Pellets might be “only” an intermediate product, but 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 now have, it never seems suited for the following challenge. A lot more products may need additional capacity. A brand new polymer or additive could be too tough, soft, or corrosive for that existing equipment. Or perhaps the job needs a different pellet shape. In such cases, compounders need in-depth engineering know-how on processing, and close cooperation using their pelletizing equipment supplier.
The initial step in meeting such challenges starts with equipment selection. The most prevalent classification of pelletizing processes involves two categories, differentiated by the state the plastic material back then it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that is almost immediately cut into pvc compound that happen to be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt from a die head is changed into strands that happen to be cut into pellets after cooling and solidification.
Variations of such basic processes may be tailored to the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps as well as other degrees of automation might be incorporated at any stage in the process.
For the greatest solution to your production requirements, start with assessing the status quo, as well as defining future needs. Create a five-year projection of materials and required capacities. Short-term solutions fairly often turn out to be higher priced and fewer satisfactory after a period of time. Though almost every pelletizing line in a compounder need to process a number of products, any given system may be optimized only for a little array of the full product portfolio.
Consequently, the rest of the products will need to be processed under compromise conditions.
The lot size, together with the nominal system capacity, will have a very strong impact on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibility of the equipment is usually a serious problem. Factors include easy accessibility for cleaning and repair and the capability to simply and quickly move in one product to another. Start-up and shutdown of your pelletizing system should involve minimum waste of material.
A line by using a simple water bath for strand cooling often will be the first selection for compounding plants. However, the average 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 are transported by way of a water bath and cooled. Right after the strands leave the water bath, the residual water is wiped in the surface through a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled in to the cutting chamber through the feed section with a constant line speed. Inside the pelletizer, strands are cut from a rotor along with a bed knife into roughly cylindrical pellets. These could be put through post-treatment like classifying, additional cooling, and drying, plus conveying.
In the event the requirement is made for continuous compounding, where fewer product changes are involved and capacities are relatively high, automation can be advantageous for reducing costs while increasing quality. Such an automatic strand pelletizing line may utilize a self-stranding variation of this kind of pelletizer. This is observed as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and supply automatic transportation to the pelletizer.
Some polymer compounds are usually fragile and break easily. Other compounds, or some 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 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 great deal of flexibility.
Once the preferred pellet shape is a lot more spherical than cylindrical, the 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 of materials with thermoplastic behavior. Functioning, the polymer melt is split right into a ring of strands that flow with an annular die in a cutting chamber flooded with process water. A rotating cutting head within the water stream cuts the polymer strands into soft pvc granule, which can be immediately conveyed out of the cutting chamber. The pellets are transported being a slurry for 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. Water is filtered, tempered, and recirculated straight back to the procedure.
The main components of the program-cutting head with cutting chamber, die plate, and begin-up valve, all with a common supporting frame-are one major assembly. All of the other system components, including process-water circuit with bypass, cutting chamber discharge, sight glass, centrifugal dryer, belt filter, water pump, heat exchanger, and transport system could be selected from the comprehensive variety of accessories and combined into a job-specific system.
In every single underwater pelletizing system, a fragile temperature equilibrium exists inside the cutting chamber and die plate. The die plate is both continuously cooled from the process water and heated by die-head heaters and also the hot melt flow. Decreasing the energy loss from your die plate on the process water results in 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 or move to a fluid-heated die.
Many compounds are usually abrasive, resulting in significant wear on contact parts for example the spinning blades and filter screens from the centrifugal dryer. Other compounds might be sensitive to mechanical impact and generate excessive dust. For the two of these special materials, a new form of pellet dryer deposits the wet pellets with a perforated conveyor belt that travels across an air knife, effectively suctioning off of the water. Wear of machine parts in addition to harm to the pellets might be reduced in comparison with a direct impact dryer. Because of the short residence time about the belt, some kind of post-dewatering drying (such as having 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 all 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.
Some other pelletizing processes are rather unusual inside the compounding field. The most convenient and cheapest strategy for reducing plastics to a appropriate size for even more processing generally is 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 and the free-flow properties of your bulk would be poor. That’s why such material are only acceptable for inferior applications and should be marketed at rather inexpensive.
Dicing have been a frequent size-reduction process considering that the early 20th Century. The importance of this procedure has steadily decreased for up to 3 decades and currently creates a negligible contribution to the current pellet markets.
Underwater strand pelletizing is actually a sophisticated automatic process. But this process of production is used primarily in many virgin polymer production, such as for polyesters, nylons, and styrenic polymers, and possesses no common application in today’s compounding.
Air-cooled die-face pelletizing can be a process applicable only for non-sticky products, especially PVC. But this product is a lot more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible levels of PVC compounds are transformed into pellets.
Water-ring pelletizing is also an automated operation. However it is also suitable simply for less sticky materials and finds its main application in polyolefin recycling as well as in some minor applications in compounding.
Deciding on the best pelletizing process involves consideration in excess of pellet shape and throughput volume. By way of example, pellet temperature and residual moisture are inversely proportional; that may be, the greater the product temperature, the low the residual moisture. Some compounds, like various kinds 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 can be generated in just two ways. First, soon after the cut, the surface temperature in the pellet is merely about 50° F above the process water temperature, even though the core of the pellet is still molten, and the average pellet temperature is simply 35° to 40° F beneath the melt temperature. If two pellets come into contact, they deform slightly, creating a contact surface between the pellets which might be free from process water. Because contact zone, the solidified skin will remelt immediately because of heat transported from your molten core, and also the pellets will fuse to one another.
Second, after discharge in the transparent pvc compound in the dryer, the pellets’ surface temperature increases due to heat transport from your core to 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 may well be intensified with smaller pellet size-e.g., micro-pellets-because the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration could be reduced with the help of some wax-like substance for the process water or by powdering the pellet surfaces immediately after the pellet dryer.
Performing a variety of pelletizing test runs at consistent throughput rate will give you a solid idea of the most practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will increase the level of agglomerates, and anything below that temperature boosts residual moisture.
In a few cases, the pelletizing operation can be expendable. This is true only in applications where virgin polymers can be converted straight to finished products-direct extrusion of PET sheet coming from a polymer reactor, for instance. If compounding of additives along with other ingredients adds real value, however, direct conversion will not be possible. If pelletizing is necessary, it is always advisable to know your choices.