2015/16 Paper Flash cards

Question 1

With the use of sketches explain the steps in a photo-chemical machining process as it is applied to the chemical blanking of a flat part

Answer 1

1. Clean raw blank part
2. Apply resist (maskant) by dipping, spraying & painting
3. Place negative on resist.
4. Expose to UV light
5. Develop to remove resist from areas to be etched.
6. Etching in progress.
7. Etching process complete
8. Remove resist & clean part.

Question 2

How does the screen resist method differ from the photographic resist method in chemical machining?

Answer 2

Screen resist:

• Maskant is applied by silk screening method
• Mask is painted onto the workpart surface through a silk or stainless steel mesh.
• Embedded in the mesh is a stencil that protects the areas to be etched from being painted

Photographic resist:

• Uses photographic techniques to perform the masking using photosensitive chemicals
• Applied to the surface & exposed to light through a negative image of the desired areas to be etched.
• Desired area is protected.

Question 3

Discuss how the etch factor is related to the depth of cut in a
chemical machining operation?

Answer 3

Fe = d / u
Fe = etch factor, d = depth of cut; u = undercut

 Undercut is directly related to the depth of cut.

 The constant of proportionality for the material is called the etch factor

Question 4

Electro-chemical machining is, similarly to chemical machining,
classed as a non-traditional machining technology. Explain its
principal method of material removal.

Answer 4

• Electrolysis -the reverse of electroplating
• Molecules of an acid, base or salt are dissociated into positively & negatively charged ions; these are the charge carries in the solution & allow electric current.
• ECM removes metal from a conductive workpiece by anodic dissolution .

-Material is deplated from the workpiece (anode) with the tool (mirror image of
desired component shape) being the cathode.

-Plating of cathode is prevented by deplated material being washed away by fast
flowing electrolyte solution in a water solution

Question 5

Give 2 reasons explaining why chemical milling is not the

preferred choice to generate deep cavities.

Answer 5

-Very slow process with small material removal rates. In order to improve
production rates the bulk of the work should (if possible) be shaped by other
processes (such as machining) prior to chemical machining

-Surface finish depends on the depth of penetration and with increasing depth the
surface finish becomes worse

• Dimensional inaccuracy increases with deeper cavities due to undercut

Question 6

Discuss fundamental technologies which were necessary to

successfully establish Stereolithography.

Answer 6

Series of technological advances include
 high-speed computers
 computer-aided design (CAD) – 3D solid modelling suites
 precise motion control
 UV lasers
 photo-curable polymers

Question 7

A referee’s whistle containing a “pea” and a lanyard hole, similar
to Figure Q2(b), is to be fabricated using the Fused Deposition
Modelling technology. Describe how you would build this part
(sequence, orientation etc.). Explain your reasoning.

Answer 7

• As FDM requires a support structure, one main consideration apart from achieving dimensional accuracy is to:
• minimise the amount of layers
• the amount of support structure (material cost, extra time to deposit support structure with dual head, removal time of support structure.

-Building the pea requires a thin support structure to the main body of the whistle
which can be broken off.

- Build sequence and support structure which depends on orientation will be
different and the following needs to be considered to produce the part
o Main internal resonance cavity
o Open entry for air (mouth piece)
o Open exit for air outlet
o Outer round shaped body
o Attachment to build internal ball
o Lanyard hole

Question 8

The technologies of Stereolithography and Fused Deposition
Modelling may both require support structures depending on the
part geometry. Discuss the principal differences between the
support structures used by these two technologies.

Answer 8

Stereolithography
 Support structure uses the same material as the one used to make the
component
 One laser to polymerise component structure and support structure

Fused Deposition Modelling
 Support structure can be generated from a different material than the one used
to make the component
 Support structure can be made of material which is soluble in a solution of
water/detergent
 Multiple print heads allow different materials to be used

Question 9

Explain how the build packet (build setup) differs between

Selective Laser Sintering and Stereolithography.

Answer 9

Selective Laser Sintering
 Build packet for Selective Laser Sintering can be stacked and the entire build
chamber can be used as the uncured powder supports the components
Stereolithography
 Parts made by Stereolithography must originate on the base plate of the build
chamber
 Parts cannot be stacked due to the liquid resin which in its uncured state cannot
be used as support structure

Question 10

How does the casting industry make use of Rapid Prototyping
(Additive Manufacturing) technologies? Explain 2 applications in
the casting industry, each using a different Rapid Prototyping
technology.

Answer 10

Pattern making (rapid tooling – indirect tooling)
 “Quickcast” pattern making using Stereolithography for Investment Casting
 Pattern making for Sand/Plaster Casting (pattern gets removed prior to casting)
Tool making (generally for short runs)
 AM/RP technologies use silicone rubbers for mould making used in vacuum
casting
 AM/RP technologies use their basic starting material and the mould halves spray
painted or plated with a metal coating
 3D powder printing technology uses a starting material with casting sand like
properties and ejects binder to produce the mould halves

Question 11

The supplier of the aluminium sheet metal does not use kanban.
Discuss whether setting up an internal kanban for the delivery of
the sheet metal to the aluminium casing manufacturer is a
sensible alternative to the current weekly delivery

Answer 11

Benefit
 Production control can order aluminium sheets based on actual usage; prevents
“guessing” using MRP data
 Moving to a more frequent delivery of aluminium sheets (similar to a milk run)
can eliminate large inventories at goods-in
 Can provide a smooth and steady demand for supplier

Question 12

Inventory is one of the Seven Wastes and often categorised by
“its position” in the value stream and by “its purpose”. Give
examples of 3 different types of physical inventory in either
category.

Answer 12

1) By position in the value stream
o Raw materials
o Work-in-process
o Finished goods

2) By purpose
o Safety stock
o Buffer stock
o Shipping stock

Question 13

Compare and contrast the failure mechanisms of ‘fatigue’ and ‘stress
corrosion cracking’.

Answer 13

• Brief comparison of fatigue failure versus stress corrosion cracking failure.
• Fatigue failure produced by oscillating stress and at values less than short term yield/fracture stress.
• Cracks initiate at surface strictions, defects etc grow initially down slip bands before turning perpendicular to major tensile stress sub-ortical crack growth proceeds leaving smooth surface showing possibly ‘beach’ in clause shell’ markings land strictions at finer level), before crack accelerates when it reaches critical size, giving more brittle appearance.
• Stress corrosion: static loader internal stress. susceptible material and environmental agent. e.f. ammonia/brass, hydrogen/steel. Inter-granular cracks is usually a dent with fairly brittle failure, etc.

Question 14

Define all the terms in the equations and outline how these equations are
applied to obtain the number of cycles to failure caused by a pre-existing
sub-critical defect.

Answer 14

Define terms k > kc - stress intensity factor > fracture toughness of material.

Question 15

Compare and contrast the characteristic appearance of ‘fast fracture’ in
metals with those of ‘fatigue failure’.

Answer 15

Fast fracture:- crack propagates rapidly. At macro level (low powered optical microscope) - bright appearance - faceted due to cleavage or inter-granular fracture or for very ductile material, dull and fibrous due to microvoid coalescence. Generally, rougher than fatigue, possibly more evidence of ridges or in the specimen, chevron markings that point back to origin. At higher magnification the microvoid coalescence and cleavage facets can be seen.

Fatigue fracture:- At macro level - two zones visible - sow sub-criticial crack growth region and fast track growth region [“beack markings”] or [“clamshell”} markings may be visible and radiate from crack origin. At micro level (eg in SEM) striations may be seen, indicating successive jumps of crack size in slow growth region - which is generally smooth, while the fast growth region is as above.

Question 16

Explain what is meant by the term ‘plane strain’

Answer 16

Plain strain exists at the tip of a crack in a loaded thick specimen.

The high stress at the crack tip in the “x” direction causes poisson’s contraction in the “y” and “z” direction, but this is restricted by the lower stressed region further from the crack - thus inducing a triaxial stress, with stresses σx, σy & σz - the only strain is in the yz plane. Near the surface or in thin specimens , σz > zero and a state of plane stress exists.

Question 17

Define the other terms in the equation and comment on the relative
values of Gc for metals, polymers and ceramics.

Answer 17

Gc = toughneess
E = Modulus
C = Crack length
v = Poisson's ratio

For ductile metals Gc is high; for ductile polymers Gc is mediocre and for ceramics.

Question 18

Define or explain, using sketches where appropriate:

Anelasticity and mechanical hysteresis.

Answer 18

where strain lags behind stress but is still elastic (recoverable). Cycling the stress produces a hysteresis loop with energy lost on each cycle equal to area in loop.

Question 19

Define or explain, using sketches where appropriate:The thermo-elastic effect.

Answer 19

this is rapid tensile straining which is adiabatic as temperature of material falls, compression raises the temperature.

Question 20

Define or explain, using sketches where appropriate: Relaxation modulus

Answer 20

ER = stress applied/strain after given time

Question 21

Define or explain, using sketches where appropriate:

specific damping capacity

Answer 21

Damping capacity is important in minimising machinery noise or vibration or to minimise amplitude of vibration - less danger of fatigue.

• In rubber, absorbs energy and increases friction: cast iron has superior damping capacity to mild steel - the large graphite flakes help absorb vibration - weak interface.
• In rubber, damping even higher due to molecular movement.

Specific damping capacity=
ΔW /W i.e. energy absorbed per cycle divided by total strain energy input.

Question 22

Difference between Isotactic, atactic & syndiotactic

Answer 22

• Isotactic has functional (side) groups on the same side.
• Atactic - side groups are random.
• Syndiotactic - is alternate on each met.
• Isotactic is easier to crystallise; closer packing leads to higher bonding forces between molecules - higher molecules.
• Atactic, due to random side groups, is non crystalline, less densely packed, so lowest modulus.

Question 23

Explain how a typical Parrots beak rupture shape in a medium density polythene pipe is formed

Answer 23

• Parrots beak rupture shape: since hoop stress is > longitudinal stress, the molecules flow firstly more in the circumferential direction and become more aligned.
• The weak inter-molecular bonds are then split by the lower longitudinal stress.