Lecture 8

SHAPING PROCESSES FOR POLYMERS

PolymersA polymer is a compound consisting of long-chain molecules, each molecule made up of 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:

Acrylics (Plexiglas): lenses, window glazingFluorocarbons (Teflon): nonstick coatings, bearings, sealsPolyamides (Nylons, Kevlar): fibersPolycarbonates (Lexan): helmets, bullet-resistance windows, wind shieldsPolyesters (Dacron, Mylar, Kodel): gears, cams, rollersPolyvinyl chloride (PVC): pipes, cable insulation, packaging, flooring, toysPolyethylene: 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: Epoxies: fiber-reinforced materials Phenolics (Bakelite): knobs, handles, cases Polyesters: fiber-reinforced materials Silicones: waterproof and heat resistance materials

Elastomers (rubbers) Exhibit extreme elastic extensibility under low mechanical stresses.The most important rubbers are: Natural rubber (Latex): tires, shoes, seals Silicones: seals, thermal insulation, electronics 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.Cross linking

Manufacturing processes for polymersExtrusionIn twin-screw extruders both screws are parallel and side-by-side inside the barrel. These extruders aresuitable for extrusion of difficult-to-extrude polymers, and for materials that require greater mixing.

Extrusion

Dies for extrusion

Injection MoldingInjection 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 cavityThe production molding cycle time is in the range 10 to 30 sec

Typical molding cycle

Mold Features for Injection Molding

Blow MoldingBlow 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 moldBlowing is done with a hot-air blast at a pressure of 350~700 kPa.

Compression MoldingIn 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.

Transfer MoldingTransfer 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.

ThermoformingThermoforming 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.

Calendering- used for shaping high melt viscosity thermoplastic sheet 1-feed pass2-metering pass3-sheet formation-gauging-finishing

Crown-crossing-bending