Plastics

Plastics
 Plastics are everywhere.
 Plastics have become the most
dominant engineering material
for most products.
 All plastics are polymers.
 Some are naturally occurring,
but most are man-made.
Created By Milan Rajawat

Classification
Plastic are mainly classified into three types:-
1. Thermosets :- Thermosets harden when they are heated, if heated further ,they will break down
chemically and lose their properties.
Properties :- More durable, harder, tough, light.
Uses :- Automobile parts, construction materials.
Examples of Thermosets :-
 Unsaturated Polyester :- varnishes, boat hulls, furniture.
 Epoxies and Resins :- glues, coating of electrical circuits, composite materials like fiberglass used
in helicopter blades, boats etc.

2. Elastomers :- Some thermosets have properties very similar to rubber, and are used as synthetic rubber;
they are categorized as elastomers.
Properties :- These are thermosets, and have rubber-like properties.
Uses :- Medical masks, gloves, rubber-substitutes.
Examples of Elastomers :-
 Polyurethanes: mattress, cushion, insulation, toys, shoe.
 Silicones: surgical gloves, oxygen masks in medical.

3. Thermoplastics :- Thermoplastics melt when heated – so they can be melted and re-formed again and
again.
Properties :- low melting point, softer, flexible.
Uses :- bottles, food wrappers, toys ,automobile parts etc.

Engineering Plastics
1. Polyethylene :- Widely used plastic. It is a polymer of ethylene. These are of two types low and high
density, and are very bad conductor of heat and electricity. Polyethylene is resistant to water, acids,
alkalises, and most solvents.
Uses :- Its many applications include films or sheets for packaging, shower curtains, unbreakable bottles
pipes, pails, drinking glasses, tubing, and insulation for wire and cable.
PE used for Packaging As a metal protector

2. Polypropylene(PP) :- Plastic noted for its lightweight, being less dense than water; it is a polymer of
propylene. It resists moisture, oils, and solvents. Since its melting point is 121°C (250°F), it is used in the
manufacture of objects that are sterilized in the course of their use. Impact and wear resistant. Flexible,
can have very high elongation before breaking.
Uses :- Polypropylene is used to make textiles, coating of wire and cable, packaging material, tubing,
carpet fibres, automotive bumpers, document holders, prosthetic body parts for disabled people.
Car Bumper made of PP Document holders

3. Polyvinyl chloride (PVC) :- Thermoplastic that is a polymer of vinyl chloride. Resins of polyvinyl
chloride are hard, but with the addition of plasticizers a flexible, elastic plastic can be made. PVC
possess fire retarding properties, durability, easy mould-ability and oil/chemical resistance
Uses :- This plastic has found extensive use as an electrical insulator for wires and cables, tubing,
gaskets, floor and wall coverings, credit cards, pipes, automobile instrument panels.
Pipes made of PVC Dashboard panels made of PVC

4. Nylon (Polyamides) :- Good combination of mechanical properties like Fatigue creep strength,
stiffness, toughness and resilience, Good abrasion resistance, Self lubricating characteristics, frictionless,
good dimensional stability, high heat resistance, good impact resistance.
Uses :- Automotive uses:- Radiator fan, Radiator grill, Instrument housings, Speedometer gears.
Other uses:- Circuit breaker housing, power capacitor housing, switch/switch parts
battery casing, potentiometer spindle, insulator housings, ventilator fan,
gears/bushes, door handles, latched/hinges, rollers, fasteners, coupling
components, impellers/fans, bearings
Nylon parts used in automobiles Fasteners made of nylon

5. Polycarbonate (PC) :- Group of clear, thermoplastic polymers used mainly as moulding compounds.
transparency, excellent toughness, thermal stability and a very good dimensional stability, non-
flammable, self extinguishing, high heat deflection temperature, good dimensional stability, resistance to
weak acids, hydrocarbons and oils, good electrical properties, stable over wide range of temperature,
frequency and humidity.
Uses :- Housing and coloured lenses for traffic signals, Reflectors, Ventilation and radiator Grills,
Bumpers, Wind shields for two Wheelers Headlight, Lenses, Tail lamps, Compact discs, baby
feeding bottles, Housing for computers, blood bottles, Mixers and blenders, Vacuum cleaner
Housings, Goggles, Green house.
CD’s made of PC Automobile parts made of PC

6. Polyurethanes (PU) :- High wear resistant, Flexibility over a wide range of temperatures, Good
resistance to oils, greases and many solvents, Good weather ability, Absence of plasticisers, Excellent
resistant to high energy radiation, Excellent resistant to decay.
Uses :- Automotive :- Bearings and bushes for oscillating parts, vibration-damping components,
diaphragms, sleeves, control cable guides, covers for flexible hydraulic hoses, shock absorber
components.
Other Uses :- Low-noise gearwheels, seals, sleeves, wipers, clutch components, toothed belts,
Hammer heads, wipers, Sports shoe soles, studs and bars, heel tips, ski boots.
PU bush kit for Landrover Defender Other examples

7. Polyethylene Terephthalate (PET) :- Polyethylene terephthalate produced by the condensation
reaction of ethylene glycol with either terephthalic acid (TPA) or dimethyl terephthalate (DMT).
Excellent wear resistance, low coefficient of friction, perfect for blow moulding and superior
dimensional stability make it a versatile material for designing mechanical and electro-mechanical parts.
Uses :- Carbonated drinks bottles, peanut butter jars, plastic film, microwavable packaging, Screen and
sewing thread, Magnetic tape, X-ray and photographic film, Electrical insulation tapes, Food
packaging, Blow molded containers, Appliance bases, Medical device packaging
Examples of PET

8. Polystyrene: It is a polymer of styrene. Polystyrene is a colourless, transparent thermoplastic. It is
resistant to acids, alkalise, oils, and alcohols. It is produced either as a solid or as a foamed plastic
marketed under the trade name Styrofoam. High impact resistance, flame retardant, food contact
acceptable, good proccesability, high heat resistance, expandable, fast moulding cycle.
Uses :- Its many uses include electrical and thermal insulation, translucent window and signage panels,
storage-battery cases, toilet articles, display signage, disposable spoons, forks, glasses etc.
Example of Styrofoam Disposable Cups

9. ABS (Acrylonitrile butadiene styrene) :- A class of thermoplastic terapolymers including a range of
resins, all prepared with usually more than 50% styrene and varying amounts of acrylonitrile and
butadiene. Impact resistance and toughness, ability to be injection moulded and extruded, ability to be
coated like steel.
Uses :- Telephone casings, housing for domestic appliances e.g. vacuum cleaners, TV cabinets, car fascia
and instrument panels, furniture, food mixture housings, automobile radiator grills, refrigerator
door and tank liners, pipe systems, musical instruments, automotive bumper bars, buffer edging
for furniture, luggage and protective carrying cases, small kitchen appliances, and toys.
Examples of ABS in Automobile parts

Industrial Application of ABS :-
Door Glossy part Rapid Prototype parts (ease of machining)
PP Part ABS Part

10. High impact polystyrene (HIPS) :- Widely used plastic; it is a polymer of styrene. Polystyrene is a
colourless, transparent thermoplastic that softens slightly above 100°C and becomes a viscous liquid at
around 185°C . It is resistant to acids, alkalis, oils, and alcohols. It is produced either as a solid or as a
foamed plastic marketed under the trade name Styrofoam. HIPS is a versatile, economical and impact-
resistant plastic that is easy to machine and fabricate.
Uses:- Its many uses include electrical and thermal insulation, translucent window and signage panels,
storage-battery cases, zippers, display signage, Refrigerator liners, food packaging.
Examples of HIPS Zips made of HIPS

Industrial Application of HIPS :-
Apps of HIPS in Refrigerator Refrigerator Liner

11. General Purpose Polystyrene (GPPS) :-
Properties:- Excellent Transparency
Good Dimensional Stability
Ease of Injection Molding
Alternative for glass.
Industrial Applications of GPPS :-
Ref Door Shelf Ice Box Freezer Grill

12. Expandable Polystyrene (EPS) :-
Properties:- Light Weight (93% Air composition)
Excellent Durability
Thermal Insulation
Shock absorption.
Industrial Applications of EPS :-
Packaging & Construction
SS Freezer
Shroud(Providing air path
and Insulation from Eva)
Freezer

Plastic forming process
The main plastic forming process are :-
1. Injection molding
2. Extrusion
3. Blow molding
4. Vacuum/Pressure forming or thermoforming
5. Compression and Transfer Molding

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 mold.
 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. The simplest form of injection
molding is shown in the schematic below.

Some examples of injection molded parts :-

Extrusion
 Extrusion can be used for thermoplastics. The raw material is in the form of pellets (~10mm sized
pieces), granules (~5 mm), or powder.
 Extrusion machines are used to make long pieces of constant cross-section. The cross-section geometry
can be solid or hollow, and may be quite complex in shape.
 Usually, extruded parts are used as raw stock for use in manufacture of other products (e.g. channels on
the sides of windows, etc. You can find plastic extruded parts in many bathroom and kitchen fittings).
Figure below shows a typical extrusion machine.

Blow molding
 Blow molding is a manufacturing process by which hollow plastic parts are formed.
 The blow molding process begins with melting down the plastic and forming it into a parison. The
parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.
 The parison is then clamped into a mold and air is blown into it. The air pressure then pushes the plastic
out to match the mold. Once the plastic has cooled and hardened the mold opens up and the part is
ejected.
Simple blow molding process

Examples of Blow molded parts :-

Thermoforming
 Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming
temperature, formed to a specific shape in a mold, and trimmed to create a usable product.
 This process is also known as vacuum forming.
 In an alternate form, called pressure forming, the pressure is applied using high pressure air from above
the plastic sheet.

Examples of Thermoformed products :-

Compression Molding
 This process is used mostly for thermosetting polymers.
 In compression molding, the raw material is placed inside the mold in semi-solid or solid (i.e. as granules,
or a single piece called a plug). The mold is heated and closed using pressure, and the plastic flows to fills
the cavity.
 If the part shape is more complex, transfer molding may be used. Here, the charge are placed in a heated
cylinder till they are soft; a hole at the bottom of the cylinder is connected to the die cavity by a sprue.
 A plunger pushes the semi-solid plastic into the die through the sprue, using high pressure.
Compression molding process Transfer molding process

Examples of compression and Transfer molding process :-

PREFACE TO INJECTION MOLDING :-
 Injection molding is a manufacturing technique for making parts from polymer materials. By injecting
molten plastics into mold, it enables us to produce variety of products.
 Injection molding is widely used in industry production, from the smallest
component to entire body panels of cars.
This process of injection molding is divided in four units namely :-
 INJECTION UNIT
 CLAMPING UNIT
 CONTROL UNIT
 MOLD UNIT
Now we will discuss mold unit in detail .

FUNCTIONS OF MOLD UNIT:-
 Accommodation and Distribution of the Melt
 Shaping of the Molded Part
 Cooling/Heating and Solidification of the Melt
 Ejection ( De-molding ) of the Molding
 Mechanical Functions
 Accommodation of forces
 Transmission of motion
 Guidance of the mold components
PARTS OF MOLD UNIT:-
 Runner
 Gate
 Cavity
 Plates
 Cooling channels
 Ejector pin

DELIVERY SYSTEM:-
The function of the delivery system is to provide a channel for the molten
polymer from injection unit to the molding cavity.
UNITS IN DELIVERY SYSTEM:-
 SPRUE
 RUNNER
 GATE
SPRUE :-
It is a channel which transfers molten
plastics injected from the injector
nozzle into the mold.

RUNNER SYSTEM:-
A runner is a channel that guides molten plastics into
the cavity.
They are of two types:-
COLD RUNNER
It is the general runner which is not heated
only the hot melt from injector flows in it
Advantages
1. Simple and cheap
2. Less maintenance and less skill to
set up and operator
Disadvantages
1. Trimming process for runner
2. Waste plastic generated
3. Recycled plastic may decrease
the plastic's mechanical
properties

HOT RUNNER :-
In this type of runner system the
runner is heated throughout by heating
elements.
Advantage :-
 No runners to sort from the parts
 Eliminate the trimming process.
 Less regrind is generated.
 Less plastics is heated in each cycle.
 Less cooling is required.
Disadvantage :-
 More expensive than cold runner
 Requires costly maintenance
 Requires more skill to operate

TYPE OF RUNNER CROSS SECTION :-
Runner cross section that minimizes liquid resistance and temperature reduction when molten plastics flows
into the cavity.
A. Circular Runner-It provides a maximum volume-to-surface ratio
and hence offers the least resistance to flow and heat loss from
the runner.
B. Parabolic Runner-It is the next best thing to circular runner but
is much easier to work with.
C. Trapezoidal Runner-It is similar to parabolic and mostly used
in 3 plate system.
The remaining two are rarely used because or their large
resistance to the flow of molten polymers.
D. Half Round Runner
E. Rectangular Runner

GATE:-
DEFINATION AND FUNCTIONALITY :-
Gate provides the connection between the runner and the mold cavity. It must permit enough material to
flow into the mold to fill out the cavity, raises melt temperature by viscous (frictional) heating, and freezes-
off when the holding stage is over. It should be smaller in the cross section so that it can be easily separated
from the molded part ( degated ).
Gating scheme and location of gates are crucial to the quality of the molding. Filling pattern and cavity
pressure profile are closely related to the final properties of molded parts, such an mechanical properties,
cosmetics (surface properties), dimensional accuracy. A gate should provide appropriate filling pattern and
heating effect, permit effective packing and holding of the material within the mold.

SOME COMMONLY USED GATING SCHEMES :-

Mold plate types:-
Injection molding can be classified based on the number of plates used.
 TWO PLATE MOLDING SYSTEM :-
This consists of two halves fastened to the two platens of the molding
machine's clamping unit. When the clamping unit is opened, the mold
halves separate. Molds can contain one multiple cavities to produce one
or multiple parts in a single shot . The parting surface is the surface
shared by the two mold halves .

 THREE PLATE MOLD SYSTEM :-
This design has some advantages. The molten plastic flows through a gate located at the base of the cup-
shaped part, rather than at the side. This allows more even distribution of melt into the sides of the cup. In
the side gate design in the two-plate the plastic must flow around the core and join on the opposite side,
possibly creating a weakness at the weld line. Secondly, the three-plate mold allows more automatic operation
of the molding machine. As the mold opens, the three plates separate; this forces the runner to break from
the parts, which drop by gravity or using air-blower into collecting containers put under the mold.

It is the process of removal of molded parts from the mold by the use of
mechanical method.
 EJECTOR PIN:
These are pins situated on the mold which push the parts out when
the mold opens.
Ejection pins
Three plates mold Two plates mold
Ejection system

THE MOST DIFFICULT TO SOLVE QUALITY PROBLEMS ARE:-
 Parts have Dark Specks
 Blisters and/or Bubbles
 Flow Marks
 Burn Marks
 Delaminating
 Dimensional Irregularities
 Parts are Discolored
 Flash
 Jetting
 Pitted
 Rough
 Sticking to Cavity
 Sticking to Core
 Parts are Short
 Sinks or Voids
 Parts Display Splay
 Parts are Streaked
 Parts are Stringing
 Warped
 Weld Lines

PARTS HAVE DARK SPECKS:-
The finished product contains dark particles.
(When transparent resins are usually used.)
Probable Cause:
 Presences of impurities in raw material.
 Barrel offline for extended period
 Barrel improperly purged
 Contamination in plasticizer
 Gate and/or runner has dead spots
Solution (In sequence):
 Purge system with appropriate material
 Trace source of contamination and repair, remove
or discard
 Adjust melt temperature if necessary

BLISTERS AND BUBBLES :-
The finished product contains small gas
or air filled pockets or cooling voids.
Probable Cause
 Low Injection Pressure
 Overheating of resin
 Suck-back cycle too long
 Rapid plasticizing
 Trapped air in feed
 Feed error
 Low mold temperature
Solutions
 Increase back pressure
 Increase mold temperature
 Inspect back flow valve
 Ensure proper venting
 Increase gate size
 Decrease vent land length

FLOW MARKS :-
The finished product exhibits blush and flow marks, bushing. Cold material in the nozzle tip section
results in a halo around the direct sprue. Due to variations in material temperature from gradients
between the machine nozzle and mold sprue
Probable Cause
 Wrong injection speed
 Wrong injection pressure
 Hold pressure too long
 Insufficient mold cooling
 Temperature of mold too high around the gate
 Temperature of mold too cool
 Melt temperature is too low
Solution
 Adjust injection speed
 Add a large cold slug area
 Add cold wells at the end of the
runner system
 Use hot sprue bushing
 identify and eliminate dead
pockets /sections

BURN MARKS :-
The finished product display brown streaks. This is from the material being overheated
due to trapped air (diesel effect), which can lighten or darken the colour.
Probable Cause:
 High injection speed
 Backflow/check ring valve malfunctioning
 High back pressure
 Overheated/under-heated melt, possible shear
Solution
 Lower injection speed
 Lower injection pressure
 Check heater functionality
 Reduce feed screw rotation
 Reduce melt temperature

PARTS ARE DISCOLORED:-
Probable Cause
 Contamination
 Incorrect sprue diameter
 Inadequate venting
 Physical contamination of raw
material
 Chemical contamination of raw
material
 Incompatible color dye
 Residence time too high
Solution
 Heating cylinder purged
 Residence time adjusted
 Cycle time adjusted
 Check for external contamination
sources
 Ensure proper cooling in all areas.

PARTS CONTAIN FLASH :-
Also known as "Fins" or "Spew“ The finished product contains a thin film of material attached at the
mold parting line.
Probable Cause:
 Low clamping pressure
 High injection pressure
 Inadequate mold supports
Solution:
 Reduce the injection speed
 Reduce the injection pressure
 Reduce the injection time
 Increase clamping force

PARTS SURFACE HAS JETTING :-
The finished product exhibits serpentine flow patterns on the surface as a result of the melt
cooling prior to complete filling of the mold
Probable Cause
 Cold mold
 Small gate
 Wrong gate land length
 Wrong gate location
 Cold melt
Solution
 Decrease injection speed
 Verify nozzle temperature
 Increase mold temperature
 Increase melt temperature
 Increase gate size
 Modify gate location

PARTS ARE PITTED :-
The finished product contains unmelted particles or small holes on the surface
Probable Cause:
 Improper or worn out feed screw
 Low melt temperature
 Low injection speed
 Resin used is not homogeneous
Solution:
 Reduce back pressure
 Reduce injection speed
 Modify temperature
 Modify regrind ratio
 Modify shot size
 Inspect hot runner and nozzles

PARTS ARE WARPED :-
The finished product has pressure differences/stress on its surface, causing the part to be disfigured
Probable Cause:
 Wrong cooling time
 Undercuts too big
 Cavity too hot
 Incorrect material
Solution
 Verify part weight following ejection
 Increase injection hold time
 Increase cooling time
 Adjust injection pressure
 Check gate dimensions, quantities and locations
 Re-design part if necessary

UNIFORM WALL THICKNESS:-
Generally range from 2 mm to 4 mm. Thin wall
injection molding ~ 0.5 mm.
 Avoids warpage and build-up of stress.
 Easier to fill in the mold cavity .
 Any transitions should be uniform .
 Thinner wall faster cooling of part,
short cycle times =>lowest part costs
Uniform wall thickness eliminates air entrapment.
Design Considerations

CORING OUT:-
 Coring out helps maintain uniform wall thickness
 Leads to lesser usage of material
 Often stronger design
 Lesser cost
Coring out aids in eliminating air entrapment

CORNERS:-
 Fillet radiuses provide streamlined flow paths for the molten plastic.
 Sharp corners greatly increase the stress concentration .

PARTING LINES:-
 Parting line is the place at which the core and cavity meet.
 More specifically the moving and stationery parts of the mold .
 The part should be designed so as to present a good parting
line to the mold maker.
 A flash is usually produced at the P/L and has to be finished
manually or mechanically.
 If the part is aesthetically important the P/L should be
concealed on a thin inconspicuous edge of the part.
Parting line selection

DRAFT:-
 Draft is the angle between the direction of ejection
of a part from the mold and the surface of the part
 Facilitates part removal from the mold
 General draft 0.5 - 2 degrees
 Additional 1.5 degrees per 0.25mm depth of texture
Example of using multiple drafts
to permit ease of part removal
Using parallel drafts with heavy walls.

UNDERCUTS:-
 Undercuts are any design features that inhibits the normal
opening of the mold
 Inside undercuts can be made with removable wedges or
ejector pins and ejector wedges
 Undercuts can be avoided by simple changes in design of part
 Complicated undercuts need special mechanisms in mold

BOSSES:-
Bosses are projections on a plastic part designed to
 Add strength
 Facilitate alignment
 Attach fasteners such as screws or accept
threaded inserts.
Wall thickness < 60 % of nominal wall
Isolate bosses to eliminate sinks

RIBS:-
 Ribs increase the bending stiffness of a part.
 Rib thickness should be ~60% the wall thickness.
Examples of rib design characteristics.

THREAD AND INSERTS :-
 Threads can be molded or tapped into a plastic.
 Internal threads usually need a rotary action in the mold.
 External threads can be molded splitting the mold.
 Screw threads should be of the coarse type and have
the outside of the thread rounded.
 Metal inserts can be used for quality threads.

SNAP FIT :-
 Snap fits rationalizes the assembly of plastic parts
 Takes over other functions like bearing, spring
cushioning, fixing
 Fixing of the joined parts is weaker than in welding,
bonding, and screw joining
Examples of different snap-fit designs

LIVING HINGES :-
 Materials such as Polypropylene can be molded
with a living hinge
 Direction of flow of material should be perpendicular
to hinge
 Used primarily in packaging products but can have
engineering applications too

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