Plastics Reference book

CHAPTER CHAPTER NAME

1. Plastics

History

Composition

Polymerization process

Composition

Properties of plastics

Common plastics & uses

Different manufacturing processes for Plastics:

Injection Moulding.

Compression Moulding.

Blow Moulding.

Compression Moulding.

Plastics Extrusion.

Thermoforming.

Slush Moulding.

Transfer moulding.

Calendaring.

Gate type

Design Rules For Plastic Parts:

Maximum wall thickness

Corners

Draft

Ribs

Bosses

Undercuts

Threads

TYPICAL NOMINAL THICKNESS FOR VARIOUS

CLASSES OF THERMOPLASTICS

DEFECTS of PLASTICS

Welding Techniques

Hot gas welding

Heat seal

Freehand welding

Speed tip welding

Extrusion welding

Contact welding

Hot plate welding

High frequency welding

Injection welding

Ultrasonic welding

Friction welding

Spin welding

Laser welding

Transparent Laser Plastic Welding

Solvent welding

Welding rod

PLASTICS Growing demand in the automotive sector

Plastics In Automotive Markets Today

Abbreviations

PLASTICS:

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids

that are mouldable. Plastics are typically organic polymers of high molecular mass, but they

often contain other substances. They are usually synthetic, most commonly derived from

petrochemicals, but many are partially natural.

History

Early plastics were bio-derived materials such as egg and blood proteins, which are

organic polymers. Treated cattle horns were used as windows for lanterns in the Middle

Ages. Materials that mimicked the properties of horns were developed by treating milk-

proteins (casein) with lye. In the 1800s the development of plastics accelerated with Charles

Goodyear's discovery of vulcanization as a route to thermoset materials derived from natural

rubber. Many storied materials were reported as industrial chemistry was developed in the

1800s. In the early 1900s, Bakelite, the first fully synthetic thermoset was reported by

Belgian chemist Leo Baekeland. After the First World War, improvements in chemical

technology led to an explosion in new forms of plastics. Among the earliest examples in the

wave of new polymers were polystyrene (PS) and polyvinyl chloride (PVC). The

development of plastics has come from the use of natural plastic materials (e.g., chewing

gum, shellac) to the use of chemically modified natural materials (e.g., rubber,

nitrocellulose, collagen, galalite) and finally to completely synthetic molecules

(e.g., bakelite, epoxy, polyvinyl chloride).

Bakelite

The first so called plastic based on a synthetic polymer was made

from phenol and formaldehyde, with the first viable and cheap synthesis methods invented in

1907, by Leo Hendrik Baekeland, a Belgian-born American living in New York state.

Baekeland was looking for an insulating shellac to coat wires in electric motors and

generators. He found that combining phenol (C6H5OH) and formaldehyde (HCOH) formed a

sticky mass and later found that the material could be mixed with wood flour, asbestos, or

slate dust to create strong and fire resistant "composite" materials. The new material tended

to foam during synthesis, requiring that Baekeland build pressure vessels to force out the

bubbles and provide a smooth, uniform product, as he announced in 1909, in a meeting of the

American Chemical Society.[12]

Bakelite was originally used for electrical and mechanical

parts, coming into widespread use in consumer goods and jewelry in the 1920s. Bakelite was

a purely synthetic material, not derived from living matter. It was also an early thermosetting

plastic.

ASTM: American Standard Test Methods; is a scientific organisation defining standards on

physical and mechanical testing of materials to obtain objective characteristics used for

comparison purposes and for design of articles. The standards are partly used to formulate

ISO (International Standards Organisation) ones.

Composition

Most plastics contain organic polymers. The vast majority of these polymers are

based on chains of carbon atoms alone or with oxygen, sulphur, or nitrogen as well. The

backbone is that part of the chain on the main "path" linking a large number of repeat units

together. To customize the properties of a plastic, different molecular groups "hang" from the

backbone (usually they are "hung" as part of the monomers before linking monomers

together to form the polymer chain). The structure of these "side chains" influence the

properties of the polymer. This fine tuning of the properties of the polymer by repeating unit's

molecular structure has allowed plastics to become an indispensable part of the twenty-first

century world.

Additives

Additives: these are in general low molecular weight chemicals added to plastics and

rubbers to improve certain characteristics such as ultraviolet absorbers, antioxidants and heat

stabilisers, lubricants, plasticisers, flame retardants, cross-linking and blowing agents,

pigments and dyes. Impact modifiers are polymeric materials added to improve the impact

resistance of e.g. PVC, PP, PBT, PA. A separate class of additives are the fillers such as

talcum, wood flour, and reinforcing agents like glass and carbon fibres.

Alloys: strictly speaking, alloys refer to metals and do not exist in plastics. The term is used

interchangeably with blends for mixtures of two or more polymers. Examples are alloys or

blends of polycarbonate (PC) with ABS or with polybutylene terephthalate (PBT).

Blend" an intimate mixture of two or more polymers to obtain the good properties of each,

for example semi-crystalline polypropylene (PP) mixed with 10 to 30% rubbery EPDM

results in a blend with good heat resistance and extraordinary impact resistance. Also the mix

of polycarbonate (PC) with ABS terpolymer results in a blend with the good heat resistance

of the PC part and the low temperature impact resistance of the ABS. Instead of the term

blend, trade literature and producers also use alloy. Blends are made passing the components

in powder or pellet form in a dry blender followed by a heated twin screw extruder to obtain

an intimate blend.

Polymerization process Plastics are one group of polymers that are built from relatively simple units called

monomers (or mers) through a chemical polymerization process. This process is illustrated

below. Processing polymers into end products mainly involves physical phase change such as

melting and solidification (for Thermoplastics) or a chemical reaction (for Thermosets).

Structure of polymers The basic structure of a polymer molecule can be visualized as

a long chain of repeating units, with additional chemical groups forming pendant branches

along the primary "backbone" of the molecule. Although the term plastics has been used

loosely as a synonym for polymer and resin, plastics generally represent polymeric

compounds that are formulated with plasticizers, stabilizers, fillers, and other additives for

purposes of processability and performance. Other polymeric systems include rubbers, fibers,

adhesives, and surface coatings. A variety of processes have been employed to produce the

final plastic parts,

Polymer family, the formation of plastics, and the polymerization process

Classification

Plastics are usually classified by their chemical structure of the polymer's backbone

and side chains. Some important groups in these classifications are

the acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. Plastics can also

be classified by the chemical process used in their synthesis, such as condensation, poly-

addition, and cross-linking.

There are two types of plastics: thermoplastics and thermosetting polymers.

Thermoplastics are the plastics that do not undergo chemical change in their composition

when heated and can be molded again and again.

Thermoplastics

A Thermoplastic, also known as a thermosoftening plastic, is a polymer that

becomes pliable or moldable above a specific temperature, and returns to a solid state upon

cooling. Most thermoplastics have a high molecular weight, whose chains associate

through intermolecular forces; this property allows thermoplastics to be remolded because the

intermolecular interactions spontaneously reform upon cooling. In this way, thermoplastics

differ from thermosetting polymers, which form irreversible chemical bonds during the

curing process; thermoset bonds break down upon melting and do not reform upon cooling.

Thermoplastic materials can be formed into desired shapes under heat and pressure and

become solids on cooling.

If they are subjected to the same conditions of heat and pressure, they can be reprocessed

into new shapes.

Thermoplastics based on their crystallization are classified into

Amorphous Thermoplastics

Some thermoplastics donot crystallise on heating and are termed as amorphous plastics

Used in applications where clarity is important.

They a