Chapter 13: Forming Processes. Forming Processes Forming Processes: the choice of manufacturing...

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


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


PERMANENT MOLD CASTING


INVESTMENT MOLD CASTING


CASTING EQUIPMENT AND PROCESSES

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

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”


COOLING EFFECTS ON MOLD CAVITIESFILLED 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


DESIGN MEMBERS SO THAT ALL PARTS INCREASEPROGRESSIVELY TO FEEDER RISERS


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 casting1⁰ for external surfaces and 2⁰ for internal surfaces


CAST PART WORKING DRAWING


PATTERN WORKING DRAWING


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– 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

Chapter 13: Forming Processes. Forming Processes Forming Processes: the choice of manufacturing...

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