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Thursday, 31 July 2008

Shop Information Media Blasting with Ground Walnut Shells

By Chris Betjemann of Barrington Motor Works

Our first experience with media blasting was, as it is with many shops, with sand blasting.  Our first blasting unit was a simple open topped siphon feed device purchased from the Sears Roebuck tool department.  We later moved on to a heavy and bulky wheel about pressurized unit.  Both of those units had to be used outdoors because of the “total loss” nature of the system.  That is, once the sand, or more correctly in our shop, the “Black Beauty”, was used, it was not to be used again.    

To use such a system outdoors one needed tolerant neighbors and a place to work in which it didn’t matter how much of the Black Beauty and debris from the objects being blasted built up on the ground.  No matter the clothing, hood, gloves and respirator worn it was an obnoxious and dirty process.  Eventually we moved on to a large media blasting cabinet that allowed the Black Beauty to be recycled and used multiple times while sparing the operator the mess of total loss sandblasting.    

At about the same time that we purchased the cabinet for blasting with Black Beauty we also purchased a much smaller cabinet to be used for media blasting with glass beads.  To those not familiar with glass beads a bit of a description is in order to understand both glass beads and their nature.    

All media blasting can be thought of as a stream of some sort of abrasive propelled by pressure against the object to be blasted.  The propulsion is usually by pressurized air although other propellants are in use as well. Many materials have been used as the abrasive including sand, Black Beauty (a slag byproduct), glass beads and others.  The aggressiveness of the process of media blasting is dependent upon the abrasive being used and the air pressure at which it is propelled against the object to be blasted.  Products such as sand and Black Beauty are extremely aggressive, especially at the typical 100-125 psi air pressures being used.  Sand blasting at such pressures can easily erode soft metals, etch glass quickly and warp sheet metal. Glass beads are an abrasive that is much less aggressive than sand and are typically used at a line pressure of 50-100 psi.    

Sand blasting is particularly good at the removal of rust, scale and old paint or powdercoat.  But it is much too aggressive to be used on soft metals such as aluminum or on thin objects.  Glass beads are much more suitable for the latter.  Our typical uses of glass bead media blasting included such things as the cleaning of cylinder heads, cylinders, carburetor bodies and internal parts and, occasionally, engine blocks, transmission cases and final drive cases.  

I say “occasionally” with reference to the use of glass bead media cleaning on engine blocks, transmission cases and final drive cases because of the extreme danger of doing so.  Although glass bead media blasting works very nicely on such aluminum it is very time consuming and difficult to adequately clean up after doing the job.  Too often times after blowing off the blasted part with compressed air and then washing it with hot soapy water and multiple rinses some of the glass bead residue can still be found in the nooks and crannies of complex castings such as these.  And that remaining glass bead residue can cause catastrophic damage if left behind in any of the mentioned items.  Additionally glass beading aluminum castings can give it a bit too much of an unnatural white and “sparkling” appearance to those castings.  

Having heard reference to walnut shell media blasting for years, but having no personal contacts with experience with the use of it, we decided to embark upon some in shop experiments with the use of walnut shell media blasting.    

After a very brief Google search for pertinent information I was fortunate to make contact with Helen Cantrell, Director of Sales and Marketing at Eco-Shell, Inc. in Corning, California.  After a number of email communications it was agreed that Ms. Cantrell would provide us with some samples of their walnut shell media to try and evaluate for our purposes.  

We were stunned several days later to receive three fifty pound bags of walnut shells.  Each bag was a different grit and so we were able to evaluate each grit in our applications.  In order to do so we emptied the glass beads from our small blasting cabinet and used the several grits of walnut shells under various pressures for various purposes.  

It became clear that walnut blasting does not replace all applications of glass bead blasting due to the much more gentle nature of the walnut blasting.  However, that more gentle nature of walnut blasting has made it a favorite of ours now for a number of cleaning operations.  It is important when using walnut blasting to do a thorough job of washing the parts to be cleaned and to have those parts dry before walnut blasting.  

When walnut blasting aluminum castings we typically do the blasting and follow it with a quick cleaning with a phosphoric acid preparation and a copious water rinse.  It seems that when the “pores” in the aluminum are “open” following the walnut blasting the aluminum is very amenable to such chemical cleaning.    

Helen provided us with three different grits of walnut shells for our evaluation.  We settled on a rather fine grit called 18/40 “fine blasting grit”.  Our theory is that the finer grit finds its way into the roughness of the aluminum casting better than the coarser grits.  

During our trials with the various walnut shell grits we also experimented with air pressures and eventually settled on a line pressure of 100 psi.  That seemed to work well without undue destruction of the media.  

Once the object being cleaned is finished in the cabinet most of the residue is easily removed with a few blasts of an air jet.  That can be followed with some brake cleaner or soap and water.  But whatever the method used the cleanup seems to be much easier and safer than the cleanup after using glass beads.  

In addition to cleaning aluminum castings we have found walnut blasting to be good for cleaning of dimmer switch and main switch board circuit boards, carburetors and internal engine parts.

At this time we would like to add one caution in the use of any media blasting in a cabinet.  However, this caution is particularly important when walnut blasting.  The pressurized stream of media creates a charge of static electricity that must somehow be discharged.  That discharge can be uncomfortable when it occurs through the skin of your hands or arms.  But it can be life threatening if it causes the explosion of the media in the air of the cabinet, a decided risk when using walnut shells.  The risk is easily eliminated by the simple expedient of grounding the cabinet to earth.  

Should there be those of you out there in VMCA-land who would like to incorporate walnut blasting into your shop armamentarium of procedures I would suggest that you contact  Helen Cantrell, Director of Sales and Marketing. 

Email: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
Office Phone: (530) 824-8794
Cell Phone: (530) 624-6522.   

They have a minimum order of 50 pounds, although Helen recommends a 200 pound order to reduce product per pound cost and shipping per pound cost.    

Grit Sizes Available and Grit Descriptions provided by Helen Cantrell: 

8/12 "Coarse Blasting Grit":  This would be used for large components with extreme build up.  May be used for the initial once over.  Can damage surfaces if not used properly.  Very aggressive. Used in very few blasting applications.  Commonly used for polishing large components in a tumbling application.  

12/20 "Medium Blasting Grit":  This would be used for medium to large components. May also be used for the initial heavy build up followed by a fine grit for a final surface preparation. Can also be aggressive, but okay when aggressive is needed.  Most common grit sizes used in polishing in a tumbling application.  

 

18/40 "Fine Blasting Grit": This would be the most common grit size for most blasting applications where fine metals are involved.  This grit size when used properly will not create excess heat that can damage fragile metal surfaces.  Used commonly on paint removal on car bodies, log cabin homes, stainless steel, and aluminum components.  

20/40 is also available as a Fine Blasting Grit.  It would be very similar to the 18/40. Slightly less aggressive, but by far 18/40 is more commonly used.  

Chris Betjemann  

Barrington Motor Works is an active VMCA sponsor. They specialize in restoration work on our old BMW’s. THEY ARE HELPING US, LET’S HELP THEM! 

Barrington Motor Works
90 Canaan Back Road
Barrington, NH 03825
603 664 2673
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Last Updated ( Thursday, 31 July 2008 )
 
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