Same grade, same settings, different parts. The cause is usually one of three things: the new lot sits at a different point inside the producer’s specification than your last one, it picked up moisture on the way to your press, or something else changed at the same time. Less often, the lot is contaminated or is not the grade on the label. Start with the paperwork and a moisture reading, which cost almost nothing, and go to lab tests only if needed.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Splay on the new lot only. The old lot ran clean through the same dryer. | The lot arrived wetter: a torn liner, a package left open, or condensation from a cold delivery opened before it warmed up. See splay. | Moisture at the feed throat; liner condition; whether the package was cold when opened |
| Short shots or flash with the settings that worked last month. | Melt flow is inside the specification but at the other end of the range from the last lot. Wet nylon, PC, PET or PBT also runs thinner. See short shots and flash. | Melt flow on both COAs, at the same test condition; part weight |
| Color shift, most visible in natural, white, pastel or clear parts. | Natural resin varies slightly in shade from lot to lot; one may read yellower. A new concentrate lot or a drifting let-down can arrive at the same time. | Natural plaques from both lots side by side; color values on the COAs, if reported; the concentrate lot |
| Parts look normal but break. The resin is PC, PBT, PET or nylon. | Molded wet: water breaks the chains (hydrolysis) and leaves no mark. Also possible: a higher-flow lot, regrind in the feed, or the wrong grade. See brittle parts. | Moisture at the feed throat; melt flow on both COAs; the label against the COA |
| Dimensions moved by a similar amount on every part. | Melt flow changed how the part packs, filler content moved within its tolerance, or a nucleating agent or pigment changed how a semi-crystalline resin shrinks. | Part weight; ash or filler on both COAs; the concentrate lot |
| Black specks or gels from the first shots of the new lot. | Contamination or degraded material in the lot as received. A start-up or color change that day is the more common cause. See black specks. | Pellets from a freshly opened package on a white tray; a pressed plaque |
| The new lot smells different at the press or in the parts. | Overheating at the press, a change in the additive or color package, or resin stored near fumes. | Melt temperature with a probe; residence time; pellets from both lots side by side |
Checks in order, cheapest first
- Compare the new COA with the last good lot’s COA, field by field. Melt flow and its test condition, moisture, ash, density, color and viscosity. A shift that matches your symptom tells you where to look. See what a COA shows.
- Check that the label, the lot number and the COA agree. Every package should show grade, color and lot, matching the COA. A covered or replaced label deserves a question before the resin goes into the hopper.
- Measure moisture as received and at the feed throat. The first shows whether the lot arrived wetter than usual; the second, whether your dryer caught up. See storing resin and moisture pickup.
- Run the new lot and a retained sample of the good lot side by side. Same press, settings and day, both dried alike. If both run the same, look at the dryer, concentrate, regrind and machine. If only the new lot misbehaves, you have evidence.
- Measure melt flow on both lots. Test dried resin at the COA’s temperature and load (ASTM D1238 or ISO 1133). Many molders flag a result more than about 10–20% from the COA value; the producer’s range is the limit that counts. See melt flow index explained.
- Run an ash test on filled grades. A burn-off test (ASTM D5630 or ISO 3451) shows whether glass or mineral content moved, which changes stiffness, shrinkage and warp.
- Use DSC or FTIR if the material itself is in doubt. FTIR identifies the polymer family. A DSC melting point separates close relatives, such as nylon 6 at about 220°C / 428°F and nylon 66 at about 260–265°C / 500–509°F. Both usually go to a lab.
- Take it to the supplier with data. Send lot numbers, both COAs, your test results, photos and retained pellets of both lots. Hold the remaining packages until you have an answer.
What to compare on the two COAs
Each producer chooses what goes on its COA, and the limits that count are the ranges printed on it. This table shows what a move usually means; it does not set limits.
| Property | Typical spec type | What a shift means |
|---|---|---|
| Grade, color and lot number | Must match the label and your PO | The COA may not describe what you received. Hold the lot until it is explained. |
| Melt flow (MFR or MVR) | A minimum and maximum, or nominal ± tolerance, at a stated temperature and load | Higher: easier fill, more flash, sometimes less toughness. Lower: short shots. Compare at the same condition only. |
| Moisture, where reported | A maximum, as packed | Your usual drying time may not be enough. After drying, hold the data sheet target, for example ≤0.02% for PC and PBT, <0.02% for injection-grade PET, 0.03–0.12% for unfilled nylon 6 and 0.03–0.2% for unfilled nylon 66, depending on the producer. |
| Ash or filler content | Nominal ± tolerance | Changes stiffness, shrinkage and warp. Lots at opposite ends of the tolerance can both pass and mold differently. |
| Color values (L*, a*, b* or yellowness index), on some grades | A range, or a match to a standard | A yellower natural lot shows first in whites, pastels and clear parts. |
| Intrinsic or solution viscosity, on some PET, PBT and nylon COAs | A range | Tracks molecular weight. Lower usually means easier flow and less toughness. |
| Test methods | ASTM or ISO method for each result | A different method, temperature or load makes two numbers impossible to compare. |
Drying targets are typical values from our resin drying chart; confirm against the data sheet for your grade. The full receiving procedure is in incoming resin inspection.
Why the same grade changes from lot to lot
- A specification is a range, not a point. A lot near the top of the melt flow range and one near the bottom both pass and are the same grade, but they fill and pack differently.
- Your process was tuned to the lots you ran. Settings developed on lots near one end of the range can run out of window on a lot from the other end.
- Moisture arrives after the resin leaves the producer. A torn liner, an open package or a cold delivery opened in a warm plant lets water in. The lot is fine; it needs more drying.
- Fillers and additives have tolerances too. Glass, mineral, nucleating agents and stabilizers are each added within a range, and some change shrinkage without showing on the COA.
- Tolerances add up. Resin lot, concentrate lot, regrind and dryer each vary a little. A new lot can push parts out of tolerance with every input inside its own limits.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| Re-center the process for the new lot | Brings parts back without changing the material. | Setup time for each lot, and it does not fix the next one. Some customers want notice of changes outside the validated window. |
| A narrower melt flow range, or other limits, on your purchase order | Screens out lots at the far ends of the range before they ship. | Not every producer will agree. When one does, it may cost more, fewer lots qualify and your choice of sources narrows. |
| A lot-specific COA reviewed before the lot ships | You can compare it with your last good lot and reject it before it reaches your dock. | A step in every order. It catches only what the COA reports. |
| Moisture-barrier packaging, where offered | The resin arrives drier, so your usual drying time is more likely to be enough. | Not offered for every grade or package size, and the resin still needs drying once the seal is broken. |
| A second source of the same grade | Another pool of lots, and cover during a shortage. See backup source. | More suppliers to manage and a trial on the first lot. Normally not a material change, but some customers want notice of a supplier change. |
| A retained sample from every lot | Evidence for a side-by-side run and any supplier discussion. | Shelf space and a little time at receiving. Nothing in the part. |
Agree on the test before a dispute. Melt flow and ash results differ between labs, so settle the method with your supplier while every lot is running well.
Often confused with
- A process change on the same day. A dryer fault or a heater band can coincide with a lot change. If the retained old lot now runs badly too, the lot is not the cause. See dimensions varying.
- A new concentrate lot. Color that shifted with the concentrate, not the base resin. See color that varies.
- Old stock. A lot that sat long or was stored poorly can run differently from fresh resin. See resin shelf life.
- An equivalent grade. A grade from a different producer is a material change, even with matching data sheet values. See finding an equivalent grade.
Quick answers
Why does a new lot of the same resin run differently?
Usually the lot sits at a different point inside the producer’s specification, most often for melt flow or filler, or it picked up moisture in transit or storage. Compare the two COAs and measure moisture at the feed throat before you change the process.
How much melt flow difference between lots is normal?
Any value inside the producer’s range passes, and two passing lots can differ noticeably. For in-house checks, many molders flag a result more than about 10–20% from the COA value. Dry hygroscopic resins first, or the result reads high.
Can I get the same grade with a COA for each lot?
Yes. Every lot from our suppliers ships with a COA, and a polymer chemist reviews every quote. Send the grade and the COA of your last good lot. We aim to send your quote within one to three business days. Samples of about 50 lb are available upon credit approval.
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