DS Twin-shaft Shredder (double-rotor)

DS twin-shaft shredder (double-rotor)

A twin-shaft shredder is the machine you put in front of everything else. It does not produce a finished product and it is not trying to. Its job is to take material that no other machine in the plant can physically swallow — a 200-litre HDPE drum, a bale of woven sacks, a pallet, a length of PVC profile — and turn it into pieces small enough and uniform enough that a crusher, a washing line or a granulator can accept them.

The DS series runs from 400 kg/h to 4,000 kg/h across six rotor lengths. What follows is the reasoning behind the numbers rather than a restatement of them: why the machine turns as slowly as it does, what the rotors are actually made of, and where it stops being the right tool.

Why it turns at 8–15 rpm

The first thing that surprises people watching a twin-shaft shredder is how slow it looks. A plastic crusher spins its rotor at several hundred rpm; a DS shredder turns at 8–15 rpm. That is not a limitation, it is the entire operating principle.

Shredding and crushing remove size in two different ways. A crusher works by impact: a fast rotor throws material against fixed blades until it breaks. That is efficient for brittle, clean, already-small feedstock. A shredder works by shear: two counter-rotating shafts grip a piece between opposing blades and tear it apart. Shear needs torque, not speed, and torque is what you get when you put a planetary gear reducer between a large motor and a slow shaft.

Three practical consequences follow from that choice, and they are the reason a shredder sits upstream rather than downstream:

It tolerates what a crusher cannot. A stray bolt, a stone in a bale, a metal handle on a drum — at 15 rpm these stall the machine or pass through. At 500 rpm they become projectiles and destroy blades.

It does not generate dust or heat. Low rotational speed means low friction energy. Thin film shredded at high speed melts and welds into lumps; sheared slowly, it stays as film.

It grips instead of bouncing. Bulky, springy items — foam board, empty drums, big bags — are thrown back out of a fast rotor. Slow intermeshing hooks pull them down.

The two rotors, and what they are made of

How the discs intermesh

Each shaft is not a single machined part. It is a stack of individual cutting discs bolted onto a square or splined shaft, with spacers between them. The discs on one shaft run in the gaps between the discs on the other, and the fixed comb plates on the chamber walls run in the same gaps. Material caught between a rotating disc and a fixed comb has nowhere to go except through.

Building the rotor from bolted discs rather than machining it whole is what makes the machine maintainable. A damaged disc is unbolted and replaced individually. On a welded or single-piece rotor, the same damage means removing the whole shaft.

Disc count scales with rotor length: 12 discs on the DS600, 32 on the DS1600. More discs across a longer rotor means a wider bite and more simultaneous shear points, which is where the capacity increase comes from — not from turning faster.

Knife steel: three grades, three situations

Blades are available in 9CrSi, D2 or tungsten carbide steel, and the choice is an economic one rather than a quality one.

9CrSi is the general-purpose grade: tough, forgiving of shock loading, cheap to replace, appropriate for clean rigid plastic and film. D2 holds a working edge substantially longer under abrasive conditions — agricultural film carrying soil, woven sacks with sand in the weave, post-consumer material of unknown provenance. Tungsten carbide is for the abrasive extremes, typically glass- or carbon-fibre-reinforced plastics, where the fibre itself is doing the cutting to your blades rather than the other way round.

The wrong direction to optimise is picking the hardest steel available for everything. Harder grades are more brittle; on feedstock with occasional metal contamination, a hard blade chips where a tougher one deforms.

What the hopper has to swallow

The hopper is sized for material that arrives in awkward shapes: plastic drums, big bags, PE film, EPS and EPE foam board, wood pallets, aluminium profiles, household waste. This is a different design problem from feeding a crusher, where the material is already granular and simply falls in.

Two things go wrong when bulky material meets a slow rotor. It bridges — forming an arch above the shafts that holds the rest of the load off the blades — or it rides on top of the rotors without being gripped. A hydraulic pusher is available as an option for exactly this: a ram in the hopper wall that presses the charge down onto the shafts and keeps the bite consistent. On light, springy, low-bulk-density feedstock such as foam or empty containers it is the difference between rated throughput and half of it. On dense rigid material that falls under its own weight, it is often unnecessary.

Output size is set by the screen, not by the blades

A common misunderstanding is that blade geometry determines the size of the pieces coming out. It does not. The discharge screen or grate under the rotors does. Material circulates in the chamber until it is small enough to fall through the openings, and typical output runs 20–100 mm depending on which screen is fitted.

That range is wide because the target depends entirely on what comes next. Feeding a crusher for regrind, a coarse screen is enough — the crusher does the fine work and a coarse shred keeps shredder throughput high. Feeding a washing line directly, the shred has to be small enough for the separation stages to work on individual pieces. Shredding purely for volume reduction before transport, output size barely matters and the coarsest available screen is the cheapest to run.

Fitting a finer screen than the downstream process needs is the most common way to lose capacity on a machine that is otherwise correctly sized.

What happens when it jams

It will jam. A piece of steel too large to shear, an overloaded charge, a lump that wedges between the shafts — on post-consumer feedstock this is routine, not exceptional, and the machine is designed around it rather than against it.

The Siemens PLC control cabinet monitors motor load. When current climbs past the threshold, the drive stops and reverses automatically, backing the obstruction out of the mesh, then resumes forward rotation. Most jams clear in this cycle without an operator touching anything. If reversal does not clear it, the machine shuts down rather than continuing to draw current, which protects the motors and the gearboxes — the two most expensive components on the machine. A safety lockout system prevents the chamber being opened while the shafts can still turn.

This matters commercially more than it looks. A shredder that trips a breaker on every jam and needs a manual restart costs you hours of production per week on dirty feedstock.

What it will and will not take

Plastics it handles as routine

HDPE bottles, drums, barrels, containers and film. LDPE and LLDPE films including shopping bags, agricultural film and stretch film. PP automotive parts and crates. PVC pipe and window profile. ABS and PS electronics housings, toys and expanded EPS. PET trays, yarn, fibre, packing straps and belts. Woven PP sacks and bulk bags. Nylon (PA) and polycarbonate (PC). Fibre-reinforced plastics in GFRP and CFRP. Plastic-metal composites from e-waste and automotive parts.

The tolerance for contamination is what distinguishes this list from a crusher’s. Dirt, labels and small metal pieces pass through without stopping production — they cost blade life, not uptime.

Where a twin-shaft shredder is the wrong tool

If your material is already smaller than about 100 mm and reasonably clean, a shredder adds a process step and an electricity bill without adding value; a crusher alone will do it. If you need output under 20 mm, a shredder cannot get you there on its own and you are looking at a shredder-plus-crusher pair. And if throughput is the only constraint on clean, uniform, in-house scrap, the higher speed of a crusher will beat a shredder on cost per tonne.

Choosing between the six DS models

Two things in this table are worth reading rather than skimming.

Capacity does not scale with power. From DS600 to DS1600, installed power rises five times (15 kW×2 to 75 kW×2) while rated capacity rises between six and seven times. The larger machines are more efficient per kilowatt, because a longer rotor shears more material per revolution at the same speed.

Rotor diameter does not increase in step with length. DS1200 and DS1400 share the same ⌀450 rotor; only the length and disc count change. Diameter governs the largest single piece the machine can bite, so if your constraint is the size of what you are feeding rather than tonnes per hour, the jump that matters is DS1400 to DS1600 (⌀450 to ⌀550), not DS1200 to DS1400.

Quoted capacities assume material of typical bulk density fed at a steady rate. Light, bulky feedstock such as foam or empty containers will run at the lower end of each band, or below it without a hydraulic pusher.

Where it sits in a recycling line

Three positions account for nearly all installations.

Pre-shredding ahead of size reduction. Large items — pallets, crates, automotive parts — are reduced to uniform pieces that a crusher or granulator can then take to final size. Mixed rigid and flexible waste is opened up here so that downstream separation and washing can act on individual pieces rather than on bales.

Volume reduction for storage and transport. Bulky waste such as EPE panels, HDPE drums and EPS foam occupies far more space than its weight justifies. Shredding before baling or loading cuts the cost of moving it, and for many operations that alone pays for the machine.

Contaminated and composite material. Plastic-metal hybrids from e-waste and automotive scrap go through a shredder first precisely because it tolerates the metal. Separation happens after shredding, once the composite has been broken apart.

See the machine running

Frequently asked questions