Glass Bead Blasting?

Q. We have a 2-ft2 tumble style wheel blaster with a rubber belt conveyor. We are blasting small (1-inch diameter × 3/4-inch) zinc die castings to remove light flash and impart an isotropic finish. The shot we use is S-70 and it does a good job, finishing 1,000 lbs of parts per hour. Our customer has seen some similar parts that were airblasted with approximately 100 mesh glass beads, and he prefers that finish. Could we provide a similar finish if we changed from the small steel shot to glass beads?

A. The short answer to your question is “no.” I will explain why not, and then give you and your customer reasons why you may be much better off with your current process.

Nothing in surface altering methodology is cheaper than wheel blasting. I introduce the phrase, “surface altering” because it is possible to put a finish on a surface concurrent with the part forming operation. For example, you can have a knurled surface on forming dies that will impart something other than a smooth surface on the formed parts. But when you add a finishing operation you are altering a given surface. Wheel blasting does that so much more efficiently than any other method that if the resulting surface is acceptable, the economics pretty much preclude any other method.

The first thing to understand is that wheel blasting propels the abrasive much more slowly than does airblasting. The impact on a surface is related to the mass and velocity of the particles being thrown. Glass beads have a specific gravity of 2.5–3.0, while steel shot has a specific gravity of 7.85. An average wheel throws the media around 200 mph, and an average suction-type airblast gun will throw the lighter glass abrasive about twice that fast depending on air pressure, media density, and other factors not necessary to this discussion. The wheel has the advantage of throwing any abrasive at the same velocity, but that velocity is much lower than a suction gun, and only one-third or less the velocity of a pressure blaster. (I’ll save the discussion of those two airblast options for another time.)

If it were possible for the wheel to effectively use glass beads as a media, the surface deformation, and hence the finish, would be far less desirable than that observed by your customer. This can be easily demonstrated in your machine by running one load with glass bead media. In addition to observing that the finish is not what you expected, you will find that almost all the beads will be used up in a single pass. Bead consumption is very high because: 1) The mechanism that accelerates the media and feeds it onto the face of the wheel blades will crush much of the glass; and 2) the air separation method for removing contaminants from the abrasive stream is going to treat the crushed beads as contaminants and remove most of them from the system. (This is adjustable, but it will be compromised when the abrasive itself has such a low specific gravity.)
Assume for a minute that the finish is acceptable to your customer. If you adjust the air separator well enough to retain the majority of the abrasive, you will soon find out that the very expensive wheel parts will wear out faster than you ever thought possible. Glass is very hard, and very abrasive. You will probably reduce the life of blades, impellers, and control cages by 80%.

All that said, your customer might suggest that you change to an airblast style of equipment. This would be the time to get a blast finishing equipment salesman into the act. The initial investment may be about the same as replacing your wheel blast machine with a new one, and that may not discourage you. Note, however, that it will take eight to 12 nozzles to approach the production requirement. Each nozzle, depending on the air pressure, will use somewhere around five horsepower to compress the air. So, at a minimum, you will need a 50-HP air compressor. It is a good idea to blast with dry air, and that will require additional expensive equipment—such as a desiccant tank or a refrigerated dryer.

Take a look at the operating costs of these two systems. You are probably using about 3 lb/hr of steel shot. Add to this the electrical cost of about 10 HP—the wheel, conveyor, elevator, and dust collector. Factor in the annual cost of maintenance parts—the wheel parts on a regular basis, and new barrel heads and belt conveyors about every five years. Calculate your cost for finishing that 1,000 lb/hr.

Now, get estimates for the operating cost of the airblast approach. It will include a 50-HP or larger air compressor, another 2 HP for dust collection, perhaps 10 HP to refrigerate the air, staggering nozzle replacement costs, and high abrasive consumption rates. Since this will be a new capital investment you may also include the cost of money. Calculate your projected cost for finishing that 1,000 lb/hr. My best guess is that neither you nor your customer will feel this to be justified given the nature of the product being finished. “Nice looking” is hard to sell when there is no return on the investment.
To those of you who are already airblasting with glass beads, I recommend you test crushed glass as an alternate. This product makes use of discarded glass products such as bottles and windows, crushing them and sizing the grit to correspond with the usual glass bead selection. This is good for the environment, and the product is a good performer in blasting applications. The finish is good, and the consumption rate is somewhat lower.