chapt 19 Flashcards
define shrinkage, what’s it due to
reduction in linear size
cooling from molding to room temperature
do polymers have low or high thermal expansion coefficients, what does this entail?
high
significant shrinkage occurs during solidification and cooling in mold
Fillers in the plastic tend to {1} shrinkage
High injection pressures reduce shrinkage by…
Increasing compaction time, will {increase, decrease} shrinkage due to…
Finally, molding temperature will reduce shrinkage if {low, high} since…
1: reduce
…: forcing more material into mold cavity
{}: decrease
…: longer time forces more material into cavity
…: high, higher temps lower polymer melt viscosity, allowing more material to be packed into mold
Describe 3 aspects of thermoplastic foam injection molding
-molding of thermoplastic parts that have dense outer skin surrounding a lightweight foam centre
-part has high stiffness-to-weight ratio–suited for structural applications
-small amount of melt is injected into the mold cavity, where it expands to fill the cavity
For {1} foam injection molding, produced from introducing a gas into {2} plastic in injection unit, or mixing gas producing ingredient with starting pellets. The foam is in contact with {cold, hot} mold surface, collapses to form {light, dense} skin, while core {changes, retains} cellular structure.
1: thermoplastic
2: molten
cold
dense
retains
Most widely used molding process for thermosetting plastics is { 1 }, which is also used for {2}. The molding compound is available in several forms e.g. {3} & {4}. Amount of {5} must be precisely controlled to obtain {6} consistency in the molded product.
1: compression molding
2: thermoplastics
3, 4: power and preshaped volume
5: material
6: uniform
In compression molds, feed or charge is ….
required to fill the {1} before the molding process begins. It results in {low, high, zero} material waste during the process. These molds are ideal for … The charge is charge is usually preheated to ensure it …
…: an pre-measured amount of preheated material
1: cavities
zero
…: expensive raw materials
…: flows more easily, filling mold completely when pressure applied
2 ways that charge reduces material wasteage
exact amount of material premeasured, meaning no excess material after molding
consistent charge weights lead to uniform moldings, reducing defective parts that would’ve been discarded due to inconsistencies
Compression molding, due to lower flow capabilities of TS materials, are limited to {…}
The mold must be heated, usually by {1}, {2}
more simpler part geometries
1,2 : electric resistance, steam, hot oil circulation
2 examples of compression molding materials, and two compression molding products
1) epoxies, elastomers
2) electric plugs, pot handles, sockets
Thermal expansion i.e dimensional changes due to { 1 } for plastic have a {lesser/ greater} impact than that of metals.
1: temperature changes
greater
{1} wall thickness is desirable in an extruded cross section. {2} in wall thickness results in { 3 } flow and { 4 } which tend to warp extrudate
1: uniform
2: uneven
3: non uniform plastic
4: uneven cooling
{ blunt, sharp} corners, internally and externally, should be avoided in extruded cross sections. They result in { 2 } during processing and { 3 } in the final product.
sharp
2: uneven flow
3: stress concentrations
An advantage to plastic molding, is it allows mulitple functional features to be { … }. Although more {2} part geometries mean more costly molds, nevertheless it may be economical to design {3} molding if the alternative involves…
…: combined into one part
2: complex
3: complex
…: combining multiple simple parts into one
{thin, thick} cross sections are wasteful of material, more likely to cause warping due to {1} and takes longer to {2}
thick
1: shrinkage
2: harden
