Somebody sends me a photo most weeks. A purge lump sitting on a concrete floor, a boot or a drink can next to it for scale, and one line underneath: “can we feed this as-is?”
That’s the right question to ask, and it’s the one nobody answers clearly. Brochures print throughput numbers and motor power. They rarely print what actually decides whether your Monday morning goes smoothly — how big a single block the machine will bite.
So let’s answer it properly. Here is where the real limit sits, what the feed openings actually measure on our machines, and the exact point where you have to reach for a saw first.
Most of the time you don’t. One machine takes your lumps whole. But the cases where it doesn’t are worth knowing before you sign anything, because “we’ll just cut the big ones by hand” is a decision that costs you a wage every single week.
“Purge Lump” covers three different things
First, the word is doing too much work. When a buyer says purge lumps, they usually mean one of three, and they behave differently in a shredder.
Color-change and grade-change purge. The classic one. You push the next material through until the color runs clean, and what comes out is a rope or a slab that cools into a hard irregular block. Usually PP, PE, ABS, sometimes PC. Fist-sized up to a shoe box. Call it 2 to 8 kg. Soft when it’s still warm, genuinely hard once it’s cooled overnight.
Die and head purge. The material you drag out of the die head or screen pack during a teardown. Denser, often darker, sometimes partly degraded. 5 to 15 kg, and much harder than a color-change block. This is the stuff that tells you whether your rotor and cutters were specified honestly.
Startup and shutdown blocks. The big ones. A full purge-out at the end of a run, sometimes dumped into a drum or a mold box and left to set. 300 to 400 mm on the long side is common. Over 20 kg isn’t unusual. Occasionally there’s a bolt, a heater band fragment, or a piece of tooling in there.
I ask which one you have because the answer changes the machine. A shop feeding fist-sized color-change blocks all day is an easy spec. A shop with 25 kg die-head blocks every Friday is a different conversation.
The limit isn’t the hopper. It’s the bite.
This is where people get it wrong. They look at the feed opening, see a number, and assume anything smaller than that number goes in fine.
The hopper only decides whether the block fits. What decides whether it breaks is whether the rotor can get purchase on it.
Here’s the mechanism on a single shaft shredder. A hydraulic ram pushes the block against a slow, high-torque rotor. The rotor knives hook the surface and drag it across a fixed counter knife, tearing a layer off with each pass. Pieces leave when they’re small enough to fall through the screen.
So two things matter:
- Can the knives hook it? A hard, smooth, rounded block has very little for a knife to grab. If the cutter projection is small and the surface is glassy, the block skates. Bigger rotor knives with more projection bite better.
- Can the ram hold it there? The block has to stay pressed into the rotor while it’s being worked. If it’s heavier than the ram can control, it bounces and rides up instead of being cut.
That’s the real failure mode. Not “it didn’t fit.” It fits, and then it sits there spinning, getting hot, and polishing itself round.
A granulator fails differently and more expensively. It runs fast and cuts by impact, so a dense block either hammers the knives, stalls the rotor, or trips the motor. I’ve seen a 12 kg block crack a granulator screen and take out three rotor knives in about four seconds. That machine was never meant to see it.
What the feed openings actually measure
Enough theory. Here are the numbers off our own machines.
O Triturador de eixo único Série S is sized by rotor width, and the feed opening follows it:
| Modelo | Rotor width | Practical max lump (long side) |
|---|---|---|
| S600 | 600 mm | ~500 mm |
| S800 | 820 mm | ~700 mm |
| S1000 | 1030 mm | ~900 mm |
| S1200 | 1230 mm | ~1100 mm |
| S1400 | 1450 mm | ~1300 mm |
| S1600 | 1650 mm | ~1500 mm |
| S1800 | 1850 mm | ~1700 mm |
O SC Series is the shredder-and-crusher unit, and there the hopper is a stated dimension:
| Modelo | Hopper size | Shredder screen | Crusher screen |
|---|---|---|---|
| SC600 | 800 × 500 mm | Φ50 mm | Φ10 mm |
| SC800 | 850 × 630 mm | Φ50 mm | Φ10 mm |
Read that hopper number carefully. On the SC800, the short side is 630 mm. So a block with its longest edge under 630 mm drops straight in. On the SC600, that number is 500 mm.
Put your actual lumps against that:
- Fist-sized color-change block, 150 mm — nowhere near the limit. Any machine in the range.
- Shoe box, roughly 300 × 200 × 150 mm — straight in, no drama.
- Basketball-sized, ~240 mm across — straight in.
- Die-head block, 400 × 300 × 200 mm — fits the SC800 and anything from the S800 up. On an SC600 you’d be turning it to find the right angle.
- A 450 mm startup block — SC800 fine, S600 no.
Most shops find their worst block lands comfortably inside the SC800 or S800. That’s why the honest answer to “can we feed it whole” is usually yes.
When you do have to pre-cut
Three signals. Any one of them and you’re cutting first.
1. The longest edge beats the short side of the hopper. 630 mm on the SC800, 500 mm on the SC600, roughly rotor width minus 100 mm on the S Series. Nothing clever about it — if it won’t lie in there, it won’t lie in there.
2. The block is heavier than the ram wants to control. My working rule: past about 30 to 40 kg for a single piece, stop and split it. The ram can shove it, but it can’t keep a block that heavy planted on the rotor, and you’ll get bouncing instead of cutting. Two halves at 20 kg each run better than one at 40.
3. It isn’t a size problem, it’s a hardness problem. Glass-filled purge is the usual offender. 30% glass-filled PA or PP shreds fine, but it eats cutters, and it does it quietly — you won’t notice until your blade life drops by half. Metal is different again: a bolt or a heater band fragment isn’t a cutting job, it’s a downtime job. Pick those out by hand.
On glass-filled material, the fix isn’t pre-cutting, it’s budgeting. Factor shorter blade intervals into your cost per ton from the start. If you want the numbers on what wear actually costs, the blade article goes through it.
What pre-cutting actually costs you
This is the part people skip. “We’ll just cut the big ones” sounds like a one-line fix.
Run it out. One worker with a bandsaw on a 20 kg block: three to five minutes each, including lifting it onto the table and stacking the halves. Call it 12 to 20 blocks an hour, so 240 to 400 kg/h. On a decent day.
An S600 on purge lumps runs 300 to 500 kg/h, and it doesn’t take breaks or need a saw blade budget.
So your cutting station is roughly keeping pace with the shredder — with a full wage attached. And you’re paying that wage forever, not once. Add the safety side: you’ve got someone handling a saw on hard, springy, awkward plastic that can kick. It’s not the worst job in a plant, but it isn’t one you’d design in on purpose.
The honest framing: pre-cutting is a patch for buying a machine with a feed opening 100 mm too small. Spend that money on the bigger hopper instead and you’ve deleted a task rather than staffed it.
How to measure your own lumps
Do this before you talk to any supplier. It takes fifteen minutes and it removes most of the back-and-forth.
Pull five pieces. Deliberately pick your worst ones — biggest, hardest, the ones everyone walks past.
For each, write down:
- Longest edge in mm. A tape measure, not a guess.
- Weight in kg. Bathroom scale is fine.
- Material. PP, PE, ABS, PC, glass-filled? If you genuinely don’t know, say so — it matters more than the size.
- Hardness, roughly. Fingernail marks it? Hammer makes it crack? Hammer leaves a white mark and nothing else? That third one is hard material and I want to know about it.
Then one photo with a drink can or a tape measure in frame. The scale reference is what makes a photo useful — a lump on its own tells me nothing.
Send those four numbers and the photo. We’ll tell you whether it feeds whole, and which model. Twenty-four to forty-eight hours, usually.
Picking the size
- Longest edge under 500 mm, one shift, under 2 tonnes a day — an S600 or SC600 covers it. This is most injection shops doing their own purge.
- Longest edge 500 to 700 mm, or higher throughput — S800 or SC800.
- You want regrind you can feed back without a second machine — the SC Series. It shreds through a Φ50 screen and then granulates through Φ10 in one footprint, so you get usable regrind out of one machine instead of sizing a shredder and then buying a triturador to follow it.
- Lumps over 700 mm — S1000 or up, and send us the photo first.
One thing worth saying. If you size to your average lump, you’ll have a bad day eventually, because the big ones don’t arrive on a schedule. Size to the biggest block you’ve ever produced, then check what that model costs. Usually it’s one step up, and usually that step is cheaper than the alternative.
The rest of the decision — how much you actually get out per hour, and what it costs you to run — is covered in the capacity article e the running cost piece.
Perguntas comuns
Can a shredder take purge lumps whole?
Usually yes. Any block with its longest edge under the feed opening goes in as-is, and for most injection shops that’s everything they produce. The exceptions are oversized startup blocks and anything hiding metal.
Do I need to pre-cut plastic purge lumps?
Only if the longest edge beats the hopper, the block runs past roughly 30–40 kg, or there’s metal in it. Pre-cutting is otherwise paying someone to do work the machine should be doing.
What size purge lump is too big for a shredder?
On our SC Series, longer than 630 mm on the SC800 or 500 mm on the SC600. On the S Series, roughly rotor width minus 100 mm. Bigger machines exist for bigger blocks — that’s what the S1000 and up are for.
Can a granulator handle purge lumps?
Not the dense ones. A granulator cuts by impact at high speed, and a solid block hammers the knives or trips the motor. That’s exactly why the shredder goes first in the line.
Does a bigger feed opening cost more?
Yes, but less than you’d think, and less than a cutting station costs over a year. Get quotes for the two sizes side by side before deciding.
Tell us what you’re actually feeding
Send the four measurements and a photo with something for scale. Material, longest edge, weight, hardness. We’ll tell you straight whether it feeds whole and which model handles it — including when the answer is “you don’t need the bigger one.”
Pedir um orçamento and include the photo. Typical reply within 24–48 hours.