Chapter 20

Question 1

Plastics, ceramics, and composites

Answer 1

Large complex shapes can be formed as a single unit. Materials can provide integral and variable color. Finishing operations are often unnecessary.

Question 2

Fabrication of an acceptable product requires

Answer 2

The selection of an appropriate material, and a companion method of processing.

Question 3

Thermoplastic polymers

Answer 3

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.

Question 4

Thermosetting polymers:

Answer 4

One the polymerization reaction takes place, no further deformation can occur. Polymerization reaction must take place during the shape forming operation.

Question 5

Process to shape polymers

Answer 5

Casting, blow molding, compression molding, transfer molding, cold molding, inject molding, etc.

Question 6

Casting for plastics

Answer 6

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.

Question 7

Castable thermoplastics

Answer 7

Acrylics, nylons, urethanes, and PVC plastisols.

Question 8

If the polymer is thermoplastic

Answer 8

It is melted and the liquid is gravity poured into a contained having the desired shape, and it cools and solidifies.

Question 9

Plate glass used to

Answer 9

Cast individual pieces of thick plastic sheet.

Question 10

Continuous sheets and films can be produced by

Answer 10

Injecting the liquid polymer between two moving belts of highly polished stainless steel.

Question 11

Thin sheets made by

Answer 11

Ejecting molten liquid from a gap slot due into a temperature controlled chill roll.

Question 12

Producing hollow or tubular shapes

Answer 12

Molten plastic is spun against a rotating mold wall.

Question 13

Castable thermoses

Answer 13

Phenolics, polyesters, epoxies, silicones, and urethanes, as well as any resin that will polymérisé at low temperatures and atmospheric pressure.

Question 14

Casting plastics advantages

Answer 14

Often simpler, and less expensive, and is suited for low production quantities. Have a distinctly lustrous appearance, and many colors including translucent is available.

Question 15

Typical products from plastic casting

Answer 15

Sheets, plates, films, rods, tubes, jewelry, ornamental shapes, gears, and lenses. Dimension precision can be quite high, but quality problems can occur.

Question 16

Use of blow molding processes

Answer 16

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.

Question 17

Parison

Answer 17

A solid bottom hollow tube preform. Is made from heated plastic by either extrusion of injection molding.

Question 18

Primary products of blow molding

Answer 18

Disposable bottle for beverages and other liquids. Large shopping drums, storage tanks, gasoline cans, automotive parts: fuel tanks, seat backs, ductwork, and bumper beams.

Question 19

Compression molding for plastics

Answer 19

One of the most widely used molding processes for thermosetting polymers

Question 20

Compression molding process

Answer 20

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.

Question 21

Advantages of compression molding

Answer 21

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.

Question 22

Products of compression molding

Answer 22

Gaskets, seals, exterior automotive panels, aircraft fairings, and a wide variety of interior panels.

Question 23

Compression molding has been used to form

Answer 23

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.

Question 24

Transfer molding for plastics

Answer 24

Sometimes used to reduce the turbulence and uneven flow that can result from the high pressures of hot compression molding.

Question 25

Process of transfer molding

Answer 25

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.

Question 26

Advantages of transform molding

Answer 26

Excellent detail, good tolerances, and good finish. Attractive for producing small to medium sized parts with relatively complex shapes and variable wall thickness.

Question 27

Disadvantages of transfer molding

Answer 27

Loss of material because design left in the pot or well, sprue, and runners also cures and must be discarded.

Question 28

Products of transfer molding

Answer 28

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.

Question 29

Injection molding

Answer 29

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

Question 30

Process of injection molding

Answer 30

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.

Question 31

Advantage of injection molding

Answer 31

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.

Question 32

Extrusion

Answer 32

Long plastic products with uniform cross sections can be produced by the extrusion process.

Question 33

Extrusion process

Answer 33

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.

Question 34

Advantages of extrusion

Answer 34

The process is continuous so it’s cheap and rapid.

Question 35

Products of extrusion

Answer 35

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.

Question 36

A combination of extrusion and blowing has made

Answer 36

Thin plastic bags, used in both kitchens and bathrooms.

Question 37

Thermoforming for plastics

Answer 37

Thermoplastic sheet material is heated to a working temperature and then formed into desired shapes.

Question 38

Thermoforming process

Answer 38

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.

Question 39

Products produced by thermoforming

Answer 39

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.

Question 40

Rotation molding for plastics

Answer 40

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.

Question 41

Process of rotational molding

Answer 41

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.

Question 42

Rotational molds

Answer 42

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.