Pellets can be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes much more important when contemplating the ever-increasing demands positioned on compounders. No matter what equipment they now have, it never seems suited for the following challenge. An increasing number of products may require additional capacity. A new polymer or additive could be too tough, soft, or corrosive to the existing equipment. Or maybe the job takes a different pellet shape. In such cases, compounders need in-depth engineering know-how on processing, and close cooperation because of their pelletizing equipment supplier.
Step one in meeting such challenges starts off with equipment selection. The most prevalent classification of pelletizing processes involves two categories, differentiated by the state of the plastic material during the time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that is certainly quickly cut into pvc compound which can be 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 these basic processes could be tailored for the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps and different degrees of automation could be incorporated at any stage of your process.
For the greatest solution to your production requirements, start with assessing the status quo, as well as defining future needs. Create a five-year projection of materials and required capacities. Short-term solutions very often turn out to be more expensive and less satisfactory after a period of time. Though just about every pelletizing line at a compounder will have to process various products, any given system may be optimized simply for a compact range of the entire product portfolio.
Consequently, the rest of the products will need to be processed under compromise conditions.
The lot size, together with the nominal system capacity, will possess a strong effect on the pelletizing process and machinery selection. Since compounding production lots tend to be rather small, the flexibleness in the equipment can be a big issue. Factors include quick access for cleaning and repair and the cabability to simply and quickly move in one product to the next. Start-up and shutdown in the pelletizing system should involve minimum waste of material.
A line utilizing a simple water bath for strand cooling often is definitely the first selection for compounding plants. However, the patient layout can differ significantly, due to demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and are transported through a water bath and cooled. Following the strands leave the liquid bath, the residual water is wiped in the surface by means of a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled into the cutting chamber by the feed section in a constant line speed. Inside the pelletizer, strands are cut between a rotor as well as a bed knife into roughly cylindrical pellets. These may be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
If the requirement is designed 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 is observed as a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and provide automatic transportation to the pelletizer.
Some polymer compounds are usually fragile and break easily. Other compounds, or a selection of their ingredients, could be very responsive to moisture. For such materials, the belt-conveyor strand pelletizer is the ideal answer. A perforated conveyor belt takes the strands from 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 the best value of flexibility.
Once the preferred pellet shape is much more spherical than cylindrical, the most effective alternative is definitely an underwater hot-face cutter. Having a capacity range from from about 20 lb/hr to a number of tons/hr, this product is relevant to any or all materials with thermoplastic behavior. Operational, the polymer melt is split right into a ring of strands that flow via an annular die right into a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into rigid pvc compound, which can be immediately conveyed out from the cutting chamber. The pellets are transported as a slurry towards the centrifugal dryer, where these are separated from water through 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 procedure.
The primary parts of the device-cutting head with cutting chamber, die plate, and initiate-up valve, all over a common supporting frame-are 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 might be selected coming from a comprehensive selection of accessories and combined in 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 through the process water and heated by die-head heaters along with the hot melt flow. Lowering the energy loss through the die plate to 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 select a thermally insulating die plate or change to a fluid-heated die.
Many compounds are usually abrasive, leading to significant wear and tear on contact parts including the spinning blades and filter screens from the centrifugal dryer. Other compounds might be understanding of mechanical impact and generate excessive dust. For both these special materials, a fresh sort 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 in addition to problems for the pellets may be reduced compared to an impact dryer. Because of the short residence time in the belt, some kind of post-dewatering drying (for example having a fluidized bed) or additional cooling is generally required. Benefits associated with this new non-impact pellet-drying solution are:
•Lower production costs because of long lifetime of all the parts getting into experience of pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is important.
Various other pelletizing processes are rather unusual in the compounding field. The easiest and cheapest way of reducing plastics with an appropriate size for further processing generally is a simple grinding operation. However, the resulting particle size and shape are exceedingly inconsistent. Some important product properties may also suffer negative influence: The bulk density will drastically decrease and also the free-flow properties of your bulk can be bad. That’s why such material are only suitable for inferior applications and should be marketed at rather affordable.
Dicing had been a typical size-reduction process ever since the early 20th Century. The value of this procedure has steadily decreased for pretty much three decades and currently constitutes a negligible contribution to the current pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this procedure of production is used primarily in some 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 is a process applicable exclusively for non-sticky products, especially PVC. But this material 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 likewise an automatic operation. But it is also suitable simply for less sticky materials and finds its main application in polyolefin recycling and then in some minor applications in compounding.
Choosing the right pelletizing process involves consideration of over pellet shape and throughput volume. For instance, pellet temperature and residual moisture are inversely proportional; which is, the larger the product temperature, the lower the residual moisture. Some compounds, like many types of TPE, are sticky, especially at elevated temperatures. This effect could be measured by counting the agglomerates-twins and multiples-in the majority of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets could be generated in just two ways. First, right after the cut, the top temperature of your pellet is simply about 50° F higher than the process water temperature, as the core in the pellet continues to be molten, and also the average pellet temperature is merely 35° to 40° F underneath the melt temperature. If two pellets enter into contact, they deform slightly, building a contact surface between your pellets which may be free from process water. In that contact zone, the solidified skin will remelt immediately as a result of heat transported in the molten core, as well as the pellets will fuse to each other.
Second, after discharge of the pvc compound through the dryer, the pellets’ surface temperature increases due to heat transport from your core for 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-since the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration can 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 a variety of pelletizing test runs at consistent throughput rate will give you an idea of the utmost practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will heighten the quantity of agglomerates, and anything below that temperature will increase residual moisture.
In a few cases, the pelletizing operation might be expendable. This is true only in applications where virgin polymers may be converted instantly to finished products-direct extrusion of PET sheet from a polymer reactor, for example. If compounding of additives along with other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is necessary, it will always be best to know your choices.