December 28 2017 0Comment

Blown film & Sheet/film extrusion

Blown film extrusion:The manufacture of plastic film for products such as shopping bags and continuous sheeting is achieved using a blown film line.

This process is the same as a regular extrusion process up until the die. There are three main types of dies used in this process: annular , spider, and spiral. Annular dies are the simplest, and rely on the polymer melt channeling around the entire cross section of the die before exiting the die; this can result in uneven flow. Spider dies consist of a central mandrel attached to the outer die ring via a number of “legs”; while flow is more symmetrical than in annular dies, a number of weld lines are produced which weaken the film. Spiral dies remove the issue of weld lines and asymmetrical flow, but are by far the most complex.

The melt is cooled somewhat before leaving the die to yield a weak semi-solid tube. This tube’s diameter is rapidly expanded via air pressure, and the tube is drawn upwards with rollers, stretching the plastic in both the transverse and draw directions. The drawing and blowing cause the film to be thinner than the extruded tube, and also preferentially aligns the polymer molecular chains in the direction that sees the most plastic strain. If the film is drawn more than it is blown the polymer molecules will be highly aligned with the draw direction, making a film that is strong in that direction, but weak in the transverse direction. A film that has significantly larger diameter than the extruded diameter will have more strength in the transverse direction, but less in the draw direction.

In the case of polyethylene and other semi-crystalline polymers, as the film cools it crystallizes at what is known as the frost line. As the film continues to cool, it is drawn through several sets of nip rollers to flatten it into lay-flat tubing, which can then be spooled or cut.

Sheet/film extrusion:Sheet/film extrusion is used to extrude plastic sheets or films that are too thick to be blown. There are two types of dies used: T-shaped and coat hanger. The purpose of these dies is to reorient and guide the flow of polymer melt from a single round output from the extruder to a thin, flat planar flow. In both die types ensure constant, uniform flow across the entire cross sectional area of the die. Cooling is typically by pulling through a set of cooling rolls. In sheet extrusion, these rolls not only deliver the necessary cooling but also determine sheet thickness and surface texture. Often co-extrusion is used to apply one or more layers on top of a base material to obtain specific properties such as UV-absorption, texture, oxygen permeation resistance, or energy reflection.

A common post-extrusion process for plastic sheet stock is thermoforming, where the sheet is heated until soft (plastic), and formed via a mold into a new shape. When vacuum is used, this is often described as vacuum forming. Orientation is highly important and greatly affects forming cycle times for most plastics.


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