Outstanding features of powder injection molding for micro parts manufacturing

Transcription

1 RIKEN Review No. 34 (April, 2001): Focused on Advances on Micro-mechanical Fabrication Techniques Outstanding features of powder injection molding for micro parts manufacturing Kenichi Yoshikawa 1 and Hitoshi Ohmori 2 1 Nexsys Corporation (RIKEN Venture) 2 Materials Fabrication Laboratory, RIKEN When powder injection molding first appeared on the market, it was very well received for the potential it offered. However, the market did not grow as expected at that time. In recent years, the perseverance of manufacturers in this field has borne fruit, and today, the market is about 10 billion yen. However, looking at the market, one is still painfully aware of how low its recognition remains and the degree of improvement required in the general technological level of the manufacturers. In this paper, we attempt to report the process of using powder injection molded products and the advantages by introducing some actual examples. Powder injection molding process First, the powder injection molding process is described. (1) PIM Powder injection molding is called PIM for short. PIM can be classified into MIM (metal injection molding) and CIM (ceramics injection molding). There are only a few manufacturers who deal with both metals and ceramics. (2) Invention 1) (i) CIM CIM is said to have been developed in the 1920s, meaning that it has been in existence for some time now. It was apparently used for the first time in the production of a ceramic sparkplug body. (ii) MIM MIM remained relatively uncommon until the latter half of the 1970s. Its patent holders are Rivers (1976) and Wicch (1980), who were the first to apply thermal plastics to binders. Dr. Wicch used thermal plastics as binders with the aim of enhancing the accuracy of the powder metallurgy method, and developed a method to inject mold parts and degrease the parts by heat decomposition. *Powder metallurgy When metal powder is filled into a mold, the filling density is inconsistent, and the accuracy is ±0.8% of the specified value. *Measures to improve the accuracy of the powder metallurgy method It is important to fill the metal powder evenly into the mold fully to each corner. *Method: Used for plastic injection molding Metal powder is placed in thermal plastics and kneaded. When the resultant compound is injection-molded, the metal powder is evenly injected fully into the mold to every corner. *Advantages: Enables machining to an accuracy of ± 0.2 to 0.4% of the specified value. (3) Importance of powder injection molding method It is called the fifth generation machining technology, following machining, die casting, powder metallurgy, and precision casting. (4) Japanese manufacturers Apparently, there used to be about 40 manufacturers in Japan, but now this number has been reduced to about 20. One wonders why the companies stopped their business, given the interest that is being shown in the method. It seems that poor business was to blame. For example, in one case, as a large output was required, products had to be sold cheaply, which led to the collapse of the business. Some manufacturers also did not have any experience in the fabrication of parts. On the other hand, there are some manufacturers who are doing well, and this perhaps indicates that we are in a very competitive age. Market There are mainly two main categories of products in this market. (1) Substitutes: Parts that are manufactured by a conventional machining method to which PIM has been applied. In this case, the parts involved are high-cost parts that are difficult to machine by conventional methods. (2) Custom-designed parts: These are parts jointly designed by the user (engineers) and engineers at the injection-molding manufacturer. Since there is a high degree of freedom in the shape which can be designed, complicated three-dimensional parts are often jointly designed in this case. The market for the latter is difficult to predict, but it has the potential to increase depending on the effort of the manufacturers. Market scale (1) Shift in Japanese market scale and forecast (metal powder market) 2) Table 1 shows that the Japanese market has been growing at a rate of 15% in recent years, and projections for the near future are about the same level. The market is bigger when CIM is included. The market including ceramics for 1998 is projected to be around 10 billion yen. (2) Shift in world market scale and forecast 1) 13

2 Table 1. Shift in Japan s market scale and forecast (metal powder market). Year One hundred million yen Table 2. Shifts in world s market scale and forecast. Year One hundred million yen ,100 Table 2 indicates that the world market may grow very rapidly in the near future. (3) Number of worldwide PIM manufacturers and related subcontractors (1998) 1) There are about 300 manufacturers and about 5,000 subcontractors. (4) Growth rate of PIM in the world 1) Thirty-two percent growth over the past 10 years. (5) Advanced countries in the world that conduct PIM 1) Japan, America, Germany, Taiwan. Technological features (1) Machining of complicated three-dimensional parts is easy Now, complicated three-dimensional parts that could not be manufactured or incurred high costs when manufactured by conventional machining methods, can be mass-produced easily with the use of molds. (2) Integrated production of composite parts assembled using several parts We have had the experience of building a composite part made up of fourteen parts. The (1) and (2) above are examples of the application of PIM to the manufacturing of complicated three-dimensional parts. Until now, at Citizen, production technology engineers would usually recommend, for complicated three-dimensional parts, the method of not designing the parts individually, by breaking up that part into multiple simple parts and assembling them in a later process to cut costs. This technology, however, runs counter to current part design concepts. This factor promoted the technology to engineers worldwide and contributed to a deepening of the recognition of the method. This is one of the reasons why there are no drawings of PIM from the beginning. (3) Parts can be given custom-designed specification functions Functions such as magnetic resistance, heat conductivity, and thermal expansion coefficient can be adjusted to user specifications. For instance, in the case of magnetic characteristics, when soft magnetic material Permendule (Fe-Co-V alloy) is used, its characteristics can be changed by changing the content ratio of Fe to Co. They can be set as the grade of customizedspecifications content ratio. For heat conductivity and thermal expansion coefficient, for example, the content ratio of W to Cucan be changed; by increasing the Cucontent ratio, heat conductivity increases. However, as the content ratio of W decreases, the thermal expansion coefficient increases. Within the manufacturable range, heatsink specifications can be adjusted to the respective customized specifications of companies. If customized specification materials were to be procured by conventional welding materials, a large amount must be placed, and numerous restrictions exist in reality. However, for the powder injection mold, the mold can be made easily using mixed powder, enabling users to develop materials unique to their companies and use those materials. As the market, including general consumables, is changing, it should be useful for users to use their own materials and manufacture products with unique characteristics. (4) Machining of hard-to-machine materials is easy Hard-to-machine materials include ceramics, the Kovar (Fe- Ni-Co alloy), permendule, SUS, Ti, etc. Until recently, it was almost impossible to mass produce complicated three-dimensional ceramic products, which are typical hard-to-machine materials. Despite some shape restrictions in powder injection molding, mass production may be realized if the mold can be made. The same applies to metal materials such as Kovar and Ti. For metal materials, unlike ceramics, some types may be moldable for small-lot production. Otherwise, mass production will encounter various problems. With PIM, these are possible without any problems. Manufacturing process Only the key points of each step are indicated. (1) Steps Compound manufacturing Injection molding Degreasing Annealing (2) Compound The compound is a composite powdered material mixed with binder which is composed of several types of plastics from low to high-molecular-weight polymers. In order to obtain a stable compound, it is necessary to implement the following evaluation and management. *Powder Powder testing: density, specific surface volume, abrasive size distribution, oiling rate, etc. *Compound kneading evaluation: flow value, viscosity, etc. Some representative plastic binder components include WAX, EVA, PBA, PS, PE, PP, POM, and PA, etc. (3) Ejection molding The basis of injection molding technology is the injection molding of plastics. Many of the routine production technology problems encountered lie in this process. When evaluating the potential of powder injection molded parts in terms of design, basically it is important to look for the potential as plastic molded parts. The injection molding machine is basically the same as the plastic injection molding machine. In recent years, we have seen the emergence of models with specifications tailored for 14

3 Fig. 1. Injection molding machine and automatic sorter. tables on properties and molding conditions. Such effort on the part of the material manufacturers apparently helps spread the technology. *For PIM More and more PIM manufacturers are manufacturing their own compounds. There are several manufacturers that supply compounds. The technical services provided are, however, not as reliable as those of the above plastic injection molding material manufacturers. However, if this market grows, we should be able to look forward to a supply system that comes close to that of the plastic injection molding industry. (4) Typical technologies Required technologies: mold technology, plastic injection molding technology, thermal treatment technology, and material technology (ceramics, metal, plastics). It is not easy to secure engineers for these technologies. use in PIM. Figure 1 shows an automatic sorter set up next to the injection molding machine. This automatic sorter is used for parts that are mass produced, or parts, that are weak in terms of shape and difficult to handle. This automatic sorter was made by Citizen. Parts requiring the use of the automatic sorter are usually industrially manufactured parts. (4) Degreasing This process is unique to the PIM process. The plastic used as the binder becomes redundant after the powder is conveyed and the product is made. This plastic is therefore heated and broken down in this degreasing process. Degreasing may take 1 to 3 days for some parts and this amount of time taken is one of the shortcomings of degreasing. Another key point is minimizing the gravitational deformation of products which may occur in this process. Manufacturers are therefore exerting effort in studying the composition of the binder and accumulating know-how on the temperature rise patterns of the furnaces used. (5) Annealing Like degreasing, this process is also time-consuming and takes about one day. The environment in the furnace vacuum, hydrogen, argon, etc. is determined according to the material to be used. The annealing technology conventionally used for powder metallurgy can be applied. The use of a continuous furnace must be considered when mass production is involved to improve productivity and reduce cost. Tasks (1) There are limits to the dimensional accuracy. In one special case, clock parts with a dimensional accuracy of ±5 µm are mass-produced. Usually, more than ±10 µm is required, and ±0.2% is the limit with respect to the specified value. High accuracy of the machining level is not possible. (2) There are limits to the materials. (3) There are limits to the followup service of material manufacturers. *For plastic injection molding Plastic injection molding material manufacturers provide services to deal with individual problems by providing various Advantages of powder injection molding The ultimate advantage is cost reduction. Sample Some actual examples are clock parts, optical communication parts, automobile parts, floppy disk drive head parts, printer parts, etc. One example product at Citizen weights 20 mg to 26 g, and measures 90 mm. One product made by another manufacturer weighs about 1 kg maximum. In the following, we describe typical mass production samples. For reference, they have been classified as substitutes and custom-designed products. (1) Optical-communication-related parts (i) Heatsink (Fig. 2) Material: Cu-W Design concept: This was a typical custom-designed product jointly developed with engineers at the manufacturer. Initially, the part shape at the right of Fig. 2 was proposed. The difference between the left and right photos is that one has a heat radiation fin, and the other does not. Through discussion, the top shape was selected. The user side considers the fin necessary to improve heat radiation, while the manufacturer side considers the fin ideal for stability when placing the ceramics setter. The absence of the fin will mean no support at the fin, and in gravitational deformation during degreasing, resulting in defective parts. It is more or less impossible to machine such shapes using conventional methods. The use of this part helped improve radiation characteristics, contributed to downsizing, and reduced cost. Fig. 2. Heatsink. 15

4 Fig. 3. The ceramic panel. (ii) Ceramics panel (Fig. 3) During discussion on the ceramic panel, the conventional powder pressing method was recommended. However, due to poor dimensional accuracy, the ceramic panel was eventually decided to be fabricated by PIM. The external accuracy, and the accuracy of the hole position in the middle of the panel are important. The external accuracy was ±20, 50 µm. In particular, the hole position must be ±10 µm from the exterior. This is considered to be a custom-designed part. (2) Clock part 1 (Module part) (i) Operating cam (Figs. 4 and 5) Though it has been five years since the start of mass production, there have been no problems to date. The mold is an eight-component semihot-runner-type mold. The graph shows the experimental data over a six-month period for a hole diameter of φ653 ± 5 µm. Including the cavity gap tolerance, the machining accuracy is ±2 µm. The surface rough- Fig. 5. Fe powder used for operating cam. ness is R max < 1 µm. The part is finished by nonelectrolytic Ni plating. Some of the reasons for the high accuracy in finishing are as follows: the flat thin part, the Fe powder used is spherical and has an average diameter of 1.4 µm, and goodquality binder is used. Refer to Fig. 4 for the part s shape, and to Fig. 5 for the Fe powder. This is classified as a substitute part. (ii) weight 1 (Fig. 6) Although quartz clocks are now the mainstream, mechanical clocks are still being produced. This photograph shows a weight for a self-winding mechanical clock. The material is a heavy metal (W is 97 wt%) and weighs 18.5 g. Conventionally, a donut-shaped sintered part is split into two parts, and each part is welded to manufacture the weight. Using PIM, two parts are treated as one to reduce cost, obtain the desired shape, and improve performance. This is classified as a custom-designed part. Fig. 6. Clock parts (weight 1). Fig. 4. Clock parts: part shape of operating cam. (iii) Weight (2) (Fig. 7) This is a weight for a self-winding clock. This part was designed from the beginning with the use of PIM in mind. A thin part was therefore realized while improving performance. This is classified as a custom-designed part. (iv) Antenna part (Fig. 8) This is a part for an electronic wave correction clock. It is made of ferrite. It was not possible to use conventional machining methods to give ferrite this shape due to the ex- 16

5 Fig. 7. Clock parts (weight 2). Fig. 11. Printer parts. Fig. 8. Clock parts (antenna part). cessively long time required for fabrication. By using PIM, this part can be made at a low cost and productivity can be improved. This is classified as a custom-designed part. (v) Clock parts 2 (Exterior parts) (Figs. 9 and 10) These parts are made of SUS and Ti, respectively. If any of these parts become complicated, conventional forging cannot be applied, the number of machining steps increases, and cost becomes high. PIM is useful for reducing the cost of complicated parts. For SUS, SUS materials that omit Ni from the powder components to produce nonallergic effects are being developed. Such SUS materials are used for band parts, resistors, cases, and labels. These parts are classified as belonging to both the substitute part and the custom-designed part categories. Fig. 12. FDD head parts. (3) Printer parts (Fig. 11) These are made of a soft magnetic material called permendule, and are used for the yoke. They are typical examples of complicated three-dimensional shape parts and fall under the category of custom-designed part. (4) FDD head parts (Fig. 12) These are made of titanium acid calcium. Previously, they were made up of three parts, but now they have been integrated into one part. Despite the long time required for development, cost has been sharply reduced including the reduction of assembly allowance. Today, about 9 million of these parts are manufactured each month, and the production lines work 24 hours a day. Annealing is carried out with about 160,000 per batch. Non-manned operations are realized by using an automatic sorter. These parts come under the custom-designed part category. Process performance is very high, as shown below. Fig. 9. Clock parts (exterior part 1). Slit dimensions: f :3, 820 ± 15 µm g : 335 ± 10 µm Process performance index (Cp value): f: 2.57, g: 1.64 (5) Automobile parts (Fig. 13) Air bag parts: material: Kovar (Fe-Ni-Co alloy) Conventionally, these were made by cutting, but now PIM is applied instead due to such reasons as debarring and bite life. These are classified as substitute parts. (6) Machine parts (Fig. 14) Decelerator parts: material: Fe Fourteen parts have been integrated into one to realize marked cost reductions. These are classified as substitute parts. Fig. 10. Clock parts (exterior part 2). 17

6 Fig. 13. Automobile body parts. Future market trends *Automobiles, ABS, airbags, injection nozzles, etc. *Optical communications: ferrule, heatsink, optical tool change machine parts, etc. *Medicine: Dental tools, etc. *General machines *Others These parts are applicable almost everywhere. However, the effort of the person exploring the market plays a key role. If the market explorer is familiar with the areas of molds, materials, and general production technologies, there will be potential for development and introduction, and expansion of the market. As described in the previous section, PIM extends over numerous areas yet is technically very difficult. For this reason, this technology is suited for Japan, a country strong in production technologies. The authors have reported on these trends at every opportunity available thus far. Though activities to further spread the awareness of this technology to users (engineers) have been simple, they must be continued. It is very encouraging to know manufacturers who have expe- Fig. 14. Machine parts. rience applying this technology understand and continue to use it. It is the authors aim to spread this technology while obtaining opinions from others in order to further expand the market. The authors would like to thank Citizen Corp. (Production Headquarters, Development and Sales Division Honcho, Tanashi-shi, Tokyo , Japan. TEL: (Direct Line). FAX: ) for supporting this study. References 1) R. M. German: in Powder Injection Molding Technologies Proc. Powder Injection Molding 98 (1998), Chapter 1. 2) 1998 Current Situation and Prospects of Powder Metallurgy Market, Report 3, Current Situation of Metal Powder Injection Molding Machine. 18