Journal of Materials Processing Technology 137 (2003) 70–73
A new type of binder for metal injection molding Songlin Li*, Baiyun Huang, Yimin Li, Xuanhui Qu, Shaojun Liu, Jianglian Fan The National Key Laboratory for Powder Metallurgy, Central South University of Technology, Changsha 410083, PR China
Abstract A new wax–oil–polyethylene (PE) binder was developed for Fe–2Ni injection molding. The advantages of both wax-based and oil-based binders were acquired. Moreover, the debinding solvent could be recycled. The miscibility of its ingredients and the characteristics of the binder were evaluated and preferable performances were obtained for this new binder. The maximum powder loading could reach 60 vol.%, and the debinding rate was over 2 mm/h. The drying time for 6 mm thick samples was less than 20 min and the recycle ratio of the debinding solvent was over 95 wt.%. # 2002 Published by Elsevier Science B.V. Keywords: Wax–oil–polyethylene binder; Metal injection molding; Debinding
1. Introduction Metal injection molding (MIM) is a newly developed technology to form complicated-shape metals and alloys [1,2]. The research of binder is the core of this technique. Debinding is the controlled step in MIM. The usual method for removing the binder is thermal debinding. However, the great quantity of gas is produced by evaporation and degrading of the binder produces many defects, such as bubbles and cracks in the molded parts. Further, thermal debinding consumes quite a large amount of time [3]. Solvent debinding is an effective means by which to reduce degreasing time. The solvent dissolves the binder ingredients from the surface to the interior of the green part. Thus, defects are not easily formed and the binder can be removed at a rate of several millimeters per hour [4]. However, the organic solvent used also results in environmental pollution. The effective recycling of the solvent is very important in practical use. However, there has not been work reported on this topic to date. Wax–polyolefin binder systems are used widely in practice, because these binders have quite good rheology and green strength. Thermal debinding is often used for waxbased system. In some papers, solvents are used also, but the degreasing temperature is high and the debinding rate is lower [5–7]. The oil–polyolefin binder is reported to have a high degreasing rate. However, the multiple performance of this binder is not reported [8,9]. * Corresponding author. Tel.: þ86-731-8877-404. E-mail address:
[email protected] (S. Li).
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 6 9 - 5
In this paper, oil is added into the wax–polyethylene (PE) binder in order to secure a quickly solvent-removable binder, and the drying and recycling of the solvent selected to dissolve the binder are also investigated.
2. Experimental method Carbonyl iron and nickel powders are used as the element powders. The wax, oil and polymer are mixed in a self-made mixer for 2 h to form the binder. Feedstocks of mixed Fe– 2Ni powders are injected. The green strength is measured in a three-point bending test using an Instron mechanical tester. Solvent debinding takes place by immersing the green parts into the thermostatic solvent. The X-ray diffraction patterns are measured using a Japanese Rigaku 3014z diffraction instrument. The heat capacity of the binder and ingredients are obtained using a Duppon 910 DSC instrument. The volume swell ratio is obtained by comparing the volume of the polymer before and after immersion in the solvent.
3. Results and discussion 3.1. Selection of the wax ingredients Oil and polyolefin is used first, then different kinds of wax are added to the oil–PE system. The performance of wax–oil–PE is given in Table 1. Although carbonate wax and microcrystal wax are reported to increase the adhesion of the binder and powder,
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Fig. 1. Debinding ability comparison of CH2Cl2 with CHCl3. Fig. 3. X-ray diffraction of PW–VO–PE binder and ingredients.
i.e. leading to higher powder loading [10], these two kinds of wax are not suitable for CH2Cl2 or CHCl3 debinding system on account of their insolubility in the two solvents. 3.2. Determination of the debinding solvent It is preferable for the solvent to be dried and recycled at a lower temperature. CH2Cl2 and CHCl3 can both fulfill these needs. The boiling point of CHCl3 is 65–68 8C, while that of CH2Cl2 is 39–41 8C. The debinding abilities of the two solvents at room temperature are compared in Fig. 1. The graph indicates that CH2Cl2 dissolves wax and oil faster than CHCl3, so that CH2Cl2 is selected as the debinding solvent. 3.3. Influence of polymer kinds on the debinding process Two kinds of polymer, PE and EVA are chosen to investigate its influence on the debinding process. The part cracks after half an hour of immersion when containing 20– 30 wt.% of EVA as the bone polymer. Samples containing
20–30 wt.% of PE suffer no defect after immersion in CH2Cl2, which may be due to the different volume swell ratio of PE and EVA in CH2Cl2. Fig. 2 indicates that the volume swell ratio of EVA is much greater than that of PE. As the solvent diffuses into the molded part, it causes the polymer molecules to swell. When the volume swell ratio is sufficiently large, the stress causes bubbles or cracks in the parts. 3.4. The structure and compatibility of the binder Fig. 3 shows the X-ray diffraction patterns for the PW– VO–PE and its constituents. This figure indicates that PW, VO and PE retain their own structure in the binder system, that no chemical interaction exists among these constituents, and that the binder is simply a physical mixture of the ingredients. 3.5. Influence of oil addition on mixing torque and maximum powder loading Fig. 4 indicates effects of oil addition on the mixing steps. The torque of the mixing steps decreases with increase of the oil content. Oil flows and covers the powder to act as a lubricant to reduce the friction between the feedstock and the rotating turbine. The barrels are protected by the oil during mixing and injection. The maximum powder loading (fm) decreased by about 1 vol.% with the addition of oil. This may be due to the slightly different rheological behavior between wax and oil. When the molecule weight of the PE is decreased, fm is increased from 57 to 60. 3.6. Effect of oil addition on the cooling step and green strength
Fig. 2. The volume swell ratio of EVA and PE in CH2Cl2 at 35 8C.
A DSC is used to measure the heat capacity (Cp) of the oil and feedstocks. The Cp of the oil is about 0.24 J/g for 50– 100 8C, whilst the Cp of the feedstock is about 0.8 J/g.
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Fig. 4. The Influence of oil on the mixing torque and maximum powder loading: (a) PE with high molecular weight; (b) PE with low molecular weight.
Fig. 5. Effect of oil addition on the green strength.
Different from wax and PE, there is no phase transition and volume shrinkage for the oil during cooling after the mold is filled. The oil absorbs a part of the heat from the powder and decreases the volume shrinkage of the binder, which leads to the reduction of heat and mechanic stresses. The effect of oil addition on the green strength is illustrated in Fig. 5. The green strength decreases substantially with the increase of the oil content in the binder. This is because oil is a liquid with no strength. The addition of the oil not only causes the percentage of wax and PE to decrease, but also it decreases the forces between molecules by dispersing in PW crystal and PE molecule chains. In [11] it is shown that with the addition of oil to PP or PS, the green strength decreases by about 2 MPa in most cases. German [1] concludes that the suitable green strength should be greater than 5 MPa. The oil content in the binder should thus be less than 30 wt.% in this binder system.
When the oil content is 40 wt.%, it fills the wax crystal and PE chains, so that any further oil will just flow out of the binder system. 3.7. Effect of oil on the debinding rate Parts of 6 mm thickness were molded to investigate the debinding rate. The results are shown in Fig. 6. The debinding rate is slow without oil. With the addition of oil, the debinding rate increases substantially because liquid oil dissolves and diffuses quickly in liquid solvent. However, dissolution of solid-state wax is slower. As the temperature is increased, the dissolution and diffusion of wax in CH2Cl2 becomes quicker, so that the debinding rate increased. At 35 8C, 3 h debinding can remove 45 wt.% of the binder. Inter-connected tunnels are formed in the binder, so that the debinding rate can exceed 2 mm/h.
Fig. 6. Effect of oil addition and temperature on the debinding rate.
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the drying temperature lead to an increase in diffusion rate so that the drying becomes quicker. The recycling of the solvent is the key in MIM production. CH2Cl2 solution containing different concentrations of wax and oil were distilled and condensed to obtain pure CH2Cl2. The data is shown in Fig. 8. With wax and oil content increase in the solution, the solvent recycle ratio decreases a little. However, it is still as high as 95 wt.% even when the concentration of wax and oil reaches 160 g/l. The remains form a gel, which comes from the interaction of wax, oil and solvent.
4. Conclusion Fig. 7. The drying rate for 6 mm thick parts.
1. PW–oil–PE is a physical mixing binder system. Every ingredient retains its own structure. 2. The addition of oil decreases the green strength and maximum powder loading, but it also decreases the torque during mixing and increases the debinding rate greatly. 3. Feedstock with wax–oil–EVA as the binder cracks when debinding in CH2Cl2. This is due to the great volume swell ratio of EVA in CH2Cl2. 4. When using CH2Cl2 as the debinding solvent to remove PW–VO–PE binder, the debinding rate reaches 2 mm/h the drying time for 6 mm thick parts is less than 20 min, and the solvent recycle ratio is over 95 wt.%.
References
Fig. 8. The recycle ratio of CH2Cl2.
3.8. Drying of brown parts and recycling of solvent The quick drying of brown parts after solvent debinding is also important for decreasing ‘‘debinding’’ time. The 6 mm thick brown parts were dried and weighed to calculate the solvent remaining in the samples. Fig. 7 indicates that the drying time is less than 25 min at 40 8C for samples of 6 mm thickness. When the drying temperature is increased to 50 8C, the drying time drops to 20 min. This time is much less than the immersion-debinding time. This can be explained as the solvent boiling into the ‘‘vapor’’ state and moving out through the inter-connected tunnels. Increasing
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