Pellets could be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes even more important when considering the ever-increasing demands put on compounders. Irrespective of what equipment they now have, it never seems suited for the next challenge. A lot more products might require additional capacity. A whole new polymer or additive may be too tough, soft, or corrosive for that existing equipment. Or maybe the job takes a different pellet shape. In these instances, compounders need in-depth engineering know-how on processing, and close cooperation using their pelletizing equipment supplier.
Step one in meeting such challenges begins with equipment selection. The most frequent classification of pelletizing processes involves two categories, differentiated by the state the plastic material at the time it’s cut:
•Melt pelletizing (hot cut): Melt originating from a die that may be quickly cut into pvc pellet that are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt coming from a die head is converted into strands which are cut into pellets after cooling and solidification.
Variations of such basic processes could be tailored towards the specific input material and product properties in sophisticated compound production. Within both cases, intermediate process steps as well as other levels of automation could be incorporated at any stage in the process.
For the greatest solution for the production requirements, start with assessing the status quo, along with defining future needs. Create a five-year projection of materials and required capacities. Short-term solutions often prove to be more expensive and much less satisfactory after a time period of time. Though just about every pelletizing line with a compounder will have to process a number of products, any system could be optimized simply for a little variety of the full product portfolio.
Consequently, all of those other products will need to be processed under compromise conditions.
The lot size, in conjunction with the nominal system capacity, will have a very strong influence on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibleness in the equipment can be a big issue. Factors include easy access to clean and service and the opportunity to simply and quickly move from one product to another. Start-up and shutdown of your pelletizing system should involve minimum waste of material.
A line using a simple water bath for strand cooling often is the first choice for compounding plants. However, the average person layout may vary significantly, due to demands of throughput, flexibility, and degree of system integration. In strand pelletizing, polymer strands exit the die head and therefore are transported via a water bath and cooled. Following the strands leave the liquid bath, the residual water is wiped from the surface by means of a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled in the cutting chamber with the feed section at a constant line speed. In the pelletizer, strands are cut from a rotor plus 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 for continuous compounding, where fewer product changes are participating and capacities are relatively high, automation can be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may employ a self-stranding variation of this particular pelletizer. This really is seen as a a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and provide automatic transportation in to the pelletizer.
Some polymer compounds are quite 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 best answer. A perforated conveyor belt takes the strands through the die and conveys them smoothly for the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-enable a good price of flexibility.
If the preferred pellet shape is much more spherical than cylindrical, the very best alternative is definitely an underwater hot-face cutter. Having a capacity vary from from about 20 lb/hr to many tons/hr, this product is applicable to all materials with thermoplastic behavior. In operation, the polymer melt is split into a ring of strands that flow via an annular die right into a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into soft pvc granule, which are immediately conveyed from the cutting chamber. The pellets are transported as 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 returning to the method.
The main elements of the device-cutting head with cutting chamber, die plate, and commence-up valve, all with a common supporting frame-is one major assembly. The rest of the system components, such as 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 your comprehensive selection of accessories and combined in to a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists within the cutting chamber and die plate. The die plate is both continuously cooled with the process water and heated by die-head heaters as well as the hot melt flow. Reducing the energy loss from your die plate for the process water generates a considerably more stable processing condition and increased product quality. To be able to reduce this heat loss, the processor may go with a thermally insulating die plate and/or change to a fluid-heated die.
Many compounds can be abrasive, causing significant wear on contact parts for example the spinning blades and filter screens in the centrifugal dryer. Other compounds could be understanding of mechanical impact and generate excessive dust. For both these special materials, a new form of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an air knife, effectively suctioning away from the water. Wear of machine parts in addition to harm to the pellets may be reduced compared with a positive change dryer. Considering the short residence time about the belt, some type of post-dewatering drying (such as having a fluidized bed) or additional cooling is usually required. Great things about this new non-impact pellet-drying solution are:
•Lower production costs because 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 needed.
Some other pelletizing processes are rather unusual from the compounding field. The best and cheapest way of reducing plastics for an appropriate size for more processing can be quite a simple grinding operation. However, the resulting particle size and shape are incredibly inconsistent. Some important product properties will likely suffer negative influence: The bulk density will drastically decrease and also the free-flow properties from the bulk will be lousy. That’s why such material will only be acceptable for inferior applications and should be marketed at rather low priced.
Dicing ended up being a frequent size-reduction process because the early 20th Century. The importance of this procedure has steadily decreased for almost thirty years and currently creates a negligible contribution to the current pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this technique of production is commonly used primarily in many virgin polymer production, like 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 exclusively for non-sticky products, especially PVC. But this material is a lot 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 also a computerized operation. Yet it is also suitable exclusively for less sticky materials and finds its main application in polyolefin recycling and also in some minor applications in compounding.
Deciding on the best pelletizing process involves consideration greater than pellet shape and throughput volume. By way of example, pellet temperature and residual moisture are inversely proportional; that is certainly, the larger the product temperature, the lower the residual moisture. Some compounds, for example various types 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 might be generated in 2 ways. First, just after the cut, the outer lining temperature of the pellet is only about 50° F over the process water temperature, while the core in the pellet remains molten, as well as the average pellet temperature is only 35° to 40° F below the melt temperature. If two pellets enter into contact, they deform slightly, creating a contact surface between your pellets which may be clear of process water. Because contact zone, the solidified skin will remelt immediately due to heat transported through the molten core, as well as the pellets will fuse to one another.
Second, after discharge of your transparent pvc compound from the dryer, the pellets’ surface temperature increases as a result of heat transport from your core to 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-since the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration could be reduced by having some wax-like substance towards the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate provides you with a sense of the most practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will heighten the amount of agglomerates, and anything below that temperature increases residual moisture.
In some cases, the pelletizing operation may be expendable. This is true only in applications where virgin polymers might be converted directly to finished products-direct extrusion of PET sheet from your polymer reactor, by way of example. If compounding of additives and also other ingredients adds real value, however, direct conversion is not possible. If pelletizing is needed, it will always be advisable to know your choices.