MEMS 0040

What is relationship between average molecular weight, degree of polymerization, and the weight of the repeat unit?

$$W_{avg}=n\cdot W_{mer}$$

What is tacticity?

Irregularly repeating side groups

What the effect of branching on density and stiffness?

Branching increases density and stiffness.

What is the repeat unit for Polyethylene?

H H

C - C

H H

What is the repeat unit for Polypropylene?

H H

C - C

H CH_3

What is the repeat unit for Polyvinylchloride?

H H

C - C

H Cl

What is the repeat unit for Polystyrene?

H H

C C

H C_6H_5

What is the repeat unit for Polytetrafluoroethylene?

F F

C - C

F F

What is the relationship between specific volume and temperature for an amorphous material?

Linearly Proportional, changes slope at the glass transition temperature

What effect would an increase in viscosity have on the resultant overall forward volume flow rate in the extrusion of a thermoplastic polymer.

It decreases the back flow, which increases the overall forward volume flow rate.

How could the viscosity of a thermoplastic polymer be decreased during extrusion without changing the rotational speed of the screw?

Increasing the temperature

Name a process used to make thermoplastic fibers. Why does the fiber diameter decrease during this forming process?

Melt spinning is used for thermoplastics.

In melt spinning, the fibers are under tension when extruded and they lengthen, so the diameter also decreases.

Why is the elastic modulus of polymer fibers very high in the axial direction but much lower (approximately 10%) in the transverse direction after melt spinning?

The fibers are pulled in tension and elongate, which causes the carbon chains to align parallel to the axis. The transverse direction primarily has the lower stiffness intermolecular bonds.

What is the definition of the operating point for an extrusion process for thermoplastic polymers?

The operating point is the combination of head pressure and flow rate for the process, given by the intersection of the lines representing the extruder characteristic and the die characteristic.

Define die swell, and state the origin of this effect in the structure of the polymer.

Die swell is the ratio of extrudate diameter Dx to die orifice diameter Dd.

It is caused by chain alignment during extrusion and rekinking after extrusion.

List two types of defects exhibited by extruded thermoplastics.

Any two of the following:

(1) Melt fracture.

(2) Sharkskin.

(3) Bamboo.

List the steps in blow-film extrusion.

1) Extrude thin-wall tube and draw it upward using rollers while molten.

(2) Expand molten tube by blowing air into it through extruder die mandrel.

(3) Reduce separation between two films by collapsing rolls and pinch rollers.

(4) Collect flattened films on windup reel.

State the expected effects of the following processing changes on the volume flow rate in the extruder barrel for extrusion of a thermoplastic polymer:

(i) Decrease in filler volume fraction.

(ii) Introduction of a plasticizer.

(iii) Increase in polymer melt temperature.

All of these changes decrease viscosity, so they will increase backflow and decrease resultant forward flow rate in the extruder.

If polypropylene were injection molded into a shape using a die cavity of volume V, why would the volume of the final injection molded shape be expected to have a volume less than V?

Polymers have high thermal expansion coefficients, so the injection molded shape would shrink during cooling.

List the steps in extrusion blow molding of thermoplastic bottles.

1) Tube is extruded and blow pin is retracted.

(2) Mold is closed and blow pin is inserted.

(3) Parison is blown to inside surface of mold to make container.

(4) Mold is opened and blow pin is retracted. The molded part is ejected.

What is the mechanism responsible for the deformation of the polymer during extrusion blow molding of thermoplastic bottles?

The thermoplastic polymer is deformed by filling the parison with pressurized air to inflate the soft polymer into the rigid mold cavity.

List two causes of flashing defects on injection molded parts.

Any two of the following:

(1) Low viscosity of melt.

(2) Low clamping pressure.

(3) Improper mating of tooling.

(4) Excessively large shot volume.

Polymers are commonly joined using thermosetting resins. List three ways in which the crosslinking can be achieved in joining.

Any three of the following:

(1) Mixing just prior to application.

(2) Heating to catalyze the reaction.

(3) Radiation curing, such as ultraviolet light.

(4) Curing by evaporation of solvent.

What is pressure thermoforming used for? Describe the steps in the process.

Pressure thermoforming is used to make shapes from thermoplastic polymer sheets. The steps are:

(1) Preheat polymer sheet.

(2) Clamp sheet over mold.

(3) Apply pressure to form sheet.

(4) After cooling, remove finished shape.

Thermoplastic polymer components manufactured by injection molding commonly shrink on ejection from the clamping unit. How can this shrinkage be reduced, and what are the consequences for the injection molding process?

Add a filler to the polymer. This will increase the injection pressure required, because it increases polymer viscosity.

Describe the steps in injection molding of a thermoplastic polymer.

(1) Mold is closed.

(2) Screw moves forward while clamping unit is under pressure. Shot of polymer of correct volume is forced into mold.

(3) Polymer solidifies in mold, and screw is retracted while spinning to move more polymer into throat of injection machine.

(4) Clamping unit is depressurized and split, allowing ejection of molded piece.

List two characteristic defects associated with injection molding.

Any two of the following:

(1) Short shots.

(2) Flashing.

(3) Sink marks and voids.

List the names of two types of thermoforming processes used on thermoplastic polymer sheet.

Any two of the following:

(1) Vacuum thermoforming.

(2) Pressure thermoforming.

(3) Mechanical thermoforming.

State the name of a manufacturing method used to produce plastic bottles.

Extrusion blow molding.

List the steps in hand-layup, open mold shaping of a boat hull, starting with unimpregnated sheets of 0/90 cross-ply continuous fiber preforms.

1. Clean mold surface.

2. Apply gel coat.

3. Impregnate sheets with resin.

4. Apply impregnated sheets to mold surface.

4. Roll out air bubbles.

5. Repeat steps 3 and 4 until final thickness.

6. Cure.

7. Demold.

How must the composite differ if it were made by a spray-up method?

Discontinuous fibers must be used in the spray-up method.

List the steps in compression molding of a composite bicycle wheel rim, starting with impregnated sheets of unidirectional fibers.

1. Cut prepreg sheets and lay them up against mold surface, controlling fiber orientation.

2. Repeat Step 1 until final thickness.

3. Insert bladder (in this case), and close mold.

4. Apply pressure and heat to bond the layers.

5. Rim is demolded.

What distinguishes resin transfer molding from compression molding of composites?

In resin transfer molding, resin is transferred to the mold after the mold is closed.

In compression molding, resin or a prepreg mat is added to the mold cavity before the mold is closed.

Why does elastic reservoir molding produce composites suited to loading in bending?

In the final composite, the fiber layers are near the external surfaces, where they can efficiently resist bending.