Regrind is your own sprues, runners and rejected parts, ground and fed back into the hopper. It runs well in many jobs when it is clean, dry and added at a steady ratio. No single percentage is safe for every job. The right amount depends on the resin, the part and what the specification allows, and many jobs set a limit in the specification. Check that first.
Also described as: regrind ratio, regrind percentage, reprocessed material, runners and sprues back in.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Parts crack at snap fits, or fail a drop or impact test, as the regrind share rises. | Repeated heat has shortened the polymer chains. In glass-filled grades the fibers have also been broken shorter. | A short run on virgin resin only; melt flow of the blend against virgin |
| Flash appears, parts get heavier or fill pressure falls. | The blend has a higher melt flow than virgin resin, so the same settings over-pack the mold. | Fill pressure trend; regrind share on the good run and the bad run |
| Splay or silver streaks appear when regrind goes in. | The regrind is wet. It sat in an open bin and needs longer in the dryer than pellets do. | How the regrind was stored; drying time; a moisture reading |
| Black specks or dark streaks. | Fines and dust that melt early and overheat, regrind that was already degraded, or dirt from the grinder. | Amount of dust in the regrind; the grinder, bins and magnet |
| A yellow or darker shade than parts made from virgin resin. | Heat history has built up over several passes. | Regrind share; whether burned or discolored parts went into the grinder |
| Shot-to-shot variation, slow or uneven screw recovery, or bridging at the feed throat. | The ratio changes from hour to hour, or the regrind is coarse and feeds differently from pellets. | How the regrind is metered; particle size; the grinder screen |
| Parts look good and fail a flame test or a mechanical test. | Regrind above what the rating or the specification allows, or regrind of another grade mixed in. | The limit in the specification; labels on the regrind bins |
Checks in order, cheapest first
- Read the specification before the press. The drawing, the customer’s material specification, a flame rating or a food-contact requirement may limit regrind or rule it out. That limit overrides anything the part can tolerate.
- Run a short trial on virgin resin only. If the problem goes away, the regrind share or the regrind handling is the cause. If it stays, look elsewhere before you blame the regrind.
- Hold the ratio constant. Meter regrind with a blender or by weight. Do not add it by the scoop or when the bin is full. A steady ratio gives a steady melt, and a ratio that wanders gives a process that wanders, even when the average is low.
- Dry it, and dry it longer than pellets. Regrind of a moisture-sensitive resin picks up water quickly in an open bin. Grind and reuse it promptly, or store it sealed. The table below gives the longer drying times that are published.
- Keep it clean and labeled. Cover the bins and label each one with the resin, grade and color. Clean the grinder between jobs. Keep purge lumps, burned parts and parts with oil, inserts or labels out of the grinder.
- Control the dust and the particle size. Sharp knives and the right screen give even particles with little dust. Dull knives make fines. If dust builds up in bins and loader filters, remove it before it reaches the screw.
- Measure what the regrind has done. Compare the melt flow of the blend with the virgin lot, and test the property the part depends on. That tells you how much room is left. See melt flow index explained.
- Change the resin last. Regrind problems are usually handling problems and not resin problems. If the handling is right and the job still needs more regrind than the part will take, a tougher starting grade is the next place to look.
The numbers
Typical drying temperature, drying time and moisture target. Where a longer time for regrind is published, it is shown. Where it is not, expect regrind to need at least the long end of the range.
| Resin | Drying temperature | Drying time | Moisture target |
|---|---|---|---|
| Nylon 6, unfilled | 176°F / 80°C | 2–6 h | 0.03–0.12% |
| Nylon 66, unfilled | 175–180°F / 80–82°C | 2–4 h from a fresh bag | 0.03–0.2% |
| Polycarbonate | 248–250°F / 120–121°C | 2–4 h | ≤0.02% |
| PC/ABS | 212–230°F / 100–110°C | 2–4 h; 4–6 h for regrind or filled grades | 0.02–0.04% |
| ABS | 176–203°F / 80–95°C | 2–4 h | 0.1% general; 0.02–0.05% for plating |
| PBT | 250°F / 121°C typical | 2–4 h | 0.02% |
| PET, injection grade | 250–275°F / 120–135°C | 2–4 h virgin; 4–6 h regrind | <0.02% (200 ppm) |
| TPV | 176°F / 80°C | 3 h virgin; 4 h with regrind | ≤0.08% |
| Polystyrene and HIPS | 160–167°F / 71–75°C, only if needed | 1–2 h; never more than 12 h | No published maximum. Dry for regrind or humid storage. |
Typical values from published drying guides. Confirm against the data sheet for your grade. One nylon molding guide gives 3 hours for a fresh bag, 20 hours for a bag left open several days and 60 hours for regrind stored in containers that are not moisture-proof. One producer’s guide says that doubling the particle weight raises drying time by roughly 60%. Clear PET regrind is amorphous and has to be crystallized before it is dried, or it will clump in the hopper. The full list is in the resin drying chart.
What each pass through the barrel changes
- Melt flow goes up and toughness goes down. In most resins, each heat history shortens some of the polymer chains. The melt gets thinner. Impact strength and elongation fall first, while stiffness and tensile strength change much less. That is why regrind parts can pass a pull test and still crack at a snap fit.
- The size of the loss depends on the resin and on moisture. Polycarbonate, PBT, PET and nylon lose the most if they are remelted wet, because water breaks the chains at melt temperature. Polypropylene, polyethylene and polystyrene are more forgiving. PVC, acetal and flame-retardant grades have little spare heat stability to give.
- Glass fibers get shorter. The grinder and the screw both break fibers. Shorter fibers mean lower strength, stiffness and impact strength, and a small change in shrinkage.
- A steady ratio settles down. A changing one does not. When regrind returns at a fixed ratio, most of the blend is virgin or has been through once, and only a small share has been through many times. The properties level off. Putting a week of saved regrind in at once breaks that balance.
- Fines and dust behave differently from pellets. Small particles melt early and overheat against the barrel wall, which gives black specks. Dust also holds moisture, clings to hoppers by static and clogs loader filters.
- Coarse regrind feeds differently. Flakes and chunks have a different bulk density from pellets and carry more air into the screw. Recovery time and shot size can vary as the mix in the throat changes.
- Stabilizers and colors get used up. Heat stabilizers are consumed a little on every pass, and some colors shift. Light and natural colors show it first. See discolored parts off the press.
When the job limits regrind
- Flame-rated parts. The rating covers the grade as supplied. UL’s rules for molders (UL 746D) generally allow up to 25% regrind of the same grade by weight, and more than that needs separate evaluation. Your customer’s specification may be tighter. See flame ratings.
- Food-contact parts. Everything that stays in the part has to be within the rules. Use only your own clean regrind of the same grade, kept separate. Regrind from outside sources is ruled out. See FDA food contact.
- Medical, automotive and other controlled parts. Many of these specifications state a maximum regrind share, and some allow none. If the document is silent, ask in writing before you assume.
- Recycled-content claims. Your own regrind fed back into the same process does not count as recycled content. See recycled content.
Material and handling changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| Less regrind in the critical part, or none | More margin on toughness, color and flow. | More virgin resin used, and regrind to store, sell or use elsewhere. |
| Regrind moved to a less demanding part | The scrap is still used, and the part that needs the toughness runs on virgin resin. | A second material stream to store, label and track. The other part’s specification has to allow it. |
| A tougher or lower-flow starting grade | It leaves room for the loss that regrind brings. | Higher fill pressure and sometimes a longer cycle. A new grade for your customer to approve, with its own shrinkage. |
| A grade with more heat stabilizer | It loses less on each pass. | Usually a higher price, and a new grade to approve. |
| Regrind repelletized before reuse | Melting, filtering and pelletizing removes fines and gives pellets that feed and dry like virgin resin. | One more heat history, a processing charge and a minimum quantity. |
| A hot runner or a smaller runner | Less regrind is made in the first place. | A tooling cost, and color changes take longer through a hot runner. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Wet resin. Splay and brittle parts from moisture look the same as regrind damage, and wet regrind causes both at once. Dry the virgin resin and the regrind properly and run each. See splay and silver streaks and storing resin.
- Another resin mixed in. Regrind from the wrong bin gives layers that peel, streaks or sudden brittleness, often on parts that ran well the day before. See delamination.
- A change in the virgin lot. If parts turned brittle at the same regrind share you have always run, compare the COAs before you blame the regrind. See brittle or cracking parts.
- Heat from the process. A barrel that runs too hot or a long residence time degrades virgin resin the same way. A virgin-only trial separates the two. See discolored parts off the press.
Quick answers
How much regrind can I use?
No single percentage is safe for every job. The right amount depends on the resin, the part and what the specification allows, and many jobs set a limit in the specification. Read the drawing, the customer’s material specification and any flame rating or food-contact requirement before deciding.
Does regrind weaken plastic parts?
It can. In most resins each pass through the barrel shortens some of the polymer chains, so melt flow goes up and toughness goes down. Impact strength and elongation fall first, while stiffness and tensile strength change much less. In glass-filled grades the fibers also get shorter.
What percentage of regrind is safe?
No one percentage is safe for every job, and the specification limit overrides anything the part can tolerate. For flame-rated parts, UL’s rules for molders generally allow up to 25% regrind of the same grade by weight. Many medical and automotive specifications state a maximum, and some allow none.
Does regrind need to be dried longer than virgin resin?
Yes, for moisture-sensitive resins. Regrind picks up water quickly in an open bin and needs longer in the dryer than pellets do. Grind and reuse it promptly, or store it sealed. Where no longer time is published, expect regrind to need at least the long end of the drying range.
Are regrind problems caused by the resin?
Regrind problems are usually handling problems and not resin problems. Run a short trial on virgin resin only to confirm the regrind is the cause. Then hold the ratio constant, dry the regrind longer than pellets, keep it clean and labeled, and control dust and particle size.
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