Effective blastcleaning: pay attention to the basics.
While the past two decades have brought new developments and continuous refinements to foundry blastcleaning equipment and materials, the basic function process remains the same: to remove sand, scale and other surface contaminations from the casting prior to finishing operations.
So, the best advice to the metalcaster in regard to shotblasting is to pay attention to the basics.
The five most important components comprising any shotblast machine are:
* dust collector;
* abrasive throwing wheel;
The separator is the most important and, usually, the most ignored component of the blast machine. It is ignored because its location at the very top of the blast machine is inaccessible.
The separator, however, controls much of the costs related to the blast machine. It controls the size distribution of the abrasives in the operating mix, the amount of sand present in the operating mix, the dust going to the dust collector and the removal of tramp metal from the operating mix.
The separator consists of a rotating scalping drum and a movable baffle weight. The rotating scalping drum removes the large tramp metal that is knocked off the work parts during the blasting operation and any large debris that enters the blast machine during the loading process. The efficient removal of these large metal particles prevents damage to the abrasive throwing wheel. When properly adjusted, the movable baffle plate (with adjustable counterweights) controls the width and thickness of the abrasive curtain cascading down from the plate. It is important to properly adjust these counterweights so the baffle will not swing open until there is a full width of abrasives pushing against the back side of the baffle. Air from the dust collector passes through this curtain and removes the light dust particles and carries them to the collector. In this same process, the small, worn abrasive particles are removed and deposited in the separator discharge chute. A properly adjusted separator is of paramount importance to the efficient blastcleaning operation. Each 2% of sand retained in the operating mix will result in a 50% reduction in the life of the wear parts, such as blades, impeller, control cage and wear plates. Also, a poorly adjusted separator that results in the removal of one larger size from the operating mix will increase abrasive consumption by as much as 25%. Experience shows more than half of all blast operations have misadjusted separators that are wasting abrasives. The separator and the abrasive's breakdown characteristic control the operating mix, or the distribution of abrasive sizes thrown by the abrasive throwing wheel. An operating mix consists of nearly new whole-sized abrasives and various worn particles.
Although exact size distribution varies with the application, most cleaning operations require four to five sizes. The largest size in the mix is controlled by the size of the new abrasives added to the machine. The smallest size in the mix is controlled by the separator setting.
The larger abrasive particles in the mix perform most of the loosening of the surface contaminants because they hit with the greatest impact. The smaller particles clean in the hard-to-get-at areas of the castings and also provide the coverage to sweep away the contaminants as they are loosened. An operating mix that contains only large, nearly whole-size particles lacks the coverage provided by the finer particles. An operating mix that contains only small particles lacks the impact provided by the larger particles.
Here are some simple tips to control the operating mix:
* make sure the blast machine is free of leaks;
* eliminate the abrasive loss due to carry out with the workpieces;
* adjust the separator baffle plates to assure a full, thin curtain of abrasives;
* adjust the draft gates from the dust collector to assure optimum air flow;
* make frequent additions of relatively small amounts of abrasives.
The dust collector provides the vacuum cleaning process needed to remove the dust from the blast cabinet and elevators. It also provides the air movement needed to make the separator function properly.
The air flow from the collector is controlled by the size and capacity of the collector and by the settings on the draft gates. Regular inspections of the collector will identify potential problems such as leaks and dust buildup. An operating manometer is vital to efficient dust collecting because it indicates pressure loss due to plugged filter bags.
Abrasive Throwing Wheel
The abrasive throwing wheel is a subject in and of itself. The key components of the wheel are the impeller, control cage and blades. Each should be inspected frequently and replaced as soon as unusual wear is indicated. When the wheel is properly adjusted and in good working order, the full effect of the blast stream is directed at the parts being blasted. Problems with long cleaning cycle times, poor cleaning and high maintenance generally can be traced back to poor directional control over the blast pattern.
As little as a 10% misadjustment of the blast pattern can result in a 25% reduction in blast efficiency. The aiming of the blast is done by the adjustable control cage and the actual "throwing" of the abrasives is done by the blades. Wear in either of these critical components can cause serious drops in efficiency.
Look at the ammeter as the speedometer on the blast machine. The ammeter indicates at a glance, how much abrasive is flowing from the wheel and at what efficiency level the wheel is working. Each electric motor has a "no load" and "full load" amps rating, and the difference between these two numbers gives the "usable amps" rating for each motor. Knowing this number indicates how much abrasive can be thrown by each wheel. To operate at peak efficiency, the abrasive flow must be set so that the ammeter is kept at the "full load" level. For example, a standard wheel with an electric motor with a "no load" rating of 10 amps and a "full load" rating of 33 amps gives 23 usable amps. The wheel will throw 690 lb per minute. If a shot size such as $330 is used, a drop of just one amp will result in a loss of 21 million impacts per minute.
After making sure the blast machine is maintained properly, which abrasive should be put in the machine? There are 11 commercial sizes of shot, nine sizes of grit and three or four hardness options for each grit size. This means there are 47 abrasive choices from which to select.
Understanding the abrasive grades and sizes and how each compares to the others makes the selection process much easier. The rule for determining size and grade selection is: always use the smallest size abrasive that hits with sufficient impact to dislodge the surface contamination on your castings.
Although this is the most important rule, there are other factors to consider. These include:
* the larger the shot, the rougher the surface profile of the work surface;
* the larger the shot, the deeper the layer of compressive stresses left on the work surface;
* the larger the shot, the shorter its life;
* the larger the shot, the greater its impact;
* the harder the shot, the shorter its life;
* the harder the shot, the deeper the layer of compressive stresses;
* the smaller the shot, the smoother the surface profile of the work surface;
* the smaller the shot, the greater the coverage;
* the smaller the shot, the longer its life;
* the smaller the shot, the less its impact.
The most important words are impact and coverage. Impact is the amount of energy transmitted to the target by each particle while coverage is the number of times the workpiece is struck by the abrasives.
All abrasive size selection is a compromise between impact and coverage. Like liquid in a "U" shaped tube, as one side goes up the other goes down. As impact is increased, the coverage is decreased. As coverage is increased, the impact is decreased.
Shot or Grit?
New shot particles are spherical and remain primarily round throughout their useful life. Shot sizes are denoted by the letter "S" followed by the dimension in ten thousandth of an inch, such as S330. Shot leaves a surface profile of rolling hills and valleys. Shot is needed if strengthening of the casting is desired, as in shot peening or stress relieving. Applications include: simple sand/scale removal; where smooth surface profiles are preferred; cleaningsoft, non ferrous castings; and shot peening or stress relieving. Grit particles are angular because they are produced by crushing round and semiround particles. Standard hardness (42-50 Rc) grit will rapidly become rounded after repeated impacts and will remain round, thereafter, in the work mix. Midrange hardness (52-58 Rc) grit and high hardness (62+ Rc) grit remain angular longer and will break down through a series of fractures, remaining blocky in the work mix.
Grit sizes are denoted by the letter "G" followed by the ASTM screen number corresponding to the particle size, such as G25. Grit leaves a surface profile of sharp peaks and deep gorges. Some degree of metal removal will occur when using grit. Grit applications include: removing tough scale or burned-on sand; removing metal to some degree; achieving a rough surface profile or etch; and preparing the surface before coating, such as rubber coating of pipe.
Table 1. Shot vs Grit Aggressiveness and Durability Aggressiveness Shot (40-50 HRC) Factor of 1 Grit (40-50 HRC) 1.2 Grit (50-60 HRC) 2 Grit (60+ HRC) 4 Durability Shot (40-50 HRC) Factor of 1 Grit (40-50 HRC) .8 Grit (50-60 HRC) .5 Grit (60+ HRC) .25 Table 2. Impact Effects of Increasing Shot Size Size Impact Degree of Change Value From Next Smaller Size S70 1 - S110 5 5x S170 10 2x S230 20 2x S280 30 1.7x S330 50 1.7x S390 90 1.8x S460 150 1.7x S550 250 1.7x S660 450 1.8x S780 750 1.7x
Table 1 shows the difference in how shot and grit perform. Grit is more aggressive than shot but does not last as long as shot. And the harder the grit, the shorter its life. This is because the harder the abrasive, the more brittle it is and the more prone it is to fracture. Remember: because grit is more aggressive, it causes greater wear on the blast machine parts: blades, impeller, control cage, wear plates, work handling pieces, etc. And, the harder the grit used, the greater the wear.
Table 3. Relative Coverage by Shot Size Average Number of Nominal Pellets Per Lb Size Dimension (Inches) New Material Work Mix S780 0.0787 18,000 88,000 S660 0.0661 31,000 151,000 S550 0.0555 57,000 265,000 S460 0.0460 99,000 475,000 S390 0.0394 150,000 715,000 S330 0.0331 250,000 1,250,000 S280 0.0280 460,000 2,000,000 S230 0.0232 800,000 3,950,000 S170 0.0165 1,200,000 5,900,000 S110 0.0117 4,000,000 17,000,000 S70 0.0070 13,500,000 56,000,000
Table 2 shows that the relative impact value from the smallest SAE shot size to the largest is roughly 750 times. If one shot diameter is double that of another (such as $330 vs. $660), the relative impact is nearly nine times. A change of one shot size has the effect of increasing the impact by 70-80%.
Table 3 illustrates the relative coverage from one size to another. Because an abrasive throwing wheel operates with pounds rather than count, the smaller the shot size the greater number of particles thrown. Table 2 shows the effect on impact of a change in shot size. Here we see the effect on coverage of that same shot size change. A change from $390 to $460 increases the impact by 70% but the coverage has been reduced 35%. As in the "U" shaped tube example described earlier, as one side goes up, the other side goes down. Experience shows most blast operations use abrasives that are too large. As sand, molding and shakeout practices have changed, so has the amount of sand left on the casting before blastcleaning. What at one time required $550 size shot to remove can now be accomplished with $460 or even $390. Foundries that have used the same size abrasive for many years are now finding that a reduction in abrasive size gives excellent results. To experiment with a smaller size does not require you to purchase anything. Ask your blast machine serviceman or your abrasive supplier to explain how.
Besides the rule mentioned earlier, no hard and fast rules govern abrasive size selection. Much of the selection process is based on trial and error over many years of work. However, an understanding of the size and grades available, and how each compares to the other is helpful.
Some factors to consider when selecting the ideal abrasive size and grade would be: hardness, chemistry and configuration of the workpiece, finish required, composition of the surface contamination to be removed, type of blast machine to be used and time constraints.
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|Title Annotation:||foundry blastcleaning|
|Article Type:||Cover Story|
|Date:||Feb 1, 1994|
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