Pellets can be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes much 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 next challenge. An increasing number of products might require additional capacity. A brand new polymer or additive may be too tough, soft, or corrosive for that existing equipment. Or maybe the job demands a different pellet shape. In these cases, compounders need in-depth engineering know-how on processing, and close cooperation with their pelletizing equipment supplier.
The first step in meeting such challenges starts with equipment selection. The most common classification of pelletizing processes involves two classes, differentiated by the state the plastic material back then it’s cut:
•Melt pelletizing (hot cut): Melt from a die that may be quickly cut into pvc pellet which 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 the basic processes can be tailored towards the specific input material and product properties in sophisticated compound production. In both cases, intermediate process steps as well as other levels of automation might be incorporated at any stage from the process.
For the greatest solution for your production requirements, start out with assessing the status quo, as well as defining future needs. Create a five-year projection of materials and required capacities. Short-term solutions frequently show to be more pricey and fewer satisfactory after a period of time. Though almost every pelletizing line at a compounder will need to process various products, virtually any system can be optimized exclusively for a tiny variety of the whole product portfolio.
Consequently, all the other products will need to be processed under compromise conditions.
The lot size, in conjunction with the nominal system capacity, will have a very strong influence on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibility in the equipment can be a big issue. Factors include comfortable access for cleaning and service and the cabability to simply and quickly move in one product to another. Start-up and shutdown of the pelletizing system should involve minimum waste of material.
A line using a simple water bath for strand cooling often will be the first selection for compounding plants. However, the person layout may differ significantly, due to the demands of throughput, flexibility, and standard of system integration. In strand pelletizing, polymer strands exit the die head and therefore are transported using a water bath and cooled. After 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 at the constant line speed. From the pelletizer, strands are cut between a rotor plus a bed knife into roughly cylindrical pellets. These can be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
When the requirement is perfect for continuous compounding, where fewer product changes come to mind and capacities are relatively high, automation could be advantageous for reducing costs while increasing quality. This type of automatic strand pelletizing line may use a self-stranding variation of this type of pelletizer. This is seen as a a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and offer automatic transportation in the pelletizer.
Some polymer compounds are usually fragile and break easily. Other compounds, or some 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 in 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 a great deal of flexibility.
Once the preferred pellet shape is a lot more spherical than cylindrical, the ideal alternative is an underwater hot-face cutter. With a capacity cover anything from from about 20 lb/hr to several tons/hr, this method is relevant to any or all materials with thermoplastic behavior. Functioning, the polymer melt is divided in to a ring of strands that flow through an annular die in a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into upvc compound, which can be immediately conveyed out of your cutting chamber. The pellets are transported being a slurry towards the centrifugal dryer, where these are separated from water by the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The liquid is filtered, tempered, and recirculated to this process.
The primary elements of the program-cutting head with cutting chamber, die plate, and initiate-up valve, all on the 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 may be selected from the comprehensive selection of accessories and combined in to 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 by the process water and heated by die-head heaters along with the hot melt flow. Reducing the energy loss from your die plate for the process water produces a much more stable processing condition and increased product quality. As a way 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 deterioration on contact parts such as the spinning blades and filter screens in the centrifugal dryer. Other compounds can be responsive to mechanical impact and generate excessive dust. For both these special materials, a brand new kind of pellet dryer deposits the wet pellets with a perforated conveyor belt that travels across an air knife, effectively suctioning from the water. Wear of machine parts and also damage to the pellets can be greatly reduced in contrast to a direct impact dryer. Given the short residence time about the belt, some sort of post-dewatering drying (such as with a fluidized bed) or additional cooling is generally required. Benefits associated with this new non-impact pellet-drying solution are:
•Lower production costs due to long lifetime of all parts getting into contact with pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is needed.
A few other pelletizing processes are rather unusual inside the compounding field. The most convenient and cheapest strategy for reducing plastics for an appropriate size for additional processing may well be a simple grinding operation. However, the resulting particle size and shape are exceedingly 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 will be poor. That’s why such material are only appropriate for inferior applications and should be marketed at rather affordable.
Dicing was a frequent size-reduction process since the early twentieth century. The necessity of this method has steadily decreased for nearly 30 years and currently creates a negligible contribution to the present pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this procedure of production can be used primarily in many 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 actually a process applicable exclusively 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 amounts of PVC compounds are transformed into pellets.
Water-ring pelletizing is likewise a computerized operation. Yet it is also suitable just for less sticky materials and finds its main application in polyolefin recycling as well as in some minor applications in compounding.
Selecting the best pelletizing process involves consideration of more than pellet shape and throughput volume. For example, pellet temperature and residual moisture are inversely proportional; that may be, 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 may be measured by counting the agglomerates-twins and multiples-in a bulk of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets might be generated in two ways. First, immediately after the cut, the top temperature of the pellet is just about 50° F over the process temperature of water, while the core from the pellet remains molten, along with the average pellet temperature is just 35° to 40° F below the melt temperature. If two pellets enter in to contact, they deform slightly, creating a contact surface between your pellets that may be without any process water. In this contact zone, the solidified skin will remelt immediately because of heat transported through the molten core, and the pellets will fuse to each other.
Second, after discharge of your transparent 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 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 might 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 addition of some wax-like substance for the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing numerous pelletizing test runs at consistent throughput rate will provide you with a concept of the maximum practical pellet temperature for that material type and pellet size. Anything dexrpky05 that temperature will raise the level of agglomerates, and anything below that temperature increases residual moisture.
In some cases, the pelletizing operation could be expendable. This is true only in applications where virgin polymers may be converted straight to finished products-direct extrusion of PET sheet coming from a polymer reactor, for example. If compounding of additives and other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is essential, it usually is better to know the options.