Pellets might be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes more important when contemplating the ever-increasing demands added to compounders. Whatever equipment they now have, it never seems suited for the following challenge. A lot more products may require additional capacity. A whole new polymer or additive could be too tough, soft, or corrosive to the existing equipment. Or maybe the job needs a different pellet shape. In these instances, compounders need in-depth engineering know-how on processing, and close cooperation using their pelletizing equipment supplier.
Step one in meeting such challenges begins with equipment selection. The most common classification of pelletizing processes involves two categories, differentiated by the state the plastic material during the time it’s cut:
•Melt pelletizing (hot cut): Melt from a die that may be very quickly cut into pvc granule that happen to 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 such basic processes can be tailored towards the specific input material and product properties in sophisticated compound production. Both in cases, intermediate process steps and various degrees of automation can be incorporated at any stage of the process.
For the greatest solution for your personal production requirements, start with assessing the status quo, in addition to defining future needs. Establish a five-year projection of materials and required capacities. Short-term solutions very often prove to be more pricey and less satisfactory after a period of time. Though nearly every pelletizing line at a compounder must process various products, any given system can be optimized simply 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 from the equipment can be a big issue. Factors include easy accessibility to clean and service and the cabability 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 utilizing a simple water bath for strand cooling often will be the first selection for compounding plants. However, the patient layout may vary significantly, due to demands of throughput, flexibility, and degree of system integration. In strand pelletizing, polymer strands exit the die head and so are transported through a water bath and cooled. Following the strands leave water 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 from the feed section at the constant line speed. In the pelletizer, strands are cut between a rotor and a bed knife into roughly cylindrical pellets. These may be exposed 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 involved and capacities are relatively high, automation might be advantageous for reducing costs while increasing quality. This type of automatic strand pelletizing line may employ a self-stranding variation of this kind of pelletizer. This is seen as a 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 very fragile and break easily. Other compounds, or a selection 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-enable the best value of flexibility.
When the preferred pellet shape is a lot more spherical than cylindrical, the ideal alternative is definitely an underwater hot-face cutter. By using a capacity cover anything from from about 20 lb/hr to many tons/hr, this system is relevant for all materials with thermoplastic behavior. Operational, the polymer melt is split into a ring of strands that flow using an annular die into a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into soft pvc granule, that happen to be immediately conveyed from the cutting chamber. The pellets are transported like 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. This type of water is filtered, tempered, and recirculated to the procedure.
The principle aspects of the machine-cutting head with cutting chamber, die plate, and start-up valve, all on a common supporting frame-is 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 may be selected from your comprehensive variety of accessories and combined right into a job-specific system.
In every underwater pelletizing system, a fragile temperature equilibrium exists in the cutting chamber and die plate. The die plate is both continuously cooled from the process water and heated by die-head heaters along with the hot melt flow. Lowering the energy loss from the die plate to the process water produces a much more stable processing condition and increased product quality. So that you can reduce this heat loss, the processor may choose a thermally insulating die plate or move to a fluid-heated die.
Many compounds are usually abrasive, resulting in significant deterioration on contact parts such as the spinning blades and filter screens from the centrifugal dryer. Other compounds could be sensitive to mechanical impact and generate excessive dust. For both these special materials, a brand new kind of pellet dryer deposits the wet pellets with a perforated conveyor belt that travels across an air knife, effectively suctioning from the water. Wear of machine parts along with problems for the pellets could be greatly reduced compared with a direct impact dryer. Due to the short residence time in the belt, some type of post-dewatering drying (such as by using a fluidized bed) or additional cooling is generally required. Advantages of this new non-impact pellet-drying solution are:
•Lower production costs on account of long lifetime of all parts getting into exposure to pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is necessary.
A few other pelletizing processes are rather unusual in the compounding field. The simplest and cheapest means of reducing plastics with an appropriate size for more processing might be a simple grinding operation. However, the resulting particle shape and size are exceedingly inconsistent. Some important product properties will likely suffer negative influence: The bulk density will drastically decrease and the free-flow properties in the bulk could be lousy. That’s why such material are only appropriate for inferior applications and must be marketed at rather inexpensive.
Dicing have been a standard size-reduction process ever since the early 20th Century. The value of this procedure has steadily decreased for up to thirty years and currently will make a negligible contribution to the present pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this method of production is commonly used primarily in a few virgin polymer production, like for polyesters, nylons, and styrenic polymers, and it has no common application in today’s compounding.
Air-cooled die-face pelletizing is really a process applicable only for non-sticky products, especially PVC. But this product is much more commonly compounded in batch mixers with heating and cooling and discharged as dry-blends. Only negligible numbers of PVC compounds are transformed into pellets.
Water-ring pelletizing is also an automatic operation. Yet it is also suitable only for less sticky materials and finds its main application in polyolefin recycling and in some minor applications in compounding.
Deciding on the best pelletizing process involves consideration of over pellet shape and throughput volume. As an 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 various kinds of TPE, are sticky, especially at elevated temperatures. This effect might be measured by counting the agglomerates-twins and multiples-in a bulk of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets might be generated in two ways. First, soon after the cut, the top temperature in the pellet is merely about 50° F on top of the process water temperature, even though the core of your pellet remains molten, along with the average pellet temperature is simply 35° to 40° F below the melt temperature. If two pellets come into contact, they deform slightly, building a contact surface between the pellets which may be free of process water. Because contact zone, the solidified skin will remelt immediately as a result of heat transported in the molten core, and also the pellets will fuse to one another.
Second, after discharge from the clear pvc granule in the dryer, the pellets’ surface temperature increases because of heat transport from 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 area to volume increases with smaller diameter.
Pellet agglomeration can be reduced with the addition of some wax-like substance to the process water or by powdering the pellet surfaces just after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate will give you a solid idea of the highest practical pellet temperature for your material type and pellet size. Anything dexrpky05 that temperature will heighten the volume 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 could be converted instantly to finished products-direct extrusion of PET sheet from the polymer reactor, for example. If compounding of additives along with other ingredients adds real value, however, direct conversion is just not possible. If pelletizing is needed, it is always wise to know your options.