W3 Transformative Manufacturing Technologies

Question 1

Transformative processes

Answer 1

Transformative processes are those that involve changing the shape of materials without removal
or additions. Some of the main processes categorised as Transformative are metal casting, metal
forging, metal rolling, metal extrusion and plastic manufacturing techniques.

Question 2

Metal casting

Answer 2

Casting involves pouring molten metal into a mould cavity; allowing it to solidify; and, removing
the part from the mould. This manufacturing process produces intricate shapes in one piece with
internal cavities.

Question 3

Advantages and disadvantages of using casting technologies

Answer 3

Advantages of casting
* Complex shapes with internal cavities
* Economically competitive
* (Near) Net-shape process
* Scalable
Disadvantages?
* High energy requirements
* Inhomogeneous material properties
* Costly lead times
* Postprocessing often required

Question 4

Solution for ensuring adequate flow

Answer 4

Successful casting requires proper mould design to ensure adequate fluid flow in the system. The
typical solution for ensuring adequate flow is the use of riser-gated casting where risers serve as
reservoirs and supply molten metal as the casting shrinks during solidification.

Question 5

Casting technologies

Answer 5

The basic types of casting can be categorized as expendable or permanent mould, depending on
whether the mould is destroyed at the end of the process. Similarly, an expendable or a permanent
pattern might be used to create the necessary moulds.

Question 6

Expandable mould technologies

Answer 6

• Sand casting; Shell mould; Evaporative pattern (lost-foam) casting; Investment casting (lostwax)

Question 7

Permanent mould technologies

Answer 7

• Permanent mould casting; Die casting; Centrifugal casting

Question 8

True Stress and True Strain

Answer 8

True stress is defined as the ratio of the load, P, to the actual or instantaneous crosssectional area, A, of the specimen. Remember than in the definition of engineering stress,
the initial cross-sectional area, A0, is used instead. For small values of strain, engineering and
true strain are similar, but they diverge as deformation increases.

Question 9

Elastic vs Plastic deformation

Answer 9

Metals deform when a load is applied to them. Elastic deformation disappears when the load is
removed. Plastic deformation takes place past the elastic range and is permanent. There is no
universal model to predict yielding of metals but the Tresca and Von Mises criteria are often used.
Other than the state of stress, flow of metal during yielding also depends on temperature and strain
rate.

Question 10

Metal Forging

Answer 10

Forging is a basic process in which the workpiece is shaped by compressive forces applied through
various dies and tooling.Forged parts now include large rotors for turbines;
gears; bolts and rivets; cutlery; hand tools; numerous structural components for machinery, aircraft,
and railroads; and a variety of other transportation equipment.

Question 11

Forging advantages

Answer 11

Because the metal flow in a die and
the material’s grain structure can be controlled, forged parts have good strength and toughness, and
are very reliable for highly stressed and critical applications

Question 12

Forging equipment

Answer 12

Simple forging operations can be performed with a heavy hammer and an anvil, as has been done
traditionally by blacksmiths. However, most forgings require a set of dies and such equipment as a
press or a powered forging hammer. Forging may be carried out at room temperature (cold forging)
or at elevated temperatures (warm or hot forging).

Question 13

Cold vs hot forging

Answer 13

Cold forging requires higher forces (because of
the higher strength of the workpiece material), and the workpiece material must possess sufficient
ductility at room temperature to undergo the necessary deformation without cracking. Cold-forged
parts have a good surface finish and dimensional accuracy. Hot forging requires lower forces, but the
dimensional accuracy and surface finish of the parts are not as good as in cold forging

Question 14

Post forging

Answer 14

Forgings generally are subjected to additional finishing operations, such as heat treating to
modify properties and machining to obtain accurate final dimensions and a good surface finish.
These finishing operations can be minimized by precision forging, which is an important example of
net-shape or near-net-shape forming processes.

Question 15

Open-die Forging

Answer 15

Open-die forging is the simplest forging operation. Although most open-die forgings generally weigh
15 to 500 kg, forgings as heavy as 275 metric tons have been made. Part sizes may range from very
small (the size of nails, pins, and bolts) to very large (up to 23 m, long shafts for ship propellers).
Open-die forging can be depicted by a solid workpiece placed between two flat dies and reduced in
height by compressing it; a process that is also called upsetting or flat- die forging. The die surfaces
also may have shallow cavities or incorporate features to produce relatively simple forgings.

Question 16

Impression-die and closed die forging

Answer 16

In impression-die forging, the workpiece takes the shape of the die cavity while being forged between
two shaped dies . This process usually is carried out at elevated temperatures to lower the required forces and attain enhanced ductility in the workpiece.
* Typically a hot process
* Draft angles necessary
* Parting line can be a critical aspect
* Subsequent trimming operations
needed

Question 17

Forging Parting line

Answer 17

For most forgings, the parting line is located at the largest cross section of the part. For simple
symmetrical shapes, the parting line is normally a straight line at the centre of the forging, but for
more complex shapes, the line may not lie in a single plane. A general guideline for flash thickness
is 3% of the maximum thickness of the forging. The length of the land is usually two to five times
the flash thickness.

Question 18

Draft Angles

Answer 18

Draft angles are necessary in almost all forging dies in order to facilitate removal of the part
from the die. Upon cooling, the forging shrinks both radially and longitudinally, so internal draft
angles (about 7
◦
to 10◦
) are made larger than external ones (about 3
◦
to 5
◦
).
Selection of the proper radii for corners and fillets is important in ensuring smooth flow of the
metal into the die cavity and improving die life. Small radii generally are undesirable because of their
adverse effect on metal flow and their tendency to wear rapidly (as a result of stress concentration
and thermal cycling)

Question 19

Rolling

Answer 19

rollers are used to reduce the thickness of a strip of metal (and in some cases shape
it to a pre-defined profile).

Question 20

Flat rolling

Answer 20

Flat rolling is the simplest form of rolling where the thickness of a strip is reduced between
powered rollers (see diagram below). A metal strip of thickness h0 enters the roll gap and is reduced
to thickness lay by a pair of rotating rolls, each powered individually by electric motors. The surface
speed of the rolls is Vr. The velocity of the strip increases from its entry value of V0 as it moves
through the roll gap; the velocity of the strip is highest at the exit from the roll gap and is denoted
as Vf . The metal accelerates in the roll gap in the same manner as an incompressible fluid flowing
through a converging channel

Question 21

why do frictional forces which oppose motion act between two sliding

Answer 21

Because the surface speed of the rigid roll is constant, there is relative sliding between the roll
and the strip along the arc of contact in the roll gap, L. At one point along the contact length
(called the neutral point or no-slip point) the velocity of the strip is the same as that of the roll. To
the left of this point, the roll moves faster than the strip; to the right of this point, the strip moves
faster than the roll. Consequently, the frictional forces–which oppose motion between two sliding
bodies-act on the strip.

Question 22

Low and controlled friction is induced in rolling through the use of effective
lubricants.

Answer 22

Although friction is necessary for rolling materials, energy is dissipated in overcoming friction.
Thus, increasing friction also increases rolling forces and power requirements. Furthermore, high
friction could damage the surface of the rolled product (or cause sticking). Thus, a compromise is made in practice

Question 23

Metal Extrusion and Drawing

Answer 23

In
extrusion, a cylindrical billet of material is force pushed through a die. Various solid or hollow
shapes can be produced as the result of the process.
Typical products manufactured using this process include railings for sliding doors, aluminium
ladder frames, window frames and structural and architectural shapes. Extrusion can take place at
room temperature (called cold extrusion and often combined with forging) or the material might be
heated.