Runners, Purge and Rejects: What Regrind and Scrap Really Cost an Indian Moulding Plant (₹)
"We regrind everything" is the most expensive sentence in a plastics plant. Regrind recovers the polymer, not the machine-hours behind the reject, the value lost on purge lumps, or the regrind the cap won't let back in. On an illustrative 30-press polypropylene plant, those leaks come to about ₹6.7 lakh a month. Roughly half is material and half is machine time.
By the Mama Editorial Team. This piece is for owners of Indian plastics injection-moulding units making closures, containers, crates and technical parts. All rupee figures are assumptions to replace with your own. Cycle drift and response time are covered in our injection-moulding camera guide.
Key takeaways
- Scrap has two price tags. One is material: the gap between what you paid for virgin dana and what the reject is worth as regrind or as scrap-dealer lumps. The other is machine-hours: every rejected shot used a full cycle of press, mould, power and operator.
- Regrind isn't free. It costs grinding, handling, drying and fines loss, and it has a ceiling: UL 746D (a safety standard for plastic parts in electrical equipment) allows in-house sprue and runner regrind at up to 25%, the rest the same virgin grade (UL Solutions).
- Start-up shots are a known, budgetable loss. A 1992 Sumitomo Wiring Systems patent, from trials on PBT connector resin, records part weight climbing for the first 20 to 30 shots after start-up before it stabilises, and calls discarding those shots "right" (US 5,133,910).
- Colour-change purge is the worst kilogram you make. It can't be reground into the next colour, it often goes out as low-value lumps, and the press makes nothing while it runs.
- Running rejects cost more in time than in plastic. In our worked example, running rejects at 1.5% cost about ₹86,000 a month in material and about ₹2.3 lakh in machine-hours.
- A camera can't weigh scrap. It shows when and where purge, start-up shots and rejects pile up; the scale does the weighing.
The five scrap streams in a moulding shop
Most plants report one "rejection %". It hides five streams that behave very differently.
1. Runners and sprues. On a cold-runner mould, the feed system is moulded and ejected every cycle. Hot-runner maker Mold-Masters says a cold runner can weigh 50% to 250% of the part (Mold-Masters). It's a vendor figure, but it shows the scale. Clean, single-colour runners are your best regrind.
2. Regrind over the cap. If a mould's runner is 30% of shot weight and your cap is 25%, the surplus can't go back into that job. It piles up in bags behind the granulator.
3. Start-up shots after a mould change or restart. In the patent above it took 20–30 shots for the mould to come up to temperature and part weight to settle; your number depends on the mould and the start-up routine. They're usually clean and regrindable, but they cost machine time.
4. Colour-change and material-change purge. These are the purge lumps (patties) and the transition parts made while the old colour clears. Chem-Trend, which sells purging compounds, describes a red-to-white PP change that took three hours when purged with virgin resin (Chem-Trend case study). It's a supplier's case, not a benchmark.
5. Running rejects. These are short shots, flash, sink, black specks, warpage and wrong colour shade. Some regrind cleanly. Contaminated or degraded parts don't.
Why "we regrind it" is not the end of the story
Regrind brings three costs that a reject percentage doesn't show.
It degrades. Each melt history shortens polymer chains. A lab study of two commercial high-impact PP grades (one unfilled, one talc-filled), ground and re-extruded up to 12 times, found a continuous decrease of elastic modulus and failure strain as reprocessing increased, attributed to chain scission (Wang et al., Materials, 2019). A steady 25% blend always carries some material that has been melted several times: fine for a crate, maybe not for a thin-wall hinged lid.
It has a ceiling. UL 746D allows up to 25% (UL Solutions). Mold-Masters, a hot-runner vendor, writes that regrind is "typically limited to 15% at most" in the jobs it sees (Mold-Masters). Food-contact, pharma and automotive buyers often set their own limit in the drawing, so read yours.
It costs energy, handling and drying. The direct grinding energy is small. Granulator maker CMG quotes 20–25 Wh/kg for its units against "over 50 Wh/kg" for conventional ones (Recycling Inside, Feb 2022). At an assumed ₹9/kWh, 50 Wh/kg is under ₹0.50/kg. Most of the cost comes from what surrounds the grinding:
- a helper carrying runners to the grinder;
- fines that are lost or that foul the dryer;
- re-drying for hygroscopic grades such as nylon and PET;
- the occasional batch of mixed-colour regrind that becomes unsaleable.
In the example below we put the whole bundle at an assumed ₹6/kg. Track your own figure for a month.
A packaging-specific wrinkle: recycled content under EPR
If you mould rigid packaging (containers, tubs, jars, closures), the Plastic Waste Management Rules define you as a "producer": a person engaged in manufacturing plastic packaging (PWM Rules 2016, consolidated to March 2024, via UNEP LEAP). The latest amendment, G.S.R. 237(E) of 31 March 2026, restates the recycled-content table in Schedule II:
- Category I (rigid) packaging: 30% in 2025-26, 40% in 2026-27, 50% in 2027-28, 60% from 2028-29.
- It applies to producers (share of packaging manufactured), brand owners (packaging used) and importers. Flexible and multi-layer categories have lower targets.
- Targets don't apply where a law, FSSAI/CDSCO rule or mandatory Indian Standard bars recycled plastic; claim it in the annual return.
- An unmet 2025-26 target for food-contact packaging can be carried forward over 2026-27 to 2028-29, clearing at least a third each year.
- Recycled packaging must conform to IS 14534:2023 and carry the recycled-content marking.
Don't assume your own runner regrind counts. The rules define recycling as transforming segregated plastic waste, and don't say whether in-house regrind qualifies. CPCB was given six months to issue audit and verification guidelines, so check its current EPR guidance and your brand-owner contracts first.
The worked ₹ example: a 30-press PP plant
Assumptions (all illustrative, replace with your own):
- 30 presses running 3 shifts, 26 working days a month.
- About 22 productive hours per press per day, so 17,160 machine-hours a month.
- 250 tonnes of PP processed a month.
- Virgin PP landed at an assumed ₹110/kg. For reference, one index put India PP at US$1.28/kg in September 2026 (businessanalytiq), about ₹113 at an assumed ₹88/USD. Domestic prices move monthly; domestic PP list prices, Reliance's and IOC's included, went up ₹1,000/MT from 1 October 2026 (Plastemart price list).
- Clean single-colour regrind sold to a scrap dealer at an assumed ₹50/kg. Mixed purge lumps at an assumed ₹20/kg.
- In-house regrind reuse cost an assumed ₹6/kg.
- Machine-hour rate, excluding material, an assumed ₹900/h. Work out yours with our machine hour rate guide.
| Scrap stream | Volume assumption | Material ₹/month | Machine-time ₹/month | Total ₹/month |
|---|---|---|---|---|
| A. Runners reground and reused | 12 t × ₹6 reuse cost | 72,000 | — | 72,000 |
| B. Runner regrind over the cap, sold out | 2 t × (₹110 − ₹50) | 1,20,000 | — | 1,20,000 |
| C. Start-up shots after mould change | 130 changes × 30 shots × 350 g = 1.37 t reground; 15 min each | 8,190 | 29,250 (32.5 h) | 37,440 |
| D. Colour-change purge | 78 changes × 10 kg lumps sold at ₹20 (loss ₹110 − ₹20); 45 min each | 70,200 | 52,650 (58.5 h) | 1,22,850 |
| E. Running rejects at 1.5% | 3.75 t: 80% reground at ₹6, 20% sold at ₹20 (loss ₹90/kg); 1.5% of machine-hours | 85,500 | 2,31,660 (257 h) | 3,17,160 |
| Total | 3,55,890 | 3,13,560 | 6,69,450 |
What the table says:
- Material leak is about 1.3% of a ₹2.75 crore monthly resin bill: easy to ignore, big enough to pay a quality engineer several times over.
- Over-cap regrind (row B) is pure planning loss. The plastic is clean. You sell it at less than half price because the runner-to-part ratio and the regrind cap don't match. A hot-runner conversion, runner redesign or routing it to a job that can take it fixes that; floor discipline won't.
- Purge (row D) is the most expensive line per event. Each colour change costs about ₹1,575 in material and machine time in this example. Light-to-dark sequencing, grouping colours across the week and dedicating presses to dark colours are the usual levers.
- Running rejects (row E) are mostly a time problem. Material is about a quarter of that row. The rest is 257 press-hours a month that produced nothing saleable.
Honest caveat on machine-time rupees. Machine-hours are a real cash loss only when the plant is capacity-bound: turning away orders, running overtime or adding a shift. If presses already sit idle half the night, the cash leak from rows C–E is closer to power, labour and wear than the full ₹900/h. The material column is real either way. The ₹900/h rate is also a big lever: at ₹375/h, the machine-time column falls to about ₹1.3 lakh and the total to about ₹4.9 lakh. The same logic appears in our piece on what a 3% rejection rate costs a CNC shop.
Where the kilograms actually leak on an Indian floor
Plastic rarely leaves by theft. It leaves in ways nobody writes down.
- The purge heap. Night-shift lumps go into a gunny bag nobody weighs.
- "Thoda aur chalao." The operator runs a few more shots "to be safe" after start-up because nobody defined when the first-off is approved.
- Mixed-colour regrind. One tip of black runner into a natural bin turns 200 kg of ₹50 regrind into ₹20 lumps.
- The scrap sale. Scrap leaves the gate as "mixed plastic" by estimate, not weight. Our gate and weighbridge guide covers why the weighbridge slip and the scrap register must reconcile.
- Regrind without a ratio. The proportioning valve is set by feel; on a busy night 25% becomes 40%.
What a camera can and cannot tell you here
A camera cannot:
- weigh anything, or tell 8 kg of purge from 12 kg;
- see the regrind ratio inside a hopper loader or confirm a material grade;
- detect black specks, sink or an off-shade part from a ceiling mount;
- tell clean regrind from contaminated regrind in a bag;
- explain why a mould makes short shots.
A camera can show:
- how long a press ran between mould-in and steady production (the time side of row C);
- how many colour changes happened per press per day, and how long each one ran;
- whether purge piles up at one press or on one shift, in aggregate terms;
- whether the granulator area and runner bins are used as intended: colour-segregated, not mixed;
- when scrap leaves the plant, to check against the scrap register.
The pairing that works is a scale for the kilograms and video for the hours: weigh purge, start-up shots and rejects per press per shift, and let video show when each event happened and how long it lasted.
On people: this is about presses and material flow, not about watching operators. Aggregate by press and shift, tell the workforce what the cameras are for, and follow the notice requirements in our DPDP worker CCTV notice guide.
Where Mama fits
Mama reads the cameras a plant already has and each morning sends the owner a short plain-language note on where press-hours went yesterday. It covers which presses spent long stretches on start-up or colour change, where downtime clustered, and which shift had the most stop time. It watches machines and flows, not individual workers, and it doesn't weigh scrap. If you'd like Mama to look at your floor and suggest where cameras would earn their keep, leave an email or phone in the form below.
FAQ
What percentage of regrind can I use in injection moulding? The most cited reference is UL 746D: in-house sprue and runner regrind up to 25% (UL Solutions). Your customer's drawing may set less or forbid it; for thin-wall, hinged or food-contact parts, validate the blend on the actual part.
Is regrind free if I grind my own runners? No. Grinding energy is small, but handling, drying, fines loss and degradation with repeated melting add up (Wang et al., 2019), and regrind over your cap gets sold at a deep discount.
How many shots should I scrap after a mould change? There is no universal number; one patent saw part weight settle after 20–30 shots (US 5,133,910). A defined first-off approval rule beats "run until it looks right". Count and weigh your start-up shots for a month.
How do I reduce colour-change purge cost? Schedule colours light to dark, group the same colour across days, dedicate presses to dark or difficult colours, and trial a commercial purging compound against your current virgin-resin purge. Measure kilograms and minutes per change before and after.
Does in-house regrind count toward EPR recycled-content targets? Don't assume it does. Under the PWM Rules as amended on 31 March 2026, Category I (rigid) packaging needs 30% recycled plastic in 2025-26 and 40% in 2026-27, rising to 60% from 2028-29, and a packaging moulder counts as a producer. The rules don't say whether in-house regrind qualifies; confirm with current CPCB guidance and your brand-owner customer.