Extruder tube, blown

Extrusion blow moulding (the simplest), in which the parison is an extruded tube that is blown with air. Various types of machinery are marketed shuttle, reciprocating and wheel machines. 

This is based on the unit in Figure 7.1. An extruded tube flowing downwards is enclosed by a cooled finishing mould with a nip-off/weld at the top and bottom of the bottle (trim/waste) where it is then compressed air blown and cooled to the final shape. The mould(s) may be neck up or down in a straight line or as rotating carousel (see Figure 7.2). 

Most blown film operations extrude the resin in an upward direction. However, blown polypropylene film is generally extruded downwards and water or mandrel quenched. The extruded tube is then reheated, to a point still below its melt temperature, before it is blown. The collapsed bubble can be fed over a series of heated rollers to reheat it and relieve thermal stresses if a heat-stabilized film is wanted or it can be heated and reinflated in what is known as the double bubble process, which will be discussed in Section 7.3.7. In either case, the film is restrained until cooling is complete, to keep it from shrinking. 

Most films are produced by extrusion. Thermoplastic materials are heated and pushed through a die to form a flat tube. Warm air is blown into the extruded tube to produce a balloon which is then cut open and laid flat. Calendering can also be employed to produce film. Hot thermoplastic is passed through a series of temperature-controlled metal rolls with progressively smaller gaps to produce a continuous sheet (Figures 3.27 and 3.28). The technique is used to make shower curtains, food wrap films, carrier bags and protective films. The most widely used films are polyethylene, nylons, polypropylene, cellulose acetate, PVC and polyesters. 

Thin film is inflated from extruded tube consequently to the die exit. The blown film dies could be in general divided to spider and spiral types, as presented in Fig. 4.3. Since spiral dies provide more uniform melt distribution and eliminate weld or knit lines caused by the spider holding the mandrel, they almost replaced... 

The thinnest nominal walls are created by the stretching processes thermoforming and blow molding. In each case, the material is first processed by some other method before it is shaped. Thermoforming uses extruded sheet which is then heated and stretched over a tool. Blow molding uses an extruded tube or injection moldment which is then blown into a mold. Plastic thins as it is stretched, and therefore, the effect of this thinning must be carefully considered in the part design. 

Coextrusion is the simultaneous extrusion of two or more polymers through a single die where the polymers are joined together such that they form distinct, well-bonded layers forming a single extruded product. Coextrusion has been applied in film, sheet, tubing, blown film, wire coating, and profile extrusion. 

Figure 1 Scanning electron micrograph of the surface of extrusion-blown LDPE film containing 10% of maize starch. As the extruded tube is inflated and stretched, characteristic mounds are raised by the solid starch particles, average diameter 15gm. Magnification circa x350,...

In film blowing it is essential to achieve excellent uniformity of the gap thickness and flow rate around the entire die circumference, as well as to minimise material inhomogeneity for very sophisticated designs. Crosshead dies of the bottom-fed/spider type tend to cause problems with weld lines, while it is difficult to obtain flow rate uniformity with side-fed/spiderless types. Modern film-blowing dies are increasingly of the bottom-fed/spiral mandrel type. In order to distribute across the width of the final wound product the inevitable circumferential thickness variations in the extruded tube or the blown bubble, a slow continuous or oscillatory rotation of the mandrel, the bushing, the entire die, and even the haul-off tower must be used. 

Many articles, bottles and containers in particular, are made by blow moulding techniques of which there are many variations. In one typical process a hollow tube is extruded vertically downwards on to a spigot. Two mould halves close on to the extrudate (known in this context as the parison ) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould. As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product. 

In blown film extrusion, the molten polymer enters a ring-shaped die either through the bottom or from the side. It is then forced around a mandrel inside llie die, shaped into a sleeve, and extruded through llie die opening in the form of a comparatively thick-walled tube. 

Tubes and blown Aims can be produced as multilayer structures by employing multiple extruders and coextrusion manifolds and dies. Figure 12.44 is a schematic representative of a conventional and new spiral coextrusion die. The designs can be used for both blown-film and blown-molding parison dies. In the extrusion of tubes, such as rigid PVC or PE pipe, the extrudate passes over a water-cooled mandrel and enters a cold-water bath whose length depends on the tube thickness the tube leaves the bath well below its Tm (if it is crystalline) or Tg (if it is amorphous) and is sectioned to the desired lengths. 

The blown film process involves extruding a relatively thick tube that is then expanded or blown by the usual internal air pressure or the water quench process to produce a relatively thin film (Figure 5.6). The tube can be collapsed to form double-layer layflat film or can be slit to make one or two single-layer film webs. The water quench process is the generally preferred method of producing blown PP type film. 

Blown film is usually extruded vertically upward through a circular die. This forms a tube that is then blown into a bubble that thins or draws down to the required final gauge. Orientation takes place in two directions horizontally (transverse direction/TV) as the bubble is formed, and in the machine direction (MD) as controlled by adjustable-speed haul-off nip rolls. 

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film, which is drawn down along the extrusion direction only. 

Blown film extrusion is perhaps the most widely used extrusion technique, by production volume. Billions of pounds of polyethylene are processed annually by this method to make products such as grocery sacks and trash can liners. In a blown film system (Figure 14-30), the melt is generally extruded vertically upward through an annular die. The thin tube is filled with air as it travels up to a collapsing frame that flattens it before it enters the nip rollers, which pull the film away from the die. The flattened tube then travels over a series of idle rollers to a slitter,... 

The plastic is heated to a melted or viscous state and a section of molten polymer tubing (parison) is extruded usually downward from the die head into an open mold. The mold is closed around the parison, sealing it at one end. Compressed air is blown into the open end of the tube, expanding the viscous plastic to the walls of the cavity, thus forming the desired shape of the container. The material cools in the cavity and solidifies. The mold is opened and the molding is removed. This technique is used for the manufacture of bottles, toys, and large containers. 

Several unique products are made by extrusion and the dies needed to make these products are classified as 1) sheet dies 2) flat-film and blown-film dies 3) pipe and tubing dies 4) profile extrusion dies and 5) co-extrusion dies. Furthermore, each product type has unique hardware downstream of the die to shape and cool the extruded melt. To aid the reader, detailed illustrations of the various die designs and the complementary downstream cooling and shaping hardware are shown. 

Water quenching can be used for wire insulation, tubing, and pipe. Sheet and film extruded from slit dies are frequently crystallized on polished steel rolls operating at 65°C-145°C. Extruded, blown, or flat film can be uniaxially or biaxially oriented to submil thickness. Monofilament is extrusion spun into a water bath, and then oriented and heat set at an elevated temperature. 

The blown film process involves the extrusion of a polymer melt through an annular die and the subsequent blowing of the tube shaped. The materials used to produce films can be neaf componenfs, blends of fwo or more polymers, virgin materials, recycled materials, or blends of those. Additives such as slip, antiblock, antistatics, or pigments can also be added into the extruder feed. 

In blow molding, a tube of molten plastic material, the parison, is extruded over an apparatus called the blow pipe and is then encased in a split mold. Air is injected into this hot section of extruded stock through the blow pipe. The stock is then blown outward, where it foUows the contour of the mold. The part is then cooled, the mold opened, and the molded part ejected. In very heavy sections, carbon dioxide or Uquid nitrogen may be used to hasten the cooling. This process is widely used in molding high- and low-density polyethylene, nylon, polyvinyl chloride (PVC), polypropylene, polystyrene, and polycarbonates. 

The blown-film technique is widely used in the manufacture of polyethylene and other plastic films [14,15]. A typical setup is shown in Figure 2.23. In this case the molten polymer from the extruder head enters the die, where it flows round a mandrel and emerges through a ring-shaped opening in the form of a tube. The tube is expanded into a bubble of the required diameter by the pressure of internal air admitted through the center of the mandrel. The air contained in the bubble carmot escape because it is sealed by the die at one end and by the nip (or pinch) rolls at the other, so it acts like a permanent shaping mandrel once it has been injected. An even pressure of air is maintained to ensure urufoim thickness of the film bubble. 

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