Polymers III (DON'T STUDY) Flashcards
What does a technique used to form a polymer depend on?
Whether it is thermoplastic or
thermosetting, final geometry and size
When are thermoplastic materials normally fabricated and what is applied to shape the material?
Normally fabricated at elevated temperatures, with
application of pressure to shape the material
What is the extrusion of thermoplastics?
Squeezing, pressing or pushing a material through a die to form a
product with a consistent cross-sectional profile.
What do heater bands provide during the extrusion of thermoplastic polymers?
Electrical
heat energy
Why is the screw shape important for the extrusion of thermoplastics?
To control the
pressure at which the
polymer is extruded
What is blown film extrusion?
A process that uses air pressure instead of a solid mold allows for precise control of very thin
film production
What are the steps for blow molding of thermoplastics?
- Heated plastic is extruded into a hollow tube(parison)
- Mould closes - parison is gripped in place
- Compressed air blown into parison which inflates.
- Parison fills mould
5.Product is trimmed and removed from mould
6.Finished product ready for next production stage
How does vacuum forming from thermoplastic sheets work?
A sheet of plastic is heated to a forming temperature, stretched onto a
mold, and a vacuum applied to force the sheet against the form
What is the limitation of vacuum forming from thermoplastic sheets?
Cannot form around holes or surfaces that go beyond 90°
to the vertical (e.g. narrow-necked container, as in blow moulding)
What can compression molding be done with?
Softened sheets or
with a viscous plastic similar to
blow molding
What types of temperature-related polymers can be formed by compression molding?
Both thermoplastics and thermosets can be formed by
compression molding.
Compare the pressure and capital cost between compression molding and injection molding.
Pressures are lower than for injection molding so the
capital cost is less
What are the steps for injection molding?
- Polymer granules are heated until molten
- Compressed by a ram or a screw
- Injected into a cold, split mold under pressure
- The molded polymer is cooled below Tg
- The mold opens and the product is ejected
What is done during injection molding to compensation for contraction?
Excess material is often pumped into the mold to
compensate for contraction
What is the range of length of cycle time for injection molding and what does the time range depend on?
Cycle time can be seconds or up to 5 minutes,
depending on part size
Why is precise machining of interlocking parts needed for injection molding?
Molds are complex and exceedingly expensive
($10k - $100k for some parts)
Compare the pricing of vacuum and compression molds to injection molds.
Injection molds are far more expensive than vacuum or
compression molds
Which molding method requires the highest level of detail?
Injection Molding
Which molding method has the highest expense for fabrication?
Injection Molding
What does blow molding produce?
Thin uniform films, bags and
other end products formed after film is
produced
What is vacuum forming formed from?
Formed from sheet stock
Which molding method yields the lowest level of detail?
Vacuum Forming
Compare the cost of compression molding to injection molding
Compression Molding has a much lower cost than injection moulding dies.
Describe the relative cost of equipment for vacuum forming?
Low cost of equipment
Compare the level of detail between compression molding and vacuum forming
Compression Molding has more detail than vacuum forming.
What are polymer additives?
Polymer additives are chemicals that improve
ductility and prevent degradation
What is the function of plasticisers?
Increases flexibility, ductility, and toughness to brittle plastics and reduces hardness and stiffness.
What are the chemical properties of plasticizers?
- Often liquids having low molecular weights, fitting in between large polymer chains
Where are most plasticizers used?
90% of plasticizers are used for PVC, in films and cables
What is the function of stabilizers?
They counteract the deterioration of plastics
What do stabilizers protect against?
- UV break-down of cross-linkages
- Oxidation of bonds, particularly at high-temperature
How do stabilizers prevent thermal degradation?
By acting as heat and flame retardants (prevents ignition of material, breakdown at higher
temperatures)
What are all of the polymer additives discussed in class?
Plasticizers, stabilizers, colorants, flame retardants, and bulk filler material
