Transcript Manufacturing with Plastics - College of Engineering | SIU

Manufacturing with
Plastics
Chapter 18
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Chapter 18
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Competencies
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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
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Chemistry of Polymerization
The carbon atom must always have four covalent
bonds. The four noted carbon compounds are
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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
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Chemistry of Polymerization
Every 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)
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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.
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Chemistry of Polymerization
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A molecule will polymerize if
• it has at least two reaction points
• maintains at least two reaction points after
each joining of the compound.
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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.
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Chemistry of Polymerization
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This type of polymerization is called addition, chaingrowth or chain reaction polymerization.
Condensation polymerization also known as stepgrowth or step-reaction.
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One characteristic of this reaction is that reaction byproducts such as water are condensed out.
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Polymers
The reactions form what is know as polymers.
Polymers are grouped into 3 categories
 Thermoplastic polymers (TP)
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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
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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)
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Polymers
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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
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Elastomers (rubbers)
• exhibit extreme elastic extensibility when subjected to low
mechanical stress.
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Polymers
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Plastics
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Polymers compounded with various additives.
Additives
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Agents that enter the molecular structure and are
designed to change its properties (Antioxidants, flame
retardants, lubricants, etc.)
Fillers
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Incorporated to improve mechanical properties, often
called reinforcing agents. (Increase bulk, stiffness,
etc.)
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POLYMER STRUCTURES
AND COPOLYMERS
There 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
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POLYMER STRUCTURES
AND COPOLYMERS
Stereoregularity
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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
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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)
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POLYMER STRUCTURES
AND COPOLYMERS
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Branched polymer
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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
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POLYMER STRUCTURES
AND COPOLYMERS
Cross-linked polymers
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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.
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POLYMER STRUCTURES
AND COPOLYMERS
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POLYMER STRUCTURES
AND COPOLYMERS
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Copolymers
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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)
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Crystallinity
Amorphous 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)
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As crystallinity is increased in a polymer, so does
• Density
• Stiffness, strength, and toughness
• Heat resistance
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Crystallinity
Crystallization 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
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Crystallinity
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Thermal & Mechanical Properties
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Manufacturing With Plastics
Classification 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)
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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)
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Molding Process
Extrusion
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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
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Extruder barrel and screw
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Molding Process
Process can produce
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Solid profiles
Hollow profiles
Wire and cable coating
Defects in Extrusion
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Melt Fracture
Sharkskin
Bambooing
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Defects in Extrusion
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Molding Process
Injection molding
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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
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Injection Molding
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Molding Process
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Defects in Injection Molding
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Short Shots
Flashing
Sink marks and voids
Weld lines
Molds in injection molding are usually expensive and
complex
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Molding Process
Other 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:
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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.
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Molding Process
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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.
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Reaction Injection Molding
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Compression molding
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Transfer Molding
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Molding Process
Blow Molding
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Extruded tube or preform is expanded by internal
pressure (most bottles)
Extrusion blow molding
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continuous tube is extruded, pinched off, placed in a
mold and blown into shape
Injection blow molding
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part is injected into a mold then transferred and blown
into shape
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Molding Process
Thermoforming - Heating a thermoplastic sheet and
using a vacuum to pull the sheet over perforated
mold.
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Fast food containers, advertising signs, panels for
shower stalls
Hot air pressure and vacuum forming - drive sheet into
female die cavity
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