Optimizing Compound Flow Rates

Q. We operate six round vibratory finishers that range in size from 3 to 25 cu ft. Compound is fed to each machine from a central mixing system, each machine having from one to four nozzles to feed the compound. Four of the machines are hand loaded and unloaded, while the largest machines have internal separators that discharge the parts automatically. My concern regards the usage of compound, which varies considerably from shift to shift, and from week to week. We have used flow-control valves with sight gauges, but no matter how these are set it seems that individual operators change them frequently, and there is considerable difference of opinion as to how much compound is enough compound. How can we better control this operation? J.M.

A. Good question. One of the most out-of-control expenses in mass finishing is compound use, and it happens exactly as you have described. Well over half of the mass finishing systems I have visited were using too much compound.

First of all, how much should you expect to use? Speaking of deburring operations in round machines with flow-through compounding – that is, one pass and out the drain for the compound – the optimum flow rate is from 1.0 to 1.5 gph for each cubic foot of work and media in the machine. Tub-style machines may need as much as 3.0 gal/cu ft, depending on drain locations and some other factors I will address.

It is extremely important to introduce the compound in the right place(s). In single-drain machines, it should be introduced upstream from the drain. Observe the forward motion of the parts to determine how many inches they progress in one cycle of their spiraling action. That is how far upstream from the drain you want to introduce the compound. In multiple-drain machines, there are usually as many nozzles as there are drains, so use this same technique for each drain. If there are several drains and only one nozzle, locate that nozzle upstream from the last drain that the parts will pass before they are removed. This assures the cleanest parts coming out of the machine. With an internal separator and spray nozzles over the screen deck, you can put all the compound in through the spray nozzles, thereby assuring rinsed parts at the end of the cycle.

In an experiment to illustrate the importance of nozzle placement, we put carbon black powder in a 10-ft foot round machine with a single drain. The machine had previously been set so that parts made one pass all the way around in 90 sec, about normal for a 10-ft foot bowl. Introducing the compound immediately downstream from the drain, it took 45 min to get most of the powder rinsed out and, even then, there was a trace amount left. Putting the compound in the proper place (in this case 10 inches upstream), most the powder was removed in 12 min, and by 15 min it was essentially all removed. In a real-life example, one customer was removing forging scale from parts while introducing the compound just past the drain. I told the plant manager what to do, and he said, “No way do I want to send fresh compound right down the drain.” He agreed to try it one time, and the time cycle was reduced more than 1/3. After all 18 of his machines were changed, the flow rates were also reduced substantially.

Tub machines are a little different. In closed tubs with a single drain in the middle, the compound flow should be split, with half being introduced at each end of the machine. This way the flow goes toward the middle and out the drain, keeping the system as clean as possible. If there is only one drain and it is at one end, put all the compound in the other end. Such a system will often require 2 to 3 gph/cu ft. With continuous, straight-through tub designs, introduce about 1/3 of the compound at the beginning, and 2/3 near the discharge of parts. This results in cleaner parts, and much of the compound from the discharge end will be transported with the media back to the beginning.

It is not only the cost of compound that is involved in flow rates. There is also the effect on cutting performance, and that can be rather dramatic when flow rates are varied from one extreme to the other. It has been demonstrated that, as flow rates are increased from 0 to 6 gph/cu ft, the cutting rate increases from very, very low in the dry operation, to a peak rate starting around 1 gph and remaining somewhat constant to 2 gph. As flow is increased, the cut rate gradually decreases until, at 6 gph, it is only slightly better than the dry cut rate. The cutting curve vs. flow rate chart is strongly influenced by the ability of the machine to drain the solution. The reason the cut rate diminishes with increased flow rate is that the floating action of the mass increases as fluid volume increases because of inadequate draining. With excellent drainage, therefore, you can maintain a fairly good cut rate with higher flow volumes; conversely, if drains are inadequate, or partially plugged with debris, the penalty for high flow rates will be much lower cutting rates.

Some may argue that higher flow rates result in cleaner parts. This is true, but higher flow rates are only one consideration for getting cleaner parts. Introducing the solution in the right place, as discussed above, is at least as important. Dirty parts coming out of the machine result from incoming contamination (Is the compound capable of removing the surface soil on the parts?); incorrect compound flow; incorrect input location of the nozzles; and rapidly wearing media. Sometimes very fast wearing/cutting media is necessary to get the desired deburring action, and this leads to media dust on the parts that may be so heavy that a subsequent washing operation is required. Finally, if you are properly using the right compound, you may be able to use a lower-cut, longer-wearing media.

Now that you know how much compound solution should be flowing, let’s address the issue of changes that occur because different operators make their own adjustments to the flow valves. This is, of course, an educational issue, and it will help if you discuss the information provided in this article. I am a firm believer in the benefits of classroom sessions to present the theories to operators and supervisors, followed by a brief demonstration at the machine. Properly instructed, an operator or supervisor can identify a proper flow rate just by looking at it.

Upon further discussion with the reader who submitted this question, I learned that his six machines total 83 cu ft of machine capacity. The company’s records show that it was using almost 5 gal of solution/cfh. Much of this high flow is because the parts are not clean enough when the management-stipulated flow rates are used. The company made the changes I recommended and report that the usage has dropped to 2 gph/cu ft — 60 percent savings! Furthermore, staff are now evaluating cut media in the expectation that they can equal the previous cutting rate with significantly lower media usage.