Pellets may be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.

This becomes even more important when considering the ever-increasing demands placed on compounders. No matter what equipment they currently have, it never seems suited for the next challenge. An increasing number of products may require additional capacity. A new polymer or additive may be too tough, soft, or corrosive for the existing equipment. Or perhaps the job requires a different pellet shape. In such cases, compounders need in-depth engineering know-how on processing, and close cooperation with their plastic pelletizing machines supplier.

The first step in meeting such challenges begins with equipment selection. The most common classification of pelletizing processes involves two categories, differentiated by the state of the plastic material at the time it’s cut:

• Melt pelletizing (hot cut):

Melt coming from a die that is almost immediately cut into pellets that are conveyed and cooled by liquid or gas.

• Strand pelletizing (cold cut):

Melt coming from a die head is converted into strands that are cut into pellets after cooling and solidification.

Variations of these basic processes may be tailored to the specific input material and product properties in sophisticated compound production. In both cases, intermediate process steps and different degrees of automation can be incorporated at any stage of the process.

To find the best solution for your production requirements, start with assessing the status quo, as well as defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions very often prove to be more expensive and less satisfactory after a period of time. Though almost every plastic pelletizing line at a compounder will have to process a variety of products, any given system can be optimized only for a small range of the entire product portfolio.

Consequently, all the other products will have to be processed under compromise conditions.

The lot size, in combination with the nominal system capacity, will have a very strong impact on the pelletizing process and machinery selection. Since compounding production lots tend to be rather small, the flexibility of the equipment is often a big issue. Factors include easy access for cleaning and service and the ability to simply and quickly move from one product to the next. Start-up and shutdown of the pelletizing system should involve minimum waste of material.

A line utilizing a simple water bath for strand cooling often is the first option for compounding plants. However, the individual layout can vary significantly, due to the 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 water bath, the residual water is wiped from the surface by means of a suction air knife. The dried and solidified strands are transported to the plastic pelletizing machine, being pulled into the cutting chamber by the feed section at a constant line speed. In the pelletizer, strands are cut between a rotor and a bed knife into roughly cylindrical pellets. These can be subjected 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 may be advantageous for reducing costs while increasing quality. Such an automatic strand pelletizing line may utilize a self-stranding variation of this type of pelletizer. This is characterized by a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and provide automatic transportation into the plastic pelletizing machine.

Some polymer compounds are quite 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 to 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.

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Article Source: http://www.ptonline.com/articles/how-to-select-the-right-pelletizer