cleaning, pretreatment & surface preparation

IMPACT BLASTING WITH GLASS BEADS

BY ROBERT C. MULHALL AND NICHOLAS D. NEDAS

POTTERS INDUSTRIES INC., VALLEY FORGE, PA.; WWW.POTTERSBEADS.COM

Glass beads were originally used for decorative applications. Their use as a mediumin impact blasting came about largely as a result of the aerospace buildup ofthe 1950s. At that time, a need developed for multipurpose media that combinedthe advantages of coarse, organic, metallic, and fine angular abrasives. Table Ishows a comparison of glass beads with other impact abrasives for cleaning,

finishing, peening, and deburring applications.Impact blasting with glass beads is well placed to satisfy demands of the 1990sfor an energy-efficient and environmentally acceptable method of metal finishing.When properly controlled, the system is safe for workers and spent media

presents no disposal problems.

PROCESS BENEFITS

Glass beads are virtually chemically inert. This factor, combined with theirspherical shape, provides several key benefits. Media consumption is minimized;Table II compares consumption data of impacting media on differentmetal surfaces of varying hardnesses. On both metals tested, glass beads offerthe lowest consumption per cycle. In addition, close tolerances are maintainedand glass beads remove a minimal (if any) amount of surface metal.Impacted surfaces are free of smears, contaminants, and media embedments;high points are blended and pores sealed. A wide range of finishes frommatte to bright satin are achievable. The peening action of the media furtheracts to impart a layer of compressive stresses on the surface of the part. Thisincreases fatigue life, decreases susceptibility of the part to stress corrosion,

and enhances surface strength.

PROCESS ENGINEERING

Proper design of impact blasting equipment is essential for each application toachieve the full benefits of high productivity and low costs. Most important,the system should be easily controllable to produce consistent results.Key to this control is determination and maintenance of the “arc heightpeening intensity” of the operation. To measure the peening intensity in a particularapplication, special steel strips are bombarded on one side only by theblasting media. The compressive stress induced by the peening action causesthe strip to bow in the direction of the blast. A series of values of arc heightversus blasting time are obtained, and when plotted on a graph, yield a saturation

curve. From this curve, the arc height peening intensity can be obtained.Environmental factors, operator skill, OSHA standards, and equipmentcapabilities are the process parameters involved in all glass bead blasting operations—whether they are cleaning, finishing, peening, or deburring. Once allthe variables are optimized and the arc height peening intensity determined,process control is achieved by maintaining that arc height peening intensity.Any change indicates some modification in the system operation, away from

optimum performance. System control via arc height peening intensity is applicable to all cleaning,finishing, peening, and deburring operations. In cleaning, the arc heighttechnique can be used to maintain process speed. In finishing, profilometer

measurements of root mean square (rms) microinch finish can be correlatedto peening intensity, thereby eliminating any subjective evaluation of performance.In peening, the degree of compressive stress induced is directly related

to the arc height peening intensity. By such control, significant benefits areachieved in terms of labor productivity, reduced supervision requirements, anddecline in the number of rejected parts.As indicated in Table I, both steel shot and glass beads are available for

peening applications. Steel shot with its heavier density offers a deeper depthof compression, but requires more energy to propel while leaving dissimilarmetallic smears (i.e., various forms of contamination) on the part’s surface.Glass beads are often used as a secondary peening medium, removing contaminationwhile improving surface texture and finish (lower rms) of the part.Glass beads are also used extensively as a peening medium, achieving awide range of arc height peening intensities in a variety of applications andindustries (see Fig. 1).Typical glass bead peening applications take place before plating and aftergrinding and welding on aerospace, automotive, and machine tool components.

KEY FACTORS IN USE OF GLASS BEADS

There are a few key considerations that will help the user to enjoy the benefitsof glass bead impact media to the fullest.Whether for cleaning, finishing, peening, or deburring, the work actuallydone depends upon the amount or weight of abrasive thrown against the

target surface in a given time. It also depends upon the speed with which the

material is thrown against the target. The formula:I = MV2/2indicates that impact energy (I) equals one half the mass or weight (M) times

the square of the velocity (V) at a 90o nozzle angle. Correction factors should beused for other angles.

As a general rule, the smallest particle that will provide the desired effect onthe surface is the most efficient

one to use, as this gives the greatestnumber of impacts per poundof glass spheres.When the nozzle is at a 90oangle to the surface being treated,the bounceback of beads hasa “blinding” effect. This interfereswith the effectiveness of theblast stream and tends to increase

the rate of bead consumptionthrough breakage. Generally, anangle between 45 and 60o will givethe most effective performance. Insome cleaning applications, stilllower angles may help speed thework.The work energy of the flyingparticles is also affected by thedistance from the nozzle to thework surface. It is usually best tokeep this between 4 and 8 in. toavoid loss of velocity, and to gainmaximum acceleration and properdiffusion of particles into themost desirable pattern.

 BEAD CONSUMPTION

Because beads can become broken after repeated impacts on the work surface,controlling bead consumption is of critical importance. It is affected by five keyfactors:

1. Bead size—the larger the bead, the more durable and resistant tobreakage it is at a given impact intensity. This preference for larger beadsmust be balanced against the greater efficiency of smaller size beads, whichare capable of the work required.

2. Uniformity of size—proper sizing also affects efficiency of operations.The wider the range of bead sizes in a particular “charge,” the higher therate of consumption at given conditions.

3. Roundness or sphericity of beads—the more spherical the individualbeads, and the freer the “charge” from nonspherical particles, the lower therate of bead consumption.

4. Surface hardness of material being treated—the harder the surfacebeing treated, the higher the rate of bead consumption.

5. Angle of impingement—the closer to 90o the stream of beads is to thework surface at a given arc height peening intensity, the greater the rate ofbead consumption.

APPLICATION NOTES

Cleaning

Because of the wide variety of different materials that must be removed in cleaningoperations—including mill scale, rust, carbon buildup, and the like—it isoften best to experiment with different nozzle angles to find which works mostefficiently. Where there are internal recesses and other difficult areas, the useof the smaller bead sizes may be particularly helpful. Because a high cleaningspeed usually minimizes labor cost, bead size and nozzle angle are the key considerations.Normally, a velocity that optimizes cleaning speed with a given size

of bead will optimize consumption, to give the lowest total cost.

Finishing

Where appearance is of prime importance, bead size is normally the key consideration.Velocity, nozzle angle, and other factors should be adjusted, first togive maximum finishing speed, and second, to minimize consumption. This willprovide the lowest total labor and material cost per unit of production. As a generalrule, large beads at high intensities provide a deep matte; at low intensitieslarge beads give a smooth, bright surface; small beads at high intensity give a dullmatte, and at low intensities a bright satin. Selective masking of surfaces, the useof multiple nozzles, and a “painting’’ motion may be employed for highly specializeddecorative effects. Automated machines are generally used for finishing.

Peening

Peening to increase fatigue resistance or to increase stress corrosion resistanceis essentially a uniform “hammering” operation. Uniformity of bead size andcontrol of the number of nonround and angular particles included is critical toprocess performance. The key consideration is impact intensity, which must bespecified as minimum and maximum. Nozzle angles should be as close to a rightangle as possible without excess bead consumption. In general, the larger beadsizes, because of their resistance to breakdown, will prove most cost effective.In peening fillet areas, it is a standard rule that beads no larger than one halfthe radius should be used.

Deburring

The key considerations in deburring are usually a combination of programmingsurface finish, while achieving sufficient impact intensity to remove or depress theburr. Bead size, which governs finish, must be adjusted to an adequate peening

intensity with velocity. Proper nozzle angle will optimize consumption