Downwards extrusion

In addition to the upward blown film there is also the downward extrusion. It allows the tube to be quenched rapidly in a water bath after which it is collapsed as a layflat for passage over nip and idler rollers. 

Cast film is generally produced by downward extrusion of the melt onto chilled chrome rollers, which are highly polished to impart good surface characteristics to the film, as shown in Fig. 7.4. The extrudate contacts the first chill roll tangentially, and then typically travels in an S-pattern around two or more chill rolls. The first chill roll typically operates at a temperature of at least 40°C (104°F), with subsequent rolls operating at successively lower temperatures to cool the film enough that it can be trimmed and wound. An air knife is typically used to pin the plastic against the first chill roll. The film dimensions are controlled primarily by the die dimensions, extrusion rate, and take-off speed. Film produced in this manner is... 

Figure 3.15 Water-cooled blown film bubble utilizing downward extrusion Brampton Engineering, Inc.)...

Figure 14.1 illustrates the general principles of extrusion blow molding. A molten tube of polymer, known as a parison, is extruded vertically downwards from an extrusion die, as shown in Fig. 14.1 a). The two halves of the blow mold surround and then close on the... 

In summary, fixed-bed processes have advantages in ease of scaleup and operation. The reactors operate in a downflow mode, with liquid feed trickling downward over the solid catalyst concurrent with the hydrogen gas. The usual catalyst is cobalt/molybdenum (Co/Mo) or nickel/molybdenum (Ni/Mo) on alumina (A1203) and contain 11-14% molybdenum and 2-3% of the promoter nickel or cobalt. The alumina typically has a pore volume of 0.5 ml/g. The catalyst is formed into pellets by extrusion, in shapes such as cylinders (ca. 2 mm diameter), lobed cylinders, or rings. 

Extrusion coating is the process of applying a thin bonded film of a plastic to a continuous substrate (web) such as paper, board, foil, fabric, as well as a plastic. A high melt temperature is employed with a downwardly directed slit die to produce a low viscosity melt web that adheres to the substrate (Figure 5.15). The plastic must adhere well to the substrate and exhibit good sealing properties. If necessary, the substrate may be given a pretreatment to improve adhesion. 

The parison head, sometimes called the die head or simply the die, is a specialized form of tube extrusion die (Chapter 17). Its function is to deliver a straight parison in the correct diameter, length, wall thickness, and at the correct temperature for BM. Prior to being clamped in the mold, the parison is suspended unsupported in free air. To avoid undue deformation, it is necessary to extrude the parison vertically downwards. 

Discs use large conical-type deflectors usually mounted in the vertical downward position so that powder is deflected in a horizontal position in a 360°radius. An omega (Q) loop conveyor system normally surrounds the powder disc that is usually mounted on an oscillator or reciprocator for vertical up-and-down motion. The powder can be charged by a corona edge or by tribocharging guns. Typical applications for the powder disc are vertical extrusions or wire goods. 

This is a further extension of the extrusion process. A tubular film emerging from a circular die is internally pressurized with air. This extends the film which, together with the temperature and draw off rate, decides the material gauge and degree of orientation. The film may be blown upwards, downwards, or horizontally (Fig. 7.4). 

The basic extrusion process used for metal collapsible tubes is identical with that used to produce rigid aluminium containers except that the slugs used are rings rather than discs and the female die which holds the slug is shaped so that the metal is forced downwards into the die to form the shoulder and nozzle as well as upwards around the plunger (Figure 10.2). 

The third and most successful approach was accomplished on a wheel-type blow molding machine. Unlike the shuttle machine where only gravity acts on a downward extruded parison, a wheel machine draws or pulls the parison from the die. This allows extension in the extrusion direction and the opportunity to thin down the barrier layer and reorient the mica platelets. 

Fully rotating nips with the winder attached are used for HMW-HDPE film because they result in better roll formation for the very low gauge film. If these are used with upward extrusion, it requires dealing with roll handling on the second story. If extrusion is downward, roll handling is easier, but extrusion is complicated. 

The double-bubble process involves the extrusion and blowing of a tube of molten plastic in a downward direction. The tube is then cooled, most often using a water bath, reheated to just below the melt temperature, and reinflated. The reinflation along with the increase in haul-off speed provides biaxial orientation. Typically the next step is annealing to relieve thermal stresses and stabilize the film. The double-bubble process is most often applied to PP film, but is also used with multilayer PP or PE-based films. One of the major advantages is that this process can deliver a high-clarity film with precise shrink characteristics and very uniform flatness. 

Vertical extruder n. An extruder arranged so that the barrel is vertical and extrusion is usually downward or upward. 

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