Pellets might be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when considering the ever-increasing demands placed on compounders. Irrespective of what equipment they currently have, it never seems suited for the next challenge. Progressively more products might require additional capacity. A new polymer or additive may be too tough, soft, or corrosive to the existing equipment. Or perhaps the job takes a different pellet shape. In these instances, 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 common classification of pelletizing processes involves two classes, differentiated by the state the plastic material at that time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that may be almost immediately cut into pvc compound which are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt coming from a die head is converted into strands which can be cut into pellets after cooling and solidification.
Variations of the basic processes can be tailored to the specific input material and product properties in sophisticated compound production. Within both cases, intermediate process steps as well as other degrees of automation may be incorporated at any stage of the process.
To get the best solution for your production requirements, get started with assessing the status quo, and also defining future needs. Develop 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 almost every pelletizing line with a compounder must process various products, any system could be optimized simply for a little variety of the entire product portfolio.
Consequently, all of those other 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 typically rather small, the flexibility in the equipment is often a serious problem. Factors include easy accessibility to clean and repair and the opportunity to simply and quickly move from a single product to another. Start-up and shutdown of your pelletizing system should involve minimum waste of material.
A line working with a simple water bath for strand cooling often is the first option for compounding plants. However, the person 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 are transported by way of a water bath and cooled. After the strands leave the liquid bath, the residual water is wiped through the surface through a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled into the cutting chamber with the feed section with a constant line speed. Inside the pelletizer, strands are cut from a rotor along with a bed knife into roughly cylindrical pellets. These could be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
In the event the requirement is 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 sort of automatic strand pelletizing line may utilize a self-stranding variation of this type of pelletizer. This is certainly observed as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and give automatic transportation in the pelletizer.
Some polymer compounds are quite fragile and break easily. Other compounds, or some of their ingredients, may be very understanding of moisture. For such materials, the belt-conveyor strand pelletizer is the best 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 the best value of flexibility.
As soon as the preferred pellet shape is more spherical than cylindrical, the ideal alternative is an underwater hot-face cutter. By using a capacity vary from from about 20 lb/hr to a few tons/hr, this method is relevant to all materials with thermoplastic behavior. Functioning, the polymer melt is divided into a ring of strands that flow via an annular die in a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into soft pvc granule, which can be immediately conveyed out of the cutting chamber. The pellets are transported like a slurry for the centrifugal dryer, where they can be separated from water through 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 key components of the machine-cutting head with cutting chamber, die plate, and initiate-up valve, all over a common supporting frame-is one major assembly. All of 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 may be selected from your comprehensive variety of accessories and combined in a job-specific system.
In each and every underwater pelletizing system, a fragile temperature equilibrium exists inside 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. Decreasing the energy loss in the die plate for the process water produces a far 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 change to a fluid-heated die.
Many compounds are usually abrasive, causing significant wear on contact parts like the spinning blades and filter screens from the centrifugal dryer. Other compounds could be understanding of mechanical impact and generate excessive dust. For both of these special materials, a brand new sort 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 could be greatly reduced in contrast to a direct impact dryer. Because of the short residence time on the belt, some kind of post-dewatering drying (such as 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 exposure to pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is necessary.
Various other pelletizing processes are rather unusual inside the compounding field. The simplest and cheapest way of reducing plastics to an appropriate size for additional processing might be a simple grinding operation. However, the resulting particle size and shape are extremely inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease and also the free-flow properties in the bulk would be poor. That’s why such material will only be acceptable for inferior applications and must be marketed at rather low priced.
Dicing was a typical size-reduction process ever since the early twentieth century. The significance of this method has steadily decreased for nearly thirty years and currently makes a negligible contribution to the current pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this technique of production is utilized primarily in certain virgin polymer production, like for polyesters, nylons, and styrenic polymers, and contains 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 a lot more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible amounts of PVC compounds are transformed into pellets.
Water-ring pelletizing is likewise a computerized operation. However it is also suitable just 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 of over pellet shape and throughput volume. By way of example, pellet temperature and residual moisture are inversely proportional; that is, the larger the product temperature, the less the residual moisture. Some compounds, for example many types of TPE, are sticky, especially at elevated temperatures. This effect may 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 above the process water temperature, while the core of the pellet remains molten, and the average pellet temperature is just 35° to 40° F underneath the melt temperature. If two pellets come into contact, they deform slightly, making a contact surface between the pellets which may be free from process water. Because contact zone, the solidified skin will remelt immediately as a result of heat transported from the molten core, and also the pellets will fuse to each other.
Second, after discharge of the transparent pvc compound from your dryer, the pellets’ surface temperature increases as a result of heat transport through the 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 probably intensified with smaller pellet size-e.g., micro-pellets-ever since the ratio of area to volume increases with smaller diameter.
Pellet agglomeration could be reduced with the addition of some wax-like substance to the process water or by powdering the pellet surfaces immediately after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate will give you an idea of the utmost practical pellet temperature for this material type and pellet size. Anything dexrpky05 that temperature will raise the amount of agglomerates, and anything below that temperature improves residual moisture.
In some cases, the pelletizing operation might be expendable. This really is only in applications where virgin polymers might be converted instantly to finished products-direct extrusion of PET sheet from the polymer reactor, as an example. If compounding of additives and also other ingredients adds real value, however, direct conversion is not possible. If pelletizing is needed, it usually is better to know your choices.