PLASTICS• The plastic is an organic substance which consists of

natural or synthetic binders or resins with or without moulding compounds.

Mechanical Engineering 2Ken Youssefi

Why Design with Plastics?

• Corrosion resistance• Low electrical and thermal conductivity, insulator • Easily formed into complex shapes, can be formed,

casted and joined.• Wide choice of appearance, colors and transparencies

Mechanical Engineering 3Ken Youssefi

Disadvantages of using Plastics

• Low strength• Low useful temperature range (up to 600 oF)• Less dimensional stability over period of time (creep

effect)• Aging effect, hardens and become brittle over time• Sensitive to environment, moisture and chemicals• Poor machineability

PROPERTIES OF PLASTICS

• It is light in weight.• Specific gravity of plastic is 1.40.• They have low electrical conductivity.• They have low thermal conductivity• They can absorb shocks. USES• To make waterproof doors, pipes, bags.• To make furniture .• To make optical lenses, frames.• For electrical & thermal conductivity.

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Classification of polymers

Thermoplastics

As the temperature is raised above the melting point, the secondary bonds weaken, making it easier to form the plastic into any desired shape. When polymer is cooled, it returns to its original strength and hardness. The process is reversible. Polymers that show this behavior are known as thermoplastics.

Thermosetting Plastics (thermosets)

Thermosetting plastics are cured into permanent shape. Cannot be re-melted to the flowable state that existed before curing, continued heating for a long time leads to degradation or decomposition. This curing (cross-linked) reaction is irreversible. Thermosets generally have better mechanical, thermal and chemical properties. They also have better electrical resistance and dimensional stability than do thermoplastics.

There are two major classifications of polymers

General properties: low melting point, softer, flexible.

Typical uses: bottles, food wrappers, toys, …

Examples:

Polyethylene: packaging, electrical insulation, milk and water bottles, packaging film

Polypropylene: carpet fibers, automotive bumpers, microwave containers, prosthetics

Polyvinyl chloride (PVC): electrical cables cover, credit cards, car instrument panels

Polystyrene: disposable spoons, forks, Styrofoam™

Acrylics (PMMA: polymethyl methacrylate): paints, fake fur, plexiglass

Polyamide (nylon): textiles and fabrics, gears, bushing and washers, bearings

PET (polyethylene terephthalate): bottles for acidic foods like juices, food trays

PTFE (polytetrafluoroethylene): non-stick coating, Gore-Tex™ (raincoats), dental floss

Plastic types: Thermoplastics

General properties: more durable, harder, tough, light.

Typical uses: automobile parts, construction materials.

Plastic types: Thermosets

Examples:

Unsaturated Polyesters: varnishes, boat components, furniture

Epoxies and Resins: glues, coating of electrical circuits,composites: fiberglass in helicopter blades, boats, …

Plastic types: Elastomers

General properties: these are thermosets, and have rubber-like properties.

Typical uses: medical masks, gloves, rubber-substitutes

Examples:

Polyurethanes: mattress, cushion, insulation, toys

Silicones: surgical gloves, oxygen masks in medical applicationsjoint seals

Manufacturing Processes of plastics

Casting

molding

Injection molding

lamination

Compression molding

Extrusion

Chapter 18 10IT 208

Manufacturing With PlasticsCasting - filling a mold

• Liquid resins (epoxy)• Hot melted plastic poured into a casting (nylon)• Slush casting for thin walled products (snow boots, gloves,

toys)• Wet spinning -fibers formed by spinning the solution through

multi-hole dies• Rotational molding - variation of slush casting, mold is heated

as it is rotated. Works exceptionally well for larger parts (IDOT barrels, chemical containers)

Chapter 18 11IT 208

Manufacturing With PlasticsMolding - melt processing-polymers are deformed with the aid of

applied pressure• Results in a finished part• Need three things to be a molding operation (Time,

temperature, and pressure)

Chapter 18 12IT 208

Molding Process

Injection molding • Similar to die casting metal. Pellets are fed into heated

cylinder where they are melted. • The screw rotates much like extrusion molder (it moves back

as material in front of it are melted) then it rams forward pushing the melted material into the die.

• most wide spread technique for making 3-D configurations• uses either reciprocating screw or reciprocating plunger

(RAM)• rated by clamping pressure of die

Chapter 18 13IT 208

Injection Molding

Compression Moulding

1. The mould is charged with a measured amount of powder or granules ready to be compressed. Sometimes plastic charge is first compacted into a shape called a preform.

2. When the two halves of the mould are brought together the plastic material is forced under compression to flow rapidly around the cavity. heat from the platens causes the plastic to cure resulting in a permanent change in shape.

Compression Moulding The component is ejected from the mould and any excess material formed at edges (flash) is removed.

Materials used.

Typical thermosetting plastics used in compression moulding are urea formaldehyde and phenol formaldehyde.

ExtrusionContinuous process used to produce both solid and hollow products that have a constant cross-section. E.g. window frames, hose pipe, curtain track, garden trellis.

Thermoplastic granules are fed from a hopper by a rotating screw through a heated cylinder.

The tapered screw compacts the plastic as it becomes elasticised. The die which is fitted to the end of the extruder barrel determines the cross-section of the extrusion.

Thicker cross-sections are extruded more slowly as more time is required for the initial heating and subsequent cooling of the larger quantities of material which are involved. As the extrusion leaves the die it is cooled by passing through a cooling trough (below) containing cold water.

Extrusion

Plastics Processing: Extrusion

pelletsopen cross-sections (channels) closed cross-sections (tubes, pipes) pelletsopen cross-sections (channels) closed cross-sections (tubes, pipes)

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Epoxy is the cured end product of epoxy resins, as well as a colloquial name for the epoxide functional group

Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees.

Molding or moulding (see spelling differences) is the process of manufacturing by shaping liquid or pliable raw material using a rigid frame called a mold or matrix. This itself may have been made using a pattern or model of the final object.

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