Binder Treatment of Metal Powder Mixes

This technique consists of binding the fine additives to the coarser ones by adding a small quantity of binding agent to the powder. This binder creates a thin film that covers the particles causing the fine additives to adhere to the base powder. Binders can be of two types: liquid or solid (Ref 1). In the case of liquid binder (Fig. 5a), a dry mix is produced and the liquid binder is subsequently added to the mixture. Further homogenization is required to ensure that the liquid is evenly dispersed throughout the mix. This operation can be done in conventional blender units commonly used in the P/M industry. Kerosene is a well-known liquid binder, but tall oil (Ref 2), types of glycol, glycerin, and polyvinyl alcohol (Ref 3) may also be used to prevent segregation.

Fig. 5 Processing techniques of binder-treated mixes (Ref 1)

As shown in Fig. 5(b), solid binders must be dissolved in an appropriate volatile solvent that acts as a carrier. The solution is then sprayed onto the mix. After homogenization, the solvent is extracted and the binder itself remains as a thin coating with the fine additives on the surface of powder particles. The solvent can be recovered in a condensation chamber and recycled. For iron powder mixes, many types of polymeric binders have been evaluated and proven their efficiency (Ref 4, 5, 6, 7, and 8). Another derivative of liquid binder is described in Ref 9. Oleic acid is sprayed onto an iron-powder-base mix containing zinc stearate, graphite, and other additives. The blender is then heated above the melting point of the lubricant, 110 to 130 °C, and the liquid lubricant acts as a binder. The mixture is then cooled to room temperature and packaged.

Effect of Blending Techniques on the Physical and Green Properties of Metal Powder Mixes Dust Resistance. One attribute of binder-treated mixes is a better resistance to segregation and dusting. A test method to evaluate the efficiency of the binder agent is described in Ref 8. This is a measure of the resistance of the powder mix to dusting when fluidized by a stream of gas, generally air or nitrogen. Figure 6 illustrates a schematic of the apparatus used in this test method. An air stream is injected at a constant flow rate of 6.0 L/min for 10 min through a 2.5 cm diam tube with a 400 mesh screen upon which the mix sample is placed. This causes the mix to bubble and the fine particles, such as graphite, to be entrained as a result of a large surface to volume ratio and low specific gravity. The graphite and other similar materials are then deposited in the dust collector. The dust resistance factor is the ratio of the chemical content of the element before and after the test.

Fig. 6 Dust-resistance apparatus

Figure 7 illustrates the effect of binder concentration, solid or liquid, on the graphite and lubricant dust resistance of a F0008 mix containing 0.8% zinc stearate. Without the addition of a binder, the typical dust resistance of the mix is 45%. This means that only 45% of the graphite and the lubricant was retained in the test sample. The various binders improve the dust resistance up to a concentration of 0.10%. The kerosene has the lowest dust resistance factor, 70% for a 0.10% addition. Glycol, which is also a liquid binder, shows better results: 90% for an addition of 0.10%. A solid binder, such as polyvinyl pyrrolidone (PVD), shows the best results. Dust resistance of about 95% is reached for a 0.10% binder addition.

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Binder concentration, wt%

Fig. 7 Effect of binder concentration on carbon dust resistance of iron powder mixes (F0008 with 0.80% lubricant) (Ref 1)

Flow rate is an important parameter in the production of metal powder mixes because it directly affects the quality and the productivity of compacting presses. As shown in Fig. 8, liquid binders deteriorate mix flow rate, potentially limiting their use. However, the use of a solid binder significantly improves the flow rate of powder mixes. This is an important attribute because it directly impacts on the productivity and the quality of the parts (Ref 10). A better flow rate results in increased press productivity because the die cavity can be filled faster than with a regular mix. Also, because of the superior flow characteristics, complex and thin-walled parts can be produced with better control of weight and density. An example of how the quality and productivity are enhanced by the use of binder-treated mixes is illustrated in Fig. 9. In this example, an index of 100% was assigned to the standard condition. The part weight (Cp) index in this condition was 1.3 for the regular mix and 2.4 for the binder-treated mix. The Cp index is a direct measurement of the scatter, which indicates that the binder-treated mix achieved a reduction of nearly 55% in weight variation. Another interesting point is that even with an increase in press productivity to 113%, the Cp index is nevertheless 25% higher than the regular mix processed at a lower production rate.

Fig. 8 Effect of binder concentration on flow rate of iron powder mixes (F0008 with 0.80% lubricant) (Ref 1)

Fig. 9 Comparison of regular (reg) and binder-treated mixes (BTM) processed on a production press (FC0208 modified)

Green properties --mix compressibility and green strength—are also critical parameters in the production of P/M parts and can be affected by the blending technique. The film rigidity of the binder can be controlled by using a plasticizer (Ref 11), which significantly improves the green strength of P/M parts (Fig. 10) and maintains good mix compressibility. For multilevel parts, the powder mix must adequately fill the die cavity with a constant quantity of powder for each press stroke. If the different sections of the die cavity are not well filled, the powder transfer during the compacting cycle can create density gradients and negatively affect the green strength. As shown in Fig. 11, laboratory results show a 25% green strength improvement, while for production parts the improvement is nearly 45%. This is caused by a lower density gradient throughout the different sections of the part.

Fig. 10 Effect of binder treatment on green properties of laboratory specimens (FC0205 modified mixes)

Fig. 11 Effect of binder treatment on relative green strength of laboratory specimens and production parts (FC0205 modified mixes)

Binder-Treated Mixes for High-Density Parts. Recent innovations (Ref 12, 13, 14, 15, 16, 17, 18, and 19) have adapted the binder-treatment technology to successfully heat and press metal powder mixes at temperatures up to 150 °C. This compaction technique can increase green densities by about 0.1 to 0.2 g/cm3 as compared with parts compacted at room temperature. However, these mixes must show good physical characteristics, particularly flow rate and apparent density, over a wide range of temperatures. To withstand these temperatures, powder mixes are produced with a high-melting-point lubricant and binder treated to improve flow rate and stabilize apparent density. As shown in Fig. 12, binder-treated mixes must be used for these specific temperature conditions because they show consistent apparent density at the working temperature. As previously mentioned, stable apparent density and good flow rate are key parameters to achieve constant die filling from stroke to stroke and hence minimize press adjustment and maintain good part-to-part consistency.

Fig. 12 Effect of powder temperature on apparent density of powder mixes in a die (FN0205 with 0.6% lubricant)
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