PROCESSING OF VARIOUS MATERIALS

Transcription

1 4 PROCESSING OF VARIOUS MATERIALS CHAPTER CONTENTS 4.1 Shaping Processes for Polymers Polymers Manufacturing Processes for Polymers 4.2 Rubber Processing Technology Processing of rubber into finished good 4.3 Processing of Ceramics Processing of Traditional Ceramics Processing of New Ceramics 4.4 Glassworking Process Sequence in Glassworking Shaping Processes 4.5 Composite Materials Starting Materials Shaping Processes SHAPING PROCESSES FOR POLYMERS Polymers A polymer is a compound consisting of long-chaim molecules, each molecule made up pf repeating units connected together. Polymers can be separated into three major groups: Thermoplastic polymers (thermoplastics) Thermosetting polymers (thermosets) Elastomers (rubbers) Thermoplastic polymers (thermoplastics) Solid materials at room temperature, become viscous liquids when heated. The most important thermoplastics are: v Acrylics (Plexiglas): lenses, window glazing v Fluorocarbons (Teflon): nonstick coatings, bearings, seals v v Polyamides (Nylons, Kevlar): Polycarbonates (Lexan): fibers helmets, bullet-resistance windows, wind-shields v Polyesters (Dacron, Mylar, Kodel): gears, cams, rollers v Polyvinyl chloride (PVC): pipes, cable insulation, packaging, flooring, toys v Polyethylene: bottles, cans, packaging materials Thermosetting polymers (thermosets) When initially heated, soften and flow for molding. After cooling, harden into an infusible solid. No repeated heating cycle is possible. The most important thermosets are: v Epoxies: fiber-reinforced materials v Phenolics (Bakelite): knobs, handles, cases v Polyesters: fiber-reinforced materials v Silicones: waterproof and heat resistance materials 45

2 46 Shaping Processes for Polymers Valery Marinov, Manufacturing Technology Elastomers (rubbers) Exhibit extreme elastic extensibility under low mechanical stresses. The most important rubbers are: v Natural rubber (Latex): tires, shoes, seals v Silicones: seals, thermal insulation, electronics v Polyurethane: seals, gaskets The difference in properties of the polymers are attributable to so-called cross-linking, which occurs in thermosets and partially in elastomers: (a) (b) (c) Models of long polymer molecules with (a) no cross-linking in thermoplastics, (b) intensive cross-linking in thermosets, and (c) low degree of cross-linking in elastomers Manufacturing processes for polymers Extrusion In polymer extrusion, the feedstock is fed into an extrusion barrel where it is heated, melted, and forced to flow through a die opening by means of rotating screw: Components and features of a single-screw extruder for thermoplastics and elastomers The raw materials not only in extrusion but also in most polymer processes are plastic pellets: Plastic pellets are the raw material in many shaping processes for polymers Twin extruder screws In twin-screw extruders both screws are parallel and side-by-side inside the barrel. These extruders are suitable for extrusion of difficult-to-extrude polymers, and for materials that require greater mixing.

3 Valery Marinov, Manufacturing Technology Shaping Processes for Polymers 47 Dies are not a component of the extruder and must be designed and fabricated for the particular profile to be produced: Side view cross-section of extrusion die for solid cross sections such as round stock Extruders Side view cross-section of extrusion die for hollow cross sections such as tubes and pipes After extrusion, the long, continuous product is subsequently cut into desired lengths. The illustration shows an extruder for plastic pipe. The specific tooling for this product is attached to the machine on the left side (in black): Plastic extrusion machine for plastic pipe The internal diameter of the barrel is usually 25 to 150 mm, and the L/D ratio is between 10 and 30. The screw rotates at about 60 rev/min.

4 48 Shaping Processes for Polymers Valery Marinov, Manufacturing Technology Injection Molding Injection Molding is a process, in which a polymer is heated to a highly plastic state and forced to flow under pressure into a mold cavity, where it solidifies. The part, called a molding, is then removed from the cavity: Diagram of an injection molding machine, reciprocating screw type The production molding cycle time is in the range 10 to 30 sec: Typical molding cycle: (1) mold is closed, (2) melt is injected into cavity, (3) screw is retracted, and (4) mold opens and part is ejected Injection-molding machines The illustration shows an injection-molding machine with CNC control: 250-ton CNC injection molding machine Large parts with complex shapes are easily produced by injection molding

5 Valery Marinov, Manufacturing Technology Blow Molding Shaping Processes for Polymers 49 Blow molding is a modified extrusion and injection molding process, wherein a tube is extruded, clamped into a mold with a cavity much larger than the tube diameter, and then blown outward to fill the mold. (1) (2) (3) (4) Extrusion blow molding: (1) extrusion of parison, (2) mold is closed, (3) the tube is blown to take the shape of the mold, and (4) mold is open to remove the part Blowing is done with a hot-air blast at a pressure of 350~700 kpa. Further modifications of the process are injection blow molding and rotational molding. Blow molding machine for plastic containers showing die cavities Compression Molding In compression molding, the workpiece (pre-shaped part, volume of powder, mixture of liquid resin and fillers) is placed in the heated mold and is formed under pressure: Compression molding of thermosets: (1) charge is loaded, (2) charge is compressed and cured, and (3) part is ejected and removed (1) (2) (3)

6 50 Shaping Processes for Polymers Valery Marinov, Manufacturing Technology Transfer Molding Transfer molding is a similar to compression molding process, but the charge is placed not in the die cavity but into a chamber next to the die cavity. Pressure is then applied to force the material to flow into the heated mold where curing occurs. (1) (2) (3) Transfer molding of thermosets: (1) charge is loaded, (2) soften polymer is compressed into mold cavity and cured, and (3) part is ejected and removed Thermoforming Thermoforming is a process in which a flat thermoplastic sheet is heated and deformed into desired shape. The classical process involves the use of vacuum and is called vacuum forming: Vacuum thermoforming: (1) a flat plastic sheet is softened by heating, (2) soften sheet is placed over the mold cavity, (3) a vacuum draws the sheet into the cavity, and (4) after cooling and solidification, part is removed and subsequently trimmed Other similar processes are pressure thermoforming and mechanical thermoforming. Mechanical thermoforming uses positive and negative molds that are brought together against the heated plastic sheet, forcing it to assume their shape.