Experimental Investigation of Pattern-less Casting Solution Using Additive Manufacturing Technique

Transcription

1 ISSN No MIT Publications 16 Experimental Investigation of Pattern-less Casting Solution Using Additive Manufacturing Technique Munish Chhabra Associate Professor, Mechanical Engineering MIT, Moradabad Dr. Rupinder Singh Associate Professor, Mechanical Engineering G.N.D.E.C, Ludhiana Abstract- The developments in additive manufacturing (AM) have opened up new avenues for fabrication of functional prototypes to the fabrication of tools and moulds for direct metal castings. Many AM techniques are commercially available to fabricate prototypes and casting tooling directly from a computer model. Three dimensional printing (3DP) is one such technology which builds prototypes, parts for end-use and for use as tooling, patterns and moulds for metal castings relatively fast and at a low cost. Rapid casting (RC) is one of the most important applications of 3DP, due to its ability to generate patterns, other casting tooling and direct production of pattern less moulds that can be used for to cast non-ferrous metals. This paper describes briefly the concept of additive manufacturing, three dimensional printing and the ZCast RC solution. In addition, feasibility of ZCast RC solution using 3DP technique to generate Al casting has also been investigated by reducing the shell wall thickness from the recommended one, 12(mm). The feasibility of RC solution has been assessed in terms of dimensional accuracy and surface roughness of the castings. The castings generated with this process showed good process stability in the casting of Al with the shell mould thickness from 12(mm) down to 2(mm). Keywords: Additive manufacturing, Three dimensional printing, Casting, IT grades. List of Abbreviations 3DP Three Dimensional Printing Al Aluminium AM Additive Manufacturing CAD Computer Aided Design CMM Coordinate Measuring Machine IT International Tolerance Ra Roughness average RC Rapid Casting RP Rapid Prototyping RT Rapid Tooling STL Standard Triangulation Language ZCorp ZCorporation I. INTRODUCTION To reduce the product development time and reduce the cost of manufacturing, the new technology of additive manufacturing (AM) has been developed, which offers the potential to completely revolutionize the process of manufacturing [1]. In the early development of additive manufacturing (AM) technologies, the emphasis was directed towards the creation of touch-and-feel models to support design [2]. Originally seen as most suitable for AM, these processes are no longer used for that purpose any longer [3]. With the advent of new materials along with new processes, each technology has been applied in diverse fields [4]. In particular, additive construction applied to the production of dies and electrodes, directly from digital data, is defined as rapid tooling (RT). The RT goal is to produce complex parts quickly of the required accuracy and quality, so it can be used in concurrent engineering [5]. RT is the use of AMgenerated objects (with or without special postprocessing) as an experimental or even regular tooling in industrial manufacturing, particularly in metal casting [1]. The purpose of RT is not the manufacture of final parts, but the preparation of the means to manufacture final parts: mass production tools such as molds, dies etc. can be ready in very short times [6]. A variety of tooling can currently be produced using different RP technologies. For the purpose of classification, tooling is divided into direct or indirect tooling [7]. Pattern, cores and cavities can be obtained through rapid casting (RC) techniques [1, 8]. Many RC solutions have been developed and commercialized. Great efforts have been made to develop new RC and RT technologies that combine the recently emerged AM processes with one or more subsequent processes [5]. In both cases, since the tooling phase is highly timeconsuming, great competitive advantages can be achieved. Moreover, RT and RC processes allow the simultaneous development and validation of the product and of the manufacturing process. Technological prototypes can constitute a strategic means, not only for functional and assembly tests or to obtain the customer s acceptance, but mainly to outline eventual critical points in the production process. The relevance of RC techniques consists, above all, in a short time for parts availability. Traditionally, in order to produce prototype a model and eventual core have to be created, involving time and costs that hardly match the rules of the competitive market. For this reason, functional tests are typically performed on prototypes obtained by metal cutting, which are not effective in outlining issues related to the manufacturing process. The possibility to verify the usefulness of a technological solution, in the early stages

2 ISSN No MIT Publications 17 of the product development, ensures a concurrent engineering approach and minimize the risk of late modifications of the definitive production tools. The initial increase in costs can thus be repaid through a reduction of costs and time for the following phases of development, engineering and production, as well as through non-monetary advantages [8]. In particular, for relatively small and complex parts, the benefits of additive construction can be significant [9]. In addition, models built with the help of AM processes are used as tools for casting and molding i.e. dies for an injection molding process and pattern for a casting process. In recent years three dimensional printing (3DP) came to the foreground as a very competitive process in terms of cost and speed [10 ].The process of 3DP was patented in 1994 under U.S. patent number [11]. It was developed at Massachusetts Institute of Technology (MIT) based on inkjet technology [12] and licensed to Soligen Corporation, Extrude Hone and Z- Corporation of Burlington. It is classified as a typical concept modeller, a low-end system, and represents the fastest RP process [13]. The 3DP prototypes may be used directly to produce moulds for casting or used as a pattern to produce mould indirectly. In this field, innovative solutions are now available based on 3DP process, which can extend RC possibilities thanks to the lower costs with respect to previous technologies such as selective laser sintering of sand [14]. One such technological solution is component printed in starch- based powder produced on 3DP process and infiltrate with surgical wax are extensively used as pattern for investment casting [3]. A second solution is the ZCast TM process, in which 3D-printing technology with the use of a ceramic material allows the production of complex cavities and cores, suitable for casting light alloys. Further, three casting methods make up the ZCast TM process: direct pour, the shell method, and production intent casting [15]. Both the 3DP based RC solutions proved to be effective for the production of cast technological prototypes, in very short time, avoiding any tooling phase and with dimensional tolerances that are completely consistent with metal casting processes. Regardless of the casting method, the foundry industry has as its central process the utilization of a physical pattern to produce moulds into which to cast metal. Although, this is true for both the design and production cycles, it is mainly the design stage that will benefit from rapid patterns. The use of AM technologies in the creation of casting patterns allows a foundry to manufacture a metal part without the use of tooling for small quantities. It also helps in optimizing the casting design in terms of process and gating parameters. All of this reduces the cost and time required to produce prototype parts [1]. A key issue regarding the shell casting process is the production of the pattern in the case of a prototype casting, for which the traditional die casting is uneconomical. AM techniques can meet this requirement, producing few parts in short times and without tooling costs [16-18]. The ZCast process using 3DP technique represents an innovative low-cost solution among RC technologies. Cast metal parts can be obtained significantly faster and are less expensive than other methods [19]. The present work is an attempt to introduce the basic concept of AM, 3DP technique and RC solution (ZCast501) and also to investigate the feasibility of the RC solution based on 3DP technique to produce non ferrous casting. Experimental studies regarding this solution are lacking in literature, in particular the technological feasibility in the case of thin-walled parts needs to be assessed. Fig.1. Additive Manufacturing (AM) concept

3 ISSN No MIT Publications 18 A. AM Concept AM is a fabrication method whereby physical objects are constructed by depositing material layer by layer under computer control. AM 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. Fig.1 shows the concept of AM. B. The Basic Steps of Additive Manufacturing Although several AM techniques exist, all employ the same basic five-steps [6]. These steps are: 1. Design: Create a 3D CAD solid model of the design 2. Converting: Convert the CAD model to Standard Triangulation Language (STL) format 3. Pre-Process: Slice the STL file into thin crosssectional layers (Generated by a dedicated Software) 4. Building process: Construct the model one layer atop another 5. Post-Process: Clean and finish the model A large number of AM techniques have been used commercially in metal casting industries such as: Stereo lithography (SLA); Selective Laser Sintering (SLS); Laminated Object Manufacturing (LOM); Fused Deposition Modeling (FDM); Solid Imaging (SI) or Multi-jet Modeling and 3D printing (3DP) [20]. In the present research, in order to investigate the feasibility of 3DP technique to produce non ferrous casting (Aluminium), the direct metal RC solution ZCast501was used. A brief description of 3DP technique is presented in section C. C. Three Dimensional Printing (3DP) Technique As shown in Fig.2 (a) 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. Then 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, CA glue, or other sealants to improve durability and surface finish. Fig.2 (b) shows the complete 3DP process cycle. D. 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 [21]. 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. (i) 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 - Color printing (ii) Composite Based Material: These materials are ideal for - Thin- walled enclosures - Assembly applications (iii) 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 - Patterns for investment casting E. ZCast 501 Direct Metal Casting ZCast501 Direct Metal Casting is an RC solution developed by ZCorp for direct metal casting of non ferrous alloys. ZCast process creates the shell molds directly from CAD data by using 3DP technology. Conventionally, metal castings are produced by using sand casting tooling, techniques and procedures. With respect to traditional sand casting, limited by the pattern extractability, layer by layer construction allows obtaining complex part, without any restrictions in terms of undercuts provided only that the unconsolidated powder can be removed from the cavity [16]. It eliminates the pattern creation phase of the traditional sand casting process in a revolutionary way, resulting in a drastic reduction of the casting lead time from weeks to

4 ISSN No MIT Publications 19 days [22]. ZCast provides three basic methods to fabricate moulds to produce casting rapidly [23]. Reference [15] reported that the accuracy and surface finish are consistent with sand casting by using ZCast process. Feed Roller Binder Cartridge Binder Component F. Major Features of ZCast: ZCast501 mold is recommended for non ferrous metals with pouring temperature below 1100 o C The recommended shell mold wall thickness range is minimum12.5mm and maximum 25.4mm [24]. Before pouring, ZCast moulds must be baked in an oven from 180 o C to 230 o C for between 4 and 8 hours (based on volume), until it is bone dry. Customers cast metal into these 3D printed molds for prototype evaluation or fully functional parts. Feed Material (Powder) Piston Build Chamber Fig.2(a) 3DP process [18] START LAYER OF POWDER DEPOSITED AUTOMATICALLY PRINT HEAD APPLIES RESIN TO POEDER LAYER POWDER LAYER DRIES ALMOST IMMEDIATELY ADD ANOTHER LAYER? NO REMOVE COMPLETED MODEL FINISH Fig. 2(b) Complete 3DP process cycle Excess Material Chute YES G. ZCast Methods (TM) Direct Metal Casting (ZCast): The ZCast Direct Metal Casting consists of materials and processes that allow designers and engineers to build moulds (copes and drags) and/or cores directly from a CAD file. It eliminates the pattern creation phase of the traditional sand casting process in a revolutionary way, resulting in a drastic reduction of the casting lead time from weeks to days. The process involves the design of basic parting lines and coring. A 3D mould shell approximately 25(mm) thick is printed using ZCast plaster-ceramic composite material. If necessary, the shell is created with ribbing and backfilled with traditional foundry sand to give added strength while minimizing material cost. Loose Pattern Method (LP): The Loose Pattern (LP) method is a familiar and well-established technique in the casting industry. The steps of this process are shown in Figure 1b. According to conventional techniques, the patterns are usually made of wood or plastic. Implementing modern technologies, these patterns can be produced using 3D Printing. Printed parts, having been infiltrated, are aligned on a joint board. Around the pattern, a dividing surface (split line) has to be created manually by using resin mixed foundry sand or similar materials. The pattern stays loosely seated so that the process of creating the split line can be repeated for the other mould half, using the same pattern. The two joint boards are framed with wood which, along with the 3D Printed pattern, constitutes the final foundry tool. Mould halves are then created separately by interchanging the pattern between the two core boxes. Production Intent Casting (PIC): This method is a combination of the zp102 plaster material for creating patterns, and ZCast material for creating cores. It first involves the creation of the foundry tooling as it would be designed for the production foundry process, including core prints, offset partings, and clearances. The pattern equipment is then printed with the zp102 material, infiltrated with epoxy, and if necessary backfilled with a rigid plastic filler for added strength. If cores are required, they may be produced using the ZCast material.

5 ISSN No MIT Publications 20 II. EXPERIMENTAION In the present work, the 3DP machine with ZCast direct metal casting solution was used to fabricate the shell moulds. The 3DP printed shell model was used as the positive pattern around which the sand is filled in a moulding box. An effort has been made through experiments, to study the feasibility of decreasing the shell wall thickness from the recommended one 12(mm), in order to reduce the cost of production and time as well as to evaluate the dimensional accuracy and surface roughness of the Al castings obtained, for assembly purpose. Experimental studies regarding this solution are lacking in literature. Reference [6] has conducted studies for two technological solutions in this field and this study aims at evaluating 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. In present work, the layer thickness, part orientation and post curing time were considered as parameters of the 3DP machine. The out come of the experimental study will be helpful to provide data for industrial application of the considered technology. Objectives of the Research Work Following were the main objectives of this research work. 1. To verify the feasibility of decreasing the shell thickness from recommended one (12mm) in order to reduce the production cost and time. 2. To evaluate the dimensional accuracy of the Al castings obtained and to check the consistency of the tolerance grades of the castings (IT grades) as per allowed IS standards for casting process. 3. Proof of concept, to present the concept in physical form with minimum time by avoiding the cost of making dies and other fixtures for a new concept. III. METHODOLOGY A. Identification of Benchmark For the investigation, an aluminum alloy casting was chosen as a benchmark, representative of the automobile field (used in suspension system), where the application of RC technologies is particularly relevant. Fig.3 shows the drawing of benchmark. B. CAD Model Creation Firstly, using the UNIGRAPHICS software CAD model of the component was made. After making the CAD model, the upper and lower shells of the model were made by using the same software. The upper and lower shells of the component were made for different values of the thickness. The thickness values for shells were12mm,11mm,10mm,9mm,8mm,7mm,6mm,5mm,4 mm,3mm and 2mm. The upper and lower shells models were then converted into STL format. To establish the consistency, the STL format has been chosen as the standard for the AM industry. a Fig.4. CAD model of (a) upper and (b) lower shell mould b Fig.3. Benchmark dimensions

6 ISSN No MIT Publications 21 C. Machine Preparations The machine was then made ready for printing by the following settings: 1. Checking of the powder level. 2. Preheating of the printer up to 32.2ºC. 3. Prime wash- cleaning of the printer head. 4. Filling of the machine bed with powder both in automatic and manual mode. 5. Toggle roller cleaning. 6. Cleaning of the fast axis (FA) and slow axis (SA). Fig.7. Post heating of Shell moulds in oven F. Shell Casting of Selected Benchmark Starting from the CAD model of the component, shells were modeled for different shell wall thicknesses. From the analysis of geometry and volume of benchmark, single feeder and riser system was designed for pouring the molten metal. RP shell models are used as positive patterns around which the sand is filled in a moulding box. Commercial cast aluminium alloy was used for casting. Fig.5. ZCorp 3DP machine D. Shell Mould Printing After parameter settings of the machine (as obtained in pilot study), the machine was set in on-line mode and the printing of the shells for different values of the thickness was done. Fig.6 shows the 3D printing of shell moulds. Fig.6. Printing of shell moulds using ZCast501 RC solution E. Post processing of Shell Moulds After printing of shells, the shells were removed carefully from the machine bed and de powdering of the shells was done. In the de-powdering, the extra powder (which is not glued by binder) was removed with the help of soft brushes. After de-powdering of the shells, the shells were cured in an electric oven (see fig.7). The post curing time for the shells was minimum 4 hours. The upper and lower shells were placed in such a way that the central axis of both the shells was collinear. Fig.8. Al castings obtained with different shell mould thicknesses III. TESTING AND ANALYSIS OF PROTOTYPE CASTINGS The measurement paths for the internal and the external surfaces of the benchmark have been generated through the measurement software of the GEOPAK v2.4.r10 CMM. These paths direct the movements of the CMM probe along trajectories normal to the parts surface. Table1 shows variation in measured dimension of average thickness castings obtained with respect to shell thickness. The results of the dimensional measurements (component thickness) have been used to evaluate the tolerance unit (n) that derives starting from the standard tolerance factor i, defined in standard UNI EN (1995) [25]. The values of standard tolerances (also known as Fundamental Tolerances ) corresponding to IT5-IT18 grades, for nominal sizes from 3mm to 500 mm, were evaluated considering the standard tolerance factor i (µm) indicated by the following formula, where D is the geometric mean of the range of nominal sizes in mm. Tolerance Factor i= 0.45 ( D) D ( 1) 1/3

7 ISSN No MIT Publications 22 S. No. TABLE I OBSERVATIONS OF FINAL EXPERIMENTATION Shell mould thickness (mm) Component thickness (mm) Surface roughness (microns) Shell mould production time (min) In fact, the standard tolerances are not evaluated separately for each nominal size, but for a range of nominal sizes. For a generic nominal dimension D JN, the number of the tolerance unit s n is evaluated as follows: n= 1000(D JN - D JM)/ i, where D JM is measured dimension. The tolerance is expressed as a multiple of i: for example, IT14corresponds to 400i with n= 400. Table II shows classificatio n of different IT grades according to UNIEN After this for each value of thick ness of castings, corresponding value of n were calcula ted, the latter taken as a reference ind ex for evalua tion of tolerance grade. The results of dimensional measuremen ts (thickness of obtained castings) are shown in Fi g.9.the r esults of dimensional measurements are us ed to evaluate the tolerance gr ades. The obtained tolerance grades are IT1 2 and IT13. The results presented in Table II shows that the tolerance grades calculated for the consider ed RC solution (ZCast 501) are consistent with the values allowed for casting operations between (2) IT11 and IT18. All the obtained castings are with in the range of IT11 to IT18 and thus are completely acceptable at all shell wall thicknesses from 12(mm) down to 2(mm). However, better dimensional accuracy is obtained at 5(mm) shell mould wall thickness (see Table I). It should be noted that the process of solidification at different shell thickness leads to different thermal gradients, which affect the heat transfer and finally shrinkage (thickness of component) of castings as shown in Fig.9. The roughness of the surface can be measured by calculating the arithmetic mean value (Ra) as the best estimate for the true value of set of experimental measurements [26]. In this research, surface roughness of the castings obtained was measured with Mitutoyo surface roughness tester (SJ-201). The measured surface roughness of obtained castings is shown in Table I and Fig.10. The experimental results show that the surface roughness increases with the decrease in shell mould wall thickness. The surface roughness obtainable by sand casting process lies between Ra 6.3 (microns) to Ra 50(microns). All castings of current experimentation are within this range which justifies the feasibility of decreasing the shell wall mould thickness from the recommended one. Fig.9. Effect of shell mould wall thickness on thickness of casting obtained Exp. No. TABLE II CLASS OF DIFFERENT IT GRADES ACCORDING TO UNI EN I Shell Thickness Component thickness Tolerance Tolerance Unit (mm) (mm) Factor i n (µm) D JN D JM IT Grade IT IT IT IT IT IT IT IT IT IT IT 15

8 ISSN No MIT Publications 23 thickness will be different. Hence there is need to generate database of optimum shell wall thicknesses based on dimensional accuracy, mechanical properties for different volumes, weight density of materials and pouring temperature of materials. Fig.10. Effect of shell mould wall thickness on surface roughness of Al casting obtained The production time (PT) for preparing shell of different wall thicknesses is shown in table1and Fig.11.The saving in production time for 5(mm) shell thickness as compare to 12(mm) shell thickness for Al alloy is = [PT at 12(mm) - PT at 5(mm)] / PT at 12(mm) = [(77 (min) - 40 (min)) / 77] 100 = 48.05% The decrease in shell thickness reduces the volume of ZCast powder as well as the quantity of binder to be printed by the 3DP machine. So, for the same internal geometry, the time required to print the shell by decreasing the wall thickness from 12mm down to 2mm will be reduced. The saving in production time for preparing all reduced shell wall thickness moulds as compared to 12(mm) shell mould was also calculated and shown in Fig. 12. Fig.11. Effect of shell mould wall thickness on shell production time mould The present research work deals with the Rapid Casting of material (Al) having low melting temperature range of o C [27], however for casting of materials having high temperature range the result of optimum shell Fig.12. Effect of shell mould wall thickness on shell mould production time IV. CONCLUSION The following conclusions are derived from the study: 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. The present study deals with finding of optimum shell thickness as per dimensional accuracy for a component having some definite size. 3. It is feasible to reduce the shell mould wall thickness from 12(mm) to 5(mm) for generating Al casting using ZCast501 RC solution. 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 I (1995). 4. The dimensional accuracy (thickness of component) obtained with 5(mm) shell mould thickness was better than the accuracy obtained with 12(mm) recommended shell mould thickness. However, the surface roughness Ra values of Al castings obtained are lie in the range of surface roughness obtainable by sand casting process. 5. The experimental results indicate that at the 5mm shell thickness, the production time was reduced by 48.05% less in comparison to 12(mm) recommended shell mould thickness. ACKNOWLEDGEMENT The authors are grateful to Management, Director General, Director, and HOD ME of Moradabad Institute of Technology, Moradabad for motivating us for this investigation.

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