Pellets could be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes much more important when it comes to the ever-increasing demands put on compounders. Irrespective of what equipment they currently have, it never seems suited for the upcoming challenge. Progressively more products may need additional capacity. A new polymer or additive may be too tough, soft, or corrosive for your existing equipment. Or perhaps the job demands a different pellet shape. In such 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 the time it’s cut:
•Melt pelletizing (hot cut): Melt coming from a die that is almost immediately cut into pvc pellet that are 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 these basic processes can be tailored to the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps and various levels of automation could be incorporated at any stage in the process.
To find the best solution for your personal production requirements, start with assessing the status quo, in addition to defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions frequently turn out to be more pricey and less satisfactory after a period of time. Though just about every pelletizing line with a compounder will have to process various products, any given system could be optimized only for a tiny selection of the entire product portfolio.
Consequently, all of the other products will need to be processed under compromise conditions.
The lot size, in combination with the nominal system capacity, will have got a strong influence on the pelletizing process and machinery selection. Since compounding production lots are usually rather small, the flexibility of your equipment is often a serious problem. Factors include quick access for cleaning and repair and the cabability to simply and quickly move from a single product to another. 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 will be 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 are transported through a water bath and cooled. Once the strands leave water bath, the residual water is wiped through the surface by means of a suction air knife. The dried and solidified strands are transported towards the pelletizer, being pulled to the cutting chamber with the feed section with a constant line speed. Inside 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.
If 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. This type of automatic strand pelletizing line may utilize a self-stranding variation of this type of pelletizer. This really is described as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and provide automatic transportation into the pelletizer.
Some polymer compounds are usually 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 perfect 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-provide for the best value of flexibility.
When the preferred pellet shape is more spherical than cylindrical, the very best alternative is surely an underwater hot-face cutter. With a capacity cover anything from from about 20 lb/hr to several tons/hr, this method is applicable to all materials with thermoplastic behavior. Operational, the polymer melt is divided in 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 soft pvc granule, that happen to be immediately conveyed out from the cutting chamber. The pellets are transported like a slurry to the centrifugal dryer, where they can be separated from water with the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The liquid is filtered, tempered, and recirculated returning to the procedure.
The main elements of the system-cutting head with cutting chamber, die plate, and commence-up valve, all on a common supporting frame-is one major assembly. All of 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 can be selected from the comprehensive selection of accessories and combined into a job-specific system.
In every single 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 along with the hot melt flow. Decreasing the energy loss from your die plate for the process water produces a much more stable processing condition and increased product quality. In order to reduce this heat loss, the processor may choose a thermally insulating die plate and switch to a fluid-heated die.
Many compounds are very abrasive, causing significant wear and tear on contact parts such as the spinning blades and filter screens from the centrifugal dryer. Other compounds might be responsive to mechanical impact and generate excessive dust. For these two special materials, a brand new type of pellet dryer deposits the wet pellets over a perforated conveyor belt that travels across an air knife, effectively suctioning away from the water. Wear of machine parts and also injury to the pellets could be greatly reduced compared to an impact dryer. Given the short residence time on the belt, some form of post-dewatering drying (for example having a fluidized bed) or additional cooling is normally required. Benefits of this new non-impact pellet-drying solution are:
•Lower production costs because of long lifetime of most parts coming into contact 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 from the compounding field. The easiest and cheapest means of reducing plastics for an appropriate size for more processing might be a simple grinding operation. However, the resulting particle size and shape 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 poor. That’s why such material will only be appropriate for inferior applications and must be marketed at rather low cost.
Dicing was a typical size-reduction process since the early twentieth century. The value of this method has steadily decreased for up to 30 years and currently will make a negligible contribution to the current pellet markets.
Underwater strand pelletizing is really a sophisticated automatic process. But this process of production is used primarily in some virgin polymer production, such as 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 just for non-sticky products, especially PVC. But this product is far more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible quantities of PVC compounds are transformed into pellets.
Water-ring pelletizing is also a computerized operation. Yet it is also suitable simply for less sticky materials and finds its main application in polyolefin recycling and in some minor applications in compounding.
Selecting the best pelletizing process involves consideration greater than pellet shape and throughput volume. For example, pellet temperature and residual moisture are inversely proportional; that is, the higher the product temperature, the lower the residual moisture. Some compounds, including various types of TPE, are sticky, especially at elevated temperatures. This effect might be measured by counting the agglomerates-twins and multiples-inside a bulk of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets may be generated in 2 ways. First, soon after the cut, the outer lining temperature from the pellet is merely about 50° F on top of the process water temperature, while the core in the pellet remains to be molten, as well as the average pellet temperature is only 35° to 40° F below the melt temperature. If two pellets come into contact, they deform slightly, creating a contact surface between your pellets which might be free of process water. In this contact zone, the solidified skin will remelt immediately as a result of heat transported from your molten core, as well as the pellets will fuse to one another.
Second, after discharge in the transparent pvc compound in the dryer, the pellets’ surface temperature increases as a result of heat transport from your core to 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 may well be intensified with smaller pellet size-e.g., micro-pellets-ever since the ratio of surface to volume increases with smaller diameter.
Pellet agglomeration can be reduced with the help of some wax-like substance towards the process water or by powdering the pellet surfaces just after the pellet dryer.
Performing numerous pelletizing test runs at consistent throughput rate will provide you with an idea of the highest practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will raise the quantity of agglomerates, and anything below that temperature improves residual moisture.
In certain cases, the pelletizing operation could be expendable. This is correct only in applications where virgin polymers may be converted right to finished products-direct extrusion of PET sheet from your polymer reactor, for instance. If compounding of additives and also other ingredients adds real value, however, direct conversion is not possible. If pelletizing is necessary, it is usually wise to know your alternatives.