SJ Plastic Pelletizing Machine for Rigid Plastics

SJ Plastic Pelletizing Machine for Rigid Plastics

The SJ series is a single-screw pelletizing machine built for hard plastic: washed HDPE container regrind, injection runners, crate and drum flake, automotive and electronics housings. It takes that regrind, melts it, filters the melt, pushes it through a die plate and cuts it into pellets a moulder or compounder can buy.

What separates it from the film pelletizers in the same catalogue is what it does not have. Rigid regrind has enough bulk density to fall into a screw on its own, so the SJ needs no compactor and no agglomerator in front of the extruder. The engineering effort moves instead to the three places where rigid feed is genuinely harder than film: melt filtration, degassing, and holding a pellet size that stays consistent when the feed does not.

Why rigid feed changes the machine

Soft film is light and springy. Drop it into a screw and it bridges in the throat instead of feeding, which is why film pelletizers are built around a compactor that crushes and densifies the material before the extruder sees it. Rigid regrind has the opposite problem set. It free-falls, it feeds evenly and it fills the screw channel without help — so the SJ uses a wide-throat feed opening optimised for rigid fragments and nothing more elaborate than that. A vertical, horizontal or forced feeder can be fitted depending on how the flake arrives, and a side feeder is available for masterbatch.

The cost of that simplicity shows up further down the barrel. Rigid polymers run at higher melt viscosity than film, so shear heat builds faster and unmelted cores survive longer. Post-consumer rigid also carries more of what nobody wants in a pellet: label paper, a stray fragment of a different resin, glue, and the fine grit that thick wall releases when it is crushed. None of that is stopped by the feeder. All of it has to be caught by the screw design and the melt filter.

What the machine runs on

The archive of applications for this machine falls into three commercial buckets, and they behave differently on the same line.

Bottle-to-pellet recycling. HDPE container regrind is the volume application — the direct downstream of a rigid washing line. Feed is reasonably uniform, moisture is already low, and the machine can be tuned once and left alone.

Engineering plastic recovery. PP, PS and ABS automotive parts, and PC/ABS electronic housings. Higher-value material, but the melt is stiffer and the tolerance for black specks is much lower, because the buyer is putting the pellet into a visible part.

Industrial scrap and post-consumer mixed streams. Injection moulding runners and production rejects are the cleanest feed the machine will ever see and the easiest to sell back into the same factory. Mixed rigid streams are the hardest: variable melt index, variable contamination, and a screen changer that will need attention more often.

Inside the barrel: L/D, heating zones and vacuum degassing

The screw and barrel are supplied at an L/D ratio of 25:1 to 32:1, with hardened steel construction and an optional bimetallic lining. Barrel heating is split into five to six zones under PID temperature control.

Length-to-diameter ratio is not a specification to skim past. A short screw plasticises less thoroughly, and on rigid feed the consequence is unmelted cores riding through the die and showing up as hard spots in the pellet. A longer screw gives the polymer more residence time at temperature, which is exactly what a high-viscosity rigid regrind needs, at the price of more shear history. Where in the 25:1 to 32:1 band your line should sit is a function of what you are running, not of what looks impressive on a quotation.

Vacuum degassing zones handle volatiles and vapour. On rigid feed this is doing two jobs at once: pulling off whatever moisture survived the washing line, and pulling off the light volatiles that come from residual product, printing ink and adhesive. Skip it and both end up as voids in the pellet.

Melt filtration and the screen changer

Filtration is where a rigid line either keeps running or keeps stopping. The SJ uses a hydraulic screen changer taking 40 to 120 mesh, with by-pass valves for continuous operation. Plate type, piston type and laser filter are all available.

Two decisions sit inside that one line of specification. The first is mesh: a finer screen catches more, produces a cleaner pellet and blocks faster, so the mesh you choose is really a statement about how dirty your feed is and how often you are prepared to intervene. The second is the by-pass. Without it, every screen change is a line stop, a purge and a batch of off-spec pellet at restart; with it, the melt is diverted while the blocked screen is swapped and the line does not come down. On a clean industrial-scrap feed this matters little. On post-consumer mixed rigid, it is the difference between a line that runs a shift and a line that runs a week.

The laser filter option belongs to the dirtiest end of that range — feed where a conventional screen would blind too quickly to be economic.

Die head and how the pellet is cut

The die is a multi-hole die plate with 2–8 mm holes, made from nitrided 38CrMoAl steel. That material choice is about hole geometry surviving. A die hole that wears oval stops producing a round strand, and once strand diameter drifts, pellet length drifts with it.

Two cutting methods are offered, and choosing between them is the most consequential configuration decision on the machine:

Strand, or spaghetti, cutting. The melt leaves the die as strands, runs through a water bath, is dried and then cut. Described in the specification as universal for PE, PP, EPS, PA, PET, ABS and PC — which is the point of it. If your feed mix changes, or if you intend to run engineering plastics alongside commodity polyolefin, strand cutting is the configuration that does not have to be revisited.

Water-ring cutting. Pellets are cut at the die face and thrown into a ring of water. The specification scopes it to PE, PP at low melt flow index and small capacity, and PS. It is faster to start and produces a rounder pellet, but the scope note is a real limit rather than a caution.

Cooling on both routes is water cooling.

Model range: what each size actually gives you

Six sizes, named for the two screw diameters they carry. The smallest, SJ90-100, runs a ∅90/∅100 extruder at 150–250 kg/h on 175 kW of installed power. The largest, SJ180-200, runs ∅180/∅200 at 900–1100 kg/h on 565 kW.

Read down the two columns together and the shape of it is clear: output rises roughly seven times across the range while installed power rises about three times. Energy per tonne falls steadily as the machine gets bigger, which is the usual argument for buying one size up. The counter-argument is equally real — a bigger extruder wants a steady feed rate to hold a stable melt temperature, and a line that is only fed intermittently will run worse on a large screw than on a small one. Size the machine to the feed you can actually supply every day, not to the peak week.

Note that the capacity figures in the specification table are the manufacturer’s stated range. Rigid polymers differ in bulk density and melt behaviour, so treat them as a basis for sizing rather than as a guarantee for your specific material, and confirm throughput against a sample of your own feed.

Where this machine sits in the line

The SJ pelletizer is the last machine in a rigid recycling chain, and it inherits every decision made upstream of it. Flake size comes from the crusher screen. Moisture comes from the dewatering and drying section. Colour separation — the single biggest driver of what the pellet sells for — happens on the sorting floor before any of it. The pelletizer controls melt quality, filtration and pellet geometry. It cannot fix a colour mix and it cannot recover material that arrived wet.

Downstream, the pellet goes to compounders, to injection and extrusion moulders willing to run recycled content, or back into the same factory that generated the scrap. Drive is an AC motor with inverter control through a gear reducer with a high-torque thrust bearing, and the line is operated either from a touchscreen PLC interface or from a button-type panel chosen for ease of maintenance, with temperature profiling on both.

See the machine running

Models & specification

Frequently asked questions