granulation (granulating)

Granulation in the context of plastic recycling refers to the final stage of the mechanical recycling process: washed and dried plastic flake or shreds are fed into an extruder (single-screw or twin-screw), melted, and forced through a die plate with small circular holes to produce continuous strands, which are then cut by a rotating knife or water-ring pelletiser into uniform cylindrical or spherical granules (pellets) typically 3–5 mm in diameter. The resulting recycled granules (also called pellets, regrind, or regranulate) are the commercial product sold to plastic manufacturers as a secondary raw material for injection moulding, extrusion, blow moulding, or film blowing.

Granulator types in Indian recycling plants: (1) Strand pelletiser — extruded strands air-cooled on a conveyor, then cut; simple, low cost (Rs 5–15 lakh for 200–500 kg/hr), suited for rigid plastics (HDPE, PP, PVC); (2) Underwater pelletiser — strands immediately cut in a water bath; better for tacky or heat-sensitive materials (EVA, TPE, recycled LDPE film); Rs 15–30 lakh for equivalent throughput; (3) Hot-face die-face pelletiser — die plate cuts directly at the extrusion face, pellets quenched in water stream; Rs 20–50 lakh; used for PET, engineering polymers; best shape and consistency. In India, the majority of MSME-scale plastic recyclers (500 kg/hr or below) use strand pelletisers with local equipment manufacturers (prominent clusters in Rajkot, Ahmedabad, Daman).

Quality parameters for recycled granules: MFI (Melt Flow Index) — measures polymer flow at standard temperature and load; must match customer application (e.g. injection moulding PP needs MFI 8–30 g/10 min at 230°C/2.16 kg; blown film LDPE needs MFI 0.2–2 g/10 min at 190°C/2.16 kg); moisture content — must be ≤0.1% for polyolefins, ≤0.02% for PET before extrusion; contamination — metal particles (from shredder wear) detected by inline metal detectors (Rs 1–3 lakh) and removed; colour consistency — measured by colorimetry (L*, a*, b* values) for customers with colour specifications; bulk density — affects packaging, handling, and feeding consistency at the customer's plant.

For Indian recyclers, the granulation step is where the most value is added — washing transforms waste to flake (Rs 15–30/kg for HDPE flake); granulation transforms flake to pellet (Rs 55–90/kg for HDPE pellets), a 2–3x price lift. The extrusion temperature, screw design, and vacuum devolatilisation settings directly determine pellet quality. Invest in a twin-screw extruder with degassing (vacuum vent) for mixed-quality feedstock — the degassing removes moisture and volatiles that cause bubble defects in the output pellets. Single-screw extruders are fine for clean, dry, sorted streams but produce inferior results with variable or mildly contaminated inputs.