<< apppex: Weitere Artikel

6th European Conference on Rapid Prototyping and Manufacturing
Nottingham 1997

RAPID TOOLING FOR SIMULTANEOUS PRODUCT AND PROCESS DEVELOPMENT

Prof. Dr.-Ing. Gunther Reinhart, Dipl.-Ing. Frank Breitinger, iwb, TU Munich

ABSTRACT
The main emphasis of this presentation is on the significance of rapid tooling for simultaneous product and process development. This will be discussed using the example of evolution tooling (tools that grow with the development status). Furthermore, a survey will be presented of the most important rapid tooling pro-cesses that employ Rapid Prototyping technologies. A distinction is made here between the process chains for rapid prototyping and conversion processes and the direct manufacture of tool components. The iwb User Center (Anwenderzentrum) in Augsburg, Germany is the first European user of the Indirect Metal Laser Sintering process. In order to communicate what has been learned from initial experience with this procedure, sample tools that have been realized will be described.

1 INTRODUCTION
The situation on the market has changed decisively in recent years. The rapid pace of technological progress is leading to shorter product life cycles, while the products are also becoming more complex. The number of items manufactured per product is declining, so that it is becoming more difficult to apply the experience curve for reducing manufacturing costs (Reichwald & Koller, 1996, p. 242 ff.). At the same time, new suppliers have entered the market as the result of increasing globalization, with competitors from countries with low wages and more favorable labor costs putting market prices under particular pressure.
These changes in the peripheral conditions impose new demands on the innovative ability of companies competing on the market. It is becoming increasingly important to develop new, successful products and to bring them onto the market within a short period of time. Simultaneous product and process development is therefore acquiring great significance. Since they are important participating factors in the production of operational equipment, challenges arise for the tool and mold makers. They are attempting to make use of innovative production processes such as rapid tooling in order to meet changing requirements.

2 SIMULTANEOUS PRODUCT AND PROCESS DEVELOPMENT
The ability to develop products that have optimum customer benefits and can be produced inexpensively is essential. Nevertheless, the ability to develop these products and put them on the market faster (or at least just as fast) as the competition is even more important. A significant amount of profit potential lies in shorter development times. This potential can be put to use in a variety of ways.
First, it is possible to enter the market at an earlier point. For products with an average life cycle of 18 to 24 months, a six-month edge over the competition can bring profits that are three times higher than average. On the other hand, if the market window is missed, a significant loss of profit threatens. Entering the market after a delay of six months can cost all the project's profits, and further delays even result in losses (Wheelwright et al, 1992, p. 21f).
The other opportunity for profitable use of shorter development times is the option of beginning development at a later time, while still achieving the target market entry time. Due to the availability of more up-to-date market data at the time the project begins, short-term changes in the market situation or in the desires of the customers can be taken into consideration before the project starts. This reduces the likelihood of a project flop (particularly on rapidly changing markets) and makes it possible to gain larger market shares by means of superior products with greater customer acceptance (Wheelwright et al, 1992, p. 21f).

2.1 Reasons for Problems in Product Development
The classic approach to product development is notable for a rigid, strictly sequential procedure with clearly defined phases (Horváth et al, 1994, p. 43) and the consistent separation of functional areas. This often results in the following problems:

2.2 Simultaneous Engineering as a Solution Approach
A systematic approach to solving these problems and increasing the efficiency of product development has been formulated. It is known as simultaneous engineering (Wildemann, 1992, p. 20 ff.). Simultaneous engineering is based on the following principles:
Parallel Activities - All activities that are only dependent on other activities in a limited sense should be started at the earliest possible point and be performed in parallel. This is possible with simultaneous product and process development, which makes it possible to achieve two crucial improvements related to the development time:
* Conducting activities in parallel shortens the overall development time.
* Decoupling of the activities makes the overall time plan less susceptible to delays from individual subprocesses.

Integration - In order to match the products better to the processes, the knowledge possessed by all those participating in the creation of value must flow into the development process at a stage as early as possible. To accomplish this, separation between the functional areas must be removed.
Shifting knowledge processes forward - In order to avoid the complexity involved in late modification loops, problems must be discovered and eliminated as early as possible. The use of suitable methods and tools such as FMEA, QFD and DFM/DFA can help to accomplish this task. Through the use of simulations, models, and prototypes, the integration of all areas involved in the product's life cycle permits a systematic check on the product at an early stage to determine production- and assembly-friendliness, customer benefits and potential sources of faults.
Improving the organization's ability to learn - Each development project makes it possible for the organization to gain additional knowledge through tests and the errors that have been made. The faster and more frequently these responses occur during an iteration cycle, the more likely it is that new knowledge can be gained even while the project is still being worked on. This turns the development process into a learning process.
The phase concept for simultaneous engineering - A further characteristic of simultaneous engineering is the reordering and reduction of the development phases. Instead of dividing the project into phases aimed at solving individual problems, a simultaneous engineering development project is divided into just two phases. The first of these is the design phase; it is much longer than in the classic approach and notable for the involvement of all areas that have a part in the creation of value. This makes it possible to integrate product and process knowledge at an early stage of the development work. Even in this early phase, a detailed, integral product and process concept is thus developed; this concept serves as concrete input for the shortened realization phase, for which activities are undertaken in parallel.
The goal of simultaneous engineering is a reduction in avoidable modification effort (Wildemann, 1993b, p. 295). Simultaneous, integrated development of the product and production process can make it possible to reach a point that is optimal for both (Milberg, 1993, p. 179). This permits a efficient production process that can start up quickly.

3 CONTRIBUTION OF RAPID PROTOTYPING AND RAPID TOOLING TO THE IMPLEMENTATION OF SIMULTANEOUS ENGINEERING
Although the advantages of simultaneous engineering are immediately obvious, significant deficits are evident in its implementation in industry. This involves bringing the product and processes development into parallel. The aim is to reduce these deficits through the use of rapid prototyping models and prototypes created by the parallel running process development which is made possible by rapid tooling.

3.1 Use of Models
In Europe, models were not used until a very late stage, due to the long time often needed to produce them (Horváth et al, 1994, p. 42) and the high costs they were believed to incur. The main task is to make a perfect "master model" just before series production in order to verify that the result of the product development meets the requirements (Clark & Fujimoto, 1992, p. 123ff.). At this point, deviations can only be corrected with a great deal of effort. This approach wastes the actual potential that lies in the early and frequent use of models:

The employment of rapid tooling for the manufacture of prototypes and pilot series can make a valuable contribution to the implementation of simultaneous engineering. Nevertheless, this requires a change of thinking when it comes to the manufacture and use of prototypes and test models.

 

3.2 Rapid Tooling as a Measure contributing to
Simultaneous Product and Process Development
The rapid tooling process can be considered as a logical further development of the rapid prototyping process chain. All of the advantages that arise from the use of rapid prototyping models also apply to prototypes made using the rapid tooling process. Unlike the rapid prototyping procedure, in rapid tooling prototypes are made in the series process and with the series materials. Components made on the basis of rapid tooling are therefore significantly closer to series status than, for example, stereolithography models.
The decision to employ rapid tooling obliges the development team to begin the development of the series process at an early stage, and thus brings process development into parallel with product development (Illustration 1). This in turn yields the following positive effects:

3.3 Evolution Tooling
The evolution tooling concept calls for the employment of rapid tooling at an early point in the product development process. As soon as the main function and the rough dimensions of a product have been determined in the concept phase, manufacture of the tools can begin. To do this, a tool component made by a suitable rapid tooling process is employed in a standardized holding block in accordance with the current development status. A new tool component is made as soon as an iteration cycle is finished that justifies this move, and is integrated into the existing standardized holdin block. This is primarily meant for injection molding and die casting, in view of the complexity of such products. Nevertheless, the use of this procedure is also conceivable for forming or forging tools.
Prototype series and pilot series are produced with the same tools from the concept phase right up to the market introduction point. Tooling components reflect the current development status. It is possible to use different rapid tooling processes according to the current stage in development. In order to limit the number of the iteration steps, options for changing the tool later (for instance suitability for welding or erosion) also represent important criteria as well as the factors of time, quality, and cost.
Rapid tooling thus competes directly with the rapid prototyping procedures currently used today. Due to the urgent need to begin process development on time and in parallel with product development, the switch from the rapid prototyping process to the rapid tooling process is made before the end of the concept phase. In this way, the influence of tool and mold making on product design increases. This is in line with the requirements of simultaneous engineering.
Due to evolution tooling's extended range of uses, rapid tooling saves time and expense. With this integrated approach, negative influences on the cost structure - such as arise due to the isolated examination in the manufacture of special prototype tools for a limited number of units - are avoided. In particular the costs for modifying series tooling that are normally incurred in classic product development decrease significantly and should therefore be credited to evolution tooling.

4 SURVEY OF RAPID TOOLING PROCESSES
Rapid tooling is a new term without a clearly defined usage. It originally occurred only in connection with rapid prototyping, but in the meantime is used to describe all developments that are meant to lead to tools becoming available quickly. This includes machining processes such as high speed cutting (HSC) and also Rapid Prototyping technologies. We shall only discuss the Rapid Tooling Processes using Rapid Prototyping technologies here. A combination of the production processes, based on the criteria of complexity, overall height and precision, has proved to be sensible in several specimen cases.
In the case of Rapid Tooling making use of Rapid Prototyping technologies, we differentiate between "soft" and "hard" tooling. Soft tooling means the use of soft molds which are normally obtained from rapid prototyping models by using silicone molding processes. These molds are then usually filled with polyurethane or a two-component resin, which polymerizes in the mold. Nevertheless, in this method, the work is not done with the series material or by the series process.
In this narrow sense, soft tooling can be classified within the transition area between rapid prototyping and rapid tooling. The only procedures which will therefore be taken into consideration are those which make it possible to employ the series material and the series production process. The main area of application for the rapid tooling process is for injection molded or pressure die cast tools.
The scheme shown in Illustration 2 (Naber & Breitinger, 1996) relates to the manufacture of tool components for injection molded and pressure die cast tools using generative production processes. As it shows, the two main methods - rapid prototyping and conversion processes or the direct manufacture of tool components with the aid of Rapid Prototyping procedures - are possible.
Rapid Prototyping and conversion processes - In this process chain, Rapid Prototyping (RP) procedures are employed in combination with conversion processes. One possible method entails making the positive tool component as an RP model and then molding it. In this case, the RP model of the tool component serves as the original model for the investment casting or for the Keltool‘ process. Another method involves making a negative of a tool component using an RP process. The actual tool component can then be produced in the casting mold that has been produced in this way. For this process chain the Sand Laser Sintering process or Direct Shell Production Casting are potentially suitable.
Direct manufacture of tool components - For the direct manufacture of tool components with the aid of RP tech-nologies, three process variations are currently available. The first is based on the stereolithography rapid prototyping procedure, and is known as bridge tooling. Two Metal Laser Sintering processes represent additional options. One of these is the Indirect Metal Laser Sintering process (see next chapter); the characteristic values for its material permit the tool components that are created to be used for the injection molding of plastics as well as for light-metal alloy pressure casting. The other option is the Direct Metal Laser Sintering process; due to the use of a material system with a low melting point, it is currently only used for making molds for plastic injection molding.

5 THE INDIRECT METAL LASER SINTERING PROCESS
The starting material for the Indirect Metal Laser Sintering process is a ferrous alloy with a low carbon content and a particle size of 50 mm that is coated with polymer (Illustration 3). This polymer-coated metal powder is melted on in the laser sintering machine, but only in the area of the polymer layer. The green part that is created in this way is then infiltrated with a water-soluble polymer binding agent. For infiltration, it is sufficient to dip the green part approx. 5 mm into the polymer bath. Due to capillary action, components with heights of up to 100 mm are completely infiltrated after half an hour. In this state, the components possess very little dimensional stability and must therefore be handled with great care. The infiltrated green part is dried in a vacuum oven at 50 °C in a nitrogen atmosphere (Reinhart et al, 1995).
The last step of the process is the sintering furnace process (Illustration 4). First, the reinforced green part is weighed; the results are used to determine the amount of copper alloy needed for infiltrating the part later. Once provided with the corresponding amount of copper alloy, the reinforced green part is placed in a graphite crucible. The sintering oven process is divided into several substeps. In the first of these, the polymer binding-agent is expelled in two stages. Then the part is heated to a temperature at which the iron powder begins to melt and connecting necks begin to form between the individual steel particles.
Since the iron powder is not melted together completely, high porosity remains. In the next subprocess, the furnace temperature is increased further. The copper alloy that was added to the graphite crucible then melts and infiltrates the component through capillary action. After cooling down to room temperature, the tool component can be taken out of the furnace (Illustration 5). The fully dense component consists of 60% steel and 40% copper (Breitinger, 1996).

The time required to make the green part depends primarily on size, not on complexity. For a base area of 180 mm x 150 mm and a height of 50 mm, laser sintering of the green part requires approximately 24 hours. Polymer infiltration and the subsequent drying process take approx. 48 hours. The final furnace process requires approximately an additional 48 hours. The entire manufacturing process thus takes five days, which is relatively independent of the complexity. All processes are accomplished without any intervention by personnel; for the furnace processes in particular, redundant functions or the relevant safety programs are available. For this reasons, weekends can be included in the time planning. The overall setting-up time required can be estimated at five hours.
Initial experiences with the Indirect Metal Laser Sintering process show that good resolution of the individual geometric elements is achieved. Details in the 0.5 mm range are infiltrated completely. Due to the 0.13 mm size stages typical of rapid prototyping technologies, direct use as a tool component is only possible if the corresponding surface roughness on the prototype can be tolerated. Otherwise the tool components must be polished in the traditional way, which requires a great deal of time (Reinhart et al, 1996).
According to DTM, the accuracy that can be achieved lies in the ±0.25 percent range. This is insufficient for the manufacture of mold partitions in particular. For this reason, mold partitions are made with an additional size allowance and are then machine-cut at a later stage. With geometrically matched scaling factors, better accuracy can be achieved. Illustration 6 shows a test component for which special scaling factors were determined. For this part, the level of accuracy achieved was in the ±0.1% range (related to 200 mm). Deviation from the nominal size came from non-uniform shrinkage during drying of the water-soluble polymer binding-agent. The outside zone dries first and prevents shrinkage inside the component. Since the CAD data for the component were scaled uniformly before the part-making process began, this sine-wave course of deviation arises through interaction with the non-linear shrinkage. DTM has announced a new metal powder system that significantly decreases the disadvantages of the polymer binding-agent system currently being used; it should be available before the end of the year.

6 SAMPLE TOOLS

6.1 Prototype Tools for Injection Molding of Plastics – Gearbox Case for Power Drill
For the prototype tools for a drill's gearbox case the emphasis was on the employment of rapid prototyping processes in combination with machine production processes. The tool components that make up the outside contour were CNC-milled with a CAD/CAM chain. The core of the complex interior contour was created using RP processes, in some cases in combination with conversion processes, in others with the tool components produced directly. The following process chains were employed:

Fundamental evidence was obtained that rapid prototyping procedures can be used for the manufacture of tool components. Advantages in time were obtained over the conventional CNC-milling and EDM (Illustration 7) (Geuer, 1996, Gebhardt, 1996, Naber et al, 1995).

6.2 Prototype Tools for Injection Molding of Plastics – Telephone Housing
The Direct and the Indirect Metal Laser Sintering processes were employed with this prototype tool (Illustration 8). In an initial test, the mold inserts were made by the Direct Metal Laser Sintering process, infiltrated with a lead-tin solder and then nickel-plated. Twenty-five components were injection molded in ABS. The injection pressure was 750 bar, the dwell pressure 440 bar. No visible wear was observed. According to Lohner, 1996, batch sizes of 4,500 components made of non-reinforced thermoplastics have been realized in similar applications.
In the second examination, the mold inserts were made by the Indirect Metal Laser Sintering process. The surface was not coated, but merely polished after ejection of the casting. These inserts were then used to mold 140 components made from ABS at an injection pressure of 1000 bar and a dwell pressure of 900 bar. No visible wear was observed. According to the DTM company, production volumes of more than 30,000 parts have been achieved in the USA with tools set up in a comparable manner (Lorenzen & Breitinger, 1996).

6.3 Prototype Tools for Magnesium Die Casting
In mid-1996, a die casting tool for die cast magnesium alloys was realized at the iwb User Center in Augsburg (Illustration 9). The test workpiece was a ribbed, lid-shaped component with a size of 120 mm x 80 mm x 35 mm (Naber & Breitinger, 1996). The mold inserts used to form the shape, which were more difficult to make as far as the production technology was concerned, were produced by the Indirect Metal Laser Sintering process; the uncomplicated gate geometry was made by the conventional method with a CNC milling machine. Due to the high degree of strain placed on the gate area, it is advantageous to adopt a steel mold insert. The casting metal was the standardized magnesium alloy AZ91HP (Breitinger et al, 1996). Components were produced under customary die casting process conditions:
Melting speed in the gate: 34 m/s Melting temperature: 680 °C
Cavity filling time: 19 ms Solidification pressure: 440 bar
Die temperature: 250 °C Cycle time: approx. 40 s
A conventional parting agent was applied to the prototype die casting tool before each cycle. Altogether, 420 components were cast. The tool revealed slight wear in the mold partition area. With this prototype die casting tool, production volumes of up to 1,000 prototypes can be expected.

6.4 Prototype Tools for Aluminium Die Casting
The main object of this series of tests was to determine the service life of mold inserts made using the Indirect Metal Laser Sintering process when used for die casting aluminium alloys (Illustration 10). To accomplish this, a simple cuboid test-component (80mm/80mm/17mm) was used. The mold insert for reproducing the contour was made using the Indirect Metal Laser Sintering process; the flat underside consisted of a steel mold insert.
The alloy was the widely-used AlSi9Cu3 aluminum alloy. Compared with magnesium alloys, almost four times the amount of heat related to the overall volume is brought into the die per cycle with this AlSi-alloy, which leads to a corresponding increase in the thermal strain placed on the mold insert. The selected process parameters correspond to the conditions used in production die casting.
Melting speed in the gate: 34 m/s Melting temperature: 720 °C
Cavity filling time: 15 ms Solidification pressure: 440 bar
Die temperature: 240 °C Cycle time: approx. 30 s
In the area of the impression, the surface temperature of the mold insert prior to spraying on the coating was 240 °C; just prior to the start of the next cycle it was 210 °C. Illustration 10 shows the increase in mold surface damage due to hot cracking, beginning at the ejector bores.
The cracks that formed on the surface of the mold due to the cyclic thermal loading reproduce themselves on the surface of the casting. The flash on the casting that is created in this way is less than 0.2 mm high and can normally be accepted for castings that do not have to fulfill extremely high surface finish requirements. The test was terminated after 210 components had been made. Apart from the heat cracks, no damage could be seen on the mold insert. The tendency towards hot cracking can be decreased by using better polished surfaces and tempered mold inserts (Breitinger et al, 1996).

7 SUMMARY
The employment of rapid tooling will increase the effectiveness of simultaneous product and process development. This is not just because a large number of prototypes are available at an early stage, which strengthens interdisciplinary cooperation; it is also due to the fact that process development for injection molding or die casting is accomplished explicitly in parallel with product development. Furthermore, the large number of prototypes also enables subsequent areas to work on machining and assembly process development at an early juncture.
One approach to solving the problem is to combine tool components produced in different ways in a tool that changes with development (evolution tooling). The rapid tooling process best suited to the current development status is employed. This makes it possible to use a single tool for the production of prototypes and small series from the concept phase right up to the pilot series or introduction to the market. In this way product start-up costs can be reduced and reliable pro-duct start-up ensured.
The rapid tooling projects conducted at the iwb User Center in Augsburg, Germany, show the potential that can be expected from the process chains which were employed, specially the potential of the Indirect Metal Laser Sintering process. Some of the processes discussed here are not only suitable for the manufacture of prototype series but extend well into the area of small and medium-size production runs. For this reason, we can expect a technological advance in tool and mold making.

REFERENCES
Breitinger, 1996
Breitinger, F.: Schneller zu Werkzeugen für funktionale Prototypen. iwb Seminarberichte (21), Herbert Utz Verlag, S 57-68, 14. Mai 96
Breitinger et al, 1996
Breitinger, F.; German, A.; Mertz, A.: Schnelles Herstellen von Prototypenwerkzeugen zum Druckgießen. Maschienmarkt 47/96, Vogel Verlag, Würzburg, 1996, S 34 - 37.
Clark & Fujimoto, 1992
Clark K.; Fujimoto T.: Automobilentwicklung mit System. Frankfurt a.M. 1992
Gebhardt, 1996
Gebhardt, A.: Rapid Prototyping: Werkzeuge für die schnelle Produktentwicklung. Hanser Verlag München, Wien 1996. S 177.
Geuer, 1996
Geuer, A.: Einsatzpotentiale des Rapid Prototyping in der Produktentwicklung. iwb-Forschungsberichte, Band 100. Springer Verlag, Berlin 1996.
Horváth et al, 1994
Horváth, P. et al: Rapid Prototyping - der schnelle Weg zum Produkt. Harvard Business Manager, (1994) Nr. 3, S. 42 - 53
Leonard-Barton et al, 1994
Leonard-Barton D. et al: How to integrate Work and Deepen Expertise. Harvard Business Review, September-October 1994, S. 121-130
Lorenzen & Breitinger, 1996
Lorenzen, J.; Breitinger, F.: Rapid Tooling - Verfahren zur schnellen Herstellung von Prototypenwerkzeugen. Euroforum Konferenz, Rapid Prototyping, Tagungsband, 24./25. Oktober, Köln, 1996.
Milberg, 1993
Milberg, J.: Mehr Effizienz in der Produktentwicklung: Neue Methoden und Hilfsmittel. In: Wildemann H.: F & E: Markt-, montage-, fertigungs- und logistikgerechte Produktentwicklung F & E Kolloquium, München. München 1993. S. 175 - 194
Naber & Breitinger, 1996
Naber, H.; Breitinger, F.: Rapid Tooling für Magnesiumdruckguß. In: Europäische Forschungsgemeinschaft Magnesiumguss e.V. 4. Magnesiumguss Abnehmerseminar, 12./13. September, Aalen, 1996.
Naber et al, 1995
Naber, H.; Macht, M.; Geuer, A.: Fast prototype tools. Rapid Prototyping and Manufacturing ´95, Dearborn. 1995.
Reichwald & Koller, 1996
Reichwald, R.; Koller, H.: Integration und Dezentralisierung von Unternehmensstrukturen, in Lutz B et al (Hrsg): Produzieren im 21. Jahrhundert. Herausforderungen für die deutsche Industrie, Frankfurt am Main / New York: Campus, 1996 S. 225 - 294
Reinhart et al, 1995
Reinhart, G.; Macht, M.; Breitinger, F.: Rapid Tooling - Neue Wege im Prototypenbau. Plastverarbeiter 46. Jg. 1995 Nr. 12, S 12 - 14.
Reinhart et al, 1996
Reinhart, G.; Breitinger, F., Macht, M.: Schnell zu Werkzeugen. Werkzeug & Formenbau, Mai 1996. S 38 - 40.
Wheelwright et al, 1992
Wheelwright, Steven C.; Clark, Kim B.: Revolutionizing product development: quantum leaps in speed, efficiency, and quality. New York 1992
Wildemann, 1992
Wildemann H.: Simultaneous Engineering als Baustein für Just-In-Time in Forschung , Entwicklung und Konstruktion. VDI-Z 134 (12) 1992, S.18 -22
Wildemann, 1993a
Wildemann H.: Just-In-Time in Forschung & Entwicklung und Konstruktion. ZFB 63 (12) 1993, S.1251 - 1270
Wildemann, 1993b
Wildemann H.: Management von Produktänderungsprozessen in F & E: Markt-, montage-, fertigungs- und logistikgerechte Produktentwicklung H. Wildemann (Hrsg.) Tagungsband F & E Kolloquium München 1993. S. 283 - 318