National Symposium on Miniature Manufacturing in 21st Century

Indian Institute of Technology (BHU), Varanasi (UP), August 16-18, 2013

Compact Integrated Metal Casting System for Miniature Castings

Himanshu Khandelwal and B. Ravi

E-Foundry Lab, Indian Institute of Technology Bombay

Powai, Mumbai-400076

Abstract

Miniature metal parts with thin and intricate features are usually produced using either

investment casting or pressure die casting. While the former requires a large facility and

skilled labour, the latter is expensive and limited to non-ferrous metals. The widely used

sand casting process is economical, but cannot produce walls less than 2 mm thin. In this

work, we explore the use of three part no-bake sand molding with an integrated casting

system to produce small castings with thin walls. The system has been developed in-house,

and integrates induction heating and melting with direct pouring (into the mold placed

below crucible). Thermocouples can be placed inside the casting and/or mold, and the

temperature data is captured for analyzing the cooling curves. The system has been used

to produce a few castings in aluminum and zinc alloys for studying the corresponding flow

and solidification behavior. Preliminary results are presented in this paper.

Keywords: Miniature parts, thin wall, sand mold, induction furnace, cooling curves.

1. Introduction

Metal casting involves pouring liquid metal into a mold cavity, and allowing it to solidify.

Casting processes are usually classified depending on the type of mold material and filling

pressure, for example: green sand casting, gravity die casting and pressure die casting.

While nearly any metal can be cast into the desired shape using a suitable process, the best

quality at the least cost can be achieved when the designer selects the most appropriate

process and adapts the design features to lie within the corresponding process capabilities.

Increasing demand for thin wall designs (to reduce part weight) and net shape processes

(to reduce machining costs), have led to a renewed interest in precision casting techniques

(Jafari, Idris, Ourdjini, Karimian, & Payganeh, 2010). This includes investment casting and

pressure die casting. Both are suitable for small parts weighing several grammes to a few

kg. Walls can be as thin as 1-2 mm, and part dimensional tolerance about 0.5 mm over a 100

mm length.

Miniature castings can be classified as micro-casting and thin-wall casting. The production

of components in micrometer range has been achieved using free form fabrication and

micro-machining (Baumeister et.al. 2002); this is however, still a long way off in metal

casting domain. The manufacture of thin-wall castings also presents unique problems,

mainly due to premature freezing of molten metal as it flows through thin sections. The

fluidity and solidification rate of molten metal in such geometries, affecting integrity and

mechanical properties, are therefore important parameters of study in miniature castings.

Other key process parameters include alloy composition, melting technique, and mold

material (Verran, Mendes, & Valentina, 2008). Influence of these parameters on

microstructure, strength, hardness and surface characteristics of the casting need to be

investigated (Jafari et al., 2010; Ravi, Pillai, Amaranathan, Pai, & Chakraborty, 2008).

Flow of molten metal in thin sections is accompanied by a rapid drop in temperature, which

in turn affects the solidification and microstructure. Junctions cool relatively slowly, leading

to shrinkage porosity defects. The size and extent of defect region depends on the thickness

and number of elements, and the angle between them, all of which affect the rate of heat

transfer from the casting (Joshi, Ravi et.al. 2009). The effect of cooling rate on

microstructure and mechanical properties in thin wall castings has been studied by a few

researchers, based on experimental measurement of temperatures and metallurgical

examination of the cast samples (Padersen et. Al. 2008; Gorny 2012). The wall thickness

reported in these investigations is however, well above those encountered in miniature

castings.

In view of the above, a need was felt to explore the limits of sand casting process for

economical production of miniature and thin wall castings required in small numbers. This

necessitates a study of flow and solidification characteristics of molten metal in the

corresponding molds. For this purpose, an integrated melting and pouring facility coupled

with temperature data acquisition has been developed in our institute. The elements of the

system are described in the next section, followed by a description of some of the test

castings produced using the system.

2. Compact Integrated Casting System

In a typical sand casting foundry, molds and cores are prepared in separate sections, and

assembled. Metal is melted in induction furnaces, and transferred to ladles, which are

transported to the assembled molds for pouring. In contrast, we visualized a desktop

foundry, where molds could be prepared in the form of a cassette, which could be inserted

into an integrated melting and pouring unit. Molten metal would be directly led into the

mold cavity under gravity, minimizing its contact with atmosphere, which otherwise

causes oxidation and moisture pick-up. The new system comprises three main units: (a)

sand mold cassette, (b) induction melting and pouring, and (c) temperature data

acquisition, which are briefly described here.

(a) Sand Mold Cassette

The conventional green sand casting process employs molds made of silica sand mixed with

clay (Bentonite), water and a few other additives to improve their strength, permeability

and collapsibility. The sand mixture is packed around patterns placed in a metal mold box

or flask, which provides the needed support around the sand mold. While it is a very

economical process, the geometric accuracy and surface finish of the castings produced by

this route are not suitable for miniature and thin wall parts.

To overcome the above limitations, several alternate methods, used for core-making, were

explored. One was silica sand mixed with sodium-silicate and hardened by passing CO2 gas.

These molds however, required large grain sand (leading to poor surface finish) and proved

to be too hard to break after casting. Another was oil bonded silica sand molds, but this

requires a furnace to bake and harden the mold.

Finally we settled on ‘no-bake’ molds prepared using a thermosetting resin, drier and

cross-linking agent, referred to as ‘3-part’ system. First part A and B are mixed, and this

mixture is mixed with silica sand of the right grain size. Then part C is mixed in, and the

final mixture of sand and chemicals is poured around the pattern placed in a mold box. The

mixture sets hard within a short time, and the pattern is then removed from the mold.

Then the hardened mold is also removed from the mold box. The entire operation takes

less than 30 minutes. The relevant process parameters include the absolute and relative

proportion of the three chemicals, ambient temperature and humidity. For example,

increasing amount of part B increases the strength of mold, but reduces the bench life of

the sand mixture. Higher ambient temperature and lower humidity accelerates the setting

time and increases the mold hardness. The composition of the mixture used to prepare the

no-bake molds is shown in Table 1.

Table 1 Composition of 3-part system for mold preparation

Part Description Role Composition

A Alkyd Resin Binder Primary Binder 2 % of sand

B Drier Accelerator 3-10 % of Part A

C Cross Linking Agent Prepared Foundry Binder 20 % of Part A

The above approach gives superior mold hardness (compared to green sand molds),

collapsibility (compared to CO2-hardened molds), and dimensional accuracy (comparable

to investment casting). The hardened mold with the part cavity ready to be filled with

molten metal is referred to as mold cassette. By standardizing the dimensions of the

cassette, it has been possible to use an integrated melting and pouring system, described

next.

Fig. 2: Integrated melting, pouring and data acquisition system

(b) Melting and Pouring

Induction heating provides a clean, fast and efficient means for melting metals for casting

purpose. The industrial induction furnaces are however, occupy considerable floor space

and require dedicated 3-phase power supply. Further, the molten metal usually needs to

be transferred to a separate ladle for pouring into the mold, which leads to heat loss,

oxidation and moisture pick-up.

To overcome the above limitations, it was decided to explore indigenous development of a

compact and computer-controlled induction melting unit, with provision to place a mold

cassette within the furnace for direct pouring into the mold cavity (Fig. 2). To minimize the

overall size of the unit, several configurations of the induction circuit, coil and crucible, and

cooling water container were explored by the research team comprising Electrical

Engineering, Mechanical Engineering and Materials Engineering researchers, in the

Treelabs facility of IIT Bombay.

The induction unit uses the heat produced by eddy currents generated by a high frequency

alternating field. A graphite crucible covered by suitable refractory material is used,

surrounded by water cooled copper coil. The alternating magnetic field produced by the

high frequency current induces powerful eddy currents in the metal charge placed in the

crucible, resulting in rapid heating and melting. Temperatures in excess of 800 oC haven

been achieved, suitable for melting nearly 1 kg of aluminum and zinc alloys in less than 30

minutes. The top lid is closed after placing the metal charge, and a thermocouple provides

a continuous measurement of internal temperature on a digital readout. The crucible has a

hole at the bottom closed with a plug, which can be opened by pulling a connected graphite

rod that protrudes from the top lid. The computerized control enables furnace starting,

stopping, and power optimization.

(c) Temperature Data Acquisition

The data acquisition (DAQ) system is integrated with the system to enable continuous

measurement of temperatures in the casting and/or mold during the casting process. For

this purpose, a 16-channel data acquisition system was developed by researchers in the

institute, including software to store and view the data captured by the system.

For temperature measurement, K type thermocouple wires are used, which are suitable for

a wide range of temperature from -200 oC to 1350 oC. They are embedded in the mold

within ceramic sheaths, stopping within the mold material, or protruding into the part

cavity, to measure the temperature of the mold and casting, respectively. The

measurements are recorded with a time interval of 0.1 seconds, providing sufficient

accuracy to plot and interpret the cooling curves, while optimizing the amount of data to be

stored and analyzed. The data is stored in a computer connected to the DAQ system, and

used for real-time visualization as well as post-processed for creating cooling curve plots

for subsequent analysis.

The experimental castings produced using the abovementioned system and related results

are described next.

3. Experimental Castings

Part Design for Cooling Curve

Casting geometry is a key feature to determining the limits of a particular sand casting

process. So a benchmark part is designed for initial experiment to determine the

temperature profile. A part has been selected which having a thick section at middle.

Casting geometry is a key factor in determining the way solidification progresses in the

casting. Therefore a part was designed to study the cooling curves. The following part was

proposed as shown in Fig. 3. All dimensions are in mm. The characteristic features of the

geometry which makes it a suitable benchmark shape are

L-junction

T-junction

Cross junction

Thick middle section

A wooden pattern was created as shown in the Fig. 4. It consists of 3 parts, an upper half of

the casting, a lower half and the sprue.

Fig. 3: Multi Junction Part

Fig. 4: Wooden pattern for thermocouple experiments

1

1.5

2

2.5

3

D-20

D-80

D-90

15

Part Design for Flow Characteristics

For another experiments focus is on understanding melt filling behavior

in terms of minimum wall thickness and flow length. Therefore a

small circular part which carries thin wall section is designed. This

circular part has wall thickness as 1 mm, 1.5 mm, 2 mm, 2.5

mm and 3mm. This kind of circular part is taken

to evaluate the minimum thickness, can be

cast form an alloy and by a particular

casting process. The following

part was proposed as

shown in Fig. 5.

Figure shows

the

Solidworks

model and the

dimension of the

parts. Fig. 6 shows the final

pattern. Pattern is made by SLA

technique of rapid casting.

Fig. 5: CAD model of Cast part with dimension of straight ribs

Fig. 6: Final pattern made by SLA process

Symmetric geometry is taken to investigate effect of filling. We don’t want to bias for any

particular channel, so pouring is done along the central common sprue. In this way equal

opportunity has been given to fill the entire channel.

Casting Experiment for Cooling Curve

Mold is prepared by above discussed three part molding process. Two k type

thermocouple has been successfully placed at two different locations in mold at the time of

molding. And other end of the thermocouple has been inserted into the two channels of

data acquisition systems. This DAQ system attached to a laptop and controlled by

Atomburg software. Temperature reading data can be collected by this software. It shows

the curve of temperature rise with the solidification time. The attachment of

thermocouple in mold is shown in below Fig. 7.

Fig. 7: Cope & Drag section of the mold

The melt charge consists of an aluminium alloy LM6. The pouring temperature is kept at

680 oC in the experiment as the melting temperature of LM6 is around 575 oC. The molten

metal was poured directly under gravity into the pouring basin of the sand mold. A

measured quantity of 350 gm of material was used in each experiment. Final casting of

multi junction part is as shown in Fig. 8.

Fig. 8: Actual casting with thermocouple

Casting Experiments for Flow Characteristics

In another experiments to understand flow characteristics wheel type pattern is used. The

mold for the wheel part is prepared in two halves. The mold consists of a cope and a drag.

Cope contains the pouring basin and sprue. On the other hand, drag contains pattern of the

casting. A series of experiments were conducted to cast the circular part.

First casting

The melt charge consists of a zinc aluminium alloy ZA8. The chemical composition of ZA8 is

shown in Table 2. The pouring temperature is kept at 455 oC in the experiment as the

melting temperature of ZA8 is around 390 oC. The molten metal was poured directly under

gravity into the pouring basin of the sand mold. A measured quantity of 125 gm of material

was used in each experiment. Table 3 presents the process parameter for this experiment.

Table 2. Chemical composition of ZA8

Table 3 Process parameters for First casting

Cast metal ZA8

Pouring temperature 455 °C

Mold Temperature 30 °C

Sand Mesh Size AFS 55

Fig. 9: Circular casting made by first experiment

Second casting

The melt charge consists of an aluminium alloy LM6. The chemical composition of

LM6 is shown in Table 4. The pouring temperature is kept at 680 oC in all the experiments

as the melting temperature of LM6 is around 575 oC. The molten metal was poured directly

under gravity into the pouring basin of the sand mold. A measured quantity of 350 gm of

material was used in each experiment. Table 5 presents the process parameters kept for

this experiment.

Table 4 Chemical composition of LM6 alloy

Cu Mg Si Fe Mn Ni Zn Pb Sn Ti

0.1 0.1 10.0-13.0 0.6 0.5 0.1 0.1 0.1 0.05 0.2

Table 5 Process parameters for second experiment

Cast metal LM6

Pouring temperature 625 °C

Mold Temperature 30 °C

Sand Mesh Size AFS 55

Fig. 10: Circular casting made by second experiment

Observations

Cooling curves that were recorded are shown in Fig 11. Thermocouple 1 is placed on the

corner or the L-junction while thermocouple 2 is placed in middle of cross junction as

shown in Fig 7. From the experimental curves it can be observed that the freezing range of

LM6 is between 560-580 oC. The temperature profile shows the behavior of a typical metal

freezing. The metal cools rapidly until the freezing point is reached and then the cooling

rate slows down until the latent heat has been released. Once the latent heat is dissipated

the cooling rate rises again.

Both the thermocouples show similar temperature profile; however, the difference can be

observed in the initial stages of cooling as shown in Fig 8. The temperature of TC2 is greater

than TC1 because TC2 is located in the middle of cross junction while TC1 is located in the

middle of an L-junction which provides it with additional surface area for heat dissipation.

Also, the central thick region is a potential hotspot because of its high thickness. The

thermal curve proves that there is a high temperature drop in initial stage of liquid metal

cooling due to intensive heat transfer between the flowing metal stream and mold material

interface.

Fig 11: Cooling curves of TC 1 and TC 2 in multi junction part

Fig 12: Cooling curves of TC 1 and TC 2 near freezing range

Flow characteristics can be observed by studying both the castings shown in Fig 9 and Fig

10. As sown in Fig. 9. it can be seen that all thin sections have been completely filled. It can

be said that if ZA material is taken we can find the filling characteristics upto 1 mm thin

section. Also with the same parameters in LM6 casting, all sections are filled except

thinnest section as shown in Fig 10. One mm section has been filled up to 6mm length.

4. Conclusions

Economical and demand of new miniature technologies pressures continue to look for

ways to produce casting with thinner wall, light weight and yet with equal or better

mechanical properties. Therefore miniature casting comes in consideration. A integrated

compact table top casting facility have been developed which have been successfully used

for molding, melting, pouring and final thermal analysis of small cast part. One mm thin

section of ZA8 has been successfully cast. Obtain cooling curve can be further used for

comparing nucleation and growth during solidification of different casting or in different

section of a same casting. This unit can further be used for casting of miniature parts below

1 mm thickness.

References

Baumeister, G., Mueller, K., Ruprecht, R., and Hausselt, J., “Production of metallic high

aspect ratio microstructures by microcasting”, Microsystem Technologies, 8 (2-3)

(2002), 105-108.

Górny, M., “Fluidity and temperature profile of ductile iron in thin sections.” Journal

of Iron and Steel Research, 19 (8) (2012), 52–59.

Jafari, H., Idris, M. H., Ourdjini, A., Karimian, M., & Payganeh, G., “Influence of gating

system, sand grain size, and mould coating on microstructure and mechanical

properties of thin-wall ductile iron.” Journal of Iron and Steel Research, International,

17 (12) (2010), 38–45.

Joshi, D., & Ravi, B., “Classification and simulation based design of 3D junctions in

castings”, American Foundry Society, 32 (2009), 7–22.

Mandal, D., “Near net shape casting through investment, die and centrifugal casting,”

(2008), In: Training programme on Special Metal Casting and Forming Processes

(CAFP-2008), Feb. 25-26, 2008, NML, Jamshedpur, 1–19.

Pedersen, K. M., & Tiedje, N. “Temperature measurement during solidification of thin

wall ductile cast iron. Part 1: Theory and experiment,” Measurement, 41(5) (2008),

551–560.

Ravi, K. R., Pillai, R. M., Amaranathan, K. R., Pai, B. C., & Chakraborty, M., “Fluidity of

aluminum alloys and composites: A review”, Journal of Alloys and Compounds, 456

(1-2) (2008), 201–210.

Verran, G. O., Mendes, R. P. K., & Valentina, L. V. O. D., “DOE applied to optimization of

aluminum alloy die castings”, Journal of Materials Processing Technology, 200 (1-3)

(2008), 120–125.