The short answer
Many molders start at about 15–25% same-grade regrind for engineering resins and 20–30% or more for polyolefins in non-critical parts, then prove it with part testing. Resins that hydrolyze (PC, PET, PBT, nylon, TPU), resins with little heat stability (PVC, acetal) and glass-filled grades need the most care. For UL-recognized parts, UL 746D allows up to 25% in-house regrind of the same grade. A customer or regulatory limit always comes first.
This page is the by-resin reference. For diagnosing a regrind problem on the press (brittle parts, splay, specks, flash), see how much regrind to use, and what it changes.
Regrind limits by resin
The figures below are common starting points for clean, dry, same-grade regrind from your own sprues, runners and rejects, fed at a fixed ratio. They are not limits from a standard. Your part, your grade's processing guide and your customer decide the real number.
| Resin | Common starting max (same grade) | What degrades first | Special risk |
|---|---|---|---|
| PP | 20–30%; more in non-critical parts | Melt flow rises; impact and color in natural grades | Stabilizer is used up each pass; natural and light colors yellow first |
| HDPE | 20–30%; more in non-critical parts | Stress-crack resistance, gels, color | Containers and film show it first; keep film trim clean |
| LDPE / LLDPE | 10–30% (film edge trim often fed back in line) | Gels and specks, melt flow drift | Dirt and degraded trim show as gels in thin film |
| PS / HIPS | 15–25% | HIPS: impact (rubber phase). GPPS: clarity and yellowness | Clear GPPS shows specks and tint readily |
| ABS | 15–25% | Impact, then color and gloss | Dry it; plating grades usually take little or none |
| PC | Up to about 20% in some producer guides | Impact (can turn brittle), yellowness | Severe hydrolysis if remelted wet; dry to ≤0.02% |
| PC/ABS | 15–25% | Impact, splay, color | Dry regrind longer: 4–6 h at 100–110°C / 212–230°F |
| PMMA | 15–25% | Clarity, yellowness, specks | Optical parts often run little or none |
| PA6 / PA66 | 15–25% | Elongation and impact, color | Hydrolysis if wet, and over-dried regrind yellows; dry carefully |
| PA GF (glass-filled) | 15–25% | Glass fiber length, so strength, stiffness, impact | Shrinkage and warp can shift as fibers shorten |
| POM (acetal) | 15–25%; copolymer more forgiving | Thermal degradation, color, specks | Never regrind burned or overheated parts; keep residence time short |
| PBT | 15–25% | Impact and elongation | Hydrolysis if wet; dry to about 0.02% |
| PET | 15–25% in injection parts | IV (molecular weight), clarity, brittleness | Amorphous regrind must be crystallized before drying |
| PPS (usually glass-filled) | Often up to 25% | Glass fiber length, impact | Stable melt, but fibers still shorten each pass |
| PEEK | Often 10–30% | Glass or carbon fiber length in filled grades | Holds properties well over a few passes; medical and aerospace programs often allow none |
| TPU | 10–25% | Melt viscosity, hardness, strength | Very moisture-sensitive; polyester TPU hydrolyzes |
| PVC (rigid) | 10–20% | Heat stability, color (yellow to brown), black streaks | Each pass uses up heat stabilizer; never let PVC meet acetal in the barrel |
Typical starting points drawn from producer processing guides and common molding practice. Grades vary, and some producers publish a specific limit for a grade. Test the property your part depends on before you settle on a ratio.
Why the limits differ
Hydrolysis. PC, PET, PBT, nylon and polyester TPU react with water at melt temperature, which cuts the polymer chains. Wet regrind does far more damage than the extra heat history itself. Dry regrind at least as long as virgin pellets, and longer if it sat in an open bin. Temperatures and moisture targets are in the resin drying guide.
Heat stability. Rigid PVC and acetal carry a limited stabilizer reserve. Each pass uses some of it. PVC that runs out turns yellow, then brown, and releases hydrogen chloride; acetal gives off formaldehyde and specks. Never regrind burned or purged material from either. Never follow PVC with acetal, or acetal with PVC, without a full purge, since the two can decompose rapidly together.
Glass fiber breakage. The grinder and the screw both shorten fibers. In glass-filled nylon, PBT and PPS, strength and impact fall with fiber length, and shrinkage can move. See glass-filled plastics.
Melt flow drift. Chain scission raises melt flow in most resins, so a high regrind share can flash a mold set up on virgin. Polyethylene can go the other way and crosslink slightly. Compare the blend's melt flow with the virgin lot; see melt flow index explained.
The steady-state math
When regrind goes back in at a fixed ratio, the material in each shot settles into a predictable mix of heat histories. With a regrind fraction p, the share melted n times is (1 − p) × pn−1, and the average number of passes is 1 / (1 − p).
| Regrind in blend | Melted once | Twice | Three times | Four or more | Average passes |
|---|---|---|---|---|---|
| 10% | 90% | 9% | 0.9% | 0.1% | 1.11 |
| 20% | 80% | 16% | 3.2% | 0.8% | 1.25 |
| 25% | 75% | 18.8% | 4.7% | 1.6% | 1.33 |
| 30% | 70% | 21% | 6.3% | 2.7% | 1.43 |
| 50% | 50% | 25% | 12.5% | 12.5% | 2.00 |
Assumes regrind is returned at a constant ratio and mixed evenly. Dumping a week of saved regrind in at once breaks this pattern.
Two practical points follow. First, at 25% very little material sees more than three passes, which is why a steady ratio is safer than an occasional large dose. Second, your steady regrind rate cannot exceed the scrap you generate: if runners are 15% of the shot and rejects are 2%, you can only feed back about 17%. Try your own numbers in the regrind calculator.
UL-recognized parts (UL 746D)
UL describes regrind under UL 746D as in-house molding scrap, such as sprues and runners, reused at the molder that made it. It can make up to 25% of the charge, with the other 75% being the same virgin grade the scrap came from. Regrind of a different grade or color, or regrind shipped to another molder, falls outside that allowance: once it leaves the molder it is treated as post-industrial recycled plastic and needs its own evaluation (now under UL 746S). Check your own UL file and end-product standard before you rely on the 25% figure. See UL 94 flame ratings explained.
Medical, automotive and food-contact limits
- Medical: many programs allow no regrind, or allow a fixed, validated ratio only. Any change usually means revalidation.
- Automotive: many OEM material specifications state a maximum, and the approved PPAP process records the ratio. Changing it is a process change.
- Food contact: use only your own clean regrind of the same compliant grade, kept separate. See FDA food-contact compliance.
Fines, ratio and labeling
- Keep fines out. Sharp knives and the right screen size give even granules. Dust melts early, burns, holds moisture and causes black specks. A fines separator or a simple screen helps.
- Hold the ratio. Use a proportioning loader or a blender, not a scoop. Regrind has a lower bulk density than pellets, so volumetric feeders under-dose it by weight unless you calibrate.
- Label every bin with resin, grade, color and date, and keep different grades apart.
Usually tolerant
- PP, HDPE, LDPE in non-critical parts
- GPPS and HIPS kept dry and clean
- Unfilled PEEK and PPS over a few passes
Needs the most care
- PC, PET, PBT, nylon, TPU (dry it)
- PVC and acetal (heat stability)
- Any glass-filled grade (fiber length)
What to send us
If your part needs more regrind headroom than your current grade gives, a tougher or better-stabilized grade may help. Recycled-content programs are a different question; see molding with PCR resin.
- The grade you run now, or its data sheet
- Your regrind share and what the part has to pass (impact, UL, food contact)
- Quantity and ship-to city
A polymer chemist reviews every request, every lot ships with a COA, and orders start at one gaylord. We aim to send your quote within one to three business days.
Common questions
How much regrind can I use in injection molding?
There is no single safe number. Many molders start at about 15–25% same-grade regrind for engineering resins and higher for commodity polyolefins in non-critical parts, then confirm with part testing. Any limit in the drawing, customer spec, UL listing or food-contact requirement overrides that.
What does UL 746D allow for regrind?
UL describes UL 746D regrind as in-house molding scrap such as sprues and runners, reused at the same molder. It can make up to 25% of the charge, with the other 75% being the same virgin grade the scrap came from. Regrind shipped to another molder is treated as post-industrial recycled plastic and needs its own evaluation.
Which resins are most sensitive to regrind?
Resins that hydrolyze when remelted wet (PC, PET, PBT, nylon and polyester TPU), resins with limited heat stability (PVC and acetal), and glass-filled grades, where each pass shortens the fibers. Polypropylene, polyethylene and polystyrene are more forgiving.
Why does glass-filled regrind lose strength?
The grinder and the screw both break glass fibers, so each pass leaves them shorter. Shorter fibers carry less load, which lowers tensile strength, stiffness and impact strength and can shift shrinkage and warp.
How many times can plastic be reground?
At a steady regrind rate the question mostly answers itself. At 25% regrind, 75% of each shot has been melted once, about 19% twice, about 5% three times and under 2% four times or more, so very little material sees many passes.
Do I need to dry regrind?
Yes, for any resin that needs drying as pellets. Regrind picks up moisture faster than pellets in an open bin, so dry it at least as long as virgin resin, and longer if it has been sitting. Undried PC, PET, PBT or nylon regrind loses molecular weight in the barrel.
Can I use regrind in medical or automotive parts?
Only if the customer specification allows it. Many medical programs allow none or require validation of a set ratio, and many automotive material specifications state a maximum. If the document is silent, ask in writing before you assume.
Tell us the grade and your regrind share
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province