manufacturing with plastics

IT 208 Chapter 18 1

Manufacturing with Plastics

Chapter 18

IT 208 Chapter 18 2

Competencies Define key terms associated with polymers and plastics and

describe the difference between the two. Differentiate between thermoplastics and thermosets, and

identify the use for each. Identify the correct plastics process used to make uniquely

different products Define the two processes of polymerization Differentiate between linear, branched, and cross-linked

polymers

IT 208 Chapter 18 3

IT 208 Chapter 18 4

Chemistry of PolymerizationThe carbon atom must always have four covalent

bonds. The four noted carbon compounds are• Methane – has one carbon atom bonded to four

hydrogen atoms• Ethane – two carbon atoms with six hydrogen atoms• Propane – three carbon atoms with eight hydrogen

atoms• Butane – four carbon atoms with 10 hydrogen atoms

IT 208 Chapter 18 5

Chemistry of PolymerizationEvery carbon atom must have four bonds, but it is possible for two

or three of the bonds to be between the same two carbon atoms. • If two of the hydrogens are removed from the ethane

molecule, the bonds that went to the hydrogen will simply form a new double bond between the carbons. (p.340)

When double bonds are formed • the once “ane” suffix is changed to “ene”• triple bond is formed the “yne” suffix is used.

So we have the progression of: Ethane to Ethene to Ethyne (Acetylene)

IT 208 Chapter 18 6

IT 208 Chapter 18 7

Chemistry of Polymerization

An unsaturated molecule is any compound having double or triple bonds. This unsaturated state enables the compound to be reacted with itself, which is the key to polymerization.

IT 208 Chapter 18 8

Chemistry of Polymerization A molecule will polymerize if

• it has at least two reaction points• maintains at least two reaction points after

each joining of the compound. Polymerization is when these bonds are

opened, they can react with another ethane molecule and continue reacting to form chains containing thousands of carbon atoms.

IT 208 Chapter 18 9

Chemistry of Polymerization This type of polymerization is called addition, chain-

growth or chain reaction polymerization. Condensation polymerization also known as step-

growth or step-reaction. • One characteristic of this reaction is that reaction by-

products such as water are condensed out.

IT 208 Chapter 18 10

PolymersThe reactions form what is know as polymers.

Polymers are grouped into 3 categories Thermoplastic polymers (TP)

• represents 70% of the tonnage of all synthetic polymers produced.

• Can be easily and economically shaped into products• Can be subjected to this heating and cooling cycle

repeatedly without significant degradation of the polymer.

Common TP polyethylene, polyvinylchloride, polypropylene, polystyrene, and nylon


Thermoplastic(linear) - Soften repeatedly when heated (straight chains) PET

• Polyethylene Terphalate (Beverage Containers) HDPE

• High Density Polyethylene (Milk bottles, detergent bottles) PVC

• Polyvinyl Chloride (Food wrap, vegetable oil bottles) LDPE

• Low Density Polyethelene (Shrink wrap, plastic bottles) PP

• Polypropylene (Margerine and yogurt containers) PS

• Polystyrene (Egg cartons, fast food trays)


Polymers Thermosets polymers (TS)

• cannot tolerate repeated heating cycles, when initially heated, they soften and flow for molding but the elevated temperatures also produce a chemical reaction that hardens the material into an infusible solid.

• toaster parts, automotive engine parts, electrical outlet parts, handles on pots and pans, ash trays

Elastomers (rubbers) • exhibit extreme elastic extensibility when subjected to low

mechanical stress.


Polymers Plastics

• Polymers compounded with various additives. Additives

• Agents that enter the molecular structure and are designed to change its properties (Antioxidants, flame retardants, lubricants, etc.)

Fillers • Incorporated to improve mechanical properties, often

called reinforcing agents. (Increase bulk, stiffness, etc.)


POLYMER STRUCTURES AND COPOLYMERSThere are structural differences among polymer

molecules. There are three aspects of molecular structure

1. Stereoregularity 2. Branching and cross-linking 3. Copolymers. We will discuss only #2 and 3


POLYMER STRUCTURES AND COPOLYMERSStereoregularity

• Concerned with the spatial arrangement of the atoms and groups of atoms in the repeating units of the polymer molecule

Branching and cross-linking (see overhead) Linear polymers

• Because their structure is linear. Generally, a polymer consists of more than one type of structure.

• Thus a linear polymer may contain some branched and cross-linked chains. (This is the characteristic of a thermoplastic polymer)


POLYMER STRUCTURES AND COPOLYMERS Branched polymer

• side branches chains are attached to the main chain during the synthesis of the polymer.

• Branching interferes with the relative movement of the molecular chain. As a result, resistance to deformation and stress-crack resistance are affected.

• Branched polymers are like a pile of tree branches compared to straight logs of linear chains


POLYMER STRUCTURES AND COPOLYMERSCross-linked polymers

• when primary bonding occurs between branches and other molecules at certain connection points.

• Lightly cross-linked structures are characteristic of elastomers.

• When the polymer is highly cross-linked it considered thermoset.


POLYMER STRUCTURES AND COPOLYMERS


POLYMER STRUCTURES AND COPOLYMERS Copolymers

• Polymers whose molecules are made of repeating units of two different types.

• Also possible are terpolymers , which consist of mers of three different types. (ex. ABS or acrylonitrile-butadiene-styrene)


CrystallinityAmorphous and crystalline structures are possible with

polymers, although the tendency to crystallize is much less than for metals and nonglass ceramics.

Degree of crystallinity (the proportion of crystallized material in the mass)• As crystallinity is increased in a polymer, so does

• Density• Stiffness, strength, and toughness• Heat resistance


CrystallinityCrystallization in linear polymers involves the folding back and

forth of the long chains upon themselves to achieve a very regular arrangement of the mers

A number of factors determine the capacity and/or tendency of a polymer to form crystalline regions within the material.• As a general rule, only linear polymers can form crystals• Stereoregularity of the molecule is critical• Copolymers, due to their molecular irregularity, rarely form

crystals• Slower cooling promotes crystal formation and growth


Crystallinity


Thermal & Mechanical Properties


Manufacturing With PlasticsClassification by process rather than product or material is

more relevant, because most processes are suitable for making products from a large variety of plastics

Casting - filling a mold by gravity• 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)


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)


Molding ProcessExtrusion

• Pellets, granules or powder is placed into a hopper and fed into the extruder barrel.

• As screw in the barrel that turns it blends and moves the material down the barrel.

• Material then forced through a die that is in desired shape. It is cooled by water or air and cut to length.

• Largest production volume of plastics (bar, tube, sheet, film)

• Rated by the length to diameter ratio of the screw


Extruder barrel and screw


Molding ProcessProcess can produce

• Solid profiles• Hollow profiles• Wire and cable coating

Defects in Extrusion• Melt Fracture• Sharkskin• Bambooing


Defects in Extrusion


Molding ProcessInjection 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


Injection Molding


Molding Process Defects in Injection Molding

• Short Shots• Flashing• Sink marks and voids• Weld lines

Molds in injection molding are usually expensive and complex


Molding ProcessOther molding techniques: Thermoplastic Foam Injection Molding – involves the

molding of thermoplastic parts that possess a dense outer skin surrounding a light weight foam center.

Multi-Injection Molding: • Sandwich – injection of two separate polymers• Bi-injection – used to combine plastics of two different

colors or to achieve different properties in different sections of the same part.


Molding Process Reaction Injection Molding (RIM) – Two reactant

liquids are heated and brought together under high pressure

Compression molding – Pre-measured amount of polymer introduced into the heated mold then the top half comes down and applies pressure. Usually uses thermosetting plastics and produces products like dishes, container caps, etc.


Reaction Injection Molding


Compression molding


Transfer Molding


Molding ProcessBlow Molding

• Extruded tube or preform is expanded by internal pressure (most bottles)

Extrusion blow molding • continuous tube is extruded, pinched off, placed in a

mold and blown into shape Injection blow molding

• part is injected into a mold then transferred and blown into shape


Molding ProcessThermoforming - Heating a thermoplastic sheet and

using a vacuum to pull the sheet over perforated mold. • Fast food containers, advertising signs, panels for

shower stalls• Hot air pressure and vacuum forming - drive sheet into

female die cavity

manufacturing with plastics

Documents

thousands of carbon

double bonds

chain reaction polymerization

chemical reaction

reaction byproducts

reaction points

triple bonds

covalent bonds