Pellets can be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when contemplating the ever-increasing demands placed on compounders. No matter what equipment they currently have, it never seems suited for the upcoming challenge. Progressively more products may need additional capacity. A fresh polymer or additive could be too tough, soft, or corrosive to the existing equipment. Or perhaps the job requires 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 initial step in meeting such challenges starts off with equipment selection. The most frequent classification of pelletizing processes involves two categories, differentiated by the condition of the plastic material at the time it’s cut:
•Melt pelletizing (hot cut): Melt coming from a die that is almost immediately cut into pvc pellet which can be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt from a die head is changed into strands that happen to be cut into pellets after cooling and solidification.
Variations of those basic processes could be tailored to the specific input material and product properties in sophisticated compound production. In both cases, intermediate process steps and various degrees of automation could be incorporated at any stage in the process.
To get the best solution to your production requirements, begin with assessing the status quo, in addition to defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions very often end up being more costly and less satisfactory after a period of time. Though just about every pelletizing line at the compounder need to process various products, any system might be optimized only for a tiny array of the full 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 possess a strong influence on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibleness of your equipment is often a serious problem. Factors include easy accessibility to clean and repair and the capability to simply and quickly move from a product to another. Start-up and shutdown of your pelletizing system should involve minimum waste of material.
A line using a simple water bath for strand cooling often is the first selection for compounding plants. However, the person layout may vary significantly, as a result of demands of throughput, flexibility, and degree of system integration. In strand pelletizing, polymer strands exit the die head and so are transported through a water bath and cooled. After the strands leave the liquid bath, the residual water is wiped through the surface by means of a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled into the cutting chamber with 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. These may be put through post-treatment like classifying, additional cooling, and drying, plus conveying.
When the requirement is for continuous compounding, where fewer product changes come to mind and capacities are relatively high, automation can be advantageous for reducing costs while increasing quality. This sort of automatic strand pelletizing line may employ a self-stranding variation of this particular pelletizer. This can be seen as a 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 a selection 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 for the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-provide for a great deal of flexibility.
Once the preferred pellet shape is far more spherical than cylindrical, the ideal alternative is definitely an underwater hot-face cutter. By using a capacity range from from about 20 lb/hr to a number of tons/hr, this technique is applicable to any or all materials with thermoplastic behavior. Operational, the polymer melt is split right into a ring of strands that flow with 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 upvc compound, which can be immediately conveyed from the cutting chamber. The pellets are transported like a slurry towards the centrifugal dryer, where these are separated from water through the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The water is filtered, tempered, and recirculated back to the process.
The primary parts of the program-cutting head with cutting chamber, die plate, and start-up valve, all over a common supporting frame-is one major assembly. The rest of the 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 just about every underwater pelletizing system, a fragile temperature equilibrium exists throughout the cutting chamber and die plate. The die plate is both continuously cooled from the process water and heated by die-head heaters and 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. As a way to reduce this heat loss, the processor may go with a thermally insulating die plate and move to a fluid-heated die.
Many compounds are quite abrasive, leading to significant damage on contact parts like the spinning blades and filter screens within the centrifugal dryer. Other compounds could be understanding of mechanical impact and generate excessive dust. For these two special materials, a brand new sort of pellet dryer deposits the wet pellets on a perforated conveyor belt that travels across an air knife, effectively suctioning off of the water. Wear of machine parts along with problems for the pellets can be greatly reduced compared to a positive change dryer. Given the short residence time on the belt, some form of post-dewatering drying (for example 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 connection with pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is essential.
Another pelletizing processes are rather unusual inside the compounding field. The best and cheapest method of reducing plastics to an appropriate size for additional processing can be quite a simple grinding operation. However, the resulting particle shape and size are extremely inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease and also the free-flow properties from the bulk will be poor. That’s why such material are only appropriate for inferior applications and must be marketed at rather inexpensive.
Dicing was a common size-reduction process since the early 20th Century. The importance of this method has steadily decreased for pretty much three decades and currently constitutes a negligible contribution to the present pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this procedure of production is used primarily in some 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 far more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible amounts of PVC compounds are turned into pellets.
Water-ring pelletizing is additionally a computerized operation. Yet it is also suitable exclusively 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 of more than pellet shape and throughput volume. As an example, pellet temperature and residual moisture are inversely proportional; that is, the greater the product temperature, the less the residual moisture. Some compounds, including various kinds of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-in a bulk of pellets.
In an underwater pelletizing system such agglomerates of sticky pellets might be generated by two ways. First, just after the cut, the outer lining temperature in the pellet is just about 50° F higher than the process temperature of water, even though the core in the pellet remains to be molten, along with the average pellet temperature is merely 35° to 40° F beneath the melt temperature. If two pellets enter into contact, they deform slightly, developing a contact surface involving the pellets which may be clear of process water. In that 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 from the pvc compound in the dryer, the pellets’ surface temperature increases because of heat transport through the core on 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-ever since the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration might be reduced with the help of some wax-like substance for the process water or by powdering the pellet surfaces immediately after the pellet dryer.
Performing a number of pelletizing test runs at consistent throughput rate provides you with a solid idea of the most practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will heighten the level of agglomerates, and anything below that temperature boosts residual moisture.
In a few cases, the pelletizing operation may be expendable. This is correct only in applications where virgin polymers may be converted instantly to finished products-direct extrusion of PET sheet from the polymer reactor, for example. If compounding of additives as well as other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is needed, it will always be advisable to know your choices.