Selecting a muller for a green sand operation.

More often than not, when a problem occurs with a green sand system or when an existing system is upgraded or a new sand plant installation is planned, the first component considered is usually the mixer or muller. This is because the careful selection and application of this key component will have a dramatic effect on the operation of the entire sand operation and provides a starting point on which to design the rest of the system.

While there are many choices of mixers, three major types are used to perform more than 95% of all mixing requirements. These are the horizontal wheel, the vertical wheel and high-shear types. These three mixing systems have been applied successfully to virtually all types of green sand operations. So, the central issue of this discussion is not the pros and cons of any one type compared to another, but rather how to conservatively apply any of these to a given application.

The first step in matching a mixer to a given application is to list the operating parameters of the system. These may include the following:

* total sand tonnage required;

* physical properties required;

* sand-to-metal ratios (mold and core sand);

* grain fineness number and sand shape;

* type and form of the binders;

* whether or not binder preblending will be used.

This discussion will assume that some sort of sand cooling is employed so that the sand entering the mixer will be somewhere between an ideal temperature of about 10OF and not higher than about 135F. It also assumes that the return sand system will properly blend the sand into one homogeneous mixture and remove all lumps and metallics.

The total sand tonnage should include the maximum net displacement of the mold, an allowance for the spill sand appropriate for the type of molding machine being used, an allowance for strike off of the finished mold in certain types of tight flask or impact mold systems, plus any prepared sand that would by-pass the molder's hopper due to the circuit logic of the distribution system. This total should then be multiplied by the maximum gross cycle rate of the mold machine together with at least a 10% safety margin on the total amount. The principal physical properties that will affect mixer selection are green compression and shear strength, as these will determine to a large extent the required active clay content.

Mulling effort will vary inversely with the sand-to-metal ratio, however, the amounts of new sand and retained core sand will have an opposing effect on this requirement. Therefore, while increasing amounts of mold and spill sand per unit weight of metal will have a positive effect on mixer performance, the addition of new sand and/or core sand will reverse that effect.

Furthermore, some core sands are more difficult to coat than others, so allowances must be made accordingly. The less difficult to coat core sands can be regarded as new sand, whereas the more difficult core sand, such as phenolic urethanes, should be double-weighed as compared to new sand. Thus, the net sand-to-metal ratio, as it relates to mulling effort, is the sum of the mold sand plus the spill and strike-off sand less the total amount of new sand and core sand divided by the poured weight of the metal.

The amount of core sand that is carried out with the castings is not considered to affect the net sand-to-metal ratio. Extreme caution should be exercised as the amount of core sand infusion approaches and exceeds 40-50% of the mold weight, as this major reduction of sand-to-metal ratio causes impractical combinations of binder additions and mulling effort.

Grain fineness number and the grain shape both have substantial influence on mulling effort as the combination of higher gfn and angularity of the sand grains both increase surface area, which in turn demands an increase in the mulling effort.

The form and type of binders also affect the mixing effort especially in steel facing sand where cereals and fireclays are used. Although bentonites in slurry form improve the efficiency of a muller in terms of energy requirements, the slurry form of binder addition is becoming less popular in state-of-the-art systems. This is because independent control of binder level and moisture content has become more of a quality issue in green sand than is the ease of developing green strength.

Preblending of binders with small amounts of moisture addition can enhance sand systems in terms of improving physical properties and clay efficiency, while at the same time reducing the loss of binder fines in the mixer and the required mixing effort per unit of sand strength. Generally, preblending of binders into cooled sand or integrated within the cooler itself will result in a savings of 15-25% of the total mulling effort in terms of energy per unit of output.

The next step in mixer selection is to provide a better understanding of how the three major mixer types differ relative to their capacities in terms of energy and batch size. The ultimate effectiveness of any green sand mixer will be the amount of mixing work done to the sand in terms of energy rate and retention time.

Figure 1 plots motor load in terms of rated output with respect to time, with an overlay of the net average power input for each of the three major types of mixers.

The vertical wheel continuous machine is typically operated at about 95% of its full-load motor output on a continuous basis. The two batch machines, on the other hand, by virtue of their cycle design, actually spend less time working at full motor output because they are charging and discharging for substantial portions of their cycles.

The typical horizontal wheel machine will operate at a root-mean-square motor load of about 83% even though its peak power input often exceeds 1 00% of rated output for short periods of time. The high shear batch types are designed such that their maximum power input is only about 90% of rated power input but their cycle time per batch is longer than the other batch machines.

This is because of their longer charge and discharge times plus the portion of the dry-mix time they spend measuring the moisture and temperature of the sand. Their net power input per unit of time averages about 71% of rated power input.

Furthermore, the amounts of net energy produced by a given machine will vary with the mixing cycle. In the case of the continuous machine, the adjusted discharge rate will hold the motor load relatively constant. In the case of the two batch machines, the root-mean-square mulling effort is shown for three typical cycle times in Fig. 2a and b. The light, medium and heavy cycles would generally relate as follows:

Light-general purpose sands of 12-20:1 sand-to-metal ratios and green strengths in the range of 10-15 psi.

Medium-higher strength, medium-to-high pressure molding, 8-12 sand-to-metal ratios, green strength range of 15-22 psi.

Heavy-high strength, (typically for the newer high pressure molding systems), steel facing sands, 4-8 sand-to-metal ratios, green strengths of 25-35 psi.

The next step in this particular study was to generate a family of curves that plot actual conditions from a number of different mixing applications in the field for each mixer type and combine them into a composite charge as shown in Fig. 3, which plots green strength, sand-to-metal ratio and gfn against energy consumption. While this chart considers all of these variables, it does not compensate for the amount of binders, binder type, slurry or dry-bond, sand temperature or the amount of sand blending and storage prior to mixing.

The actual shape and surface area of the castings and the pouring temperatures of the various metals, no doubt, will further affect the shape of such a curve. Because all of these variables will not behave precisely as the curve suggests, this curve is intended to be used as a guideline only.

The use of this curve simply requires the sand-to-metal ratio, as previously discussed, to be plotted against green strength. The horizontal intercept of these two is plotted to the right, and the energy requirement reads from the gfn intercept on the right-hand side of the graph. The three lines on the righthand side indicate the power requirements per ton of prepared sand for the three types of machines.

The last step of this procedure is to relate the total energy requirement with sufficient retention time to produce the sand required. This can be done by referring to Fig. 4, which plots the required sand tonnage of the machine against the appropriate retention time/cycle time curves to determine the size of the machine in terms of operating sand capacity.

Thus, the last two steps approximate the motor horse-power requirement and the physical capacity of the machine by simply multiplying the energy requirement by the tonnage and then selecting the appropriate size machine in terms of cycle time.

The last link between the muller and most molding machines should include a means of aerating the sand to provide maximum flowability and eliminate "muller cake" and the detrimental increase in bulk density caused by the packing of sand through the number of transfers between the muller and the mold machine.

First, the aerator should be placed as close as possible to the introduction of sand to the machine. Second, it should be capable of reducing the bulk density down to the range of 55-60 pcf and ideally should reduce the amount of lumpy sand to less than 10-1 5% of lumps larger than 3/4-1 in. Ideally, sands for highpressure molding systems should have 5% less lumps for best flowability and intricate mold cavity detail.

In the final analysis, the manufacturers of the mixer should be consulted for their recommendations for these conditions.

The important lesson here is to carefully select a mixer for a given set of operating parameters. And, more importantly, to convince foundrymen to be more conservative in selecting their green sand mixer, as there is little or no comfort in the relative savings of capital dollars against the added cost of scrap, downtime and maintenance.