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18.6THERMOFORMING

Thermoforming- A series of processes for forming plastic sheet or film over a mold, by means of the application of heat and pressureProcess1. A sheet is heated in an oven to the sag point(softening but not melting point)2.The sheet is then removed from the oven, placed over a mold.3. Plastic sheet forced against the mold through the application of vacuum

-The mold is usually at room temperature, hence the shape of the plastic becomes set upon contact with the mold.Typical parts-advertising signs-refrigerator liners-packaging -appliance housings-tray cookies-panels for shower stalls

- The sheets used in thermoforming are made by sheet extrusion- Parts with openings or holes cannot be formedMolds-Aluminum molds, tooling is inexpensive. The holes are usually less than 0.5mm, in order not to leave any marks on the formed parts.

Various thermoforming processes for a thermoplastic sheet. These processes commonly are used in making advertising signs, cookie and candy trays, panels for shower stall, and packaging

18.7COMPRESSION MOLDING

-A pre-shaped charge of material, a pre-measured volume of powder, or a viscous mixture of liquid resin and filler material is placed directly into a heated mold cavity (200 C).

-Forming is done under pressure from upper half of die (10-150MPa).-It is used mainly with thermosetting plastics, thermoplastics and elastomers are also processed. Cross-linking is completed in the heated die.

Typical partsHandlesDishesContainer cupsElectrical and electronic componentsWashing-machine agitatorsfittings

-creates formation of flash, removed by trimming-Curing times: 0.5-5min.

3 types of compression molds:

1. Flash type: for shallow or flat parts2. Positive: for high density parts

3.Semi positive: for quality production.

Types of compression molding a process similar to forging: (a) positive, (b) semi positive, and (c) flash, which is later trimmed off. (d) Die design for making a compression-molded part with external undercuts

Design considerations Undercuts are not recommendedDies can be designed to open sideways to allow removal of the molded part Dies are typically less expensive than for injection molding, partially due to the simplicity of the dies

18.8TRANSFER MOLDING

-Development of compression molding1. Uncured material is placed in a heated transfer pot or chamber2. After material is heated it is injected into heated closed molds

3. A ram, a plunger, or a rotating screw feeder is used to force the material to flow through the narrow channels into the mold cavityPROCESS-Curing takes place by crosslinking

Typical parts:electrical and electronic components, rubber and silicon parts.-Transfer molding is suitable for intricate shapes with varying wall thicknesses

Sequence of operations in transfer molding for thermosetting plastics. This process is suitable particularly for intricate parts with varying wall thickness.

18.9CASTING

-Both thermoplastics and thermosets can be cast-Simple, inexpensive but slowTypical parts:GearsBearingsWheelsthick sheetsa. conventional casting : a mixture of monomer, catalyst, and various additives is heated and poured into the mold. Part forms after polymerization takes place at ambient pressure.

b. Centrifugal casting: the plastic resin is placed into the mold, the mold is closed, and rotated. Heat is transferred through the walls of the mold to soften thermoplastics or initiate cure in thermosets. Centrifugal force of rapid rotation forces the resin to conform to the shape of the mold

c. Potting and encapsulation: casting plastic material around electric component [transformer]. Potting is done in a housing or case, which is an integral part of product. In encapsulation, component is coated with a layer of the solidified plastic

d. Foam molding and casting: The raw material is expandable [polystyrene beads]. Polystyrene beads placed in mold with blowing agent (e.g. nitrogen) and exposed to heat (usually steam). Beads expand (up to 50 times) to take shape of the mold cavity. The amount of expansion is controlled by varying the temperature & time.

Typical productsStyrofoam cupsfood containersinsulating blocksshaped packaging materials

-Polystyrene beads are available in three sizes: small, for cups; medium, for molded shapes; and large, for the molding of insulating blocks-The bead size depends on the minimum wall thickness of the product.-Pre-expanded beads are used to the molding, in which beads are partially expanded by steam in an open- top chamber. The beads are then placed in a storage bin and allowed to stabilize for a period of fro 3 to 12 hours.