Pellets can be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes much more important when contemplating the ever-increasing demands positioned on compounders. Regardless of what equipment they now have, it never seems suited for the upcoming challenge. An increasing number of products might need additional capacity. A brand new polymer or additive could be too tough, soft, or corrosive to the existing equipment. Or maybe the job needs 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.
Step one in meeting such challenges starts with equipment selection. The most frequent classification of pelletizing processes involves two classes, differentiated by the condition of the plastic material during the time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die which is very quickly cut into pvc pellet that happen to be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt originating from a die head is converted into strands which can be cut into pellets after cooling and solidification.
Variations of such basic processes can be tailored on the specific input material and product properties in sophisticated compound production. In both cases, intermediate process steps and other degrees of automation could be incorporated at any stage from the process.
To find the best solution for the production requirements, start with assessing the status quo, along with defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions very often end up being more costly and less satisfactory after a period of time. Though just about every pelletizing line with a compounder will have to process many different products, virtually any system could be optimized just for a little range of the whole product portfolio.
Consequently, all of those other products will have to be processed under compromise conditions.
The lot size, along with the nominal system capacity, will have a very strong impact on the pelletizing process and machinery selection. Since compounding production lots are generally rather small, the flexibility of your equipment is generally a serious problem. Factors include easy access for cleaning and service and the opportunity to simply and quickly move from a product to the next. Start-up and shutdown from the pelletizing system should involve minimum waste of material.
A line using a simple water bath for strand cooling often will be the first choice for compounding plants. However, the patient layout may differ significantly, due to the demands of throughput, flexibility, and standard of system integration. In strand pelletizing, polymer strands exit the die head and therefore are transported through a water bath and cooled. Following the strands leave this type of water bath, the residual water is wiped from your surface by means of a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled in 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. These could be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
If the requirement is perfect for continuous compounding, where fewer product changes come to mind and capacities are relatively high, automation might be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may utilize a self-stranding variation of this kind of pelletizer. This can be described as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and offer automatic transportation into the pelletizer.
Some polymer compounds are quite fragile and break easily. Other compounds, or some of their ingredients, could be very sensitive to moisture. For such materials, the belt-conveyor strand pelletizer is the best answer. A perforated conveyor belt takes the strands in 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 price of flexibility.
If the preferred pellet shape is far more spherical than cylindrical, the very best alternative is surely an underwater hot-face cutter. Using a capacity range from from about 20 lb/hr to a number of tons/hr, this method is applicable for all materials with thermoplastic behavior. In operation, the polymer melt is split in a ring of strands that flow through 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, which are immediately conveyed from the cutting chamber. The pellets are transported like a slurry for the centrifugal dryer, where they can be separated from water with the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. This type of water is filtered, tempered, and recirculated back to this process.
The principle elements of the program-cutting head with cutting chamber, die plate, and start-up valve, all over a common supporting frame-are one major assembly. All of 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 array of accessories and combined in a job-specific system.
In each and 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 along with the hot melt flow. Decreasing the energy loss in the die plate for the process water generates a considerably 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 and/or change to a fluid-heated die.
Many compounds are usually abrasive, resulting in significant damage on contact parts like the spinning blades and filter screens within the centrifugal dryer. Other compounds might be sensitive to mechanical impact and generate excessive dust. For both these special materials, a new kind of pellet dryer deposits the wet pellets on a perforated conveyor belt that travels across an air knife, effectively suctioning off of the water. Wear of machine parts along with damage to the pellets may be cut down tremendously compared with an impact dryer. Given the short residence time on the belt, some form of post-dewatering drying (for example with a fluidized bed) or additional cooling is normally required. Benefits associated with this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of parts getting into contact with pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is important.
Various other pelletizing processes are rather unusual inside the compounding field. The most convenient and cheapest means of reducing plastics to an appropriate size for further processing might be a simple grinding operation. However, the resulting particle shape and size are really inconsistent. Some important product properties may also suffer negative influence: The bulk density will drastically decrease and also the free-flow properties of your bulk can be bad. That’s why such material are only appropriate for inferior applications and should be marketed at rather affordable.
Dicing was a frequent size-reduction process considering that the early 20th Century. The importance of this process has steadily decreased for almost three decades and currently creates a negligible contribution to the present pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this technique of production is utilized primarily in certain virgin polymer production, such as for polyesters, nylons, and styrenic polymers, and it has no common application in today’s compounding.
Air-cooled die-face pelletizing is a process applicable just for non-sticky products, especially PVC. But this product is much more commonly compounded in batch mixers with cooling and heating and discharged as dry-blends. Only negligible amounts of PVC compounds are turned into pellets.
Water-ring pelletizing is likewise an automatic operation. However 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.
Choosing the right pelletizing process involves consideration of more than pellet shape and throughput volume. For instance, pellet temperature and residual moisture are inversely proportional; that is certainly, the higher the product temperature, the lower the residual moisture. Some compounds, such as many types of TPE, are sticky, especially at elevated temperatures. This effect can be measured by counting the agglomerates-twins and multiples-in a bulk of pellets.
Inside an underwater pelletizing system such agglomerates of sticky pellets can be generated in 2 ways. First, right after the cut, the top temperature in the pellet is simply about 50° F on top of the process temperature of water, even though the core from the pellet is still molten, and the average pellet temperature is simply 35° to 40° F underneath the melt temperature. If two pellets come into contact, they deform slightly, developing a contact surface involving the pellets that could be without any process water. In this contact zone, the solidified skin will remelt immediately on account of heat transported from the molten core, and the pellets will fuse to one another.
Second, after discharge of the transparent pvc compound from the dryer, the pellets’ surface temperature increases due to heat transport from the 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 might be intensified with smaller pellet size-e.g., micro-pellets-because the ratio of area to volume increases with smaller diameter.
Pellet agglomeration can be reduced by having some wax-like substance towards the process water or by powdering the pellet surfaces soon after the pellet dryer.
Performing numerous pelletizing test runs at consistent throughput rate will give you a concept of the most practical pellet temperature for that material type and pellet size. Anything dexrpky05 that temperature will heighten the quantity of agglomerates, and anything below that temperature boosts residual moisture.
In certain cases, the pelletizing operation may be expendable. This is correct only in applications where virgin polymers could be converted right to finished products-direct extrusion of PET sheet from your polymer reactor, for example. If compounding of additives along with other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is necessary, it usually is wise to know your options.