2. Why plastics in Engineering?
• The term Plastics is commonly employed as a synonym for Polymers.
• Plastics are one of numerous polymeric materials having extremely large
molecules, many unique and diverse properties.
• Consumer & Industrial products made of polymers include food & beverage
containers, packaging, signs, housewares, textiles, medical devices, foams,
paints, safety shields, toys, appliances, lenses, gears, electronic & electrical
products, & automobile bodies & components.
• Polymers have increasingly replaced metallic components in applications
such as automobiles, civil & military aircraft, sporting goods, toys, appliances
& office equipment.
3. Advantages of polymers in terms of
Characteristics
• Corrosion resistance and resistance to chemicals
• Low electrical & thermal conductivity
• Low density
• High strength to weight ratio, particularly when reinforced.
• Noise reduction
• Wide choice of colors & transparencies
• Ease of manufacturing and complexity of design possibilities
• Relatively low cost
4. Design considerations for plastic
components
• Materials must be selected with the correct properties to meet design, economic, and
service conditions.
• Environment in which the plastic product will be subjected to.
• All plastics have useful Electrical insulation characteristics.
• The Chemical & electrical natures of plastics are closely related because of molecular
makeup, test of each plastic for the chemical environment to which it will be subjected.
• Mechanical factors of fatigue, tensile, flexural, impact & compressive strengths,
hardness, damping, cold flow, thermal expansion, and dimensional stability should be
considered.
• Economics must also include the method of production and design limitations of the
product, The cost of material for tooling along with the processing cost of that material &
final cost of product
.
5. Defects due to uneven thickness
Sink marks due to uneven
shrinkage
•
Stress due to uneven shrinkage
6. Defects
• due to maintenance of
non uniform thickness
• If the uniform thickness is not
maintained, the durability
&sustainability of the component
decreases & may not be useful under
few circumstances.
• It will lead to shrinkage of materials.
• Sink Marks, Stress, Voids, Warping.
in plastic components
• Low thickness
• Over heat production
7. Classification of plastics-Elastomers
• Elastomers are characterized by
wide meshed crosslinking of the
knotted molecular chains.
• This type of crosslinking means
the materials have a high level of
dimensional stability but are still
elastically malleable.
8. Classification of plastics- Thermoset
• The individual molecular chains of
thermoset polymers are characterized
by 3-D closely meshed crosslinking.
• This property means they no longer
be shaped after hardening, and it
cannot be melted.
9. Classification of plastics- Thermoplastics
• Thermoplastic polymers are plastic in
which the molecular chains are not
crosslinked.
• It consequently demonstrate plastic
elastic behavior and are thermoformable
(meltable, weldable).
• This formability is reversible, it can be
repeated as often as required as long as
the material is not thermally damaged
byoverheating.
10. SPI- Society of Plastic Industries
SPI created the resin identification coding system in the late 80s to help identify the six
most common plastics used on regular basis.
• SPI code: 1 – PETE / PET: Polyethylene Terephthalate
• It’s a polymer resin, with an excellent barrier to water, oxygen and carbon dioxide.
Uses:
Carpet,
Films,
Food packaging,
Liquids packaging
Oven ready food trays
11. SPI
• SPI Code:2 –HDPE- High Density
Polyethylene
• It is made from petroleum
Uses: Bottles, Containers, Grocery
bags, Plastic bags, Hand soap
containers.
• SPI Code: 3 – PVC- Polyvinyl
Chloride
• It has stable electric properties.
Uses: Cables, Decks, Electrical boxes,
Pipes
12. SPI
• SPI Code: 4 – LDPE – Low
Density Polyethylene
• LDPE is a thermoplastic made
from petroleum.
• Thermoplastic can be frozen and
reheated repeatedly for reshaping.
Uses: Lids, Computer components,
Tubing,Trays
• SPI Code: 5 – PP – Polypropylene
• PP is a thermoplastic polymer
Uses:
Auto accessories, Bike racks, Shipping
bins, Storage bins.
13. SPI
• SPI code: 6 – PS –Polystyrene
• A versatile plastic characterized as
either a rigid or a foamed product
with a low melting point that's great
for insulation in foam form.
Uses:
Cameras, Plastic cutlery, Protective
packaging, thermometers.
• SPI Code:7
• Code 7 plastics include those that
are made from bio based products
such as corn, potato starch and
sugar cane.
14. Injection Molding
• Injection molding is perhaps the most common and important of all plastic
processing processes.
• The process is extremely versatile, and can produce very complex shaped
parts, with the use of multi-sided molds.Even parts with metal inserts can be
produced
• While injection molding dies are expensive to produce, each die can be used
to make tens of thousands of components at very rapid rate, so that per-part
cost is very low. .
15.
16. Injection Molding
• In this process polymer is heated to a highly plastic state and forced to flow
under high pressure into a mould cavity, where it solidifies.
• The process produces discrete components that are always net shape.
• Complex and intricate shapes are possible to produce, however the challenge
is to design mould so that the part can be ejected successfully.
• Process is economical in large production as the cost of mould is very high
17. Two alternative type of Injection Moulding
machines
Screw preplasticizer Plunger type (Older machine)
18. Thermoforming
• A sheet of plastic is used to cover a shape (e.g. a die) by heating the sheet till
it is semi fluid, and then pulling it over the die using vacuum suction (this
method is called vacuum forming).
• In an alternate form, called pressure forming, the pressure is applied using
high pressure air from above the plastic sheet.
• This process is most commonly seen in packaging of food, toys etc.; it is also
used to make appliance housings, etc.
19.
20. Acrylonitrile-Butadiene-Styrene (ABS)
• ABS is a low cost engineering plastic that is easy to machine and fabricate.
• ABS is an ideal material for structural applications when impact resistance, strength, and
stiffness are required. It is widely used for machining pre-production prototypes since it has
excellent dimensional stability and is easy to paint and glue.
• A class of thermoplastic terpolymers including a range of resins, all prepared with usually
more than 50% styrene and varying amounts of acrylonitrile and butadiene.
• The three components are combined by a variety of methods involving polymerization,
graft copolymerization, physical mixtures and combinations.
• Typical applications are found in appliances, automotive parts, pipe, and business machine
and telephone components.
21. Draft angles
• Draft angle design is an important factor when designing plastic parts. Because of shrinkage
of plastic material, injection molded parts have a tendency to shrink onto a core.
• This creates higher contact pressure on the core surface and increases friction between the
core and the part, thus making ejection of the part from the mold difficult.
• draft angles should be designed properly to assist in part ejection. This also reduces cycle
time and improves productivity. Draft angles should be used on interior and exterior walls
of the part along the pulling direction.
