A stable bubble holds its diameter, its frost line and its position over the die. When it breathes, sways, snakes or breaks, gauge and layflat width go with it. Most instability comes from cooling air, melt temperature or output pushed past what the melt strength of the blend will carry. Check the air and the melt temperature first, because those checks cost nothing.
Also described as: bubble breathing, bubble dancing, snaking bubble, frost line moving, bubble breaks.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| The bubble diameter and layflat width pulse in a slow, regular cycle, and the frost line hunts up and down (breathing) | Cooling and output are out of balance. The melt is too hot or the cooling air is too little for the rate, so the frost line cannot settle. | Measured melt temperature; air ring flow and cooling air temperature; frost line height against your normal setting |
| The bubble and frost line move in step with the extruder pressure gauge or motor load | The extruder is surging, so output itself is rising and falling. The bubble is only following it. | Head pressure and motor amps over a few minutes. See extruder surging. |
| The bubble moves in a spiral around the die axis, or snakes from side to side (helical instability) | The air around the bubble is uneven or too strong for the melt. One side is pulled toward the air ring, and the pull travels around the bubble. | Air ring lips for dirt and damage; air ring centered and level on the die; air flow setting |
| Gauge varies along the machine direction in a regular cycle, mostly on thin film at high line speed (draw resonance) | The melt is being drawn down harder and faster than it can take. More common in blends rich in LLDPE, which have less melt strength, and in cast film. | Line speed; die gap against finished gauge; frost line height |
| The bubble flutters or sways with no regular cycle | Drafts across the bubble, or a bubble cage or collapsing frame that is not guiding it evenly. | Open doors, fans and heaters near the tower; cage and collapsing frame settings |
| The bubble tears off or a hole opens and the bubble is lost | A gel, an unmelted particle or contamination passed through the die and made a weak spot. Or the melt is too cold to draw, or too hot to hold its shape. | The edge of the break for a speck or lump; the screen pack; melt temperature. See gels and fisheyes. |
| The trouble started with a new lot or a change in the blend | Melt strength went down. The melt index moved up between lots, the LDPE share went down, or the regrind share changed. | COA of the good lot against the bad lot; feeder rates at the blender |
Checks in order, cheapest first
- Watch the bubble and name the motion. Mark the normal frost line height on the tower. Time the cycle, and watch the head pressure and motor load while you do. Breathing, a spiral, flutter and a gauge cycle have different causes, and a bubble that follows the pressure gauge is an extruder problem.
- Measure the melt temperature. Hotter melt has less melt strength and takes longer to freeze, so the frost line climbs and the bubble softens. Bring the melt down in small steps toward the lower end of the supplier’s range. Stop if head pressure climbs too far or the film surface turns rough.
- Clean and balance the cooling air. Clean the air ring lips, check that the ring is centered and level on the die, and check the blower filter and every air hose. Then change the air flow in small steps. Too little air lets the frost line climb. Too much can pull the bubble off center and start a spiral or a flutter.
- Remove drafts and set the bubble guides. Close doors and move fans that blow across the tower. Set the cage so it steadies the bubble without squeezing it, and check that the collapsing frame is square to the nip.
- Reduce output or line speed as a test. If the bubble settles at a lower rate, the line is past the cooling it has for this blend and gauge. You then choose between a lower rate, more cooling, or a blend with more melt strength.
- Review the blow-up ratio and the die gap. A large bubble from a small die is harder to hold steady. A wide die gap drawn down hard to a thin gauge is where draw resonance starts. Changing either usually means a different die or die gap, so make the free checks first.
- Check the blend at the blender, then compare lots. Confirm each feeder delivers what the recipe says, including regrind, which feeds less evenly than pellets. If the trouble came with a new lot, compare melt index and density on the COAs and run a retained good lot on the same line.
- Change the resin last. Bubble instability is usually a cooling, temperature or rate problem and not a resin fault. The resin sets how far the line can be pushed. If the line is right and the bubble is still unstable at the rate you need, look at the blend. The options and their costs are below.
The numbers
Melt strength is not on a COA. Melt index and density are, and they are the figures to compare between lots and between blend components. Within one resin type, a lower melt index means longer chains and more melt strength. At the same melt index, LDPE has more melt strength than LLDPE.
| Resin | Melt index (g/10 min) | Density (g/cm³) | On the bubble |
|---|---|---|---|
| LLDPE, blown film grades | ~0.5–2 | ~0.916–0.926 | Lower melt strength. Tough, but the bubble is less forgiving. Often blended with LDPE. |
| LDPE film grades | ~0.2–4 (general-purpose film ~1–2; coating grades higher) | ~0.918–0.930 | Long-chain branching gives the melt strength that keeps a bubble stable. Low melt index grades have the most. |
| 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 | High melt strength. Runs with a high stalk, on a line built for it. |
| 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. Grades vary, so confirm against the data sheet and the COA for your lot. This site publishes no blow-up ratios, frost line heights or film melt temperatures, because they depend on the die, the air ring and the grade. Set them from the data sheet and your own line records. See melt flow index explained.
Why a bubble becomes unstable
- Melt strength. Between the die and the frost line the film is molten and is held up only by its own resistance to stretching. A melt with too little of it cannot resist small changes in air or pull, so they grow.
- Cooling. The frost line is where the film freezes and stops stretching. If the cooling air cannot remove heat as fast as the extruder delivers it, the frost line rises, and a longer molten section is easier to disturb.
- LDPE and LLDPE behave differently. LDPE has long-chain branching, which makes the melt stiffen as it is stretched. LLDPE has only short branches, so it draws down easily and has less melt strength. That is why LLDPE is so often blended with LDPE.
- Draw resonance. Above a certain draw-down, the thickness of the molten film starts to cycle by itself even when the extruder output is steady. It shows as a regular gauge variation along the machine direction.
- HMW-HDPE is a separate case. It runs with a long stalk above the die before the bubble blows out. Stalk height and the air setting have to stay matched, and the supplier’s guide for the grade is the place to start.
Resin changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| Add LDPE to an LLDPE-rich blend, or raise its share | Long-chain branching adds melt strength and steadies the bubble. | Lower dart impact, tear and puncture resistance. Too much LDPE weakens the film, and the blend does not draw down as thin. |
| A lower melt index grade of the same resin | Longer chains give more melt strength. | Higher melt pressure and motor load, usually lower output, and a greater risk of melt fracture. |
| Less metallocene LLDPE, or an easier-processing LLDPE in its place | Metallocene grades are harder to process and add little melt strength. | You give back some of the dart impact, puncture resistance, clarity and sealing the metallocene grade was bought for. |
| Less regrind or recycled content, or a more consistent source | Regrind feeds unevenly and its melt index varies, so output and melt strength both wander. | More trim to dispose of or sell, and a lower recycled-content figure if the customer requires one. |
| A melt index range on the specification | Instability that comes and goes by lot often traces to melt index moving between lots. | Nothing in the film. It takes an incoming check, and it may narrow your choice of sources. |
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
- Extruder surging. If head pressure and motor load cycle in step with the bubble, the cause is in the extruder and not at the air ring. See extruder surging.
- Gauge bands from the die. Thick and thin bands that stay at the same position across the web come from the die or the air ring. Instability changes the gauge along the length of the film.
- Melt fracture. A rough, sharkskin surface on a bubble that holds steady is a flow problem at the die lips. See melt fracture and sharkskin.
- Breaks from gels. If every break starts at a lump or speck, the bubble may be stable and the gels are the problem. See gels and fisheyes in film.
Quick answers
Why is my blown film bubble unstable?
Most bubble instability comes from cooling air, melt temperature or output pushed past what the melt strength of the blend will carry. Watch the bubble and name the motion first, because breathing, a spiral, flutter and a gauge cycle have different causes. Check the cooling air and the melt temperature before anything else. It is usually not a resin fault.
How do I stop the bubble from breathing?
Breathing means cooling and output are out of balance: the melt is too hot or the cooling air is too little for the rate. Measure the melt temperature and bring it down in small steps toward the lower end of the supplier’s range. Clean and balance the cooling air, then reduce output or line speed as a test.
What causes bubble breaks in blown film?
A bubble break usually starts at a weak spot made by a gel, an unmelted particle or contamination that passed through the die. The melt can also be too cold to draw, or too hot to hold its shape. Check the edge of the break for a speck or lump, then the screen pack and the melt temperature.
Is bubble instability caused by the resin or the process?
Bubble instability is usually a cooling, temperature or rate problem and not a resin fault. The resin sets how far the line can be pushed. If the trouble started with a new lot or a blend change, melt strength may have gone down. Compare melt index and density on the COAs and confirm each feeder delivers what the recipe says.
Does adding LDPE make a blown film bubble more stable?
Adding LDPE to an LLDPE-rich blend, or raising its share, steadies the bubble because long-chain branching adds melt strength. The trade-off is lower dart impact, tear and puncture resistance. Too much LDPE weakens the film, and the blend does not draw down as thin. Run the new film through the customer’s converting step before approving a swap.
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