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 pelletizing equipment supplier.

Two Core Pelletizing Methods

All pelletizing systems fall into one of two categories based on when the material is cut:

Melt pelletizing (hot cut) — The molten polymer exits the die and is cut immediately into pellets while still hot. Pellets are then cooled and conveyed by liquid or gas. This method produces rounder pellets and suits high-throughput, continuous operations.

Strand pelletizing (cold cut) — The molten polymer is first extruded into continuous strands, cooled and solidified in a water bath or air stream, then cut into cylindrical pellets. This method offers more flexibility for smaller batches and frequent product changes.

Planning Your Selection

Start with a five-year projection of materials and capacities. No single system handles every product optimally — each line is typically efficient for only a narrow slice of your portfolio. The rest run under compromise conditions.

Lot size matters. Compounding batches are often small, so equipment flexibility — quick cleaning, fast changeover, minimal startup waste — is usually as important as raw throughput.

Strand Pelletizing (Cold Cut)

This is the typical starting point for compounding plants. Polymer strands exit the die, pass through a water bath for cooling, and have surface water removed by an air knife. The solidified strands are then pulled into a pelletizer at constant speed and cut between a rotor and bed knife into cylindrical pellets.

For higher-volume, continuous operations with fewer product changes, automated strand lines replace the water bath with a cooling water slide and perforated conveyor belt that feed strands directly into the pelletizer.
Fragile compounds or moisture-sensitive materials suit a belt-conveyor strand pelletizer. Strands travel on a perforated belt with customizable cooling — water spray, mist, compressed air, or fan cooling — minimizing breakage and water exposure.

Underwater Pelletizing (Hot Cut)

Choose this method when you need spherical rather than cylindrical pellets. It handles all thermoplastic materials from 20 lb/hr up to multiple tons per hour.

Melt passes through an annular die into a cutting chamber filled with process water. A rotating cutting head slices the strands into pellets, which are immediately conveyed out as a slurry. A centrifugal dryer separates pellets from water using rotating paddles; water is filtered, tempered, and recirculated.

Temperature control is critical. The die plate is simultaneously cooled by water and heated by die-head heaters. Using a thermally insulated die plate or fluid-heated die reduces heat loss and stabilizes processing conditions.

For abrasive compounds, standard centrifugal dryers cause rapid blade and screen wear. For impact-sensitive materials, excessive dust is a problem. A non-impact belt dryer deposits wet pellets on a perforated conveyor with an air knife for water removal. This extends part life, reduces dust, and cuts energy use, though post-drying or additional cooling may be needed.

Other Methods: When They Apply

Grinding — The cheapest size-reduction method, but produces irregular particles with poor bulk density and flow properties. Limited to low-value applications.

Dicing — Historically common, now largely obsolete in compounding. Minimal market presence.

Underwater strand pelletizing — Used mainly in virgin polymer production (polyesters, nylons, styrenics), not typical for compounding.

Air-cooled die-face — Only for non-sticky materials like PVC. Most PVC is sold as dry-blend rather than pellets.

Water-ring pelletizing — Suited for low-stick materials, mainly polyolefin recycling and niche compounding.

Product Quality Factors

Pellet temperature and residual moisture move in opposite directions — higher temperature means lower moisture. Sticky materials like TPE are especially problematic at elevated temperatures, forming agglomerates (twins and multiples).

In underwater systems, agglomeration happens two ways. First, immediately after cutting, the pellet surface is cool but the core remains molten. If two pellets touch, the contact zone reheats and fuses. Second, after drying, heat migrates from core to surface. Stored soft pellets deform, enlarge contact areas, and stick together — worse with micro-pellets due to higher surface-area-to-volume ratio.

Countermeasures include adding wax-like substances to process water or powdering pellet surfaces post-dryer.

Run pelletizing tests at consistent throughput to find the maximum practical pellet temperature for your material and size. Above that temperature, agglomerates rise; below it, residual moisture increases.

In a few cases, the pelletizing operation may be expendable. This is true only in applications where virgin polymers can be converted directly to finished products—direct extrusion of PET sheet from a polymer reactor, for example. If compounding of additives and other ingredients adds real value, however, direct conversion is not possible. If pelletizing is necessary, it is always best to know your options.

FAQ: Selecting a Pelletizer

Strand or underwater: which should I choose first?

Start with strand pelletizing if you run small batches, frequent product changes, or need simple operation and lower capital cost. Move to underwater if you need higher throughput, rounder pellets, or process sticky materials that tangle in strand lines.

How do I prevent pellet agglomeration with TPE?

Control pellet temperature precisely. Keep it below the agglomeration threshold for your specific material. Add process water additives or apply surface powder immediately after drying. Consider non-impact drying for especially soft compounds.

Can one pelletizing line handle all my products?

Unlikely. Most lines are optimized for a narrow product range. Running diverse materials on one system means compromise on quality, yield, or changeover time. Plan for a primary line and possible secondary equipment for outlier products.