Pellets could be “only” an intermediate product, however size, shape, and consistency matter in subsequent processing operations.
This becomes even more important when thinking about the ever-increasing demands put on compounders. No matter what equipment they now have, it never seems suited for the next challenge. An increasing number of products might require additional capacity. A fresh polymer or additive can be too tough, soft, or corrosive for 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 making use of their pelletizing equipment supplier.
The first step in meeting such challenges starts off with equipment selection. The most common 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 coming from a die that may be quickly cut into pvc compound which can be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt from a die head is transformed into strands that are cut into pellets after cooling and solidification.
Variations of those basic processes might be tailored on the specific input material and product properties in sophisticated compound production. Within both cases, intermediate process steps and various degrees of automation may be incorporated at any stage of your process.
To get the best solution for your personal production requirements, begin with assessing the status quo, in addition to defining future needs. Establish a five-year projection of materials and required capacities. Short-term solutions frequently prove to be more pricey and much less satisfactory after a time period of time. Though almost every pelletizing line in a compounder need to process various products, virtually any system may be optimized only for a tiny range of the full product portfolio.
Consequently, all of those other products will need 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 typically rather small, the flexibleness in the equipment is usually a big issue. Factors include quick access to clean and service and the capability to simply and quickly move from one product to another. 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 may be the first selection for compounding plants. However, the individual layout may vary significantly, because of the demands of throughput, flexibility, and degree of system integration. In strand pelletizing, polymer strands exit the die head and so 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 for the pelletizer, being pulled to the cutting chamber with the feed section with a constant line speed. Inside the pelletizer, strands are cut between a rotor along with a bed knife into roughly cylindrical pellets. These may be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
In the event the requirement is designed for continuous compounding, where fewer product changes are involved and capacities are relatively high, automation could be advantageous for reducing costs while increasing quality. Such an automatic strand pelletizing line may utilize a self-stranding variation of this sort of pelletizer. This is certainly described as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and provide automatic transportation to the pelletizer.
Some polymer compounds are very 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 ideal answer. A perforated conveyor belt takes the strands through the die and conveys them smoothly on the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-allow for a great deal of flexibility.
As soon as the preferred pellet shape is more spherical than cylindrical, the best alternative is surely an underwater hot-face cutter. With a capacity range between from about 20 lb/hr to many tons/hr, this technique is applicable for all materials with thermoplastic behavior. In operation, the polymer melt is split in to a ring of strands that flow through an annular die right into a cutting chamber flooded with process water. A rotating cutting head within the water stream cuts the polymer strands into soft pvc granule, that are immediately conveyed out of the cutting chamber. The pellets are transported being a slurry for the centrifugal dryer, where they may be separated from water by the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The water is filtered, tempered, and recirculated to the method.
The principle components of the device-cutting head with cutting chamber, die plate, and initiate-up valve, all on the common supporting frame-is one major assembly. All of those 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 might be selected from the comprehensive variety of accessories and combined in to a job-specific system.
In every single underwater pelletizing system, a fragile temperature equilibrium exists throughout the cutting chamber and die plate. The die plate is both continuously cooled through the process water and heated by die-head heaters as well as the hot melt flow. Reducing the energy loss through the die plate on the process water results in a much more stable processing condition and increased product quality. To be able to reduce this heat loss, the processor may pick a thermally insulating die plate and/or move to a fluid-heated die.
Many compounds are very abrasive, causing significant wear and tear on contact parts like the spinning blades and filter screens in the centrifugal dryer. Other compounds can be sensitive to mechanical impact and generate excessive dust. For both of these special materials, a new type of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an aura knife, effectively suctioning away from the water. Wear of machine parts and also problems for the pellets can be cut down tremendously in comparison with a direct impact dryer. Due to the short residence time on the belt, some form of post-dewatering drying (including using a fluidized bed) or additional cooling is often required. Great things about this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of parts coming 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.
A few other pelletizing processes are rather unusual within the compounding field. The easiest and cheapest means of reducing plastics to an appropriate size for additional processing generally is a simple grinding operation. However, the resulting particle shape and size are really inconsistent. Some important product properties will also suffer negative influence: The bulk density will drastically decrease as well as the free-flow properties from the bulk would be very poor. That’s why such material will only be appropriate for inferior applications and should be marketed at rather low priced.
Dicing had been a standard size-reduction process ever since the early twentieth century. The significance of this method has steadily decreased for pretty much 30 years and currently constitutes a negligible contribution to the present pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this method of production can be used primarily in a few virgin polymer production, like for polyesters, nylons, and styrenic polymers, and possesses no common application in today’s compounding.
Air-cooled die-face pelletizing is really a process applicable simply for non-sticky products, especially PVC. But this material is more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible numbers of PVC compounds are transformed into pellets.
Water-ring pelletizing can also be an automatic operation. However it is also suitable simply for less sticky materials and finds its main application in polyolefin recycling as well as 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; which is, the larger the product temperature, the low the residual moisture. Some compounds, such as various types of TPE, are sticky, especially at elevated temperatures. This effect might be measured by counting the agglomerates-twins and multiples-within a bulk of pellets.
In a underwater pelletizing system such agglomerates of sticky pellets could be generated in two ways. First, immediately after the cut, the surface temperature of your pellet is just about 50° F on top of the process water temperature, while the core of the pellet remains molten, and the average pellet temperature is merely 35° to 40° F below the melt temperature. If two pellets enter into contact, they deform slightly, creating a contact surface in between the pellets that may be free from process water. Because contact zone, the solidified skin will remelt immediately due to heat transported in 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 as a result of heat transport in the core for 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 might be intensified with smaller pellet size-e.g., micro-pellets-since the ratio of surface to volume increases with smaller diameter.
Pellet agglomeration might be reduced by adding 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 will give you a sense of the maximum practical pellet temperature for your material type and pellet size. Anything dexrpky05 that temperature will raise the amount of agglomerates, and anything below that temperature improves residual moisture.
In certain cases, the pelletizing operation might be expendable. This is true only in applications where virgin polymers can be converted right to finished products-direct extrusion of PET sheet from your polymer reactor, for example. If compounding of additives as well as other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is needed, it is usually wise to know your choices.