Pellets can be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes even more important when contemplating the ever-increasing demands put on compounders. Whatever equipment they now have, it never seems suited for the upcoming challenge. Progressively more products might need additional capacity. A whole new polymer or additive may be too tough, soft, or corrosive for the existing equipment. Or maybe the job takes a different pellet shape. In such instances, compounders need in-depth engineering know-how on processing, and close cooperation making use of their pelletizing equipment supplier.
The initial 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 during the time it’s cut:
•Melt pelletizing (hot cut): Melt from a die that is certainly quickly cut into pvc pellet which are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt coming from a die head is transformed into strands that are cut into pellets after cooling and solidification.
Variations of those basic processes might be tailored towards the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps and different levels of automation might be incorporated at any stage of the process.
To get the best solution to your production requirements, start out with assessing the status quo, as well as defining future needs. Build 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 almost every pelletizing line at the compounder will need to process a variety of products, any given system may be optimized exclusively for a compact selection of the entire product portfolio.
Consequently, all the other products will need 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 generally rather small, the flexibleness in the equipment is often a serious problem. Factors include easy accessibility to clean and service and the opportunity to simply and quickly move from a single product to the next. Start-up and shutdown from the pelletizing system should involve minimum waste of material.
A line using a simple water bath for strand cooling often may be the first option for compounding plants. However, the patient layout may vary significantly, due to the demands of throughput, flexibility, and standard of system integration. In strand pelletizing, polymer strands exit the die head and so are transported by way of a water bath and cooled. Right after the strands leave this type of water bath, the residual water is wiped from the surface through a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled in to the cutting chamber with the feed section at the constant line speed. Inside the pelletizer, strands are cut between a rotor along with a bed knife into roughly cylindrical pellets. These may be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
In the event the requirement is for continuous compounding, where fewer product changes are participating and capacities are relatively high, automation may be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may utilize a self-stranding variation of this particular pelletizer. This can be 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 are usually 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 perfect 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-enable a good price of flexibility.
Once the preferred pellet shape is much more spherical than cylindrical, the very best alternative is surely an underwater hot-face cutter. With a capacity vary from from about 20 lb/hr to several tons/hr, this technique is relevant for all materials with thermoplastic behavior. In operation, the polymer melt is divided in to a ring of strands that flow using an annular die 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. The water is filtered, tempered, and recirculated back to the process.
The principle parts of the system-cutting head with cutting chamber, die plate, and start-up valve, all on a common supporting frame-is 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 a comprehensive variety of accessories and combined right into a job-specific system.
In each and every 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 the die plate towards the process water results in a a lot more stable processing condition and increased product quality. So that you can reduce this heat loss, the processor may choose a thermally insulating die plate or switch to a fluid-heated die.
Many compounds are very abrasive, causing significant wear on contact parts including the spinning blades and filter screens inside the centrifugal dryer. Other compounds may be responsive to mechanical impact and generate excessive dust. For both these special materials, a new form of pellet dryer deposits the wet pellets over a perforated conveyor belt that travels across an aura knife, effectively suctioning off the water. Wear of machine parts as well as damage to the pellets can be reduced in comparison with an effect dryer. Due to the short residence time in the belt, some type of post-dewatering drying (for example by using a fluidized bed) or additional cooling is often required. Benefits of this new non-impact pellet-drying solution are:
•Lower production costs due to long lifetime of parts coming into exposure to pellets.
•Gentle pellet handling, which ensures high product quality and much less dust generation.
•Reduced energy consumption because no additional energy supply is important.
Various other pelletizing processes are rather unusual within the compounding field. The simplest and cheapest way of reducing plastics for an appropriate size for even more processing can be quite a simple grinding operation. However, the resulting particle shape and size 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 poor. That’s why such material are only acceptable for inferior applications and should be marketed at rather inexpensive.
Dicing ended up being a typical size-reduction process considering that the early twentieth century. The importance of this procedure has steadily decreased for up to thirty years and currently creates a negligible contribution to the current pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this procedure of production is commonly used primarily in a few virgin polymer production, like for polyesters, nylons, and styrenic polymers, and has no common application in today’s compounding.
Air-cooled die-face pelletizing is actually a process applicable simply for non-sticky products, especially PVC. But this product is much more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible quantities of PVC compounds are turned into pellets.
Water-ring pelletizing can also be an automatic operation. But it is also suitable simply for less sticky materials and finds its main application in polyolefin recycling and also in some minor applications in compounding.
Selecting the best pelletizing process involves consideration of over 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, such as many types of TPE, are sticky, especially at elevated temperatures. This effect might be measured by counting the agglomerates-twins and multiples-in a bulk of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets could be generated by two ways. First, right after the cut, the surface temperature of the pellet is simply about 50° F above the process temperature of water, while the core of the pellet is still molten, and the average pellet temperature is just 35° to 40° F underneath the melt temperature. If two pellets enter into contact, they deform slightly, making a contact surface between your pellets that may be free from process water. In this contact zone, the solidified skin will remelt immediately as a result of heat transported from your molten core, along with the pellets will fuse to one another.
Second, after discharge of your pvc compound in the dryer, the pellets’ surface temperature increases as a result of heat transport in the core to 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 most likely intensified with smaller pellet size-e.g., micro-pellets-considering that the ratio of area to volume increases with smaller diameter.
Pellet agglomeration may be reduced with the help of some wax-like substance for the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing a variety of pelletizing test runs at consistent throughput rate will provide you with a concept of the most practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will heighten the quantity of agglomerates, and anything below that temperature will increase residual moisture.
In some cases, the pelletizing operation can 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 instance. If compounding of additives along with other ingredients adds real value, however, direct conversion will not be possible. If pelletizing is needed, it will always be advisable to know your alternatives.