Pellets can 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. Whatever equipment they currently have, it never seems suited for the following challenge. A lot more products might need additional capacity. A whole new polymer or additive may be too tough, soft, or corrosive to the existing equipment. Or maybe the job demands 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 task in meeting such challenges begins with equipment selection. The most common classification of pelletizing processes involves two classes, differentiated by the condition of the plastic material at that time it’s cut:
•Melt pelletizing (hot cut): Melt from a die which is very quickly cut into pvc granule that are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt from a die head is transformed into strands that are cut into pellets after cooling and solidification.
Variations of the basic processes might be tailored on the specific input material and product properties in sophisticated compound production. In both cases, intermediate process steps and various degrees of automation might be incorporated at any stage in the process.
To get the best solution to your production requirements, get started with assessing the status quo, as well as defining future needs. Establish a five-year projection of materials and required capacities. Short-term solutions frequently prove to be more costly and less satisfactory after a period of time. Though nearly every pelletizing line in a compounder must process various products, any system could be optimized exclusively for a small range of the full product portfolio.
Consequently, all of the other products will need to be processed under compromise conditions.
The lot size, along with the nominal system capacity, will have got a strong impact on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibleness in the equipment is generally a serious problem. Factors include easy 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 the pelletizing system should involve minimum waste of material.
A line by using a simple water bath for strand cooling often is definitely the first choice for compounding plants. However, the person layout may vary significantly, as a result of demands of throughput, flexibility, and standard of system integration. In strand pelletizing, polymer strands exit the die head and they are transported by way of a water bath and cooled. Following the strands leave water bath, the residual water is wiped from your 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 by the feed section at a constant line speed. In the pelletizer, strands are cut between a rotor plus a bed knife into roughly cylindrical pellets. These can be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
In case the requirement is designed for continuous compounding, where fewer product changes are involved and capacities are relatively high, automation might be advantageous for reducing costs while increasing quality. Such an automatic strand pelletizing line may employ a self-stranding variation of this type of pelletizer. This really is observed as 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, may 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 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-enable a good price of flexibility.
As soon as the preferred pellet shape is far more spherical than cylindrical, the ideal alternative is an underwater hot-face cutter. With a capacity cover anything from from about 20 lb/hr to several tons/hr, this technique is relevant to all materials with thermoplastic behavior. Functioning, the polymer melt is split in 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 are immediately conveyed out from the cutting chamber. The pellets are transported being a slurry to the centrifugal dryer, where they may be separated from water by the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. This type of water is filtered, tempered, and recirculated to this process.
The principle parts of the program-cutting head with cutting chamber, die plate, and start-up valve, all on the 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 can be selected from a comprehensive selection of accessories and combined in a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists throughout the cutting chamber and die plate. The die plate is both continuously cooled with the process water and heated by die-head heaters as well as the hot melt flow. Reducing the energy loss through the die plate towards the process water produces a considerably 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 or switch to a fluid-heated die.
Many compounds are quite abrasive, contributing to significant wear and tear on contact parts including the spinning blades and filter screens inside the centrifugal dryer. Other compounds can be responsive to mechanical impact and generate excessive dust. For both these special materials, a brand new form of pellet dryer deposits the wet pellets with a perforated conveyor belt that travels across an aura knife, effectively suctioning from the water. Wear of machine parts as well as injury to the pellets may be cut down tremendously in contrast to a positive change dryer. Because of the short residence time around the belt, some type of post-dewatering drying (including with a fluidized bed) or additional cooling is often required. Benefits of this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of parts coming 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 necessary.
Various other pelletizing processes are rather unusual inside the compounding field. The most convenient and cheapest strategy for reducing plastics for an appropriate size for additional processing may well be 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 as well as the free-flow properties in the bulk could be very poor. That’s why such material will only be acceptable for inferior applications and should be marketed at rather inexpensive.
Dicing have been a typical size-reduction process considering that the early 20th Century. The significance of this procedure has steadily decreased for almost thirty years and currently constitutes a negligible contribution to the current pellet markets.
Underwater strand pelletizing is actually a sophisticated automatic process. But this method of production is utilized primarily in many virgin polymer production, including 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 exclusively for non-sticky products, especially PVC. But this material is more commonly compounded in batch mixers with heating and air conditioning and discharged as dry-blends. Only negligible quantities of PVC compounds are transformed into pellets.
Water-ring pelletizing is also an automatic operation. Yet it is also suitable just for less sticky materials and finds its main application in polyolefin recycling as well as in some minor applications in compounding.
Choosing the right 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 is certainly, the greater the product temperature, the reduced the residual moisture. Some compounds, including various types of TPE, are sticky, especially at elevated temperatures. This effect might be measured by counting the agglomerates-twins and multiples-within a bulk of pellets.
In a underwater pelletizing system such agglomerates of sticky pellets might be generated by two ways. First, immediately after the cut, the surface temperature in the pellet is simply about 50° F above the process water temperature, whilst the core in the pellet remains molten, as well as the average pellet temperature is only 35° to 40° F below the melt temperature. If two pellets enter into contact, they deform slightly, creating a contact surface between the pellets which might be free from process water. In that contact zone, the solidified skin will remelt immediately due to heat transported from your molten core, and also the pellets will fuse to one another.
Second, after discharge of your transparent pvc compound from your dryer, the pellets’ surface temperature increases because of heat transport in the core to 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 may well be intensified with smaller pellet size-e.g., micro-pellets-ever since the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration may be reduced by adding some wax-like substance on the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate will give you a solid idea of the most practical pellet temperature for this material type and pellet size. Anything dexrpky05 that temperature will increase the volume 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 may be converted straight 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 not possible. If pelletizing is essential, it will always be advisable to know your alternatives.