Pellets may be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when contemplating the ever-increasing demands placed on compounders. Irrespective of what equipment they now have, it never seems suited for the upcoming challenge. Progressively more products may require additional capacity. A whole new polymer or additive could be too tough, soft, or corrosive for that existing equipment. Or perhaps the job requires a different pellet shape. In such cases, compounders need in-depth engineering know-how on processing, and close cooperation with their pelletizing equipment supplier.
The first task in meeting such challenges starts off with equipment selection. The most frequent classification of pelletizing processes involves two categories, differentiated by the condition of the plastic material during the time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that is certainly almost immediately cut into pvc compound which can be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt originating from a die head is converted into strands which can be cut into pellets after cooling and solidification.
Variations of these 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 may be incorporated at any stage in the process.
To find the best solution for your production requirements, start out with assessing the status quo, and also defining future needs. Build a five-year projection of materials and required capacities. Short-term solutions frequently prove to be more expensive and much less satisfactory after a period of time. Though just about every pelletizing line with a compounder need to process a variety of products, virtually any system can be optimized exclusively for a tiny range of the whole product portfolio.
Consequently, all the other products will have to be processed under compromise conditions.
The lot size, in combination with the nominal system capacity, will have a very strong effect on the pelletizing process and machinery selection. Since compounding production lots tend to be rather small, the flexibleness of the equipment is usually a big issue. Factors include comfortable access to clean and repair and the cabability to simply and quickly move from a single product to the next. Start-up and shutdown of the 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 individual layout can differ significantly, due to the demands of throughput, flexibility, and standard of system integration. In strand pelletizing, polymer strands exit the die head and they are transported through a water bath and cooled. Once the strands leave water bath, the residual water is wiped in the surface by means of a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled in the cutting chamber from the feed section with a constant line speed. From the pelletizer, strands are cut from a rotor plus a bed knife into roughly cylindrical pellets. These could be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
When the requirement is for continuous compounding, where fewer product changes come to mind and capacities are relatively high, automation may be advantageous for reducing costs while increasing quality. This type of automatic strand pelletizing line may use a self-stranding variation of this sort of pelletizer. This is seen as a 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 some of their ingredients, may be very responsive to moisture. For such materials, the belt-conveyor strand pelletizer is the best 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 deal of flexibility.
If the preferred pellet shape is more spherical than cylindrical, the ideal alternative is definitely an underwater hot-face cutter. Using a capacity vary from from about 20 lb/hr to several tons/hr, this technique is applicable to all of materials with thermoplastic behavior. Functioning, the polymer melt is split in a ring of strands that flow through an annular die in a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into rigid pvc compound, which can be immediately conveyed from the cutting chamber. The pellets are transported like a slurry towards the centrifugal dryer, where they can be 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 back to the process.
The primary elements of the machine-cutting head with cutting chamber, die plate, and start-up valve, all on the common supporting frame-is one major assembly. All of those 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 may be selected from your comprehensive range of accessories and combined into a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists inside the cutting chamber and die plate. The die plate is both continuously cooled through the process water and heated by die-head heaters along with the hot melt flow. Decreasing the energy loss from your die plate on the process water generates 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 or change to a fluid-heated die.
Many compounds are quite abrasive, leading to significant deterioration on contact parts for example the spinning blades and filter screens within the centrifugal dryer. Other compounds might be sensitive to mechanical impact and generate excessive dust. For the two of these special materials, a brand new type of pellet dryer deposits the wet pellets on a perforated conveyor belt that travels across an air knife, effectively suctioning from the water. Wear of machine parts along with problems for the pellets may be reduced compared to a direct impact dryer. Given the short residence time around the belt, some type of post-dewatering drying (for example by using a fluidized bed) or additional cooling is generally required. Great things about this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of most 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 needed.
Another pelletizing processes are rather unusual from the compounding field. The best and cheapest method of reducing plastics for an appropriate size for even more processing might be a simple grinding operation. However, the resulting particle size and shape are exceedingly inconsistent. Some important product properties may also suffer negative influence: The bulk density will drastically decrease and also the free-flow properties in the bulk would be bad. That’s why such material will only be suitable for inferior applications and must be marketed at rather low priced.
Dicing ended up being a frequent size-reduction process because the early twentieth century. The necessity of this method has steadily decreased for up to 3 decades and currently creates a negligible contribution to the current pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this method of production is utilized 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 is a process applicable just for non-sticky products, especially PVC. But this product is much more commonly compounded in batch mixers with air conditioning and discharged as dry-blends. Only negligible levels of PVC compounds are turned into pellets.
Water-ring pelletizing is additionally an automated operation. However it is also suitable only for less sticky materials and finds its main application in polyolefin recycling and then in some minor applications in compounding.
Selecting the best pelletizing process involves consideration in excess of pellet shape and throughput volume. As an example, pellet temperature and residual moisture are inversely proportional; which is, the larger the product temperature, the less the residual moisture. Some compounds, such as many types 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.
In an underwater pelletizing system such agglomerates of sticky pellets may be generated in two ways. First, right after the cut, the outer lining temperature of the pellet is just about 50° F higher than the process water temperature, even though the core from the pellet remains molten, and also the average pellet temperature is only 35° to 40° F below the melt temperature. If two pellets come into contact, they deform slightly, developing a contact surface between your pellets which may be free from process water. In this contact zone, the solidified skin will remelt immediately due to heat transported in the molten core, and also the pellets will fuse to one another.
Second, after discharge in the clear pvc granule in the dryer, the pellets’ surface temperature increases on account of heat transport from the core on the surface. If soft TPE pellets are held 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-because the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration may 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 variety of pelletizing test runs at consistent throughput rate provides you with a solid idea of the highest practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will raise the quantity of agglomerates, and anything below that temperature improves residual moisture.
In certain cases, the pelletizing operation may be expendable. This is correct only in applications where virgin polymers could be converted straight to finished products-direct extrusion of PET sheet from your polymer reactor, for instance. If compounding of additives and other ingredients adds real value, however, direct conversion will not be possible. If pelletizing is necessary, it usually is advisable to know the options.