For a successful blasting operation, it is critical to choose the correct blasting media. The desired end result, the blasting principle and the base material that will be blasted all have to be taken into account.
Expendable or Reusable abrasives
The first distinction is between an expandable and a reusable abrasive.
Expendable abrasives are blasting media that can only be used once. These media are cheaper per kg or per ton, but only last one cycle. These expendable abrasives are often used in open blasting operations where there is no possibility to recycle. When blasting a bridge or structure outside, often abrasives like aluminum silicate, copper slag or Testra are being used.
Reusable abrasives are more suitable when there is a possibility to recycle the blasting media. In a blast room, blast cabinet or wheel blasting machine, it is possible to collect, clean and recycle the abrasive. In that case reusable abrasives, like steel shot, steel grit, fused alumina, glass beads, plastic blast media and stainless steel abrasives are used.
There are also possibilities to use a mobile abrasive recycling unit on location, for the use of garnet. Advantages of reusable abrasives are:
- The blaster has a better view at the work and is able to perform his task better, because it creates less dust.
- Cost efficient because it takes several cycles before it turns into dust. The price per ton may be higher, but the costs per square meter blasted is considerably. Besides that, the disposal of waste and dust are getting increasingly more expensive. With reusable abrasives there is less dust to dispose of, because it doesn’t turn into dust after one blasting cycle.
- More environmental friendly, because there is less waste to dispose of.
Metallic abrasives or Non-metallic abrasives
The second important distinction is between metallic and non-metallic abrasives.
Metallic abrasives
If there is a possibility to recycle the abrasive and the substrate allows it, metallic abrasives are the most cost efficient blasting media.
On steel substrates where iron contamination is not an issue, especially steelshot and steelgrit are a very favorable blasting media. Considering the cost price per ton and the extremely high recyclability, steel shot and grit are the most cost efficient abrasive.
If iron contamination is an issue, for example on stainless steel substrates, there is also the possibility to use stainless steel shot and stainless steel grit. This material has a good life time and creates little dust, but is very costly per ton. Therefor one should be very aware of drag out losses, unintentionally removing the abrasive out of the closed system, for example in the pockets of the work piece.
Metallic abrasives have a high density (approx. 7 cm3 / gr) and are therefor less suitable on delicate or very thin substrates.
In some shot blasting applications carbon steel cut wire or stainless steel cut wire can also be used.
Non-metallic abrasives
Non-metallic abrasives are synthetic, natural or mineral abrasives, that contain no metal. When iron contamination is an issue, when blasting delicate surfaces or when blasting at location outside, non-metallic abrasives are the most applicable. When blasting in a blast room or blast cabinet, often fused alumina and glass beads are being used. For very delicate cleaning, without damaging the base material plastic blast media, soda blast media and walnut shell blast media are used. When blasting on location the non-metallic abrasives garnet, aluminum silicate (coal slag), iron silicates (like copper slag) calcium silicate (Testra) can be used.
Which blasting principle and blasting equipment are used
The choice for blasting media is highly dependent on the equipment that is being used. When open blasting a bridge a different media will be used then blasting small engine parts in a blast cabinet.
- In wheel blasting machines, mainly high carbon steel shot and high carbon steel grit GP (hardness 42-52 HRC) are used. For some applications stainless steel shot or cut wire can be a good option.
- In blast rooms mainly steel grit GH (hardness 60-64 HRC), brown fused alumina, stainless steel grit and glass beads are being used. When using a high quality garnet, then this can also be suitable for a blast room in some cases.
- In blast cabinets a reusable abrasive is most suitable, because it generates less dust in the small working area. In a pressure blast cabinet all reusable abrasives can be utilized. In a vacuum injection blast cabinet, the metallic abrasives are too heavy to efficiently enter the system.
- In open blasting at location an expendable abrasive like copper- or coal slags are normally the best choice. When there is a possibility to effectively recycle the abrasive at the working site, a good quality garnet can also be used.
- In vacuum blasting a reusable abrasive like steel grit and brown fused alumina are often used. The vacuum blast machine needs to be powerful enough to process the steel grit, like our AB1070.
- In wet abrasive blasting various non-metallic abrasive can be used, like garnet, aluminium silicate, glass granules, glass beads.
Which purpose for blasting
Creating an anchor profile
Creating a surface profile is often an import goal for blasting. The anchor profile are the craters in the workpiece that creates extra surface for the coating to adhere. A correct anchor profile is critical for the life time of the coating and a heavy coating system would require a higher roughness than a standard coating system. The roughness is normally expressed in:
- Ra, or Average Surface Roughness, is the arithmetic average of the absolute values of the surface height deviations measured from the mean line over one sampling length.
- Rz, or Average Maximum Profile Height, is the average of the five highest peaks and the five deepest valleys over the length of the assessment.
The paint manufacturer will specify in his datasheet the required surface profile after blasting, for his coating system to adhere.
At Airblast-Abrasives we gladly advice on the required abrasive and grain size to reach your surface profile.
Cleaning
When only having to remove mill scale from steel plates or profiles, steel shot S330 or steel shot S390 will be suitable in a wheel blast machine.
When having to remove a thick layer of rust or a heavy coating, it is better to use a larger size of steel grit GP in combination with steel shot in a wheel blast machine and steel grit GH or fused alumina a blast room.
In the foundry industry our high carbon steel shot is used for desanding after casting, where the size of the casting determines to a large extent the size of steel shot to be used.
Gentle cleaning
When cleaning softer materials like wood or in façade cleaning a softer abrasives needs to be used, like garnet. When the purpose is to strip paint of a soft or thin base material, mainly soda, plastic blast media and walnut shell blast media can also be used.
Deburring
after production some sharp edges can remain on the product which need to be removed. This can be done by using (stainless) steel shot and grit.
Shot peening
Like a black smith hammering on the steel to strengthen it. Shot peening is the same principle only using many steel shot particles instead of the hammer. It is used to strengthen the steel of equipment, landing gear and springs for example.
Glass bead blasting
is used to make (stainless) steel surfaces smoother. The surface is blasted, reducing peaks and opening valleys without cavities (openings) at the micro level. A very fine surface profile is created, where dirt can only form deposits on the peaks. The adhesive strength is minimal, so that the surface remains cleaner and can be cleaned better.
Hardness, Shape and Size of the abrasive
The hardness, shape and size are the most important choices. During blasting with an reusable abrasive, the working mix is also an important factor.
Hardness / Toughness
The degree of hardness required is determined by:
- The cleaning speed required.
- The type of contaminant to be removed.
- the surface finish and profile required.
Abrasive hardness is important because hardness, by definition, is resistance to deformation. So the harder the abrasive grain, the less it deforms and the more it performs useful work on the surface. However, energy is wasted in the fracturing of pellets so there is no advantage in using an abrasive which is hard, but also extremely brittle. Maximum hardness combined with a low breakdown rate is required for optimum results.
Abrasive hardness correlates directly with cleaning speed in that the harder the abrasive, the faster it cleans. Hardness is also an important factor in cleaning quality, particularly when the work is intricate and has deep cavities. The harder the abrasive shot, consistent with lack of brittleness, the greater its ability to ricochet. Areas of parts that cannot be cleaned by the direct blast depend on rebounding abrasive for adequate cleaning.
In general we can say:
- The harder the abrasive, the more aggressive
- The harder the abrasive the higher the production and the faster the cleaning. Softer abrasives loose energy due to transformation of the grain. Softer abrasives have a longer life time, but are less efficient.
- Harder abrasive shots have a better ability to ricochet (bounce) and clean areas that don’t have a direct blast.
Shape
The particle shape is determined by:
- surface finish and profile requirements
- Type of contaminant to be removed
Where steel shot creates a more dimpled wavy surface, the angular grit particles creates an sharply edged profile.
The shape of an abrasive when new is not always the same as when in the operating mix. Steel shot remains round throughout and, therefore, produces minimum wear in the machine.
Most grits remain angular to give an effective etching action, though steel grit GP rapidly changes from angular to round when in use, thereby combining the performance capabilities of both grit and shot. GL grit remains angular, but loses its sharp edges during service optimising rapid cleaning without excessive wear in the plant.
Size
Given that impact treatment is based on kinetic energy, it follows that selecting a large pellet will give a greater impact than a small one and vice versa. The initial size selection, therefore, is very important for the process.
- E= ½ MV2 Kinetic Energy = ½Mass x Velocity2
- The larger steel shot S660 has 500 times the impact of a smaller steel shot S70
When removing heavy contamination this impact might be required to properly clean the work piece. In general we advise to use the smallest grain size that does the cleaning well and create the desired profile. Smaller grain sizes have much more particles that hit the surface and therefor have the following advantages:
- Smaller grain sizes have much better coverage
- Smaller grain sizes clean faster
- Smaller grain sizes have less impact fatigue to endure, so has a longer life time.
Working mix for reusable abrasives
The abrasive supplied by the manufacturer is predominantly one size. The size range in the operation is known as the ‘operating mix’ and consists of the nominal size shot or grit and the worn particles or broken down particles.
The operating mix provides a balance of particle sizes for impact and coverage. Impact is provided by the new abrasive or larger size pellets to remove heavy contaminant and coverage is provided by the medium and smaller pellets to remove light contaminant and to give the final finish to the work processed.
This balance between impact and coverage is important and can only be maintained by regular additions of new abrasive, preferably at the abrasive breakdown rate, by the wheel hour or by the golden rule of ‘little and often’. It is also necessary to return back to the system any abrasive losses or carry-out from the machine. As a result, the number of particles per kg thrown will be higher, coverage will be improved, cleaning will be faster and blast time will be reduced.
Choosing the correct abrasive, with the right shape, size and hardness, and maintaining the optimum operating mix are the most important elements in controlling costs. There are, however, other factors that must be taken into account if maximum efficiency is to be achieved. Our teams of sales engineers are ready to assist you in choosing the correct specification of abrasive for your application – please do not hesitate to contact the Airblast Office most conveniently located for your facility.
