Troubleshooting · Defect guide
Gels and fish eyes in film
Where the specks come from, and what to fix first
Gels have more than one cause. Spots that look alike in a film can be crosslinked degraded polymer, unmelted resin, contamination or poorly dispersed masterbatch, and each needs a very different fix. This guide starts by telling the types apart, then works through checks from cheapest to most expensive.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Which type of gel is it?
A gel, or fish eye, is a small raised spot in the film, easiest to see when you hold the film up to the light. It is usually lens-shaped with a tail in the machine direction, because the spot is stiffer or more viscous than the melt around it and the film stretches unevenly around it. Five types are common:
- Oxidised / crosslinked gels: hard, do not melt back when heated, often with a yellow, brown or black core. They come from melt that has spent too long in a hot machine.
- Unmelted or high-molecular-weight gels: clear and colourless, melt back when heated. They form when temperature or mixing is not enough, or when a resin lot contains more of this fraction.
- Contamination: a different polymer, dust, metal fragments, or paper fibres from recycled pellets. Cutting one open usually shows the foreign material.
- Poorly dispersed masterbatch or additives: darker or hazier than the surrounding film, and gone when you run the base resin alone.
- Moisture or volatile voids: not true gels, since they are hollow, but the shop floor often calls them the same thing.
Look-alikes that are not gels: dust on the film surface, which wipes off, and intentional antiblock particles, which are small and evenly spread (see Film layers stick together).
Mechanism: where gels form on the line
Pellets are melted and mixed in the barrel, pass through the screen pack, then flow through the die to become film. Gels can start upstream, such as gels already in the pellets or contamination in recycled feed, or inside the machine, where melt in a dead spot moves so slowly that it is heated far longer than normal, degrades, crosslinks and then breaks loose into the stream from time to time.
The practical takeaway: do not fix gels before you know the type. Degradation gels are fixed at the machine and in the heat history, unmelted gels in temperature and mixing, and contamination in the raw material and filtration. Applying the fix for one type to another wastes time and can make things worse.
Four groups of causes
1. Raw material
A resin lot that arrives with gels, recycled pellets or regrind that carry contamination or several heat histories, and wet components such as fillers or washed recycled pellets. See Polymer basics for the background.
2. Masterbatch and additives
Masterbatch whose pigment or additive was poorly dispersed at the compounding stage, a carrier that is more viscous than the base resin or a different polymer, so it does not blend in time, and an antioxidant package that is not enough for the line's temperature and residence time.
3. Machine and die
Dead spots in the adapter, screen changer and die, worn screw or barrel surfaces where melt sticks, a torn or blocked screen pack, and die build-up that breaks loose from time to time.
4. Process
Temperatures too low to melt everything, temperatures too high or a slow line speed that leaves melt sitting too long, stoppages with melt left in a hot machine, and incomplete purging when changing material or colour. For an overview of the line, see Extrusion.
Step-by-step checks, cheapest first
- Count gels as a number. Cut the same size of film every time, hold it to the light, count, and log the count with time and lot. Without a number you cannot tell whether a change helped.
- Cut a gel open and look at it under a magnifier or microscope. A black or brown core usually means degraded material; clear and colourless usually means unmelted resin; fibres or foreign particles mean contamination; a hollow centre means moisture or volatiles. If you have a hot plate, warm the cut piece: a gel that melts back points to unmelted resin, one that stays solid points to crosslinked material or contamination.
- Look at when gels appear. Mostly after start-up or a stoppage, rising day by day of running, or changing with the resin lot: each points to a different cause group.
- Run the base resin alone for a short time, with no masterbatch and no regrind. If gels disappear or drop clearly, the problem is in the blend, not the machine.
- Change the resin lot and check the masterbatch carrier. Send film samples with lot numbers to the resin supplier, and ask the masterbatch maker what polymer the carrier is and whether its viscosity is close to your base resin.
- Inspect the screen pack at every change. Check for tears or blockage, and watch whether pressure ahead of the screen rises unusually fast. A fast rise means there is a lot to catch; go back upstream to find it.
- Review temperatures and residence time, then purge or strip and clean the die. This needs a line stop, so it is the most expensive step. Do it when the checks above point to degraded material from dead spots.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Degraded melt held up in dead spots | Gels with yellow or black cores; count rises with running time | Planned purging and strip-down cleaning of adapter and die; remove dead spots when upgrading equipment |
| Temperature too high or melt held too long | Worse after stoppages or at slow line speeds | Adjust the temperature profile; do not leave melt sitting in a hot machine during stops |
| Incomplete melting or mixing | Clear, colourless gels that melt back when heated | Adjust the temperature profile, improve mixing, review screw design when changing resin grade |
| Resin lot with many gels | Gel count changes clearly with the lot | Report to the resin supplier with film samples, gel counts and lot numbers |
| Poorly dispersed masterbatch or mismatched carrier | Gone with base resin alone; spots darker or hazier than the film | Use masterbatch with a carrier matched to the base resin and good dispersion |
| Contamination in recycled pellets or regrind | Fibres, other polymers or foreign particles when cut open | Sort and filter at the pelletising stage; hold the blend ratio constant |
| Torn or blocked screen pack | Inspect removed screens; watch pressure ahead of the screen | Change on schedule; use a support screen so fine meshes do not tear |
| Moisture or volatiles | Hollow gels that drop when the suspect component is dried | Dry fillers, high-filler masterbatch or recycled pellets before use |
| Dust from conveying | Random; dark foreign matter when cut open | Cover bins and hoppers; clean conveying lines on schedule |
How much can additives help?
Unlike sink marks or flash, gels are a problem where the formulation genuinely plays a part, but only for some types. These are the options that actually exist, including ones ACS does not sell:
- Antioxidants reduce degradation during melting, so they reduce degradation gels, especially in material with several heat histories such as regrind and recycled pellets. They do not help while dead spots remain in the machine.
- Masterbatch with a carrier matched to the base resin. A carrier of the same polymer with similar viscosity blends in more easily; the same principle as in Colour streaks. If a cut spot shows a white mineral core, check the dispersion of the antiblock masterbatch as well.
- Polymer processing aids (PPA) reduce die build-up in some film applications, and die build-up is one source of spots that break loose. ACS does not supply them.
- Purging compounds push old material out when changing resin or colour. Several specialist makers offer them; they are not on ACS's product list.
With recycled pellets, most gels must be dealt with at the pelletising stage through sorting, degassing and melt filtration. See Recycled pellet production and lot QC and Recycled PCR resin.
What additives cannot fix: dead spots in the machine, a worn screw or barrel, a torn screen, and contamination already in the pellets.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Gel questions we hear often
Why do gels keep increasing after the line has run for several days?
This usually comes from melt held up in dead spots: adapter joints, corners inside the die, or worn screw surfaces. That material sees heat for far too long, degrades and crosslinks, then breaks loose from time to time, so the gel count rises with running time. The main fixes are planned purging and strip-down cleaning, keeping melt from sitting in a hot machine during stoppages, and an antioxidant package that is adequate for the process temperature.
Will a finer screen pack fix gels?
Partly. A finer screen catches more contamination and larger unmelted particles, but pressure ahead of the screen and melt temperature both rise, the screen blocks sooner, and soft gels may be squeezed through or sheared smaller instead of being caught. A screen is a help, not a root-cause fix. Find out first what type of gel you have and where it comes from.
Gels went up after we added recycled pellets. Do we have to stop using them?
Not necessarily. Cut a gel open first to see whether it is contamination, degraded material or a different polymer, then ask the recycled-pellet supplier how they filter the melt and check each lot. Common measures are keeping the blend ratio constant, topping up antioxidant for material that has been through several heat histories, and in multilayer film putting the recycled content in a core layer rather than a skin layer.
How do gels differ from antiblock particles?
Antiblock particles are designed to be small and spread evenly through the film, so they add slight haze but do not stand out as spots. Gels are larger, unevenly spread, usually lens-shaped and trail a tail in the machine direction. If antiblock agglomerates because the masterbatch is poorly dispersed, it can show as specks too. Cut one open: if the core is white mineral powder, check the dispersion of the antiblock masterbatch.
Can't find the source of the gels?
Send photos of the film against the light, a gel you have cut open, and the resin and masterbatch you use on LINE. Our technical team will help you separate raw material, formulation and machine causes.