Pellets could 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 positioned on compounders. Irrespective of what equipment they now have, it never seems suited for the next challenge. A lot more products may require additional capacity. A new polymer or additive may be too tough, soft, or corrosive for that existing equipment. Or maybe the job requires a different pellet shape. In such instances, 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 typical classification of pelletizing processes involves two categories, differentiated by the condition of the plastic material at that time it’s cut:
•Melt pelletizing (hot cut): Melt provided by a die that is certainly quickly cut into pvc granule which are conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt coming from a die head is converted into strands that happen to 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 both cases, intermediate process steps as well as other degrees of automation can be incorporated at any stage in the process.
To find the best solution for your personal production requirements, get started with assessing the status quo, in addition to defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions frequently prove to be more costly and fewer satisfactory after a period of time. Though almost every pelletizing line at the compounder will need to process a number of products, any given system might be optimized exclusively for a compact selection of the whole product portfolio.
Consequently, all of the other products will need to be processed under compromise conditions.
The lot size, in combination with the nominal system capacity, will have got a strong affect on the pelletizing process and machinery selection. Since compounding production lots are typically rather small, the flexibility of your equipment is generally a big issue. Factors include quick access for cleaning and service and the ability to simply and quickly move from a single product to the next. Start-up and shutdown from the pelletizing system should involve minimum waste of material.
A line by using a simple water bath for strand cooling often will be the first option for compounding plants. However, the average person layout may differ significantly, due to demands of throughput, flexibility, and degree of system integration. In strand pelletizing, polymer strands exit the die head and are transported through a water bath and cooled. After the strands leave the liquid bath, the residual water is wiped from the surface through a suction air knife. The dried and solidified strands are transported to the pelletizer, being pulled in the cutting chamber from the feed section in a constant line speed. From the pelletizer, strands are cut between a rotor and a bed knife into roughly cylindrical pellets. This can be exposed to post-treatment like classifying, additional cooling, and drying, plus conveying.
If the requirement is 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 kind of automatic strand pelletizing line may utilize a self-stranding variation of this particular pelletizer. This really is seen as a a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and offer automatic transportation into the pelletizer.
Some polymer compounds can be fragile and break easily. Other compounds, or a selection of their ingredients, could be very understanding of moisture. For such materials, the belt-conveyor strand pelletizer is the greatest answer. A perforated conveyor belt takes the strands in the die and conveys them smoothly to the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-provide for a good price of flexibility.
When the preferred pellet shape is much more spherical than cylindrical, the ideal alternative is undoubtedly an underwater hot-face cutter. Using a capacity vary from from about 20 lb/hr to a number of tons/hr, this method is applicable to all of materials with thermoplastic behavior. Functioning, the polymer melt is split in a ring of strands that flow using an annular die into a cutting chamber flooded with process water. A rotating cutting head in the water stream cuts the polymer strands into upvc compound, which are immediately conveyed out of your cutting chamber. The pellets are transported as being a slurry on 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 this process.
The key parts of the device-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, such as 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 variety of accessories and combined right into 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 and the hot melt flow. Reducing the energy loss through the die plate to the process water generates a far more stable processing condition and increased product quality. In order to reduce this heat loss, the processor may choose a thermally insulating die plate or move to a fluid-heated die.
Many compounds are quite abrasive, leading to significant damage on contact parts for example the spinning blades and filter screens inside the centrifugal dryer. Other compounds may be responsive to mechanical impact and generate excessive dust. For both these special materials, a whole new type of pellet dryer deposits the wet pellets on the perforated conveyor belt that travels across an aura knife, effectively suctioning from the water. Wear of machine parts in addition to problems for the pellets could be greatly reduced in comparison with a positive change dryer. Due to the short residence time around the belt, some form of post-dewatering drying (including by using a fluidized bed) or additional cooling is usually required. Benefits associated with this new non-impact pellet-drying solution are:
•Lower production costs as a result of long lifetime of parts getting into contact with pellets.
•Gentle pellet handling, which ensures high product quality and less dust generation.
•Reduced energy consumption because no additional energy supply is needed.
A few other pelletizing processes are rather unusual from the compounding field. The most convenient and cheapest way of reducing plastics to an appropriate size for additional processing can be quite a simple grinding operation. However, the resulting particle size and shape are incredibly inconsistent. Some important product properties will also suffer negative influence: The bulk density will drastically decrease as well as the free-flow properties of the bulk will be poor. That’s why such material are only appropriate for inferior applications and should be marketed at rather inexpensive.
Dicing had been a common size-reduction process since the early twentieth century. The necessity of this process has steadily decreased for nearly 3 decades and currently will make a negligible contribution to the current pellet markets.
Underwater strand pelletizing can be a sophisticated automatic process. But this technique of production is used primarily in certain 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 really a process applicable just for non-sticky products, especially PVC. But this material is 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 additionally a computerized operation. However it is also suitable exclusively 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 in excess of pellet shape and throughput volume. As an example, pellet temperature and residual moisture are inversely proportional; that is, the larger the product temperature, the less the residual moisture. Some compounds, for example various types of TPE, are sticky, especially at elevated temperatures. This effect can be measured by counting the agglomerates-twins and multiples-in a bulk of pellets.
Inside an underwater pelletizing system such agglomerates of sticky pellets could be generated in two ways. First, immediately after the cut, the top temperature from the pellet is simply about 50° F higher than the process temperature of water, while the core of the pellet remains molten, and 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 involving the pellets that could be free from process water. In this contact zone, the solidified skin will remelt immediately due to heat transported in the molten core, as well as the pellets will fuse to one another.
Second, after discharge of your clear pvc granule from your dryer, the pellets’ surface temperature increases as a result of heat transport from the core for 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 may well be intensified with smaller pellet size-e.g., micro-pellets-since the ratio of surface to volume increases with smaller diameter.
Pellet agglomeration might be reduced with the help of some wax-like substance towards the process water or by powdering the pellet surfaces right after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate gives you a concept of the highest practical pellet temperature for your material type and pellet size. Anything dexrpky05 that temperature will raise the amount of agglomerates, and anything below that temperature increases residual moisture.
In a few cases, the pelletizing operation can be expendable. This really is only in applications where virgin polymers might be converted directly to finished products-direct extrusion of PET sheet from a polymer reactor, by way of example. If compounding of additives and other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is needed, it is always wise to know your alternatives.