viability, quick tooling development and producing defect-free casting in the ... overcome by using web-based technologies, to create e-manufacturing systems.
E-MANUFACTURING ONE-OFF INTRICATE CASTINGS USING RAPID PROTOTYPING TECHNOLOGY
D. K. PAL Scientist ‘C’, DRDO, Naval College of Engineering, INS Shivaji, Lonavla-410 402, India Dr. B. RAVI Associate Professor, Mechanical Engineering Department, Indian Institute of Technology, Powai, Mumbai-400 076, India L. S. BHARGAVA Scientist ‘F’, DRDO, Naval College of Engineering, INS Shivaji, Lonavla-410 402, India E-mail: bravi@me.iitb.ac.in dineshpalb@bharatmail.com
ABSTRACT There is an industrial need for rapid manufacture of one-off intricate castings, for defense, vintage equipment and medical prosthetics. This paper presents a systematic approach for this purpose, using a combination of reverse engineering, solid modeling, rapid prototyping, rapid tooling and Internet technologies. Rapid prototyping (RP) technology, involving automated fabrication of intricate shapes using a layer-by-layer principle, has matured over the last decade. Two basic characteristics of RP make it eminently suited to e-manufacturing: (1) the main input is a solid model of the part in a facetted format stored in a STL file (generated by 3D scanning an existing part or by solid modeling), and (2) the fabrication process is highly automated; no part-specific tooling is required. In practice, there are a large number of combinations of RP and RT, besides a choice of materials and fabrication equipment. These decisions greatly influence the quality of the parts (in terms of surface finish, dimensional accuracy, strength and life) as well as the lead-time and cost. The paper also presents the experimental investigations to demonstrate the methodology and benchmarking major RP/RT methods for casting application. Key words: Casting, Solid Modeling, Reverse Engineering, Rapid Prototyping, Rapid Tooling, Internet.
1. INTRODUCTION Advances in computer and communication technologies have led to globalization and increased competition. This in turn has fuelled interest in new methodologies and technologies to improve and accelerate product development. The most promising of these is Rapid Manufacturing, a combination of Rapid Prototyping and Rapid Tooling technologies.
Rapid Prototyping (RP) involves automated fabrication of intricate shapes from CAD data using a layer-by-layer principle. These “three dimensional printers” allow designers to quickly create tangible prototypes of their designs, rather than just twodimensional pictures. Such models make excellent visual aids for communicating ideas with co-workers or customers and can also be used for testing purposes. For example, an aerospace engineer might mount a model airfoil in a wind tunnel to measure lift and drag forces. While designers have always utilized prototypes, RP is now allowing them to be made faster and less expensively. At least six different rapid prototyping techniques are commercially available, each with unique capabilities. These are: Stereolithography (SLA), Fused Deposition Modeling (FDM), Laminated object Manufacturing (LOM), Selective Laser Sintering (SLS), Solid Ground Curing (SGC) and Ink-Jet Printing (IJP). A software ‘slices’ the CAD model into a number of thin layers (~0.1 mm), which are then built up one atop another. Rapid prototyping is an additive process, combining layers of paper, wax, 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. RP’s additive nature allows it to create objects with complicated internal features that cannot be manufactured by other means. Major limitations of RP include part volume (less than 0.125 m3), and part materials (mainly polymers). Most prototypes require from three to seventy-two hours to build, depending on the size and complexity of the object. Initial investment is very high. For simple shaped metals parts required in large quantity, conventional manufacturing techniques are usually more economical. 3 RAPID CASTING DEVELOPMENT Development for one-off intricate castings involves the challenges of economic viability, quick tooling development and producing defect-free casting in the first attempt. These challenges can be overcome by making use of RP process for developing casting patterns for sand as well as investment casting. While RP may seem slow, it is much faster than the weeks or months required to manufacture the tooling by conventional machining processes, especially for complex shapes. The RP processes can be used for directly producing the casting patterns, referred to as direct rapid tooling. The RP parts can also be used as masters for ‘soft tooling’ processes such as epoxy mass casting, PU face casting, metal spray and RTV molding (Ashley, 1994; Dvorak, 1998; Chua et al). Thus, the combination of RP and soft tooling methods can give indirect routes (Figure 1) for casting tooling development (Akarte and Ravi, 2000). An example of a double step route is an ABS mold by FDM process, converted into a pattern by PU face Casting. An example of a triple step route is an ABS plastic pattern master created by FDM, then converted into an epoxy mold by mass casting and finally converted into a production pattern by PU face casting. The different machine models and materials available for each RP and RT process give a large number of potential routes.
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Figure 1: Alternate Tooling Routes for Sand Casting Several researchers have explored rapid casting development using RP and RT. Castings produced by LOM patterns were found to be well within the acceptable quality range and gave 25% cost saving (Mueller and Kochen, 1999). In another investigation, it was found that LOM tooling yielded about 50% saving in time and cost compared to aluminum tooling (Wang et al, 1999). The pattern and core box produced for a valve body directly from FDM process took 73% less time than conventional methods (Sushila et al, 1999). In a study, three alternative approaches (rapid pattern, rapid tooling and hybrid) for making molds, all starting from Stereolithography process, were compared (Chua, et al, 1998). The RP models were used in investment casting to produce foundry tooling and castings, saving time up to 60-80% and proving more cost effective than conventional methods (Warner, 1997). In another investigation, using RP processes to produce the investment casting pattern and ultimately cast metal parts (Dickens, 1995), it was found that castings were generally less accurate than the RP model. The foundry experience in producing the casting from RP pattern was the most significant factor. A similar study was carried out by NASA to evaluate various RP techniques for fabricating the pattern for a fuel pump housing (Spada, 2000). It was concluded that RP techniques are effective for complex 3D patterns. There is an industrial need for rapid manufacture of one-off intricate castings for defense, vintage equipment and medical prosthetics. The RP and RT technologies provide the solution, but are limited by the high costs of installation and maintenance. This can be overcome by using web-based technologies, to create e-manufacturing systems. A number of researchers have explored the application of Internet for engineering purposes (Tan, 1998). Most of them have mainly focused on faster and effective communications and on virtual reality (Broll, 1997). Francis et al have proposed a methodology called Internet manufacturing (IMAN) for the development of a distributed rapid prototyping system via the Internet to form a framework of Internet Prototyping and manufacturing for the support of effective product development (Francis, et al, 2001).
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Internet Client Software - STL Correction
Customer Check
Customer Database
Part parameters -
Login
-
Quotation Request
-
Quotation Order + STL
Ordering/ STL forwarding
Express Carrier
CUSTOMER
Server Software Automatic Quotation Process Planning Part Fabrication
Transport Planning
Packing & Dispatching
CARRIER
RP FACILITATOR
Figure 2: One-Day and One-Off E-Manufacturing Concept using RP Technology for Casting Development 4 E-MANUFACTURING OF CASTINGS Two basic characteristics of RP make it eminently suited to e-manufacturing: (1) the main input is a solid model of the part in a facetted format stored in a STL file (generated by 3D scanning an existing part or by solid modeling), and (2) the fabrication process is highly automated; no part-specific tooling is required (no cumbersome selection of tools, setup, processing sequence, etc.). It is possible to model the part or tooling in one location and get the part automatically fabricated in another location by sending the data over Internet. Starting from the part drawing, it is now possible to produce a medium complexity casting within a week using this approach. The first step will involve converting the part drawing into a solid model and then into the pattern model. The last step will involve using the pattern to produce the cast part. Both of these are well established. The critical middle step of casting pattern development can be compressed by web-enabling the RP/T route. The steps are described below. 1. Log on to the server of the RP company and forward the STL file of the pattern to be manufactured. 2. The STL file is checked and errors (such as missing facets and dangling edges) if any, are fixed. 3. Automatic generation of quotation depending on the pattern volume. 4. After the customer accepts the offer, process planning is automatically done (slicing, scheduling, selection of process parameters, etc.). 5. Part manufacturing is done on the RP machine, followed by post processing and then packing for delivery.
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6. The invoice and related forms are generated automatically and forwarded by email to the express carrier company, who prepares all other forms and papers and plans the transportation. The pattern is delivered to the customer.
5 CASE STUDY An industrial component (impeller casting) was selected to demonstrate rapid casting development. The pattern (Figure 3a) was reverse engineered using Renishaw Cyclone Laser scanner for getting the Cloud of points (CoPs) (Figure 3b). Using the Laser scan CoPs, CAD model (Figure 3c) was generated using Imageware surfacer software. STL format of the CAD Data was used for generating FDM Pattern in ABS plastic material (Figure 3d). The STL file was sent over Internet to an RP facility with an FDM machine. This FDM pattern was used for making Aluminium sand casting (Figure 3e). Five different patterns with Stereolithography (standard and Quickcast), Fused Deposition Modeling, Thermojet and LOM processes were also fabricated. In addition, a silicone rubber mold was created using the SLA RP part as master (Figure 4). The results of the comparative study are summarized in Table 2.
b a
d
e
c
Figure 3: Development of One-off Casting Using a FDM RP Pattern (a) Original Wooden Pattern, (b) Laser Scan CoPs Data Using Renishaw Cyclone Scanner, (c) CAD Model Developed Using Imageware Surfacer Software, (d) FDM ABS Pattern, (e) Al Casting Developed Using FDM Pattern
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a
b
c
d
e
f
Figure 4: Experimental studies on rapid pattern making (a) FDM Polycarbonate pattern, (b) SLA pattern, (c) Stereolithography QuickCast pattern, (d) Thermojet pattern, (e) LOM pattern, (f) Silicone rubber mold. Table-2: RP pattern development using other processes Object
RP Pattern
RP Machine
Material Used
Time Taken (Hrs)
LayerThickness, mm
Organisation/ Place
a
FDM Polycarbonate pattern
Stratasys FDM Titan
Polycarbonate
7
0.254
M/s Stratasys Inc., Bangalore
b
SLA pattern
SLA 250/50
Epoxy Resin
4
0.1
GTRE, Bangalore BARC, Mumbai
c
Stereolithography QuickCast pattern,
SLA 250/50
Epoxy Resin
4
0.1
GTRE, Bangalore
d
Thermojet pattern
Thermojet
Wax
4
0.1
GTRE, Bangalore
e
LOM pattern
Helysis LOM Machine
Paper
3
f
Silicone rubber mold
Vacuum casting
Silicone Rubber
N.A.
--
GTRE, Bangalore
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6 CONCLUSION Pattern development is the main bottleneck (in terms of time and cost) for manufacturing one-off intricate castings, especially for replacement purposes. This can be overcome by a combination of reverse engineering, RP/RT and web-based technologies. The approach has been demonstrated by taking up an industrial case study of an impeller casting pattern. The pattern was solid modeled based from the reverse engineering data. The model was used to fabricate five different patterns with Fused Deposition Modeling, Stereolithography (standard and Quickcast), Thermojet, and Laminated Object Manufacturing processes, and a silicone rubber mold using the RP part as master. We hope that this investigation, along with the comparative data for major RP processes, will motivate the industry to explore and adopt this new technology.
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