A Properly Balanced Abrasive Work Mix for Effective Blastcleaning

VOLUME NO. IX, ISSUE No. 4
SEPTEMBER, 2003

A Properly Balanced Abrasive Work Mix
Is The Key To Effective And Efficient Blastcleaning

True! No question about it. The above statement has been proven over and over again .. What has also been proven over and over again is that an unbalanced work mix causes extremely severe problems with Quality of Finish, Productivity, Prohibitively High Operating Costs—problems no company should live with.

So, what Is a properly balanced work mix for your application? What will constitute a problem-causing out-of-balance work mix? These important questions are addressed in this issue of Ervin's Technical Bulletin. There are two companion pieces that accompany this issue: Ervin's Technical Bulletin of January 2003, (Vol. VIII, Issue No. 7), and Ervin's Blastcleaning Analysis Card. Each of these pieces sheds important light on different aspects of the challenge of controlling the work mix size balance. You are urged to study the entire 3-part package for the best, most comprehensive understanding of the overall problem. (This issue and the companion pieces supersede the Technical Bulletin issue 8/3/84—a first effort at defining the challenge).

NOW — Right Now —

We suggest you study the January 2003 Technical Bulletin, entitled: "Challenge No. 2: Controlling the Work Mix Size." First, digest its descriptions of the many problems caused by an unbalanced, too coarse work mix, the comments on what a balanced work mix is and how it is developed, and, finally, the 5 guideline steps for maintaining work mix size control—then, come back to this issue. We'll show you an effective, simplified work mix analysis procedure that applies to whatever size(s) you use in your operation.

— TIME OUT —

(While you study the 1/03 Technical Bulletin)

Back Again?

Maybe this Murphy's Law says it best: "When everything (in the cleaning room) seems to be going well, you obviously have overlooked something."

Simply put: When you do look into it, and find everything is not going that well and you discover your work mix is out of balance and too coarse, you will realize that what you overlooked was those 5 guideline steps for maintaining proper control of the work mix.

Simply put: A properly balanced work mix happens only when you observe those 5 guideline steps. That's the only way you can control the work mix, and avoid those horror problems relating to poor quality of finish, low productivity, and out-of-sight blastcleaning costs.

A Quick Easy Way To Know Whether
Your Work Mix Is In Balance

As you learned from the 1/03 Technical Bulletin, selection and purchase of the new, original size steel shot or grit to be used automatically determines how large the largest particles in the work mix will be. For example, if SAE G-25 grit is in use, we know from the SAE specs that the original size equivalent in the work mix will pass through a #16 mesh (.0469") screen and will be retained on the nominal #25 mesh (.0278") screen. How large, then, is a known factor. But, yet to be determined is: What percentage of the total work mix should that original size portion be?

How small the smallest abrasive in your work mix will be is determined by the air wash system, which separates the usable abrasive to be recycled, from the spent abrasive that is to be discarded along with contaminant blastcleaned from the work surface. The key: Be sure all contaminant is removed from the system—no residual contaminant should be left in the work mix. That is an absolute must! The proper abrasive cut-off point for your operation is determined by trial and error on airflow settings of the air wash systems. Because the nature of contaminants involved varies from one application to another, trial and error is the only way to determine the smallest shot or grit particles that can be retained in the work mix, without also retaining contaminant.

Fortunately. Ervin's field experience over the many years, with many diverse blastcleaning applications, permits us to identify the "normal target" take-out screen openings applicable to the various sizes of shot and grit. These "targets" have been found to be both reasonable and practical.

Once the original abrasive size content in the work mix has been established and the smallest size to be retained is known, a properly balanced work mix, within the large/small parameters, will occur naturally as long as the operation is controlled via the 5 operating guideline steps referred to earlier.

A key finding in Ervin's study of work mix control was that a simple, easy control measure exists: The percentage of the original size particles. When that percentage is held within specified parameters (spelled out in the tables that are included herein), a properly balanced work mix is automatically assured.

The "original size" portion of the work mix is measured and expressed as a min/max percentage range. The use of a range gives recognition to the fact that a work mix constantly undergoes change. The natural mechanics of abrasive failure and the air wash extraction of spent abrasive fines, along with the fact of intermittent make-up additions of new abrasive, produces a swing in the work mix size distribution between additions.

The control-factor, measuring just the “original size” content, works this way:

If the min/max range is 30% to 45% and the actual screen analysis shows. for example. 37% original size content—(1) the 37% is well within the control range for a balanced work mix: (2) It means the remaining 63% of the work mix represents medium and small particles which are needed for good coverage and faster, better cleaning.

However. if the screen analysis were to show instead, 80% on that original size control screen, we see: (1) The 80% is out of the guideline range, seriously; (2) It means there is only 20% medium and small particles in the work mix—way below the 63% shown above the balanced work mix. This is the condition that leads to the 9 troubles shown on Page 1 of the 1/03 Technical Bulletin.

Food for Thought: Mother Nature generates those medium and small sizes in the work mix by “making little one out of the big ones.” It’s unavoidable, inescapable: medium and small particles will be generated. If they are not in the work mix in proper balance, you have to ask yourself: “Where did they go?” The trouble-shooting checklist on the back side of Blastcleaning Operations Analysis Card tells you what causes a coarse work mix, as well as how those medium and small particles can be lost out of the system. How to correct a work mix gone wrong? Follow through on those 5 guideline steps listed on the 1/03 Technical Bulletin (also shown on Ervin’s Trouble-shooting POSTER, which should be posted at your blast equipment). Your Ervin sales representative will welcome the opportunity of working with you on a step-by-step review of the guidelines shown on the POSTER.

Work Mix Guidelines For Blastcleaning With Steel Shot Or Grit

(Based on SAE specifications for Cast Steel Shot and Grit — J-827, J-444)

Shot
Sizes

Opening Size
Control Screen
Original Size
In Work Mix
Control Range
Normal Target*
Take-Out Screen
  Mesh (Opening) Min Max Mesh (Opening)
S-660 12-M (.0661”) 30% 45% 40-M (.0165”)
S-550 14-M (.0555”) 30% 45% 40-M (.0165”)
S-460 16-M (.0469”) 30% 45% 45-M (.0139”)
S-390 18-M (.0394”) 30% 45% 45-M (.0139”)
S-330 20-M (.0661”) 35% 50% 50-M (.0117”)
S-280 25-M (.0278”) 35% 50% 50-M (.0117”)
S-230 30-M (.0234”) 35% 50% 60-M (.0098”)
S-170 40-M (.0165”) 40% 55% 80-M (.0070”)

Grit Sizes

           
SG-12 12-M (.0661”) 30% 45% 40-M (.0165”)
SG-14 14-M (.0555”) 30% 45% 40-M (.0165”)
SG-16 16-M (.0469”) 30% 45% 45-M (.0139”)
SG-18 18-M (.0394”) 30% 45% 45-M (.0139”)
SG-25 20-M (.0278”) 35% 50% 50-M (.0117)
SG-40 40-M (.0165”) 40% 55% 80-M (.0070”)
SG-50 50-M (0.117”) 45% 60% 120-M (.0049”)

(Grit in hardness ranges 50-55 RC or 55-60 RC: min/max ranges above to be dropped by 5 percentage points ... Example—drop a 30%-45% range to 25%-40%. Full hard, untempered grit, 60 RC or more, min/max ranges above to be dropped by 15 percentage points—Example, drop 35%-50% range to 20%-35%).

• Target take-out screen opening should be determined for your operation via trial/error adjustment of separator. The indicated guideline take-out screen reflect normal conditions ... yours may be different.

Simple — Easy

To determine the status of your work mix, all that is needed is a Laboratory Test Sieve matching the indicated original size control screen and a pan to catch the material passing through the control screen ... then measure the percentage of original size, using a 25 cc graduate (or 50 cc, at most).

Example: S-280 shot or SG-25 grit

Original size control screen—
#25 mesh (.0278")
Target range—35% min, 50% max
Actual percent ______ %
(to be determined)

Remember —

These work mix guidelines are not rigid limits. A work mix that differs slightly from the guideline range is not necessarily wrong for your particular operation, but is pays to play safe and check it out from all aspects. A coarser separator takeout point than indicated may be used-but that automatically guarantees a higher than normal abrasive consumption (going up just one size in take-out can increase abrasive usage by as much as 20 to 25%, in some instances).

When your Test Sieve shows the work mix to be out of balance, too coarse, and you embark on a Sherlock Holmes’ Hawkshaw program to find the culprit, you may want to screen analyze the full work mix to see just where the actual cutoff is. You may also want to screen analyze discard material from the scalp screen and the discard from the separator and expansion trap. Your Ervin sales representative can assist you—tell you what screens to use, etc.—help you procure them—and he's a pretty good Dr. Watson for your Hawkshaw! Use him—he'll be happy to help.

It is also possible to work with a finer take-out point. As long as no harmful contaminants are retained in the work mix, and cleaning speed or effectiveness is not impaired, a finer take-out is all to the good as abrasive usage will be lowered. Trial and error on your part is still the only way you will learn what works best for you. The guidelines shown will give you a sound starting point for your trial and error efforts.

AND BE SURE TO REMEMBER THIS, TOO —

A Properly Balanced Abrasive Work Mix
Is The Key To
Effective And Efficient Blastcleaning

Download the Technical Bulletin here.

or Call Ervin Today at 1-800-748-0055

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