Pellets could be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes a lot more important when thinking about the ever-increasing demands put on compounders. Whatever equipment they now have, it never seems suited for the upcoming challenge. A lot more products may need additional capacity. A whole new polymer or additive might be too tough, soft, or corrosive for the existing equipment. Or perhaps the job demands 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 starts off with equipment selection. The most frequent classification of pelletizing processes involves two classes, differentiated by the state the plastic material back then it’s cut:
•Melt pelletizing (hot cut): Melt from a die which is almost immediately cut into pvc pellet which are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt provided by a die head is changed into strands that are cut into pellets after cooling and solidification.
Variations of those basic processes might be tailored towards the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps and different levels of automation could be incorporated at any stage of the process.
To get the best solution for the production requirements, get started with assessing the status quo, as well as defining future needs. Build a five-year projection of materials and required capacities. Short-term solutions often turn out to be more costly and much less satisfactory after a period of time. Though almost every pelletizing line in a compounder must process various products, any given system may be optimized simply for a tiny variety of the entire product portfolio.
Consequently, all the other products will have to be processed under compromise conditions.
The lot size, in conjunction 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 flexibility in the equipment is usually a serious problem. Factors include easy accessibility 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 by using a simple water bath for strand cooling often is definitely the first choice for compounding plants. However, the average person layout may differ significantly, due to demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and therefore are transported through 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 to the pelletizer, being pulled in the cutting chamber by the feed section in a constant line speed. Within the pelletizer, strands are cut from a rotor plus a bed knife into roughly cylindrical pellets. These could be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
If the requirement is for continuous compounding, where fewer product changes come to mind and capacities are relatively high, automation may 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 is characterized by 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 usually fragile and break easily. Other compounds, or some of their ingredients, may be very responsive to moisture. For such materials, the belt-conveyor strand pelletizer is the perfect answer. A perforated conveyor belt takes the strands from your die and conveys them smoothly for 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.
When the preferred pellet shape is far more spherical than cylindrical, the very best alternative is an underwater hot-face cutter. By using a capacity range between from about 20 lb/hr to many tons/hr, this method is relevant to all of materials with thermoplastic behavior. Operational, the polymer melt is split into a ring of strands that flow using an annular die in a cutting chamber flooded with process water. A rotating cutting head within the water stream cuts the polymer strands into upvc compound, which are immediately conveyed out of the cutting chamber. The pellets are transported as being a slurry to 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 to the procedure.
The primary elements of the machine-cutting head with cutting chamber, die plate, and initiate-up valve, all on the common supporting frame-are one major assembly. All 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 a comprehensive range of accessories and combined into a job-specific system.
In every single underwater pelletizing system, a fragile temperature equilibrium exists in the cutting chamber and die plate. The die plate is both continuously cooled with the process water and heated by die-head heaters and also the hot melt flow. Lowering the energy loss through the die plate on the process water results in a a lot more stable processing condition and increased product quality. To be able to reduce this heat loss, the processor may select a thermally insulating die plate and switch to a fluid-heated die.
Many compounds are usually abrasive, resulting in significant deterioration on contact parts like the spinning blades and filter screens inside 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 aura knife, effectively suctioning off the water. Wear of machine parts and also damage to the pellets could be greatly reduced in comparison with a positive change dryer. Due to the short residence time on the belt, some sort 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 due to long lifetime of all the parts getting into experience of pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is essential.
Another pelletizing processes are rather unusual in the compounding field. The most convenient and cheapest means of reducing plastics with an appropriate size for more processing generally is a simple grinding operation. However, the resulting particle shape and size are really inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease along with the free-flow properties in the bulk will be lousy. That’s why such material are only appropriate for inferior applications and must be marketed at rather low priced.
Dicing had been a standard size-reduction process considering that the early 20th Century. The significance of this technique has steadily decreased for up to 3 decades and currently creates a negligible contribution to the present pellet markets.
Underwater strand pelletizing is really a sophisticated automatic process. But this process of production is commonly used 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 a process applicable only for non-sticky products, especially PVC. But this material is 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 also a computerized operation. But it is also suitable only for less sticky materials and finds its main application in polyolefin recycling and then in some minor applications in compounding.
Deciding on the best pelletizing process involves consideration in excess of pellet shape and throughput volume. As an example, pellet temperature and residual moisture are inversely proportional; that may be, the greater the product temperature, the lower the residual moisture. Some compounds, such as various types of TPE, are sticky, especially at elevated temperatures. This effect can be measured by counting the agglomerates-twins and multiples-in the bulk of pellets.
In a underwater pelletizing system such agglomerates of sticky pellets could be generated by two ways. First, immediately after the cut, the surface temperature of the pellet is simply about 50° F on top of the process water temperature, even though the core from the pellet is still molten, as well as the average pellet temperature is just 35° to 40° F underneath the melt temperature. If two pellets enter into contact, they deform slightly, creating a contact surface between your pellets that could be clear of process water. In this contact zone, the solidified skin will remelt immediately because of heat transported from your molten core, and the pellets will fuse to one another.
Second, after discharge in the clear pvc granule from your dryer, the pellets’ surface temperature increases because of heat transport in the core to the surface. If soft TPE pellets are kept 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-considering that the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration may be reduced by having some wax-like substance to 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 a sense of the highest practical pellet temperature for your material type and pellet size. Anything dexrpky05 that temperature will raise the level of agglomerates, and anything below that temperature increases residual moisture.
In a few cases, the pelletizing operation can be expendable. This is true only in applications where virgin polymers may be converted right to finished products-direct extrusion of PET sheet coming from a polymer reactor, for example. If compounding of additives and other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is important, it will always be wise to know the options.