Pellets can be “only” an intermediate product, however size, shape, and consistency matter in subsequent processing operations.
This becomes a lot more important when it comes to the ever-increasing demands positioned on compounders. Regardless of what equipment they currently have, it never seems suited for the following challenge. An increasing number of products may need additional capacity. A new polymer or additive can be too tough, soft, or corrosive for the existing equipment. Or perhaps the job needs a different pellet shape. In these instances, compounders need in-depth engineering know-how on processing, and close cooperation because of their pelletizing equipment supplier.
The initial step in meeting such challenges begins with equipment selection. The most frequent classification of pelletizing processes involves two classes, differentiated by the state of the plastic material at the time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that is certainly very quickly cut into pvc pellet that happen to be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt originating from a die head is changed into strands that happen to be cut into pellets after cooling and solidification.
Variations of such basic processes could be tailored on the specific input material and product properties in sophisticated compound production. In both cases, intermediate process steps and different degrees of automation may be incorporated at any stage in the process.
For the greatest solution for your personal production requirements, begin with assessing the status quo, as well as defining future needs. Build a five-year projection of materials and required capacities. Short-term solutions frequently end up being more expensive and less satisfactory after a period of time. Though nearly every pelletizing line in a compounder must process a number of products, any system may be optimized only for a small selection of the full product portfolio.
Consequently, all of those other products will have to be processed under compromise conditions.
The lot size, together with the nominal system capacity, will have a very strong affect on the pelletizing process and machinery selection. Since compounding production lots are generally rather small, the flexibleness of your equipment is generally a big issue. Factors include quick access to clean and repair and the cabability to simply and quickly move from a product to another. Start-up and shutdown of the pelletizing system should involve minimum waste of material.
A line utilizing a simple water bath for strand cooling often may be the first choice for compounding plants. However, the individual layout can differ significantly, due to 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 the liquid bath, the residual water is wiped through the surface by means of a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled in to the cutting chamber by the feed section in a constant line speed. Inside the pelletizer, strands are cut from 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.
In the event the requirement is perfect for continuous compounding, where fewer product changes are involved and capacities are relatively high, automation might be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may employ 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 in the pelletizer.
Some polymer compounds are very fragile and break easily. Other compounds, or a selection of their ingredients, may be very sensitive to moisture. For such materials, the belt-conveyor strand pelletizer is the greatest 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-allow for a good price of flexibility.
When the preferred pellet shape is more spherical than cylindrical, the very best alternative is surely an underwater hot-face cutter. Having a capacity range between from about 20 lb/hr to several tons/hr, this technique is relevant to all materials with thermoplastic behavior. In operation, the polymer melt is divided in to a ring of strands that flow via an annular die in 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 like a slurry to the centrifugal dryer, where these 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 returning to the method.
The principle parts of the machine-cutting head with cutting chamber, die plate, and begin-up valve, all on a common supporting frame-are certainly one major assembly. All of those other system components, like process-water circuit with bypass, cutting chamber discharge, sight glass, centrifugal dryer, belt filter, water pump, heat exchanger, and transport system might be selected from a comprehensive array of accessories and combined in to a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists within the cutting chamber and die plate. The die plate is both continuously cooled from the process water and heated by die-head heaters as well as the hot melt flow. Reducing the energy loss through the die plate to the process water produces a a lot more stable processing condition and increased product quality. As a way to reduce this heat loss, the processor may go with a thermally insulating die plate or move to a fluid-heated die.
Many compounds are quite abrasive, leading to significant damage on contact parts including the spinning blades and filter screens inside the centrifugal dryer. Other compounds could be responsive to mechanical impact and generate excessive dust. For both these special materials, a brand new sort of pellet dryer deposits the wet pellets with a perforated conveyor belt that travels across an aura knife, effectively suctioning off of the water. Wear of machine parts as well as problems for the pellets could be reduced in comparison with a direct impact dryer. Given the short residence time in the belt, some kind of post-dewatering drying (like having a fluidized bed) or additional cooling is usually required. Great things about this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of all parts getting into connection with pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is needed.
Another pelletizing processes are rather unusual inside the compounding field. The best and cheapest method of reducing plastics to an appropriate size for further processing might be a simple grinding operation. However, the resulting particle shape and size are exceedingly inconsistent. Some important product properties will likely suffer negative influence: The bulk density will drastically decrease and the free-flow properties of the bulk would be very poor. That’s why such material are only suitable for inferior applications and should be marketed at rather inexpensive.
Dicing had been a common size-reduction process considering that the early 20th Century. The value of this technique has steadily decreased for nearly thirty years and currently constitutes a negligible contribution to the current 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, like for polyesters, nylons, and styrenic polymers, and possesses no common application in today’s compounding.
Air-cooled die-face pelletizing is a process applicable just for non-sticky products, especially PVC. But this product is much more commonly compounded in batch mixers with heating and air conditioning and discharged as dry-blends. Only negligible levels of PVC compounds are transformed into pellets.
Water-ring pelletizing is likewise a computerized operation. But it is also suitable exclusively for less sticky materials and finds its main application in polyolefin recycling and then in some minor applications in compounding.
Choosing the right pelletizing process involves consideration of over pellet shape and throughput volume. By way of example, pellet temperature and residual moisture are inversely proportional; that is, the larger the product temperature, the reduced the residual moisture. Some compounds, like many types of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-in the majority of pellets.
In an underwater pelletizing system such agglomerates of sticky pellets can be generated by two ways. First, just after the cut, the surface temperature from the pellet is only about 50° F above the process temperature of water, as the core of your pellet remains to be molten, and the average pellet temperature is simply 35° to 40° F underneath the melt temperature. If two pellets enter in to contact, they deform slightly, making a contact surface between the pellets that could be clear of process water. In that contact zone, the solidified skin will remelt immediately due to heat transported in the molten core, as well as the pellets will fuse to one another.
Second, after discharge from the transparent pvc compound in the dryer, the pellets’ surface temperature increases on account of heat transport in the core for the surface. If soft TPE pellets are saved 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 area to volume increases with smaller diameter.
Pellet agglomeration might be reduced by adding some wax-like substance towards the process water or by powdering the pellet surfaces immediately after the pellet dryer.
Performing a number of pelletizing test runs at consistent throughput rate gives you a concept of the maximum practical pellet temperature for this material type and pellet size. Anything dexrpky05 that temperature will raise the level of agglomerates, and anything below that temperature improves residual moisture.
In some cases, the pelletizing operation can be expendable. This really is only in applications where virgin polymers can be converted instantly to finished products-direct extrusion of PET sheet from a polymer reactor, for example. If compounding of additives and also other ingredients adds real value, however, direct conversion will not be possible. If pelletizing is needed, it is usually advisable to know the options.