Pellets could be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when contemplating the ever-increasing demands put on compounders. Regardless of what equipment they now have, it never seems suited for the following challenge. An increasing number of products might require additional capacity. A new polymer or additive might be too tough, soft, or corrosive for your existing equipment. Or perhaps the job needs a different pellet shape. In these cases, compounders need in-depth engineering know-how on processing, and close cooperation making use of their pelletizing equipment supplier.
The first step in meeting such challenges begins with equipment selection. The most prevalent 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 provided by a die which is very quickly cut into pvc compound which can be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt provided by a die head is converted into strands which can be cut into pellets after cooling and solidification.
Variations of these basic processes can be tailored on the specific input material and product properties in sophisticated compound production. In both cases, intermediate process steps and various degrees of automation may be incorporated at any stage of the process.
For the greatest solution to your production requirements, begin with assessing the status quo, as well as defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions often end up being higher priced and less satisfactory after a time period of time. Though virtually every pelletizing line in a compounder need to process many different products, any system might be optimized just for a tiny variety of the full product portfolio.
Consequently, the rest of the products will need to be processed under compromise conditions.
The lot size, together with the nominal system capacity, will possess a strong influence on the pelletizing process and machinery selection. Since compounding production lots are generally rather small, the flexibleness of the equipment is generally a big issue. Factors include easy access to clean and repair and the cabability to simply and quickly move from a single product to another. Start-up and shutdown from the pelletizing system should involve minimum waste of material.
A line utilizing a simple water bath for strand cooling often is definitely the first choice for compounding plants. However, the individual layout can vary significantly, due to demands of throughput, flexibility, and standard of system integration. In strand pelletizing, polymer strands exit the die head and they are transported by way of a water bath and cooled. Once the strands leave the liquid bath, the residual water is wiped in the surface by means of a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled in to the cutting chamber by the feed section with a constant line speed. From the pelletizer, strands are cut from a rotor and a bed knife into roughly cylindrical pellets. These could be put through post-treatment like classifying, additional cooling, and drying, plus conveying.
In case the requirement is made for continuous compounding, where fewer product changes come to mind and capacities are relatively high, automation could 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 really is observed as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and supply automatic transportation in the pelletizer.
Some polymer compounds are very 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 ideal 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 a good price of flexibility.
Once the preferred pellet shape is much more spherical than cylindrical, the most effective alternative is undoubtedly an underwater hot-face cutter. Using a capacity range from from about 20 lb/hr to a few tons/hr, this system is applicable to all materials with thermoplastic behavior. Operational, the polymer melt is divided in a ring of strands that flow via an annular die in to a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into soft pvc granule, that happen to be immediately conveyed out of your cutting chamber. The pellets are transported as being a slurry for the centrifugal dryer, where they may be separated from water through the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The liquid is filtered, tempered, and recirculated to the procedure.
The main parts of the system-cutting head with cutting chamber, die plate, and commence-up valve, all on a common supporting frame-is 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 could be selected coming from a comprehensive array of accessories and combined right into a job-specific system.
In each and every underwater pelletizing system, a fragile temperature equilibrium exists within 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. Decreasing the energy loss through the die plate towards the process water results in a much more stable processing condition and increased product quality. So that you can reduce this heat loss, the processor may choose a thermally insulating die plate and/or switch to a fluid-heated die.
Many compounds can be abrasive, leading to significant damage on contact parts for example the spinning blades and filter screens from the centrifugal dryer. Other compounds may be understanding of mechanical impact and generate excessive dust. For the two of these special materials, a new kind of pellet dryer deposits the wet pellets with a perforated conveyor belt that travels across an aura knife, effectively suctioning off of the water. Wear of machine parts in addition to damage to the pellets can be cut down tremendously in contrast to an effect dryer. Because of the short residence time on the belt, some kind of post-dewatering drying (such as using a fluidized bed) or additional cooling is generally required. Advantages of this new non-impact pellet-drying solution are:
•Lower production costs as a result of long lifetime of most parts getting into experience of pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is essential.
Some other pelletizing processes are rather unusual from the compounding field. The easiest and cheapest strategy for reducing plastics to a appropriate size for further processing generally is a simple grinding operation. However, the resulting particle shape and size are incredibly inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease and the free-flow properties of your bulk would be very poor. That’s why such material are only appropriate for inferior applications and should be marketed at rather inexpensive.
Dicing ended up being a standard size-reduction process because the early 20th Century. The significance of this method has steadily decreased for nearly three decades and currently creates a negligible contribution to the current pellet markets.
Underwater strand pelletizing is actually a sophisticated automatic process. But this procedure of production is utilized primarily in many virgin polymer production, including 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 simply 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 transformed into pellets.
Water-ring pelletizing is additionally a computerized operation. However 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.
Picking the right 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 greater the product temperature, the low the residual moisture. Some compounds, including various kinds of TPE, are sticky, especially at elevated temperatures. This effect can be measured by counting the agglomerates-twins and multiples-in a majority of pellets.
In a underwater pelletizing system such agglomerates of sticky pellets could be generated in 2 ways. First, soon after the cut, the outer lining temperature of the pellet is just about 50° F above the process temperature of water, even though the core of your pellet continues to be molten, and also the average pellet temperature is merely 35° to 40° F underneath the melt temperature. If two pellets come into contact, they deform slightly, building a contact surface between the pellets that may be without any process water. In that contact zone, the solidified skin will remelt immediately as a result of heat transported through the molten core, and also the pellets will fuse to each other.
Second, after discharge of the pvc compound from your dryer, the pellets’ surface temperature increases as a result of heat transport from your core for 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 is most likely intensified with smaller pellet size-e.g., micro-pellets-ever since the ratio of area to volume increases with smaller diameter.
Pellet agglomeration can be reduced with the help of some wax-like substance for the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing a variety of pelletizing test runs at consistent throughput rate will provide you with an idea of the utmost practical pellet temperature for this 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 really is only in applications where virgin polymers can be converted right to finished products-direct extrusion of PET sheet from a polymer reactor, as an example. If compounding of additives as well as other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is important, it will always be best to know your choices.