Pellets could be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes a lot more important when thinking about 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 require additional capacity. A fresh polymer or additive may be too tough, soft, or corrosive for your existing equipment. Or perhaps the job requires a different pellet shape. In such instances, compounders need in-depth engineering know-how on processing, and close cooperation because of their pelletizing equipment supplier.
The first step in meeting such challenges begins with equipment selection. The most prevalent classification of pelletizing processes involves two categories, differentiated by the state of the plastic material at the time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die which is almost immediately cut into pvc compound that happen to be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt provided by a die head is transformed into strands which can be cut into pellets after cooling and solidification.
Variations of those basic processes might be tailored to the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps and different levels of automation might be incorporated at any stage from the process.
To find the best solution for the production requirements, get started with assessing the status quo, as well as defining future needs. Establish a five-year projection of materials and required capacities. Short-term solutions very often turn out to be higher priced and fewer satisfactory after a period of time. Though just about every pelletizing line at the compounder will have to process a variety of products, virtually any system could be optimized exclusively for a little selection of the complete product portfolio.
Consequently, all the other products will have to be processed under compromise conditions.
The lot size, in combination with the nominal system capacity, will possess a strong effect on the pelletizing process and machinery selection. Since compounding production lots are usually rather small, the flexibility in the equipment is generally a big issue. Factors include comfortable access for cleaning and repair and the cabability to simply and quickly move in one product to the next. 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 may be the first option for compounding plants. However, the person layout may differ significantly, due to demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and so are transported using a water bath and cooled. Once the strands leave this type of water bath, the residual water is wiped from your surface by means of a suction air knife. The dried and solidified strands are transported to the pelletizer, being pulled in to the cutting chamber from the feed section in a constant line speed. Inside the pelletizer, strands are cut from a rotor as well as a bed knife into roughly cylindrical pellets. These can be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
When 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 use a self-stranding variation of this particular pelletizer. This is characterized by a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and give automatic transportation in the pelletizer.
Some polymer compounds are very 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 towards the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-provide for a good deal of flexibility.
If the preferred pellet shape is more spherical than cylindrical, the best alternative is undoubtedly an underwater hot-face cutter. Having a capacity range between from about 20 lb/hr to a few tons/hr, this system is applicable for all materials with thermoplastic behavior. Functioning, the polymer melt is split in a ring of strands that flow via an annular die right into a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into soft pvc granule, which are immediately conveyed out of the cutting chamber. The pellets are transported as a slurry towards the centrifugal dryer, where they are separated from water with the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. This type of water is filtered, tempered, and recirculated back to this process.
The main components of the machine-cutting head with cutting chamber, die plate, and begin-up valve, all with a common supporting frame-is one major assembly. All the other system components, such as 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 a comprehensive selection of accessories and combined in to 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 with the process water and heated by die-head heaters as well as the hot melt flow. Decreasing the energy loss through the die plate towards the process water produces a considerably more stable processing condition and increased product quality. To be able to reduce this heat loss, the processor may select a thermally insulating die plate and/or change to a fluid-heated die.
Many compounds are usually abrasive, resulting in significant deterioration on contact parts like the spinning blades and filter screens within the centrifugal dryer. Other compounds can be responsive to mechanical impact and generate excessive dust. For these two special materials, a fresh sort of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an aura knife, effectively suctioning away from the water. Wear of machine parts in addition to damage to the pellets can be reduced compared to a direct impact dryer. Because of the short residence time in the belt, some kind of post-dewatering drying (like having a fluidized bed) or additional cooling is normally required. Benefits associated with this new non-impact pellet-drying solution are:
•Lower production costs due to long lifetime of most parts coming into connection with pellets.
•Gentle pellet handling, which ensures high product quality and much less dust generation.
•Reduced energy consumption because no additional energy supply is needed.
Various other pelletizing processes are rather unusual inside the compounding field. The simplest and cheapest means of reducing plastics with an appropriate size for even more processing might be a simple grinding operation. However, the resulting particle size and shape are really inconsistent. Some important product properties may also suffer negative influence: The bulk density will drastically decrease as well as the free-flow properties in the bulk could be bad. That’s why such material are only appropriate for inferior applications and must be marketed at rather inexpensive.
Dicing ended up being a common size-reduction process considering that the early 20th Century. The significance of this process 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 process of production is commonly 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 can be a process applicable exclusively for non-sticky products, especially PVC. But this product is far more commonly compounded in batch mixers with cooling and heating and discharged as dry-blends. Only negligible levels of PVC compounds are transformed into pellets.
Water-ring pelletizing is likewise an automatic operation. But it is also suitable just for less sticky materials and finds its main application in polyolefin recycling and also in some minor applications in compounding.
Deciding on the best pelletizing process involves consideration greater than pellet shape and throughput volume. For example, pellet temperature and residual moisture are inversely proportional; which is, the better the product temperature, the reduced the residual moisture. Some compounds, including various kinds of TPE, are sticky, especially at elevated temperatures. This effect may be measured by counting the agglomerates-twins and multiples-inside a majority of pellets.
In a underwater pelletizing system such agglomerates of sticky pellets could be generated in just two ways. First, right after the cut, the surface temperature of your pellet is just about 50° F over the process temperature of water, as the core of the pellet remains molten, and also the average pellet temperature is merely 35° to 40° F underneath the melt temperature. If two pellets enter in to contact, they deform slightly, building a contact surface between your pellets that may be without any process water. In this contact zone, the solidified skin will remelt immediately due to heat transported in the molten core, along with the pellets will fuse to one another.
Second, after discharge from the clear pvc granule from your dryer, the pellets’ surface temperature increases because of heat transport from the core on 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 is probably intensified with smaller pellet size-e.g., micro-pellets-since the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration could be reduced with the addition of some wax-like substance on the process water or by powdering the pellet surfaces soon after the pellet dryer.
Performing numerous pelletizing test runs at consistent throughput rate provides you with an 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 will increase residual moisture.
In some cases, the pelletizing operation might be expendable. This is true only in applications where virgin polymers may be converted directly to finished products-direct extrusion of PET sheet coming from a polymer reactor, as an example. If compounding of additives and other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is important, it will always be best to know the options.