Abrasive Grit Blasting

Abrasive Grit Blasting Inspection

2010-06-06T08:05:00.001-07:00

Abrasive grit blasting processes are coming under increased scrutiny in situations where it is used as a precursor to more elaborate coatings such as plasma sprayed coatings. This is because the abrasive grit surface preparation techniques form an integral part of the final coating quality. A poorly grit blasted surface will produce a bad quality coated surface and hence increased importance is being placed not only on the abrasive grit blasting process itself but also on the inspection steps that provide quality assurance after the process.

Now what are the steps involved in abrasive grit blast inspection is the main thrust of this post. The first criteria is proper grit blast coverage. It is essential to check for proper and complete grit blast coverage of the required areas as well as ensuring that masking for grit blast to prevent grit blasting on not allowed areas is properly followed through.

Associated with this requirement is the inspection for uniform grit blast coverage. What this means is that the grit blasted finish must be uniform throughout the required area. This can be a problem sometimes when using manual techniques, because in such cases, sometimes the gun movement may be non-uniform in certain areas or the stand off distance may be different in some areas resulting in some areas having a different finish than other areas. This non-uniform coverage must be avoided and the inspector needs to watch for this during inspection.

Surface finish must be considered a key inspection criterion. While the best way to ensure accurate surface finish value is by the use of a profilometer, at the very least, visual grit blast surface finish standards need to be established and the inspector can visually inspect as to which standard matches closest to the component at hand. It is preferable that inspection of grit blast surfaces by comparison to visual standards be performed at least at a 10X magnification level.

Grit blasting air control

2009-08-13T13:12:00.000-07:00

Ensuring high quality in any industrial process involves control of as many variables as possible. Of course, the first task is to identify the variables that affect any process and that itself can take quite some effort. There are many instances where variables that were once thought to be not affecting the process may be deemed to be important and then they must be controlled also. Abrasive grit blasting is no different than any other industrial process in this regard. While it seems simple and easily controllable, there are many variables that affect the abrasive grit blasting process that are overlooked quite often. It is the purpose of this post to spend some time concentrating on one such variable that affects the abrasive grit blasting process, namely the air supply that is used.

When addressing control of process variables, it is important to note that there may be a series of variables within a variable family. It is then important to control every single variable within that subset. In the case of air supply for abrasive grit blasting, there are multiple variables that exist that have to be controlled.

The first and foremost variable in the family of abrasive grit blasting air supply is the moisture content in the supplied air. Excessive moisture content can affect both the abrasive media as well as the component that is being grit blasted. Ideally, one should aim for dry air that is devoid of all moisture content. In some sensitive applications, in addition to installing driers in the air line, one may need to install more sophisticated moisture absorbing equipment.

The second variable is the elimination of oil in the air supply. Oil in the air supply for grit blasting units can create havoc in the finished surface, especially when subsequent operations such as thermal spray coatings and such are planned. It is imperative to ensure that no oil exists in the air supply.

Of course, the third and final variable is the supply of air at proper pressure and proper volume. This may involve ensuring that your air compressor is of the proper capacity and has been properly maintained so there is no sudden drop of pressure when excessive demand kicks in.

Proper control of air supply for grit blasting is important if quality control is of cardinal importance.

Grit blast cabinet release

2009-07-18T15:42:00.000-07:00

In this post, I would like to discuss grit blast cabinet release. This is a term coined for something akin to releasing a cnc machine to start full production based upon inspection of the first article produced. Grit blast cabinet release therefore is very much a part of the quality control aspect of the process. Especially in today’s age of competitive manufacturing where customers expect quality to be a given, grit blast cabinet release concepts can go a long way in ensuring quality in the abrasive grit blasting process.

So what is the definition of grit blast cabinet release and what are the parameters that will form the grit blast cabinet release mechanism? Well, let us deal with them one by one. Essentially, the grit blast cabinet release mechanism is defined as a written procedure that will detail a checklist with items to be checked off prior to releasing the station for full production. Such a mechanism should be put in place before the start of every new job. And if it is a job that runs beyond one shift, then the mechanism must be restarted at the beginning of every shift.

The following parameters are recommended to be included in the grit blast cabinet release checklist. Firstly, does the operator have all the written instructions for the job at hand. Secondly, are the pressures and stand-off distances properly set. Is the correct grit blast media being used. Is a representative sample blasted with the same parameters visually acceptable.Is the surface roughness on a representative sample as measured by a profilometer correct within the specifications. Is the first actual piece blasted depicting proper masking cut off requirements. If the answers to all of the items listed above are acceptable, then the grit blast cabinet is said to have been released for production.

If any of the items falter, then production should not be allowed to go on until a re-evaluation of the items has been performed and found to be acceptable. This may sound like a lot, but the quality of your organization hinges on producing quality hardware every step of the way and the business of your organization hinges on the quality of the hardware that your organization ships.

Grit blasting tooling maintenance

2009-06-27T08:36:00.001-07:00

Abrasive grit blasting when used as a pre-cursor to subsequent processing such as coatings of the thermal spray family almost always have requirements of blasting required, blasting optional and no blast allowed areas. This is from component design criteria. Such criteria may preclude abrasive grit blasting from being existent in some areas. And how can this be accomplished? Well there are two ways in which abrasive grit blasting is prevented in certain areas: one is by masking tape and the other is by hard tooling. The former is generally used in one piece or otherwise low volume applications and the latter is used in high volume applications. In this post, we will limit ourselves to abrasive grit blasting on high volume applications.

When it comes to high volume applications, abrasive grit blasting tooling becomes very important. We are referring to the grit blast masking tooling. Such tooling is generally made out of an organic material such as ultra high molecular weight polypropylene or sometimes even polyvinyl chloride. At other times, there could be a steel masking tooling with a piece of masking tape underneath it. This latter is used when there is aggressive blasting and the tape is used as a secondary level of protection against overblasting.

In such cases, it is imperative to perform a hard tooling maintenance check on the grit blasting tooling. Because over time, the repeated abrasive blasting environment that the tooling “sees” can end up fraying the edges and the next thing you know the sharp cut offs that may be required are no longer evident. Unlike in tooling used for coatings masking operations, hard tooling used in abrasive grit blasting seldom see a build up; rather they see a wear down.

There have been cases where I have observed abrasive grit blasting tooling failing simply because of mechanical abuse. It is thus important to keep a watchful eye on such hard tooling because all of the subsequent operations will get affected if the blasting operation is not performed accurately.

Used grit blasting cabinets

2009-06-12T09:35:00.000-07:00

Companies involved in metal finishing and surface modification businesses usually have abrasive grit blasting as a process either by itself or as a pre-cursor to subsequent operations. And in this day and age as everyone tries to reduce the capital costs of equipment, more and more organizations are turning to used grit blasting cabinets as a means of reducing equipment costs. Whether you are operating a one-man shop or a multi-employee facility, you may find that investing in used grit blasting cabinets may prove to be a money saving plan.

The reason that used grit blasting cabinets have been so popular is that generally such machines are usually built quite ruggedly and not much can go wrong with them. Hence it is not as risky as buying a used cnc lathe where so many moving parts are there and many of them need to be in tight toleranced conditions. Secondly, these used grit blasting cabinets are designed to take a sizeable level of mechanical abuse right from the get go and so generally people have had few problems with these.

In proceeding to purchase used grit blasting cabinets, there are some factors that you may want to consider. For example, the age of the cabinet is something you may want to ask about. The really old ones may not necessarily suit your requirements, but cabinets that are just a couple of years old may not be that bad. Secondly, you can look at the physical appearance of the cabinet and while looks may not mean much in other areas of life, they do certainly tell a lot when it comes to used grit blasting cabinets.

Such used grit blasting cabinets can be found at auctions and plant liquidations. On-line sources such as ebay and some other on-line machinery distributors and used machinery sites may also be of value in locating them. Sometimes a bad competitor of yours who did not watch his costs or did not bother having an aggressive sales team may be a good source of finding such used grit blast cabinets. And finally the salesman that sells you your abrasive grit blast media may be a good source also, since he is in the industry that you are interested in.

Once you have purchased your used grit blast cabinets though, it may be wise to spend a couple of bucks in installing new hoses, a new nozzle and probably replace the sight window and install new gloves. A coat of paint may simply be the last step you may want to undertake to make your used grit blast cabinet look like new.

Abrasive grit blasting recycling

2009-06-06T16:14:00.000-07:00

Abrasive grit blasting processes use abrasive grit blasting media as their consumables. And needless to say there is a big interest in recycling abrasive grit blast media. Now this interest comes from several angles. First of all the accountants and bean counters always complain that too much money is spent on abrasive grit blasting media. Of course you can always retort saying that too much money is spent on the accounting department, but then that would not go too well. Second of all, there is an interest on the part of everyone to have a recycling program so that we do not waste our resources; this is therefore from the point of view of environmental impact. And of course everyone wants to be a good citizen of the world. Thirdly, the interest in recycling abrasive grit blast media comes from the business and company bottom line angle. If the spent grit from one job can be re-used again, then the company saves money which means the bottom line improves which means everyone earns more ( especially the accountants )

So in this post, we will deal with abrasive grit blasting media recycling. There are two terms in this regard that get used a lot. One is media recycling and the other is media reclamation; and over time, the two terms have kind of come to be used interchangeably. In my opinion though, the two mean two different things. The former is to do with recycling the material; the latter is to do with trapping and preventing the material from blowing all over the factory. In either case let us stick with recycling abrasive grit blast media for now in the true sense of the word.

The simplest way to recycle is to take the virgin material and after its use a specified number of times on new jobs, to take it and use it in something that is less demanding of an application within the same shop. For example, after you have finished two batches of blasting aerospace hardware and have deemed the grit to be unuseable, you use it in blasting some pump casings that are to be used in sewer pumps ( not really life threatening components ). This will save some money.

A second way is to take the spent grit and simply sell it to someone else who may have a need for even less of an exacting application. For example, you can sell the grit to an industrial flooring contractor, who may use it in developing anti-skid industrial flooring. Or you may sell it to someone who is in the business to grit blasting tomb stones Do not laugh, because I worked at a company that was an aerospace contractor who sold their used grit to a small time operator who manufactured tomb stones and gave a nice blasted finish to these monuments prior to selling them

This second way is what I get impressed about; because you could use your imagination in finding and locating companies that can use your spent grit. And the reward is that you end up making some money.

Abrasive grit blasting instructions

2009-06-05T11:36:00.001-07:00

Abrasive grit blasting is used as a surface preparation technique in many metal finishing and coating industries and is considered a significant operation by many aerospace component manufacturers. When the abrasive grit blasting procedure is not properly performed, the subsequent operations that are performed on the part may get affected from the point of view of component integrity. This is because the abrasive grit blasting process modifies the surface and makes it activated to receive subsequent treatments and if it is not performed to the necessary specifications, then the quality of the component may be suspect.

It is thus imperative that instructions to the abrasive grit blasting operator be as specific as possible and as clear as possible to avoid any potential confusion and doubts. These instructions must be in a written form using simple language. Additionally, it is preferable that an engineering department or manufacturing department person go over these instructions with the abrasive grit blasting operator and clarify any questions that may still arise.

Such abrasive grit blasting instructions must include the abrasive media to be used, any blending ratios that are to be followed, air pressure settings, stand-off distance, blast gun angle, speed of movement, maximum number of re-use for the media, etc. A quality assurance checklist that the operator signs off after every job is a necessary step to ensure that the operator did indeed follow the instructions specified in the grit blast operation sheet.

For those organizations that use different kinds of media for different jobs, it is necessary to specify the proper media to be used for each job. Additionally, it is mandatory to specify that the grit blast cabinet be completely cleared of the media from the previous job before introducing new media for the next job; this is important because otherwise media may get contaminated and that leads to quality problems.

Abrasive grit blasting media

2009-06-03T13:21:00.000-07:00

Abrasive grit blasting was defined in an earlier post as a method of surface modification hy impinging abrasive particles on the surface usually with a force. The abrasive particles are what are termed as the abrasive media. While some abrasive media are easily identifiable, there are some that the average person would not have dreamed of using as one

The most common abrasive grit blasting media are obviously silica, alumina and silicon carbide. However, silica generally creates a lot of dust and is not preferable to use as an abrasive grit blasting media from the stand-point of operator health issues. Continued exposure to silica dust can lead to pulmonary diseases. Alumina, which is basically aluminum oxide can be used either as standard purity or high purity grades. Silicon carbide is a very hard material, but is also relatively more expensive.

Steel grit which is from recycled steel is sometimes used as media for abrasive blasting. A variation of steel grit is the steel shot which is basically a round form of steel grit. Steel shot blasting is sometimes called shot peening, because the rounded steel shots do not generally create a roughened surface for subsequent coating, for example, but create a surface with large compressive stresses. This is sometimes used after heat treating or some coating operations.

All that is fine and okay you may think, but did you know that walnut shells can be used as a blasting media? Or how about ground corn cob? Surprised? Well, these materials ARE ( and have been ) used for quite some time now and offer a bio-degradable blasting media.

When it comes to abrasive media, sometimes grits of different sizes are mixed to produce the perfect surface roughness requirements. For example, a number 80 aluminum oxide grit may produce a very fine surface finish and a number 36 aluminum oxide grit may produce a very coarse surface finish, but a blend of the two in the correct proportion, under the right conditions of pressure, stand-off distance and speed of movement may produce the perfect surface roughness. This is usually determined only by trial and error.

Glass bead material is also used sometimes as an abrasive blasting media. This is generally used as far as I have seen in the finishing operation to produce a nice cosmetically pleasing appearance and not for the purpose of a roughness requirement.

Abrasive grit blasting safety

2009-06-02T13:06:00.000-07:00

Many metal finishing and coating industries incorporate abrasive grit blasting in their operations. This has resulted in abrasive sand blasting processes being used more and more in industry as more and more components undergo some form of surface modification or another in order to extend the life of components used in industry. With abrasive grit blasting being in greater use today than ever before in history, the associated safety problems become important to consider also. In this post, we will briefly talk about abrasive grit blasting safety.

Every company that is involved in abrasive grit blasting must have a special section devoted to abrasive grit blasting safety. This is because this operation can become a focal point of safety issues considering the nature of the operation itself. Some of the things that companies can require the operators to do compulsorily may sound simple, but can go a long way in reducing the danger of accidents in grit blasting departments and can additionally provide the operator with a healthy working environment.

Believe it or not, operators MUST be required to keep the grit blasting room clean by sweeping and vacuuming off the floor several times a day. This is because fallen grit, steel shots and other media used in abrasive grit blasting can pose fall hazards being that they are slippery in nature. This seemingly simple requirement can lead to lower fall risks.

All abrasive grit blasting operators must be required to wear necessary respiratory protective gear. Because of potential pulmonary health issues that can be caused by the inhalation of fine abrasive media, respiratory protection is almost always a must in grit blast departments. If face masks with breathing air supply are required to be used, then periodically check the breathing air tank for sufficient supply as well ensure that operators are properly trained to use these pieces of equipment properly.

Certain materials such as cadmium are not allowed to be blasted with pneumatic grit blasting equipment, because the dust from the process is considered to be carcinogenic. Such materials must be subjected to wet blasting only.

Use of proper safety eyewear must be made mandatory in abrasive grit blasting operations, because particles of media that may fly around can damage operator vision. Noise is another major problem in grit blasting rooms. A well certified ear protection equipment usage must also be made mandatory to the operators.

Because of the nature of the process, it is recommended that operators be required to wear overalls or aprons, be required to wear long gloves and preferably canvas blasting suits that are designed specifically for this process.

As an organization, provide sufficient ventilation to grit blast areas and avoid certain kinds of media such as silica, copper slag, staurolite, etc.

Abrasive grit blasting equipment

2009-06-01T11:58:00.000-07:00

Abrasive grit blasting processes are being used extensively in metal finishing and surface modification industries. Abrasive grit blasting is used either simply to clean the surface of scale and other contamination or as a surface preparation step for subsequent operations such as thermal spray coatings, etc. Because of increased demand on product quality and increased pressures on keeping part costs low, companies involved in abrasive grit blasting processes have to be quite vigilant and compare the various pros and cons of the different types if abrasive grit blast equipment available in the market today.

Abrasive grit blasting equipment manufacturers have come a long way in the design and fabrication of grit blasting equipment in the last thirty years or so. Advances in materials technology has resulted in better performing nozzles, longer lasting hoses and capabilities of longer continuous running machines. A key advancement that is often being overlooked is the development of sophisticated sound muffling materials that has resulted in better acoustic booths for walk-in grit blast cabinets design.

In the old days, about the only thing that abrasive grit blast equipment manufacturers made were simple cabinets with a couple of hoses attached to a gun, a view in window and two inserting holes with gloves. Gone are those days. Nowadays, sophisticated controls and automated grit blast cabinets are available that provide for automated part loading, part rotation and gun movements and manipulation.

A significant advancement in abrasive grit blast equipment in recent days is the advanced data logging that some manufacturers are beginning to offer, especially to aerospace sub-contractors who have the burden of maintaining statistical data for quality control purposes. This data that can be downloaded into a computer can then be used to provide accurate information about the performance of the equipment as well as the history of operation on every part.

Another significant advancement is the introduction of robotic abrasive grit blast equipment. This has enabled fewer operator involvement in the repetitive grit blasting operations, increased repeatability of gun motions and therefore repeatability of part quality.

Dust collector technology has made major strides in recent decades with hepa filter technology that is available for reasonable prices for companies big and small. This has resulted in the modern blast room being a more pleasant place to work than what was in existence fifty years ago. Additionally, the requirements of the environmental laws and regulations can be met comfortably by using these advanced dust collection systems.

Abrasive grit blasting parameters

2009-05-31T09:03:00.000-07:00

Abrasive grit blasting is used quite extensively both in metal finishing industries as well as in the general fabricating and outdoor fields. Limiting ourselves to the abrasive grit blasting used in metal finishing industries, one would be interested in the controlling parameters of the process. Only when the necessary parameters for abrasive grit blasting are first of all identified, can they then be controlled. And only when you have a controlled set of parameters, can you ensure a consistent surface morphology that can be repeated over and over again in order to ensure quality from batch to batch and from part to part.

So what are the abrasive grit blasting parameters that can be identified for control? The correct answer is that there are too many of them to go into very great detail in one post. However, limiting ourselves only to pneumatic abrasive grit blasting and limiting ourselves only to operations performed in a controlled grit blast cabinet, we can identify a few key items here. Of course, more discussions on this topic will always be the subject of future posts.

The first and foremost parameter is the air pressure. This pressure usually given in pounds per square inch is the controlling force parameter. Excessive pressure or very low pressure are both to be avoided and a reasonable upper and lower limit needs to be established. The second parameter is the stand-off distance. This is the distance, usually given in inches, between the nozzle and the work surface. Too high or too low stand-off distances are both to be avoided. The next parameter is the blast gun movement speed. If the grit blasting gun is moved too rapidly, then the necessary profile will not be achieved; conversely, if the grit blasting gun dwells on any one place too long or moves too slowly, then the metal peaks will get folded on to themselves resulting in a poor surface roughness again. Hence there is an optimum speed that has to be maintained and controlled.

The grit blast gun angle is another important parameter to be considered. Not all grit blasting is done at ninety degrees to the surface and so when an angle other than ninety degrees is being employed, the angle of attack of the impinging particles results in a lower force than otherwise and the surface profile will be altered. This angle that really depends upon each individual application needs to be controlled and monitored.

If the part that is being abrasive grit blasted is on a turn-table that is rotating, then the rotating speed that is usually prescribed in surface feet per minute is another important parameter that has to be controlled and monitored. An important parameter that is often overlooked is the nozzle geometry of the grit blasting nozzle itself. Many times, the nozzle diameter may have gotten eroded and therefore become larger and needs replacement, failing which the force component will change and next thing you know the quality control between batch to batch will start suffering.

Hence there are several key parameters that need to be identified and controlled in order to ensure repeatable performance in abrasive grit blast operations.

What is abrasive grit blasting

2009-05-30T06:55:00.000-07:00

Abrasive grit blasting is quite a common process used in manufacturing companies nowadays. However, unlike machining or welding, which are easily understood, the term abrasive grit blasting needs a formal definition in order to avoid confusion. Put simply, I would like to define abrasive grit blasting as the process of modifying a surface by propelling abrasive material against it, usually by force. This is the simplest definition of abrasive grit blasting.

Abrasive grit blasting also goes under different names. Commonly used terms of abrasive grit blasting include grit blasting, sand blasting, shot blasting, vapor blasting and abrasive blasting. However, some of these terms are not exactly interchangeable and in fact some of these commonly used terms for abrasive grit blasting do mean different variations of the basic process.

When used appropriately, the process of abrasive grit blasting as defined above is actually a surface modification process. It modifies a surface in that the process can be used to clean surface contaminants such as dirt, old paint, etc and thus expose a clean surface; it can also be used to modify or change the roughness of a surface by making it rougher or alter the profile of a surface.

Abrasive grit blasting is sometimes used as the only surface modification process as in the case of cleaning graffiti out of elaborate carved stone figures or of cleaning the paint out of wooden structures or cleaning tombstones. Abrasive grit blasting is many times used as a surface preparation step for subsequent coating or finishing operations. In some cases, abrasive grit blasting is used to remove or strip an existing coating and then the same process using a different set of parameters is used to prepare the surface for re-application of fresh coating. This is the case in strip and re-coat operations of some industrial components.

The second part of our definition of abrasive grit blasting has to do with propelling the abrasive material using some sort of force. The force is generated either by suction, pressure or both. And the carrier of the media can be either air, water or other liquids or both. Herein lies the opportunity for various permutations and combinations leading to different types of the process notwithstanding the variations of materials used.

There are two basic categories here: one is portable blasting and the other is stationary blasting. As the name indicates, portable blasting is generally used for on-site work and stationary grit blasting is done in a stationary cabinet or blast room. In the former, the abrasive grit is not recycled, whereas in the latter, the abrasive grit is recycled and re-enters the blast cabinet, thus saving in media usage.

A particular terminology subset of the abrasive blasting process is pencil blasting, wherein a fine nozzle with a fine but powerful abrasive is used to etch on glass and is used by artists in the decorating industry.

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2009-05-29T14:26:00.000-07:00

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