Pellets may be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when considering the ever-increasing demands placed on compounders. Whatever equipment they now have, it never seems suited for the upcoming challenge. An increasing number of products may require additional capacity. A brand new polymer or additive might be too tough, soft, or corrosive for the existing equipment. Or perhaps the job takes 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 off with equipment selection. The most typical classification of pelletizing processes involves two categories, differentiated by the condition of the plastic material at the time it’s cut:
•Melt pelletizing (hot cut): Melt originating from a die that is certainly quickly cut into pvc granule that happen to be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt coming from a die head is converted into strands that are cut into pellets after cooling and solidification.
Variations of such basic processes may be tailored towards the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps and various levels of automation can be incorporated at any stage of your process.
To get the best solution for the production requirements, start with assessing the status quo, and also defining future needs. Build a five-year projection of materials and required capacities. Short-term solutions very often end up being higher priced and much less satisfactory after a period of time. Though virtually every pelletizing line in a compounder need to process many different products, virtually any system can be optimized exclusively for a tiny variety of the full product portfolio.
Consequently, all of the other products will need to be processed under compromise conditions.
The lot size, along with the nominal system capacity, will have a very strong affect on the pelletizing process and machinery selection. Since compounding production lots are usually rather small, the flexibleness of the equipment is generally a serious problem. Factors include easy access for cleaning and service and the ability to simply and quickly move from one product to another. 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 will be the first selection for compounding plants. However, the average person layout can vary significantly, because of the demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and so are transported through a water bath and cooled. Right after the strands leave the liquid bath, the residual water is wiped in the surface by means of a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled to the cutting chamber by the feed section at the constant line speed. From the pelletizer, strands are cut from a rotor and a bed knife into roughly cylindrical pellets. These could be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
If the requirement is perfect for continuous compounding, where fewer product changes are participating 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 kind of pelletizer. This can be observed as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and give automatic transportation into the pelletizer.
Some polymer compounds can be 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 best answer. A perforated conveyor belt takes the strands from your 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 more spherical than cylindrical, the best alternative is undoubtedly an underwater hot-face cutter. With a capacity cover anything from from about 20 lb/hr to a number of tons/hr, this system is applicable to any or all materials with thermoplastic behavior. In operation, the polymer melt is divided in a ring of strands that flow with an annular die right into a cutting chamber flooded with process water. A rotating cutting head within the water stream cuts the polymer strands into upvc compound, which are immediately conveyed out of your cutting chamber. The pellets are transported as a slurry to the centrifugal dryer, where these are separated from water from the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The water is filtered, tempered, and recirculated back to this process.
The primary elements of the system-cutting head with cutting chamber, die plate, and initiate-up valve, all on a common supporting frame-are certainly one major assembly. All the 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 could be selected from a comprehensive selection of accessories and combined right into a job-specific system.
In just about every underwater pelletizing system, a fragile temperature equilibrium exists in the cutting chamber and die plate. The die plate is both continuously cooled through the process water and heated by die-head heaters as well as the hot melt flow. Decreasing the energy loss through the die plate to the process water produces a much more stable processing condition and increased product quality. So that you can reduce this heat loss, the processor may pick a thermally insulating die plate and/or move to a fluid-heated die.
Many compounds can be abrasive, contributing to significant wear and tear on contact parts for example the spinning blades and filter screens in the centrifugal dryer. Other compounds can be sensitive to mechanical impact and generate excessive dust. For these two special materials, a fresh sort of pellet dryer deposits the wet pellets over a perforated conveyor belt that travels across an air knife, effectively suctioning off the water. Wear of machine parts and also injury to the pellets could be greatly reduced compared with an impact dryer. Considering the short residence time in the belt, some type of post-dewatering drying (including having a fluidized bed) or additional cooling is normally required. Benefits of this new non-impact pellet-drying solution are:
•Lower production costs as a result of long lifetime of all parts getting into experience of pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is needed.
A few other pelletizing processes are rather unusual from the compounding field. The best and cheapest means of reducing plastics with an appropriate size for even more processing generally is a simple grinding operation. However, the resulting particle shape and size are exceedingly inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease and also the free-flow properties of your bulk will be poor. That’s why such material will only be suitable for inferior applications and must be marketed at rather inexpensive.
Dicing have been a common size-reduction process ever since the early 20th Century. The value of this procedure has steadily decreased for nearly thirty years and currently will make a negligible contribution to the current pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this process 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 exclusively for non-sticky products, especially PVC. But this product is more commonly compounded in batch mixers with cooling and heating and discharged as dry-blends. Only negligible quantities of PVC compounds are turned into pellets.
Water-ring pelletizing is also an automatic operation. But 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.
Deciding on the best pelletizing process involves consideration greater than pellet shape and throughput volume. By way of example, pellet temperature and residual moisture are inversely proportional; that is, the better the product temperature, the reduced the residual moisture. Some compounds, like various kinds of TPE, are sticky, especially at elevated temperatures. This effect might be measured by counting the agglomerates-twins and multiples-in the majority of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets can be generated in just two ways. First, just after the cut, the top temperature from the pellet is only about 50° F over the process temperature of water, even though the core of your pellet remains molten, as well as the average pellet temperature is just 35° to 40° F below the melt temperature. If two pellets enter into contact, they deform slightly, developing a contact surface involving the pellets which might be free of process water. In that contact zone, the solidified skin will remelt immediately as a result of heat transported from the molten core, as well as the pellets will fuse to one another.
Second, after discharge in the pvc compound through the dryer, the pellets’ surface temperature increases because of heat transport through the core towards 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 surface area to volume increases with smaller diameter.
Pellet agglomeration might be reduced by adding some wax-like substance for the process water or by powdering the pellet surfaces just after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate will give you a solid idea of the highest 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 will increase residual moisture.
In some cases, the pelletizing operation might be expendable. This is true only in applications where virgin polymers could be converted right to finished products-direct extrusion of PET sheet from the polymer reactor, for example. If compounding of additives along with other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is needed, it is usually advisable to know your options.