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Emerging Rapid Tooling Technologies Based on Spray Forming

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Emerging Rapid Tooling Technologies Based on Spray Forming 4th International Latin-American Conference on Powder Technology, 19 – 21 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-02044 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) ToolingTM, 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 OspreyTM 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 KeltoolTM 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 KeltoolTM, 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 die- casting. Different RT technologies are classified in Fig. 1. A comprehensive description of the 4th International Latin-American Conference on Powder Technology, 19 – 21 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/ceramic Metal spray- Metal casting Silicone powder, SLS forming moulds Microcast Electroformed KeltoolTM Epoxy tooling moulds Laminated Fig. 1. Classification of rapid tooling technologies. 2. Overview of different RT methods based on spray forming 2.1 Thermal spraying 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, resin- metal 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 oxy- fuel 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. 4th International Latin-American Conference on Powder Technology, 19 – 21 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 SmartArcTM 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 1m2 [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 ± 0.076 mm, and the costs are reduced by 25 to 30% [10]. 2.2 Rapid Solidification Process (RSP) ToolingTM RSP ToolingTM 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 0.075 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 4th International Latin-American Conference on Powder Technology, 19 – 21 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 OspreyTM 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: 4th International Latin-American Conference on Powder Technology, 19 – 21 Nov. 2003, Brazil 5(8) • A (plastic) pattern is made by converting a CAD file through a RP method or NC milling.
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