The life of a bag begins with small polyethylene pellets that are hard to make out with the naked eye. Delivered as granules, the raw material is melted and shaped in the film extrusion step and finally becomes a thin film wound onto a roll. The most common method in bag production is blown film extrusion, named after the way the melt is inflated like a balloon.
Decisions made at this step affect every later stage of the bag. The film’s thickness, width, flexibility and surface condition are set here; printing, cutting and sealing are built on this foundation.
- Extruder Granules from the hopper are conveyed and melted by a screw rotating inside a heated barrel.
- Annular die and cooling ring The melt leaves a circular gap as a tube; air from the surrounding ring starts to cool it.
- Bubble Air blown inside expands the tube, and the film thins as it travels upwards.
- Nip rollers The cooled bubble is flattened between two rollers and becomes a two-layer tubular film.
- Winder The film is wound into a roll to be sent on to the next steps.
From granule to melt: the extruder
The granules are loaded into the feed hopper at the start of the line. If a colour or a special property is required, masterbatch and additives are also added to the mix at this stage in set proportions. The pellets falling from the hopper enter the extruder barrel.
Inside the extruder is a long rotating screw. As it pushes the pellets forward, the screw compresses and mixes them; the heaters around the barrel and the heat generated by friction soften and melt the polyethylene. Temperatures set zone by zone along the barrel are monitored so that the melt is homogeneous and flows evenly.
Before reaching the die, the melt passes through a screen pack. This filter limits the unmelted particles and foreign matter carried into the film. Small bubbles or spots known as “gels” in the film are often related to the mixing and filtering conditions at this stage.
The annular die and forming the bubble
From the extruder, the melt is directed to the annular die. The annular die forces the melt through a thin circular gap, giving it the shape of a continuous tube. Even distribution around the die matters; even small variations in the gap can leave one side of the film thick and the other thin.
Air is blown into the tube through the centre of the die. The air expands the still-hot tube like a balloon; as the bubble is drawn upwards, its diameter grows and its wall gets thinner. In this way a thick ring of melt becomes a thin, wide film over a short distance.
On lines where more than one extruder feeds the same die, the film consists of overlapping layers. With this method, called co-extrusion, the outer layer can for example be given a structure suited to printing and the inner layer one suited to sealing; using recycled material in the middle layer is also possible with this method.
Cooling and blow-up ratio
The cooling ring just above the die directs a steady stream of air onto the outside of the bubble. On some lines the inside of the bubble is also cooled separately. Beyond a certain point the cooling polyethylene solidifies; this zone can be seen as a slightly hazy line on the bubble and is known in the industry as the frost line.
How far the bubble is expanded is described by the blow-up ratio: the ratio of the bubble diameter to the die diameter. The blow-up ratio determines the stretching of the film in the transverse direction: the higher the ratio, the more the film is stretched transversely. Stretching in the machine direction comes from the ratio of the speed at which the bubble is drawn upwards to the speed at which the melt leaves the die, that is, the draw-down ratio. The balance between the two ratios directly affects the film’s tear behaviour and its strength in both directions.
Film thickness is not set by a single adjustment. The die gap, melt output rate, blow-up ratio and the speed at which the bubble is drawn upwards are considered together. When the haul-off speed is increased, the film thins more in the machine direction; so a change in the thickness target requires several settings to be corrected together.
Flattening, folding and winding
At the top of the line, the bubble passes through collapsing boards and enters between two nip rollers. Flattened here, the bubble becomes a two-layer tubular film. The side folds needed for gusseted bags are also usually formed at this stage, as the bubble closes.
The tubular film can be wound as it is, slit along its edges into two separate single-layer films, or opened along one edge. Which form is chosen depends on what bag the film will later become. Films to be printed undergo corona treatment before winding so that their surface will hold the ink.
Finally, the film is collected on the winder as a roll. Winding tension needs to be balanced; on a roll wound too tight the film can stretch, while on a loosely wound roll the edges can shift. The full roll is labelled and sent to printing or directly to the cutting and sealing step.
Colour, additives and masterbatch
On its own, polyethylene is colourless or translucent. White, black or coloured bags are obtained with masterbatch added to the granules. Masterbatch is a concentrated pigment or additive formed into granules within a carrier polymer; added in small amounts, it colours the whole melt.
Additives are also used for purposes other than colour. Examples include additives that stop the film sticking to itself, ease slip, improve resistance to light or reduce static electricity. Each additive serves a purpose, but it can also affect later steps: some slip additives, for example, can make ink adhesion more difficult in printing.
Starting the line and holding the settings
When a blown film line is started up, the target film is not reached straight away. It takes time for temperatures to settle, for the bubble to become stable and for the thickness to approach the target; the film produced during this period is separated and sent for recovery. A change of raw material or product requires a similar setting-up process.
Once the settings are right, the job is to hold them. Even small differences in ambient temperature, cooling air temperature and the raw material batch can change how the bubble behaves. That is why operators monitor film width, thickness and the appearance of the bubble throughout production.
What to watch at this step
- Thickness distribution: Even thickness across the film width is important both for seal quality and for the roll to wind properly.
- Bubble stability: A bubble that sways or changes diameter causes variations in width and thickness.
- Raw material compatibility: HDPE, LDPE and LLDPE need different temperature and cooling conditions; when the blend ratio changes, the settings are reviewed too.
- Surface cleanliness: Defects such as gels, streaks and bubbles show up in printing and in use.
Film extrusion determines what the later steps will work with. A roll from a well-adjusted line lays the groundwork for even colour in printing, clean edges in cutting and a sound seam in sealing.