Polymers III (DON'T STUDY)

Question 1

What does a technique used to form a polymer depend on?

Answer 1

Whether it is thermoplastic or
thermosetting, final geometry and size

Question 2

When are thermoplastic materials normally fabricated and what is applied to shape the material?

Answer 2

Normally fabricated at elevated temperatures, with
application of pressure to shape the material

Question 3

What is the extrusion of thermoplastics?

Answer 3

Squeezing, pressing or pushing a material through a die to form a
product with a consistent cross-sectional profile.

Question 4

What do heater bands provide during the extrusion of thermoplastic polymers?

Answer 4

Electrical
heat energy

Question 5

Why is the screw shape important for the extrusion of thermoplastics?

Answer 5

To control the
pressure at which the
polymer is extruded

Question 6

What is blown film extrusion?

Answer 6

A process that uses air pressure instead of a solid mold allows for precise control of very thin
film production

Question 7

What are the steps for blow molding of thermoplastics?

Answer 7

1. Heated plastic is extruded into a hollow tube(parison)
2. Mould closes - parison is gripped in place
3. Compressed air blown into parison which inflates.
4. Parison fills mould

5.Product is trimmed and removed from mould

6.Finished product ready for next production stage

Question 8

How does vacuum forming from thermoplastic sheets work?

Answer 8

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

Question 9

What is the limitation of vacuum forming from thermoplastic sheets?

Answer 9

Cannot form around holes or surfaces that go beyond 90°
to the vertical (e.g. narrow-necked container, as in blow moulding)

Question 10

What can compression molding be done with?

Answer 10

Softened sheets or
with a viscous plastic similar to
blow molding

Question 11

What types of temperature-related polymers can be formed by compression molding?

Answer 11

Both thermoplastics and thermosets can be formed by
compression molding.

Question 12

Compare the pressure and capital cost between compression molding and injection molding.

Answer 12

Pressures are lower than for injection molding so the
capital cost is less

Question 13

What are the steps for injection molding?

Answer 13

1. Polymer granules are heated until molten
2. Compressed by a ram or a screw
3. Injected into a cold, split mold under pressure
4. The molded polymer is cooled below Tg
5. The mold opens and the product is ejected

Question 14

What is done during injection molding to compensation for contraction?

Answer 14

Excess material is often pumped into the mold to
compensate for contraction

Question 15

What is the range of length of cycle time for injection molding and what does the time range depend on?

Answer 15

Cycle time can be seconds or up to 5 minutes,
depending on part size

Question 16

Why is precise machining of interlocking parts needed for injection molding?

Answer 16

Molds are complex and exceedingly expensive
($10k - $100k for some parts)

Question 17

Compare the pricing of vacuum and compression molds to injection molds.

Answer 17

Injection molds are far more expensive than vacuum or
compression molds

Question 18

Which molding method requires the highest level of detail?

Answer 18

Injection Molding

Question 19

Which molding method has the highest expense for fabrication?

Answer 19

Injection Molding

Question 20

What does blow molding produce?

Answer 20

Thin uniform films, bags and
other end products formed after film is
produced

Question 21

What is vacuum forming formed from?

Answer 21

Formed from sheet stock

Question 22

Which molding method yields the lowest level of detail?

Answer 22

Vacuum Forming

Question 23

Compare the cost of compression molding to injection molding

Answer 23

Compression Molding has a much lower cost than injection moulding dies.

Question 24

Describe the relative cost of equipment for vacuum forming?

Answer 24

Low cost of equipment

Question 25

Compare the level of detail between compression molding and vacuum forming

Answer 25

Compression Molding has more detail than vacuum forming.

Question 26

What are polymer additives?

Answer 26

Polymer additives are chemicals that improve
ductility and prevent degradation

Question 27

What is the function of plasticisers?

Answer 27

Increases flexibility, ductility, and toughness to brittle plastics and reduces hardness and stiffness.

Question 28

What are the chemical properties of plasticizers?

Answer 28

• Often liquids having low molecular weights, fitting in between large polymer chains

Question 29

Where are most plasticizers used?

Answer 29

90% of plasticizers are used for PVC, in films and cables

Question 30

What is the function of stabilizers?

Answer 30

They counteract the deterioration of plastics

Question 31

What do stabilizers protect against?

Answer 31

• UV break-down of cross-linkages
• Oxidation of bonds, particularly at high-temperature

Question 32

How do stabilizers prevent thermal degradation?

Answer 32

By acting as heat and flame retardants (prevents ignition of material, breakdown at higher
temperatures)

Question 33

What are all of the polymer additives discussed in class?

Answer 33

Plasticizers, stabilizers, colorants, flame retardants, and bulk filler material