Pellets can be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when thinking about the ever-increasing demands positioned on compounders. Whatever equipment they now have, it never seems suited for the following challenge. An increasing number of products may require additional capacity. A fresh polymer or additive might be too tough, soft, or corrosive for your existing equipment. Or perhaps the job requires a different pellet shape. In such instances, compounders need in-depth engineering know-how on processing, and close cooperation using their pelletizing equipment supplier.
The first step in meeting such challenges starts off with equipment selection. The most prevalent classification of pelletizing processes involves two categories, differentiated by the state the plastic material back then it’s cut:
•Melt pelletizing (hot cut): Melt coming from a die that may be very quickly cut into pvc granule which are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt originating from a die head is transformed into strands which are cut into pellets after cooling and solidification.
Variations of such basic processes might be tailored for the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps and various degrees of automation might be incorporated at any stage of the process.
For the greatest solution for your production requirements, begin with assessing the status quo, and also defining future needs. Create a five-year projection of materials and required capacities. Short-term solutions often prove to be higher priced and much less satisfactory after a time period of time. Though virtually every pelletizing line in a compounder will need to process various products, any given system can be optimized only for a compact array of the whole product portfolio.
Consequently, the rest of the products will have to be processed under compromise conditions.
The lot size, along with the nominal system capacity, will have got a strong affect on the pelletizing process and machinery selection. Since compounding production lots are usually rather small, the flexibility of the equipment can be a big issue. Factors include quick access to clean and service and the cabability to simply and quickly move from a single product to another. Start-up and shutdown from the pelletizing system should involve minimum waste of material.
A line working with a simple water bath for strand cooling often is the first selection for compounding plants. However, the person layout may vary significantly, due to the demands of throughput, flexibility, and standard of system integration. In strand pelletizing, polymer strands exit the die head and so are transported through a water bath and cooled. After the strands leave the liquid bath, the residual water is wiped from the surface by means of a suction air knife. The dried and solidified strands are transported to the pelletizer, being pulled in to the cutting chamber from the feed section at the constant line speed. Within the pelletizer, strands are cut between a rotor along with a bed knife into roughly cylindrical pellets. These may be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
In case the requirement is made for continuous compounding, where fewer product changes are participating and capacities are relatively high, automation could be advantageous for reducing costs while increasing quality. This sort of automatic strand pelletizing line may utilize a self-stranding variation of this type of pelletizer. This can be seen as a a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and give automatic transportation to the pelletizer.
Some polymer compounds are very fragile and break easily. Other compounds, or some of their ingredients, could 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 towards the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-allow for a great deal of flexibility.
If the preferred pellet shape is more spherical than cylindrical, the very best alternative is undoubtedly an underwater hot-face cutter. Using a capacity vary from from about 20 lb/hr to several tons/hr, this product is relevant for all materials with thermoplastic behavior. In operation, the polymer melt is split right into a ring of strands that flow using an annular die into a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into soft pvc granule, which are immediately conveyed out of the cutting chamber. The pellets are transported like a slurry for the centrifugal dryer, where they are separated from water from the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The liquid is filtered, tempered, and recirculated returning to the process.
The main elements of the program-cutting head with cutting chamber, die plate, and commence-up valve, all over a common supporting frame-is one major assembly. All the 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 can be selected from a comprehensive selection of accessories and combined in to a job-specific system.
In each and every underwater pelletizing system, a fragile temperature equilibrium exists inside the cutting chamber and die plate. The die plate is both continuously cooled by the process water and heated by die-head heaters and the hot melt flow. Lowering the energy loss through the die plate towards the process water results in 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 and change to a fluid-heated die.
Many compounds can be abrasive, resulting in significant wear on contact parts including the spinning blades and filter screens within the centrifugal dryer. Other compounds can be understanding of mechanical impact and generate excessive dust. For the two of these special materials, a brand new form of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an air knife, effectively suctioning from the water. Wear of machine parts as well as harm to the pellets might be cut down tremendously compared to an impact dryer. Considering the short residence time about the belt, some kind of post-dewatering drying (such as using a fluidized bed) or additional cooling is normally required. Advantages of this new non-impact pellet-drying solution are:
•Lower production costs because of long lifetime of all parts getting 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 important.
Various other pelletizing processes are rather unusual within the compounding field. The simplest and cheapest means of reducing plastics to an appropriate size for further processing can be quite a simple grinding operation. However, the resulting particle size and shape are really inconsistent. Some important product properties will likely suffer negative influence: The bulk density will drastically decrease and also the free-flow properties from the bulk can be very poor. That’s why such material will only be appropriate for inferior applications and must be marketed at rather inexpensive.
Dicing had been a frequent size-reduction process because the early 20th Century. The value of this technique has steadily decreased for up to three decades and currently makes a negligible contribution to the present pellet markets.
Underwater strand pelletizing is a sophisticated automatic process. But this technique of production is used primarily in certain virgin polymer production, for example 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 exclusively for non-sticky products, especially PVC. But this product is much more commonly compounded in batch mixers with cooling and heating and discharged as dry-blends. Only negligible quantities of PVC compounds are transformed into pellets.
Water-ring pelletizing is also an automated operation. However it is also suitable just 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 example, pellet temperature and residual moisture are inversely proportional; that is, the higher the product temperature, the reduced the residual moisture. Some compounds, including various types of TPE, are sticky, especially at elevated temperatures. This effect may be measured by counting the agglomerates-twins and multiples-inside a majority of pellets.
Inside an underwater pelletizing system such agglomerates of sticky pellets can be generated by two ways. First, immediately after the cut, the surface temperature of your pellet is only about 50° F higher than the process temperature of water, even though the core of the pellet continues to be molten, as well as the average pellet temperature is just 35° to 40° F underneath the melt temperature. If two pellets enter into contact, they deform slightly, creating a contact surface between the pellets which might be clear of process water. In that contact zone, the solidified skin will remelt immediately due to heat transported from the molten core, and the pellets will fuse to each other.
Second, after discharge in the pvc compound through the dryer, the pellets’ surface temperature increases due to heat transport from the core to 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 might be intensified with smaller pellet size-e.g., micro-pellets-since the ratio of area to volume increases with smaller diameter.
Pellet agglomeration can be reduced with the addition of 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 will provide you with an idea of the maximum practical pellet temperature for this material type and pellet size. Anything dexrpky05 that temperature will raise the level of agglomerates, and anything below that temperature improves residual moisture.
In a few cases, the pelletizing operation can be expendable. This really is only in applications where virgin polymers might be converted right to finished products-direct extrusion of PET sheet from your polymer reactor, for example. If compounding of additives and also other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is essential, it is usually better to know your alternatives.