Pellets may be “only” an intermediate product, however size, shape, and consistency matter in subsequent processing operations.
This becomes much more important when thinking about the ever-increasing demands placed on compounders. No matter what equipment they now have, it never seems suited for the upcoming challenge. An increasing number of products might require additional capacity. A whole new polymer or additive might be too tough, soft, or corrosive to the existing equipment. Or maybe the job takes a different pellet shape. In these instances, compounders need in-depth engineering know-how on processing, and close cooperation with their pelletizing equipment supplier.
The first step in meeting such challenges begins with equipment selection. The most common classification of pelletizing processes involves two classes, differentiated by the state the plastic material during the time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that is very quickly cut into pvc granule which are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt provided by a die head is changed into strands that happen to 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 cases, intermediate process steps and other levels of automation might be incorporated at any stage from the process.
To get the best solution for the production requirements, get started 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 prove to be higher priced and less satisfactory after a period of time. Though just about every pelletizing line in a compounder will need to process various products, any given system might be optimized just for a little selection of the full product portfolio.
Consequently, all of the other products will have to be processed under compromise conditions.
The lot size, along with the nominal system capacity, will have a very strong impact on the pelletizing process and machinery selection. Since compounding production lots are usually rather small, the flexibility in the equipment is often a big issue. Factors include comfortable access to clean and service and the cabability to simply and quickly move from a single product to the next. Start-up and shutdown of your pelletizing system should involve minimum waste of material.
A line working with a simple water bath for strand cooling often may be the first option for compounding plants. However, the average person 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. After the strands leave this type of water bath, the residual water is wiped from your surface through a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled into the cutting chamber with 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 could be put through post-treatment like classifying, additional cooling, and drying, plus conveying.
In case the requirement is designed 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 kind of automatic strand pelletizing line may utilize a self-stranding variation of this type 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 in to the pelletizer.
Some polymer compounds are quite fragile and break easily. Other compounds, or a selection of their ingredients, may be very understanding of 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 on 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.
When the preferred pellet shape is far more spherical than cylindrical, the most effective alternative is definitely an underwater hot-face cutter. Using a capacity range from from about 20 lb/hr to a number of tons/hr, this technique is relevant for all materials with thermoplastic behavior. Operational, the polymer melt is split right into a ring of strands that flow with 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 out of the cutting chamber. The pellets are transported being a slurry for 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 returning to the method.
The key elements of the device-cutting head with cutting chamber, die plate, and begin-up valve, all over 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 can be selected from your comprehensive array of accessories and combined into a job-specific system.
In every single underwater pelletizing system, a fragile temperature equilibrium exists inside 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. Lowering the energy loss through the die plate for the process water generates a far more stable processing condition and increased product quality. So that you can reduce this heat loss, the processor may select a thermally insulating die plate or move to a fluid-heated die.
Many compounds are quite abrasive, leading to significant wear on contact parts including the spinning blades and filter screens from the centrifugal dryer. Other compounds can be responsive to mechanical impact and generate excessive dust. For both of these special materials, a new sort of pellet dryer deposits the wet pellets with a perforated conveyor belt that travels across an air knife, effectively suctioning off of the water. Wear of machine parts in addition to injury to the pellets could be greatly reduced in contrast to an impact dryer. Due to the short residence time in the belt, some type of post-dewatering drying (for example with a fluidized bed) or additional cooling is usually required. Great things about this new non-impact pellet-drying solution are:
•Lower production costs as a result of long lifetime of all the parts coming into contact with pellets.
•Gentle pellet handling, which ensures high product quality and much less dust generation.
•Reduced energy consumption because no additional energy supply is essential.
Various other pelletizing processes are rather unusual within the compounding field. The easiest and cheapest way of reducing plastics to an appropriate size for more processing may well be a simple grinding operation. However, the resulting particle shape and size are incredibly inconsistent. Some important product properties may also suffer negative influence: The bulk density will drastically decrease along with the free-flow properties of your bulk could be poor. That’s why such material will only be suitable for inferior applications and should be marketed at rather inexpensive.
Dicing have been a standard size-reduction process considering that the early 20th Century. The value of this technique has steadily decreased for nearly 30 years and currently will make 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 a few virgin polymer production, such as for polyesters, nylons, and styrenic polymers, and 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 heating and air conditioning and discharged as dry-blends. Only negligible amounts of PVC compounds are turned into pellets.
Water-ring pelletizing is additionally a computerized operation. But it is also suitable exclusively for less sticky materials and finds its main application in polyolefin recycling as well as in some minor applications in compounding.
Picking the right pelletizing process involves consideration greater than pellet shape and throughput volume. As an example, pellet temperature and residual moisture are inversely proportional; that is, the greater the product temperature, the low the residual moisture. Some compounds, including various types of TPE, are sticky, especially at elevated temperatures. This effect can be measured by counting the agglomerates-twins and multiples-in the bulk of pellets.
Inside an underwater pelletizing system such agglomerates of sticky pellets may be generated in 2 ways. First, right after the cut, the top temperature of your pellet is simply about 50° F higher than the process temperature of water, while the core from the pellet remains to be molten, and also the average pellet temperature is just 35° to 40° F beneath the melt temperature. If two pellets come into contact, they deform slightly, making a contact surface involving the pellets that may be without any process water. In this contact zone, the solidified skin will remelt immediately due to heat transported from your molten core, as well as the pellets will fuse to each other.
Second, after discharge in the transparent pvc compound from the dryer, the pellets’ surface temperature increases because of heat transport in the core on the surface. If soft TPE pellets are kept 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 to volume increases with smaller diameter.
Pellet agglomeration may be reduced by adding some wax-like substance for the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing numerous pelletizing test runs at consistent throughput rate will provide you with a sense of the maximum practical pellet temperature for that material type and pellet size. Anything dexrpky05 that temperature will increase the quantity of agglomerates, and anything below that temperature improves residual moisture.
In some cases, the pelletizing operation can be expendable. This is true only in applications where virgin polymers may be converted right to finished products-direct extrusion of PET sheet from the polymer reactor, as an example. If compounding of additives and also other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is necessary, it will always be wise to know your choices.