Pellets might be “only” an intermediate product, however size, shape, and consistency matter in subsequent processing operations.
This becomes more important when thinking about the ever-increasing demands put on compounders. No matter what equipment they currently have, it never seems suited for the upcoming challenge. An increasing number of products might need additional capacity. A brand new polymer or additive might be too tough, soft, or corrosive for that existing equipment. Or perhaps the job needs a different pellet shape. In such instances, compounders need in-depth engineering know-how on processing, and close cooperation with their pelletizing equipment supplier.
The first task in meeting such challenges starts with equipment selection. The most prevalent 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 from a die which is almost immediately cut into pvc granule which can be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt originating from a die head is converted into strands that are cut into pellets after cooling and solidification.
Variations of the basic processes might be tailored on the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps and different degrees of automation might be incorporated at any stage of the process.
To find the best solution for your personal production requirements, start with assessing the status quo, along with defining future needs. Create a five-year projection of materials and required capacities. Short-term solutions often show to be more pricey and much less satisfactory after a period of time. Though just about every pelletizing line with a compounder need to process many different products, virtually any system may be optimized just for a compact variety of the entire product portfolio.
Consequently, the rest of the products will need to be processed under compromise conditions.
The lot size, in combination with the nominal system capacity, will have a very strong impact on the pelletizing process and machinery selection. Since compounding production lots tend to be rather small, the flexibility of the equipment is usually a serious problem. Factors include quick access to clean and repair and the cabability to simply and quickly move from one product to the next. Start-up and shutdown of the pelletizing system should involve minimum waste of material.
A line utilizing a simple water bath for strand cooling often is the first choice for compounding plants. However, the individual layout may vary significantly, because of 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. Right after the strands leave the liquid bath, the residual water is wiped from 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 with the feed section at a constant line speed. From the pelletizer, strands are cut between a rotor and a bed knife into roughly cylindrical pellets. This can be subjected 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 come to mind and capacities are relatively high, automation may be advantageous for reducing costs while increasing quality. This sort of automatic strand pelletizing line may utilize a self-stranding variation of this kind of pelletizer. This is described as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and supply automatic transportation in to the pelletizer.
Some polymer compounds are quite fragile and break easily. Other compounds, or a selection of their ingredients, may be very responsive to moisture. For such materials, the belt-conveyor strand pelletizer is the greatest answer. A perforated conveyor belt takes the strands through the 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.
If the preferred pellet shape is much more spherical than cylindrical, the very best alternative is an underwater hot-face cutter. Having a capacity vary from from about 20 lb/hr to a number of tons/hr, this technique is relevant to all of materials with thermoplastic behavior. Functioning, the polymer melt is divided in to a ring of strands that flow via an annular die in to a cutting chamber flooded with process water. A rotating cutting head within the water stream cuts the polymer strands into rigid pvc compound, which can be immediately conveyed out of your cutting chamber. The pellets are transported like a slurry on 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 straight back to the process.
The key elements of the machine-cutting head with cutting chamber, die plate, and start-up valve, all over a common supporting frame-are one major assembly. All of those other system components, for example 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 selection 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 through the process water and heated by die-head heaters as well as the hot melt flow. Reducing the energy loss in the die plate to 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 go with a thermally insulating die plate or change to a fluid-heated die.
Many compounds can be abrasive, causing significant deterioration on contact parts such as the spinning blades and filter screens inside the centrifugal dryer. Other compounds could be responsive to mechanical impact and generate excessive dust. For both of these special materials, a whole new form of pellet dryer deposits the wet pellets over a perforated conveyor belt that travels across an aura knife, effectively suctioning off of the water. Wear of machine parts along with injury to the pellets can be reduced in contrast to a positive change dryer. Because of the short residence time around the belt, some kind of post-dewatering drying (including with a fluidized bed) or additional cooling is generally required. Benefits of this new non-impact pellet-drying solution are:
•Lower production costs as a result of long lifetime of all parts coming into contact with pellets.
•Gentle pellet handling, which ensures high product quality and much less dust generation.
•Reduced energy consumption because no additional energy supply is necessary.
Another pelletizing processes are rather unusual from the compounding field. The easiest and cheapest method of reducing plastics with an appropriate size for even more processing may well be 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 and the free-flow properties of your bulk could be bad. That’s why such material will only be appropriate for inferior applications and must be marketed at rather low priced.
Dicing had been a standard size-reduction process since the early twentieth century. The necessity of this procedure has steadily decreased for up to thirty years and currently will make a negligible contribution to the present pellet markets.
Underwater strand pelletizing is actually a sophisticated automatic process. But this procedure of production is commonly used primarily in certain virgin polymer production, for example for polyesters, nylons, and styrenic polymers, and has 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 much more commonly compounded in batch mixers with air conditioning and discharged as dry-blends. Only negligible quantities of PVC compounds are transformed into pellets.
Water-ring pelletizing is likewise an automated operation. Yet 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.
Deciding on the best pelletizing process involves consideration greater than pellet shape and throughput volume. For instance, pellet temperature and residual moisture are inversely proportional; that is certainly, the greater the product temperature, the low the residual moisture. Some compounds, such as many types of TPE, are sticky, especially at elevated temperatures. This effect might be measured by counting the agglomerates-twins and multiples-in the majority of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets might be generated in 2 ways. First, immediately after the cut, the outer lining temperature of your pellet is only about 50° F above the process temperature of water, whilst the core of your pellet remains to be molten, as well as the average pellet temperature is just 35° to 40° F below the melt temperature. If two pellets enter into contact, they deform slightly, building a contact surface involving the pellets that may be free of process water. For the reason that contact zone, the solidified skin will remelt immediately due to heat transported through the molten core, as well as the pellets will fuse to one another.
Second, after discharge of your pvc compound through the dryer, the pellets’ surface temperature increases on account 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 is probably intensified with smaller pellet size-e.g., micro-pellets-because the ratio of surface to volume increases with smaller diameter.
Pellet agglomeration can be reduced with the addition of some wax-like substance for the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate will provide you with a solid idea of the maximum practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will raise the level of agglomerates, and anything below that temperature improves residual moisture.
In certain cases, the pelletizing operation might be expendable. This really is only in applications where virgin polymers might be converted right to finished products-direct extrusion of PET sheet coming from a polymer reactor, by way of example. If compounding of additives and other ingredients adds real value, however, direct conversion is not possible. If pelletizing is important, it is always better to know your choices.