Chapter 12

Chapter 12
Die casting. Image courtesy of Thomas Publishing, New York.
A process = method of shaping, joining or finishing a material
There are hundreds of processes.
Which one to use for our application?
Depends on materials, its size, shape,
required precision, and how many to
be made.
i.e. design requirements
We need to first classify the processes
to be able to select
Processing has two functions
shaping and microstructure
Processing selection depends on material and
shape. The “process attributes” are used as criteria
for selection.
The classes of process. The first row contains the family of shaping processes; below lie the
secondary processes of machining and heat treatment, followed by the families of joining and
finishing (surface treatment) processes.
The taxonomy of the kingdom of process with part of the shaping family expanded. Each
member is characterized by a set of attributes. Process selection involves matching these to
the requirements of the design.
The taxonomy of the process kingdom again, with the families of joining and finishing partly
Poor surface
Best surface
Avoids turbulent flow
and inclusion of
oxides and gas
Casting processes. In sand casting, liquid metal is poured into a split sand mold. In die
casting, liquid is forced under pressure into a metal mold. In investment casting, a wax pattern
is embedded in a refractory, melted out, and the cavity filled with metal. In pressure casting, a
die is filled from below, giving control of atmosphere and of the flow of metal into the die.
Cheaper molds
Molding processes. In injection molding, a granular polymer (or filled polymer) is heated,
compressed, and sheared by a screw feeder, forcing it into the mold cavity. In blow molding, a
tubular blank of hot polymer or glass is expanded by gas pressure against the inner wall of a
split die. In polymer extrusion, shaped sections are formed by extrusion through a shaped die.
In thermo-forming, a sheet of thermoplastic is heated and deformed into a female die by
vacuum or gas pressure.
Can be hot, warm and cold
Deformation processes. In forging, a slug of metal is shaped between
two dies held in the jaws of a press. In rolling, a billet or bar is reduced in
section by compressive deformation between the rolls. In extrusion,
metal is forced to flow through a die aperture to give a continuous
prismatic shape. All three processes can be hot (T > 0.85 Tm), warm
(0.55 Tm < T < 0.85 Tm), or cold (T < 0.35 Tm). In spinning, a spinning
disc of ductile metal is shaped over a wooden pattern by repeated
sweeps of the smooth, rounded tool.
According to deformation temperature
Hot working
Cold working
Working of preheated material.
Low flow stresses (low forces and power requirements)
High ductility (large deformations, excess of 99%
can be done, complex part generate)
3. Cast structure destroyed 75% reduction in area or height.
Hot Working can be carried out by
Adv. Of both worlds. Steel
650-700oC. Temp low
avoid scaling,
Warm working
Normally refers to working at r.t..
Usually follows hot working
Adv. since no cooling problems and
scales better surface finish and
thinner walls possible`. Can control
final properties of work piece
through annealing/recrystallization.
Disadv. R.t <0.5Tm  high flow
stresses high power requirements
Non-isothermal forming. Work piece higher temp. than dies. Variable cooling  variable properties
thermal fatigue of tooling
Isothermal forming avoids above problems, but need very good die material and lubricants
Controlled Hot Working -conducted non isothermally to impart desirable properties
Cogging, Drawing out
Impression Die and Closed Die Forging
Impression die Forging
Material fills entire mold with no flash
Direct vs. indirect extrusion
Extrusion speed
Extrusion ratio
Extrusion Temperature
Initial area
Deformed area
Extrusion pressure
Extrusion constant
Extrusion ratio
Die Geometry:
Flat vs. conical die
(surface quality)
Hydrostatic extrusion
no friction, long billets
can be used.
Cold extrusion
of copper tubes
Powder processing. In die-pressing and sintering the powder is compacted in a die, often with
a binder, and the green compact is then fired to give a more or less dense product. In hot
isostatic pressing, powder in a thin, shaped shell or preform is heated and compressed by an
external gas pressure. In powder injection molding, powder and binder are forced into a die to
give a green blank that is then fired. In slip casting, a water-based powder slurry is poured
into a porous plaster mold that absorbs the water, leaving a powder shell that is subsequently
Composite forming methods. In filament winding, fibers of glass, Kevlar, or carbon are wound
onto a former and impregnated with a resin-hardener mix. In roll and spray lay-up, fiber
reinforcement is laid up in a mold onto which the resin-hardener mix is rolled or sprayed. In
vacuum-bag and pressure-bag molding, laid-up fiber reinforcement, impregnated with resinhardener mix, is compressed and heated to cause polymerization. In pultrusion, fibers are fed
through a resin bath into a heated die to form continuous prismatic sections.
FIGURE 13.10
Rapid prototyping. In deposition modeling and ballistic particle manufacture (BPM), a solid
body is created by the layer-by-layer deposition of polymer droplets. In stereo-lithography
(SLA), a solid shape is created layer by layer by laser-induced polymerization of a resin. In
direct mold modeling, a sand mold is built up layer by layer by selective spraying of a binder
from a scanning print-head. In laminated object manufacture (LOM), a solid body is created
from layers of paper, cut by a scanning laser beam and bonded with a heat-sensitive polymer.
FIGURE 13.11
Machining operations. In turning and milling, the sharp, hardened tip of a tool cuts a chip from
the work piece surface. In drawing, blanking, and stretching, sheet is shaped and cut to give
flat and dished shapes. In electro-discharge machining, electric discharge between a graphite
electrode and the work piece, submerged in a dielectric such as paraffin, erodes the work
piece to the desired shape. In water-jet cutting, an abrasive entrained in a high-speed water
jet erodes the material in its path.
FIGURE 13.12
Joining operations. In adhesive bonding, a film of adhesive is applied to one surface, which is
then pressed onto the mating one. Fastening is achieved by bolting; riveting; stapling; pushthrough snap fastener; push-on snap fastener; or rod-to-sheet snap fastener. In metal fusion
welding, metal is melted, and more is added from a filler rod to give a bond or coating. In
thermoplastic polymer welding, heat is applied to the polymer components, which are
simultaneously pressed together to form a bond.
FIGURE 13.13
Finishing processes to protect and enhance properties. In mechanical polishing, the
roughness of a surface is reduced and its precision increase, by material removal using finely
ground abrasives. In electroplating, metal is plated onto a conducting work piece by electrodeposition in a plating bath. In heat treatment, a surface layer of the work piece is hardened
and made more corrosion resistant by the inward diffusion of carbon, nitrogen, phosphorous,
or aluminum from a powder bed or molten bath. In anodizing, a surface oxide layer is built up
on the work piece (which must be aluminum, magnesium, titanium, or zinc) by a potential
gradient in an oxidizing bath.
FIGURE 13.14
Finishing processes to enhance appearance. In paint spraying, a pigment in an organic- or
water-based solvent is sprayed onto the surface to be decorated. In polymer powder-coating
a layer of thermoplastic is deposited on the surface by direct spraying in a gas flame, or by
immersing the hot work piece in a bed of powder. In silk-screen printing, ink is wiped onto the
surface through a screen onto which a blocking pattern has been deposited, allowing ink to
pass in selected areas only. In pad-printing, an inked pattern is picked up on a rubber pad and
applied to the surface, which can be curved or irregular.
FIGURE 13.15
The extent of the material bubbles on the property charts gives an idea of the degree to which
properties can be manipulated by processing.

similar documents