Walk into almost any injection molding shop and there’s a corner where the purge lumps live. Startup purges from changeovers, color-change blocks, off-spec parts, the occasional solid lump the size of a shoe box. Somebody wheels a bin out there every week and it keeps filling up.
Ask what happens to it and you’ll usually get one of two answers. Either “we sell it” or “we’re still figuring that out.”
Here’s the part people don’t sit down and calculate. That bin is full of material you already paid virgin prices for. A purge lump isn’t contaminated mystery scrap — it’s usually clean, known, single-polymer material that got squeezed out during a changeover. If you’re selling it at scrap price and buying virgin resin to replace it, you’re paying the difference twice, every single week.
A plastic lumps shredder machine is how you close that gap. And for most shops, one machine is enough to do it. Not a washing line, not a pelletizing line, not a five-stage system. One shredder, sized right for your lumps, feeding regrind straight back into production.
That’s the whole pitch for in-house recycling: keep the material you already paid for, process it on your own floor, and stop buying virgin resin to replace plastic you threw in a bin. A plastic lumps shredder is the machine that makes that possible without building a plant.
This article is about what that machine can realistically do, how to know whether one is enough for you, and how to run the numbers so you know what you’re buying.
What Comes Out Is the Whole Question
Before you think about machines, think about the output, because that decides everything else.
Purge lumps are dense, solid, and awkward. A 20 kg block of cooled PP or a hard ABS purge puck doesn’t behave like film or bottles. That’s why a conventional granulator struggles with them — the lumps are too big and too solid to be grabbed by a high-speed rotor, and they’ll stall or hammer the machine before they cut.
A plastic lumps shredder works differently. A hydraulic ram pushes the block into a slow, high-torque rotor, and the cutters tear it down against a bed knife until the pieces fit through a screen. That’s the mechanism that handles a dense lump without drama.
The output is flake, and the screen decides how coarse. Here’s where the decision gets made:
- Coarse flake (roughly 20–40 mm) — good enough to feed a granulator, or to sell as sized scrap at a much better price than whole lumps.
- Fine flake (roughly 10–15 mm) — small enough that many shops feed it straight back into the hopper mixed with virgin, or run it through a granulator for a final uniform grind.
So the honest question isn’t “can it shred my lumps.” It’s “is the flake it makes good enough for where I want it to go?” If you’re blending back into your own production at a modest percentage, a single shredder with the right screen often gets you there. If you need a precise, uniform regrind for a tight-spec product, you’d add a granulator downstream — but you still start with the shredder, because nothing downstream can take the lumps whole.
Is One Plastic Lumps Shredder Enough? Usually Yes.
I get asked this constantly, and I understand why — most equipment suppliers make their money selling lines, so the honest answer is refreshing for people.
For in-house purge recycling specifically, one machine is the normal answer, not the compromise answer.
One shredder is enough when:
- Your lumps are clean and single-polymer (typical for in-house purge from your own machines)
- You’re blending regrind back into production at a reasonable percentage, not running 100% recycled
- Your output spec can accept shredder flake, or you already own a granulator that can take that flake as feed
- Your volumes are in the range a single machine handles — for most small and mid-size shops, this is the case
You’d add a second stage when:
- You need a tight, uniform particle size for a demanding product or for resale as regrind
- You’re processing mixed or contaminated material that needs washing or separation
- Your throughput is high enough that one machine becomes the bottleneck
That’s it. For a shop recycling its own purge back into its own production, one well-sized shredder is the normal, sensible answer. I’ve talked plenty of people out of buying a second machine they didn’t need, because a second machine is more money, more floor space, and one more thing to maintain.
The flip side is worth saying too: if you buy the wrong single machine — undersized for your biggest lump, or with a screen that can’t make the flake you need — you end up needing that second stage anyway. So “one machine” only works if it’s the right one.
The Three Things That Decide Whether It Works on Your Lumps
Past the basic sizing, these are the three specifications that separate a shredder that eats your purge all day from one that frustrates you.
Rotor and cutter type
Not all rotors are built for solid blocks. A rotor and cutter set designed for film or light scrap will struggle to bite into a dense purge puck, and it’ll wear or chip doing it. For purge lumps you want a rotor with cutters that get purchase on hard, solid material — aggressive enough to grip and tear a dense block, tough enough not to chip when it hits one. Tell your supplier the hardest, densest lump you’ll ever feed, not the average one.
Screen size (and being honest about what you need)
As I keep saying, the screen sets the output — and it sets your throughput too. A fine screen makes a nicer flake but can cut your hourly rate substantially, because material stays in the chamber being re-cut. If your process can take a coarser flake, don’t spec a fine screen and then wonder why the machine feels slow.
Hydraulic ram force and motor torque
This is the pair that actually determines whether a 30 kg lump gets eaten or sits there. The ram has to push a dense, awkward block into the rotor with real force. The motor and gearbox have to deliver enough torque to keep cutting at low speed instead of stalling. A machine with a big motor and a weak ram still stalls, because the material never reaches the cutters. When we size a machine for purge, this is where we spend most of the conversation.
What One Costs, and When It Pays for Itself
Let’s do the arithmetic, because this is the part that actually justifies the purchase. The numbers below are illustrative — plug in your own and the method still works.
Say you generate 2 tons of purge scrap a month. That’s a realistic figure for a mid-size molding shop running frequent changeovers.
Selling it as scrap:
– Scrap price for mixed/unprocessed lumps: call it $200/ton (it varies a lot by polymer and region)
– Revenue: 2 × $200 = $400/month
Shredding it and blending it back:
– Virgin resin you’re replacing: call it $1,200/ton
– If you blend the regrind at even 20% into your production, you’re displacing real virgin purchases
– The recovered material is worth roughly its replacement value minus your processing cost
– Conservative value: 2 tons × ~$800/ton net = $1,600/month
The gap is about $1,200 a month, or roughly $14,000 a year, on two tons a month. If your volumes are higher — and plenty of shops generate far more than two tons — the number scales directly.
A single shaft shredder sized for this kind of duty typically pays for itself somewhere in the 12 to 24 month range on material savings alone, before you count the avoided cost of scrap haulage, the floor space you’re not using for storage, or the fact that you stop watching resin prices with dread.
If that payback window looks long for your volumes, that’s useful information too — it means either your volumes don’t justify a machine yet (keep selling the scrap), or you should be looking at a smaller, appropriately-sized unit rather than something oversized.
How We’d Spec One for Your Purge
When a customer comes to us with purge lumps, this is the conversation. Not “what’s your biggest machine,” but:
- What polymer, and what’s the biggest, hardest lump you’ll feed? A 30 kg ABS block and a 5 kg PE purging are different machines.
- How much per month? That sets the throughput, and honestly it sets whether this is worth doing at all.
- Where is the flake going? Straight back into your hopper, into an existing granulator, or out the door as sized scrap? This decides the screen.
- What’s your power and floor situation? Single phase or three, and how much space you actually have.
Our S Series single shaft shredders are the machines we build for this — slow-speed, high-torque rotors with a hydraulic ram, indexable bolt-on cutters you can rotate instead of grinding, and a screen you set for the flake size you actually need. For lumps specifically, the ram force and rotor configuration matter more than the headline motor number, and that’s what we configure per application.
But the honest answer to “which machine” is: it depends on your lumps. Send us a photo of your biggest one, tell us the polymer and your monthly volume, and we’ll tell you what size you need — and whether a single machine genuinely covers it or whether you’d be wasting money.
الأسئلة الشائعة
Can a plastic lumps shredder machine handle solid purge blocks?
Yes — that’s exactly what it’s built for. Solid, dense purge lumps are too big and too hard for a conventional granulator, which will stall or choke on them. A single shaft shredder with a hydraulic ram pushes the block into a slow, high-torque rotor that tears it down to flake, which is why it’s the standard first step for purge scrap.
What output size does a plastic lumps shredder produce?
It depends on the discharge screen. Coarser screens give roughly 20–40 mm flake suitable for feeding a granulator or selling as sized scrap; finer screens give roughly 10–15 mm flake that many shops blend back into production directly. Pick the screen based on where the flake is going, not on “smaller is better.”
Do I need a shredder and a granulator, or just a shredder?
For most in-house purge recycling, a shredder alone is enough — especially if your lumps are clean, single-polymer material and you’re blending regrind back at a modest percentage. Add a granulator only if you need a tight, uniform particle size for a demanding product or for resale. Nothing downstream can take the lumps whole, so the shredder always comes first.
Why not just use a granulator on purge lumps?
Because purge lumps are dense and solid, and a granulator’s high-speed rotor is built for smaller, lighter feed. Large solid blocks either won’t be grabbed, or they hammer the rotor and knives, causing stalls, trips, and damage. Shredding first reduces the lumps to a size a granulator can handle.
How much purge scrap do I need to justify buying a shredder?
Run the numbers on material value, not tonnage alone. Compare what you’re paid for whole lumps as scrap against the virgin resin you’d displace by regrinding and reusing them. For many shops, the gap pays for a correctly-sized machine in 12 to 24 months. If your monthly volume is very low, selling the scrap may still be the better call, and we’ll tell you that.
Can shredded purge go straight back into my machines?
Often yes, at a blend percentage, if the flake size suits your process and the material is clean, single-polymer purge from your own machines. Contaminated or mixed material usually needs more than shredding. Ask your process engineer what regrind percentage your product tolerates before you spec the machine.