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 most cost-effective, and it can be produced using the same equipment used for the production of ordinary blown polyethylene film, so industrial polyethylene heat-shrinkable film for packaging is, without exception, produced by the blow-moulding (single-bubble) method.
Production of polyethylene heat shrinkable film, the main should grasp the selection of PE resin, film stretching process control, temperature and cooling control and so on several links.
1. Selection of raw materials
Lower density: PE resin has a high density, low density two categories. Higher density PE molecular branch chain is less, the molecular regularity is better, there is a better crystallinity, this kind of PE film made of high mechanical strength, but poor transparency, especially from the heat-shrinkable film made of it, constant temperature shrinkage curve of the contraction, high temperature insensitive area is narrow, shrinkage performance is not good, so the manufacture of polyethylene heat-shrinkable film, you should choose the lower density of polyethylene (generally choose the density in the 0.920~0.925g/cm3 polyethylene).
When the melt flow rate is low, the molecular weight is high, and there are many entanglement points between molecules. When used for the production of heat-shrinkable film, it is easy to form and retain the forced elastic deformation in the film, which results in a larger shrinkage rate. Therefore, the manufacture of polyethylene heat shrinkable film, choose a lower melt flow rate of polyethylene is good, heat shrinkable film used in polyethylene melt flow rate should not be greater than 5g/10min, preferably below 1.0g/10min.
The shrinkage performance of the film is related to the molecular weight distribution of the resin. Generally speaking, the molecular weight distribution is wide, heat shrinkage performance is better, so if the conditions, the production of heat shrinkable film is best to use bimodal polyethylene resin.
2. Influence of production process and 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 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 a necessary condition for polyethylene heat-shrinkable film to obtain heat-shrinkable properties, so the longitudinal shrinkage of the film increases when the draw ratio is increased; and the larger the blowing ratio is, the transverse shrinkage increases.
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