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cleaning, pretreatment & surface preparation

SIX WAYS TO BETTER BLASTING

BY MARK HANNA, DAWSON-MACDONALD COMPANY, WILMINGTON, MASS.

If your shop regularly uses blast prep, our tips can make your blasting operationclean, effective, and profitable.

1. REDUCE BLASTING COSTS

Here’s a paradox no one wants to hear: Maybe, just maybe, your blasting is costinga lot because you’ve invested too little money (or attention).It’s easy to focus on initial capital costs. But don’t overlook operational costs,which can add up quickly. Have a payback timeline in mind, and add your capitalcosts to projected operational costs over that time horizon – and you mayfind that a little more investment in equipment can make your operation muchmore profitable.So, exactly what are the costs to factor in?

 Reduce labor costs

Many owners would guess that material or compressed air cost the most. However,in most situations it is labor. So do what is needed to cut down on labor. This canbe as simple as investing in an “abrasive upgrade,” a stationary gun holder, a differenttype of blast gun, or pre-packaged system conversion. But where warranted,a whole new system – perhaps including automation – may also be in order.Paying a worker to blast six or eight hours a day – when the same work couldbe accomplished in a fraction of that time – is wasteful. In addition, fewer hoursspent blasting will reduce overall consumption of compressed air energy, andmedia, and save wear and tear.

Understand media

How did you select the media you are using? For some, it’s easy: it is in the spec.For others, it’s what they’ve always used. Many people buy media by how muchit costs per pound – or bag. That can be a mistake.Today, there are dozens of media (type & size), and many are excellent. Theone that will give you the best results the fastest – and hold up the longest – willalso be the cheapest to use, regardless of the initial cost. Select the right mediafor the job, and your major – sometimes hidden – costs will diminish, becauseyou’re blasting fast and clean.On the other hand, “cheap media” is often slow and dusty, reducing visibility(which increases rework). Inconsistent blast quality and a poor and possiblyunhealthy working environment become factors. Add in extra needed maintenance,freight and disposal costs, and saving cents on media makes no sense atall. (See section: “How to Select Blast Media” at the end of this article.)

Optimize your compressed air

Loss of production, rework and downtime are all costly – and you may not beaware there’s a problem lurking in your compressed air supply.Compressed air for blast systems must be oil-free and “clean and dry” and thatusually means a refrigerated air dryer. Moist air is the source of many blastingills, especially in high humidity when water condenses easily, contaminatingmedia and causing flash rusting. “Economizing” on the pipe diameter to theblast machine and adding other restrictions such as quick disconnect fittingscan reduce performance.Air compressors have improved significantly. Consult with your supplier: acompressed air audit may reveal potential savings.

Use the correct blast nozzles

The right blast nozzle can greatly increase the utility of your machine, and todaythere are many that offer gains in production. New ergonomic blast guns formanual cabinets are much easier to use1. Fan nozzles provide wide spray. Airinductionnozzles eliminate the blasting “hot spot” that can otherwise warpdelicate parts and make for slow, streaky blasting. A variety of side-outlet nozzlesreach into or behind difficult-to-access areas. Extra long venturi nozzles havebeen developed to increase production up to 40% over conventional nozzles whenusing the same amountof compressed air.In terms of compressedair consumption,it can be verycostly to leave a wornnozzle in place. Be sureto use a durable, highperformance nozzle –cheap disposables needfrequent replacement,can lead to prematureblast hose replacement,tax your compressor,and drive up labor andenergy costs.Extra labor costs canbe incurred by not usinga large enough nozzle inthe first place2 – re-evaluateand realign yourCFM consumption toFigure 1. Simply upgrading to a blast gun designed for productivity your throughput needs.

2. ACHIEVE CONSISTENT QUALITY

Blasting is a recipe with many ingredients and techniques, so get to know whataffects the speed and quality of your individual application.

Reclaim your media

To economize, virtually all industrial systems reuse blast media several timesover. But media breaks down as it impacts hard surfaces. In dry blasting, debrisfrom the surface of the workpiece becomes pulverized dust. Dust and brokenmedia must be removed, or the process degrades rapidly4. This requires a dustcollector. On cheaper systems only a fraction of the ambient dust Ð nearest theblast cabinetÕs vent Ð is sucked out. ThatÕs not good enough.To maximize the extraction of dust and undesirable fines from the workingmedia, several kinds of air wash or cyclonic separators are employed in conjunction Media Make-UpAs you’d imagine on a bell curve, grit size will skew towards fi neness over time due to theconstant breaking down of the impacted media. The “seasoned abrasive” will be doingthe work for you, patterning the surface with the texture of that “working mix.” For thatreason, some operations charge the machine with a fi ner media and feed the make-upwith coarser stock to draw the bell curve back into the intended range. Automated systemscan be incorporated to constantly trickle in fresh media to “make-up” for the wornabrasive that’s been pulled out of the working mix by the reclaimer – and these “automaticmake-up systems” quickly pay for themselves. That’s because, optimally, you should neverhave to empty the entire load of media for a fresh recharge. Surface quality will remainconsistent, too. You can mimic the make-up system on simpler blast cabinets by emptyingthe dust collector and adding the poundage of dust taken away in fresh, new media – infrequent, small increments.

With wet slurry blasting, blast media can retain its shape and size for longperiods of time. One manufacturer of engines has processed more than 2 millionsmall parts while consuming very little ceramic bead.

Select the right pressure and angle

What blast pressure should you use? Depends.With one-pass (disposable) media on durable parts you may wish to go all out

and blast at the highest pressure6 in order to save labor costs.But you may find that blasting at 90 PSI gains little over blasting at 60 PSI.

Blasting at the lower pressure will increase the service life of the equipment andyour recycled media, keep the media’s “working mix” bell curve centered, andsave compressed air. Lower pressure blasting is also more forgiving and gentleron the parts. Experiment within the bounds of the blast specification.Higher operating pressures can cause rougher surface profiles. Excessive pressureis one of many reasons for “embedment” of abrasive shards, which mightlead to coating failure. Embedment can also occur if the media is too friable, or

if the wrong blasting method is used.What difference does it make if you blast at 90-degrees to the surface orsome other angle? First, it changes the finish. You “dig” blasting straight onand “scuff ” blasting at a low angle, and that affects finish. So keep a consistent

angle when possible. In addition, you may find that low angle blasting is fasterat removing a coating, since media plows beneath it to lift it off. When blastingat 90-degrees, incoming media is battling media rebounding off of the surface.Those collisions slow blasting rates and fracture media unnecessarily – leadingto lower production, higher costs, high dust levels, and inconsistent finishes.

3. CLEAN UP! BLASTING AND “LEAN PROCESS” CAN BE COMPATIBLE

No process can be considered “lean” when a part leaves the work cell, or worse,the factory, for blast processing. The extra handling and time lost in transportand waiting for parts adds unnecessary cost and complexity. With today’s plating,painting, and adhesives, many specs do not allow parts to sit around for more thanfour hours, and some instruct you to plate within 30 minutes of blasting. You maynot achieve that reliably with a blast lab in a remote part of the facility or off site.There are no two ways about it, blasting can be messy – that’s the traditionalreason for it being relegated to a distant room. But there are many very cleanblasting operations, and yours can be, too. Today, we can put modern blastequipment inside some work-cells – which was unthinkable in the past.Cabinets have evolved, dust collection can be fortified, and new techniquesimplemented. Eliminating dust through smart equipment and media selectionshould be a priority.A work cell machine can be compact and simple. For example, a poweredspindle that rotates your part allows blasting to proceed while your operatorattends to other tasks in the cell.Wet slurry blast7 machinery combining wash – blast – rinse in a single step.That is lean. And because these machines are inherently cleaner, they can fitinto the process cell, so queuing,steps, and lost time areeliminated.On other systems, mediacatch trays, some of which areself-cleaning, can be added, aswell as features and controlsthat keep dust emissions atbay. Vertical sliding doors anddoors with delay-timer lockshelp keep things compact andclean.Scrap a poorly performingor undersized9 dust collectorand upgrade to a self-cleaningcartridge model with “surfaceloading10”filters. Considerequipping it with a pulse-cleaningcontroller11 with monitoringgauge, after-filters (HEPA)and a variable frequency drivewith airflow controller12. It should have a sealed waste drum. Everypart of thesystem should be under negative pressure; some blowers pressurize their dustcollector, and a small leak there will make a big mess everywhere.

4. BLAST THROUGH THE BOTTLENECK

What’s slowing you down? Is it your material handling, the blast machine, a poorselection of process variables, or something harder to pin down?

Handle it

Whether you’re cleaning large engine housings in a walk-in room or deburringtiny injector nozzles, material handling is critical. And there is a wide variety oftools available that facilitate processing your part’s properties and batch sizesin your working environment. Some of these include:

• Robotics

• Turntables: continuously powered, indexing (satellite) and spinnerhanger

• “Lazy Susan” turntables – stationary, with dolly, or powered in-and-out

• Crane slots in the roofFigure 3. A work cell machine where a robot does the blasting.This wet slurry blast machine8 is compact and clean.

• Inline: belt or rollerconveyor, pinch belt, traintracks.

• Tumbelt, batch and basketblasters.

• Lathe-style, eitherenclosed in a cabinet, oropen with a vacuum-blastwork-head.

• Custom modifications tobetter match the cabinetto the part or process,such as tilting turntables,feed-through iris orifice,etc.

Blast efficiently

Sometimes you just need a blastcabinet on steroids, and that callsfor direct-pressure blasting, wheelblasting, or any one of severaldynamic approaches to blast prep(equipment, media, training, etc.).Each has limitations to consider,but for covering a lot of toughground efficiently, they save laborlike no other. For instance, thosewho upgrade to direct-pressureoften find a day’s worth of blasting done by morning break. It might be time toaudit your operations, call in your blasting expert and re-evaluate, using theirlocal test lab.

Less can be more

A New England machine shop blasted a forged part several times during its routing, andthey had picked up a well worn multi-gun automatic blast cabinet as well as two inexpensiveblast cabinets for the task. But blasting was still taking too much time. Although theywere using an appropriate media, the equipment was inadequate. All of these machineswere replaced with a new, centrally-located direct-pressure blast cabinet and a single,well-trained operator. Floor space was reclaimed and the hours spent blasting reducedsignifi cantly. Maintaining one machine that was wear-protected from the start becameeasy to manage.

Automate it

In air blast and wet blast systems, automation often means moving multiplenozzles along the profile of the part, maintaining the optimal stand-off distanceand angle of incidence. Nozzles may be fixed – to blast moving parts, attached tolinear oscillator or robotic arm, or arrayed in “whirlybird” fashion.When you eliminate bottlenecks, you may find yourself with lot of extracapacity. That could leave you seeking additional, profitable blasting work.

Figure 4. A simple automated blast machine for IDand OD blasting on a wide range of part sizes keepsthis job shop blasting in the fast lane. Machine has

multiple vertically oscillating guns and PLC controls. Acrane slot with powered turntable facilitates materialhandling. It is shown set up for ID blasting on a jetengine burner can13.

5. FIND A WAY

Everyone has their own idea of what blasting is.There is suction, direct-pressure, wheel and wetslurry. Manual cabinets, suit-up/walk-in blastrooms, and automation of every description.And more.Bicarb blasting, dry ice blasting, vacuum blasting,laser coating removal all have their place.Keep in mind that no tool does it all, and someare much better at a particular process thananother.Today there aremorechoices than ever, andit will pay to work with your local supplier tomake sure you have the advantages of moderntechniques. Ask for a test toprove the processbefore making a commitment.Investigate the properties of different blastmedia, such as:

• Particle shape

• Hardness & density

• Grit size

• Durability

You may find, with testing, that you can strip paint off of glass withoutetching, eliminate masking, use less topcoat, etc. A good supplier can help youdetermine the best choice.Accessing hard-to-reach recessed areas on the workpiece can make blastingfrustrating or impossible. Direct-pressure systems are especially useful for blastinginside tubes and odd shapes; specialized nozzles make this possible. Anddirect-pressure is effective at some distance, so you can effectively blast an areafar from the nozzle.Using a cabinet that is too small will restrict you from moving the end of apart in front of the operator. Custom features can be built into professional-gradeblast cabinets to fit the machine to the part or process, as mentioned earlier.

6. SEND YOUR BLAST MACHINE TO REFORM SCHOOL

Blasting with abrasives can be tough on the equipment, but that does not meanconsumables and maintenance will be an ongoing problem. The solution is toequip your machinery with appropriate wear protection from the start. Thatincludes heavy-duty hoses, protective linings, rubber curtains, and long-lifenozzles. Invest in these and your costs will decrease. YouÕll spend more timeproducing and less time fixing.

Larger blast cabinets last longer, too, because the abrasive is not going inÒhotÓ on the cabinet wall, having had time to dissipate its energy.Automated systems often have a run-time meter to keep track of actualblasting hours. Utilize it to keep a log and anticipate Ð not react to Ð your maintenanceneeds.Preventive maintenance beats fixing. Set up a schedule. Refer to manufacturerÕsmanuals, and consider farming out PM to experts on a regular, scheduledbasis, perhaps spring and fall.

 HOW TO SELECT BLAST MEDIA

Step one in your goal of a cleaner, more effective, and profitable blast prep isselecting the best blast media for the job. As noted earlier in this article, ÒThemedia that will give you the best results the fastest Ð and hold up the longest Ðwill also be the cheapest to use, regardless of the initial cost.Ó

Variety: With so much to choose from, different blasting media can handle any blast prep requirement.

Various factors go into selecting the most appropriate media, including:

Particle shape:

Round media peen and sharp media etch. Think Òball-peenhammerÓ and ÒchiselÓ (on a micro, but broad, scale). The medium imparts itsreverse image onto the substrate. For brittle coatings or removing light, burrs,etc., a round media will probably work best by ÒflexingÓ the coating loose, andit will leave the smoothest finish. To aggressively ÒcutÓ the surface and leave aprofile of Òpeaks and valleysÓ a hard, sharp or blocky media will do best. A soft,blocky media can strip without etching.

Hardness:

Relative to the substrate and coating, pick an appropriate mediahardness, which ranges from almost 1.0 (the rating for talc) to nearly 10.0 (therating for diamond, on the logarithmic moh scale).

Optimizing grit hardness

An example of thoughtful hardness selection comes from blasting a fry pan made froman aluminum bottom fused to a stainless sidewall: A melamine plastic media at 4.0 willprofi le the softer aluminum so that Tefl on® will anchor to it, all the while not etching theharder stainless portion of the pan. No masking required: how is that for “lean” manufacturing!

Grit size:

Often a too-coarse abrasive is selected, under the theory that largermedia will last longer in the system and therefore cost less. However, a finermedium often does the job faster, whereas coarse media will not only take longer,but also require copious amount of primer to cover the rough peaks andvalleys it leaves behind.The reason finer media often works faster is that, for instance, a fine #10glass bead (100-170 mesh) has over 300 million particles per pound compared toa coarser #5 glass bead (40-50 mesh) at “only” 7.9 million beads per pound. So,you see, many more particles hit every square inch every second.Fine media won’t work all the time. In circumstances when you need to“power through” a tough coating or severely rusted surface, the momentum ofa coarse, dense media particle is the only way to “cut it.”Also, medical implants such as titanium knees may require a very coarse profile

for the human body’s bones to “knit” onto successfully. So every applicationrequires a thoughtful analysis of grit size needs.And, of course, your blasting spec may dictate your mesh size.

Density:

A denser media will carry more of its kinetic energy onto the surface for morepower when needed. You can actually save some energy by lowering the PSIwhen upgrading to a denser media. And, you can safely work on many delicate

substrates with low pressure, and soft, low density media such as plastic, walnut,corn cob or bicarb.

Durability – Friability:

A friable medium is more likely to fall apart upon impact. This can be a problemor you can use it to your benefit, depending upon your aim. Dry ice blasting ispopular for specific applications, and it takes friability to an extreme – it literallyexplodes upon contact with the surface, with the expanding volumetric energygetting underneath and lifting lightly adhered contaminants, such as hot plasticdeposits inside injection mold cavities – as the media transitions to gas. (Thermalshock of CO2 at -107 degrees F against a hot residue also helps in these cases.)Generally, durable media is the best choice. And media that keeps its shapegives the most consistent blasting results and quality finishes over time. Ceramicbead and cut wire are fine examples of high-yield blasting media.

Cost:

Your selection of media directly affects blasting cost. Remember, the best mediais fastest and cleanest, and you’ll have your priorities right – and be on the roadto more profitable blasting.

Divergent needs:

There’s no two ways about it. Those shops that need to produce a variety offinishes will have to keep several kinds of media on hand. If production or scalewarrants, you may find that you dedicate one machine to aluminum oxide, oneto glass bead, etc.

Seek expertise:

Buying media from a distant catalog source that offers no expertise is no way topurchase media. Seek out your local blasting supply house for expertise. And usetheir local test lab, which should be well stocked with modern blast equipmentand a variety of different media.

CONCLUSION

Abrasive blasting can be a cleaner, faster, lower maintenance-intensive process.Labor costs dearly, so reduce it through thoughtful selection of equipment andmedia. Rely on local outside sources for expertise. Determine if you want tospend a little extra up front to gain the benefits that better equipment, media,and product support offers: higher quality, higher yield, and an improved workenvironment.

REFERENCES

1. By ditching the old trigger blast gun and upgrading instead to a moreeffective gun with a foot treadle, you’ll blast faster and lessen the chance ofworkman’s compensation squeezing that trigger for hours on end has leadto claims for carpal tunnel, tennis elbow, etc.

2. Changing nozzle size may necessitate other system changes. Consult yoursupplier.

3. Photo of ergonomic blast gun courtesy of Kennametal Abrasive FlowProducts.

4. Dust is erosive, prevents free-flow of media, clogs filters, slows blasting, andin manual operations, reduces visibility – which is important to speed andquality results, without costly rework.

5. Photo by the author.

6. Most pressure vessels are rated for 125 PSI; some newer ones for 150 PSI.

7. Older designs of wet slurry blasting machines did not have the advantages oftoday’s models. Be sure the one you select is designed for easy maintenance,durability, and clean operation – and is constructed with all-stainless steel(or roto-molded plastics when lower production economics are required). Aglandless vortex pump will beat an older design in performance, and be surethe pump is rebuild-able, without tools, in minutes. This is one case whereyou get what you pay for, and good engineering counts.

8. Photo provided courtesy Wet Technologies, Inc.

9. Sizing the dust collector involves determining the total air flow (CFM)required, friction factors through the system (including ductwork), the airto-filter ratio (CFM: FT2) appropriate for the particular contaminant, anda variety of other factors. Therefore, adapting surplus collectors to blastingprocesses should only be done after a complete evaluation. (That wouldalso include the risk of inheriting potentially hazardous dust shipped inwith a used collector). The blaster’s dust collector should be sized not justto overcome the incoming amount of compressed air, but also create severalair changes, up to 10 or more per minute, depending upon the cabinet size.

The blower that powers the dust collector must be matched in air flow ratewith its reclaimer.

10. UltraWeb® by Donaldson-Torit is the originator, and many consider thebest. This style filter prevents submicron particles from bypassing or prematurelyclogging the filter media, amongst other benefits.

11. Wild changes in filter loading caused by inattention to the basic operationaltask of filter purging will lead to fluctuations in the operating conditions,and that can affect quality, cost, and add to the dust burden. For this reason,automating the cleaning cycle, using a narrow on/off band, is recommended.

12. A VFD, when tied in with an airflow controller, keeps air flow consistent,increases the service life of the filters, saves electricity, and ensures that yourcritical reclaimer system stays “in tune.”

13. Photo by the author.

14. Photo of micro-blast courtesy of Vaniman Manufacturing.

ABOUT THE AUTHOR

Mark Hanna has been designing, troubleshooting, andapplying blast prep systems for over 30 years, He workedfor Empire Abrasive Equipment in engineering, productdevelopment and management, and established Empire’ssuccessful Custom Cabinets department. In 1993 he joinedDawson-Macdonald, and since then has been improvingblasting operations of all descriptions for his customers in

Massachusetts, NH, VT and Upstate NY.He welcomes your questions and comments.Personal website / blog: www.blastprep.com

Company website: www.dawson-macdonald.comEmail: این آدرس ایمیل توسط spambots حفاظت می شود. برای دیدن شما نیاز به جاوا اسکریپت دارید with Mark Hanna on LinkedIn at http://lnkd.in/4-amf6

© 2013 Mark Hanna