THREE DIMENSIONAL PRINTING FOR CASTING APPLICATIONS:

A STATE OF ART REVIEW AND FUTURE PERSPECTIVES

Rupinder Singh1, a 1Department of Production Engineering, GNDEC, Ludhiana, India

aemail: rupindersingh78@yahoo.com

Keywords: Three dimensional printing; rapid prototyping; casting; micro structure.

Abstract. Rapid prototyping (RP) is being widely used in diverse areas, from the building of

aesthetic and functional prototypes to the production of tools and moulds for prototypes. Many RP

techniques like stereo lithography, laminated object manufacturing, three dimensional printing etc.

are commercially available. The implication of these technologies revealed that the time and cost of

developing new foundry tools could be greatly reduced. The purpose of the present research paper

is to review three dimensional printing for generation of prototype for casting applications.

Introduction

During the development cycle, design engineer often prefer to see a physical model of the

component in hours instead of weeks. With this, extra time will be available to the design engineers

to improve quality and also to study new ways of producing the product in a more cost-effective

way [1]. In response to these challenges, a spectrum of new technologies has evolved that helps to

develop new products and broaden the number of product alternatives. One such technology is

rapid prototyping (RP), which produces parts by deposition of material, layer by layer. Today, the

key benefits of RP are mostly derived from its ability to create physical models- regardless of their

shapes and complexities- directly from computer aided design (CAD) models. In addition, models

built with the help of RP processes are used as tools for casting and moulding that is dies for an

injection moulding process and pattern for a casting process [2]. However, manual prototyping by a

skilled craftsman has been an age-old practice for many centuries and is considered as first phase of

technological development in prototyping. Second phase of prototyping started around mid-1970s,

when a soft prototype modelled by 3D curves and surfaces could be stressed in virtual environment,

simulated and tested with exact material and other properties. Third and the latest trend of

prototyping that is RP, by ‘layer-by-layer’ material deposition, started during early 1980s with the

enormous growth in computer aided design and manufacturing (CAD/CAM) technologies, when

almost unambiguous solid models with knitted information of edges and surfaces could define a

product and also manufacture it by CNC machining.

RP Concept. RP is a fabrication method whereby physical objects are constructed by depositing

material layer by layer under computer control [3]. RP takes virtual designs (from CAD or from

animation modelling software), transforms them into cross sections, still virtual, and then creates

each cross section in physical space, one after the next until the model is finished. RP is an

"additive" process, combining layers of paper, wax, powder or plastic to create a solid object. In

contrast, most machining processes (milling, drilling, grinding, etc.) are "subtractive" processes that

remove material from a solid block. Although several RP techniques exist, all employ the same

basic five-steps [4-5]. These steps are:

1. Create a CAD model of the design

2. Convert the CAD model to standard triangulation language (STL) format

3. Slice the STL file into thin cross-sectional layers

4. Construct the model one layer atop another

Advanced Materials Research Vols. 83-86 (2010) pp 342-349Online available since 2009/Dec/21 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.83-86.342

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.6.218.72, Rutgers University Libraries, New Brunswick, United States of America-08/07/14,10:40:26)

5. Clean and finish the model

RP Techniques. Dutta et al. [6] reviewed the emerging field of layered manufacturing / rapid

prototyping. Three broad areas namely, design systems for heterogeneous objects, layered

manufacturing processes and process planning techniques were considered. Several applications

were included in the survey. Further, Ferreira [7] presented some methodologies and technologies

for rapid tooling development that is by reverse engineering to control and reconstruct the

geometric models and rapid prototyping techniques to manufacture directly core-boxes for foundry.

The implication of these technologies revealed that the time and cost of developing new foundry

tools could be greatly reduced. Brief overview of different RP techniques, which have been

commercialized so far, is given below [8]:

Stereo lithography (SL): This was the first RP process to become available in the commercial

market. The system involves, using ultra violet (UV) beams to harden liquid acrylic polymer into

the desired shape. The liquid contains acrylic monomers and photo initiator. The model is built

upon a platform situated just below the surface in a vat of liquid epoxy or acrylate resin. Initially,

the platform is raised to the highest position, so that it is covered only by a surface of liquid. Then a

low power highly focused UV laser beam moves above the surface of the liquid, tracing the shape

required for the base layer and curing the model’s cross-section while leaving excess areas liquid.

The platform is then lowered equal to one slice thickness and left for short time (dip delay) so that

liquid polymer settles to a flat or even surface and inhibit bubble formation. The laser then moves

again to cure the next layer. Zhou et al. [9] conducted a scientific and experimental study to

improve the SL part accuracy through parameters tuning. SL process parameters studied were layer

thickness, resultant over cure, hatch space, blade gap, and the part location.

A special designed specimen with twenty dimensional, geometrical, and surface roughness features

had been used in the inspection of RP manufacturing processes. In terms of Taguchi experimental

design techniques, an orthogonal array of experiments had been developed which has the least

number of experimental runs and desired process parameter settings. 3-D coordinate measuring

machine (CMM) and surface profilometer was used to find the functional relationships between the

output part quality and input manufacturing process parameters. Two analysis tools, response

surface methodology and Analysis of Variance (ANOVA) have been used to evaluate the SLA

rapid prototyping process and to perform the product optimization. The optimal setups of SLA

manufacturing parameters for both individual features and a general part with various features have

been concluded from this study. Chokalingam et al. [10] optimized the process parameters to

maximize part strength for stereo lithography process and developed an empirical relationship

between process parameters and part strength through design of experiments (DOE). The process

parameters that influence the strength of parts made by SL process have been identified. A

statistical tool ‘DOE’ was used for the purposes of identification of process parameters,

optimization of selected levels and establishment of the regression equation. From the DOE study,

it was concluded that the layer thickness, post curing time and part orientation have a large

influence on the part strength. It was found that among these process parameters, orientation has the

maximum influence on the part strength. Jibin [11] established an optimizing model based on the

considerations of staircase effect, support area and production time in RP, through investigations of

the geometric issues of STL model and process planning of rapid manufacturing (RM). The optimal

part orientation during fabrication was found most critical as it could improve part accuracy,

minimize the requirement for supports and could reduce the production time. The best part-building

orientation was obtained by solving the function employing generic algorithm (GA). The

experimentation and analyses performed show that it was reasonable and feasible method to employ

multi-objective optimization in part-building orientation based on satisfactory degree theory.

Selective Laser Sintering (SLS): SLS is a procedure by which (usually) non-metallic powders are

sintered into the shape of the required prototype. The powder delivery piston moves up and the

Advanced Materials Research Vols. 83-86 343

roller spreads the exposed powder over the top of the build cylinder. The laser then sinters the

powder to produce a layer of the part according to the CAD pattern. The build cylinder then drops

to allow another cycle to take place. The final part produced is surrounded by un-sintered powder at

the end of production. Barlow et al. [12] outlined the mechanical properties of a new mould making

material proposed for producing rapidly proto-typed injection mould inserts for plastics by SLS.

The results of study suggests that, although the strength of that proposed material was far below

that of the tool steel usually used to fabricate moulds, it could still be used for mould insert

production. Thermal conductivity of proposed material was lower than that for steel but higher than

that for plastic melts. From the calculations, it was indicated that the proper choices of conduction

length and cycle time can minimize differences, relative to steel moulds, in the operational

behaviour of moulds made of the new material. Dotchev and Soe [13] described the RM of the

patterns for shell or flask investment casting using the selective laser sintering (SLS) technique with

cast form material. The conducted experiments show how the cast form material strength varies at

different process stages and temperatures. Cleaning and wax infiltration were considered the main

reason for part distortion and breakage. Song et al. [14] performed experiments on RP

manufacturing of silica sand based on SLS. Influence of process parameters such as laser power,

scanning speed and powder mixture rotation, the dimensional accuracy and sintered qualities were

investigated.

Laminated Object Manufacturing (LOM): In this technique, (developed by Helisys of Torrance,

CA,) layers of adhesive-coated sheet material are bonded together to form a prototype [2]. The

original material consists of paper laminated with heat-activated glue and rolled up on spools. The

feeder/collector mechanism advances the sheet over the build platform, where a base has been

constructed from paper and double-sided foam tape. Next, a heated roller applies pressure to bond

the paper to the base. A focused laser cuts the outline of the first layer into the paper and then

crosshatches the excess area. Cross-hatching breaks up the extra material, making it easier to

remove during post-processing. After the first layer is cut, the platform lowers out of the way and

fresh material is advanced. The platform rises slightly below the previous height, the roller bonds

the second layer to the first, and the laser cuts the second layer. This process is repeated as needed

to build the part.

Fused Deposition Modelling (FDM): In FDM process, a movable (x-y movement) nozzle deposits

thread of molten polymeric material on to a substrate. The build material is heated slightly above

(approximately 0.5˚C) its melting temperature so that it solidifies within a very short time

(approximately 0.1 s) after extrusion and cold-welds to the previous layer.

Three Dimensional Printing (3DP): 3DP is a relatively new form of RP. The process of 3DP was

patented in 1994 by Sachs et al. under U.S. patent number 005340656 [15]. It was developed at

Massachusetts Institute of Technology (MIT) and licensed to Soligen Corporation, Extrude Hone

and Z Corporation of Burlington. Parts are built upon a platform situated in a bin full of powder

material. Powdered material is distributed in form of a layer at a time and selectively hardened and

joined together by depositing drops of binder from a mechanism similar to that used for ink-jet

printing. After this a piston lowers the part, so that the next layer of powder can be applied. For

each layer, powder hopper and roller systems distribute a thin layer of powder over the top of the

work tray. Adapted continuous-jet printing nozzles apply binder during a raster scan of the work

area, selectively hardening the part's cross-section. The loose powder that wasn't hardened remains

and acts as a support for subsequent layers. The process is repeated to complete the part. When

finished, the green part is then removed from the unbound powder, and excess unbound powder is

blown off. Finished parts can be infiltrated with wax, glue, or other sealants to improve durability

and surface finish.

344 Advances in Materials and Processing Technologies

Materials for 3DP. The 3DP process is quite flexible in choice of materials. Any combination of

a powdered material with a binder that has low enough viscosity to form droplets could potentially

be used [16]. In addition to ceramics, plastic, metal, and metal- ceramic composite parts can be

made. A potential disadvantage is that the parts will always be porous because of density limitations

on the distribution of dry powder. For metal-ceramic composites, the porous ceramic shape is

produced using 3DP and subsequently pressure infiltrated with molten metal to form the composite.

The main focus with ceramics, however, has been on ceramic shells and cores that are used for

casting metal. Three basic material systems have been developed for use with the 3D printers.

Plaster based material: A powder/binder system comprises an oxidant and a reductant (a redox

couple). When the binder is applied to the powder, the oxidant and reductant react to generate an

acid that catalyzes cross linking. As a result, the strength of the 3D article builds up. The oxidant

may be in the powder, and the reductant in the binder; or the reductant may be in the powder, and

the oxidant in the binder. Plaster based materials are ideal for high strength requirements, delicate

or thin- walled parts, accurate representation of design details and colour printing.

Composite based material: These materials are ideal for thin walled enclosures and assembly

applications

Starch based material: In this system starch-based polymer powders (cornstarch, dextran and

gelatin etc.) are used for the 3DP process. Starch based materials are ideal for high speed printing,

large bulky parts and patterns for investment casting

Applications of 3DP Technology. 3DP technology is successfully used by various industries like

aerospace, automotive, jewellery, coin making, tableware, biomedical etc. The technology of 3DP

is used to fabricate concept models, functional models, patterns for investment and vacuum casting,

medical models and models for engineering analysis. 3D printers are less costly variations of RP

technology that are positioned as a design tool to create inexpensive models early in the design

process. The ability to produce quick, inexpensive models allows the designer to model multiple

concepts. Physical models produced on a 3D printer allow everyone on the design team to review

the concepts. In turn, the company gets better feedback, resulting in a better product. 3DP allows

companies to achieve real time collaboration on a global scale. Dimension ‘3D printers’ allows the

designers to test form, fit and function - and as many design iterations the designer like - right from

desktop. Currently only a few final products are produced by RP machines, but the number will

increase as metals and other materials become more widely available [17]. RM will never

completely replace other manufacturing techniques, especially in large production runs where mass-

production is more economical. For short production runs, however, RM is much cheaper, since it

does not require tooling. RM is also ideal for producing custom parts tailored to the user’s exact

specifications. Customized products, which are tailored according to specific requirements of the

customer, are usually made in quantities between one and ten. Sachs et al. [15] provided a

definition of line quality and the main parameters, variables involved in 3DP have been pointed out.

From the extensive data collection coming from the numerous experiments, the authors proposed a

methodology, which had been successfully applied to two different cases. It was found that the

formation of good quality lines is a necessary step for the process optimization, to improve the

surface finish, the mechanical resistance, and the geometrical tolerances. It has been shown that the

right powder composition, surface treatments, and drop spacing give the most relevant effects but

also less controllable or less quantitative factors, like spreading or the presence of humidity and

clumps, have a strong influence on results. The classification of line defects and the other

information included in the 3DP report represent a key for understanding the reason of unsuccessful

results and to iteratively correct them to optimize the line quality. Yao and Tseng [18] investigated

the powder material used in the 3DP. Experiments were conducted to optimize the process

parameters including binder setting saturation value, layer thickness and location of made-up parts.

Advanced Materials Research Vols. 83-86 345

It was found that these optimal parameters can shorten the part building time and reduce the use of

powder and glue about 20 percent. Stampfl and Liska [19] presented a brief overview of the

currently available rapid prototyping techniques with special emphasis on three-dimensional

printing (3DP). The advantages and drawbacks of various RP processes regarding material quality,

feature resolution, and surface quality were pointed out. The new developments in the field of

material development were discussed which allow the use of polymers for 3D printing. Using

polymers some of the drawbacks of 3D printing can be eliminated. In particular, the mechanical

strength can be increased compared to traditional powder systems used for 3D printing. This article

describes the chemical background of polymers and the relevance of these materials for future

developments in RP.

Dimitrov et al. [5] presented and evaluated the comprehensive review of the capabilities of

3DP). Variety of manufacturing applications such as rapid pattern making and RT using the 3DP

process directly or as core technology were presented. This work shows the weaknesses,

opportunities and future of this important process. A benchmark procedure was designed to assess

the dimensional as well as the geometric accuracy of currently one of the most widely used rapid

prototyping processes – the 3DP. The procedure tests not only linear accuracy but also precision,

and repeatability of the process, as well as its ability to create manufacturing features such as fillets

and draft angles. The presented research results reflect the necessity to adequately respond to

engineering requirements for clearly meeting dimensional and geometric tolerances and root on

several in house case studies proving the comparison with established high-end RP processes. The

reported experimental results indicate clearly that both the accuracy and the precision of the process

are influenced by the three factors: The material (powder) used to produce the item, the 3D printer

axis responsible for the particular dimension, and the magnitude of the nominal dimension. The

significance and consistency of this observation makes it easy to establish reliable calibration

(scaling) constants that can be used to minimize the bias in all three axes. The capability of this

specific 3D printing process in terms of IT grades, ranges from IT9 to IT16. Ferreira et al. [20]

considered the application of indirect rapid tooling (RT) technology to manufacture electrical

discharge machining (EDM) copper electrodes from investment casting, with wax prototypes made

by Thermo Jet 3D printing, a rapid prototyping (RP) technique. The reverse engineering (RE)

method was utilized to transform the point cloud data of an object surface, obtained from 3D

digitizing, in a 3D CAD surface model dataset. The methodology presented was fundamental to

verify the prototype’s geometry for tooling so as to assure its metrological accuracy and to optimize

foundry process parameters using finite element analysis (FEA). Based on a case study, some

functional conclusions were presented for the application of RT in manufacturing EDM electrodes

aided by 3D digitizing and reverse engineering, validating the accomplishment by the integration of

these technologies and methodologies in EDM manufacturing processes. The integration of reverse

engineering methodology with additive rapid prototyping and tooling techniques to manufacture

complex 3D shapes reduced the lead-time and the associated costs in the metalworking industry.

Violantte et al. [21] developed a procedure based on reverse engineering and rapid prototyping

techniques for the realization of fixtures fitting the geometry of free-form elements. The application

of the procedure has been made on a sheet metal free-form element. After the design and

manufacturing of the supporting elements, some uniformly distributed measurements have been

made on the sheet metal component. A coordinate measuring machine (CMM) has been used in

order to get dimensional information and to give the IT class location of the component. It was

found that the use of the CMM for the dimensional control of the production elements requires the

availability of an adequate supporting system above all if the control concerns free-form

components with complex forms. This influences considerably the final quality of the

measurements mainly if the control concerns free-form components with complex forms, not bound

to classic geometric entities. The commonly used supporting systems foresee the utilization of

standard elements (clamps, magnets, suction cups and plates and others) ideal for the parts with

regular geometry but that can cause inconveniences if applied to free-form elements and long times

for the part supporting. The supporting elements paper fit to the geometry of free-form component.

346 Advances in Materials and Processing Technologies

For the production of the supporting elements, the chosen technique has been the SLS with the use

of the alumide powders . Bassoli et al. [22] performed experiments to verify the feasibility,

evaluation of the dimensional accuracy of two rapid casting (RC) solutions based on 3D printing

technology: investment casting starting from 3D-printed starch patterns and the Z Cast process for

the production of cavities for light-alloys castings. Technological prototypes were produced with

the two RC processes. Measurements on a CMM allowed calculating the dimensional tolerances of

the proposed technological chains. It was found that both the investigated RC solutions were

effective in obtaining cast technological prototypes in short times, avoiding any tooling phase and

with low costs, with dimensional tolerances that are completely consistent with metal casting

processes. The research proved the possibility of realizing parts with overall dimensions exceeding

the 3D printer working volume, through a modular mould. The process limits can be identified in

the surface finish of castings, which will be the objective of future developments of the research.

Traditional shell casting vs. 3DP shell casting

In traditional, investment or lost-wax casting, ceramic shells are made by a multi-step process. First

a metal die tool is made to define ‘negative of the part’, and if the part has hollows, dies are also

made for cores that define the internal geometry. The cores are moulded, and then a wax positive is

moulded around the cores using the primary tool. Wax positives are next attached together with a

tree of wax that denotes metal inlets and gas vents. The whole wax structure is repeatedly dipped

into ceramic slurry, allowing it to dry between dipping. Then the wax is melted out and the ceramic

shell is prepared. Finally the metal can be cast in this ceramic mould. As the metal hardens and

shrinks, the shell and core crack. The shell is broken and the cores are chemically dissolved during

the final cleaning step. By using 3D printing, to produce the ceramic shells with integral cores

directly from the CAD model, a number of disadvantages of the traditional process are avoided

[23]. Most significant is that the metal dies are typically expensive and time consuming to produce,

with lead times ranging from two to six months. For small batch runs the cost of tooling can be

prohibitive. With 3D printing, designs can prototyped quickly and economically. Furthermore,

traditional lost wax casting methods require multiple pattern transfers, each with a potential loss of

accuracy, that are eliminated by printing the ceramic shell directly. Finally, printing integral cores

means that they will be precisely located and not subject to shifting when embedded in the wax,

allowing thinner walls to be cast, and the cores can be made hollow, leaving less material to be

leached out. Diverse metals, including copper, bronze, aluminium, cobalt chrome, stainless steel,

and tooling steel, have been successfully cast in the ceramic shells produced by this process.

Process parameters of 3DP

After developing the CAD model of the component, the upper and lower shells of the model were

made by using the same software. The upper and lower shells models were then converted into STL

format. The machine is then made ready for printing by the following settings: Checking of the

powder level; Preheating of the printer up to 90ºF; Prime wash- cleaning of the printer head; Filling

of the machine bed with powder both in automatic and manual mode; Toggle roller cleaning;

Cleaning of the fast axis (FA) and slow axis (SA).

The post curing time for the pilot experiments were fixed at 60 minutes (minimum

recommended by the manufacturer).The cost and time of production for different experiments were

noted. The RP models of the upper and lower shells were placed in a moulding box with help of

moulding sand support. The light metal castings were then obtained by pouring the molten metal.

From the observations of experimentation, the following combinations of the process parameters of

the machine were selected for final experimentation. Layer thickness: - 0.127 mm, Part orientation:

- horizontal and Post curing time:-60 minutes [24]. After printing of shells, the shells are removed

carefully from the machine bed and de-powdering of the shells is done. In the de-powdering, the

extra powder (which is not glued by binder) is removed with the help of soft brushes. Once de-

Advanced Materials Research Vols. 83-86 347

powdering of the shells is complete, the shells are cured in an electric oven. The post curing time

for the shells is minimum 60 minutes. The upper and lower shells are placed in such a way that the

central axis of both the shells was collinear. The co linearity of the shells was checked with the help

of surface profilometer. The results indicates that at the 5mm shell thickness, hardness of the

castings was improved by 3.79% for aluminium alloy casting. The production cost and production

time was 54.6% and 55.4% less in comparison to 12 mm recommended shell thickness. Further the

results indicate that better photomicrographs were obtained at 5mm shell wall thickness for

aluminium alloy casting. All shell thickness were confirming tolerance grades of the castings

produced and are consistent with the permissible range of tolerance grades (IT grades) as per

standard UNI EN 20286-I (1995). The results indicates that at the 7mm shell thickness, the

production cost and production time was 41% and 37% less in comparison to 12 mm

recommended shell thickness for zinc casting [25].

Conclusions

1. The adopted procedure is better for proof of concept and for the new product, for which the cost

of production for dies and other tooling is more.

2. It is feasible to reduce the shell thickness from the recommended value of 12 mm to 2 mm. The

tolerance grades of the castings produced from different thicknesses were consistent with the

permissible range of tolerance grades (IT grades) as per standard UNI EN 20286-I (1995). The

hardness (on Vickers scale) obtained with 5 mm thickness was better for aluminium alloy casting.

However marginal improvement in dimensional accuracy has been observed. So the effect of rate of

heat transfer from the shells on the shrinkage (dimensional accuracy) may be studied quantitatively

to obtain the relation between shell thickness and shrinkage.

3. The effect of different powder materials used for making shells may be studied for dimensional

accuracy and other mechanical properties. The process limits can be identified in the surface finish

of castings, which will be the objective of future developments of the research.

4. The reported results are part of an ongoing research. Aspects that must still be analysed include

among others are, influence on the accuracy of post treatment procedures on the dimensional

stability over time.

5. The present study deals with finding of optimum shell thickness as per dimensional accuracy for

a component having some definite size. A general mathematical relation between specific volume

and the shell thickness may be developed for 3DP based shell casting.

Acknowledgement: The author would like to thank AICTE, New Delhi for financial assistance

under CAYT.

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