Chapter 13: Forming Processes
Forming Processes
• Forming Processes: the choice of manufacturing process
depends on the size, shape and quality of the component
• Designers must be familiar with different manufacturing
processes (advantages, disadvantages, cost, machines
necessary) to wisely recommend a cost effective method
Casting Processes
• Casting is the process whereby parts are produced by pouring
molten metal into a mold. Casting processes classified by…
– Type of mold or pattern
– Pressure or force to fill mold
 Conventional sand, shall and plaster molds use a permanent pattern,
but the mold is used only once.
 Permanent molds and die-casting dies are machined in metal or
graphite sections and are employed for a large number of castings
 Investment castings and the relatively new full mold process involve
both an expendable mold and an expendable pattern
Sand mold casting
• Most widely used for metals
• Permanent pattern of metal or wood that shapes the mold
cavity when loose molding material (fine sand & a binder)is
compacted around the pattern
• Sections:
– Bottom (drag)
– Top (cope)
– Intermediate (cheeks) when required
 Molten metal is poured into the sprue, connecting runners
channel metal to the mold
 Riser cavities located over heavier sections of the casting
 Vents release gases
 Core for hollow castings
Sand Mold Castings
• Shrinkage allowance-Metal or wooden pattern
slightly larger than part to be cast
• Drafts or slight tapers allow for easy withdrawl
from the sand mold
• After a sand mold is used the sand is broken
and casting removed. Excess metal, gates,
risers are removed and remelted
MANUFACTURING MATERIAL
SEQUENCE IN PREPARING A SAND CASTING
MANUFACTURING MATERIAL
MOLD CASTING TECHNIQUES
Other Castings
• Shell mold refractory sand is bonded by a resin forming a thin
shell mold and a reusable, heated metal pattern plate
• Plaster mold plaster of paris and fillers mixed with water form
a slurry which is poured around a reusable metal or rubber
pattern and set to form a gypsum mold
• Permanent mold uses a permanent metal mold to produce
many castings
• Investment mold investment refers to the refractory material
used to encase the wax patterns. An expendable pattern and
mold used
• Full mold a consumable pattern is made of foamed plastic
(individual castings)
Other Casting Continued
• Centrifugal a permanent mold is rotated rapidly about
the axis of the casting. Castings are smooth, sound,
and clean on the outside
• Continuous continuously pouring molten metal into a
water-jacketed mold. The metal solidifies in the mold
& the solid billet exits continuously into a water spray
(uniform section rounds, ovals, squares, rectangles,
plates)
• Die one of the least expensive, fastest & most efficient
methods to produce metal parts. Molten metal is
formed into a die or mold. Used for nonferrous alloys.
Submerged-plunger & cold-chamber types
MANUFACTURING MATERIAL
PERMANENT MOLD CASTING
MANUFACTURING MATERIAL
INVESTMENT MOLD CASTING
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CASTING EQUIPMENT AND PROCESSES
Selection of Process
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Type of metal
Number of castings required
Shape & size of castings
Dimensional accuracy required
Casting finish required
Economics
The number of finishing operations-the processes that
provide the closest dimensions, best surface finish, & the
most intricate detail usually require the smallest number of
finishing operations
Solidification of metal in a mold
• Heat dissipates from the surface through the mold
• Solidification commences from the outside &
progresses inward in a series of layers
• Solidifying metal contracts in volume & a shrinkage
cavity may form in the center
• Final castings are smaller than the mold cavity
• Fillets and radii at corners prevent rapid
cooling/shrinkage at corners – “hot spots”
MANUFACTURING MATERIAL
COOLING EFFECTS ON MOLD CAVITIES
FILLED WITH MOLTEN METAL
General design rules
• Casting soundness-feeder heads can be placed to offset liquid
shrinkage
• Fillet or round all sharp angles
• Bring the minimum number of adjoining sections together
• Design all sections as nearly uniform in thickness as possible
• Avoid abrupt section changes-eliminate sharp corners at
adjoining sections: not exceed a 2:1 ratio
• Design ribs for maximum effectiveness-increase stiffness and
reduce mass
• Avoid bosses & pads unless absolutely necessary
General design rules continued
• Use curved spokes-less likely to crack
• Use an odd number of spokes-more resilient to casting
stresses
• Consider wall thicknesses
– Gray-iron & aluminum: .16 in minimum
– Malleable iron & steel: .18 in minimum
– Bronze,brass,magnesium: .10 minimum
 Parting lines: a line along which the pattern is divided for
molding or along which sections of the mold separate
(consider shape of casting, elimination of machining on
draft surfaces, methods of supporting cores, location of
gates & feeders)
 Drill holes in castings: small holes are drilled and not cored
MANUFACTURING MATERIAL
DESIGN MEMBERS SO THAT ALL PARTS INCREASE
PROGRESSIVELY TO FEEDER RISERS
MANUFACTURING MATERIAL
FILLET ALL SHARP ANGLES
Drafting practices-information for cast
parts
 Material-physical characteristics of the metal
 Machining allowances- sufficient excess metal should be provided
for all machined surfaces (Table 13-2 guidelines)
 Surface texture
 Draft angles
 Limits of cast surfaces that must be controlled
 Locating points
 Parting lines
 Fillets & radii-generous should be specified on the drawing
 Casting tolerances-Table 13-2 general guidelines
 Draft-a draft or taper on all surfaces perpendicular to the parting
line to facilitate removal of the pattern & ejection of the casting
1⁰ for external surfaces and 2⁰ for internal surfaces
MANUFACTURING MATERIAL
CAST PART WORKING DRAWING
MANUFACTURING MATERIAL
PATTERN WORKING DRAWING
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DRAFT ANGLES
Casting Datums
• In many cases a drawing is made of the fully machined end
product, & casting dimensions, draft, machining allowances
are left entirely to the pattern maker or foundry worker.
• For mass-production it is advisable to make a separate casting
drawing with carefully selected datums to ensure that parts
will fit into machining jigs and fixtures and will meet final
requirements after machining.
• Two sets of datum surfaces to provide reference points
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Casting
Machining
Datum surface or base surface for casting-Datum A
Secondary & tertiary surfaces at right angles to eachother & primary
datum surface- Datum B & Datum C
Machining Datum
• Primary datum surface for machining-Datum D is the first surface
on the casting to be machined
• Secondary & Tertiary datum surfaces for dimensioning purposes
• Datum-locating dimension-the dimension between eaching casting
datum surface and the corresponding machining datum surface
• Dimensions-directly from the datums to all main surfaces
– Regular point-to-point used to maintain a particular relationship
between 2 or more surfaces or features (thickness of ribs,
height of bosses, projections, depth of grooves, diameters, radii,
center distances between holes or similar features
– Dimension to surfaces or surface intersections, not radii centers
or nonexistent center lines & dimensions should not be
repeated