INVESTMENT CASTINGAlso called LOST WAX PROCESS- used during 4000-3000 BC

• Die for casting wax pattern made with allowances for wax and metal.

• Pattern and gating systems made of wax (bee wax, aera wax, paraffin) or plastic (polystyrene) by injecting -in molten condition - into the metal die

• PRECOATING- The pattern dipped in a slurry of refractory material (fine 325 mesh silica &binders, water, ethyl silicate, acids), and sprinkled with silica sand

• This pattern with initial coating dried, coated repeatedly to increase thickness

• The one piece mould is dried

• DEWAXING- Inverted and heated to 900C -1750 C for 12 hours

• Wax melts. Can be reclaimed and reused.

• Mould fired to 6500C-10500C for about 4 hours

• POURING- Metal poured, allowed to solidify

• Mould broken, casting taken out

INVESTMENT CASTING- SEQUENCES

Plus and Minus points

• Very good dimensional accuracy

• No or very little finishing

• Intricate and thin shapes possible

• About 40 kg parts cast

• Both for ferrous and nonferrous alloys

• Suited for mechanization

• Careful handling needed,as the patterns are not strong.

• Close control of process needed

• Labour and material costs high, but high melting point alloys cast with good surface finish & close tolerances.

• Eg: gears, cams, valves, ratchets, turbine blades, electrical & electronic components etc.

INTRODUCTION • Investment casting, often called lost wax casting, is

regarded as a precision casting process to fabricate near-net-shaped metal parts from almost any alloy. Although its history lies to a great extent in the production of art, the most common use of investment casting in more recent history has been the production of components requiring complex, often thin-wall castings. A complete description of the process is complex. But, the sequential steps of the investment casting process are as below, with emphasis on casting from rapid prototyping patterns.

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Fig: 1- Investment casting process NITC

• The investment casting process begins with fabrication of a sacrificial pattern with the same basic geometrical shape as the finished cast part

• Patterns are normally made of investment casting wax that

is injected into a metal wax injection die. Fabricating the injection die is a costlier process and can require several months of lead time.

• Once a wax pattern is produced, it is assembled with other wax components to form a metal delivery system, called the gate and runner system. The entire wax assembly is then dipped in a ceramic slurry, covered with a sand stucco, and allowed to dry. The dipping and stuccoing process is repeated until a shell of ~6-8 mm (1/4-3/8 in) is applied.

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Fig. 2- Investment casting process - dewaxing NITC

• Once the ceramic has dried, the entire assembly is placed in a steam autoclave to remove most of the wax.

• After autoclaving, the remaining amount of wax that soaked

into the ceramic shell is burned out in a furnace. At this point, all of the residual pattern and gating material is removed, and the ceramic mold remains.

• The mold is then preheated to a specific temperature and filled with molten metal, creating the metal casting. Once the casting has cooled sufficiently, the mold shell is chipped away from the casting.

• Next, the gates and runners are cut from the casting, and final post-processing (sandblasting, machining) is done to finish the casting.

(The CAD solid model, the shell, and the pattern produced in the QuickCast process is schematically shown)

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Fig. 3. Investment casting process –Preheating and pouring

                                                             

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SHELL MOULDING-DEVELOPED IN 1940s

• THERMOSETTING RESINS USED AS BINDERS

• PHENOL FORMALDEHYDE(3% BY WT.OF SAND)

• 15% HEXAMETHYLENE TETRAMINE ADDED TO GIVE THERMOSETTING PROPERTY

• RESIN SETS AT ABOUT 2500 C (1750 C- 3700 C)• SHELL OF 4 to 9 MM FORMS • SHELL MOULDING MACHINES

USED• PATTERN MADE OF METAL• MOUNTED ON MATCH PLATES

WITH GUIDE PINS

• PATTERN HEATED TO 2500 C• CLEANED WITH COMPRESSED

AIR, PETROLEUM SPIRIT APPLIED

• PATTERN INVERTED, PLACED IN DUMP BOX CONTAINING SAND MIX , LOCKED

• DUMP BOX INVERTED, KEPT FOR A FEW MINUTES, (1-3 MINS) SHELL FORMS

• RE-INVERTED, SHELL FORMED IS TRIMMED, REMOVED USING GUIDE PIN EJECTION,

• ANOTHER HALF ASSEMBLED, READY FOR POURING

SHELL MOULDING - SEQUENCES

+ AND - POINTSAdvantages• Least moulding material

(about 5%)

• Quick operating cycle ( <5 minutes)• Ready for pouring• Excellent surface finish (inner

surface of shell smooth, less resistance to flow of metal)

• Dimensional tolerances superb• Very thin possible• Defects minimum• Good collapsibility• Any metal can be cast• Mechanisation of process

Disadvantages• Initial cost high• Metal patterns expensive• Binder expensive• Reclamation is difficult• Even though 100 kg possible,

generally limited to 10 kg-------------------------------------------When production runs high, cost of

pattern and cost of resin can be compensated

Reduced cleaning, machining and finishing are added advantages

Small mechanical parts – gear housings, cylinder heads, cylinders, connecting rods etc made by SHELL MOULDING

V-Process 1. Pattern (with vent holes) is placed on hollow carrier plate.2. A heater softens the .003" to .007" plastic film. Plastic has good elasticity and high plastic deformation ratio.3. Softened film drapes over the pattern with 300 to 600 mm Hg vacuum acting through the pattern vents to draw it tightly around pattern.4. Flask is placed on the film-coated pattern. Flask walls are also a vacuum chamber with outlet shown.5. Flask is filled with fine, dry unbonded sand. Slight vibration compacts sand to maximum bulk density.6. Sprue cup is formed and the mold surface leveled. The back of the mold is covered with unheated plastic film.7. Vacuum is applied to flask. Atmospheric pressure then hardens the sand. When the vacuum is released on the pattern carrier plate, the mold strips easily.8. Cope and drag assembly form a plastic-lined cavity. During pouring, molds are kept under vacuum.9. After cooling, the vacuum is released and free-flowing sand drops away leaving a clean casting, with no sand lumps. Sand is cooled for reuse.

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Benefits Of Using The V-Process:

• Very Smooth Surface Finish • 125-150 RMS is the norm. Cast surface of 200 or better, based on The Aluminum

Association of America STD AA-C5-E18. • Excellent Dimensional Accuracy • Typically +/-.010 up to 1 inch plus +/-.002 per additional inch. Certain details can be held

closer. • +/-.010 across the parting line. • Cored areas may require additional tolerances. • Zero Draft • Eliminates the need for machining off draft to provide clearance for mating parts and

assembly. • Provides consistent wall thickness for weight reduction and aesthetic appeal. • Allows for simple fixturing for machining and inspection.

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With wall thickness to 0.12 in., this casting requires moderate strength, good stability and resistance to stress-corrosion cracking to 600F (316C).

This casting exhibits mechanical properties at room temperature of 32-ksi tensile strength, 24-ksi yield strength and 1.5% elongation, while maintaining a 16-ksi tensile strength and 4% elongation at 600F.

The component's as-cast surface finish meets the customer's requirements, and the invest casting process reduced the customer's finishing and machining costs.

SEMI-PERMANENT MOLD CASTING

Semi-permanent mold is a casting process - producing Aluminum alloy castings - using re-usable metal molds and sand cores to

form internal passages within the casting. Molds are typically arranged in two halves - the sand cores being put into place before the two halves are placed together. The molten metal flows into the mold cavity and surrounds the sand core while filling the mold cavity. When the casting is removed from the mold the sand core is removed

from the casting leaving an internal passage in the casting.

The re-usable metal molds are used time and again, but the sand cores have to be replaced each time the product is cast, hence the term semi-permanent molding.

Semi-permanent molding affords a very high precision quality to the

casting at a reduced price compared to the sand casting processes.

NO BAKE CASTING

The No-Bake Sand Casting process consists

of sand molds created using a wood, metal or plastic pattern. Sand is mixed with a urethane

binder and deposited into a box containing the

pattern (and all necessary formers and inserts) for

pouring. Filling a wood mold with sand

PRODUCTS 15. TENSIONER PULLEY

Material: Gray iron

Process: Nobake sand

Casting Supplier: Wellsville Foundry, Wellsville, Ohio

This 175-lb component is used as a brake that puts tension on a 4 ft. wide roll of rubber feeding into a tire press.

Converted from a steel fabrication (two ring burn-outs with spokes), the

foundry provided the end-user with a 50% cost savings.

Previously made from two steel stampings welded together with two tube sections and subsequently tin-plated for corrosion resistance (r), this bronze cast component (l) now is a one-piece permanent mold casting.

The cast component (l) exhibits good corrosion resistance (without plating or painting), 50 ksi yield strength and 95 ksi tensile strength.