Equipment for Blending and Premixing

Many different types of machines are available for blending and premixing solids. The high density, abrasive nature, frictional characteristics, and tendency to segregate of metal powders, however, reduce the suitability of some machines. A blender suitable for metal powders should:

• Achieve a maximum blending in minimum time

• Have a gentle mixing action to avoid particle degradation

• Provide repeatability of blending

• Provide for complete batch discharge

• Provide accessibility for ease of cleaning

• Have low power consumption

• Require low maintenance

• Provide dust-tight operation

The above list indicates that the use of high-shear paddle- or plough-type devices is inappropriate for metal powders, with few exceptions. Thus, the choice narrows down to tumble-type and low-shear agitated-type machines.

Tumble-type blenders meet the greatest number of the aforementioned criteria, making them the most desirable. Furthermore, scale-up from laboratory size tumblers is very precise. Drum-type, double cone, twin shell, and cross-flow, as well as other tumbler shapes, all produce some quality or degree of blending and also permit straightforward scale-up. Some of these have obvious shortcomings, such as lack of material movement along the axis of rotation, incomplete discharging, and lack of accessibility to the internal surfaces for cleaning. Figures 5 and 6 illustrate the effects of the ratio of total powder volume to mixer volume and the effect of baffle plates in cylindrical mixers on mixing quality.

Volume ratio, powder/mixer

Fig. 5 Effect of powder fill in mixer on quality of mixing

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Number of revolutions

Fig. 6 Effect of baffles on quality of mixing in cylindrical mixer. Radius of cylindrical mixer: R, rotational speed: 25 rpm. a, no baffle plates; b, height of baffle plate: 0.66 R; c, height of baffle plates: 0.5 R; d, height of baffle plates: 0.33R

Low-shear agitated-type blenders, using ribbons, slow-speed paddles, screw-type augers, and other means of moving components in a stationary vessel, are inferior to tumblers. Excessive power consumption resulting from driving the submerged mixing device through the mass of dense metal powders, grinding action caused by the relative motion between the moving and stationary parts, excessive wear of the agitator in an abrasive medium, and lack of precision mixing capability are the main drawbacks. Also, ribbon blender effectiveness does not scale up in direct proportion to laboratory models.

The ideal mixer has not yet been invented. Material variables have such an overwhelming influence on the results obtained from any mixing operation that it is mandatory to sample and analyze to arrive at the best combination of materials and equipment for each application.

Number of revolutions

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Responses

  • Don
    Why twin shell is used for blending in powder metallurgy?
    2 years ago

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