4 Powder Processing, Polymers And Composites

Question 1

Positives of powder processing

Answer 1

Can form high melting point materials into final shape, near net shape process therefore few additional manufacturing stages

Low material wastage

Good dispersion of phases (fine particle dispersions), good for hardness and avaoids segregation effects

Can control porosity

Relatively cheap for high production runs

Question 2

What’s powder processing

Answer 2

Material in fine powder form is pressed into the required shape and then heated to bond the particles together by interdiffusion to form components, which generally require very little further processing.

Question 3

How is the powder manufactured

Answer 3

By atomisation

in which high pressure jets of water (water atomization) or gas (gas atomization) are directed at
a stream of molten metal, causing it to break up into droplets and solidify.

Ceramic powders are made by crushing or grinding (followed by sieving)
– the particles are very irregular.

Question 4

Difference between atomisation with water or gas

Answer 4

Water atomization (fast quenching in a high heat-capacity medium) leads
to irregularly shaped particles; gas atomized particles tend to be more
spherical. For very reactive materials, inert gas or fluid may be used.

Question 5

What is the point of the lubricant added to powder

Answer 5

• Powder flows more easily, so it fills the mould better
• Die friction is reduced, so more uniform product density is achieved,
part is ejected without cracking and die life is increased.

Question 6

Define sintering

Answer 6

Sintering is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.

Question 7

Which shrinks less on sintering, ceramics or metals

Answer 7

Ceramics due t9 greater porosity of the green compact

For ceramic materials the linear dimensions of the green compact will be
15-20% greater than those of the finished part because of the remaining
porosity (up to 50%). With metals the powder particles themselves
deform during compaction and the green compact has a similar density to
the final product (up to 95% of bulk metal); there is little or no shrinkage
on sintering.

Question 8

How can a more uniform distribution be achieved

Answer 8

• better lubrication between powder and die
• multiple punches, e.g. with punches moving at both top and bottom
of die. Get highest compaction close to moving punch, so average
compaction increased and compaction
variation is reduced.

You want a constant compression ratio (hence density distribution), can be hard for compacts of non-uniform shape

Question 9

What is HIPing

Answer 9

Hot Isostatic Pressing

In rudimentary terms, the HIPing process uses the combination of high temperatures and high pressures to densify engineering ceramics and hard metals. The densification and removal of porosity thereby leading to improved mechanical properties such as strength and reliability.

HIPing provides components with good mechanical properties (low porosity), but dimensional accuracy is low. The
improvement in ductility and tensile strength can be very significant.

Question 10

Why do the parts shrink so much during sintering for Metal injection moulding and powder injection moulding

Answer 10

Process uses conventional polymer injection moulding technology with a blended
polymer-metal or polymer-ceramic feedstock.
A binder is added to the powers of a volume fraction of 30-50% which before sintering is removed, so the green part becomes a brown part with v high porosity, hence lots of shrinking on sintering.

Question 11

Advantages of PIM and MIM

Answer 11

low die wear rates

complex shapes (variations in wall thickness,
moulded-in decoration) with high dimensional tolerances can be
made

low and uniformly distributed porosity means products take high surface polish and have excellent mechanical properties.

Particularly useful for high-volume production.

Question 12

Do thermoplastics have permanent cross links between chains

Answer 12

No

Question 13

Are thermoplastics amorphous are crystalline

Answer 13

They may be amorphous (= glassy i.e. with randomly oriented chains) or semi-crystalline
(containing crystalline regions mixed with some amorphous component).

Question 14

4 methods for polymer processing

Answer 14

Extrusion, injection moulding, thermoforming, rotational moulding

Question 15

How can the alignment of polymers be controlled in film blowing

Answer 15

Align polymer chains axially by tension of windup rolls
Align polymer chains in hoop direction
by expanding the bubble.

Question 16

4 important variables in injection moulding

Answer 16

Injection pressure, component thickness, hold on times and mould temperature

Hold-on times:
Time that the die is under pressure. Longer time allows for counterbalancing of shrinkage (as
long as the polymer is still molten at gate).

Question 17

2 things that influence polymer properties

Answer 17

Chain geometry and bonds between polymers

Geometry means: chain length, shape (branched or linear), side groups

Bonds between: VDW are weak, Entanglements are friction between polymer chains, secondary bonds (strong chemical bonds) determined by nature of polymer. Thermometer have many secondary bonds (can’t melt), elastomers have some, thermoplastics have NONE

Question 18

2 ways in which processing affects properties of thermoplastics

Answer 18

Crystallinity and Molecular alignment

Question 19

What does crystallinity do to volume on solidification

Answer 19

Crystalline regions are more dense so cause volume reduction.

Density can be used to indicate degree of crystallinity.

Question 20

What affects the degree of crystallinity on cooling of a thermoplastic

Answer 20

Cooling rate, faster equals more crystalline

Question 21

What does crystallinity do to elastic modulus and tensile strength

Answer 21

Increases them

100% crystalline doesn’t even have a glass transition on a modulus temp graph.

Question 22

Are optically transparent polymers crystalline

Answer 22

No they’re amorphous

Question 23

Purpose of Cold drawing

Answer 23

To align the chains of the molecules in the direction of drawing, which vastly improves strength and elastic modulus in this direction.
there is an equivalent
deterioration in properties normal to this
direction.
Chain alignment allows very strong fine
threads of polymers to be created: the
effect of draw-strengthening.

Question 24

What is the principle that PMCs use to get good mechanical properties

Answer 24

For good properties, need strong, high-modulus fibres to which load is transferred from the weaker, low-modulus matrix.
Load transfer happens via shear stress at the fibre-matrix interface. Bonding at the interface is therefore important.

Elastic modulus is effected by the fibre orientation

Question 25

What causes fibres to break

Answer 25

Load transfer from the matrix to the fibre causes the tensile stress to peak in the middle (linear, like a triangle distribution). If the peak exceeed the fracture strength of the fibre, it breaks