymer reactor, for example. If compounding of additives and other ingredients adds real value, however, direct conversion is not possible. If pelletizing is important, it is always best to know the options.