Pellets might be “only” an intermediate product, however size, shape, and consistency matter in subsequent processing operations.
This becomes much more important when considering the ever-increasing demands put on compounders. Irrespective of what equipment they now have, it never seems suited for the upcoming challenge. A lot more products may need additional capacity. A brand new polymer or additive may be too tough, soft, or corrosive for the existing equipment. Or maybe the job needs a different pellet shape. In such cases, 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 condition of the plastic material back then it’s cut:
•Melt pelletizing (hot cut): Melt from a die that is certainly almost immediately cut into pvc granule that are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt provided by a die head is changed into strands that happen to be cut into pellets after cooling and solidification.
Variations of those basic processes might be tailored for the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps and various levels of automation may be incorporated at any stage of the process.
To get the best solution for the production requirements, get started with assessing the status quo, and also defining future needs. Create a five-year projection of materials and required capacities. Short-term solutions fairly often end up being more pricey and fewer satisfactory after a period of time. Though just about every pelletizing line at a compounder will need to process many different products, any system can be optimized simply for a little selection of the full product portfolio.
Consequently, all the other products will have 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 tend to be rather small, the flexibility of the equipment is generally a serious problem. Factors include easy access to clean and service and the capability to simply and quickly move in one product to another. Start-up and shutdown in the pelletizing system should involve minimum waste of material.
A line by using a simple water bath for strand cooling often will be the first choice for compounding plants. However, the average person layout may differ significantly, as a result of demands of throughput, flexibility, and level of system integration. In strand pelletizing, polymer strands exit the die head and are transported by way of a water bath and cooled. Following 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 for the pelletizer, being pulled to the cutting chamber from the feed section at a constant line speed. In the pelletizer, strands are cut from a rotor and a bed knife into roughly cylindrical pellets. These may be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
If the requirement is perfect for continuous compounding, where fewer product changes are involved and capacities are relatively high, automation could be advantageous for reducing costs while increasing quality. This sort of automatic strand pelletizing line may employ a self-stranding variation of this type of pelletizer. This can be seen as a a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and give automatic transportation in to the pelletizer.
Some polymer compounds are quite fragile and break easily. Other compounds, or a selection of their ingredients, could be very understanding of moisture. For such materials, the belt-conveyor strand pelletizer is the best answer. A perforated conveyor belt takes the strands from the die and conveys them smoothly to the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-permit the best value of flexibility.
As soon as the preferred pellet shape is a lot more spherical than cylindrical, the very best alternative is definitely an underwater hot-face cutter. By using a capacity range between from about 20 lb/hr to a few tons/hr, this method is relevant to all materials with thermoplastic behavior. Functioning, the polymer melt is split in to 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 rigid pvc compound, which can be immediately conveyed out from the cutting chamber. The pellets are transported as a slurry to the centrifugal dryer, where these are separated from water from the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The water is filtered, tempered, and recirculated returning to the process.
The main aspects of the system-cutting head with cutting chamber, die plate, and start-up valve, all on a common supporting frame-are one major assembly. All of the 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 the comprehensive variety of accessories and combined into a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists within the cutting chamber and die plate. The die plate is both continuously cooled with 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 a lot 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 quite abrasive, leading to significant deterioration on contact parts like the spinning blades and filter screens from the centrifugal dryer. Other compounds could be responsive to mechanical impact and generate excessive dust. For these two 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 from the water. Wear of machine parts in addition to harm to the pellets might be reduced in contrast to a positive change dryer. Given the short residence time around the belt, some kind of post-dewatering drying (for example with a fluidized bed) or additional cooling is often required. Advantages of this new non-impact pellet-drying solution are:
•Lower production costs as a result of long lifetime of most 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.
Another pelletizing processes are rather unusual within the compounding field. The simplest and cheapest strategy for reducing plastics to a appropriate size for more processing might be a simple grinding operation. However, the resulting particle size and shape are incredibly inconsistent. Some important product properties will likely suffer negative influence: The bulk density will drastically decrease as well as the free-flow properties of your bulk would be lousy. That’s why such material are only acceptable for inferior applications and must be marketed at rather low priced.
Dicing have been a frequent size-reduction process since the early 20th Century. The importance of this technique has steadily decreased for up to 3 decades and currently creates a negligible contribution to the current pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this technique of production is used primarily in some virgin polymer production, for example for polyesters, nylons, and styrenic polymers, and contains 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 material is more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible levels of PVC compounds are turned into pellets.
Water-ring pelletizing is additionally 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.
Selecting 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 larger the product temperature, the low the residual moisture. Some compounds, such as various types of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-in the bulk of pellets.
In a underwater pelletizing system such agglomerates of sticky pellets can be generated by two ways. First, right after the cut, the top temperature in the pellet is only about 50° F on top of the process water temperature, even though the core of the pellet continues to be molten, and the average pellet temperature is merely 35° to 40° F beneath the melt temperature. If two pellets enter into contact, they deform slightly, making a contact surface in between the pellets that may be free of process water. For the reason that contact zone, the solidified skin will remelt immediately on account of heat transported from your molten core, and also the pellets will fuse to one another.
Second, after discharge of your transparent pvc compound through the dryer, the pellets’ surface temperature increases because of heat transport from the core towards 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-considering that the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration may be reduced by adding some wax-like substance on the process water or by powdering the pellet surfaces just after the pellet dryer.
Performing a number of pelletizing test runs at consistent throughput rate will give you a concept of the highest practical pellet temperature for your material type and pellet size. Anything dexrpky05 that temperature will raise the amount of agglomerates, and anything below that temperature will increase residual moisture.
In a few cases, the pelletizing operation could be expendable. This is correct only in applications where virgin polymers could be converted right to finished products-direct extrusion of PET sheet from the polymer reactor, for example. If compounding of additives and other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is important, it is always better to know the options.