Machinery's Handbook, 31st Edition
584 MANUFACTURING WITH PLASTICS very high pressure (1,000–8,000 psi, or 14–55 MPa) needed to pump the melt through the die. This produces a continuous length of product whose cross section resembles that of the die opening. Extruders are widely employed in intermediate processing of material in the plastics industry. Some final consumer products made by this process are pipes, sheets and films, garden hoses, electrical wires, netting, fishing lines and leaders, and house sidings. Many extruded products are parts of other industrial or consumer products. For example, huge quantities of ABS sheet and flexible vinyl gasketing go into refrigerators and freezers each year. Mop handles are extruded tubing cut to the length needed. Produce bags are made from extruded tubular blown polyethylene film that is laid flat, printed, heat-sealed across and perforated for tear-off at intervals, then wound onto roll cores for delivery to supermarkets. An important class of extruded products includes rods, bars, and tubes from which machinists fabricate smaller quantities of objects, such as prototypes for testing prior to mass production. Sheet Thermoforming.— Also known as vacuum or pressure forming, these widely used molding processes respectively operate at significantly less than or greater than atmospheric pressure to form feedstock in the form of sheet. The process begins with a sheet of plastic from which a single product or sometimes up to several dozen small products are to be made per operating cycle. Mounted in a frame, the sheet to be formed is preheated in an oven between paired radiant heaters at the thermoforming press until it begins to sag. The temperature is still well below that at which the same resin would be injection molded. The sheet is then positioned over the mold, which, in the original and simplest variant of the process, is a female mold made porous by drilling many small holes through it. The mold is enclosed in a chamber (vacuum or pressure box) so that a vacuum can be drawn from below (or a pressure applied from above). The softened sheet is clamped to the mold around its edges, and the pressure difference is applied to press the sheet against the mold surface. Cooling and solidification of the part is done with fans, water mist, or cooling channels in the mold. The unclamped molding is then indexed to a die-cutting table where the edges are trimmed and the part(s) cut out. Trim scrap, which may be as much as 20 percent of the sheet area, is recycled to the sheet extruder. Because the drawn sheet contacts the upper areas of the mold early in the mold cycle and the bottom last, the wall thickness of the finished part is thinner at the bottom than the upper areas when using a female mold. Often, at least average thickness is needed in the bottom areas to satisfy the design service and service life requirements. Over the years, techniques such as plug assist, male forming, and others have been developed that permit draw depths of more than twice the clamping width with much more uni- form wall thickness. Polystyrene cups for cold drinks are now made in this way. Fig. 14 shows the steps in a more sophisticated variant vacuum air-cushion forming, in which a male mold component provides plug assistance without actually touching the softened plastic membrane. Clearly, though, thermoforming cannot match the tight tolerances of injection molding. Nor can it produce intricate components that require substantial or abrupt variations in wall thickness. Because of the low pressure, thermoforming molds can be made much more cheaply of aluminum—even plaster for short runs—and usu - ally do not need the extensive polishing and hard-chrome plating as typical with injec - tion molds. Also, thermoforming machines are much less massive for equal part sizes than injection machines. Production efficiency is higher in multi-station machines but requires a higher financial investment. Blow Molding.— In its most common form, blow molding couples an extruder with a chilled aluminum mold to make a hollow object such as a 2-liter soda bottle. A cross head die attached to the extruder extrudes a tube of hot melt ( parison ) downward be- tween the mold halves, as in Fig. 15 . The parison-inflating air injection at stage 4 is done through a needle that typically, but not always, penetrates the pinched-off melt at the inverted top of the bottle. Bottles with capacities from 2 ml to 10 gallons (38 liters) and myriad other hollow objects are produced by this process. Close tolerances are
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