Term
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Definition
| Used a measure of laminar v. turbulent flow. Flow is turbulent if R>2000 which is impossible in polymers due to their high viscosities |
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Term
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Definition
| Means there is a need for high pressure processing and that flow will always be laminar |
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Term
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Definition
| chain length is very importatn because at the engtanglement point, reptation becomes a requirement for flow. Therefore MWT has a high effect on viscosotiy |
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Term
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Definition
| viscosity decreases as strain rate increases |
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Term
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Definition
| strain rate = shear stress/viscosity |
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Term
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Definition
| when velocity speeds up with time as a material gets closer to an orifice |
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Term
| what is the difference between thermosets and thermoplastics in terms of preparing them |
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Definition
thermoplastics: heat to melt>change shape>solidify
thermosets: mix>heat to melt>change shape>chemical reaction>cool |
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Term
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Definition
| describes the progress of crystallization into-semi-infinite space and is used to describe cooling polymers as there are often inhomogeneities in structure, skin-core texture, and transcrystallization. Often there is a delay in the curve because undercooling is necessary. |
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Term
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Definition
| a measure of the extent to which elastic processes can be expected. N>>1 elastic memory effects, N<<1 viscous, N~1 viscoelastic. At low stress N~1 and the extrudate would tend to curve back to horizontal but through a die there is much higher stress therefore N<<1 and viscous with plenty of relaxation time therefore no memory effects and extruadate is straight |
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Term
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Definition
| this mathematically describes the pre-tube pressure drop as polymer approaches die and is treated by assuming an increasing effective tube length (L*=L+Nr, N-constant, r-radius). This is then put into the equation for and plotted with the negative x-intercept which corresponds to Nr |
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Term
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Definition
| the flow curve is a straight line and these tend to be approximations over versy small log or low . There are analytical expressions describing the flow curve: and n is related to the slope by different ratios |
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Term
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Definition
| major application of these is the measurement of flow characteristics. Ie we know r3 therefore we can find Q, P, dQ/dP, , . The flow curves compare strain rate, shear stress, and apparent viscosity in various combinations. At times there are horizontal regions which correspond to Newtonian flow and areas where the curve flips back upwards which correspond to shear crystallization sometimes due to flow causing chains to line up and crystallize but this is only close to small T-Tm |
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Term
| Tensile/Troutonian viscosity |
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Definition
| comparing shear strain and shear stress parallel to the load - . This has strong implications for film drawing as tension stiffening can lead to a uniform draw and tension thinning leads to necking when there is a stress concentration. |
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Term
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Definition
| Most polymers are near-Newtonian at low shear rates and show shear thinning behavior at high shear rates. Also high MWT polymers orient more readily therefore show greater shear thinning |
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Term
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Definition
| ex. Ketchup viscosity decreases with time at constant shear strain rate |
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Term
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Definition
| ex. Non drip paint viscosity increases with time at constant shear strain rate |
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Term
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Definition
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Term
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Definition
τ/γ*
often the difference between apparent and true viscosity is no more than 15%
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Term
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Definition
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Term
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Definition
Relaxation of elastic component of deformation upon die exit leading to increased thickness
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Term
| how can die swell effects be reduced? |
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Definition
-T↑-decreased elastic deformation and less viscous
-Shear rate↓-decrease the degree of orientation
-Post extrusion draw down (continue to apply a force on the extrudate by pulling it out or through sizing die)
-L↑-the less swelling b/c there is a loss of memory of extreme deformation at die entry (extensional flow)
-Compensation for profile distortion
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Term
| how can melt fracture effects be reduced? |
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Definition
σ↓-as Q and α decrease also
λ ↓-as T increases or MWT decreases
careful choice of die material for improved adhesion
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Term
| derive the equation for melt fracture |
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Definition
dε/dt=1/d×dd/dt=1/d×dd/dz×dz/dt
dd/dz-can be found using angle in system, tanα
dz/dt-strain rate/velocity
Assume incompressible flow
ν=0.5, ε*z+2ε*r=0
σ=λεz* |
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Term
What is melt fracture?
What does it look like? |
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Definition
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Looks a bit like turbulent flow (but that never happens in polymers). It is a result of extensional flow at die entry where a point is reached where no further orientation is possible and if chains can not slip past entanglements fast enough to relieve stress, there is failure of adhesision to die walls and fracture of melt
[image]
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Term
| How can shark skin be reduced |
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Definition
Q↓
P↓
T↑
Heat surface upon exit |
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Term
| what is shark skin and what does it look like? |
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Definition
Looks like mild melt fracture but is always perpendicular to flow – not helicoidal. As the melt leaves the die, acceleration generates a tensile stress which leads to surface orientations. May be seen at low Q.
die exit effect
[image]
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Term
| what is the term for a 'bad case' of shark skin and what does it look like? |
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Definition
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Term
| how can orientation be reduced? |
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Definition
- Reduced by high T (relaxation)
- lower MWT (low orientation to start with),
- low strain rate (shorter relaxation time)
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Term
| advantages of orientation |
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Definition
- High strength parallel to the fibers,
- decreased permeability (especially if biaxial)
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Term
| disadvantages of orientation |
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Definition
- Tendency to tear/crack along orientation,
- retraction or warping on heating
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Term
| Describe injection moulding |
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Definition
Ideal I.M. has uniform wall thickness (minimize internal stress, shrinkage, voids, sink, and manes), avoids large flat surfaces (hard to keep flat, lack rigidity), and avoids weld lines
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Term
| what are the advantages of injection moulding |
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Definition
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Can make complex shapes quickly (low dT/dt), dimensional precision
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Term
| what are the disadvantages of injection moulding |
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Definition
High pressure, can’t make large objects (want even cooling), expensive moulds, must make MANY objects to counteract costs
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Term
| how can injection mould effects be altered |
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Definition
melt/mould temperatures (cheaper) and position/number of gates (expensive and leads to weld lines)
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Term
what are the advantages of blow moulding
how can thickness be controlled |
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Definition
| Can be used for multi layering, moving core can be used to control thickness, tends to lack orientation leading to isotropic bodies |
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Term
| disadvantages of blow moulding |
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Definition
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- Every time you heat up the polymer you lower MWT therefore can only use so much scrap material,
- unequal thicknesses depending on complexity of shape
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Term
| describe rotation moulding |
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Definition
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Term
| describe/sketch injection moulding |
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Definition
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Term
| describe/sketch blow moulding |
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Definition
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Term
| describe/sketch extrusion |
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Definition
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Term
| describe/sketch two-roll mixing |
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Definition
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Term
| what are the advantages of two-roll mixing |
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Definition
- High shear,
- good mixing,
- pressure ensures good contact,
- good temperature control,
- lots of surface area
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Term
consider a rectangular plate injection moulded with one gate in the center
describe the flow fields and the warping over time |
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Definition
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Term
consider a rectangular plate with two gates
describe the flow fields and the warping |
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Definition
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Term
| consider two situations in which gate movement reduces warping effects |
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Definition
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