CHAPTER CONTENTS
45
PROCESSING OF VARIOUS MATERIALS
4
4.1 Shaping Processes for Polymers Polymers
Manufacturing Processes for Polymers 4.2 Rubber Processing Technology
Introduction
Processing of rubber into finished good 4.3 Processing of Ceramics
Introduction
Processing of Traditional Ceramics Processing of New Ceramics
4.4 Glassworking Introduction
Process Sequence in Glassworking Shaping Processes 4.5 Composite Materials Introduction Starting Materials Shaping Processes
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 Polyamides (Nylons, Kevlar): fibers
v Polycarbonates (Lexan): 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
SHAPING PROCESSES FOR POLYMERS
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
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. The raw materials not only in extrusion but also in most polymer processes are plastic pellets:
Twin extruder screws Plastic pellets are the raw material in
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
Side view cross-section of extrusion die for hollow cross sections such as tubes and pipes
Extruders
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):
After extrusion, the long, continuous product is subsequently cut into desired lengths.
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.
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
Blow Molding
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:
(1) (2) (3) Compression molding of
thermosets: (1) charge is loaded, (2) charge is compressed and cured, and (3) part is ejected and removed
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:
Other similar processes are pressure thermoforming and mechanical thermoforming. Mechanical thermo-forming uses positive and negative molds that are brought together against the heated plastic sheet, forcing it to assume their shape.
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
