Section 3: Polymer Processing

What type of polymer can be considered “plastic”?

thermoplastic polymers, thermosetting polymers

Which type of polymer can be considered “rubber”?

elastomers

Which type of polymer can be recycled?

thermoplastics only

What are the properties of thermoplastics?

solid at room temperature, viscous when heated; easily shaped; can be melted

What are the properties of thermosets?

cannot be repeatedly melted; will degrade and char if repeatedly melted; elevated temperatures activates cross linking, turning material into infusible solid

What are the properties of elastomers (rubber)?

extremely elastic; share a similar molecular structure with thermosets (have irreversible chemical bonds)

What polymer type follows a linear structure?

*most* thermoplastics

What polymer type follows a loosely cross-linked structure?

elastomers

What polymer type follows a tightly cross-linked structure?

thermosets

degree of crystallinity

the proportion of crystalized material in the mass

What happens as the crystallinity in a polymer is increased?

* density:
* stiffness/strength:
* heat resistance:

* density: increases
* stiffness/strength: increases
* heat resistance: increases

What is the purpose of additives in polymer processing?

alter the molecular structure or add a second phase in transforming the polymer into a composite

fillers

a type of additive; strengthen the polymer and helps reduce overall cost

plasticizers

a type of additive; soften the polymer and improve flow

colorants

a type of additive; pigments or dyes

lubricants

a type of additive; reduce friction and improve flow in polymers

flame retardants

a type of additive; reduce flammability of polymers

cross-linking agents

a type of additive; used for thermosets and elastomers

ultraviolet light absorbers

a type of additive; reduce sunlight degradation in polymers

antioxidants

a type of additive; reduce oxidation damage in polymers

extrusion

heated plastic is forced through a die orifice to provide a long, continuous product; cross-sectional shape is determined by the die orifice; extrudate is then cut into desired lengths

What are the three “zones” in an extruder?

feed, compression, metering

equation for die swell

r_s = D_x/D_d

* D_x = diameter of extrudate
* D_d = diameter of die

What is the purpose of a screen pack?

filters the melt before the die, builds pressure in the metering section, straightens flow (removes “memory” of the extruded polymer)

During extrusion, hollow profiles require a ________ mandrel.

spider

calendering

feedstock is passed through a series of rolls to reduce thickness to desired gage; product has good surface finish

fiber

a long, thin strand whose length is at least 100 times its cross-section

filament

a fiber of continuous length

melt spinning

molten polymer is pumped through spinneret

What are the parts of an injection molding machine?

injection unit and clamping unit

What are the parts of a two-plate mold?

cavity, distribution channel, ejection system, cooling system, air vents

What are the advantages of three-plate mold over a two-plate mold?

runner and parts disconnect and drop into two separate bins; material can be injected at the base of the mold or the middle

What are the advantages of a hot runner mold (runner-less mold)?

eliminates solidification of sprue and runner by adding manifold heaters; saves material

compression molding