Emerging Rapid Tooling Technologies Based on Spray Forming

Transcription

1 4 th International Latin-American Conference on Powder Technology, Nov. 2003, Brazil 1(8) Emerging Rapid Tooling Technologies Based on Spray Forming Yunfeng Yang and Simo-Pekka Hannula VTT Industrial Systems, P.O.BOX 1703, FIN VTT, Finland Keywords: rapid tooling; spray forming; near-net-shape manufacturing; tool steel Abstract In this paper rapid tooling (RT) methods based on spray forming technologies are reviewed. These include plasma spray, electric-arc spray, RSP (Rapid Solidification Process) Tooling TM, and PSF (Precision Spray Forming) rapid tooling processes, among which PSF process represents the latest development. This technology developed at VTT (Technical Research Centre of Finland) is based on Osprey TM spray deposition equipment to spray molten tool steel onto ceramic moulds to form net-shaped die inserts. PSF process directly converts steel melt into high-quality net shape die inserts for mass production with rapid prototyping timing. Techno-economical features of PSF process are analysed in comparison with other spray forming methods. The spray forming methods are also compared with some other major hard tooling processes such as Keltool TM process, RT methods based on SLS, and high-speed CNC machining. 1. Introduction Ranges of mass-produced articles from cell phone parts to automotive components are made by using metal moulds or dies through such processes as pressure die-casting, die forging, and injection moulding. Conventionally, the dies and die inserts are made of a block of forged tool steel by machining. Because of increasing severity of working conditions such as heavy mechanical and thermal cyclical stresses, wear and erosion and/or corrosion, and complexity of geometry and often high requirement of surface finish and dimensional accuracy, manufacturing of the dies is very time consuming and costly. Manufacturers are continuously searching for ways to cut down the costs and long lead times of tool making. Rapid tooling is also promoted by an estimation that total profits on new products are often reduced by as much as 60% because of the company s inability to get the product to market quickly enough [1]. There are numerous RT technologies, which can broadly be classified into direct and indirect tooling methods. Direct approaches use a rapid prototyping (RP) process to make tooling inserts directly, whereas indirect tooling methods use the RP process to generate a pattern from which the tooling inserts are made. Direct tooling includes different resin tools, selective laser sintered metal and/or ceramic tools, microcast tools and laminated tools. Indirect tooling include silicone moulds, epoxy moulds, metal spray tooling, metal casting tooling, electroformed tooling, and Keltool TM, etc. Based on the lifetime of the indirect tools, they can also be divided into soft and hard tooling. However, the lifetime is generally measured in injection moulding of plastic components. Some hard tools may be still too soft in more demanding processes such as hot forging and pressure diecasting. Different RT technologies are classified in Fig. 1. A comprehensive description of the

2 4 th International Latin-American Conference on Powder Technology, Nov. 2003, Brazil 2(8) various tooling technologies can be found, e.g. in ref. [2]. This article will focus on analysing the major RT processes based on spray forming in comparison with other hard tooling methods. Rapid tooling Indirect tooling Direct tooling Hard tooling Soft tooling Resin tools Metal sprayforming Metal casting Silicone moulds Metal/ceramic powder, SLS Electroformed tooling Keltool TM Epoxy moulds Microcast Laminated 2. Overview of different RT methods based on spray forming 2.1 Thermal spraying Fig. 1. Classification of rapid tooling technologies. In thermal spraying, materials in the form of wire or powder are melted in a flame generated by electric arc, combustion flame, plasma flame etc. The molten droplets are accelerated by the combustion gas or compressed air or nitrogen and sprayed on to a substrate to form a coating or shell of up to a few millimeters thick. Deposition rates in thermal spraying are relatively small, in the order of tens of grams per minute. The deposition also has a laminated structure that always contains infusible particles, metal oxides and porosity. Due to the fast cooling and great temperature variation in the relatively large deposition area, thermal stresses are also a concern causing distortions in the mould shell, or even cracks. The shell moulds must be backed with resin, resinmetal composite, or low-melting-point metals in order to achieve adequate rigidity for various applications. Thermal spraying methods used for RT include HVOF spray, plasma spray, and electric-arc spray. Early applications thermal spraying were limited to spraying of injection moulding prototyping tools with low-melting-point alloys, such as zinc alloy. It can be directly sprayed onto RP resin models or masters. Recently, high-melting-point alloys have been employed for longer tooling life by the thermal spraying approaches [3-5]. By energy source, plasma spray is similar to electric arc spray, but metal powder is normally used in plasma spray, whereas electric arc spray uses metal wires. HVOF utilizes a high velocity oxyfuel flame to fuse a powdered feedstock and propel it onto the pattern surface. Although the coatings are of higher quality in terms of density and hardness than arc spray, HVOF requires both a greater initial capital spend and has higher operating costs [4]. On the other hand, methods utilizing powders are more capable to produce high quality tooling materials than wire based methods because spraying wires can be produced only at limited compositions not necessarily optimal for a particular tooling application.

3 4 th International Latin-American Conference on Powder Technology, Nov. 2003, Brazil 3(8) Nevertheless, the most significant advances in thermal spraying have been made by electric arc spray, e.g., with the so called Ford Rapid Tooling process, developed mainly by Ford Motor Company and Oxford University, for both prototype tooling and production tooling. The Ford Rapid Tooling process (also known as Sprayform Tooling process [6], and Novarc are spraying process [7]) uses electric arc guns to spray steels on to a substrate that is a Freeze Casting ceramic. Four to six SmartArc TM guns are mounted to a Kuka 6-axis programmable industrial robot, at different angles towards the substrate, as shown in Fig. 2, for forming tools with surface area exceeding 1m 2 [8], and for good deposition soundness adaptable to different geometry features of the mould. The robot moves the gun cluster in a programmed manner termed path-plan [6] to optimize the robot path for minimizing thermal variations and stresses by utilizing the volumetric expansion in martensite transformation to compensate for the steel thermal contraction of the deposition during and after sprayforming. This stress control enables steel shells up to 20 mm thick to be produced with negligible distortion of loss or dimensional accuracy. Fig. 2. The electric arc spray apparatus at Oxford University [9]. Ford Rapid Tooling process has been used to make stamping dies, and so far 750,000 sheet-forming parts have been made with the same die by the process. The lead-time is reduced from 4-18 weeks to 1-2 weeks, tool geometry tolerance is ± mm, and the costs are reduced by 25 to 30% [10]. 2.2 Rapid Solidification Process (RSP) Tooling TM RSP Tooling TM is developed by INEEL (The Idaho National Engineering and Environmental Laboratory, USA), and the initial patent for the process was written in 1990 [11-14]. Its concept involves converting a mould design in CAD file to a tooling master using a suitable RP technology. A pattern transfer is made to a castable ceramic. This is followed by spray forming a thick deposit of tool steel or other alloys to duplicate the desired mould. The process is schematically shown in Fig. 3. The process can replicate details that cannot easily be machined, including mm features, and claims 30 50% cost savings comparing to conventional machining-on-ingot process. The overall turnaround time for tooling is about three days starting with a master. Wide ranges of tooling materials and applications have been tested with good results, including injection-moulding moulds, die casting dies and forging dies. Latest information shows that capacity of RSP process remains in the 100 mm range of insert size. Thus made H13 die casting inserts have 25% longer lifetime than

4 4 th International Latin-American Conference on Powder Technology, Nov. 2003, Brazil 4(8) conventional inserts. However, such an increase in lifetime has not been realised with forging dies [15]. A major limitation for the process is that it is difficult to fill narrow and deep mould cavities, and the aspect ratio (the ratio of depth of a mould cavity against the width) is so far limited within 3-4 [15]. As a fact, this is a common issue for all the spray forming processes. Other common limitations are that it is impossible to make undercut, and only one-sided inserts can be made by spray forming. Fig. 3. Spray forming system of RSP process [16]. 2.3 Precision Spray Forming (PSF) Rapid Tooling Process PSF Rapid Tooling process, in which Osprey TM spray deposition equipment is used to spray molten tool steel or other alloys onto ceramic moulds to form net-shape tools has been developed in the last two years at VTT (Technical Research Centre of Finland). The process is schematically shown in Fig. 4. Tundish Atomizer Ceramic mould Fig. 4. Principle of the PSF process. The PSF process consists of the following major processing steps:

5 4 th International Latin-American Conference on Powder Technology, Nov. 2003, Brazil 5(8) A (plastic) pattern is made by converting a CAD file through a RP method or NC milling. A pattern can be obtained also by silicone RP to copy from an objective, or by certain conventional pattern making methods. Ceramic moulds are made with the pattern. VTT has developed a special castable ceramic moulding process that fulfils requirement of strength, thermal shock resistance, and it is relatively simple and low in costs comparing to other ceramic moulding processes. Tool steel or any other alloy is spray deposited onto the ceramic mould. VTT has also developed a series of mould steels and hot work steels that have better processing properties in spray deposition and improved wear resistance as compared to the commercial ones. The as-sprayformed tools have hardened structure with hardness up to 60 HRC, so only tempering is needed after spray forming. Also porosity of the deposit can effectively be controlled and minimised by using the developed alloys [17]. Normally the working surfaces of the forging and pressure die casting die inserts do not need machining, but only the side and back surfaces should be trimmed to fit into insert holders. Minor finish grinding and/or surface polishing can be carried out if needed. The whole process takes about 5 to 7 days. If it is started with a ready pattern, it will be a couple of days less. A forge die insert and an insert for pressure die-casting made by PSF process are shown in Fig. 5. Fig. 5. A forge die-insert (left) and a pressure die-casting insert (right) made by PSF process. 3. Comparison of the RT processes based on spray forming Capacity of metal-spray tooling methods in terms of deposition area and thickness depends on the metal spray rate. At a fixed deposition rate, the increase in the deposition area would be at a sacrifice of the cohesive strength between the sprayed layers, deposition soundness, and the deposition thickness. Metal flow rates of the electric arc spraying process are normally under 100 g/min. The spray formed tools have a high porosity in the deposit, 2-5% at the best. In the difficultto-control deposition locations, e.g., by the vertical walls of the deposition mould, the porosity can be as high as 30 %. The spray deposit also contains a large amount of oxide [18]. The authors are not aware of this process to have any industrial applications with higher thermal and mechanical stresses than sheet forming or stamping. A common challenge for spray forming processes is to get rid of the porosity in the deposition adjacent to the vertical walls of the mould. When a mould surface becomes nearly parallel to the spray direction, porous columns are formed. These columns are more pronounced as the surface

6 4 th International Latin-American Conference on Powder Technology, Nov. 2003, Brazil 6(8) plane approaches the spray direction and mould cavities become deeper. The mechanism of porous column formation on a slanted surface is obviously the results of competing growth of the metal deposition and a shadowing effect of the vertical surfaces. In the PSF process measures have been developed to solve this problem [19]. As can be seen in Fig. 3, RSP process uses a Venturi type spray nozzle, and the atomizing efficiency and capacity is limited. The metal flow rate is about 4 kg/min. It may be difficult for this process to increase the tool sizes from the current capacity of about 100 mm. PSF process utilizes a two-stage free-fall nitrogen atomizer. The deposition rate is currently 30 kg/min per atomizer. So far, inserts of 270 mm in diameter have been produced. The melting and spraying are carried out in nitrogen protection. Steel tools made by PSF process normally have porosity less than 0.5%. PSF tools also have a fine and homogenous microstructure due to the rapid solidification process. The forging die shown in Fig. 4 has reached more than 25% longer lifetime than conventionally made tools. Comparing to electric arc spraying, RSP and PSF process enjoy more material benefits. They are more flexible in spraying special tooling alloys for different applications. PSF process is particularly developed in hot work tool steels for sprayforming, so thus made tools have gained longer lifetimes even in hot forging of steel components than the conventionally made. 4. Comparison of PSF process with other RT processes 4-1 Keltool TM Process In Keltool process, a metal mixture with a polymer binder is poured into a silicone mould or other mould that contains the positive master of the component shape, and heated up at 100 C to form a green part duplicating the master. The green part is fired to remove the binder and to sinter the metal particles together. Finally, copper alloy is infiltrated into the sintered part that has about 30% pores, to form a fully dense hard tool. Depending on the powder and infiltration alloy, Keltool inserts can reach fairly high hardness (35-55 HRC), and withstand more than one million shots in injection moulding process [2,20]. Compared to PSF process, the strength and toughness of Keltool would not be high enough to satisfy the requirements of the more demanding process such as hot forging and pressure die casting. Keltool also seems to have more process steps generating dimensional variations, which are critical for applications of large tools. 4-2 Direct tooling from laser sintered metal powder Selective Laser Sintering (SLS) has the advantage to be able to process metal powder so that it can make tools directly from CAD data. EOS DirectTool TM is a commercial SLS process using proprietary metal powders, including steel powders, which are selectively sintered in a specially developed machine. The sintered part usually undergoes infiltration with an epoxy resin or metal to increase their strength. After infiltration, further polishing of the part surfaces is possible to achieve the quality required for injection moulding inserts. DTM RapidTool TM is another commercial RT process utilizing SLS approach. It takes use of ironbased powders coated with a thermoplastic binder. A "green" tool insert is built layer by layer through fusion of the binder. The green part is then sintered and subsequently infiltrated with a second metal, copper for example, to form a fully dense part. Hardness is about 75 HRB and 22 HRC. Lifetimes of more than 50,000 shots in injection moulding are reported. The accuracy of the process is claimed to be ±0.25%. An interesting application of direct laser sintering is producing copper EDM electrodes for injection mould tooling and forging dies [20].

7 4 th International Latin-American Conference on Powder Technology, Nov. 2003, Brazil 7(8) A particular problem with the direct tooling methods is their relatively poor surface finish, which results from the layered structure inherent to the building method. Normally the inserts should be finished by machining (milling, grinding or polishing). Similar to Keltool, the tool strength depends on the powder type and infiltration materials. 4-3 High-speed CNC-machining There is a tendency to treat high-speed CNC-machining as a part of RT, but there is also strong opinion that it should be viewed as a competing technology to RT. It is claimed that only RP related techniques, with RP based on adding not removing material, should be referred to RT [20]. High-speed machining is competitive in many applications of relatively simple shaped tooling or producing tools with complicated geometries, but involving little material removal. Competitiveness of PSF process, in this case, should be pronounced in manufacturing of repeating tools especially with complicated geometry, in possibilities of laying cooling channels during spray forming, in net-shape forming of special alloys that are very difficult to machine, and in the tooling material quality for longer lifetime. The authors have found applications demonstrating that these two technologies can be complementary or good combination rather than competing. 5. Future Development of PSF process Development of PSF process started with a small spray-forming machine. So far, some hot forging die and pressure die casting die inserts have been tested only up to 270 mm in diameter. However, the dimension capacity will be enlarged in the future development. Further development of PSF process in the near future aims to the following goals: Larger inserts up to 500 mm in dimension. Placement of cooling channels into the deposit during spray forming. Making of tools with narrow ribs or fins, or other difficult tool structures. Tooling materials and process reliability for more applications. Dedicated spray forming machine for PSF rapid tooling process. 6. Summary Capacity of metal-spray tooling methods in terms of deposition area and thickness depends on the metal spray rate. The increase in the deposition area is at a sacrifice of the mould strength, deposition soundness, and the deposition thickness. Metal spray rates of the metal spray tooling methods in a decreasing sequence are PSF, RSP, and thermal spray methods. PSF and RSP processes have the greatest flexibility in tooling materials. They can use not only commercial alloys, but also those that would be very difficult for conventional steel making processes. Thus made tools can have better lifetimes than those by conventional processes. This is also the major advantage over other RT methods. Metal-spray tooling by gas atomizing of a bulk melt produces higher soundness of the deposition. For example, steel tools made by PSF process normally have porosity less than 0.5%, whereas it is 2-5% in electric arc sprayformed tools. PSF tools also have a pure and homogenous microstructure. The high oxides content with the porosity would exclude applications of thermal sprayed tools from those of high thermal and mechanical impact, such as hot forging and pressure die casting. Tooling cost reduction of spray forming methods is quoted to be 25-50% compared to traditional machining-on-ingot process. Lead times for die making can be shortened from months to weeks. In

8 4 th International Latin-American Conference on Powder Technology, Nov. 2003, Brazil 8(8) most cases, an insert can be made in a few days. It is more rapid and the costs are significantly lower with follow-on tools. Dimensional accuracy of the PSF tools should be ± 0.05 mm in mm. Surface finishing should satisfy most cases in die forging and pressure die casting. High-speed CNC machining with pre-hardened steels claims to have the same level of cost and lead time reduction. Competitiveness of PSF process, in this case, should be pronounced in manufacturing of repeating tools especially with complicated geometry, in possibilities of laying cooling channels during spray forming, in net-shape forming of special alloys that are very difficult to machine, and in the tooling material quality for longer lifetime. Combination of PSF process with those new machining methods sometimes might be of great potential and advantages. Major limitations of spray forming tooling methods are that it is difficult to fill narrow and deep mould cavities, and the aspect ratio is so far limited within 3-4; it is impossible to make undercut; and only one-sided inserts can be made. References [1] T. J. Weaver, J. A. Thomas, S. V. Atre, R. M. German, Materials and Design 21 (2000), p [2] D.T. Pham, S.S. Dimov, Rapid Manufacturing - The Technologies and Applications of Rapid Prototyping and Rapid Tooling. Springer-Verlag, 2001, 214 p. [3] H. Zhang, G. Wang, Y. Luo, T. Nakaga, Thin Solid Films 390 (2001), pp [4] D.I. Wimpenny, G.J. Gibbons, Journal of Materials Processing Technology 138 (2003) [5] R.B Heimann, Plasma-Spray Coatings: Principles and Applications, Weinheim, New York, 1996, 138. [6] S. Duncan, P. Jones, P. Grant, T. Rayment, Z. Djuric, S. Hoile, A. Roche, D. Field: The Sprayform Tooling Process. Key note presentation at the 6 th International Tooling Conference, Karlstad, Sweden, September [7] T. Shelley, Eureka, June 2003, p. 19. [8] D. Field, Materials World, Dec. 2002, pp [9] P. Grant, [10] N. Asnafi: Tooling in manufacturing of car bodies today & tomorrow. Key note presentation at the 6 th International Tooling Conference, Karlstad, Sweden, September [11] J. DeGaspari: Tools to die for. Mechanical Engineering, June 2000, pp [12] K. M. McHugh: Spray forming system for producing molds, dies and related tooling. US Patent 6,074,194, June 13, [13] K. M. McHugh, B.R. Wickham: Spray-formed tooling for injection molding and die casting applications. Proc. Spray Deposition and Melt Atomization SDMA 2000, Bremen, Germany, June [14] J. Knirsch, Advanced Materials & Processes, Jan. 2003, pp [15] K. M. McHugh, J. E. Folkestad: Production of molds and dies using the RSPTM tooling approach. Proceedings of SDMA 2003/ICSF V, Bremen, Germany, June 22-25, 2003, pp. (5) [16] J. R. Knirsch, Spray Forming, Rapid Solidification Process An Update International Die Casting Congress, Oct. 29 Nov. 1, 2001, Cincinnati, Ohio. [17] Y. Yang, S-P. Hannula: Soundness of spray formed disc shape tools of hot work steels. Proceedings of SDMA 2003/ICSF V, Bremen, Germany, June 22-25, 2003, pp. (4) [18] S. Hoile, T. Rayment, P. Grant, A. Roche: Oxide formation in the sprayform tool process. Proceedings of SDMA 2003/ICSF V, Bremen, Germany, June 22-25, 2003, pp. (4) [19] Y. Yang, The Origin of PSF Rapid Tooling Process. VTT Research Report No. BTUO , 28 th Aug. 2003, 24 p. [20] A. Roscochowski, A. Matuszak, Journal of Materials Processing Technology 106 (2000), pp