Pellets could be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when it comes to the ever-increasing demands added to compounders. Whatever equipment they currently have, it never seems suited for the next challenge. An increasing number of products may require additional capacity. A fresh polymer or additive can be too tough, soft, or corrosive to the existing equipment. Or maybe the job requires a different pellet shape. In these cases, compounders need in-depth engineering know-how on processing, and close cooperation using their pelletizing equipment supplier.
The initial step in meeting such challenges begins with equipment selection. The most common classification of pelletizing processes involves two classes, differentiated by the state of the plastic material at that time it’s cut:
•Melt pelletizing (hot cut): Melt coming from a die that is quickly cut into pvc compound which are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt originating from a die head is transformed into strands which are cut into pellets after cooling and solidification.
Variations of those basic processes may be tailored on the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps and other levels of automation might be incorporated at any stage of the process.
For the greatest solution for your production requirements, start with assessing the status quo, and also defining future needs. Build a five-year projection of materials and required capacities. Short-term solutions fairly often turn out to be more pricey and much less satisfactory after a period of time. Though nearly every pelletizing line with a compounder will have to process various products, virtually any system might be optimized exclusively for a compact selection of the whole 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 have got a strong impact on the pelletizing process and machinery selection. Since compounding production lots are generally rather small, the flexibility in the equipment can be a big issue. Factors include easy accessibility to clean and service and the capability to simply and quickly move from a single product to the next. Start-up and shutdown of your pelletizing system should involve minimum waste of material.
A line utilizing a simple water bath for strand cooling often is the first option for compounding plants. However, the person layout can differ significantly, due to demands of throughput, flexibility, and level of system integration. In strand pelletizing, polymer strands exit the die head and therefore are transported via a water bath and cooled. Following the strands leave water bath, the residual water is wiped through the surface through a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled in the cutting chamber through the feed section in a constant line speed. Inside the pelletizer, strands are cut from 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 might be advantageous for reducing costs while increasing quality. Such an automatic strand pelletizing line may use a self-stranding variation of this particular pelletizer. This is described as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and provide automatic transportation in to the pelletizer.
Some polymer compounds are usually fragile and break easily. Other compounds, or some of their ingredients, could 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 to the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-allow for the best value of flexibility.
As soon as the preferred pellet shape is far more spherical than cylindrical, the most effective alternative is surely an underwater hot-face cutter. Having a capacity cover anything from from about 20 lb/hr to many tons/hr, this product is applicable to all materials with thermoplastic behavior. Operational, the polymer melt is split in a ring of strands that flow using an annular die into a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into upvc compound, that are immediately conveyed out of your cutting chamber. The pellets are transported as a slurry for the centrifugal dryer, where they are separated from water through the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. Water is filtered, tempered, and recirculated to this process.
The principle elements 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 those 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 can be selected from the comprehensive variety 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 by the process water and heated by die-head heaters along with the hot melt flow. Lowering the energy loss from the die plate for 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 pick a thermally insulating die plate and/or move to a fluid-heated die.
Many compounds are very abrasive, resulting in significant deterioration on contact parts including the spinning blades and filter screens inside the centrifugal dryer. Other compounds can be responsive to mechanical impact and generate excessive dust. For both of these special materials, a fresh kind of pellet dryer deposits the wet pellets over a perforated conveyor belt that travels across an air knife, effectively suctioning off the water. Wear of machine parts as well as injury to the pellets may be reduced compared to an impact dryer. Because of the short residence time on the belt, some form of post-dewatering drying (for example with a fluidized bed) or additional cooling is normally required. Benefits of this new non-impact pellet-drying solution are:
•Lower production costs due to long lifetime of all parts getting into connection with pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is needed.
A few other pelletizing processes are rather unusual within the compounding field. The best and cheapest way of reducing plastics to an appropriate size for further 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 also the free-flow properties in the bulk would be very poor. That’s why such material will only be suitable for inferior applications and must be marketed at rather inexpensive.
Dicing had been a common size-reduction process because the early twentieth century. The significance of this technique has steadily decreased for up to 3 decades and currently makes a negligible contribution to the present pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this method of production can be used primarily in certain virgin polymer production, like for polyesters, nylons, and styrenic polymers, and has no common application in today’s compounding.
Air-cooled die-face pelletizing is actually a process applicable simply for non-sticky products, especially PVC. But this material is more commonly compounded in batch mixers with heating and air conditioning and discharged as dry-blends. Only negligible numbers of PVC compounds are transformed into pellets.
Water-ring pelletizing is also an automated operation. But it is also suitable just for less sticky materials and finds its main application in polyolefin recycling and then 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, the better the product temperature, the low the residual moisture. Some compounds, for example many types of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-in the majority of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets could be generated in 2 ways. First, soon after the cut, the surface temperature of your pellet is only about 50° F above the process temperature of water, whilst the core from the pellet remains molten, as well as the average pellet temperature is just 35° to 40° F underneath the melt temperature. If two pellets enter in to contact, they deform slightly, developing a contact surface between your pellets that may be free from process water. For the reason that contact zone, the solidified skin will remelt immediately on account of heat transported from your molten core, along with the pellets will fuse to one another.
Second, after discharge of your pvc compound in the dryer, the pellets’ surface temperature increases as a result of heat transport from the core for 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-since the ratio of surface to volume increases with smaller diameter.
Pellet agglomeration can be reduced by adding 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 an idea of the most 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 a few cases, the pelletizing operation can be expendable. This is correct only in applications where virgin polymers could be converted straight to finished products-direct extrusion of PET sheet from a polymer reactor, for example. If compounding of additives along with other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is needed, it will always be better to know your choices.