introduction to microcasting techniques for micromanufacturing
TRANSCRIPT
INTRODUCTION TO MICROCASTING TECHNIQUES FOR MICROMANUFACTURING
Dr. Arvind Kumar
Department of Mechanical Engineering
Indian Institute of Technology Kanpur
Introduction to Microcasting• Based on permanent mold investment casting techniques
• Manufacture micro parts in large quantities
• Process is versatile in manufacturing very complicated structures, scalable, economically efficient for mass production and needs minimal subsequent machining
• The process can fabricate curved structured surfaces more easily and with higher throughput than other processes
• This technology has been successfully applied for manufacturing of instruments for surgery, dental devices, biotechnology, miniaturized devices for mechanical engineering applications, jewellery and dental casting
Techniques for Casting StructuresCapillary action
MicrocastingMicrocasting based on Investment casting
• Capillary action microcasting technique uses a permanent mold which can be opened in order to remove the cast structure.
• The cavities in the mold are shaped by high-precision grinding.
• Two different principles to fill these cavities exist the suction principle and the displacement principle.
• In the first method, the melt is sucked into a specially coated mold by the capillary pressure.
• In the second method, the casting alloy is melted inside the divisible mold and fills the microstructured cavities owing to the capillary force.
• In capillary action microcasting, the castable geometries are limited to structures which can be filled by the application of capillary forces.
• This is also known as the lost-wax/lost-mold technique.
• Molding from a microstructured grand master pattern, parts of arbitrary geometry can be manufactured.
• An injection molded plastic master is embedded in a gypsum slip. After drying the gypsum, mold is sintered. During this process the master is lost by melting and pyrolysis. The preheated gypsum mold is then filled with a metal melt by centrifugal or pressure casting.
• After removing the mold from the casting, the cast structure is cleaned and the micro parts are separated from the gate system.
• This technique has the advantages of simple process, high material utilization ratio, possibility of castings with complicated shape and mass production at relatively low cost compared to common ultra precision technologies .
Sand Casting
Investmen
t Casting
Die
Casting
High Temperature Alloy, Complex Geometry, Rough
Surface Finish
High Temperature Alloy, Complex Geometry,
Moderately Smooth Surface Finish
High Temperature Alloy, Moderate Geometry,
Smooth Surface
Casting Methods
Sand Casting
Tolerances:Non-Ferrous ± 1/32” to 6”Add ± .003” to 3”, ± 3/64” from 3” to 6”.Across parting line add ± .020” to ± .090” depending on size. (Assumes metal patterns)Surface Finish:Non-Ferrous: 150-350 RMSFerrous: 300-700RMSMinimum Draft Requirements:1° to 5°Cores: 1° to 1 1/2°Normal Minimum Section Thickness:Non-Ferrous: 1/8” - 1/4" Ferrous: 1/4" - 3/8”Ordering Quantities: All quantitiesNormal Lead Time:Samples: 2-10 weeksProduction 2-4 weeks A.S.A.
Description: Tempered sand is packed into wood or metal pattern halves, removed form the pattern, and assembled with or without cores, and metal is poured into resultant cavities. Various core materials can be used. Molds are broken to remove castings. Specialized binders now in use can improve tolerances and surface finish.Metals: Most castable metals.Size Range: Limitation depends on foundry capabilities. Ounces to many tons.
Sand Casting Mold FeaturesCourtesy: www.the-warren.org
Investment CastingDescription: Metal mold makes wax or plastic replica. There are sprued, then surrounded with investment material, baked out, and metal is poured in the resultant cavity. Molds are broken to remove the castings.Metals: Most castable metals.Size Range: fraction of an ounce to 150 lbs.
Tolerances:± .003” to 1/4"± .004” to 1/2", ± .005” per inch to 3” ± .003” for each additional inchSurface Finish:63-125RMSMinimum Draft Requirements: NoneNormal Minimum Section Thickness:.030” (Small Areas).060” (Large Areas)Ordering Quantities: Aluminum: usually under 1,000Other metals: all quantitiesNormal Lead Time:Samples: 5-16 weeks (depending on complexity)Production 4-12 weeks A.S.A. (depending on subsequent operations).Talbot Associates Inc.
Investment Casting Produce very complicated formed parts in metals even with undercuts.
Shaping processes is the rapidity of the casting procedure itself and the low loss of material due to the possibility of recycling the runner and sprues.
Best suited for small and medium series & for parts with high complex shape.
For Jewelry and dental casting, the sizes of the produced parts are in the millimeter up to the centimeter range with structural and details in the millimeter
and sub millimeter ranges.
Recent progresses in the development of investments however, opened the possibility of casting microstructures with bronze as a non-precious alloy.
This technique has the advantages of simple process, high material utilization ratio, possibility of castings with complicated shape and mass production at
relatively low cost compared to common ultra precision technologies .
Investment Casting (Contd ..)
Methods and state-of-the-art
Microcasting (Process Description)Microcasting process itself is based on the lost-wax, lost-mold technique.
Uses wax patterns
Microtechnology mostly works with injectionmolded plastic patterns which have much higher mechanical strength.
The improved mechanical properties permit easier handling and assembling of the pattern during the manufacturing process.
Scheme of Investment casting process
Methods and state-of-the-art (Contd ..)
a) Mold insert for injection molding b) PMMA pattern c) cast part made in gold based alloy
Investment casting procedure for manufacturing microparts is influenced by parameters
Casting alloy
Ceramic investment
Preheating temperature of the moldCasting
pressure
Methods and state-of-the-art (Contd ..)Pattern Design• In microcasting, single polymer patterns are normally fixed with wax.
• Single microstructured patterns should be made at least with a small runner owing to the difficult handling of the small parts.
• Especially patterns which are injection-molded on a substrate plate proved to be advantageous because the substrate plate can be used as feeder.
• In order to produce faultless patterns, different wall thicknesses and sharp edges should be avoided.
• The cross-sectional thickness of the sprue system should increase in the direction of the sprue bottom, because solidification must begin in the microparts and end in the bottom of the tree.
• The heat capacity of the mold should also be taken into account because the compact molds used for microcasting show a comparatively high heat capacity.
• Thin-walled parts should be positioned in the outer and thick-walled parts in the inner area of the mold. The melt will then remain liquid in the thick-walled parts for the longest time so that they can work as feeder for the thinner parts.
• A special aspect in microcasting is the flow behavior in very fine channels. Owing to the much higher surface to volume ratio in microchannels compared with macrostructures and the distinct influence of surface roughness, the occurrence of turbulent flow needs to be taken into account.
• Another aspect is the extremely high cooling rate and therefore extremely fast solidification in the small structures, which hinders form filling much more than in macrostructures.
Methods and state-of-the-art (Contd ..)Melting
• Resistance heating
The method is used for alloys with casting temperatures up to about13000 C. Such set-ups with resistance heating are predominantly used for casting precious metals.
• Open flame technique
The metal is melted by a propane–oxygen flame in a ceramic crucible. The method is limited to relatively small amounts of metal and is especially used in dental casting workshops for heating high-melting alloys to temperatures between 13000 and 15000 C.
• Induction heated casting
The amount of energy injected depends on the alloy and the frequency (of the furnace). Modern equipment works with high frequencies in the region of 100 kHz. A benefit of this method is the very high melting rate. The higher melting alloys requiring casting temperatures above 13000C can be cast, compared with electrical resistance furnaces which are limited to 13000C . The induced eddy currents result in a strong convection in the melt. Hence good mixing and homogenization are achieved.
• Arc flame
A pure argon atmosphere is necessary because the gas atoms work as charge carriers for the current flow. The melting crucibles are made of ceramic or graphite. At high temperatures the graphite crucibles produce a reducing CO atmosphere as a result of the reaction of the carbon with the oxygen in the air. This is especially beneficial for precious metals because the melt is protected against oxidation.
Methods and state-of-the-art (Contd ..)Casting
Casting
Methods and state-of-the-art (Contd ..)Solidification• The solidification starts with nucleation and crystal growth at the cooler mold wall. At the same time, the volume
of the melt decreases owing to the normal shrinkage process, which may then cause casting defects.
• It is important that the cast part solidifies first while the metal in the sprue still remains liquid.
• Another important aspect is the changing of the chemical composition during the solidification due to segregation.
• This segregation can occur in the center of cast blocks because companion elements and inclusions are pushed aside by the solidification front and accumulate in the rest of the melt.
• Graduated microsegregation inside dendrites or in general inside one phase, formed during solidification, also known as coring , is found in alloys which show a solidification interval.
• In this case, a difference in alloy composition between the center and the extremities of dendrite arms occurs owing to an enrichment of one element in the forming crystals at the expense of an impoverishment of the same element in the liquid.
• Alloys are prone to coring if the solidification is too fast to reach an equilibrium state according to the phase diagram.
• The chemical composition can be homogenized by a subsequent long heat treatment at relatively high temperature.
Methods and state-of-the-art (Contd ..)Vapor Pressure Casting
Scheme of the vaccum pressure casting processLeft: Vaccum condition with mold a top the heating chamberRight: Pressure condition with melt discharged into the mold by gravity
Vaccum pressure casting machine
At top of the crucible the open
mold is fixed
Metal is melted in the
crucible
Melt flows into the mold by gravity
Machine
turns itself
upside down
Complete form filling even of small cavities
is achieved by application of pressure
to the melt
Vacuum pressure casting machines typically work at pressures of 3.5–4 bar.
Extremely high aspect ratios are to be cast, a higher pressure is necessary.
Calculations of the form filling behavior show that the pressure which is necessary for the melt to enter an extremely small pinhole increases hyperbolically with decreasing radius.
Fibers with a diameter of 1µm a pressure of 20 bar is necessary to overcome the negative capillary forces which hinder the melt entering a small hole owing to the bad wetting behavior of the melt on the mold.
Experiments showed that for parts with high aspect ratios, high pressure is beneficial even for parts with a diameter of 100µm .
It is very difficult to provide a pressure higher than 4 bar because the pressure needs to be applied to the melt in a very short time interval t, where t ranges from 10–3 to 1 s.
A high pressure is not always desired because with increasing pressure the surface roughness determined by the generally porous investment increases owing to better replication of the surface structure of the mold.
Methods and state-of-the-art (Contd ..)Centrifugal Casting
Scheme of the Centrifugal process
Mold rotates and the melt
fills the mold by centrifugal
forces
The rotational speed of centrifugal casting machines is in the range 350–3000 rpm.
Modern centrifugal casting machines produce a higher pressure for form filling compared with vacuum pressure casting machines, which is beneficial for the casting of very small structures.
The higher pressure can give rise to various defects in the casting and the high turbulences may cause gas entrapment and favours gas porosity .
• High form filling ability and flowability of the melt.
• Influenced by
Viscosity of the melt,
Wetting behavior of the form,
Reaction with the mold,
Atmosphere and solidification behavior.
• A high form filling ability & good flowability are mainly guaranteed for precious alloys such as are used in jewelry and dental casting, for bronze (handcraft arts) .
• High-strength alloys, such as CoCrMo alloys, which are especially used for dental castings.
• Steels are not widespread in microcasting because of their oxidation and corrosion sensitivity.
Methods and state-of-the-art (Contd ..)Casting alloys
Methods and state-of-the-art (Contd ..)Casting alloys (Contd ..)
•Relatively low melting and casting temperature•Excellent castability & low oxidation affinity during the casting process•High resistance against corrosion and tarnishing•Good chemical properties and biocompatibility
Gold base casting alloy
•corrosion-resistant & wear resistance•In addition to aluminum, these alloys often contain iron, nickel or manganese in order to improve their mechanical properties•Al-bronze for microcasting provide the formation of a thin, transparent oxide layer, which already gives the casting a metal brightness without any chemical cleaning or mechanical polishing.
Bronze base casting alloy
•Medical and Dental applications•CoCrMo alloys a yield stress of450 MPa , ultimate tensile strength of 655 MPa and an elongation at fracture of 8% can be expected.•Excellent corrosion and biodegradation resistance•Melting temperature of the alloy lies between 1320 and 13800C. •The recommended preheating temperature for this alloy is 1000C and the recommended casting temperature 15000 C
CoCrMo base castingalloy
Methods and state-of-the-art (Contd ..)Investment Materials
• Good flowability and processibility. • At high temperature sustain Mechanical and Thermal stability. • The resulting mold should be:
chemically inert with regard to the metal melt, Dimensional stable, Sufficient mechanical strength, Low surface roughness , Porosity & Easy removal of the mold from the cast part is desirable.
• Fillers are in all cases ceramic powders.• Typical minerals used as fillers in investments are:
quartz, Cristobalite, Aluminum oxide, Zirconium oxide, Zirconium silicate and Burned potter’s clay minerals such as mullite and molochite.
• In dental techniques, the filler materials mainly used are quartz and cristobalite, which are both modifications of SiO2.
Methods and state-of-the-art (Contd ..)Investment Materials ( Contd ..)
Phosphate Bonded Investments• The choice of the investment material depends on the cast metal and the required strength of the
mold.
• Phosphate bonded investments were initially used for dental alloys with a high melting temperature at casting temperatures between 1200 and 15000C.
• Properties:
High heat resistance,
Good mechanical strength &
Convenient workability.
• They consist of magnesium oxide and ammonium hydrogen phosphate as binder and the two different SiO2 modifications quartz and cristobalite as filler.
• Special investments for gold base alloys sometimes also contain graphite powder in order to produce a reducing atmosphere in the mold.
• The powdery binder and filler components are mixed with a liquid which mainly consists of aqueous silica sol.
Plaster Bonded Investments
Methods and state-of-the-art (Contd ..)Investment Materials ( Contd ..)
• When casting microparts, a great difficulty is the removal of the investment.
• For gold base alloys, phosphate bonded investment can be removed by acid owing to the higher chemical resistance of the metal compared with the investment, but for base alloys such as bronze or CoCrMo alloys this procedure is not applicable.
• This is achieved by using plaster as binder. Like phosphate bonded investments, plaster bonded investments contain quartz and cristobalite as refractory filler materials.
• The material is commonly employed in casting gold alloys with high gold content and with liquidus temperatures not higher than 10800C.
• In dental applications, plaster bonded investments are not used as often as in jewelry casting owing to the danger of sulfur release by reactions of the melt with the investment and hence resulting low strength of the casting.
• In jewelry and artistic casting, however, most investments are plaster based, because they achieve a low surface roughness and can be easily removed also from complicated-shaped structures.
Methods and state-of-the-art (Contd ..)Investment Materials ( Contd ..)
Influence of the Investment on the Surface Roughness
The investment has a significant influence on the surface roughness of the cast part.
Increasing form filling ability of the casting alloy, the surface roughness of the cast part approaches more and more that of the surface of the mold.
A very smooth surface of the mold with only a few pores is necessary.
Three different ways are possible to meet these requirements:
Coating the pattern with an extremely fine ceramic,
Infiltrating the sintered mold with a ceramic suspension and
Modifying the investment by addition of fine ceramic particles.
Methods and state-of-the-art (Contd ..)Coating the Pattern• Investigations on surfaces improved by coating the pattern were carried out with a 1% aqueous
methylcellulose solution containing 5% fine-scale SiO2powder.
• The pattern was dipped into the liquid and dried on air. Subsequently, the coated pattern was embedded in an investment slip, dried and burned.
• The hollow form was then filled with the molten gold base , the applied pressure was 4 bar.
• The profile for surface roughness for cast parts that were replicated in the commercial investment shows large amplitudes with roughness values of Ra= 1.13 µm and Rmax= 8.41 µm whereas a coated specimen achieves values of only Ra= 0.74 µm and Rmax= 6.19 µm. Therefore, coating of the mold is beneficial to a low surface roughness of the cast part.
• It must be stressed that coating a plastic or wax pattern is difficult owing to the poor adhesion of the liquid on the pattern.
• Dipping the PMMA pattern in the slurry once leads to a coating which delivers relatively good results. Repeated dipping two or three times, however, often results in flaking of the coating.
Methods and state-of-the-art (Contd ..)Infiltrating the Mold• Infiltration was examined using suspensions of fine-grained SiO2powder stired in distilled water (1 : 10). In
further experiments it was diluted to 1 : 20 and1 :40.
• The hollow form was then infiltrated with the suspension and sintered in a second burning process followed by the final casting process. Compared with the coating technique, the infiltration of a microstructured sintered form is much more difficult.
• If the viscosity of the suspension is too high, it does not infiltrate the pores of the mold but instead forms a thick layer on the surface, leading to rounded edges and differences in tolerance.
• However, when the viscosity of the infiltration liquid is too low, then there is no visible effect on the mold.
• It fully penetrates the porous investment without leaving a layer on the mold surface. Additional investigations showed that it is almost impossible to realize a homogeneous infiltration in a closed mold with internal microstructures
Methods and state-of-the-art (Contd ..)Cast Microparts and Their Properties
Methods and state-of-the-art (Contd ..)Microstructure/Grain Size
Methods and state-of-the-art (Contd ..)Surface Roughness
The surface roughness is influenced by various parameters.
• Investment,
• Mold temperature and
• Casting pressure.
The influence of these parameters was measured on tensile test specimens made of the gold base alloy using both a confocal white light microscope and a scanning optical system .
Comparing specimens cast at a mold temperature of 10000C but with different casting techniques, it can be seen that the surface roughness achieved by vacuum pressure casting is significantly lower than that by centrifugal casting.
Centrifugal casting machine works with a filling pressure of about 20–25 bar, Vacuum pressure casting machine fills the mold with only 4 bar.
The high pressure in the centrifugal casting machine leads to a better replication of the mold surface but at the same time results in a higher surface rough-ness or worse surface quality, respectively. In comparison, the lower filling pressure of the vacuum die cast machine produces significantly lower surface roughnesses.
Methods and state-of-the-art (Contd ..)Mechanical Properties
Methods and state-of-the-art (Contd ..)Achievable Structure Size, Flow Length and Aspect Ratio• The smallest achievable structure size depends on the aspect ratio, which is defined as the ratio of flow length to wall
thickness. • Investigations showed that very small structures can be cast with the gold base alloy owing to its good flowability and
form filling behavior.• Wall structures down to 20µm width were produced with an aspect ratio of 6.• The flow length and aspect ratio achievable are mainly influenced by the preheating temperature of the ceramic mold
and by the filling pressure.
Theoretical and Experimental Investigations
Due to decrease in the dimensions of casting part, some challenges like complete mold filling, suitable operational pressure and other parameters become important in microcasting technique.
The casting parameters, such as casting material, investment's preheating temperature and filling pressure determine the entire form filling process, and, as a consequence, the achievable grain size and the resulting mechanical properties of the microcast .
Materials to be microcast must have sufficient castability. This embraces properties such as flowability and form filling ability, little contraction and shrinkage, reduced segregation, low porosity, little hot crack susceptibility, high surface quality and good mechanical properties.
The form filling ability and flowability of the melt are influenced by the viscosity of the melt, the wetting behaviour of the form, the reaction with the mold and the atmosphere and, of course, by the solidification behaviour.
THEORETICAL AND EXPERIMENTAL INVESTIGATIONS(CONTD..)
The research and development concerning microcasting was started with precious alloys, such as gold alloy, Sn–36Pb–2Ag alloys due to relatively low melting and casting temperature and good fluidity.
Previous investigations led to the choice of the particle hardened gold base alloy ,which is used in the dental applications. To expand the applications domain of microcasting in other industrial applications Aluminium base alloys, which have good fluidity, can be potential candidates (e.g., Zn–4%Al alloys).
Another typical material can be bronze which shows good slip properties. A high form filling ability and a good flowability are guaranteed for these materials to be used in jewellery, dental casting, handcraft arts made of bronze, and in scenario where high-strength materials (e.g., CoCrMo alloys) are required. Steels are not widespread in microcasting because of their oxidation and corrosion sensitivity.
Theoretical and Experimental Investigations (Contd ..)
Another aspect is the fast solidification in the small structures due to very high cooling rate. This hinders form filling much more than in macrostructures.
Further, the occurrence of turbulent flow needs to be taken into account due to higher surface to volume ratio in microchannels and the distinct influence of surface roughness.
In microcasting, length scale is very low so issues, such as mold filling, progress of solidification, shrinkage, segregation and surface roughness (finish) which were of macroscale in macro casting, now should be treated as meso and microscale phenomena.
Fluid flow in cavities larger than 10 times of fluid molecular diameters, the continuum hypothesis will still be valid and Navier-Stokes equation can be used to analyze the flow. The channels’ dimension in microcasting is in order of hundreds of micro meters that is sufficiently far from fluid molecular diameter.
THEORETICAL AND EXPERIMENTAL INVESTIGATIONS (CONTD ..)
Thus, continuum hypothesis and Navier-Stokes equation along with some suitable solidification models should still be valid for modelling microcasting fluid flow, filling behaviour, solidification, as-cast grain morphology, and nucleation and growth kinetics of grains.
Modelling and simulations can be used to overcome existing challenges in the microcasting process and to optimize the process. For example, a combined volume of fluid and solidification modelling method [Yadav et al., 2009] can be used to study the microcasting process.
Such computational method based on deterministic solidification heat transfer and fluid flow models can provide predictive capability and deeper insights of the filling behaviour (role of viscosity, capillary, surface tension, mold roughness and preheat temperature, centrifugal forces and applied pressure), progress of solidification (interface location, solidification time and local cooling rate), segregation behaviour, and casting defects.
Theoretical and Experimental Investigations (Contd ..)
In order to obtain more insights and deeper understanding of the
various transport phenomena taking place during the microcasting
process and data for validation of numerical models, suitable
experimental work concentrating on local temperature and
composition measurements, and defects analysis are also needed.
Existing Challenges and Strategies to Overcome Challenges
As decrease in the dimensions of casting part, some challenges like complete filling of the mold and determining suitable operational pressure are come into picture.
As the flow channel size becomes smaller, some of the conventional theories for (bulk) fluid, energy, and mass transport may need to be revisited for validation.
casting real three-dimensional microstructures with undercuts is most complex problem.
Existing Challenges and Strategies to Overcome Challenges (contd..)
To introduce two new approaches: microcasting in permanent mold and composite casting.
In the permanent mold case, the focus is laid on the suitability of metal or graphite molds for casting of extremely small structures.
Graphite molds are more challenging then metal molds, because of the brittleness of graphite.
In the composite casting case, the investigations are focused on the connecting or assembling of two different materials or structures.
Existing Challenges and Strategies to Overcome Challenges (contd..)
Reduction of surface roughness as much as possible.
This can be done by casting into mold inert i.e. permanent mold.
Permanent mold casting includes some special challenges regarding the form filling of molds with micro-sized cavities for example
1. Gas-tight i.e. trapped air or inert gas in the cavities cannot penetrate through the wall.
2. Precise replication of small cavities
Existing Challenges and Strategies to Overcome Challenges (contd..) Therefore an elevated mold temperature is beneficial for precise form filling and replication of
the structure in the cavity.
Figure below shows the effect of temperature.
Existing Challenges and Strategies to Overcome Challenges (contd..)
In composite casting, there is also some difficulties as following
1. Different coefficients of thermal expansion that lead to different shrinkage during cooling .
2. Chemical reactions between the partners need to be taken into account, which lead to reaction layers or even to the occurrence of oxide layers during processing.
These above problems can be solve by using several modes of fitting between the partner materials: force fitting by utilizing the different thermal expansion coefficients or shrinking rates, form fitting dependent on the geometry or even an adhesive joining or metallurgical bonding by intended boundary reactions or boundary alloying.
Future Research to be Done
Research on limitation of process regarding the obtainable aspect ratio.
Development of special investment for microcasting, which is suitable for best alloys and easily removable from cast parts.
Get more smooth and dimensionally cast structure.
Research on solidification structure and thermal behavior of metal/mold system because casting method influences the microstructure.
3-D microstructure with undercuts in microparts.
Many microstructured surfaces are known in nature, such as shark skin, lotus leaf and insect feet structures. By understanding these effects and using this knowledge for technical applications, several material properties such as drag, friction, adhesion, hydrophobia can be adapted in components manufactured by microcasting .
Possible applications include cast parts where a reduced wettability or a self-cleaning effect is desired. Also, cast parts which become rapidly dirty, such as wheel rims or the underbody of cars and motorbikes, represent very interesting applications for microstructured surfaces, especially in terms of corrosion protection.
Another big field is industrial facilities such as chemical or power plants with complex piping and pumping systems which are very difficult to maintain and clean.
Furthermore, micro holes can be used for tribological applications, for example as reservoirs for lubricant films in cylinder-piston-pairings or bearing blocks in engines.
The various investigations have demonstrated that microcasting is a potential fabrication method for metal parts in microdimensions.
Since this technique is at developmental stage, many works are needed for this technology for suitability in industrial production.
CONCLUSION