investment castings

LATEST TRENDS IN INVESTMENT CASTING TECHNOLOGY

Dr.Ing.Milan Horek,PhD

Brno University of Technology, CZ

PAPER STRUCTUREA/ Survey of foundry technologies available for casting manufacture B/ Basic principles of lost wax process investment casting C/ Brief history of the investment casting technology D/ Description of individual investment casting process phases- wax pattern manufacture - ceramic shell manufacture - de-waxing - shell heat treatment - metal pouring - finishing operations

E/ Accuracy of investment casting technology F/ Capabilities of modern lost wax technology (examples of castings)

A/ SURVEY OF CASTING TECHNOLOGIES USING SAND MOULDING Ist . GENERATION - clay binders IInd. GENERATION chemical binding a/ Self-hardening processes (plaster,cement,sodium silicate,resin) b/ External hardening processes (sodium silicate, resin) c/ Ceramic moulds (mould firing before pouring) 1. Permanent pattern 2. Lost pattern

LOST FOAM

LOST WAX

IIIrd. GENERATION physical binding

B/ BASIC INVESTMENT CASTING PROCESSWax pattern makingDie

Wax pattern assemblyWax pattern

Dipping into ceramic slurry

Wax pattern Gating system

Ceramic coating

Finished shellCeramic shell

De-waxing

Metal pouring

Removal of shell

Casting cut-off

C/ BRIEF HISTORY OF LOST-WAX PROCESS

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5000

4000

3000

2000

1000

0

1000

2000

EXAMPLES OF HISTORICAL CASTINGS

gold comb casting (300g, Dniepr area, 4th cent.A.D.)

bronze head (Benin 12th.cent.A.D.) bronze statue of Perseus with Meduza head (Cellini, 1540)

D/ DESCRIPTION OF INDIVIDUAL PHASES OF INVESTMENT CASTING PROCESS1. WAX PATTERN MANUFACTURE a/ Die manufacture b/ Wax pattern injection c/ Pattern assembly 2. CERAMIC SHELL MANUFACTURE a/ Step-by-step shell building and drying b/ Shell de-waxing c/ Ceramic shell firing 3. METAL POURING a/ On air b/ Under vacuum 4. FINISHING OPERATIONS a/ Gating system removal b/ Surface finish cleaning, blasting, etc.

1. WAX PATTERN MANUFACTURE a/ Master die making -Using master pattern(using low-melt point alloys, by metal spraying, by galvanization)Master pattern Sand mould Low melting point alloy Plaster METAL SPRAYING Master die

Low melting point alloy Sn,Bi,Pb,Cd Tmel=70C soft Tmel= 138C hard Master die

GALVANIZATION Low melting point alloy Metal master pattern

Ni,Cu,Fe by galvanization

-By machining from solid metal blocks( the most often used method )

1. WAX PATTERN MANUFACTURE b/ Wax pattern makingWAXES USED Present waxes complex materials containing following components:- natural waxes - syntetic waxes - natural resins - syntetic resins - organic fillers - water TYPES of WAXES - Straight (non-filled)

- Filled (30% of filler) - Emulsified(by water, air)

Combination of different raw materials properties to achieve optimal wax characteristics:-melting and congealing point -hardness -viscosity -expansion/contraction -solidification rate - ash content ( 0,05%) - flexibility - surface quality - stability to oxidation - possibility to recycle

BASIC PRINCIPLE OF INJECTION MACHINE FOR WAX PATTERN MAKING

Valve

Wax storage

Sp (piston surface) Transport wax hose

Piston pump

vcInjection nozzle Piston Injection cylinder with wax Master die

EQUIPMENT FOR WAX PATTERN MAKING injection machine +wax melter + conditioner

INJECTION CYCLESpeed controlled phase1000

Pressure controlled phase

2U (pre-set packing pressure)

3

E die cavity filling time K packing time N holding time Z cycle time (till the die opening)

Pressu re [bar]

NOTE: Points 1, 2, 3, 4 a 5 correspondent to the points in following picture

1a

Die opening

11

4 E K Z N

5

Time

INJECTION CYCLE

1 bar

Specif ic volum [cm3/ g]

1200

1a600 1000

4 3

21500

1 2 pressure increase in die cavity decrease of spec.volume 2 3 packing pressure slight decrease of spec.volume 3 4 pressure decrease during wax solidification constant spec. volume 4 5 wax cooling in die decrease of spec.volume (contraction)

5NOTE:

vSP = 1/ specific volume [cm3/g] = 1/vSP density [g/cm3]

TE

(wax temperature during pattern stripping from die)

TM

(wax temperature during injection)

Temperature [C]

EXAMPLES OF INJECTION MACHINES

WAX PREPARATION FOR INJECTION

EXAMPLES OF WAX PATTERNS

1. MANUFACTURE OF WAX PATTERNS c/ Wax pattern assemblyWax pattern assembliesa) Horizontal placed patterns on a special gating rings

Assembly Design Influenced by:-Technique of pattern assembly (glueing/welding) -Shelling technique -De-waxing technologyb) Patterns straight on gating sprue

-Pouring system -Casting cutting-off technique -Standartization of gating systems

2. MANUFACTURE OF CERAMIC SHELL a/ Shell building and dryingI/ DEGREASING OF WAX PATTERNS (removal of remaining separator from the wax pattern surface) II/ DIPPING INTO CERAMIC SLURRY (ceramic slurry consists of filler and binder) Filler heat resistant ceramic flour (fused silica,molochite,zircon,..) Binder colloidal silica sols based on alcohol (alcosols) or water (hydrosols) III/ SHELL DRAINING IV/ STUCCO APPLIED WITH CERAMIC GRIT ( fluid or rainfall systems) Stucco materials silica, molochite, alumina, zircon, atd. Grain size according to the coat number : - first 1-2 prime coats fine particles 0,175-0,25 mm (CASTING SURFACE FINISH) - next 3- x back-up coats coarser 0,25-0,5 mm (MOULD GAS PERMEABILITY) V/ SHELL DRYING (in aircondition room 2-4hours temp. 20 C 1 C , relative humidity 30-60% -according to the type of binder used, sufficient air flow) VI/ REPEATING ( II V) (till the needed number of coats 8-12)

SLURRY MIXING TANKS

SANDERSRainfall Fluidized bed

ROBOTIZIED SHELLING LINES

FULLY ROBOTIZED SHELLING LINE

2. MANUFACTURE OF CERAMIC SHELL b/ Shell de-waxing

KEY PROBLEM : different wax and shell expansion ! Wax expansion bigger,therefore danger of shell cracking during de-wax process. NECCESITY of dilatation gap building on wax pattern surface through THERMAL SHOCK see picture bellow.

TECHNIQUES USED FOR DE_WAXING I/ By overheated steam in boilerclaves II/ By firing flash fire systemCERAMIC SHELL Thermal expansion WAX PATTERN

III/ By microvave heating

2. MANUFACTURE OF CERAMIC SHELL b/ Shell de-waxing Majority foundries use boilerclave systemMAIN REASONS: -Ideal steam properties for heat transfer -Easy to collect de-waxed wax -High efficiency

Typical working parameters:-Steam temperature 160-170C -Working pressure 6-8 atm -Reaching work.pressure in 4-5 s -Controlled speed of pressure drop

DE-WAXING IN BOILERCLAVE

WAX RECYCLING AFTER ITS DE-WAXING WAX RECYCLINGREMOVAL OF IMPURITIES + NEW WAX ADDITION

AUTOCLAVE

BOILER TANK 16 hours IMPURITIES

STORAGE TANK GEAR PUMP

MASTER DIE

2. MANUFACTURE OF CERAMIC SHELL c/ Shell firingGOAL: transfer of amorphous type of SiO2 binder layer into a crystallic one + removal of volatiles matters (waxes remains)1000900 - 1080 C

90060 80 min

800 700

5 C/min

Teplota [ C]

600 500 400 5 C/min

575 C

30 min

300 200 100 0 0 60 120 180 240 300 360100 C 30 min

as [min]

DIFFERENT TYPES OF CERAMIC MOULDS MADE BY LOST WAX PROCESSa) SOLID CERAMIC MOULD COMPACT METHOD

CERAMIC SLURRY a) SOLID CERAMIC MOULD COMBINED METHOD

SLURRY SAND (CEMENT)

2-3 SHELLS

c) BACKFILLED SHELL

d) SELF SUPPORTING SHELL

2-3 SHELLS

5-8 SHELLS

POSSIBILITIES OF USING CERAMIC CORES

Ceramic core manufacturing:-Preparation of ceramic mass -Forming into a needed shape -Heat treatment

3. METAL POURING GRAVITYa/ Classical pouring b/ Roll-overpouringGRAVITY POURING

UNDER VACUUMa/ Melting and gravity pouring under vacuum

b/ Counter-gravity pouring (CLA , CLV)POURING UNDER VACUUM

VACUUM

VACUUM

VACUUM

POSSIBILITIES OF MOLTEN METAL FILTRATIONFILTRATION POSSIBILITIES 1.STRAINER CORES 3.EXTRUDED FILTERS

FILTER 2.FOAM FILTERS 4.GLASS FIBRES FILTERS

4. -

FINISHING OPERATIONS casting cut-off from gating system(by vibration , cutting )

casting surface cleaning(by blasting , grinding )

casting heat treatment

5. -

CASTING QUALITY INSPECTION chemical composition, structure(spectrometr, metalography )

internal casting quality(X-ray, ultrasonic )

casting surface finish(capillar methods )

mechanical properties(tensile strength, hardness, ductility, etc. )

dimensional accuracy

E/ DIMENSIONAL ACCURACY OF INVESTMENT CASTINGS

INVESTMENT CASTING PROCESS

(Lost wax process)CASTING TECHNOLOGY A FINAL MACHINING SHOULD BE ELIMINATED NET SHAPE CASTING AS CAST CONDITIONS DIMENSIONAL AND TOLERANCE REQUIREMENTS

is

where

i.e. our goal is to achieve

to be ready for usage in

complying with all

The average tolerance exhibited by various casting processes (by J.Campbell)

THE FINAL CASTINGS DIMENSIONS

dependent on

Dimensional changes during individual stages of investment casting technology INVESTMENT CASTING PROCESS FINAL CASTING DIMENSIONS

PATTERN DIE DIMENSIONS

PATTERN DIE DIMENSIONmust comply w