Pellets might be “only” an intermediate product, however size, shape, and consistency matter in subsequent processing operations.
This becomes more important when contemplating the ever-increasing demands added to compounders. Irrespective of what equipment they now have, it never seems suited for the upcoming challenge. An increasing number of products might need additional capacity. A new polymer or additive can be too tough, soft, or corrosive for that existing equipment. Or maybe the job takes a different pellet shape. In these cases, compounders need in-depth engineering know-how on processing, and close cooperation making use of their pelletizing equipment supplier.
The first step in meeting such challenges starts with equipment selection. The most common classification of pelletizing processes involves two categories, differentiated by the condition of the plastic material at that time it’s cut:
•Melt pelletizing (hot cut): Melt coming from a die that may be very quickly cut into pvc granule that are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt provided by a die head is transformed into strands which can be cut into pellets after cooling and solidification.
Variations of those basic processes can be tailored for the specific input material and product properties in sophisticated compound production. In both cases, intermediate process steps and various levels of automation can be incorporated at any stage of the process.
To get the best solution for your production requirements, start out with assessing the status quo, in addition to defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions often end up being more pricey and fewer satisfactory after a period of time. Though almost every pelletizing line in a compounder will need to process a number of products, any given system might be optimized just for a compact variety of the whole product portfolio.
Consequently, all of those other products will have to be processed under compromise conditions.
The lot size, in combination with the nominal system capacity, will have got a strong effect on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibility in the equipment is usually a big issue. Factors include comfortable access to clean and service and the capability to simply and quickly move in one product to another. Start-up and shutdown in the pelletizing system should involve minimum waste of material.
A line working with a simple water bath for strand cooling often will be the first choice for compounding plants. However, the average person layout may differ significantly, due to demands of throughput, flexibility, and degree of system integration. In strand pelletizing, polymer strands exit the die head and are transported using a water bath and cooled. After the strands leave this type of water bath, the residual water is wiped in the surface through a suction air knife. The dried and solidified strands are transported to the pelletizer, being pulled to the cutting chamber by the feed section in a constant line speed. Within the pelletizer, strands are cut between 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.
When the requirement is designed for continuous compounding, where fewer product changes come to mind 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 sort of pelletizer. This is described 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 can be 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 ideal 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-enable the best value of flexibility.
If the preferred pellet shape is more spherical than cylindrical, the ideal alternative is an underwater hot-face cutter. With a capacity range from from about 20 lb/hr to a few tons/hr, this technique is applicable to all materials with thermoplastic behavior. Functioning, the polymer melt is split into a ring of strands that flow via an annular die into a cutting chamber flooded with process water. A rotating cutting head within the water stream cuts the polymer strands into soft pvc granule, which are immediately conveyed out from the cutting chamber. The pellets are transported as being a slurry towards the centrifugal dryer, where they can be separated from water by 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 procedure.
The principle elements of the program-cutting head with cutting chamber, die plate, and commence-up valve, all with a common supporting frame-are certainly one major assembly. All the other system components, such as 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 range of accessories and combined in to 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 with the process water and heated by die-head heaters and the hot melt flow. Reducing the energy loss through the die plate to the process water produces a far more stable processing condition and increased product quality. So that you can reduce this heat loss, the processor may go with a thermally insulating die plate and/or switch to a fluid-heated die.
Many compounds are very abrasive, resulting in significant deterioration on contact parts including the spinning blades and filter screens in the centrifugal dryer. Other compounds could be responsive to mechanical impact and generate excessive dust. For both these special materials, a new type of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an aura knife, effectively suctioning off of the water. Wear of machine parts along with problems for the pellets may be reduced in comparison with an effect dryer. Because of the short residence time around the belt, some type of post-dewatering drying (including by using a fluidized bed) or additional cooling is generally 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 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 essential.
Another pelletizing processes are rather unusual from the compounding field. The most convenient and cheapest way of reducing plastics for an appropriate size for additional processing can be quite a simple grinding operation. However, the resulting particle size and shape are incredibly inconsistent. Some important product properties will 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 will only be appropriate for inferior applications and should be marketed at rather affordable.
Dicing ended up being a common size-reduction process ever since the early 20th Century. The necessity of this procedure has steadily decreased for pretty much thirty years and currently creates a negligible contribution to the present pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this procedure of production is used primarily in many virgin polymer production, including for polyesters, nylons, and styrenic polymers, and possesses no common application in today’s compounding.
Air-cooled die-face pelletizing is a process applicable only for non-sticky products, especially PVC. But this material is a lot more commonly compounded in batch mixers with air conditioning and discharged as dry-blends. Only negligible numbers of PVC compounds are turned 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 and also in some minor applications in compounding.
Picking the right pelletizing process involves consideration of over pellet shape and throughput volume. For instance, pellet temperature and residual moisture are inversely proportional; that may be, the greater the product temperature, the lower 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 the majority of pellets.
In an underwater pelletizing system such agglomerates of sticky pellets may be generated in just two ways. First, immediately after the cut, the outer lining temperature of your pellet is merely about 50° F higher than the process temperature of water, while the core of the pellet remains molten, along with the average pellet temperature is merely 35° to 40° F beneath the melt temperature. If two pellets come into contact, they deform slightly, making a contact surface between the pellets that could be free of process water. In that contact zone, the solidified skin will remelt immediately on account of heat transported from your molten core, along with the pellets will fuse to each other.
Second, after discharge in the pvc compound from the dryer, the pellets’ surface temperature increases as a result of heat transport from your core on 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 surface area to volume increases with smaller diameter.
Pellet agglomeration could be reduced with the addition of some wax-like substance to the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing a number of pelletizing test runs at consistent throughput rate gives you a sense of the maximum practical pellet temperature for your material type and pellet size. Anything dexrpky05 that temperature will increase the volume 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 can be converted straight to finished products-direct extrusion of PET sheet from the polymer reactor, for example. If compounding of additives and also other ingredients adds real value, however, direct conversion is not possible. If pelletizing is essential, it will always be best to know the options.