Forming processes like casting involve pouring molten metal into molds to form parts, with sand casting being the most common method using sand molds around a pattern; designers must understand casting processes to select the most cost effective for the component's size, shape, required quality, and number needed while considering factors like draft angles, parting lines, and machining allowances in the casting design.

1. Chapter 13: Forming Processes

2. 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

3. 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

4. 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

5. 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

6. MANUFACTURING MATERIAL
SEQUENCE IN PREPARING A SAND CASTING

7. MANUFACTURING MATERIAL
MOLD CASTING TECHNIQUES

8. 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)

9. 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

10. MANUFACTURING MATERIAL
PERMANENT MOLD CASTING

11. MANUFACTURING MATERIAL
INVESTMENT MOLD CASTING

12. MANUFACTURING MATERIAL
CASTING EQUIPMENT AND PROCESSES

13. Selection of Process
• 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

14. 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”

15. MANUFACTURING MATERIAL
COOLING EFFECTS ON MOLD CAVITIES
FILLED WITH MOLTEN METAL

16. 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

17. 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

18. MANUFACTURING MATERIAL
DESIGN MEMBERS SO THAT ALL PARTS INCREASE
PROGRESSIVELY TO FEEDER RISERS

19. MANUFACTURING MATERIAL
FILLET ALL SHARP ANGLES

20. 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

21. MANUFACTURING MATERIAL
CAST PART WORKING DRAWING

22. MANUFACTURING MATERIAL
PATTERN WORKING DRAWING

23. MANUFACTURING MATERIAL
DRAFT ANGLES

24. 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
– 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

25. 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