Regrind plastic is the cheapest raw material in any moulding or extrusion shop, because you already paid for it once. It is also the material most likely to produce a batch of parts nobody can explain, and the reason is usually not the one operators reach for first.
This article stays on the material side of the question. What changes in the polymer when it is ground and remelted, what accumulates rather than degrades, what actually caps a blend ratio, and how the crusher settings that produce the regrind decide how well it behaves later.
Regrind is material you already paid for once, returning to the hopper it came from.
What Regrind Plastic Is and Where It Comes From
Regrind is plastic that has been through at least one processing cycle and has then been ground back into particles so it can be fed again. In most factories it is generated in house, from sprues and runners, start-up purgings, edge trim, off-cuts and parts that failed inspection. Its defining feature is a known history.
That known history is worth more than it sounds. When regrind comes from your own line you know the exact grade, the exact colour, the additive package and how many times the material has been through a barrel. None of those are knowable for material bought on a spot market, and every one of them affects how the batch will behave.
The physical form is what separates regrind from a pellet. Grinding produces irregular, angular fragments with a wide size distribution and a fines fraction, where virgin resin arrives as uniform cylinders or lenses. That difference in shape, not any difference in chemistry, causes most of the practical problems attributed to regrind.
Regrind, Repro and Recyclate Are Not the Same Material
Three words get used interchangeably and they describe genuinely different products. Regrind is ground, unmelted material from a known internal source. Repro is regrind that has been remelted and pelletized, so it has one extra heat cycle but a uniform shape. Recyclate is externally sourced recycled material with a history you did not observe.
| Term | What it is | Heat cycles | What you know about it | Typical use |
|---|---|---|---|---|
| Regrind | Ground fragments of your own processed material, never remelted after grinding | One, the process that made the scrap | Everything, if your own records are kept | Blended straight back into the same product |
| Repro or reprocessed pellet | Regrind that has been extruded and pelletized | Two | Everything, plus a homogeneous, measurable material | Where feed consistency matters more than the extra heat cycle |
| Recyclate, PCR or PIR | Recycled material bought in, post-consumer or post-industrial | Unknown, at least one | Only what the supplier certifies | Wherever the specification permits recycled content |
| Virgin resin | Newly polymerised material | Zero before you process it | Full data sheet, repeatable next month | The reference everything else is measured against |
The real difference is not quality. It is how much of the history you can verify.
The distinction between the first two rows is the one that costs money when it is missed. Choosing repro over regrind buys uniform particle shape and a homogeneous melt at the price of one extra thermal cycle. On a shear-sensitive or heavily stabilised material that trade is not automatically worth taking, and on a dimensionally critical part it usually is.
What Actually Degrades When Plastic Is Reground and Remelted
Two things degrade and they are frequently confused. The polymer chains themselves get shorter, mostly from shear and heat in the barrel rather than from the grinding. Separately, the additive package that protects the polymer gets consumed. The second is often the limiting one, because it happens quietly and leaves no visible sign.
Chain scission is the mechanism behind the first. Each pass through an extruder or an injection barrel exposes the melt to shear and temperature, and both break molecular chains. Shorter chains mean lower average molecular weight, which shows up as an easier-flowing melt, lower melt strength and reduced impact resistance. The material has not become a different polymer; it has become a lower grade of the same one.
Chains shorten and stabilisers get used up. The second one leaves no visible warning.
Additive depletion is the quieter mechanism. Antioxidants and heat stabilisers work by being consumed, so a material on its third pass carries less protection than the same material on its first. It may test acceptably on delivery and then degrade faster in service or during the next processing cycle. This is why some grades tolerate repeated regrinding far better than others.
| Polymer | Dominant degradation route | How it shows on the line | What limits reuse first |
|---|---|---|---|
| PP and PE | Chain scission from shear and heat | Melt flows more easily, melt strength drops | Loss of impact strength and melt strength |
| PET | Hydrolysis if any moisture is present at melt temperature | Sharp viscosity loss, brittle parts | Drying discipline, long before pass count |
| PVC | Stabiliser consumption, then dehydrochlorination | Yellowing, then discolouration and corrosive gas | Remaining thermal budget in the stabiliser package |
| ABS and PS | Chain scission with some crosslinking | Colour shift and falling impact strength | Appearance, usually before mechanical properties |
Reading that table across explains why a single blanket rule about regrind never works. PET is limited by whether it was dried, PVC by how much stabiliser is left, and PP and PE by accumulated shear. A percentage that is safe for one is meaningless for the others.
Why Colour and Contamination Accumulate Faster Than Strength Is Lost
Mechanical properties fall gradually, but colour and contamination behave differently. They accumulate, and accumulation is not reversible. In most shops a regrind loop is stopped by a batch of parts that look wrong long before anyone measures a strength figure that fails, and that ordering surprises people.
Colour drift is the clearest case. Every regrind pass adds a small amount of thermally darkened material, plus whatever pigment was in the parts being ground. The shift is always in one direction, towards darker and greyer, because nothing in the loop removes colour. A shop running a light colour with a high regrind fraction is on a one-way trip.
Properties decline gradually. Colour and contamination only go one way.
Contamination concentrates by the same logic. Anything that does not leave the loop stays in it, so a low level of foreign material in the scrap becomes a higher level in the regrind and a higher one again next cycle. Dust from the grinding operation behaves the same way, and it matters because fines melt and convey differently from the coarse fraction they travel with.
How contamination is removed once it is in the melt is a separate subject with its own hardware and its own trade-offs. The material-side point is simply that the removal is downstream and partial, and that the concentration effect runs whether or not the removal exists. Keeping the loop clean at source is cheaper than cleaning it later.
What Sets the Ceiling on a Regrind Blend Ratio
There is no universal safe percentage and anyone quoting one without asking about your material is guessing. The ceiling comes from four separate constraints, and whichever binds first sets the number. On most jobs the binding constraint is appearance or dimensional consistency rather than any mechanical property.
| Constraint | What it caps | How it is settled |
|---|---|---|
| Product standard or load rating | Whether regrind is permitted at all, and at what level | By the standard the part is certified against |
| Appearance and colour tolerance | Usually the first constraint to bind on visible parts | Sample production judged against the customer’s own limit |
| Feed and dimensional consistency | Shot weight and wall thickness variation | Trial running with the actual particle size distribution |
| Remaining stabiliser and chain length | How many further cycles the material can take | Melt flow and mechanical testing across successive passes |
Whichever constraint binds first sets the number. It is rarely the mechanical one.
Equipment documentation reflects this honestly when it is written well. SUHUI’s HDPE pipe extrusion line states that the usable recycled percentage depends on material quality, pipe standard, pressure grade and customer requirements. The PP profile extrusion line distinguishes structural profiles, where the ratio must be confirmed by material testing and sample production, from decorative and non-structural ones that tolerate more.
The third constraint deserves emphasis because it is the one that catches shops out. Injection moulding machines meter by volume, so inconsistent bulk density shows up directly as shot-weight variation. A regrind fraction that is chemically fine can still produce scrap simply because the material occupies a different volume per kilogram than the pellets it is mixed with.
Controlling Regrind Particle Size With the Crusher Screen and Blades
Particle size consistency is the single most controllable variable in the whole regrind question, and it is set at the crusher rather than anywhere downstream. Output size is governed by the perforated screen under the cutting chamber, because only fragments small enough to pass through its openings leave the machine at all.
That gives a direct adjustment. Different screen sizes produce different target dimensions, so a plastic crusher can be configured for whatever the downstream process needs, whether that is direct feeding into an extruder, an injection machine or a pelletizing line. Fitting a smaller screen tightens the distribution but increases residence time in the chamber and generates more fines.
Only fragments small enough to pass the screen leave the chamber. That is the whole control.
Blade condition matters as much as the screen and is easier to neglect. Sharp rotating and fixed blades cut cleanly against each other. Dull blades tear and smear instead, which widens the size distribution, raises the fines fraction and adds heat to the material at the very moment you are trying not to. Hardened alloy blades can be resharpened rather than replaced, which is what keeps the maintenance interval reasonable.
Feed material also decides which machine belongs in the loop. A shredder does primary size reduction on bulky or thick-walled items, producing irregular chunks in the 20–100 mm range. A crusher then takes those down to the uniform size a process can actually feed on. Consistent output geometry improves feeding stability and reduces screw wear downstream, which is a quiet but real return on getting this stage right.
When Regrind Should Be Pelletized Rather Than Fed Directly
Direct blending is the right answer whenever the scrap is single grade, clean and generated at a rate the process can absorb. Pelletizing earns its extra heat cycle when the material is mixed, when the size distribution cannot be held tight enough, or when a downstream process meters by volume and needs uniform bulk density.
| Situation | Blend regrind directly | Pelletize first |
|---|---|---|
| Single grade, clean, from your own process | Yes, this is the cheapest correct answer | Only if bulk density consistency is critical |
| Mixed grades or mixed colours | No, the melt will not be homogeneous | Yes, so the blend becomes one measurable material |
| Wide particle size distribution with heavy fines | Risky, feed rate and shot weight will vary | Yes, or fix the crusher first and reconsider |
| Material carrying residual contamination | No | Yes, and specify the filtration against a sample |
| Thick-wall abrasive regrind at volume | Depends on the receiving line’s wear protection | Usually, on a line built with wear-resistant elements |
Where pelletizing is the answer, the receiving line has to be built for regrind rather than adapted to it. A rigid plastic granulating pelletizing line takes 3–8 mm crushed regrind and produces 2–5 mm pellets at 300–3,000 kg/h, using a high-torque twin screw to homogenise polymers that soften at different temperatures, and wear-resistant screw elements because thick-wall regrind abrades in a way film never does.
Pelletizing buys homogeneity and uniform bulk density at the cost of one heat cycle.
PVC is the exception that proves the point. PVC regrind at 3–6 mm goes to a dedicated PVC granulating pelletizing line rather than a general-purpose one, because the material needs a controlled 160–200°C window, corrosion-resistant contact parts and fresh stabiliser dosed in. Running PVC regrind through equipment not built for it damages both the batch and the machine.
Frequently Asked Questions
What is regrind plastic?
Plastic that has already been processed once and then ground back into particles so it can be fed again. It usually comes from a factory’s own sprues, runners, trim, purgings and reject parts. Its distinguishing feature is a known history, since the grade, colour and heat cycle count are all traceable.
What is the difference between repro and regrind?
Regrind is ground but never remelted after grinding, so it stays irregular in shape with a wide size distribution. Repro is regrind that has been extruded and pelletized, giving it one extra heat cycle but a uniform particle shape and a homogeneous, measurable melt. Repro costs more and feeds better.
How much regrind can you add to virgin material?
No universal figure exists. The ceiling is set by whichever constraint binds first, usually the product standard, the appearance tolerance, feed and dimensional consistency, or the remaining stabiliser in the material. It is settled by testing and sample production against that specific part, not by a general percentage.
How many times can plastic be reground?
It depends far more on the polymer and the application than on a count. PET is limited by drying discipline, PVC by remaining stabiliser, PP and PE by accumulated shear. In practice the loop is usually stopped by colour drift or contamination build-up before any mechanical property fails a test.
What is PVC regrind used for?
Mostly back into conduit, trunking, non-pressure pipe, trim and moulded fittings, after being granulated to 3–6 mm and re-compounded with fresh stabilisers on a dedicated PVC line. Pressure-rated products are the usual exclusion, because a pressure rating depends on knowing the original formulation and its long-term behaviour.
What size should regrind be?
Matched to what the receiving process feeds on, which is why crusher screens are changeable. Pelletizing lines commonly take 3–8 mm rigid regrind. What matters more than the nominal figure is the width of the distribution and the fines fraction, because both affect feed rate stability and bulk density.
Why does regrind cause colour variation?
Because colour only accumulates. Each pass adds a little thermally darkened material plus whatever pigment was in the parts being ground, and nothing in the loop removes any of it. The drift is always towards darker and greyer, which is why light colours tolerate much lower regrind fractions than dark ones.
From Crusher Screen to Pellet Die
- Crusher and Shredder Range — the machine classes that make regrind, chosen by material size and hardness
- Plastic Granulator vs Shredder vs Crusher — which of the three cutting actions gave your regrind its particle shape
- Single Stage vs Double Stage Pelletizing — what the line looks like once you stop feeding regrind directly
- Rigid Plastic Granulating Pelletizing Line — the one-pass route for regrind that no longer feeds well on its own
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