Pellets can be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when it comes to the ever-increasing demands positioned on compounders. Whatever equipment they currently have, it never seems suited for the next challenge. A lot more products may require additional capacity. A whole new polymer or additive can be too tough, soft, or corrosive for that existing equipment. Or perhaps the job takes a different pellet shape. In such cases, compounders need in-depth engineering know-how on processing, and close cooperation with their pelletizing equipment supplier.
The initial step in meeting such challenges begins with equipment selection. The most frequent classification of pelletizing processes involves two categories, differentiated by the state of the plastic material back then it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that may be 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 converted into strands which can be cut into pellets after cooling and solidification.
Variations of the basic processes may be tailored for the specific input material and product properties in sophisticated compound production. In cases, intermediate process steps and other degrees of automation could be incorporated at any stage of your process.
To get the best solution for the production requirements, start out with assessing the status quo, as well as defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions often end up being more pricey and much less satisfactory after a time period of time. Though nearly every pelletizing line at the compounder will have to process many different products, any given system can be optimized just for a small variety of the full product portfolio.
Consequently, all of those other products will need to be processed under compromise conditions.
The lot size, in conjunction with the nominal system capacity, will possess a strong affect 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 quick access for cleaning and repair and the ability to simply and quickly move from one product to another. Start-up and shutdown in the 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 individual layout can vary significantly, due to the demands of throughput, flexibility, and level of system integration. In strand pelletizing, polymer strands exit the die head and they are transported by way of a water bath and cooled. Right after the strands leave the water bath, the residual water is wiped from your surface by means of a suction air knife. The dried and solidified strands are transported on 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 as well as a bed knife into roughly cylindrical pellets. This can be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
If the requirement is made for continuous compounding, where fewer product changes are participating and capacities are relatively high, automation may be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may use a self-stranding variation of this type of pelletizer. This really is characterized by a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and offer automatic transportation in to the pelletizer.
Some polymer compounds are very fragile and break easily. Other compounds, or a selection of their ingredients, may be very responsive to moisture. For such materials, the belt-conveyor strand pelletizer is the best answer. A perforated conveyor belt takes the strands from the die and conveys them smoothly for the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-permit a good deal of flexibility.
When the preferred pellet shape is a lot more spherical than cylindrical, the best alternative is an underwater hot-face cutter. Using a capacity range between 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 divided into a ring of strands that flow through an annular die right into a cutting chamber flooded with process water. A rotating cutting head within the water stream cuts the polymer strands into upvc compound, which can be immediately conveyed from the cutting chamber. The pellets are transported as a slurry on 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 water is filtered, tempered, and recirculated straight back to this process.
The key components of the machine-cutting head with cutting chamber, die plate, and begin-up valve, all with a common supporting frame-are one major assembly. The rest of the 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 coming from a comprehensive range 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 with the process water and heated by die-head heaters along with the hot melt flow. Reducing the energy loss from the die plate to 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 change to a fluid-heated die.
Many compounds are usually abrasive, resulting in significant wear on contact parts for example the spinning blades and filter screens from the centrifugal dryer. Other compounds could be responsive to mechanical impact and generate excessive dust. For both of these special materials, a brand new form of pellet dryer deposits the wet pellets with a perforated conveyor belt that travels across an air knife, effectively suctioning off the water. Wear of machine parts in addition to injury to the pellets might be cut down tremendously in comparison with an impact dryer. Given the short residence time on the belt, some kind of post-dewatering drying (such as with a fluidized bed) or additional cooling is usually required. Benefits of this new non-impact pellet-drying solution are:
•Lower production costs as a result of long lifetime of all the parts getting into connection 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 method of reducing plastics to a appropriate size for more processing can be quite 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 in the bulk can be bad. That’s why such material will only be appropriate for inferior applications and should be marketed at rather inexpensive.
Dicing have been a common size-reduction process considering that the early twentieth century. The importance of this method has steadily decreased for pretty much 3 decades 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 utilized primarily in a few virgin polymer production, such as for polyesters, nylons, and styrenic polymers, and contains no common application in today’s compounding.
Air-cooled die-face pelletizing is a process applicable only for non-sticky products, especially PVC. But this material is a lot more commonly compounded in batch mixers with heating and air conditioning and discharged as dry-blends. Only negligible levels of PVC compounds are transformed into pellets.
Water-ring pelletizing is additionally an automatic operation. But it is also suitable only for less sticky materials and finds its main application in polyolefin recycling and then in some minor applications in compounding.
Deciding on the best pelletizing process involves consideration of more than pellet shape and throughput volume. By way of example, pellet temperature and residual moisture are inversely proportional; that may be, the higher the product temperature, the reduced the residual moisture. Some compounds, like many types of TPE, are sticky, especially at elevated temperatures. This effect can be measured by counting the agglomerates-twins and multiples-in a majority of pellets.
Inside an underwater pelletizing system such agglomerates of sticky pellets might be generated in two ways. First, right after the cut, the surface temperature in the pellet is just about 50° F on top of the process temperature of water, whilst the core of the pellet continues to be molten, and the average pellet temperature is merely 35° to 40° F beneath the melt temperature. If two pellets enter into contact, they deform slightly, creating a contact surface in between the pellets that could be without any process water. In that contact zone, the solidified skin will remelt immediately on account of heat transported through the molten core, and the pellets will fuse to one another.
Second, after discharge of the pvc compound through the dryer, the pellets’ surface temperature increases due to heat transport from your core on 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-considering that the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration could be reduced by having 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 provides you with a sense of the utmost practical pellet temperature for that 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 could be expendable. This is correct only in applications where virgin polymers might be converted straight to finished products-direct extrusion of PET sheet coming from a polymer reactor, as an example. If compounding of additives and also other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is essential, it is always advisable to know your choices.