Fabrication of Cemented Carbide Molds with Internal Cooling Channels Using Hybrid Process of Powder Layer Compaction and Milling*

Transcription

1 Materials Transactions, Vol. 46, No. 11 (25) pp to 253 #25 The Japan Institute of Metals Fabrication of Cemented Carbide Molds with Internal Cooling Channels Using Hybrid Process of Powder Layer Compaction and Milling* Yoshiaki Mizukami 1 and Kozo Osakada 2 1 Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi , Japan 2 Graduate School of Engineering Science, Osaka University, Suita , Japan Layered compaction manufacturing (LCM), which is a hybrid process of powder compaction and milling in layers, is applied to the fabrication of a cemented carbide mold (WC 9 mass%co) for the forming of optical glass lenses with internal cooling channels, which are placed along the molding cavity. The mold is produced by repeating the process of powder compaction with the subsequent creation of grooves filled with paraffin wax as a sacrificial material. The channels placed along the molding cavity are formed during the sintering process by dewaxing. In the sintering process, the extent of deformation in the shape of the internal channels and molding cavity is measured. It is found that the shapes of the channels and cavity exhibit uniform linear shrinkage in the range from 17 to 19%. By filling the molding cavity with epoxy resin, the cooling capability of the mold by air is investigated by performing experiments as well as two-dimensional finite differences simulation. The cooling effect of the mold in the glass lens-forming process is also estimated by the simulation. For both resin and glass, when air is supplied to the channels, the obtained cooling rate is approximately ten times higher as compared to natural cooling at an ambient temperature. (Received May 27, 25; Accepted September 22, 25; Published November 15, 25) Keywords: powder forming, freeform fabrication, layered manufacturing, milling, cemented carbide, mold and die, optical lens, cooling channel 1. Introduction With the increasing demand for improving the dimensional accuracy and the quality of molded precision products, such as optical glass lenses, the thermal and elastic distortion of the mold cannot be neglected. WC Co alloys are extremely useful for such applications due to its low thermal expansion and high elastic modulus as well as high resistance to wear. Furthermore, it is important to efficiently cool the surface of the molding cavity with uniform temperature in order to reduce the shape deformation and cycle time of the molded products. The creation of internal cooling channels along the molding cavity, which is difficult to realize by conventional machining, is most effective. Layered manufacturing 1) (or solid freeform fabrication), which is a technique to produce physical prototypes in layerby-layer directly form their CAD models, has been investigated. Stereolithography (SLA), 2) selective laser sintering (SLS), 3) laminated object manufacturing (LOM), 4) fused deposition modeling (FDM), 5) and three-dimensional printing (3DP) 6) are some of the most popular method. For the fabrication of the mold with internal cooling channels, SLS or 3DP has been applied. 7,8) SLS is a method for forming models by locally scanning a focused laser beam to the powdered materials in layers. Although these techniques enable the fabrication of models with complicated geometry, such as internal channels, directly from powder, there are limitations on the configuration of the cooling channels because it is necessary to remove non-sintered or non-bonded powder from the channels after fabrication. A mold with straight channels or that assembled from several parts with grooves for the cooling channels are generally used. However, some problems exist in the molds, e.g., the cooling *This Paper was Originally Published in Japanese in J. Jpn. Soc. Technol. Plast (24) efficiency is not sufficient, and manufacturing period is large. A solid free-form fabrication method named layered compaction manufacturing (LCM) 9) has been proposed to fabricate three-dimensional models of difficult-to-machine materials. A WC 9 mass%co carbide model with internal structures, which is difficult to obtain with conventional machining methods, was successfully fabricated using paraffin wax as the sacrificial material in the LCM process. The LCM method based on layered manufacturing is a hybrid process combining powder compaction and CNC milling in a layer-by-layer additive manner. Each layer is created by compacting powder with the subsequent milling of grooves corresponding to the cross-sectional profiles of the designed model. The grooves are filled with paraffin wax that acts as a separator or sacrificial material. After the parts of the model are separated according to the grooves, the green models are sintered. In this study, the LCM method is applied to the fabrication of a cemented carbide mold (WC 9 mass%co) with internal cooling channels, which are placed along the molding cavity, for the forming of optical glass lenses. In addition, the cooling efficiency of the mold is investigated. Furthermore, the applicability of this mold and die fabrication method for precision products is discussed. 2. Principle of the Layered Compaction Manufacturing (LCM) The procedure for the LCM method is schematically shown in Fig. 1. (a) First, the sliced data of a designed solid model is formulated using a CAD/CAM system. (b) The green model is then constructed using a compacting tool composed of a container, lower punch, and upper punch. After the powdered metal is supplied into the container, (c) the powder is compacted as a thin layer. (d) The grooves are then milled according to the coordinate information of the

2 2498 Y. Mizukami and K. Osakada (a) Model design with CAD/CAM system.5 4 φ14 φ9 Wiper Powdered metal Container Lower punch (b) Powder supply Upper punch Separator (paraffin wax) 5.5 (a) 4 φ14 φ14 (e) Filling grooves with wax (c) Powder compaction Endmill Grooves 9 (b) φ19 Molding cavity φ7 Cooling channels Core Cavity (d) Creation of grooves (c) 3.5 Support part Heating Fig. 2 Schematic drawing of the mold with internal cooling channels for the forming of optical glass lenses: (a) core, (b) cavity, (c) cross section. Model part Inner structures Sintering (f) Finish (g) Model separation (h) Completion slice data. (e) The created grooves are filled with a molten material having a low-melting point, e.g., paraffin wax as the sacrificial material (separator), to separate the model part or to create cavities. (f) A layered green object that consists of the model and support parts is obtained by repeating these processes for the final layer of the slice data. (g) The support parts are then removed from the model part by remelting the separator in a furnace. At this time, the cavities such as internal channels are also created in the model because the separator is completely removed from the grooves. (h) Finally, the green model is sintered and polished. The removed powder is reused. 3. Fabrication of a Tungsten Carbide Mold Final model Fig. 1 Principle of the LCM method combining powder compaction and milling in a layer-by-layer additive manner. 3.1 Mold design Figure 2 shows the schematic diagram of a mold with internal cooling channels for the forming of a plano-convex glass lens fabricated using the LCM method. The mold was composed of a core part with a single cooling channel [Fig. 2(a)] and a cavity part with two symmetric cooling channels placed along the molding cavity [Fig. 2(b)]. The cross section of the channels before sintering was 1.2 mm wide 1 mm deep. The three channels were independent of each other. The core and cavity parts were formed by six and nine layers of 1 mm thickness, respectively. The compacting pressure was 1 MPa. 3.2 Materials and apparatus In this study, WC 9 mass%co carbide powder with a particle size in the range of 1 3 mm and an apparent density of 18.2% (true density: 14:7 1 3 kg/m 3 ) was used. The paraffin wax with a melting temperature of approximately 6 C, which completely evaporates in a sintering process, was utilized as the sacrificial material for the forming of internal channels. For powder compaction, a tool composed of a container with an inner diameter of 4 mm, lower punch, and upper punch was used. The powdered material was compressed using a hydraulic press machine with a maximum load of 1 MN (PHC, Maruto). The grooves were created by employing a CNC milling machine (CAMM-3 PNC3, Roland DG) using a square-end mill coated with diamond (1.2-mm diameter). 3.3 Fabrication result Figure 3 shows the fabrication process of the cavity part. Figure 3(a) is the photograph of the milled grooves with a cross section of 1.2 mm wide 1 mm deep for the internal cooling channels at the seventh layer. The grooves were filled with paraffin wax using an injection syringe with a.4-mm diameter needle [Fig. 3(b)]. In this process, the wax heated to approximately 14 C was injected into the grooves in order to improve the wettability with the compacted tungsten carbide powder. Although the injected wax penetrates into the compacted carbide layer, it is completely dewaxed in the

3 Fabrication of Cemented Carbide Molds with Internal Cooling Channels Using Hybrid Process of Powder Layer Compaction and Milling 2499 Grooves for internal channels Grooves filled with paraffin wax (a) (b) (c) Molding cavity (d) (e) (f) Fig. 3 Fabrication procedure of the WC 9 mass%co mold (cavity) with internal cooling channels using the LCM method. A B Cooling channels A Molding cavity B 1 mm Layered direction (a) AA cross section Layered direction (b) BB cross section 5 mm Fig. 4 WC 9 mass%co sintered mold with internal cooling channels fabricated using the LCM method: (a) cavity and cross section of the mold, (b) core and cross section of the model. 1 µm 1 µm 1 µm (a) Tungsten (W) (b) Cobalt (Co) (c) Carbon (C) Fig. 5 EDX analysis of W, Co, and C elements of the sintered cavity part.

4 25 Y. Mizukami and K. Osakada sintering process. The excessive wax was then removed after solidification [Fig. 3(c)]. Figure 3(d) shows the layered green object that consists of the cavity and support parts obtained by repeating these processes for the final layer. The cavity part was completely formed by milling the molding cavity and removing the support parts [Figs. 3(e) and (f)]. The surface of the molding cavity was finished using a ball-end mill coated with diamond (1.2-mm diameter). The core part was fabricated by the same process. Figure 4 represents the photographs of the cavity part (left side) and the core part (right side) after sintering. Both parts were sintered at a temperature of approximately 135 C after dewaxing at 3 C. The paraffin wax was completely removed from the grooves and the internal channels were thus obtained. Figure 5 shows the EDX analysis images of the cavity part after sintering. Each element shows uniform distribution. We reported 1) that the relative density of the models fabricated by the LCM method is close to 1% after sintering, and the transverse strength of the sintered models measured by a three-point bending test was greater than 2 GPa, which is sufficiently high for WC Co alloys. From the results, it is considered that both parts of the mold possess sufficiently high mechanical strength. 4. Evaluation of the Fabrication Accuracy In the sintering process, the shape deformation of the internal channels and molding cavity was investigated by measuring the linear shrinkage and fabrication accuracy of the LCM method was discussed. The linear shrinkage of the channel width and depth was measured using test specimens with internal channels fabricated under the same conditions as that of the mold (section 3.3). Before sintering, the cross sections of the three channels were.5 mm wide 1 mm deep, 1.2 mm wide 1 mm deep, and 2 mm wide 1 mm deep. From the plotted experimental results, as shown in Fig. 6, the channel width and depth show uniform linear shrinkage in the range from 17 to 19%. Figure 7 is an example of the cross-sectional view before/after sintering (cross section before sintering: 1.2 mm wide 1 mm deep). It is clear that the channel was uniformly shrunk without shape deformation. Since the mechanical strength of wax was not sufficient, a hyperbolic shape deformation at the top of the channel wall was produced when the powdered material was compacted on the groove (channel) filled with paraffin wax, as shown in Fig. 7(a). The ratio of the shape deformation with respect to the channel depth at a compacting pressure of 1 MPa was in the range of 5 1%. It is considered that this shape deformation produced in the layered processes is reduced by using materials with sufficiently high compressive strength. The cross-sectional profiles of the molding cavity before/ after sintering measured by a laser displacement meter (LK- 8, Sigma Koki) are shown in Fig. 8. The measuring pitch was.1 mm. The linear shrinkage of the molding cavity with respect to the diameter, height, and curvature radius was 17.9, 16.7, and 17.4%, respectively, and it showed approximately the same ratio as that of the internal channels. From the results, it is considered that the entire part of the mold was uniformly shrunk in the sintering process. Linear shrinkage (%) Fig Compacting pressure : 1 MPa : Width : Depth Channel width before sintering, w /mm Linear shrinkage of the channel width and depth after sintering. Powder compaction Sintering (a) (b) 1 mm Fig. 7 Cross-sectional view of the internal channel: (a) before sintering, (b) after sintering.

5 Fabrication of Cemented Carbide Molds with Internal Cooling Channels Using Hybrid Process of Powder Layer Compaction and Milling 251 Height, H/mm H g H s D g Diameter, D/mm After sintering Linear shrinkage Diameter: ((D g -D s )/D g ) : 17.9% Height: ((H g -H s )/H g ) : 16.7% Curvature radius: ((r g -r s )/r g ) : 17.4% Fig. 8 D s In general, when a green model with a high aspect ratio is formed by conventional powder molding, a non-uniform density distribution is generated in the model. Therefore, in the sintering process, a substantial shape deformation occurs because of the non-uniform shrinkage of the model. The LCM method enables the fabrication of green models with a uniform density distribution independent of the aspect ratio of the model because the green models are formed by repeating the compaction of powdered material in a layerby-layer additive manner. Consequently, the cracks and/or shape deformation caused by the non-uniform shrinkage in the sintering process are reduced. After sintering, although post-machining such as abrasion is required at the model surface for optical finishing, it is considered that the LCM method is applicable for near-net-shape/net-shape manufacturing. r s r g Before sintering Cross-sectional shape of the molding cavity. 5. Evaluation of the Cooling Efficiency The cooling capability of the fabricated WC 9 mass%co mold for the forming of a plano-convex glass lens was investigated. However, it was difficult to directly measure the cooling efficiency in the forming process of the glass lenses. Therefore, the cooling characteristics with regard to the molding cavity filled with epoxy resin was measured when air was supplied to the internal channels. By employing the results, the cooling efficiency in the forming process of glass lenses was estimated by numerical simulation using the finite differences method. 5.1 Experimental method The experimental setup for measuring the cooling efficiency is illustrated in Fig. 9. In this experiment, a mold cut into a 24-mm square was used. A stainless pipe with an inner diameter of 3 mm was attached to each channel of the mold and air was supplied to the internal channels after the flow rate was regulated. The mold was heated by four cartridge heaters (maximum power: 1 W) installed on the top and bottom of the mold. The temperature exchange was measured by four thermocouples placed on the surface of the molding cavity, as shown in Fig. 9. The molding cavity was filled with a liquid-type epoxy resin after the installation of the thermocouples. The resin was then solidified in the ambient temperature. 5.2 Simulation method The heat transfer of the fabricated mold by air cooling was numerically calculated using the two-dimensional finite differences method as an axis-symmetric non-steady heat conduction. The simulation model, which is represented by the cross section AA indicated in Fig. 9(c), is shown in Fig. 1. The grid interval in the r- and z-direction was.5 MPa Air Cartridge heater (1 W) Exhaust Thermocouple wires T c4 T c3 T c2 T c1 A A Exhaust Cooling channel Epoxy resin : Thermocouple : Flow control vale (a) Cavity (b) Core (c) Cross section Fig. 9 Experimental setup for measurement of the cooling capability of the fabricated WC 9 mass%co mold.

6 252 Y. Mizukami and K. Osakada Axis of symmetry 37 Cooling channels (h c ) WC-9mass%Co h w =18 W (m 2 K) -1 T c1 T =25 C j+1 j z r T c3 Resin/Glass j-1 4 : Evaluation points i-1 i i+1 Fig. 1 Heat transfer simulation model of the mold with internal channels used for finite differences analysis. Table 1 rates. Heat transfer coefficient of the cooling channel at various airflow Airflow rate Q/1 5 m 3 s Heat transfer coefficient of cooling channel h c /W(m 2 K) Table 2 Thermal properties. WC-9Co Resin Glass Thermal conductivity (W(mK) 1 ) Density (1 3 kgm 3 ) Specific heat capacity (kj(kgk) 1 ) mm and the number of cells was 4 (r) 37 (z). The time interval of the calculation was.78 ms. The heat transfer coefficients of the cooling channels h c, when air at room temperature with a flow rate from 3:3 1 5 to 8:3 1 5 m 3 /s was supplied to the channel, were determined by both experiments and numerical simulations (Table 1). The boundary between the WC 9 mass%co mold and molding cavity filled with epoxy resin (or optical glass) was assumed as heat transfer by conduction. The heat transfer coefficient of the mold surface h w and outside air temperature T w were 18 W/(m 2 K) and 25 C, respectively. The thermal properties of the WC 9 mass%co alloy, epoxy resin, and optical glass used in the numerical simulation are shown in Table 2, and these properties were assumed to be constant in the range from 25 to 3 C. Temperature, T / Experiment Natural cooling Q= m 3 s -1 Q= m 3 s -1 Q= m 3 s -1 Q= m 3 s -1 Calculation (1) Natural cooling (2) h c =1 W (m 2 K) -1 (3) h c =179 W (m 2 K) -1 (4) h c =271 W (m 2 K) -1 (5) h c =375 W (m 2 K) -1 T =25 h w =18 W (m 2 K) -1 2 (5)(4) (3) (2) (1) T c1 and T c3 almost coincide Cooling time, t/s Fig. 11 Experimental and simulation results of the cooling capability of the mold when the molding cavity is filled with epoxy resin. 5.3 Results and discussion Figure 11 shows the experimental and simulation results of the cooling characteristics at the surface of the molding cavity filled with epoxy resin when the airflow rate was (natural cooling), 3:3 1 5, 5: 1 5, 6:7 1 5, and 8:3 1 5 m 3 /s. The cooling rate was improved by increasing the airflow rate. When the airflow rate was 8:3 1 5 m 3 /s, the obtained cooling rate was approximately ten times higher as compared to natural cooling at the ambient temperature. The cooling characteristic obtained on the surface of the core part T c1 was similar to that of the cavity part T c3 (T c1 and T c3 almost coincide in Fig. 11). In the experiments, the largest temperature difference between T c1 and T c3 was 2.5 C (3:3 1 5 m 3 /s), 3.5 C (5: 1 5 m 3 /s), 4. C(6:7 1 5 m 3 /s), and 4.5 C(8:3 1 5 m 3 /s). This result stated that the surface of the molding cavity was cooled with a uniform temperature. Moreover, in the cooling process, it was possible to locally control the surface temperature of the molding cavity when the airflow rate supplied to each channel was variable (three channels were independent). In Fig. 11, the cooling characteristics obtained from the simulation were similar to the experimental results even though the results varied by 1 2% in the time required to

7 Fabrication of Cemented Carbide Molds with Internal Cooling Channels Using Hybrid Process of Powder Layer Compaction and Milling 253 Temperature, T / Cooling time, t/s reach room temperature. Hence, the cooling effect of the fabricated WC 9 mass%co mold in the glass lens-forming process was estimated by the same simulation procedures. In the simulation, the cooling curves T c1 and T c3 were numerically calculated when air was supplied to the internal channels after press molding of the optical glass material at 3 C. The thermal properties of the WC 9 mass%co alloy and optical glass, as shown in Table 2, were used in the simulation. Figure 12 represents the simulation results, and it was found that a sufficient cooling effect was obtained by air cooling in the glass lens-forming process. The cooling efficiency can be improved by using a coolant or cooled air. 6. Conclusions (1) Natural cooling (2) h c =1 W (m 2 K) -1 (3) h c =179 W (m 2 K) -1 (4) h c =271 W (m 2 K) -1 (5) h c =375 W (m 2 K) -1 (2) (1) (3) (4) (5) T c1 and T c3 almost coincide (Q= m 3 s -1 ) (Q= m 3 s -1 ) (Q= m 3 s -1 ) (Q= m 3 s -1 ) T =25 h w =18 W (m 2 K) -1 Fig. 12 Simulation results of the cooling capability for the forming of optical glass lenses. Layered compaction manufacturing (LCM), which is a hybrid process of powder compaction and milling in layers, is applied to the fabrication of a cemented carbide mold (WC 9 mass%co) with internal cooling channels, which are placed along the molding cavity, for the forming of optical glass lenses. The results are summarized below as follows: (1) A WC 9 mass%co mold with internal channels for temperature control, which is difficult to obtain with conventional machining methods, is successfully fabricated using paraffin wax as the sacrificial material in the LCM process. (2) In the sintering process, the shape deformation of the internal channels and molding cavity is measured. It is found that the shapes of the channels and cavity exhibit uniform linear shrinkage ranging from 17 to 19%. It is considered that the LCM method is applicable for nearnet-shape/net-shape manufacturing. (3) By filling the molding cavity with epoxy resin, the cooling capability of the mold is investigated by both experiments and two-dimensional finite differences simulation. When air is supplied to the channels, the obtained cooling rate is approximately ten times higher as compared to natural cooling at the ambient temperature. The cooling effect of the mold in the glass lensforming process is also estimated by the simulation. Acknowledgements The author would like to thank Dijet Co., Ltd. (Japan) for the sintering of the WC Co green models as well as for providing WC Co carbide powders. REFERENCES 1) J. P. Kruth and T. Laoui: Int. J. Electr. Mach. 6 (21) ) H. Kodama: Rev. Sci. Instrum. 52 (1981) ) DTM Corp.: Proc. 1st Int. Conf. on Rapid Prototyping, (199) pp ) M. Feygin: Proc. 2nd Int. Conf. on Rapid Prototyping, (1991) pp ) S. S. Crump: Proc. 3rd Int. Conf. on Rapid Prototyping, (1992) pp ) E. Sachs, M. Cima and J. Cornie: Ann. CIRP 39 (199) ) T. Yoneyama, H. Kagawa, T. Ito, T. Ito, A. Iwane, Y. Kuramoto, K. Nishimoto and C. Yan: J. Jpn. Soc. Prec. Eng. 67 (21) ) E. Sachs, E. Wylonis, S. Allen, M. Cima and H. Guo: Polym. Eng. Sci. 4 (2) ) Y. Mizukami and K. Osakada: Proc. 13th Symp. on Solid Freeform Fabrication, (22) pp ) Y. Mizukami and K. Osakada: J. Jpn. Soc. Prec. Eng. 69 (23)