Composition, Control and Use of
Investment Casting Wax
Richard Hirst FIMMM
Agenda
• Composition and Structure
• Properties
• Categories
• Recycling
• Selection
• Control and Testing of Wax
• Relationship between Wax and Injection
Characteristics
Investment Casting Wax
• Wax is the oldest thermoplastic material known to man
• Beeswax was utilized in the lost wax process by
craftsmen in the ancient civilizations of China & Egypt • Today the name ‘wax’ applies to any substance
having wax-like properties
– better described as industrial moulding compounds
• If the pattern is wrong, the casting will be wrong • It follows that the choice of wax is critical
Composition of Investment Casting Wax
• Investment casting waxes are complex
formulations containing many components :
• Paraffin wax
• Microcrystalline wax
• Resins - strong, viscous, brittle • Polymers - ductile
• Fillers - reduce contraction, improve mechanical strength
• The % addition and control of these additives is
critical in determining the properties of the wax,
they make each wax unique
Structure of Investment Casting Wax
• Many variations are formulated to suit differing requirements
• Key properties such as melting point, hardness,
viscosity, expansion and contraction, setting rate, etc are all influenced by the structure and composition of the wax compound
• Understanding the properties of the individual components and how they interact is essential in meeting foundries individual requirements
• The complex composition manifests itself in a physical behaviour different to that of other substances
Phase Changes of a Typical Wax
• Unlike other homogeneous chemical
compounds, wax does not melt immediately
on heating but passes through several
intermediate states :
Expansion & Contraction of Wax
• The structure and components used in an investment casting wax will influence the expansion and
contraction
• Like other materials wax expands on heating and contracts on cooling
• In comparison with a metal the expansion is relatively high
• Wax expansion and contraction rates are not uniform but vary with phase and structure changes during
Categories of Investment Casting Wax
• Pattern wax
– straight (unfilled) – emulsified – filled• Runner wax
• Water Soluble wax
• other Special wax
– dipping/patching/adhesive
Wax Recycling
• Customers used wax from autoclave returned
to wax manufacturer for reprocessing
– cleaned & filtered
– additives to adjust properties to specification
• Reclaim wax for runner systems
• Reconstituted wax for pattern production
• Environmental and economic benefits
Wax Selection
• Wax selection is determined by the particular application and the process criteria are :
• Injection
– state/setting rate/rheology/geometry/equipment/surface finish
• Removal,handling,assembly
– strength/hardness/setting rate/stability/ability to make joins
• Dimensional control
– thermal expansion/contraction/cavitation/distortion
• Shelling/mould making
– strength/wetability/resistance to binders and solvents
• Dewax and burnout
– melting point/viscosity/thermal expansion/thermal diffusivity/ash content
Process Control of Wax Products
Close control is achieved by :
• Automatic temperature control of the
melting process
• Data monitoring of both temperature of the
melt and process equipment
• Extensive testing
• Statistical Process Control of both raw
materials and finished products
Statistical Process Control
O b s e r v a t io n In d iv id u a l V a lu e 3 4 2 3 4 0 3 3 8 3 3 6 3 3 4 3 3 2 3 3 0 3 2 8 3 2 6 3 2 4 3 2 2 3 2 0 3 1 8 0 .0 3 0 .0 2 0 .0 1 0 .0 0 -0 .0 1 _ X= 0 .0 0 8 1 1 U C L= 0 .0 2 7 0 9 LC L= -0 .0 1 0 8 7 O b s e r v a t io n M o v ing R a ng e 3 4 2 3 4 0 3 3 8 3 3 6 3 3 4 3 3 2 3 3 0 3 2 8 3 2 6 3 2 4 3 2 2 3 2 0 3 1 8 0 .0 3 0 .0 2 0 .0 1 0 .0 0 _ _ M R = 0 .0 0 7 1 4 U C L= 0 .0 2 3 3 2 LC L= 0 1 1C ontr ol c ha r t of inc oming a s h le v e ls on fille r us e d.
Both incoming test results and in-house test results on raw
materials are fed into a control chart, and use based on the results determined.
The use of SPC also allows tracking of trends in suppliers processes, and allowing pre-emptive corrective action.
Out of Control (Not used)
Wax Specification
• Products are tested to
establish whether they meet specification
• The specification is determined during the development stage
• it is a reflection of the capability of the process • a provisional specification is
introduced which is then reviewed after 12 batches • The data is used to generate
a process capability which is built into the formalised
specification 74 73 72 71 70 69 68 LB UB Process Data Sample N 13 StDev(Within) 0.679669 StDev(Overall) 0.851196 LB 68 Target * UB 74 Sample Mean 70
Potential (Within) Capability
Overall Capability Pp * PPL * PPU * Ppk * Cpm Cp * * CPL * CPU * Cpk * Observed Performance PPM < LB 0.00 PPM > UB 0.00 PPM Total 0.00
Exp. Within Performance PPM < LB * PPM > UB * PPM Total *
Exp. Overall Performance PPM < LB * PPM > UB * PPM Total *
Within Overall
Process Capability of DMP (12 batches)
SPECIFICATION WOULD NEED ADJUSTING IN LINE WITH RECORDED DATA.
Test Methods
Industry recognised physical tests are :
• Congealing Point
• Drop Melt Point
• Viscosity
• Penetration
• Ash
• Filler content
DSC, Oscillation
DSC analysis gives an indication of the melting phases of different compounds. 40 45 50 55 60 65 70 75 80 85 T [°C] 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 ƒ ' [P a s ] 0 10 20 30 40 50 60 70 80 90 Ì [°] 060085A Osc ƒ' = f (T) Ì = f (T) 060107 (XL21) Osc ƒ' = f (T) Ì = f (T)
Oscillation curves give an
indication of the shear properties of wax at different temperatures.
Gives a good indication of the setting properties of a wax.
Volumetric Expansion
Com parison of Microcrystalline w axes and Parrafin w axes (Volum teric Expansion.)
-5.000% 0.000% 5.000% 10.000% 15.000% 20.000% 25.000% 25.1541 30.1761 35.036 40.058 45.242 50.102 55.2859 60.1459 65.1679 70.0279 75.0498 80.0718 85.2558 90.4398 95.1377 100.16 LMP from Korea MMP FROM KOREA WAX REX 2460 LMP FROM BLAYSON Parraffin 56 - 58 Parraffin 62 - 64 Parrafin 68 - 70 C0310
Blayson Japan has developed a volumetric expansion test to dewax temperatures.
A unique feature is that data capture is automatic and not dependent on operator
Relationship between Wax and
Injection Characteristics
Wax Fluidity
• Wax fluidity is key to control of many problems
– flow lines
– surface finish
– non fill and air entrapment • A relationship exists between
wax temperature and fluidity. Important when operating near the congealing point
• Effect of die temperature
should not be underestimated
60 65 70 75 80 85 T [°C] 0 1 2 3 4 5 6 7 8 9 10 11 ƒ [Pas] 040178B ƒ = f (T) l0049 ƒ = f (T)
Fluidity Measurement
• Wax Fluidity can be measured using the ‘Fluidity Spiral’
• Injected at :
– fixed wax temperature – fixed die temperature – fixed injection pressure – fixed flow rate
• Can be used with each new batch to estimate its flow characteristics and allow
adjustments to be made prior to start of production
Factors Affecting Fluidity
WAX FLUIDITY INJECTION TEMPERATURE DIE TEMPERATURE AIR RESISTANCE/ PRESSURE IN DIE SURFACE TENSION OF DIE VISCOSITY TURBULENCE AIR TEMPERATURE CAVITY GEOMETRY INJECTION PRESSURE CONGEALING POINT/ RATE OF CRYSTALLISATION DIE THERMAL CONDUCTIVITYRelationships with Injection Pressure
Pressure Fl o w li n e s 1.0 0.5 0.0 -0.5 -1.0 4.0 3.5 3.0 2.5 2.0 1.5 1.0 S 0.290832 R-Sq 95.5% R-Sq(adj) 94.8%Fitted Line Plot Flow lines = 2.638 + 1.163 Pressure
Pressure Su rf a ce f in is h 1.0 0.5 0.0 -0.5 -1.0 5 4 3 2 1 S 0.890225 R-Sq 50.3% R-Sq(adj) 42.0% Fitted Line Plot
Surface finish = 3.150 + 0.7750 Pressure
Recent trials suggest a relationship between flow lines and pressure, higher pressures minimise flow lines. Evidence exists for a similar
relationship between pressure and surface finish (orange peel).
Obviously care must be taken when using cored parts (core breakage)
Temperature and Cavitation
Me a n o f S in ki n g 1 -1 5 4 3 2 1 -1 1 -1 5 4 3 2 Flow Pressure TemperatureMain Effects Plot (data means) for Sinking
Temperature Si n ki n g 1.0 0.5 0.0 -0.5 -1.0 5 4 3 2 1 S 0.677003 R-Sq 84.6% R-Sq(adj) 82.1% Fitted Line Plot
Sinking = 3.375 - 1.375 Temperature Injection trials suggest a strong link between wax
temperature and cavitation.
Indications are that the cause is the time that the wax is liquid/semi liquid, increasing the
temperature increases the time.
This explains why sprue size and location affects sinking, and also that sinking is probably related to the setting rate of the wax.
Filler reduces liquid by around a third and reduces contraction. 40 45 50 55 60 65 70 75 80 85 T [°C] -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 ƒ ' [Pa s ] 20 40 60 80 100 Ì [°] L0049 OSC ƒ' = f (T) Ì = f (T) 040178B OSC ƒ' = f (T) Ì = f (T)
Summary
• Investment casting wax compounds are complex
– consist of many different components
– consequently exhibit a range of properties
• Wax properties influence pattern behaviour in the foundry and ultimately the quality of castings
produced
• Correct product choice allied with strict process & quality control procedures is essential
• More information available at www.investmentcasting.com
