Blown film lines

A blown film line extrudes molten plastic through an annular die, inflates the tube into a bubble, cools it with air and winds the flattened film. Choose a mono-layer line for simple HDPE or LDPE bag film, an ABA line to put filler or recycled resin in the core, and five or more layers for barrier packaging.

How the process works

Pellets and additives are fed to an extruder, where a heated barrel and screw melt and mix them. The melt passes a screen changer and flows into a die with an annular opening. Dies come in several designs (spider, crosshead and spiral); Rajoo's Heptafoil line, for instance, uses a bottom-fed cylindrical spiral die. Air introduced through the die inflates the tube, and the tube is pulled away faster than it leaves the die, so the film thins in both directions at once.

An air ring at the die lip blows cooling air onto the outside of the bubble. The point where the melt solidifies shows as a hazier band called the frost line. Above it, a collapsing frame flattens the bubble into lay-flat tube, nip rolls pull it at a controlled speed, and a winder builds the rolls. Corona treaters, gusseting boards, slitters and automatic reel changers sit in between as the product requires.

Main line types

Mono-layer lines

One extruder feeds one layer. The Chyi Yang HPE-55FS is a typical bag film line: a 55 mm screw, 50 to 105 kg/h depending on film width, thickness, die size and resin, 1,000 mm take-up rollers, a double-lip air ring and an automatic reel-change winder. An optional die lets it switch from HDPE to LDPE and LLDPE.

Water-quenched PP lines

The Chyi Yang PP-55H extrudes the bubble downward into a water quench ring. Chyi Yang says the fast cooling gives the clarity, gloss and stiffness wanted for bread, garment and food bags, and a 360-degree rotating die spreads thickness variation across the roll so it winds flat. A dryer chamber removes water before winding.

ABA three-layer lines

Two extruders of different sizes make an A-B-A film with identical skins and a separate core. On the Polystar Eco-X, the core can carry more calcium carbonate filler or in-house recycled pellets while the skins carry colour and surface quality. Polystar pairs 45 and 55 mm screws for T-shirt bag film and 55 and 65 mm screws for garbage bag film, and points out that ABA needs one extruder fewer than an ABC line.

Multilayer barrier lines

Food film that must keep oxygen or moisture out combines polyolefins with barrier resins such as EVOH or nylon, joined by tie layers that bond to both, as engineers from Macro Engineering describe in Plastics Technology. Lines such as the W&H VAREX II are configured to order with 1 to 11 layers, dies from 160 to 900 mm and line widths from 1,300 to 3,600 mm. W&H cites an 11-layer barrier film with a 1 micron EVOH layer as one example.

How to choose: key specs explained

• Output (kg/h). Makers state output as a range because it depends on film width, gauge, die size and resin. Compare lines at your film, not at their maximum.
• Screw diameter and L/D. Larger screws give more output. Polystar quotes L/D ratios of 28:1 or 30:1; Rajoo uses grooved feed sections with fluid temperature control.
• Die diameter and blow-up ratio. Blow-up ratio (BUR) is bubble diameter divided by die diameter, and it sets the balance of strength along and across the film. Lingfeng, a film machine builder, gives about 2.0 to 3.5 for general LDPE and LLDPE packaging film. Lay-flat width is about 1.57 times the bubble diameter, so die size and BUR together fix the widths you can make.
• Bubble cooling. Cooling often decides how fast a line can run. Single-lip air rings suit stable, long-stalk HDPE, which runs a frost line about 8 to 10 times the die diameter high; dual-lip rings suit less stable resins. Internal bubble cooling (IBC) circulates air inside the bubble. Plastics Technology reported add-on cooling elements giving 10 to 15% more output than a dual-flow air ring alone.
• Thickness range and gauge control. The Eco-X is listed for 12 to 100 micron. Rotary dies, IBC and automatic thickness control with capacitive or gamma backscatter sensors (listed by Rajoo) tighten gauge.
• Winder. Surface, centre and gap winding and automatic reel change affect roll quality and labour.
• Resin flexibility. Check that the line is rated for the recycled content, filler and bio-based materials you plan to run.

What your plant needs

• Power. Chyi Yang lists 32 kW for the HPE-55FS and 34 kW for the PP-55H; multilayer lines need far more. Plan for 600 V supply and transformers; see our 600 V power guide.
• Building height. Even the compact HPE-55FS stands 5.2 m (about 17 ft) tall. Larger dies and high-stalk HDPE need taller towers, so confirm clear height, crane access and floor loading early; our floor preparation guide helps.
• Cooling. Air ring blowers, IBC where fitted, extruder feed zone cooling and, for water-quenched PP, a water supply with drying before the winder.
• Resin handling. Dry storage for bags, gaylords or silos, vacuum loaders and dosing. Polystar and Rajoo list automatic loaders, masterbatch dosing and gravimetric blending.
• Downstream. Most film plants feed their own bag making machines or sell rolls to converters.

Running costs

Resin is bought by the tonne, so every film decision (gauge, filler level, recycled content, layer split) shows up directly in cost per kilogram of good film. This is the reason for ABA structures: cheaper material goes where it matters least. Other drivers:

• Energy for barrel heaters, extruder drives, blowers and IBC.
• Start-up and changeover scrap, and edge trim, reduced by in-line regrind.
• Screens, die cleaning and screw and barrel wear, which filled compounds accelerate.
• Quality losses from gels, contamination and gauge bands, which the Plastics Technology article links to dies, air-ring cooling and temperature control.
• Labour for reel changes, reduced by automatic winders.

Typical applications

Blown film feeds T-shirt, garbage and produce bag production, bread and garment bags, liners, stretch and sticky film, and barrier packaging for products such as edible oil, UHT milk, meat and frozen food, the uses Rajoo lists for its seven-layer lines. Canada's federal single-use plastics regulations, in force as of 2026, restrict plastic film checkout bags but not garbage, compost, recycling or bulk food bags; the single-use plastics guide explains what is covered. More on film and bag plants is on our plastic film and bags page.