Technical Information

Plastic is an extract from oil.
Plastic is manufactured by the polymerization of ethylene molecules to form long chain molecules having carbon atoms linked together with hydrogen atoms attached to each side of the carbon atoms. Low-density polythene has a number of side branches where a hydrogen atom is replaced with a carbon atom to create a new chain.

Because of this side branching the main chains cannot pack as close together and thus the polymer has a density of around .922 grams per cc. High density polythene has very few side branches and thus the main chains can pack closer together giving this polymer a density around .95 grams per cc.

As HDPE has so few branches, it produces a very linear product which when extruded into film is extremely strong when pulled in the machine direction along the length of the film. However, the HDPE film will split easily along the transverse direction of the film. In order to reduce this weakness, the extrusion equipment used is designed to enable some of the long chains to be trapped in a transverse direction (across the web) as the molten polymer freezes in the manufacturing process.

Low density and linear low density films have enough side branches on their main chains to enable these branches to become entwined and give the film additional strength in the transverse direction, negating the need for a very high wine glass shaped bubble. A low density bubble is very smooth and rounded in shape and consequently is a lot more stable than a HDPE bubble.

The extruder barrel is heated using gas or electric heaters clamped to it. The plastic raw material entering the barrel from the hopper is melted by both the heater elements and the shearing action of the screw. At the end of the barrel the molten plastic is pushed through fine stainless steel gauzes, which filter any dust out of the melt, and into a neck which turns the plastic at right angles up into the base of the die.

The die consists of an outer circular steel pipe with a matching inner steel pipe, with a 1mm gap at the exit, between the two pipes.

The plastic exits the die as a circular tube and is drawn upward by a set of nip-rollers situated on a tower, several metres above the die. This forms “a bubble” with the die centre at the bottom, the plastic forming the walls and trapped by the top nip-rollers. Air is forced into the centre of the bubble, through a hole in the die, until the bubble has the desired diameter to give the required lay-flat measurement of the film exiting the top nip-rollers.

The film is drawn down from the top nip-rollers by the bottom wind-up system that rolls the flattened tube onto a cardboard core. At this point knives can be inserted into the edges of the lay-flat tube to form either a centre-fold film or two lay-flat sheets.

Situated just above the die, is a circular air ring, surrounding the base of the bubble, through which a fan blows cooling air to freeze the plastic as quickly as possible in order to stabilize the bubble.

Once extruded, the plastic film may be printed using either flexo-graphic or gravure printing presses. These work on a roll-to-roll basis or can be mounted inline between the extruder and the wind-up station.

In order to ensure that the ink will adhere to the plastic, the film is "treated". This consists of passing the film close to a very high frequency and high voltage electric current, which effectively oxidizes the film - roughening the surface and allowing the ink to settle on the plastic.

All plastic bag making machinery uses a heated sealing bar or heated resistance wire to melt the polythene which when combined with pressure will form a seal

There are a number of different ways to manufacture a plastic bag, some of these are:

Side Weld
This bag is manufactured using centrefold film, which is fed past a sharp sealing bar to the required width of the bag. The film is then stopped and the heated sealing bar pulled down to compress the film between the bar and a Teflon coated roller. The sealing bar is unprotected metal and consequently melts the plastic to such an extent that it parts on either side of the bar forming a seal on both sides. The bar lifts up and the cut off bag is drawn away from the sealing area by a belt conveyor, whilst the required length for the next bag to be formed, once again feeds the film forward.

The strength of the seal is a function of the temperature of the bar, the time the plastic is under the bar and the pressure used to compress the plastic under the bar.

Bottom weld
This uses tubular film and has the sealing bar protected by PTFE coated tape so that the plastic is not in direct contact with the hot steel.

As with the side-welding process the film is fed out under the sealing bar for the required length and then the sealing bar closes either onto a roller or onto a heated bottom-sealing bar. The sealing bar is not as sharp as that used for a side-weld and can often be a flat surface. After the seal has been formed the plastic is cut across the web directly behind the seal either with a guillotine or some form of moving blade. The bag is then either drawn away by a belt conveyor or stacked on a packing table.

As an alternative, the film can be perforated behind the seal and wound on to a cardboard core to form what is known as "bag on the roll".

Double end weld
Vest type carrier bags are made by a combination of the above welding methods. The ends of the plastic tube is welded on both front and back by protected sealing bars and cut off between the two by a sharp, exposed sealing bar. When the bag is fed forward and the sealing process repeated a bag is formed which is welded on both ends, the tubing used is folded into gussets on each side so that when the "u" shaped cutting punch cuts through the bag it leaves two arms which form the handles of the bag.