Pellets could be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes much more important when contemplating the ever-increasing demands placed on compounders. Whatever equipment they currently have, it never seems suited for the next 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 perhaps the job needs a different pellet shape. In these cases, compounders need in-depth engineering know-how on processing, and close cooperation using their pelletizing equipment supplier.
The first task in meeting such challenges begins with equipment selection. The most prevalent classification of pelletizing processes involves two classes, differentiated by the condition of the plastic material during the time it’s cut:
•Melt pelletizing (hot cut): Melt originating from a die which is almost immediately cut into pvc granule 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 are cut into pellets after cooling and solidification.
Variations of such basic processes may be tailored on the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps as well as other degrees of automation may be incorporated at any stage of the process.
To find 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 often show to be higher priced and fewer satisfactory after a period of time. Though virtually every pelletizing line at the compounder need to process a number of products, any system could be optimized exclusively for a small variety of the full product portfolio.
Consequently, all of those other products will have to be processed under compromise conditions.
The lot size, in conjunction with the nominal system capacity, will have a very strong impact on the pelletizing process and machinery selection. Since compounding production lots tend to be rather small, the flexibility of the equipment is usually a serious problem. Factors include easy access to clean 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 is definitely the first option for compounding plants. However, the patient layout may vary significantly, due to demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and therefore are transported via 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 to the pelletizer, being pulled into the cutting chamber from the feed section in a constant line speed. Within the pelletizer, strands are cut from a rotor as well as a bed knife into roughly cylindrical pellets. These could be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
In case the requirement is perfect 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 use a self-stranding variation of this particular pelletizer. This is seen as a a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and provide automatic transportation in to the pelletizer.
Some polymer compounds can be fragile and break easily. Other compounds, or a selection of their ingredients, may be very understanding of moisture. For such materials, the belt-conveyor strand pelletizer is the best answer. A perforated conveyor belt takes the strands through the die and conveys them smoothly towards the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-allow for a good deal of flexibility.
As soon as the preferred pellet shape is much more spherical than cylindrical, the best alternative is an underwater hot-face cutter. By using a capacity range between from about 20 lb/hr to a few tons/hr, this method is applicable to all materials with thermoplastic behavior. In operation, the polymer melt is split in to a ring of strands that flow through an annular die right into a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into upvc compound, that happen to be immediately conveyed out of the cutting chamber. The pellets are transported as being 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. Water is filtered, tempered, and recirculated straight back to the procedure.
The main components of the system-cutting head with cutting chamber, die plate, and begin-up valve, all on a common supporting frame-are one major assembly. The rest of the 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 in a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists throughout 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. Reducing the energy loss in the die plate on the process water produces a much more stable processing condition and increased product quality. To be able to reduce this heat loss, the processor may choose a thermally insulating die plate and/or move to a fluid-heated die.
Many compounds are usually abrasive, leading to significant wear on contact parts for example the spinning blades and filter screens within the centrifugal dryer. Other compounds may be understanding of mechanical impact and generate excessive dust. For these two special materials, a new kind of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an air knife, effectively suctioning from the water. Wear of machine parts and also harm to the pellets can be greatly reduced compared with an impact dryer. Considering the short residence time on the belt, some type of post-dewatering drying (such as by using 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 the parts getting 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 essential.
Another pelletizing processes are rather unusual in the compounding field. The simplest and cheapest way of reducing plastics for an appropriate size for further processing may well be a simple grinding operation. However, the resulting particle shape and size are incredibly inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease and also the free-flow properties of the bulk will be very 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 considering that the early twentieth century. The significance of this technique has steadily decreased for almost 3 decades and currently creates a negligible contribution to the present pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this procedure of production is used primarily in a few virgin polymer production, such as for polyesters, nylons, and styrenic polymers, and it 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 air conditioning and discharged as dry-blends. Only negligible amounts of PVC compounds are turned into pellets.
Water-ring pelletizing is additionally an automatic operation. But it is also suitable simply for less sticky materials and finds its main application in polyolefin recycling and in some minor applications in compounding.
Deciding on the best pelletizing process involves consideration of more than pellet shape and throughput volume. For instance, pellet temperature and residual moisture are inversely proportional; which is, the larger the product temperature, the less the residual moisture. Some compounds, such as various types of TPE, are sticky, especially at elevated temperatures. This effect might be measured by counting the agglomerates-twins and multiples-in a majority of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets can be generated in 2 ways. First, right after the cut, the surface temperature of your pellet is just about 50° F on top of the process temperature of water, even though the core from the pellet remains molten, and also the average pellet temperature is simply 35° to 40° F below the melt temperature. If two pellets come into contact, they deform slightly, developing a contact surface involving the pellets which might be clear of process water. In that contact zone, the solidified skin will remelt immediately on account of heat transported from the molten core, as well as the pellets will fuse to one another.
Second, after discharge of the transparent pvc compound from your dryer, the pellets’ surface temperature increases as a result of heat transport in the core for the surface. If soft TPE pellets are stored 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 area to volume increases with smaller diameter.
Pellet agglomeration could be reduced by adding some wax-like substance towards the process water or by powdering the pellet surfaces just after the pellet dryer.
Performing a variety of pelletizing test runs at consistent throughput rate will give you a concept of the maximum practical pellet temperature for your material type and pellet size. Anything dexrpky05 that temperature will increase the level of agglomerates, and anything below that temperature boosts residual moisture.
In some cases, the pelletizing operation might be expendable. This is true only in applications where virgin polymers could be converted straight to finished products-direct extrusion of PET sheet from the polymer reactor, for instance. If compounding of additives along with other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is needed, it will always be advisable to know your choices.