October 23, 2003
If you are a fabricator of metal parts that need to be primed or painted, then you more than likely will have to prepare the part surface before finishing to produce the desired end result.
Many options are available that can achieve the necessary prefinish requirements of any part. Of those options, blast cleaning can be an efficient and economical choice. Blast cleaning requires no chemicals and does not remove any surface metal in its process. Some facts however, should be considered if you select the blast cleaning option.
Not all the equipment is the same. The price of the equipment alone does not account for maintenance requirements, part replacement intervals, efficiency, or part finish quality. As a purchaser of such equipment, what must you know or understand about blast cleaning to make an informed decision before you buy?
|Air Injected blast cleaning wheel|
Selecting the proper equipment requires consideration of the following:
The metal condition and finish specifications are linked. The condition of the metal—the amount of scale, rust, or corrosion and the desired surface profile specification—determines the amount of blast material and the length of blasting time required to achieve the desired outcome. The surface profile specification is based on the depth of the part's surface cross section versus the thickness of the applied finish. The Steel Structures Painting Council (SSPC) for surface cleaning quality has set standards. For example:
The SSPC provides written as well as visual guidelines that show the physical differences among cleaning standards. You can use these guidelines to help you specify equipment. The targeted specification determines how much blast cleaning is required on the workpiece and will ultimately affect the equipment design and manufacturing costs. Knowing how clean a part needs to be sets the groundwork for determining what type of equipment you will need.
|Impeller control/cage blast cleaning wheel|
The SSPC equipment design criterion also addresses other factors, such as part size and surface configuration—even or uneven. The part size or size and weight range has a major impact on the type of blasting equipment that will be needed to do the job. Complex fabricated parts or assemblies will require very different productivity and part quality.
A material handling system that can turn the part, load, unload, and provide a conveying system also adds to the complexity in determining the overall equipment requirements needed for a specific application. The production rate and finish quality are other factors that help determine blasting requirements.
One of the more important components of blast cleaning equipment is the blasting wheel, an integral part of the blasting process that determines part quality. The number of wheels used, their size and horsepower, and how they are positioned on the blast cabinet directly affect fabrication cost.
Blasting wheels propel the shot or grit onto the workpiece in specific patterns. The size and RPM of the blast wheel determine the amount of blast media that can be thrown at the workpiece within a given time frame. The type of wheel used and wheel design can greatly affect the efficiency of the process. Blast patterns and impact angles affect productivity and quality.
Wheels should incorporate wear-resistant materials. The abrasive nature of the shot media can cause extreme wear on wheel blades unless they are made of hard, durable metal. The wheels direct the abrasive media against the workpiece in a steady, high-velocity stream. Blast patterns should be adjustable for optimizing blast cleaning effectiveness.
The majority of centrifugal blasting wheels used throughout the industry are standard impeller, control cage wheels. The blast media is held in a storage hopper and gravity-fed to the impeller control cage at the center of the wheel. As the media feeds into the wheel, it is centrifugally thrown onto the workpiece.
Some manufacturers provide other types of wheel designs, such as air-injected blasting wheels, which utilize turbine-driven air that preaccelerates the abrasive and delivers it to the blades. This method fluidizes and spreads the abrasive uniformly over the face of the blade and eliminates hot spots that occur with conventional wheel designs. Hot spots occur when abrasive dumps onto the wheel blades in a single area. This limits the spread and shape of the blast pattern and can affect the efficiency and quality of the blasting process, producing unacceptable results.
Air-injected blast wheels also can use different types of abrasive materials, allowing for special-application use such as with aluminum and stainless steel. Part shape and size will be the determining factor in selecting a wheel design that is best-suited for your specific application.
Other factors that determine blast cleaning equipment cost are horsepower ratings, the number of blades per wheel, and the number of wheels per system or unit. Bigger isn't necessarily better. Some large wheels are less efficient than some smaller-horsepower-rated units. As an example, some wheels are rated at 25 HP yet provide only the same efficiency as units rated at 15 HP. Study carefully the ratings information provided by the equipment manufacturer. The most important factors to consider are the amount of blast material that is delivered to the workpiece and where the wheels are positioned for the most effective and efficient cleaning.
Maintaining the blast cleaning equipment properly can have a significant impact on operating costs. If the abrasive blast material is not checked periodically for breakdown in its size and shape, the grit can eat away blades and other components at an alarming rate. Also, with loss of desired surface profile, visual inspections should be made at regular intervals, specified by the manufacturer to ensure the longest wheel components life.
Recommendations for correct abrasive size and composition for a specific application should be provided by the manufacturer. Many abrasive options are available, and choosing the correct one for a specific application is critical to minimizing the operating cost of the equipment and maximizing longevity and successful cleaning. Each manufacturer has its own maintenance schedules and training procedures for its equipment. These schedules and procedures should be strictly adhered to for maximum equipment efficiency.
Michael Gignac is a Marketing Manager at Jet Wheelblast Equipment, 401 Miles Drive, Adrian, Michigan 49221, 517-263-0502, fax 517-263-0038, firstname.lastname@example.org.