Plastic film classification and film blowing process

In daily life, you will find that plastic films are different, such as plastic bags, some are transparent and soft, and some are stiff; plastic wrap is different from plastic bags. In fact, this is because there are so many kinds of plastic films, and the uses of various films are different.

There is no uniform provision for the classification of plastic films. The usual classification methods are:

( 1 ) According to the classification of raw materials for plastic film forming : polyethylene film, polypropylene film, polyvinyl chloride film, polyester film, etc.

( 2 ) Classification according to the use of plastic films: there are agricultural films (here, they can be divided into mulch films and greenhouse films according to the specific uses of agricultural films); packaging films (packaging films can be divided into food packaging films and various industries according to their specific uses Packaging films for products, etc.) and breathable films, water-soluble films and piezoelectric films for special environments and special purposes.

( 3 ) According to the classification of plastic film forming methods: there are films that are extruded and then blow-molded, called blown films; films that are extruded and plasticized, and then the molten material is cast from the die orifice, called Cast film; there are several rollers on the calender that roll the film made of plasticized materials, called calendered film.

There are many plastic film forming methods, among which the three processes of casting, extrusion and blow molding are more common.

The casting method is a non-stretched, non-oriented flat-extruded film produced by melt-salting and quenching. There is no obvious orientation structure.

Common extrusion methods include unidirectional stretching and biaxial stretching, and biaxial stretching is a production method that has been paid close attention in recent years. The basic principle of the bidirectional stretching technology is that the polymer raw material is heated and melt extruded into a thick sheet through an extruder, and then in a suitable temperature range above the glass transition temperature and below the melting point, through the stretching machine to carry out a certain lengthwise and transverse direction The multiple stretching makes the molecular chains oriented and arranged in a plane parallel to the film, and then heat-set under tension to fix the oriented macromolecular structure, and finally formed a film by cooling and subsequent processing.

The blow molding method is usually manufactured by extrusion blow molding, the material is plasticized and extruded to form a tube blank to be inflated, and then cooled, pulled, and coiled. The blown film is oriented.

The different forming methods of the above three processes cause the film's conductivity to be significantly different; at the same time, compared with the unstretched film, the barrier performance of the stretched film is improved, the permeability to gas is reduced, and the optical performance, transparency and surface gloss are improved. Good dimensional temperature and uniform thickness performance.