Processing Chapter 3
Compression Molding
Introduction
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In 1907 Dr. Leo Baekeland achieved a reaction
between Phenol, a caustic, crystalline acidic
compound and Formaldehyde, a colorless,
pungent gas, by heating these ingredients
under pressure in a reaction kettle.
The resultant, hot liquid when cooled, became
a hard brittle solid. It lacked physical strength,
but possessed excellent thermal, chemical and
electrical insulating properties
Introduction
• The hard brittle resin in a pulverized from
could be mixed with fillers, reinforcement,
colorants, lubricants and catalyst. The end
product was a molding compound capable
of further processing on heated mills and
calendar mills.
• Thus the first thermosetting molding
compound was created.
Introduction
• In the following years additional thermoset
resin systems were developed to meet the
requirements of diverse applications in
automotive, communications, construction,
transportation, household appliances,
business machines, aircraft and
aerospace fields.
Thermoset Definition:
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A polymeric material which can be
formed by the application of heat and
pressure, but as a result of chemical
reaction permanently crosslinks and
cannot be reformed upon further
application of heat
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Chemical crosslinking must occur for the
resultant product to be called a thermoset.
Monomeric precursors may or may not be
polymeric
Thermoset Families
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Allyls
Aminos
Epoxies
Phenolics
Polyesters
Polyurethanes
Silicones
Crossed linked thermoplastics
Classifications
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General Purpose – Average mechanical
properties, lower resistance to heat, higher
coefficient of expansion, low cost and
commodity production.(Phenolics, Aminos and
polyesters)
Engineering – Higher mechanical properties
and temperature resistance and more durable.
More expensive with moderate production
rates. (Epoxies and Polyurethanes)
Classifications
• Specialty – one or more highly specific
and unusual properties. Very expensive
and produced in small quantities.
(Silicones, Allyls, Crossedlinked
Thermoplastics)
General Properties
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Thermal and dimensional stability
Electrical insulation
Chemical resistance
High flexural modulus
Toughness
Surface hardness
Moldablilty
Cost effectiveness
Phenolics
Introduction
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First thermosetting plastic, first synthetic
commercially available plastic resin
Produced by chemical reaction between
phenol (resin) and formaldehyde (curing
agent)
First made by Dr. Leo Baekeland in 1907
By far the most widely used of all
thermosets for molding applications
Chemistry
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Phenol – Formaldehyde resins are general
purpose thermosets formed mainly by the
polycondensation reaction between phenol
and formaldehyde
Resole process, other main process is
Novalac
Raw Materials
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Phenol
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Show phenol structure
Formaldehyde
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Show formaldehyde structure
Phenol
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Cumene process for making phenol
Formaldehyde
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Formaldehyde is produced by the
controlled catalytic oxidation of methanol
Phenolic Reaction
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Phenolic resin reaction process
Phenolic curing process
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The curing process to the final thermoset
material can be initiated by just heating the
resin in a mold above its gel temperature
When heated, the resin forms larger
molecules without the use of additional
catalyst
The reaction is a typical polycondensation,
since water is given off as a by product
Phenolic curing process
Processes that use Phenolics
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Compression Molding
Transfer molding
Injection Molding
Properties of Phenolics
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Physical
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Excellent dimensional stability
Low water absorption
High surface hardness
High creep resistance
Mechanical
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High compressive strength
High modulus, stiffness
Properties of Phenolics
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Thermal
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Phenolics retain a high percentage of properties
at elevated temperatures
Low coefficient of thermal expansion
Good heat resistance
Properties of Phenolics
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Electrical
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Chemical
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Good electrical resistance
Good chemical resistance
Other
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Good weather resistance
Easily molded
Good machining properties
Applications for Phenolics
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Appliances
Closures
Electrical
Housewares
Industrial
Transportation
17%
7%
38%
23%
4%
11%
Molding and Molding
Techniques
Introduction
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There are three fundamental molding
processes for thermoset compounds.
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Compression molding
Transfer molding
Injection molding
The process used will be determined by
which process can produce the part to
appropriate specifications at the most
economical rate
Production Methods and
Equipment
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The choice of methods and equipment
for use in the production of thermosetting
molding compounds is usually
determined by the type of reinforcement
being used
Reinforcements are added to thermosets
to improve properties. Impact, flexural
strengths, shrinkage, etc.
Compression Molding
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Introduction
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Uses a hydraulically operated press
Press consists of two heated mold platens, one
stationary and one movable
Platens have either “T” slots or tapped holes to fix
the mold halves to the platens
Tie Rods, usually 4, provide for precise alignment
as the press is opened and closed
The press open and shut height and the platen
temperature are controlled.
Compression Molding
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Compression presses are rated by their
closing force capacities
Can be manual, semiautomatic or fully
automatic
Preform temperature, molding temperature,
molding pressure, molding time and cooling
time are the most important design
parameters
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Cavity depth is very important to achieve the proper
molded density
Transfer Molding
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Similar to compression molding
Four basic types
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Pot type transfer molding – old technology
Plunger transfer molding
Automatic Transfer molding
Screw transfer molding
Pot Type Transfer Molding
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Mold design provides a “pot” into which the
compound, heated or unheated, is placed and
the press closed
The clamping pressure exerted by the press
also forces the compound from the pot into a
runner system, then into the cavity
Was innovative, providing an improved means
for certain parts and producing less flash at the
parting line
Pot Type Transfer Molding
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Also made it possible to feed the molding
compound for a single location
Produced a lot of waste due to pot
volume being 15% higher than the
runner and cavities volume, to overcome
the pressure needed to force material to
the runners and cavities
Plunger Transfer Molding
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Developed in early 1940’s to overcome
difficulties/expenses of the pot type
A hydraulic cylinder is mounted on the top
platen of a standard compression press. It’s
the plunger
The clamping pressure is used to keep the
mold totally closed while the top cylinder
provides the pressure required to force the
compound into the runner system and cavity
Automatic Transfer Molding
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An attempt to integrate perform and
preheat as a function of press operations
An overhead performer capable of
feeding a precise weight of perform
feeds a dielectric preheater directly
below
The preheater feeds the compound to
the plunger
Automatic Transfer Molding
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The plunger feeds the mold
Completely automatic, but confined to
the use of free flowing granular materials
They have been slightly redesigned for
other materials
Screw Transfer Molding
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Uses an extruder to supply a precise
weight, temperature and density, shot,
that feeds into the mold on a completely
automatic basis
Screw Transfer Molding