Pellets can be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes even more important when it comes to the ever-increasing demands put on compounders. Irrespective of what equipment they currently have, it never seems suited for the following challenge. A lot more products may need additional capacity. A fresh polymer or additive might be too tough, soft, or corrosive for the existing equipment. Or perhaps the job requires a different pellet shape. In these 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 starts off 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 coming from a die that may be almost immediately cut into pvc pellet that are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt from a die head is converted into strands that are cut into pellets after cooling and solidification.
Variations of such basic processes may be tailored towards the specific input material and product properties in sophisticated compound production. Within both cases, intermediate process steps and various levels of automation may be incorporated at any stage of your process.
To find the best solution for the production requirements, begin with assessing the status quo, in addition to defining future needs. Establish a five-year projection of materials and required capacities. Short-term solutions often turn out to be higher priced and fewer satisfactory after a time period of time. Though almost every pelletizing line at the compounder need to process a number of products, any system can be optimized just for a little selection of the complete product portfolio.
Consequently, all 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 influence on the pelletizing process and machinery selection. Since compounding production lots tend to be rather small, the flexibility of the equipment is generally a serious problem. Factors include comfortable access for cleaning and service and the opportunity to simply and quickly move in one product to another. Start-up and shutdown of your 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 average person layout can differ significantly, as a result of demands of throughput, flexibility, and amount of system integration. In strand pelletizing, polymer strands exit the die head and so are transported by way of a water bath and cooled. Right after 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 on the pelletizer, being pulled to the cutting chamber through the feed section with a constant line speed. Within the pelletizer, strands are cut between a rotor as well as a bed knife into roughly cylindrical pellets. These could be subjected to post-treatment like classifying, additional cooling, and drying, plus conveying.
When the requirement is made for continuous compounding, where fewer product changes are involved and capacities are relatively high, automation could be advantageous for reducing costs while increasing quality. This sort of automatic strand pelletizing line may use a self-stranding variation of this type of pelletizer. This really is described 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 very fragile and break easily. Other compounds, or a selection of their ingredients, may be very sensitive 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-allow for a good deal of flexibility.
As soon as the preferred pellet shape is more spherical than cylindrical, the ideal alternative is surely an underwater hot-face cutter. Using a capacity range between from about 20 lb/hr to a number of tons/hr, this product is applicable for all materials with thermoplastic behavior. Operational, the polymer melt is split into a ring of strands that flow through an annular die right into a cutting chamber flooded with process water. A rotating cutting head in water stream cuts the polymer strands into rigid pvc compound, which can be immediately conveyed from the cutting chamber. The pellets are transported being a slurry on 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. Water is filtered, tempered, and recirculated returning to the process.
The primary aspects of the system-cutting head with cutting chamber, die plate, and commence-up valve, all with a common supporting frame-are one major assembly. All of those other system components, for example 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 variety 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 with the process water and heated by die-head heaters as well as the hot melt flow. Reducing the energy loss from your die plate for 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 and/or move to a fluid-heated die.
Many compounds can be abrasive, causing significant wear on contact parts such as the spinning blades and filter screens from the centrifugal dryer. Other compounds may be responsive to mechanical impact and generate excessive dust. For both these special materials, a brand new type of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an air knife, effectively suctioning from the water. Wear of machine parts in addition to injury to the pellets may be reduced compared with a direct impact dryer. Given the short residence time on the belt, some form of post-dewatering drying (like having a fluidized bed) or additional cooling is normally required. Great things about this new non-impact pellet-drying solution are:
•Lower production costs because of long lifetime of all parts getting into connection with pellets.
•Gentle pellet handling, which ensures high product quality and fewer dust generation.
•Reduced energy consumption because no additional energy supply is important.
Some other pelletizing processes are rather unusual from the compounding field. The best and cheapest strategy for reducing plastics to an appropriate size for further processing may well be a simple grinding operation. However, the resulting particle shape and size are extremely inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease and also the free-flow properties in the bulk can be poor. That’s why such material will only be suitable for inferior applications and should be marketed at rather low priced.
Dicing ended up being a standard size-reduction process ever since the early 20th Century. The importance of this procedure has steadily decreased for almost thirty years and currently will make a negligible contribution to the present pellet markets.
Underwater strand pelletizing is actually a sophisticated automatic process. But this process of production is utilized primarily in a few virgin polymer production, like for polyesters, nylons, and styrenic polymers, and has no common application in today’s compounding.
Air-cooled die-face pelletizing is actually a process applicable just for non-sticky products, especially PVC. But this material is far more commonly compounded in batch mixers with heating and air conditioning and discharged as dry-blends. Only negligible quantities of PVC compounds are turned into pellets.
Water-ring pelletizing can also be an automatic operation. However it is also suitable simply for less sticky materials and finds its main application in polyolefin recycling as well as in some minor applications in compounding.
Selecting the best pelletizing process involves consideration of more than pellet shape and throughput volume. For example, pellet temperature and residual moisture are inversely proportional; which is, the larger the product temperature, the lower the residual moisture. Some compounds, for example various kinds 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.
In a underwater pelletizing system such agglomerates of sticky pellets might be generated by two ways. First, right after the cut, the surface temperature from the pellet is simply about 50° F over the process temperature of water, even though the core in the pellet remains to be molten, as well as the average pellet temperature is merely 35° to 40° F beneath the melt temperature. If two pellets enter into contact, they deform slightly, building a contact surface between the pellets that may be free of process water. Because contact zone, the solidified skin will remelt immediately because of heat transported from your molten core, along with the pellets will fuse to one another.
Second, after discharge from the clear pvc granule in 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-considering that the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration can be reduced by having some wax-like substance towards the process water or by powdering the pellet surfaces soon after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate will provide you with a sense of the most practical pellet temperature for the material type and pellet size. Anything dexrpky05 that temperature will heighten the volume of agglomerates, and anything below that temperature boosts residual moisture.
In some cases, the pelletizing operation could be expendable. This really is only in applications where virgin polymers may be converted instantly to finished products-direct extrusion of PET sheet from the polymer reactor, for example. If compounding of additives and other ingredients adds real value, however, direct conversion will not be possible. If pelletizing is needed, it will always be best to know your alternatives.