Gels and Fish Eyes in Blown Film: Causes and How to Trace Them

A gel is any small inclusion in a film that has different optical or mechanical properties from the material around it. In a 25 micron film it is glaringly visible, in a 3 mm moulding it would be invisible, and that is why film converters spend time on a problem injection moulders never think about.

Gels are not one defect. They are at least four different defects that look similar and have completely different causes. Identifying which one you have is most of the work.

The four types

### 1. Crosslinked gels

Polymer that has been thermally degraded to the point of crosslinking. The chains have bonded to each other into a network that no longer melts.

How to identify: hard, will not melt on a hot stage, often has a slight yellow or brown tint, frequently irregular in shape.

Where it comes from: material sitting in a dead spot in the extruder or die and cooking. Look at flow path transitions, the area behind a worn non-return valve, breaker plate edges, die lips and any place where a seal or a diameter change creates stagnation.

The tell that confirms it: the rate rises steadily over a production run and drops sharply after a purge or a die clean. Crosslinked gel accumulates until it sloughs off.

### 2. Unmelts

Polymer that never melted. Chemically identical to the surrounding material, simply not processed.

How to identify: melts cleanly on a hot stage and disappears – the single most diagnostic test available. Usually roughly spherical, no colour.

Where it comes from: insufficient melting – screw profile, barrel temperature, throughput too high for the screw, or a very high molecular weight fraction in the resin that needs more energy than it is getting.

Also from masterbatch, and this is the legitimate case: a carrier with much lower MFI than the host resin may not fully melt and disperse in the residence time available. It survives as a discrete domain, which in film is a fish eye. This is exactly why the carrier MFI belongs on the specification – see melt flow index explained.

### 3. Contamination

Foreign material – a different polymer, dust, paper fibre, metal, insect.

How to identify: does not melt, often has a distinctive colour or shape, may be visible as an inclusion with a clear boundary. FTIR analysis will name it in an hour if you have access to a lab.

Where it comes from: contaminated regrind, poor housekeeping around the hopper, damaged bags, and the environment. A cross-polymer contamination – PP in PE – is one of the most common and one of the most damaging, since PP will not melt at PE processing temperature.

### 4. Additive and pigment agglomerates

Undispersed additive or pigment.

How to identify: coloured, usually, and concentrated. A carbon black agglomerate is an obvious dark speck; an anti-block or filler agglomerate is white or translucent.

Where it comes from: inadequate dispersion during compounding, or a masterbatch overloaded beyond what the carrier can properly wet.

The identification procedure

This takes an afternoon and settles arguments that otherwise run for weeks.

1. Cut out the gel with a margin of surrounding film.

2. Look at it under magnification. Shape, colour, boundary and whether there is a visible core.

3. Hot stage test. Heat it above the polymer’s melting point. If it melts and disappears, it is an unmelt. If it stays, it is crosslinked or contamination.

4. Note the position across the web. Gels concentrated in one lane point at a specific die location or a specific extruder in a coextrusion. Gels spread evenly point at the material.

5. Track the rate over time. Rising through a run and dropping after a purge means accumulation in the flow path. Constant means it is arriving with the feed.

6. Run the resin alone, with no masterbatch, no regrind, no additive. This single step eliminates most of the possibility space in one production trial, and it is the step people skip.

The order that matters: run resin alone, then resin plus masterbatch, then production formulation. Two extra trial runs cost far less than three weeks of correspondence between a converter and a supplier who each believe it is the other’s problem.

Where masterbatch is and is not implicated

Being precise about this is useful to both sides.

Masterbatch is a genuine cause when: – The carrier MFI is far below the host resin and does not fully melt – The masterbatch is loaded beyond what the carrier can wet, so pigment agglomerates survive – Dispersion during compounding was inadequate – The masterbatch itself contains degraded material from the compounder’s own dead spots – The carrier is a different polymer family that does not melt at your processing temperature

Masterbatch is not the cause when: – Gels are present at the same rate running natural resin – Gels are crosslinked and accumulate over a run – that is your flow path – Gels are concentrated in one die lane – Gels are contamination with a clear foreign identity

The half-dosage test is quick and informative: run the same masterbatch lot at half the loading. If gel count roughly halves, the masterbatch is contributing. If it does not change, it is not.

Prevention

Screen changers and filtration. A melt filter removes gels above its rating. Finer screens catch more and raise head pressure, so there is a limit. Continuous screen changers avoid the pressure spikes that dislodge accumulated material – which is why a manual screen change is often followed by a burst of gels.

Purging discipline. Regular purging with a proper purging compound removes accumulated degraded material before it sloughs. On a line running dark colours followed by natural film, this is not optional.

Screw and die inspection. Wear creates the clearances where material stagnates. A worn non-return valve or a scored die lip is a permanent gel generator and no process setting compensates.

Temperature profile. Excess temperature accelerates degradation; too little leaves unmelts. Both ends of the range produce gels by opposite mechanisms.

Regrind control. Regrind is thermally aged and carries whatever contamination the process introduced. It is a frequent gel source and the percentage should be a controlled variable, not whatever is available.

Storage and handling. Resin and masterbatch stored in damaged bags, in dusty conditions, or open to the atmosphere pick up contamination that arrives as gels.

Why this matters commercially

Gels are not only cosmetic.

They are stress concentrations. In a stretched film a hard inclusion is where a tear starts. Gel count correlates with unexpected web breaks, and web breaks are lost production.

They fail printing and lamination. A gel disrupts ink lay-down and creates a void in a laminate bond.

They fail specification. Many film specifications set a maximum gel count per unit area, measured by an inline gel counter. Failing that specification is a rejection regardless of whether the film performs.

They cost most on thin gauge. As film gets thinner, a gel of a given size occupies a greater fraction of the wall. Downgauging for cost – a constant pressure in Indian packaging – makes an acceptable gel level unacceptable without anything having changed.

Talk to us

We supply colour, white, black and additive masterbatches for blown and cast film, matched to the host resin’s MFI and dispersion-tested, under ISO 9001:2015.

If you are chasing gels and want to establish whether the colour is contributing, send us your resin grade, your masterbatch lot and your film gauge – and run the natural-resin trial before you call anyone, because it will tell you more than any of us can.

Related: masterbatch for blown film, packaging, black specks in plastic parts, film blocking on the reel.

FAQs

What causes gels in blown film? Four distinct things – crosslinked degraded polymer from dead spots in the flow path, unmelted polymer that never fully melted, foreign contamination, and undispersed pigment or additive agglomerates. They look similar and have completely different remedies.

How do I tell a gel from an unmelt? Heat it on a hot stage above the polymer melting point. An unmelt melts and disappears because it is chemically identical to the surrounding material. A crosslinked gel or a contaminant does not melt.

Why do gels increase during a production run? Because degraded material is accumulating in a dead spot in the extruder or die and periodically sloughing off. The confirming sign is that the rate drops sharply after a purge or a die clean, then climbs again.

Can masterbatch cause fish eyes? Yes, in specific ways – a carrier with much lower melt flow index than the host resin may not fully melt, loading beyond what the carrier can wet leaves pigment agglomerates, and inadequate dispersion during compounding survives into the film. Test by running the same lot at half dosage.

What is the fastest way to find out if the masterbatch is the problem? Run the resin alone with no masterbatch, no regrind and no other additive. If gels persist at the same rate, the masterbatch is not the cause. Then add back one component at a time.

Do gels concentrated in one part of the web mean anything? Yes. Gels in one lane point at a specific die location, a worn die lip, or one extruder in a coextrusion. Gels spread evenly across the web point at the material or the general flow path instead.

Do melt filters remove gels? They remove gels above the screen rating. Finer screens catch more but raise head pressure, so there is a practical limit. Note that a manual screen change is often followed by a burst of gels, because the pressure change dislodges accumulated material.

Why are gels worse on thinner film? Because a gel of a given size occupies a larger fraction of the wall thickness. Downgauging can turn a previously acceptable gel level into a rejection without anything in the process or material having changed.