Every injection molder and extruder has the same problem. Startup purges, color-change scrap, off-spec parts, runners, sprues. It piles up fast. You can sell it as mixed scrap at $50–100 a ton. Or you can shred it, granulate it, and feed it back into production at a fraction of virgin material cost. The difference is the machine that does the first cut.
A single shaft shredder is the right machine for this job. Not a granulator — those choke on purge lumps. Not a double shaft — those produce coarse, irregular strips that need another pass. A single shaft with the right rotor, the right ram, and the right screen gives you uniform flake in one stage. This article covers how to match the machine to your scrap, what specs matter, and the mistakes that cost throughput.
What Actually Happens Inside a Single Shaft Shredder
One rotor. Indexable knives bolted around it. Stationary counter-knives fixed to the housing. A screen underneath. A hydraulic ram pushes material against the rotor. The knives cut. Material passes through the screen holes when it’s small enough. That’s it.
For purge lumps — solid, dense blocks of plastic that can weigh 10–50 kg each — the ram is what makes it work. Without hydraulic pressure, a lump sits on the rotor and bounces. The knives skim the surface instead of biting in. Throughput drops to nothing. The ram forces contact. That’s the whole secret.
Rotor speed for this application: 70–85 RPM. Slower than a granulator (300–600 RPM), faster than a double shaft (10–40 RPM). Fast enough to cut, slow enough that a missed bolt doesn’t turn into shrapnel. The knives are indexable — four cutting edges per insert. When one dulls, rotate 90 degrees. When all four are used, replace the insert. No welding. No sharpening. Unbolt, rotate, bolt back.
What Kind of Scrap We’re Talking About
| Tipo de chatarra | Fuente | Typical Size | Reto |
|---|---|---|---|
| Purge lumps | Startup/shutdown purge from extruder or injection machine | 5–50 kg solid blocks | Too large for granulator; dense, hard, needs hydraulic ram to engage rotor |
| Startup cakes | Material that solidified in barrel/nozzle during startup | 1–10 kg irregular masses | Irregular shape makes gravity feed unreliable; ram fixes this |
| Runners and sprues | Cold runner molds, hot runner startup | Long thin strands, branched shapes | Bridges across hopper opening; needs consistent feed pressure |
| Off-spec parts | Rejected molded parts, dimensional fails, color mix-ups | Varies — from small caps to large crates | Mixed shapes and thicknesses; screen defines uniform output |
| Pipe and profile cutoffs | Pipe extrusion line startup, cut-to-length scrap | 100–2,000 mm lengths | Long pieces need chamber width to match; can’t be fed crosswise |
| Film edge trim and roll scrap | Film and sheet lines | Rolls, bales, continuous trim | Light and springy; needs swing-arm ram to prevent floating |
Purge Lumps: The Toughest Feed, and What Handles It
Purge lumps are the hardest scrap to shred in a plastics factory. They’re dense. They’re irregular. They come in materials ranging from soft LDPE (easy) to glass-filled nylon (eats knives for breakfast). And they arrive hot from the machine — or cold and rock-hard the next morning. Same machine has to handle both.
The key specs for purge lump shredding:
- Rotor diameter: 400–520 mm minimum. A 300 mm rotor doesn’t have enough bite on a 30 kg lump. The lump sits on top and the knives skim it. Bigger rotor = deeper bite = faster reduction.
- Motor power: 37–75 kW, depending on throughput and material. HDPE and PP purges are manageable at 37 kW. Glass-filled nylon and PC need 55–75 kW. Underpowered motor = rotor stalls under load = operator runs smaller batches = throughput tanks.
- Ram type: Horizontal ram for blocky purges. Swing-arm ram if you also run film or fiber scrap. Gravity feed won’t work on lumps over 2–3 kg.
- Knife material: D2 for standard polyolefins (PP, PE). DC53 if you run engineering plastics with fillers. Tungsten carbide inserts if glass-filled nylon or PBT is a daily thing.
| Material | Recommended Motor | Knife Steel | Screen Size | Notes |
|---|---|---|---|---|
| PP / PE purge | 37–45 kW | D2 | 20-40 mm | Straightforward. Screen size depends on granulator downstream. |
| HDPE purge | 45–55 kW | D2 | 25–50 mm | Tougher than PP. Bigger screen = faster throughput. |
| ABS / PS purge | 45–55 kW | DC53 | 20-40 mm | Brittle, but impact-resistant grades need torque. |
| PA (nylon) purge | 55–75 kW | DC53 or carbide | 15–30 mm | Abrasive when glass-filled. Knife life drops 50–70% vs unfilled. |
| PC / PMMA purge | 55–75 kW | DC53 | 15–30 mm | Hard, brittle. High impact force on knives. |
| Glass-filled PP/PA | 55–90 kW | Tungsten carbide | 15–25 mm | Extreme abrasive. Standard D2 lasts weeks, not months. |
Screen Size: The Number Nobody Checks Until Output Is Wrong
The screen under the rotor controls what comes out. Holes from 15 mm to 80 mm. For feeding a granulator downstream, 20–40 mm is standard. Too small and throughput drops because material sits in the chamber longer. Too large and your granulator works harder and the knives wear faster.
If you’re shredding purge lumps that go directly to a granulator, 30–40 mm is the sweet spot. The granulator handles the rest. If you’re shredding for direct extrusion or molding (no granulator), go 15–25 mm — but expect throughput to drop 30–40%. Smaller screen = more time in the chamber = less output per hour.
Buy two screens when you order the machine. Start with the larger one. If your granulator or extruder handles it fine, keep it. If not, swap to the smaller one. Screens are a few hundred dollars. Finding out your screen is wrong after the machine is installed costs days of downtime.
The S Series: Which Model for Your Scrap
| Modelo | Rotor (mm) | Motor (kW) | Real Throughput | Ideal para |
|---|---|---|---|---|
| S600 | φ300 × 600 | 22 | 100–300 kg/h | Small molding shop, sprues and runners, small purge lumps |
| S800 | φ400 × 820 | 37/45 | 300–600 kg/h | Mid-size plant, pipe cutoffs, standard purge lumps |
| S1000 | φ450 × 1030 | 55 | 500–900 kg/h | Larger molding operations, mixed scrap types |
| S1200 | φ450 × 1230 | 75 | 800–1 500 kg/h | High-volume plants, large purges, multiple machine feeding |
| S1400 | φ450/520 × 1450 | 90 | 1,200–2,000 kg/h | Central recycling for multi-plant operations |
The S600 handles most small to mid-size injection molders. If your largest purge lump is under 15 kg and you run one or two shifts, the S600 or S800 is the right size. The S1200 and above are for plants generating 500+ kg of scrap per shift, or processing for multiple facilities.
Three Ways People Get This Wrong
Buying a granulator instead of a shredder for purge lumps. A granulator is not a shredder. Feed it a 20 kg HDPE purge lump and the rotor stalls or the knives shatter. The granulator needs pre-shredded material under 40–50 mm to work. The shredder makes that material. You need both, in series. Shredder first, granulator second.
Undersizing the motor for the material. PP and PE are forgiving. Nylon, PC, and glass-filled anything are not. A 37 kW motor that handles PP purges all day will trip on overload when you switch to glass-filled nylon. Spec the motor for the toughest material in your mix, not the average.
Not matching chamber width to the largest piece. If your longest purge lump or pipe cutoff is 800 mm, and your shredder chamber is 800 mm wide, it fits — barely. But it wedges. The ram can’t push it evenly. Throughput drops. Rule of thumb: chamber width should be at least 20% larger than your largest feed piece. 800 mm piece → 1,000 mm chamber minimum.
PREGUNTAS FRECUENTES
Do I need a shredder and a granulator, or can one machine do both?
You need both. The shredder takes large scrap (whole purges, pipes, parts) down to 20–50 mm flake. The granulator takes that flake to 8–12 mm regrind for direct reuse in molding or extrusion. A granulator fed whole purges will destroy itself. A shredder can’t produce the fine, uniform regrind a molding machine needs. They’re two different machines for two different jobs.
What’s the smallest purge lump a single shaft shredder can handle?
No minimum. The machine handles everything from small sprues to 50 kg lumps. The ram adjusts pressure automatically based on load. But if all your scrap is under 5 kg and you never have large purges, you might be able to use a heavy-duty granulator with a large hopper instead of a shredder. Tell your supplier your actual scrap size range before you buy.
How often do I need to rotate or replace the knives?
On clean PP/PE purge: rotate edges every 400–600 hours, replace inserts after all four edges used (1,600–2,400 hours total). On filled materials (glass, mineral): rotate every 150–300 hours, replace after 600–1,200 hours. Watch the amp meter. When power draw at the same feed rate climbs 10–15%, the edges are dull. Rotate them before throughput drops.
Can I shred hot purges straight from the machine?
Yes, within limits. Hot PP/PE straight from an extruder is soft and cuts easily. But it also sticks to the rotor if it’s too hot. Let purges cool to below 80°C before feeding. Hot nylon and PC are too hard when cooled — they’re actually easier to shred warm. Know your material behavior. Test if you’re not sure.
What about noise and dust?
Single shaft shredders at 70–85 RPM are relatively quiet compared to granulators (300–600 RPM). Expect 75–85 dB at the operator position. Dust depends on material: PP and PE generate almost none. Glass-filled materials generate fine dust that needs extraction. Wet shredding (water injection into the chamber) kills dust and cools the knives but adds water handling. Most in-house operations don’t need it for standard polyolefins.
Single Shaft Shredders From Streamline Eco Tech
The S Series handles everything from small molding shop purges (S600) to multi-plant central recycling (S1400). We spec the rotor, motor, ram type, knife steel, and screen size to your actual scrap — not a generic default. Send us photos of your largest purge lumps and your weekly scrap volume. We’ll tell you which model fits and what throughput to expect. No guesswork.
More Questions
How do I keep different materials separate during shredding?
Dedicate time blocks. Run all the PP scrap, clean the machine, then run HDPE. Or run separate bins for each material type at the molding machine and shred one material per shift. Mixing PP and HDPE in the shredder produces contaminated regrind that’s worth less than either material alone. The extra handling pays for itself in regrind value.
What happens if a metal insert gets into the shredder?
Best case: the auto-reverse triggers, the rotor backs up, and the operator removes the metal. Worst case: the insert chips a knife edge. D2 knives are brittle and don’t like impact. DC53 is tougher but still not indestructible. Put a magnet on the infeed if metal contamination is a risk. A $500 magnet saves $2,000 in knife replacements.
Can the shredder handle film scrap along with purge lumps?
Yes, but you need the swing-arm ram. The horizontal ram lets film float above the rotor. The swing-arm presses it down from above. If your scrap mix includes film rolls, edge trim, or bags, don’t buy the horizontal ram. The swing-arm costs more upfront and pays back in throughput on the first batch of film you run.