I. Production of polythene heat-shrinkable film
The production of polyethylene heat-shrinkable film for packaging can be carried out either by the single-bubble method (direct blow-moulding, with only one blister) or by the double-bubble method (firstly making a blank film, which is then heated and blown up a second time to produce two blisters). The single-bubble method is the easiest and cheapest to produce, and it can be produced using the same equipment used to produce ordinary polyethylene blown film, so industrial polyethylene heat-shrinkable film for packaging is, without exception, produced using the blow moulding (single-bubble) method.
2. Production process and the influence of process parameters
(1) The production process of polyethylene heat-shrinkable film is basically the same as that of ordinary polyethylene blow-moulding film, which can be expressed as follows.
PE resin
Extrusion plasticisation
Blow and stretch moulding
Cooling
Drawing
Winding
Polyethylene heat shrink
Film rolls
(2) Influence of the production process on the shrinkage properties of polythene heat shrinkable film
To produce polyethylene heat shrinkable film with high heat shrinkage performance, in addition to the selection of polyethylene raw materials, the conditions of the production process are also very important factors. In particular, the tensile multiplier (blowing ratio and drafting ratio), the bubble tube cold curing line of the high and low, the heat shrinkage performance of the product has a very obvious impact.
① Relationship between shrinkage and stretch ratio
The forced elastic deformation within the film originates from the molecular orientation within the film during the production process, which is necessary for polyethylene heat-shrinkable film to obtain heat-shrinkable properties, so the longitudinal shrinkage of the film increases when the draw ratio is increased; the larger the blowing ratio is, the transverse shrinkage increases.
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