Blocking is film sticking to itself. A bag will not open, a tube will not separate, or a roll tears when it is unwound. Smooth, soft film pressed together while it is warm will block, so look first at how hot and how tight the film is wound. Then look at the antiblock and slip in the film.
Also described as: film sticking to itself, bags that will not open, rolls that will not unwind.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| The tube will not open straight off the line, or the two sides are stuck together at the winder | The film is still too warm when it reaches the nip rolls, and the nip presses the two inside faces together. | Film temperature at the nip; frost line height; nip pressure |
| Rolls unwind well when made and are blocked days later, worst near the core | The roll was wound too tight or too warm. Pressure builds toward the core, and a warm roll holds its heat for a long time. | Winding tension and taper; film temperature at the winder; roll hardness |
| Blocking in summer, or in rolls that sat in a hot trailer or near a heater | Storage or shipping temperature. Warm film is softer, and additives and low molecular weight material move to the surface faster. | Where the rolls were stored and for how long |
| Fresh film is tacky and drags, and the same film is better a day or two later | The slip has not reached the surface yet. Migrating slip takes time to bloom, and takes longer when the film is cold. | Test a retained sample at a fixed age before you change anything |
| Blocking is worse on corona-treated film, or began when the treater power went up | Treatment makes the surface more active, so it sticks to the next layer more readily. Treatment that reaches the reverse side makes it worse. | Treatment level against what the printer needs; a dyne test on the side that should be untreated |
| Blocking rose after a change to a lower density resin, more metallocene LLDPE or more EVA | Softer resin gives a tackier surface with more contact between layers. The additive level that worked before is no longer enough. | Density on the COA; vinyl acetate content of the EVA; antiblock and slip level in the new blend |
| Blocking started with a new lot or a new concentrate, with no process change | The additive level changed. A grade without slip and antiblock was supplied, the concentrate is a different strength, or the feeder is off. | Grade name and additive package on the COA; concentrate let-down and feeder calibration |
Checks in order, cheapest first
- Find where and when the film blocks. Check at the nip, at the winder and after storage, and note whether it is the inside faces of the tube or the wraps on the roll that stick. Film that blocks at the line is a cooling problem. Film that blocks later is a winding, storage or additive problem.
- Measure the film temperature at the nip and at the winder. Cooler film blocks less. More cooling air, a lower frost line or a lower output all bring it down, and on a cast line so does a colder chill roll. A roll wound warm stays warm inside for a long time.
- Reduce nip pressure and winding tension. Use only the nip pressure needed to hold the air in the bubble. Wind with taper tension so the pressure does not build toward the core. Go in steps, because a roll wound too soft will telescope.
- Check storage and shipping. Let rolls cool before they are wrapped and stacked. Keep them away from heaters, sunlit walls and the roof space, and ask what the trailer temperature was if the complaint came from a customer.
- Check the additive feed. Calibrate the concentrate feeder and confirm the let-down against the recipe. Confirm the grade in the hopper has the slip and antiblock package you expect, because many film grades are sold both with and without it.
- Allow for slip bloom. Test friction and blocking at the same film age every time. Film tested minutes after it is made and film tested the next day are not comparable.
- Check the corona treater. Treat to the level the ink or adhesive needs and no higher. Check that the film lies flat on the treater roll, because air trapped under it lets the reverse side be treated as well.
- Compare lots, then change the resin or the additive package. If the line, the winding and the feed are right and a retained good lot runs clean while the current one blocks, look at the material. The options and their costs are below.
The numbers
Density is the figure to watch. The lower the density, the softer the film and the more readily it blocks. In EVA, vinyl acetate content does the same job: more of it means a softer, tackier film.
| Resin | Melt index (g/10 min) | Density (g/cm³) | Tendency to block |
|---|---|---|---|
| LLDPE, blown film grades | ~0.5–2 | ~0.916–0.926 | Moderate to high. Rises as density falls, and metallocene grades usually need more antiblock. |
| LDPE film grades | ~0.2–4 (general-purpose film ~1–2; coating grades higher) | ~0.918–0.930 | Moderate. Clear, glossy film has smooth surfaces that sit close together. |
| 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 | Low. The film is stiffer and has a rougher, hazier surface. |
| EVA, film grades | See the grade sheet | Not listed. Sold by vinyl acetate content: ~2–9% VA for film, ~9–20% VA for heat-seal layers | High, and higher as vinyl acetate content rises. |
Typical published ranges from our LLDPE, LDPE film, EVA 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. The last column is a general direction, not a measured value. This site publishes no antiblock or slip loadings, because the right level depends on the resin, the gauge and the additive. Take it from the concentrate supplier’s data sheet.
Why film blocks
- Smooth surfaces in close contact. Two smooth, clean polyethylene surfaces pressed together push out the air between them and hold. The clearer and glossier the film, the smoother it is, and the more it blocks.
- Soft resin. Lower density resins and EVA are softer. Under the pressure in a roll the surfaces press into each other, and the low molecular weight fraction that these resins carry is tacky.
- Heat, pressure and time. All three raise blocking. A roll wound warm and tight and stored in a hot building has all three at once.
- What antiblock does. Antiblock is a fine mineral, such as silica or talc, that stands slightly proud of the surface. It keeps a little air between the layers so they cannot sit flat against each other.
- What slip does. Slip is usually a fatty amide that migrates to the surface after the film is made and lowers friction. It helps layers slide apart, but antiblock is the main tool against blocking. The two are normally used together.
- What corona treatment does. Treatment oxidizes the surface so that ink and adhesive will wet it. The same change makes the surface stick more readily to the next wrap.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| More antiblock | More particles at the surface keep the layers apart. | Clarity. Haze goes up and gloss goes down. Poorly dispersed antiblock also shows as specks or gels. |
| More slip, or a faster-blooming slip | Lower friction lets the layers slide apart. | Slip at the surface can interfere with ink adhesion, lamination and heat sealing, and it can make rolls telescope. The level at the surface keeps changing as the film ages. |
| A higher density resin in the blend or in the outer layers | A harder surface presses together less under roll pressure. | Stiffer film, lower dart impact and tear, less clarity, and a higher sealing temperature. |
| Less EVA, or EVA with a lower vinyl acetate content | A less tacky surface. | You give back some of the softness, clarity, cling and low-temperature sealing the EVA was there for. |
| A grade with slip and antiblock built in, in place of a concentrate added at the line | The additive level is set by the resin maker and does not depend on a feeder. | A new grade to approve, and less freedom to adjust. One grade may not suit both a job that needs slip and a job that must print or laminate. |
Any blend change moves film properties. Antiblock costs clarity, and slip can cost you printing and sealing. Run the new film through the customer’s printing, sealing and filling steps before you approve a change.
Often confused with
- High friction. Film that separates but drags on the bagging or wrapping machine has too little slip at the surface. Blocked film will not separate at all without force.
- Static cling. Layers that jump back together, pick up dust and let go when the charge is removed are held by static. See static and ESD.
- A welded edge. Bags stuck only along the cut or sealed edge were welded there by the knife or the seal bar. The rest of the bag opens freely. See heat seal failures.
- Ink set-off. Printed film that sticks only where the ink is, and leaves ink on the next wrap, was wound before the ink was dry. That is a press problem, not a resin one.
Quick answers
Why is my film sticking together?
Film sticks to itself (blocks) when smooth, soft film is pressed together while it is warm. Look first at how hot and how tight the film is wound, then at the antiblock and slip in the film. Film that blocks at the line is a cooling problem. Film that blocks later is a winding, storage or additive problem.
How do I stop film blocking?
Start by cooling the film and winding it with less pressure. More cooling air, a lower frost line or a lower output bring film temperature down. Use only the nip pressure needed to hold the air in the bubble, and wind with taper tension. Then check the concentrate feeder and the slip and antiblock package in the grade.
Why will my bags not open?
Bags that will not open are most often blocked: smooth, soft film was pressed together while it was warm, and the faces now hold. If the bags are stuck only along the cut or sealed edge, they were welded there by the knife or the seal bar, and the rest of the bag opens freely.
Is film blocking caused by the resin or the process?
Check the process first: film temperature at the nip and winder, nip pressure, winding tension and storage heat. The resin matters when the blend got softer, such as a lower density resin, more metallocene LLDPE or more EVA, or when a new lot or concentrate changed the slip and antiblock level.
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