Polymers & Composites
Ch. 10
March 25 and 30, 2020
Instructor: Dr. Jingjing Li
2020 The Pennsylvania State University All Right Reserved
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Outline
1. Processing techniques for polymers
2. Processing techniques for composites
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1. Processing Techniques for Polymers
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Plastic Processing Methods (& Composites)
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Extruder
sheets, shapes, tubes, coatings, fibers, etc.
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Extrusion of Fibers
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Extrusion Defects
Differential
axial stresses
b/w inner and
outer layers of
the tubing
http://www.sylvin.com/News/Our-News/view_1497/detail/cid_1497/1312/page_1497/3
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Injection Molding
most widely used
molding process for
plastics
https://gminsig
hts.wordpress.c
om/2017/08/03/
injectionmolded-plasticmarket/
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A 2.2-MN (250-ton) Injection-Molding Machine
The tonnage is the force applied to keep the dies closed during the injection of
molten plastic into the mold cavities, and hold it there until the parts are cool and
stiff enough to be removed from the die.
Source: Courtesy of Cincinnati Milacron, Plastics Machinery Division.
Products Made by Injection Molding
Electronic housings, Toys, Agricultural components, Household, Machinery and
automotive components, Healthcare industry, …
https://www.qualityplasticsnz.com/plastic-injection-molding/
Mold Features for Injection Molding
Important Parameters in Injection Molding
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Mold temperature
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Injection pressure
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Holding pressure
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Holding time
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Mold open time
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Injection Molding Defects
Problem
Flow lines
Sink
marks
Image
Cause
Solution
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•
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Varying speed
Varying wall thickness
Injection speed is too low
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•
Cooling is insufficient for
plastic to fully cool and cure
Inadequate pressure
Excessive temperature at the
gate
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Mold is not clamped together
with enough force
Excessive injection pressure
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•
Increase the clamp pressure
Ensure that the mold is
properly maintained and
cleaned
Inadequate materials to fill the
cavities
Inadequate degassing or gas
venting
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Increase mold or melt
temperature
Increase the material feed
Account for gas generation
by designing the mold13
•
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Flash
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Short shot
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Increase injection speeds
and pressure
Avoid sudden thickness
changes
Lower mold temperatures
Increase holding pressure,
holding time
Reduce the thickness of the
thickest wall sections
Ex: Effects of injection molding
parameters on quality
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Blow Molding
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Thermoforming
Thermoforming
The thermoforming process converts plastic
sheet into products using heat. It is a
process where heated thermoplastic sheets are
molded to conform the sheet to a mold and once
the shape is formed and allowed to cool, it is
trimmed to create a part or product.
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Advantages and Disadvantages
Advantages
A good speed
Almost all types of thermoplastic can be
used in this process
High quality control
The costs involved in tooling are low
Disadvantages
During this process, the plastic sheets that
are in a pliable state can break due to
excessive stretching under certain
temperatures
Due to the use of higher-quality plastic
sheets, this method is costly (about 50 %
more) as compared to other methods.
In this process, only one side of part is
defined by the mold
Internal stresses are common
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Molding of Thermosets
Reaction Injection Molding
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Compression and Transfer Molding of Thermosets
Compression Molding
The compression process starts with
granular bulk or sheet materials added
into the mold cavity. Closing the mold
delivers the force needed for the
material to flow in the mold which is
heated to 300 degrees or higher. This
process starts the curing.
Difference is that the mold is enclosed rather than
open to the fill plunger
Transfer Molding
Transfer molding applies a preheated
preform of material into the mold pot
before a plunger applies pressure to
the material loading, forcing it
through a runner and gate into the
mold cavity.
Thermoset transfer molding is
considered for more precise precision
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geometrics.
Advantages and Disadvantages of Thermoset
Compression Molding
Advantages
• Heavy plastic parts can be
molded
• The molding process is
cheaper as compared to
injection molding
Disadvantages
• Production rate lower than
injection molding and transfer
molding
• Limited largely to flat or
moderately curved parts with
no undercuts
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Advantages and Disadvantages of Thermoset
Transfer Molding
Advantages
Disadvantages
• High temperature thermoset
materials
• Dimensionally stable
• Uniform thickness of parts
• Plastic parts with metal inserts
can be made
• Production rate lower than
injection molding
• Waste of material
• Air can be trapped in the mold
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Variations of Transfer Molding
Resin transfer molding
uses a liquid thermoset resin to saturate a fiber preform placed in a closed mold. The
process is versatile and can fabricate products with embedded objects such as foam
cores or other components in addition to the fiber preform
https://www.slideshare.net/HinaIbrahim1/resin-transfer-molding
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Rubber Molding
Rubber molding creates molded rubber
parts by pressing a block of rubber into a
rubber molding metal cavity. The rubber
is then exposed to heat, activating a
chemical reaction. While there are
variations in specific methods, all rubber
manufacturers use heat and pressure
method to form molded rubber products.
https://www.mnrubber.com/Design_Guide/2-17.html
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All polymers cannot be processed using all
processing methods.
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2. Types of Composites
• Matrix à polymer, metal, or ceramic
• Reinforcement à ceramic, metal,
polymer
• Reinforcement Shape
– Fiber
• 1-D, 2-D, 3-D à $$, $$$, $$$$
• High performance
• Special processing methods à $$, $$$,
$$$$
– Particulate
• Lower performance
• Can use conventional processing methods
à Low $
https://www.sciencedirect.com/science
/article/pii/S1359835X17302051
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Fiber-Reinforced Composites
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Manufacturing with Prepregs
In making fiber-reinforced plastic prepregs (pre-impregnated with resin), the
continuous fibers are aligned and subjected to a surface treatment to enhance
the adhesion to the polymer matrix. They then are coated by dipping them in a
resin bath and are made into a tape.
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Vacuum-Bag Forming & Pressure-Bag Forming
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Open-Mold Processing
hand layup/spray up
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Filament Winding
The process involves winding filaments under tension over a male mandrel. Once
the mandrel is completely covered to the desired thickness, the resin is cured. Once
the resin has cured, the mandrel is removed, leaving the hollow final product.
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Sheet Molding
(b)
Paste reservoir dispenses a measured amount of specified resin paste onto a plastic
carrier film. This carrier film passes underneath a chopper which cuts the fibers onto
the surface. Once these have drifted through the depth of resin paste, another sheet is
added on top which sandwiches the glass. The sheets are compacted and then enter
onto a take-up roll, which is used to store the product whilst it matures. The carrier
film is then later removed and the material is cut into charges. Depending on what
shape is required determines the shape of the charge and steel die which it is then
added to. Heat and pressure act on the charge and once fully cured, this is then
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removed from the mold as the finished product.
Summary of The Video Links
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Extrusion: https://www.youtube.com/watch?v=WaB-dsB1Kfk
Extrusion of fibers: https://www.youtube.com/watch?v=cn6K1m7yH0I
Injection molding: https://www.youtube.com/watch?v=RMjtmsr3CqA
Blow molding: https://www.youtube.com/watch?v=8W6P5KU5ONQ
Thermoforming: https://www.youtube.com/watch?v=alq3RDZN4jo
Compression molding:
https://www.youtube.com/watch?v=kDQ7ZXGWhCA
Resin transfer molding: https://www.youtube.com/watch?v=1u2GvhghQA&t=47s
Open-mold processing: https://www.youtube.com/watch?v=ZotUR_GiVK8
Sheet molding: https://www.youtube.com/watch?v=J9ChVcR58aA
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