Plastics And Composites 2

Question 1

What does video-elasticity mean when applied to a polymer melt?

Answer 1

Polymer melts are viscoelastic materials that have viscous, fluid properties as well as elastic. Viscous because when a force is applied the strain increases til removal. Elastic in that a step change in strain happens with a load applied and then is recovered after removal.

Question 2

Define die swell in extrusion

Answer 2

Hot plastic expands when exiting the die opening due to the viscoelastic properties

Question 3

How do you reduce the amount of die swell for a set material and why?

Answer 3

Increase die length or duration of material in die because of the viscoelastic nature of the material. The longer in the die, the more material will relax into the shape and remember it.

Question 4

Describe the blown-film extrusion process

Answer 4

Begins with extrusion of a tube that is drawn upward while molten and expanded in size by air inflated into it through the die mandrel. A frost line marks the position of the upward moving bubble where polymer solidifies. Air pressure kept constant for uniform film thickness and diameter. Air is contained by pinch rolls that squeeze tube back together after cooling. Guide rolls and collapsing rolls restrain and direct pinch rolls. Flat tube is then collected onto a windup reel.

Question 5

Would you expect the Young’s modules of a blown film to be greater or less than of a raw material

Answer 5

Blown is higher because as you stretch the polymer the chains untangle and break the VDW. With thermoplastics as you increase strain, stiffness increases due to influence of stronger covalent bonds.

Question 6

Describe extrusion process

Answer 6

It’s a compression process where material is forced to flow through a die hole to provide long continuous product whose CSA is the same as the hole.

Question 7

What are the 3 sections in the barrel/screw of an extruder?

Answer 7

1: feed section, where stock is moved from hopper port and preheated
2: compression, where polymer is transformed into liquid, air trapped among the pellets is extracted and material is compressed
3: metering, where the melt is homogenised and sufficient pressure is developed to pump it through die opening

Question 8

Disadvantages of increasing extrusion rate?

Answer 8

Increased die swell from lower residence time. Potentially not enough heat applied as duration is lower or too much heat made as shear rate is increased. (Material/screw shape dependent)

Question 9

Describe injection moulding process:

Answer 9

1) mould is closed
2) melt is injected into cavity
3) screw is retracted
4) mould opens
5) remove part

Question 10

Why are reciprocating screws better than older rams used in injection moulding?

Answer 10

More homogenised polymer melt

Question 11

Describe extrusion blow moulding process

Answer 11

1) extrusion of parison
2) parison pinched at top and sealed at bottom around metal blow pin as two mould halves join
3) tube is inflated so it takes mould cavity shape
4) mould opened and part is removed

Question 12

How can you tell a plastic bottle has been extrusion blow moulded?

Answer 12

It has a pinch point at the bottom from the mould trimming the parison.

Question 13

What’s the difference between extrusion blow moulding and injection moulding?

Answer 13

Parison is made in separate mould and then transferred to another mould

Question 14

Why might extrusion blow moulding be more suitable for making a bottle for medicines?

Answer 14

Easier to make a 2 layer parison. Do duel extrusion and then expand out

Question 15

Describe the rotational moulding process

Answer 15

1) polymer power loaded into cavity of split mould
2) mould heated and rotated (quite slow) on two perp. axis so powder coats all surfaces and forms a fused layer of uniform thickness
3) while rotating, the mould is cooled so that the plastic skin solidifies
4) mould is opened and part removed

Question 16

Are milk bottles with handles made by extrusion blow moulding or injection blow moulding?

Answer 16

Extrusion blow moulded. Handle is added by the form of the mould to create the home in the bottle to act as a handle. Not possible with injection as blow pin would obstruct.

Question 17

Difference between positive and negative mould in thermoforming?

Answer 17

Positive: internal dimension is the most accurate one and smooth since it takes form of mould.
Negative: opposite

Question 18

What are there no filament winding for short fibre PMCs

Answer 18

Because the fibres are discontinuous and can’t wind or pull a long strand

Question 19

What’s the difference between a roving and a yarn?

Answer 19

A roving is a collection of parallel continuous strands, this is a convenient form for handling and processing. Rovings typically contain 12-120 strands. Yarn is twisted filaments. Continuous rovings are used in several PMC processes including filament winding and pultrusion.

Question 20

What’s a mat in fibre reinforcement?

Answer 20

A felt with randomly oriented short fibres held by a binder sometimes in a carrier fabric

Question 21

In what order would you expect the fibre density to be for a woven yarn, woven roving and a mat?

Answer 21

(Highest) woven roving, woven yarn, mat.

Because of how easily the fibres pack together.

Question 22

What is sheet moulding compound? SMC

Answer 22

A combination of TS polymer resin, fillers and other additives, and chopped glass fibers (randomly oriented) all rolled into a sheet of thickness 6.5mm.

Question 23

What is a prepreg and what are it’s advantages

Answer 23

Prepreg consists of fibres impregnated with partially cured thermosetting remains to facilitate shape processing. Completion of curing must be accomplished during/after shaping. The advantage of prepregs is that they’re fabricated with continuous filaments instead of chopped random which increases strength and modulus of the final product. Distribution of polymer matrix can also be more closely controlled.

Question 24

Describe the hand lay up process

Answer 24

1) mould is cleaned and treated with a mould release agent
2) thin gel coat (resin maybe with pigment) is applied which will become outside surface of moulding
3) when the gel is almost set, successive layers of resin and fibre are applied, the fibre being in the form of mat or cloth, each layer is rolled to fully impregnate fibre and resin and remove air bubbles.
4) part is cured and 5) removed when fully hard

Question 25

If you made a bath tube with open moulding processes, would you use a positive or negative mould and why

Answer 25

Positive as you want the inner surface to be smoother

Question 26

Why are laminated FRP products made by the spray up method not as strong as similar ones made by hand lay up?

Answer 26

Spray up fibres are short and randomly oriented, whereas hand lay up may use prepregs which uses continuous and aligned fibres

Question 27

If I was to make a pressure cylinder using filament winding which type of winding would I use and why

Answer 27

Bi-axial winding as in a pressure vessel there are forces in all directions and therefore windings need to have a combination for radial strength and axial strength.