Chapter 20 Flashcards
Plastics, ceramics, and composites
Large complex shapes can be formed as a single unit. Materials can provide integral and variable color. Finishing operations are often unnecessary.
Fabrication of an acceptable product requires
The selection of an appropriate material, and a companion method of processing.
Thermoplastic polymers
Can be heated to produce either a soft, formable solid, or a liquid. The material can then be cast, injected into a mold, or forced into or through dies to produce a desired shape.
Thermosetting polymers:
One the polymerization reaction takes place, no further deformation can occur. Polymerization reaction must take place during the shape forming operation.
Process to shape polymers
Casting, blow molding, compression molding, transfer molding, cold molding, inject molding, etc.
Casting for plastics
Simplest shape forming processes, no fillers or pressure required. Not all plastics can be cast, but there are castable members of both the thermoplastic and thermosetting families.
Castable thermoplastics
Acrylics, nylons, urethanes, and PVC plastisols.
If the polymer is thermoplastic
It is melted and the liquid is gravity poured into a contained having the desired shape, and it cools and solidifies.
Plate glass used to
Cast individual pieces of thick plastic sheet.
Continuous sheets and films can be produced by
Injecting the liquid polymer between two moving belts of highly polished stainless steel.
Thin sheets made by
Ejecting molten liquid from a gap slot due into a temperature controlled chill roll.
Producing hollow or tubular shapes
Molten plastic is spun against a rotating mold wall.
Castable thermoses
Phenolics, polyesters, epoxies, silicones, and urethanes, as well as any resin that will polymérisé at low temperatures and atmospheric pressure.
Casting plastics advantages
Often simpler, and less expensive, and is suited for low production quantities. Have a distinctly lustrous appearance, and many colors including translucent is available.
Typical products from plastic casting
Sheets, plates, films, rods, tubes, jewelry, ornamental shapes, gears, and lenses. Dimension precision can be quite high, but quality problems can occur.
Use of blow molding processes
Used to convert thermoplastic polyethylene, polyvinyl chloride, polypropylene, polyethylene terephyhalaye, and poly ether ether ketone resins into bottles and other thin wall seamless bellow shaped containers.
Parison
A solid bottom hollow tube preform. Is made from heated plastic by either extrusion of injection molding.
Primary products of blow molding
Disposable bottle for beverages and other liquids. Large shopping drums, storage tanks, gasoline cans, automotive parts: fuel tanks, seat backs, ductwork, and bumper beams.
Compression molding for plastics
One of the most widely used molding processes for thermosetting polymers
Compression molding process
A preshaped charge of ore washers amount of molding compound is introduced into a heated cavity. A heated plunger applied pressure, and the material flows into all portions of the cavity. Multiple cavities can be placed within a mold to produce more than one part.
Advantages of compression molding
Tool and machinery costs are lower, and the dimensional precision and surface finish are high. Most economical and is applied to small production runs of parts requiring close tolerances, high impact strength, and low mold shrinkage.
Products of compression molding
Gaskets, seals, exterior automotive panels, aircraft fairings, and a wide variety of interior panels.
Compression molding has been used to form
Fiber reinforced plastics, both thermoplastic sand thermosetting, into parts with properties that rival the engineering metals. Typical products include wash basins, bathtubs, equipment housings, and various electrical components.
Transfer molding for plastics
Sometimes used to reduce the turbulence and uneven flow that can result from the high pressures of hot compression molding.
Process of transfer molding
Unpolymerized raw material is placed in a plunger cavity where it is heated. The plunger then descends, forcing the molten plastic through channels or runners into adjoining die cavities.
Advantages of transform molding
Excellent detail, good tolerances, and good finish. Attractive for producing small to medium sized parts with relatively complex shapes and variable wall thickness.
Disadvantages of transfer molding
Loss of material because design left in the pot or well, sprue, and runners also cures and must be discarded.
Products of transfer molding
Electrical switchgear, wiring devices, parts of household appliances that require heat resistance, structural parts that require hardness and rigidity under load, under hood automotive parts, and parts that require good resistance to chemical attack.
Injection molding
Most used process for the high volume production of relatively complex thermoplastic parts, and is considered to be the polymer equivalent to metal die casting
Process of injection molding
Granules of raw material are fed by gravity from a hopper into a cavity. The plunger forces the material into a preheating chamber and in through a torpedo section where it is mixed melted and superheated. The superheated material is driven through a nozzle that seats against a mold. Spruces and runners channel the molten material into one or more die cavities. The die remains cold.
Advantage of injection molding
Different materials or materials with different colors can be coinjected or sequentially injected to produce single products with differing characteristics. Like the multicolor light assemblies of current cars and trucks. Quality is improved, and there is less turbulence.
Extrusion
Long plastic products with uniform cross sections can be produced by the extrusion process.
Extrusion process
Thermoplastic pellets or powders are fed through a hopper into the barrel chamber of a screw extruded. A rotating screw propels the material through a preheating section , where it is heated. Then it forces it through the a heated die. The material is cooled and hardened by jets of air.
Advantages of extrusion
The process is continuous so it’s cheap and rapid.
Products of extrusion
Solid shapes (window and trim molding), hollow shapes (tubes and pipes), cable coatings and wires, sheets and films, and small diameter fibers or filaments. Thermoplastic foam shapes can also be produced.
A combination of extrusion and blowing has made
Thin plastic bags, used in both kitchens and bathrooms.
Thermoforming for plastics
Thermoplastic sheet material is heated to a working temperature and then formed into desired shapes.
Thermoforming process
If continuous material is used, it is heated by passing through an oven, and then emerging over a mold and is formed by the application of vacuum.
Products produced by thermoforming
Small packaging containers for food products, plastic luggage, interior automotive door and body liners, panels for light fixtures, interior panels for refrigerators, shower enclosures, bathtubs, and boat hills. Thermoforming of thing films has been used to produce pages Of Braille text for the blind.
Rotation molding for plastics
Can be used to produce hollow, seamless products of a wide variety of sizes and shapes, including storage tanks, bins and refuse containers, swimming looks, boat and canoe hills, luggage, football helmets, garbage cans, portable outhouses, septic tanks, and numerous automotive and truck panels and parts.
Process of rotational molding
A split mold or cavity is filled with a premeasured amount of thermoplastic power or liquid. The molds are either preheated or placed in a heated oven and are then rotated simultaneously about two perpendicular axes. The resin melts and is distributed by gravity to reduce a uniform thickness coating over all of the surfaces of the mold. The mold is transferred to a cooling chamber where the motion is continued.
Rotational molds
Lightweight molds made from cast aluminum, but sheet metal is often used for larger parts, and electroformed of vapor formed nickel is used when fine detail is to be reproduced. Production times are long.
