Pellets may be “only” an intermediate product, however their size, shape, and consistency matter in subsequent processing operations.
This becomes even more important when it comes to the ever-increasing demands placed on compounders. Irrespective of what equipment they currently have, it never seems suited for the upcoming challenge. A lot more products may need additional capacity. A whole new polymer or additive may be too tough, soft, or corrosive to the existing equipment. Or perhaps the job requires a different pellet shape. In these instances, 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 during the time it’s cut:
•Melt pelletizing (hot cut): Melt coming from a die that is very quickly cut into pvc granule that happen to be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt from a die head is converted into strands which can be cut into pellets after cooling and solidification.
Variations of those basic processes might be tailored to the specific input material and product properties in sophisticated compound production. Within both cases, intermediate process steps as well as other degrees of automation could be incorporated at any stage of your process.
For the greatest solution for the production requirements, get started with assessing the status quo, along with defining future needs. Create a five-year projection of materials and required capacities. Short-term solutions frequently show to be more costly and less satisfactory after a time period of time. Though virtually every pelletizing line at the compounder need to process various products, any system could be optimized exclusively for a little array of the complete 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 got a strong effect on the pelletizing process and machinery selection. Since compounding production lots tend to be rather small, the flexibility from the equipment is usually a serious problem. Factors include easy access for cleaning and service and the capability to simply and quickly move from a product to the next. Start-up and shutdown from the pelletizing system should involve minimum waste of material.
A line by using a simple water bath for strand cooling often may be the first choice for compounding plants. However, the patient layout may differ significantly, as a result of demands of throughput, flexibility, and level of system integration. In strand pelletizing, polymer strands exit the die head and therefore are transported using a water bath and cooled. Right after the strands leave the water bath, the residual water is wiped from your surface through a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled to the cutting chamber with the feed section at a constant line speed. Within the pelletizer, strands are cut between a rotor and a bed knife into roughly cylindrical pellets. These could be put through post-treatment like classifying, additional cooling, and drying, plus conveying.
In the event the requirement is perfect for continuous compounding, where fewer product changes come to mind and capacities are relatively high, automation can be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may use a self-stranding variation of this particular pelletizer. This is certainly observed as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and offer automatic transportation in the pelletizer.
Some polymer compounds are usually fragile and break easily. Other compounds, or some 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 through 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 the best value of flexibility.
Once the preferred pellet shape is more spherical than cylindrical, the most effective alternative is an underwater hot-face cutter. By using a capacity range between from about 20 lb/hr to many tons/hr, this system is relevant to all of materials with thermoplastic behavior. Functioning, the polymer melt is split right into a ring of strands that flow via an annular die in to a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into soft pvc granule, that happen to be immediately conveyed from the cutting chamber. The pellets are transported as being a slurry on the centrifugal dryer, where they can be separated from water through the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. Water is filtered, tempered, and recirculated back to the process.
The key parts of the machine-cutting head with cutting chamber, die plate, and commence-up valve, all on a common supporting frame-is 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 may be selected from a comprehensive range of accessories and combined right into a job-specific system.
In each and every underwater pelletizing system, a fragile temperature equilibrium exists within the cutting chamber and die plate. The die plate is both continuously cooled by the process water and heated by die-head heaters as well as the hot melt flow. Decreasing the energy loss in the die plate on 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 go with a thermally insulating die plate and switch to a fluid-heated die.
Many compounds can be abrasive, causing significant wear on contact parts such as the spinning blades and filter screens inside the centrifugal dryer. Other compounds may be understanding of mechanical impact and generate excessive dust. For these two special materials, a fresh kind of pellet dryer deposits the wet pellets over a perforated conveyor belt that travels across an aura knife, effectively suctioning off of the water. Wear of machine parts and also damage to the pellets may be reduced in comparison with an impact dryer. Considering the short residence time in the belt, some type of post-dewatering drying (including using a fluidized bed) or additional cooling is usually required. Benefits associated with this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of most parts getting into exposure to pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is essential.
Various other pelletizing processes are rather unusual in the compounding field. The best and cheapest way of reducing plastics to an appropriate size for additional processing may well be a simple grinding operation. However, the resulting particle shape and size are really inconsistent. Some important product properties will also suffer negative influence: The bulk density will drastically decrease along with the free-flow properties of the bulk can be very poor. That’s why such material are only suitable for inferior applications and must be marketed at rather affordable.
Dicing was a common size-reduction process since the early 20th Century. The necessity of this procedure has steadily decreased for up to three decades and currently makes a negligible contribution to the current pellet markets.
Underwater strand pelletizing is really a sophisticated automatic process. But this method of production is commonly used primarily in many virgin polymer production, such as for polyesters, nylons, and styrenic polymers, and it has no common application in today’s compounding.
Air-cooled die-face pelletizing can be a process applicable just 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 amounts of PVC compounds are transformed into pellets.
Water-ring pelletizing is also an automatic operation. But it is also suitable only 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 in excess of pellet shape and throughput volume. For instance, pellet temperature and residual moisture are inversely proportional; that is certainly, the better the product temperature, the low the residual moisture. Some compounds, for example various kinds of TPE, are sticky, especially at elevated temperatures. This effect may be measured by counting the agglomerates-twins and multiples-within a bulk of pellets.
Inside an underwater pelletizing system such agglomerates of sticky pellets can be generated in just two ways. First, just after the cut, the outer lining temperature in the pellet is merely about 50° F above the process water temperature, while the core in the pellet remains molten, and also the average pellet temperature is merely 35° to 40° F below the melt temperature. If two pellets enter into contact, they deform slightly, creating a contact surface involving the pellets that could be clear of process water. In that contact zone, the solidified skin will remelt immediately because of heat transported from the molten core, along with the pellets will fuse to each other.
Second, after discharge in the clear pvc granule through the dryer, the pellets’ surface temperature increases as a result of heat transport from your 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 might be intensified with smaller pellet size-e.g., micro-pellets-considering that the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration may be reduced with the help of some wax-like substance on the process water or by powdering the pellet surfaces soon after the pellet dryer.
Performing a variety of pelletizing test runs at consistent throughput rate will give you a solid idea of the utmost practical pellet temperature for this material type and pellet size. Anything dexrpky05 that temperature will raise the quantity of agglomerates, and anything below that temperature boosts residual moisture.
In certain cases, the pelletizing operation might be expendable. This is true only in applications where virgin polymers may be converted straight to finished products-direct extrusion of PET sheet from the polymer reactor, by way of example. If compounding of additives as well as other ingredients adds real value, however, direct conversion will not be possible. If pelletizing is needed, it is always wise to know your alternatives.