Maximum blastcleaning machine efficiency starts with the basics.

Shotblast unit maintenance and abrasive segregation and handling help lower cleaning costs and avoid production bottlenecks.

These days, with casting cost reduction on the mind of every foundryman and purchaser, an analysis of current cleaning room practices is in order. Scrutinizing your finishing operations - from maintenance to products flow - can yield significant savings.

Blastcleaning equipment in particular is not only an obvious bottleneck in the production process, but a major drain on maintenance time and money. By examining your current maintenance practices, as well as your load mix, most foundries will find "going back to basics" will generate savings and help cut the overall cost of their castings.

Blast Unit Maintenance

It's pretty well known that blasting equipment is self destructive. As such, the machine is often down for emergency repairs, rather than planned preventive maintenance shutdowns. To get the machine back up and running quickly, maintenance personnel are forced to fix only what immediately needs to be fixed, skipping the normal operating adjustments that should be made.

There are some basic but critical areas of the machine that are often overlooked.

The Blast Wheel - Efficient blasting begins with consistent operation of the blast wheel. Most cycle time problems begin here because the "hot spot" is misdirected, the amps are low or the feed parts are worn out. All these problems can be easily corrected.

The hot spot is the area in the machine that receives the highest concentration of shot thrown by the wheel, with the spot's length and direction controlled by the control cage. The location of the hot spot can be altered in length by wear on the control cage opening, or in direction by improper location of the window opening.

Both of these situations may sound minor, but if the control cage opening is out of position by as little as 3/4 in., the hot spot can move on the workpiece by as much as 12 in. Therefore, if the opening is worn back by as little as 1/4 in., the hot spot will move about 4 in. Thus, the machine itself may receive more blasting than the actual part, causing premature wear on the machine and an increase in cycle time.

Two things can be done to correct this problem. First, the control cage should be checked twice weekly for wear on the leading edge. Second, once the hot spot has been correctly aimed, pick a reference point on the control cage flange and mark the wheel housing where that point is. This helps keep the hot spot consistent through future control cage changeouts. The hot spot can be checked by positioning a piece of steel plate where the work load would be and blasting the sheet for about 30 sec. The hot spot can actually be seen and felt.

The Ammeter - This is the most overlooked troubleshooting device on the blast machine. In fact, at any given time, the ammeter on most machines is either broken or improperly calibrated. The ammeter tells the operator how many amps the wheel is drawing, and indicates when the abrasive level is low, when the wheel is being under-amped or when the wheel is flooded.

The blast wheel should be set up to run consistently at full load amps. If not, full wheel efficiency is not reached, causing dramatic increases in cycle times. If, for example, a 60 hp/460 V wheel specified for 72 full load amps is running at 60 amps, it is only 74% efficient, meaning it is throwing only 74% of the abrasive it is capable of (Fig. 1). In terms of the number of impacts on the workpiece, this results in about 36,270,000 fewer impacts per minute of blasting time.

If the abrasive level is low, the ammeter will show a sudden drop from full load amps to no load amps sometime during the cleaning cycle. Low abrasive levels are a common reason for longer cycle times, and they typically go unnoticed as the operator prepares the next load for blasting and doesn't see the drop in the amps. This is an easy problem to overcome just by watching a couple of the longer loads cycle through the machine. If a drop is noticed, check the storage hopper for the abrasive level.

Flooding the wheel is a condition caused by an excess of abrasive trying to enter the wheel at one time. A sudden drop in amps will also occur in this scenario, and cycle time again increases. If, upon checking the storage hopper, the abrasive level is high enough, then the abrasive valve is allowing too much media in and must be readjusted.

The Feed Parts - The blades, control cage and impeller are obviously the machine parts most subject to wear. If these parts are worn, they affect the abrasive pattern and increase cycle times.

Blade wear causes the abrasive to remain on the blade longer, releasing later and moving the hot spot toward the "tailing" side of the pattern. Besides increasing cycle time, this also causes wear to the cabinet and around the wheel opening. Wear to the impeller and control cage causes poor feeding of the abrasive onto the blades, as well as causes the pattern to move.

Figure 2 shows the effects of a worn impeller on blade and wheel wear. To control the problems associated with feed component wear, all feed parts should be checked at least twice weekly and replaced when necessary.

Abrasive Separation/Storage

Because of its location in the machine, the separator is often the most neglected part of a blast unit - though it may perform the machine's most important internal operation. The separator removes contaminants such as heavy tramp metals, sand and broken-down abrasive - all of which cause major machine wear.

The separator also plays an important role in classifying the used abrasive, which impacts machine cycle times. For the separator to operate at peak efficiency, special attention must be paid to the rotary screen, abrasive curtain and the air draw.

The rotary screen is the first part of the separator the abrasive must pass through. It is designed to remove tramp metal from the system. The most important preventive maintenance that can be performed on the screen is checking for holes - holes can cause wheel blowup and long downtimes.

A full curtain of abrasive over the entire length of the "airwash" portion of the separator is extremely important. During this process, the shot is dropped over a lip on the separator and allowed to fall anywhere from 3-5 in. Ductwork from the dust collector is attached behind this area, pulling air through the "waterfall" of shot, thus cleaning the abrasive of any fines (sand and broken down shot).

If a full curtain of abrasive is not maintained, the air drawn from the dust collector takes the path of least resistance and moves around the abrasive waterfall, leaving the sand and fines mixed with the shot. This leads to increased cycle times, since the cleaning is no longer being done with well-sized abrasive. It also causes machine wear. For example, if the current sand content in the abrasive is 1%, an increase to 2% decreases blade life by about 50%.

Preventive maintenance to the separator consists of a combination of mechanical adjustments and adjustments of the air draw of the dust collector.

Two easy and important steps are adjustments to the stationary and swinging baffles. The stationary baffle should be manipulated to bring the abrasive across 75% of the separator lip. The swinging baffle should then be adjusted to bring the abrasive across the rest by setting the counterweights (Fig. 3).

Once this full curtain is attained, attention should turn to the air flow from the dust collector. The correct air flow is 400-600 ft per min through the abrasive curtain. Most times, this can be attained by moving the blast gates on the back of the separator. When the separator is working perfectly, a small amount of abrasive is mixed with the sand and dust exiting the fines discharge tube.

It is also important to remember the importance of the abrasive mix. A balanced mix of large and small abrasive keeps the cycle times down and has a direct effect on the finished part's cosmetics. The larger shot impacts harder on the surface of the part, penetrating the layer of sand that may be on the casting, and knocking off the hard scale. The smaller shot that is the product of the fracturing of the large shot then comes in and puts a finish on the casting. This balance can only be kept with regular and thorough additions of abrasive, thereby making certain there is enough shot in the hopper to last the entire cycle.

Process Improvement

In most foundries, there is plenty of room for improvement to the blasting process, particularly in cycle time and the incidence of over-blasting. The object of setting cycle times is obviously to clean the part with the least amount of machine time, while incurring as little reblast as possible.

Typically, cycle times are determined the first time a part is run, without any adjustments after that. Over-blasting is common, and is usually due to the fact that nobody has determined exactly what a "clean" casting is. Studies on the cycle time should be conducted on each part run to determine exactly what the customer will or won't accept as clean.

There should then be periodic reviews of the cycle times to reflect periodic adjustments in the casting process. Changes in mold wash, binders, or even the way the parts are molded can impact cleaning times. Once cycle times are established and the machine is running consistently, increases in reblast can help point out other problems in the casting process that may need attention.

New Technology

Changes in product mix have a tremendous effect on machine efficiency. Most machines in the field are quite old, and were usually bought to clean parts the foundry no longer makes. Using direct drive blast wheels and magnetic separators are two ways to update your blast machine.

Generally, these direct drive blast wheels can cut up to 50% from the machine's cycle time. By eliminating the bearing unit, belts and sheaves on a blast wheel, more working amps can be applied to throwing the abrasive.

Cycle time efficiency can be improved either by throwing more abrasive at the part, or by throwing the abrasive harder. Large diameter, low RPM direct drive wheels throw more abrasive, while small diameter, high RPM wheels throw it harder. This means that correctly sizing the wheel to the application is critical. Knowing as much as possible about the current operation helps ensure that the right size wheel is selected.

Another means to reducing cycle time and machine maintenance is using a magnetic separator. More and more foundries are turning to magnetic separation for their blast units. The purpose of magnetic separation is to remove as much sand as possible from the abrasive mix and help classify the shot. It also helps extend the life of wear parts on the machine, and it is not uncommon for magnetic separation to extend the life of a set of blades up to 2000 hr.

Developing a program that helps maximize uptime and minimize cycle times all starts with taking a basic look at the current state of the machine and working to improve the process. Once a starting point is established and all the possible efficiencies of the current machine are gained, upgrades in current technology will be easier to justify and be more accurate - all adding up to a more competitively priced casting.