A gel is a small lump in the film. A fisheye is the thin, lens-shaped ring that forms around a gel as the film draws down. There are three kinds: crosslinked resin made inside the extruder or die, unmelted resin, and contamination. Each has a different fix, so find out which one you have before you change anything.
Also described as: fisheyes, unmelts, lumps in film, pinpoint defects, hard specks in film.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Hard, clear or amber gels, often with brown or black specks, worse after a shutdown or a long run | Crosslinked gels. Resin has sat hot in a dead spot, oxidized and broken loose. | Time since the screw and die were last cleaned; shutdown and startup procedure |
| Soft, clear gels that melt flat when a piece of film is reheated | Unmelts. A high molecular weight or higher-melting fraction did not melt and mix. | Measured melt temperature; screen pack; resins of very different melt index in the blend |
| Gels with a fiber, a colored center or a gritty core | Contamination: cardboard or liner fiber, dust, or a different resin. | Resin handling, from the gaylord to the hopper |
| A shower of gels just after startup or a resin change, then clearing | Deposits disturbed by the change in temperature or viscosity, or an incomplete purge. | Purge procedure and how long the changeover was run |
| Gel count rose when the regrind or recycled content went up | Each pass through the extruder adds heat history, and trim and recycled resin bring their own contamination. | Regrind and recycled percentage; how trim is stored and handled |
| Gels in a streak at the same position across the web | Buildup or a hang-up at one place in the die or adapter. | Die lips and die internals at that position |
| Gels started with a new lot | Gels in the resin as supplied, or a poorly dispersed additive such as antiblock. | Run a retained sample of the good lot on the same line; compare the COAs |
Checks in order, cheapest first
- Cut out a few gels and identify them. Look at them under a magnifier, then reheat a piece of film on a hot plate. Unmelts melt and flow into the film. Crosslinked gels soften but keep their shape, and come back when the film cools. Fibers and dirt do not melt at all. Note also when the gels appear and where across the web.
- Check melt temperature against the supplier’s range. Measure the melt. Too cold, and resin passes through unmelted. Too hot, or too long in the barrel, and resin oxidizes and crosslinks.
- Check the screen pack. A torn or blown screen lets everything through. Change the pack and look at what it caught. A finer mesh holds back more, at the cost of higher pressure and more frequent changes.
- Review purge and shutdown procedure. Polyethylene left hot in contact with air crosslinks on the metal and comes off later as gels. Follow the resin supplier’s shutdown advice, purge fully at changeovers and do not leave the line soaking at full temperature.
- Check resin handling. Keep gaylords covered, clean hopper loaders and filters, and keep cardboard, liner scraps and other resins away from the feed. A few pellets of a higher-melting resin are enough to cause unmelts.
- Check regrind, then compare lots. Run a short trial without regrind or recycled content. If the gels arrived with a new lot, run a retained good lot on the same line. If the gels stay, the cause is the line and not the resin.
- Pull and clean the screw and die. Dead spots, worn flights and carbon buildup keep feeding gels until they are removed. This is the expensive check, so confirm the cheap ones first.
- Change the resin. If the line is clean, the temperatures are right and a known good lot runs clean while the current one does not, the resin is the next place to look.
The numbers
Know the melt index and density of every resin in the blend. Components with very different melt indexes are harder to mix, and the stiffer one can show up as unmelts.
| Resin | Melt index (g/10 min) | Density (g/cm³) | Melt range on our processing chart |
|---|---|---|---|
| LLDPE, blown film grades | ~0.5–2 | ~0.916–0.926 | Not listed |
| LDPE film grades | ~0.2–4 (general-purpose film ~1–2; coating grades higher) | ~0.918–0.930 | 320–450°F / 160–232°C (LDPE) |
| HMW-HDPE film grades | Very low, often <0.1 at 190°C/2.16 kg. High-load melt index ~5–15 at 190°C/21.6 kg | ~0.945–0.960 | 380–525°F / 193–275°C (HDPE) |
| A typical bimodal HMW-HDPE bag-film grade | ~0.06, with a high-load melt index of ~9.5 | 0.950 | As above |
Typical published ranges from our LLDPE, LDPE film and HMW-HDPE pages. Melt index is measured at 190°C/2.16 kg unless stated. The melt ranges come from the processing chart, which is written for injection molding; use them only as an outer check and set a film line from the data sheet for your grade. Polyethylene does not need drying, but pellets that have collected condensation need the surface moisture removed (see the drying chart).
Three kinds of gel
- Crosslinked gels. Polyethylene held hot for a long time, especially with air present, oxidizes and crosslinks. It collects in dead spots in the screw, adapter and die, darkens with time and breaks loose in pieces. These gels do not melt out, and the same deposits later turn up as black specks.
- Unmelts. A fraction of the resin, or one component of a blend, is too high in molecular weight or melting point to melt and mix in the time it has. These gels are the same material as the film and will melt out with enough heat and mixing.
- Contamination. Fibers, dust, degraded resin from an earlier job or a different polymer. Poorly dispersed antiblock, pigment or other additives look similar in the film.
- Why it matters. Every gel is a weak spot where a tear or pinhole can start, and gels spoil printing and lamination.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A grade with a stronger stabilizer package | Antioxidants slow the oxidation that makes crosslinked gels, which helps on long runs and lines with long residence time. | A new grade to approve. Additive changes can affect food-contact status, sealing and printing, so check each one again. |
| Blend components closer in melt index | Resins of similar viscosity mix more completely, so fewer unmelts. | Melt strength, bubble stability and film toughness all move with melt index. |
| Less regrind or recycled content, or a better filtered source | Fewer heat histories and less contamination going in. | More trim to dispose of or sell, and a lower recycled-content figure if the customer requires one. |
| Prime resin in place of wide-spec or off-grade | Gel level is one of the properties that puts a lot outside prime specification. See prime vs. off-grade. | A higher price. |
| A different antiblock, slip or color concentrate | A concentrate made on a carrier resin close to the film resin disperses better. | Slip, blocking and color have to be checked and approved again. |
Any blend change moves film properties. Seal temperature, slip, clarity and strength all move with it. Run the new film through the customer’s converting step before you approve a swap.
Often confused with
- Melt fracture. A rough, sharkskin surface over the whole film is a flow problem at the die. Gels are separate lumps with clear film between them.
- Moisture or trapped gas. A lens-shaped void with nothing solid at its center is a bubble, not a gel. Check for condensation on cold resin brought into a warm plant.
- Black specks. Dark specks are degraded resin or contamination, and often come from the same deposits that produce crosslinked gels. See black specks and contamination.
- Weak film. If film tears with no gel at the start of the tear, the cause is the blend or the bubble settings. See film tearing or puncturing.
Quick answers
What causes gels in blown film?
Gels come from three sources: crosslinked resin made inside the extruder or die, unmelted resin, and contamination such as fiber, dust or a different resin. Each has a different fix. Cut out a few gels, look at them under a magnifier and reheat a piece of film on a hot plate to find out which kind you have.
How do I get rid of fisheyes in film?
Identify the gel at the center of the fisheye first, because each kind has its own fix. Then check melt temperature against the supplier’s range, change the screen pack, review purge and shutdown procedure and check resin handling. Pulling and cleaning the screw and die is the expensive check, so confirm the cheap ones first.
What is the difference between a gel and an unmelt?
An unmelt is one kind of gel. It is the same material as the film and will melt out with enough heat and mixing, so it melts flat when a piece of film is reheated. A crosslinked gel softens but keeps its shape and comes back when the film cools. Fibers and dirt do not melt at all.
Are gels in film caused by the resin or the line?
Run a retained sample of a known good lot on the same line to find out. If the gels stay, the cause is the line and not the resin. If the line is clean, the temperatures are right and the good lot runs clean while the current one does not, the resin is the next place to look.
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