PLASTICS
• The plastic is an organic substance which consists of
natural or synthetic binders or resins with or without
moulding compounds.
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
Ken Youssefi
Mechanical Engineering
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Disadvantages of using Plastics
• Low strength
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• Low useful temperature range (up to 600 F)
• 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
Ken Youssefi
Mechanical Engineering
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PROPERTIES OF PLASTICS
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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.
Classification of polymers
There are two major classifications 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.
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Plastic types: Thermoplastics
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: Thermosets
General properties: more durable, harder, tough, light.
Typical uses: automobile parts, construction materials.
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 applications
joint seals
Manufacturing Processes of plastics
Casting
molding
Injection molding
Compression molding
Extrusion
lamination
Manufacturing With Plastics
Casting - 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)
IT 208
Chapter 18
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Manufacturing With Plastics
Molding - 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)
IT 208
Chapter 18
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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
IT 208
Chapter 18
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Injection Molding
IT 208
Chapter 18
13
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.
Extrusion
Continuous 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
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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