4 Powder Processing, Polymers And Composites Flashcards
Positives of powder processing
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
What’s powder processing
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.
How is the powder manufactured
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.
Difference between atomisation with water or gas
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.
What is the point of the lubricant added to powder
• 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.
Define sintering
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.
Which shrinks less on sintering, ceramics or metals
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.
How can a more uniform distribution be achieved
• 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
What is HIPing
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.
Why do the parts shrink so much during sintering for Metal injection moulding and powder injection moulding
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.
Advantages of PIM and MIM
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.
Do thermoplastics have permanent cross links between chains
No
Are thermoplastics amorphous are crystalline
They may be amorphous (= glassy i.e. with randomly oriented chains) or semi-crystalline
(containing crystalline regions mixed with some amorphous component).
4 methods for polymer processing
Extrusion, injection moulding, thermoforming, rotational moulding
How can the alignment of polymers be controlled in film blowing
Align polymer chains axially by tension of windup rolls
Align polymer chains in hoop direction
by expanding the bubble.
4 important variables in injection moulding
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).
2 things that influence polymer properties
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
2 ways in which processing affects properties of thermoplastics
Crystallinity and Molecular alignment
What does crystallinity do to volume on solidification
Crystalline regions are more dense so cause volume reduction.
Density can be used to indicate degree of crystallinity.
What affects the degree of crystallinity on cooling of a thermoplastic
Cooling rate, faster equals more crystalline
What does crystallinity do to elastic modulus and tensile strength
Increases them
100% crystalline doesn’t even have a glass transition on a modulus temp graph.
Are optically transparent polymers crystalline
No they’re amorphous
Purpose of Cold drawing
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.
What is the principle that PMCs use to get good mechanical properties
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
What causes fibres to break
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
