Blown sheet

In the blown sheet extrusion process, after leaving the extmder, gear pump and screen pack, the resin moves to a circular spiral mandrel die in which a tube of molten material is formed and inflated with air from the inside. The tube is then lifted in a haul-off tower up to 40 m high where it is cooled by an internal and external flow of cool air. It then proceeds to... 

Fig. 2.11. View of a blown sheet extmsion line. In the background, bottom left, the circular die can be seen of which an HOPE hose, supported by supporting air, exits. The cut tube runs down from the top and covers the view onto the closed tube running upward into the layflat boards. The cut hose enters the take-up mechanism (middle ground) and is finally wound on a spool in the winding mechanism (foreground). The plant tower may be as high as 40 m and is visible on the factory site from a great distance. Figs 2.10 and 2.11 courtesy Serrot International...

Plug descends Plug into sheet Blown sheet Sheet... 

Figure 16.4 Schematic of free-blown sheet method.

Melt Extrusion. By far the most important method for producing film and sheeting materials reties on one or another of the various melt extmsion techniques (5). The main variations of melt extmsion are the slot (or flat) die-cast film process, the blown films process, and the flat die sheeting-stack process. These may be combined with one or more steps such as coextmsion wherein multilayer film or sheet is formed, biaxial orientation, and in-line coating (6). 

Many polymers, including polyethylene, polypropylene, and nylons, do not dissolve in suitable casting solvents. In the laboratory, membranes can be made from such polymers by melt pressing, in which the polymer is sandwiched at high pressure between two heated plates. A pressure of 13.8—34.5 MPa (2000—5000 psi) is appHed for 0.5 to 5 minutes, at a plate temperature just above the melting point of the polymer. Melt forming is commonly used to make dense films for packaging appHcations, either by extmsion as a sheet from a die or as blown film. 

If a thermoplastic sheet is softened by heat and then pressure is applied to one of the sides so as to generate a freely blown surface, it will be found that the shape so formed has a uniform thickness. If this was the case during thermoforming, then a simple volume balance between the original sheet and the final shape could provide the wall thickness of the end product. 

An 80 m3 tank fiberglass-reinforced plastic acid tank was blown apart at the base as the result of overpressure. The vent had been slip-plated so the tank could be entered for inspection. The steel slip-plate was covered with a corrosion-resistant sheet of polytetrafluoroethylene. Afterward, when the slip-plate was removed, the sheet was left behind. This did not matter at the time, as the tank was also vented through an oveiflow line,... 

Polyethylene is an inexpensive thermoplastic that can be molded into almost any shape, extruded into fiber or filament, and blown or precipitated into film or foil. Polyethylene products include packaging (largest market), bottles, irrigation pipes, film, sheets, and insulation materials. 

An extrusion-blown hinge can be compared to a stamping (Fig. 3-21). Hot- and cold-stamped hinges are made by compressing a sheet of material down to the desired thickness [12 to 20 in. (3 to 5 mm)]. Stamped... 

The pumping pressure required on the melts entering the different designed die heads differs to meet their melt flow patterns within the die cavities. The pressure usually varies as follows (1) blown and lay-flat films at 13.8-41.3 MPa (2000-6000 psi) (2) cast film, sheet, and pipe at 3.5-27.6 MPa (500-4000 psi) (3) wire coating at 10.3-55.1 MPa (1500-8000 psi), and (4) monofilament at 6.9-20.7 MPa (1000-3000 psi). 

As cast or blown films we encounter polyethylene in packaging films, bags, cling film, greenhouse skins, diaper back sheets, and high altitude scientific balloons. In these applications... 

Polypropylene film is found in a variety of applications. Films are thin plastic sheets with thicknesses less than 250 pm. Applications for these films include candy mappers, plastic label material, electrical capacitors (which requires very pure polymer), and fiber glass insulation backing. There are two general categories of polypropylene films unoriented and oriented. Films are manufactured through either a cast or a blown film extrusion process. Oriented films exhibit improved strength relative to unoriented films. This means that they can be... 

The mixture is then run on to a sheeting table which is made on the same principle as a paper-making machine, but with the wire replaced for safety by a suitable cloth filter. The pulp passes over suction tubes and between rollers in order to remove as much water as possible, so that the paste when stripped from the cloth contains 20-25% of water. It is dried on trays or else in trucks through which air at 50°C is blown. 

The dispersibility of an organic pigment which is to be incorporated in a polyolefin is of particular importance. This is especially true for the coloration of extrusion films and for HDPE or PP ribbon made from stretched blown film or from sheeted extrusion film, as well as for the coating or melt spin dyeing. 

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