Pellets might be “only” an intermediate product, however size, shape, and consistency matter in subsequent processing operations.
This becomes more important when thinking about the ever-increasing demands added to compounders. Regardless of what equipment they now have, it never seems suited for the following challenge. Progressively more products may require additional capacity. A fresh polymer or additive may be too tough, soft, or corrosive for the existing equipment. Or maybe the job requires 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 task in meeting such challenges starts off with equipment selection. The most common classification of pelletizing processes involves two classes, differentiated by the state of the plastic material during the time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that may be very quickly cut into pvc compound that are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt from a die head is converted into strands which are cut into pellets after cooling and solidification.
Variations of those basic processes could be tailored to the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps and other levels of automation may be incorporated at any stage from the process.
To get the best solution for your personal production requirements, begin 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 expensive and fewer satisfactory after a time period of time. Though just about every pelletizing line at the compounder will have to process many different products, virtually any system may be optimized only for a compact range of the whole product portfolio.
Consequently, all of those other products will need to be processed under compromise conditions.
The lot size, together with the nominal system capacity, will have a very strong impact on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibility of your equipment can be a big issue. Factors include easy access for cleaning and service and the opportunity 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 by using a simple water bath for strand cooling often is definitely the first selection for compounding plants. However, the average person layout can differ significantly, due to demands of throughput, flexibility, and standard of system integration. In strand pelletizing, polymer strands exit the die head and so are transported via a water bath and cooled. Following the strands leave the water bath, the residual water is wiped through the surface through a suction air knife. The dried and solidified strands are transported for the pelletizer, being pulled in to the cutting chamber from the feed section in a constant line speed. Inside the pelletizer, strands are cut from a rotor and a bed knife into roughly cylindrical pellets. This can be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
In case the requirement is for continuous compounding, where fewer product changes are participating and capacities are relatively high, automation can be advantageous for reducing costs while increasing quality. Such an automatic strand pelletizing line may employ a self-stranding variation of this kind of pelletizer. This really is observed 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 are usually 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 ideal answer. A perforated conveyor belt takes the strands in the die and conveys them smoothly towards the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-provide for a good deal of flexibility.
As soon as the preferred pellet shape is a lot more spherical than cylindrical, the most effective alternative is an underwater hot-face cutter. By using a capacity cover anything from from about 20 lb/hr to a few tons/hr, this technique is applicable for all materials with thermoplastic behavior. Functioning, the polymer melt is split in a ring of strands that flow using an annular die in to a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into upvc compound, that happen to be immediately conveyed out of the cutting chamber. The pellets are transported as a slurry for the centrifugal dryer, where these are separated from water with the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. The water is filtered, tempered, and recirculated straight back to the method.
The principle components of the machine-cutting head with cutting chamber, die plate, and start-up valve, all with a common supporting frame-are one major assembly. All of the 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 may be selected from a comprehensive range of accessories and combined in to a job-specific system.
In just about 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 and also the hot melt flow. Reducing the energy loss from your die plate towards the process water generates a much more stable processing condition and increased product quality. To be able to reduce this heat loss, the processor may pick a thermally insulating die plate and/or change to a fluid-heated die.
Many compounds are very abrasive, causing significant deterioration on contact parts like the spinning blades and filter screens inside the centrifugal dryer. Other compounds may be sensitive to mechanical impact and generate excessive dust. For both these special materials, a new kind of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an air knife, effectively suctioning off the water. Wear of machine parts and also problems for the pellets could be reduced in contrast to an impact dryer. Because of the short residence time on the belt, some form of post-dewatering drying (like using a fluidized bed) or additional cooling is usually required. Advantages of this new non-impact pellet-drying solution are:
•Lower production costs as a result of long lifetime of parts coming into experience of pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is essential.
A few other pelletizing processes are rather unusual in the compounding field. The most convenient and cheapest way of reducing plastics for an appropriate size for further 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 along with the free-flow properties in the bulk can be lousy. That’s why such material are only suitable for inferior applications and must be marketed at rather low cost.
Dicing had been a standard size-reduction process since the early 20th Century. The significance of this process has steadily decreased for up to three decades and currently creates a negligible contribution to the present pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this technique of production is used primarily in certain 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 actually a process applicable only for non-sticky products, especially PVC. But this material is more commonly compounded in batch mixers with air conditioning and discharged as dry-blends. Only negligible quantities of PVC compounds are transformed into pellets.
Water-ring pelletizing is additionally a computerized operation. But 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 greater than pellet shape and throughput volume. For example, pellet temperature and residual moisture are inversely proportional; that is, the larger the product temperature, the lower the residual moisture. Some compounds, including various types of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-inside a majority of pellets.
In an underwater pelletizing system such agglomerates of sticky pellets can be generated by two ways. First, soon after the cut, the surface temperature of your pellet is just about 50° F above the process water temperature, whilst the core from the pellet remains molten, as well as the average pellet temperature is simply 35° to 40° F underneath the melt temperature. If two pellets enter in to contact, they deform slightly, developing a contact surface between the pellets which may be free from process water. In that contact zone, the solidified skin will remelt immediately because of heat transported through the molten core, and the pellets will fuse to one another.
Second, after discharge in the transparent pvc compound through the dryer, the pellets’ surface temperature increases as a result 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 might be intensified with smaller pellet size-e.g., micro-pellets-since the ratio of surface to volume increases with smaller diameter.
Pellet agglomeration may be reduced with the addition of some wax-like substance on the process water or by powdering the pellet surfaces just after the pellet dryer.
Performing numerous pelletizing test runs at consistent throughput rate will provide you with a sense of the most practical pellet temperature for that material type and pellet size. Anything dexrpky05 that temperature will heighten the level of agglomerates, and anything below that temperature will increase residual moisture.
In certain cases, the pelletizing operation may be expendable. This is true only in applications where virgin polymers can be converted right to finished products-direct extrusion of PET sheet from a polymer reactor, as an example. If compounding of additives and other ingredients adds real value, however, direct conversion will not be possible. If pelletizing is essential, it usually is best to know your options.