Most resins keep for a long time in a sealed container. The trouble starts when the container is opened. Moisture-sensitive resins take up water from the air, dust and stray pellets get in, and cold resin brought into a warm plant collects condensation. The result shows at the press as splay, brittle parts or a process that will not hold steady. Look at the opened containers first.
Also described as: shelf life, old resin, opened gaylords, moisture pickup, resin sitting in the warehouse.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Splay or silver streaks after the normal drying time, from a gaylord that has been open for a while. | The resin went into the dryer wetter than the drying time assumes. | A moisture reading before and after the dryer; how long the container has been open |
| Parts look good and are brittle. The resin is polycarbonate, PBT, PET or nylon. | The resin was molded wet. In these resins water breaks the polymer chains and may leave no mark on the surface. | A moisture reading at the dryer outlet; dryer temperature, airflow and dew point |
| Splay on polyethylene, polypropylene or polystyrene, which normally need no drying. Often in cold weather or just after a delivery. | Condensation. Cold pellets were opened in warm, humid air and water formed on the surface. | Whether the pellets are cold or damp to the touch; when the load arrived |
| Parts are bad from the top of the box and improve further down. | The top layer sat exposed under an open liner. | Whether the liner was closed after the last use |
| Specks, odd pellets, fibers or dust in the part. | An open or torn liner let in dirt, cardboard dust or pellets from another job. | The liner, the lid and the scoop or wand used |
| Old stock runs differently from a new lot: a yellower color, a different flow or more odor. | The resin has aged in long or hot storage, or two lots have been mixed. | Lot number and date on the label; the COA of each lot; a melt flow check |
| Pellets are stuck together in lumps or bridge in the hopper. | Soft grades can stick together under heat and stacking weight. Damp pellets can also clump. | Storage temperature; how high the pallets were stacked |
Checks in order, cheapest first
- Read the label and use the oldest lot first. Note the lot number and the date on every container. First in, first out keeps any one box from sitting for a year. Do not top up a part-used gaylord with resin from another lot.
- Measure the moisture. Do not guess. Check a sample before the dryer and after it. Simple shop-floor analyzers lose accuracy at very low moisture levels, so for the tightest targets confirm the method with your supplier.
- Check the dryer before you blame the storage. Confirm the air temperature at the hopper inlet, the airflow, the dew point and that the hopper holds enough resin for the time required. A dryer that is not working makes well-stored resin look bad.
- Dry exposed resin longer. Resin that sat open needs more time than resin from a fresh bag. Several resins also have an upper limit on drying time, so check the data sheet before you leave resin in the dryer for a day.
- Close every liner, every time. Take what the job needs, press the air out, twist the liner shut and tie or tape it, and put the lid back on. This costs nothing and prevents most of the problems on this page.
- Let cold resin warm up before you open it. Leave the container sealed in the plant until the resin is near room temperature. A full gaylord warms slowly, so in cold weather bring resin in well ahead of the job.
- Store it indoors, off the floor and away from heat. Keep containers out of sunlight and away from dock doors, heaters and roof leaks. Do not stack boxes higher than they are built for. Outdoor storage exposes bags and liners to sun and water.
- Retest old stock before you run it, and change the resin last. If a lot is old or its storage is unknown, check moisture and melt flow and run a short trial. Storage problems are almost never a reason to change the resin. A change in package size or order size is the usual fix.
The numbers
Typical drying temperature, drying time and moisture target for the resins most affected by storage. The drier the target, the less time an open container can be left.
| 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% |
| 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) |
| TPU | 194–230°F / 90–110°C; aliphatic grades 104–140°F / 40–60°C | 1–4 h | <0.02–0.05% |
| ABS | 176–203°F / 80–95°C | 2–4 h | 0.1% general; 0.02–0.05% for plating |
| Acrylic | 158–208°F / 70–98°C | 2–4 h | <0.02% strict; 0.05–0.1% non-critical |
| Polypropylene | 167°F / 75°C, for cosmetic parts only | 2 h when required | Up to 0.2% has no effect |
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. Polyethylene, polypropylene, polystyrene and PVC carry surface moisture only, which seldom exceeds 0.01%. The full list is in the resin drying chart.
How stored resin picks up moisture
- Some resins absorb water into the pellet. Nylon, polycarbonate, PET, PBT, TPU, ABS and acrylic are hygroscopic. They take water from the air until they are in balance with it, and humid air gets them there sooner. Polyethylene, polypropylene and polystyrene do not, and hold water on the surface only.
- An open gaylord is a large surface left out for a long time. A gaylord holds roughly 1,000 to 1,600 lb. On a small job that can be weeks of resin, and the top layer is exposed every hour the liner is open.
- Dried resin gets wet again quickly. Published guidance puts the window after the dryer at 15 to 30 minutes. For ABS, one producer notes that as little as 15 minutes of exposure on a humid day can cause moisture defects. A polycarbonate guide limits time in an unheated hopper to 30 minutes, and TPU should be used within 15 to 30 minutes. A nylon guide says to keep no more than one hour’s supply in the hopper.
- Cold resin condenses water like a cold glass. When pellets are colder than the dew point of the air around them, water forms on the surface. This affects every resin, including those that never need drying. Keeping the container sealed while it warms keeps the water off the pellets.
- The liner slows moisture. Only a barrier bag stops it. A polyethylene liner keeps out dust and liquid water and slows water vapor. Moisture-sensitive resins such as nylon are often packed dry in foil-lined bags or liners, and they stay dry only until the seal is opened or damaged.
- Regrind picks up moisture faster than pellets. It has more surface, and it usually sits in an open bin. See how much regrind to use.
What “aged” resin means
- Most resin has no fixed expiry date, though some producers give a shelf life for certain grades, so check the data sheet. Sealed, cool, dry and out of sunlight, most pellets keep their properties for a long time. “Aged” resin is older or surplus stock. The date matters less than how it was stored and whether the package is still sealed. See prime, wide-spec and aged resin.
- Moisture-sensitive resins age by getting wet. Old nylon, PET or polycarbonate from a damaged or opened package can usually be dried and run, but it needs a longer drying time and a moisture check.
- Some resins are more sensitive to time and heat. TPU, flexible PVC, EVA and soft elastomer grades can stick together in warm storage. Grades with slip, antistatic or similar additives can change as the additive moves to the pellet surface. PLA and other compostable resins are sensitive to moisture and heat in storage. For any of these, ask for the producer’s storage guidance.
- Natural and white resin can yellow in the box. Polyolefins with certain stabilizers can turn yellow or pink when exposed to exhaust fumes from fuel-powered forklifts and heaters. The color is on the pellet surface and the properties are usually unchanged, but it can fail a color check.
- Sunlight and heat shorten the life of everything. Resin stored outdoors or against a hot wall loses stabilizer before it is ever molded, and the bags and liners break down.
Material and packaging changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| Smaller packages for slow-moving, moisture-sensitive grades | A 55 lb bag is used up soon after it is opened, so less resin sits exposed. | More handling and more packaging to dispose of. Not every grade is offered in bags. |
| Moisture-barrier packaging | Resin packed dry in a sealed foil-lined bag or liner stays dry until it is opened. | Not offered for every grade or package size. Most resins still need drying once the seal is broken. |
| Smaller, more frequent orders | Less stock on the floor and less time for any lot to age. | More freight per pound. A single gaylord shipped LTL carries far more freight per pound than the same resin in a full truck. |
| A resin that absorbs less water | Nylon 11 and nylon 12 absorb much less than nylon 6 and 66, and polyolefins hold surface moisture only. | A different price, different properties and different shrinkage, and a full part approval. This is rarely worth doing for a storage problem alone. |
| A larger dryer, or a sealed drying hopper at the press | More drying capacity covers resin that arrives wetter, and a sealed hopper stops dried resin from picking moisture back up. | Equipment cost, floor space and energy. |
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
- A dryer that is not drying. A failed heater, spent desiccant, a clogged filter or a hopper that is too small gives wet resin no matter how it was stored. A moisture reading at the dryer outlet settles it. See the resin drying chart.
- Splay from heat or trapped air. Silver streaks also come from an overheated melt, from shear at the gate and from air drawn in at the screw. Dry resin that still splays points there. See splay and silver streaks.
- Regrind. Brittle parts and splay that appear only when regrind is added are a regrind handling problem. See how much regrind to use.
- Nylon parts that change size after molding. That is moisture going into the finished part, which is normal for nylon. Moisture in the pellets before molding is a separate matter. See nylon and moisture.
Quick answers
Does plastic resin go bad?
Most resin has no fixed expiry date, though some producers give a shelf life for certain grades, so check the data sheet. Sealed, cool, dry and out of sunlight, most pellets keep their properties for a long time. How it was stored and whether the package is still sealed matter more than the date. If a lot is old, check moisture and melt flow and run a short trial first.
How should I store resin?
Store it sealed, indoors, off the floor and away from heat, sunlight, dock doors and roof leaks. Close the liner every time: press the air out, twist it shut, tie or tape it, and put the lid back on. Use the oldest lot first, and let cold resin warm up sealed before opening it.
How long can resin sit in an open gaylord?
There is no single time limit, and the drier the moisture target, the less time an open container can be left. The top layer is exposed every hour the liner is open. Resin that sat open needs a longer drying time than resin from a fresh bag, so measure moisture before and after the dryer.
Why does resin that needs no drying show splay in cold weather?
The usual cause is condensation. When cold pellets are opened in warm, humid air, water forms on the surface. This affects every resin, including polyethylene, polypropylene and polystyrene. Leave the container sealed in the plant until the resin is near room temperature. A full gaylord warms slowly.
Is a storage problem a reason to change resin?
Very rarely. Check the dryer before blaming the storage, because a dryer that is not working makes well-stored resin look bad. The usual fix is a change in package size or order size, such as smaller packages for slow-moving, moisture-sensitive grades.
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