Six Factors Impacting Blast Cleaning Performance
Abrasive Blasting is a process that requires precision and reproducibility. Therefore, the equipment designed to accomplish such a task must be properly maintained and calibrated.
In many applications, accuracy and repeatability can be guaranteed with the use of automated systems and fully automatic process control.
In automated systems, blast nozzles can be mounted on robotic arms in order to ensure accurate blasting distance, ideal travel blast stream exposure. Blast nozzles can also be fixed or mounted on a rotary head as parts pass through the media stream by the use of conveyor or rotary tables.
However, most of the time, the sandblasting process is handled manually with versatile equipment that can be used for various applications. Therefore, some basic process parameters must be set carefully in order to achieve desirable results. Here are the six most prominent factors to keep in mind when defining the process parameters.
- Abrasive Media:
Abrasive media is critical to abrasive blasting processes, as it has a direct impact on the workpiece and produces expected outcomes.
Shape: Typically, there are two types of media. Angular-type media is usually used for creating an anchoring profile on metallic surfaces, while spherical-type media is usually preferred for peening and some cleaning applications.
Grit Size: Media average particle size (so-called MESH) can vary from one application to another. Usually, the bigger the media particles, the greater the anchoring profile will be. This could result in excessive coating material required to cover the surface, leading to a costly and longer coating process. Using a proper-sized grit is critical to ensuring both quality and efficiency in the process.
Hardness Level: Workpiece composition and process objectives define the right media to use for a given application. In cleaning-only applications, the hardness level of the media should be less than that of the workpiece in order to avoid modifying its structure too much. When creating an anchoring profile on a surface, a media with a hardness level that is higher than the workpiece is appropriate.
Refer to the Abrasive Media Selection Guide for more information on available media and their benefits.
- Blast Nozzle:
Many different designs of nozzles are available. The design impacts the stream pattern as well as the velocity of the media as it exits the nozzle.
Straight Bore nozzles produce a very narrow pattern. They are suitable for high-precision applications and are widely used in sandblast cabinets.
Long Venturi nozzles have a tight throat opening and a large exit end, which results in a wider pattern and significantly increases the velocity of the media leaving the nozzle for more aggressive blasting action.
Double Venturi nozzles can be up to twice more efficient than a regular nozzle due to the introduction of air right at the exit end of the nozzle, which reduces the loss of velocity.
Angle bore nozzles allow the operator to blast inside cavities and hard to reach spots of complex workpieces.
Also, many different materials are used to produce nozzles – Boron Carbide, Tungsten Carbide, Ceramic, etc. However, the material will not affect the short term performance, but rather the durability and weight of the nozzle. Premature wear on nozzle, on the other hand, could result in loss of velocity and/or uneven stream pattern.
The design of the nozzle, the interior diameter (I.D.) of its orifice, the air pressure at the nozzle (PSI), and the blasting distance from the workpiece are factors which directly impact the stream pattern, coverage of the blast stream, abrasive media consumption, and efficiency of the blasting action.
As the nozzle begins to wear out, the orifice I.D. expands and produces a larger coverage along with weaker impact. This happens because the amount of air leaving the nozzle remains the same, while the opening becomes wider. The rule of thumbs is that the nozzle should be changed as soon as its I.D. becomes one size larger than its original size.
- Velocity of the Media Leaving the Nozzle:
The velocity of the media leaving the nozzle (FPM), as well as the nozzle design and orifice (I.D.), are key factors impacting the media stream and velocity onto the workpiece. In other words, it is the fuel which allows the system to perform well for a given application.
These two factors combined will determine the air requirements of the system (CFM). Consequently, the minimum air requirement of the air compressor is determined by the required pressure at the nozzle (PSI) and its orifice size (I.D.).
Furthermore, it is recommended to supply 50% more air than is required at the nozzle in order to ensure a stable performance at the nozzle tip. That way, the air supplied at the nozzle is consistent, and the nozzle remains efficient even as it wears out (an adjustment needs to be made in order to compensate the velocity loss by a pressure increase).
To avoid velocity fluctuation at the nozzle, an air regulator should be installed at the air inlet of the system and set to the desired pressure.
The physical properties and size of the abrasive media could also affect the media stream and must be taken into consideration when calibrating blasting equipment.
Refer to the Air Consumption Chart to calculate the minimum air requirement that the compressor should be able to supply for a given nozzle ID and PSI.
- Quality of air supplied by compressor:
To optimize the performance of the equipment and avoid premature system failure, it is critical to supply clean, dry air to the blasting equipment.
Unfortunately, air compressor naturally supplies air with excessive moisture content because of the condensation of water present in ambient air, and humid atmospheric air conditions will produce even more humidity in air exiting the compressor. This is a natural occurrence that unfortunately cannot be avoided.
If humidity enters the plumbing system of the blasting equipment, moisture will eventually contaminate abrasive media and could cause air/media valve blockage and premature wear on system plumbing, as well as affect the blasting performance. Fixing a valve blockage takes time and could lead to complete failure and replacement of the valve.
For these reasons, it is very important to fit an air dryer and a water separator at the compressor air outlet to get rid of water and other contaminants that could be found in compressed air (such as oil). There are many solutions out there to clean and dry air as it exits the compressor. Some devices can be installed directly on the outlet of the compressor and remove humidity throughout the entire compressed air network, but these solutions could be very expensive and not all production equipment can be damaged by humidity. We recommend at least using a simple and cost-effective Super-Dry system on the specific airline that supplies the sandblasting equipment.
- Operator’s technique:
When all aforementioned process configurations have been carefully observed and selected, the next major factor impacting the blasting result is the operator’s technique. More precisely, here are different variables of a technique that could impact the results of a sandblasting application:
Blasting distance from the workpiece: as the blast nozzle moves away from the workpiece, the media stream becomes wider, but the velocity of the media impacting the workpiece decreases.
Angle of impact: forms of media which impact perpendicularly on the workpiece have a greater impact than the ones impacting at an angle. Also, angle blasting results in an uneven stream pattern where some areas of the pattern have a greater impact than others.
Blast Pattern: the blast pattern can be wide in order to maximize productivity on large surfaces, or very tight for spot blasting and precise blasting applications such as parts cleaning, stone carving, and weld seam grinding. The design of the nozzle will determine the pattern of the media stream.
Time Spent on Area: the speed at which the blast stream moves on the surface, or similarly, the number of passages or the blast path, are all factors affecting the amount of media particles hitting the workpiece. As the time spent or passages on the area increase, the amount of media impacting on the surface increases at the same pace.
Blasting Path: The blasting path used by the operator to expose the part’s surface to the abrasive media stream has a major impact on the overall process performance. A poor blast technique severely affects the performance of the process by either increasing the overall process time spent by the operator to cover the whole surface (labour cost), media consumption (raw material cost), and system wear and tear (maintenance cost), or by damaging the surface of the workpiece (rejection rate cost).
Refer to the Overlapping Gap Method Blasting Path for the optimal blasting pattern and passage technique that will optimize the process time and costs.
- Maintenance of the equipment
As mentioned earlier, precision and reproducibility are crucial in abrasive blasting, and this can only be achieved with properly maintained and calibrated equipment and components.
Blast Nozzle: should be checked periodically to make sure that the orifice is not oversized due to wear caused by abrasives passing through it. When the orifice I.D. of the nozzle expands one size over its original size, it should be replaced to ensure consistent results and optimal blasting performance. Refer to the Blasting Nozzle Selection Guide.
Blast Hose: must also be examined regularly to make sure that there is no air leak that could result in loss of pressure. When the blast hose is easy to fold, it means that the wall is becoming very thin and will eventually fail – it should be replaced right away.
Fittings and Couplings: must be installed properly to avoid premature wear on couplings and fittings. The hose must be cut at 90º to make sure it stands evenly on coupling, without a gap that would either result in loss of air pressure through the system and/or premature wear on the coupling by leading abrasives onto an exposed area.
Abrasive Media Reclaim System: should be tuned properly to make sure it cleans abrasive mix by filtering out dust, fine particles, and shattered media from reusable media.
Dust collectors: must be maintained to avoid impacting the operator’s visibility and the system’s wear and tear. Dust drawers and solid screens must be emptied regularly to ensure air is circulating without restriction through the dust collection system. Filtering cartridges or bags should be cleaned regularly and replaced when needed to guarantee optimal working performance.
Refer to your equipment’s Maintenance Sheet or contact your IST technical representative to learn more about maintenance routines that should be performed from time to time.