THE 4 STAGES OF BAG MAKING
(2) FILM
For us, the first stage is to make polythene film. We take the granules, blend them with colouring agents (known as Masterbatch) and other additives if required. This mixture of cold granules is then conveyed to a film extruding machine, in which the granules are heated until they are molten and again forced through a die - but in this case, of an annular shape (circular) with about a 1mm gap between the inner and outer parts of die, so a molten tube of polyethylene emerges vertically with a wall thickness of 1mm and a diameter the same as that of the die (typically 60-300mm)
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extruding machine
This molten tube has air pumped into it, so that it inflates like an elongated balloon. This stretches the tube outwards, whilst at the same time the tube is being pulled by a pair of nip rollers, which stretch it the other direction. This process is known as biaxial orientation, as it causes the molecular chains to orientate both crossways and lengthways, thus giving it strength.
The tube is cooled by cold air blowing onto it; it emerges beyond the nip rollers, still as a flattened tube, but now of a much larger diameter and with a wall thickness of typically 10-100 micron (Mu = 1/1000th mm). this flattened tube (known as layflat tubing LFT) is then wound on to a cardboard or plastic tube (known as a core).
The finished rolls of film, typically 100kg each and of up to 10,000 metres (10km) in length of tube depending on its thickness, are removed for the next stage. See Printing
raw material | film | printing | final product