Everything You Need to Know About Sand Blasting
What is sand blasting and what is it used for?
Sand blasting, also known as abrasive blasting, uses air pressure to shoot small abrasive particles at high speeds at any surface needing to be cleaned. The term “sand blasting” is used because the process typically uses particles of sand as the media. As the sand particles hit the surface, they create a smooth and more even texture that can then be painted or treated with a coating.
Sand blasting is one of the first steps in preparing a surface for painting or a coating. It is used in industries such as construction and auto body work where it is used to strip the existing paint and rust from a surface, such as the body of a vehicle or side of a house. Sand blasting can also be used to clean sidewalks and roads, as well as remove scale from steel.
Surface preparation is the most important part of a project and without a solid start, the rest of the project is likely to have issues. Not only does sand blasting clear away paint, rust, dust, and dirt, but it smooths the surface so that paint or coatings will not end up peeling. Learning how sand blast correctly will enable you to build a solid foundation for work in surface prep.
In this article you will learn about this surface finishing process, the different types of sand blasting, and how it’s done.
Sand Blasting vs Hand Sanding
One of the primary advantages of sandblasting over hand sanding is its efficiency. Cleaning and surface preparation can take three times as long if you sand by hand or use a brush. Sand blasting is also very thorough compared with hand sanding. While sanding by hand will smooth a surface, the end result will be dependent on a person's strength and attention to detail.
Sand blasting can also cover a far larger area than hand sanding and do it much more evenly. This permits more work to be done without sacrificing quality, in addition to maximizing speed. Sand blasting is much more likely to raise the quality above that of other surface preparation methods.
Physics 101 - How Sand Blasting Works
For those of you who liked Physics in high school. Here is a run down of how sand blasting actually works and some of the factors that affect performance, sound production, and cost.
The principle of operation of a sandblaster is extremely simple: A strong jet of air is generated which is mixed with a blasting agent. The required air pressure is generated by special compressors and the powder-like abrasive can either be mixed in dry or with water from a collecting container.
This special sand-air mixture is then blasted at high speed via a hose and nozzle system onto the surface to be treated. In the process, surface particles are loosened out due to the abrasive blasting effect and then removed. Surfaces are thoroughly cleaned of impurities, rust, paint or scale. Depending on the process, different degrees of cleaning can be achieved.
Sand Blasting Equipment
Below are the main components of a sand blasting machine:
- The blast pot can be considered the media reservoir. It also maintains the pressure necessary for blasting. Blast pots come in varying sizes, depending on productivity needs, and typically range from 0.5 cubic feet to as large as 20 cubic feet.
- Nozzles accelerate the air/abrasive as the mixture exits the end of the blast hose. The nozzle’s taper and inlet length determine the pattern and velocity of the abrasive exiting the nozzle.
- The inlet and outlet valves monitor the inflow and outflow of air and determine whether the pot is pressurized. They are integral components of the remote control system and overall blast machine setup.
- The pop-up valve responds to pressure put into the system and “pops up” to pressurize the system.
- The media valve is located at the bottom of the machine and regulates the abrasive flow from the blast pot. Abrasive media valves have two inlets and one outlet: one inlet is for abrasive, the other is for air, and the outlet is for the mixture of air and abrasive that will be carried through the blast hose. They are available as either manual or automatic (the former does not have a shutoff to stop the flow of abrasive).
- The abrasive trap prevents abrasive from traveling through the outlet valve.
- A blast machine uses a number of hoses: twin-line hoses, blast hose, and an 18-inch hose.
Sand Blasting Nozzles Nozzles
For vapor abrasive blasting, the most commonly-used nozzle orifice sizes range from 3/8" inner diameter to 3/4", increasing by increments of 1/16". A 3/8" nozzle is sufficiently constricted to produce a effective blast pressure with a 185 CFM compressor. A 1/2" nozzle is sufficient to produce an effective blast pressure with a 375 CFM compressor.
It’s important to note that when you double the diameter of the orifice, you quadruple the size of the orifice and the volume of air and abrasive that can pass through the nozzle.
To find your optimally productive nozzle, determine what nozzle pressure (PSI) you need to maintain for productive blasting, and what volume of air your available compressor can supply per minute (CFM), then consult the following chart to find the nozzle orifice size that meets those parameters.
Nozzle shapes and materials: Nozzles come in two basic shapes: straight bore and Venturi, with several variations of Venturi nozzles.
Example: Long Venturi Nozzle
Safety Equipment and Considerations
During sandblasting operations, industrial painting contractors are required to wear personal protective equipment, including safety goggles, respirators, coveralls, and helmets that are especially designed and inspected during the manufactured process to ensure adequate protection from potential job hazards.
Additionally, industrial painters have a responsibility to take reasonable care of their own health and safety and that of others. But for this, they need to be aware of the specific hazards that may occur during sandblasting as well as know all the safety precautions they can take against harmful exposures. Below are a few of the major safety considerations to be aware of when sandblasting.
- Air Contaminants
- Lack of Ventilation
- Vision Impairment
- Exposure to Elevated Sound Levels
Types of Sand Blasting:
Sandblasting, abrasive blasting, grit blasting, and media blasting all refer to the same method of surface preparation. They differ in the type of abrasive used to perform the work.
In the past, sand (silica dioxide) was used as the primary abrasive media. Now there are substantial other abrasives that do the cleansing. Below is a short list of different types of sand blasting:
Silica Sand Blasting: Silica is popularly known as silicon dioxide, which is ordinary sand that is used in the process of shearing and scraping any surface. It is commonly used to remove all the surface impurities and provide a clean and finished look. Just because silica consists of sharp edges, it provides a grit effect in abrasive blasting/sandblasting. Today, Silicon sand is not the primary choice for sandblasting because it causes respiratory issues. Most of the benefits of using silica sand can be met with the use of other materials. Its previous use was due to its sharp edge and uniform size.
Soda Sand Blasting: Soda sandblasting is a technique that provides a cleaning effect without causing any damage to the surface of the metals. This technique is commonly used for soft metals because it doesn’t disrupt the outer layer of the metal or chemically react with them. In this method, sodium bicarbonate particles are blasted against a surface using compressed air
Soda sandblasting is extensively used to remove rust or corrosion from the metal surfaces without creating any sort of depression or damage to the underlying soft delicate layer of metal.
Steel Grit Blasting: Steel Grit Blasting is a type of abrasive blasting technique that uses spherical steels as abrasives. This method is commonly used when attempting to clean metal surfaces. It’s very effective in removing paint or rust on other steel surfaces. The use of steel grit also has added advantages such as providing smoother surface finish and helping in peening which strengthens the metal.
Bead Blasting: Bead blasting is yet another technique that uses air pressure powered abrasive blasting methods along with glass beads. These glass beads are effective at cleaning, deburring, and peening metal surfaces. The glass beads are spherical in shape and when impact the surface creates a micro-dimple. The aim is to provide a much more uniform finish. These glass beads are 100% recyclable making it a cost-effective method.
The use of glass beads also results in a much cleaner and bright finish. The use of glass beads also ensures the abrasive is non-toxic and not harmful to the environment. Bead blasting is generally used for providing surfaces with a bright smooth finish.
Bristle Blasting: In the Bristle blasting method, no separate medium is used. Rather, the steel wire bristles are rotated on the surface of the metal. The rotation process helps in the removal of impurities, hence leaving the surface smooth. This method is commonly used to clean metal surfaces with some form of corrosion.
Wet Blasting: The key features of wet abrasive blasting include the ability to use very fine or coarse media with densities ranging from plastic to steel and the ability to use hot water and soap to enable synchronous degreasing and blasting to eliminate dust and remove hazardous material or waste (asbestos, radioactive, or other poisonous products) without danger to surfaces and structures, thereby achieving effective decontamination.
Choosing the right sand blasting media
There are many types of abrasive blasting applications, each of which requires the use of different types of blasting media:
- Glass Beads: Glass is not as aggressive a blasting media as other materials, such as steel shot or silicon carbide.
- Aluminum Oxide: Aluminum oxide is characterized by its superior hardness and strength.
- Plastics: Plastic abrasive is a dry thermoset cleaning media made from crushed urea, polyester or acrylic. Plastic is generally regarded as the best media for mold cleaning, blasting of plastic parts, or in applications where the removal of the substrate material is not permitted.
- Silicon Carbide: Silicon carbide is the hardest abrasive blasting material available, making it the best choice for your most challenging surface finishing applications.
- Steel Shot & Steel Grit: Steel abrasive is a cost-effective alternative to other abrasives due to its toughness and high recyclability.
- Starblast: Starblast™ is a mined loose blend of coarse and fine staurolite sands with extremely low levels of silica making it an ideal general purpose blasting abrasive.
- Walnut Shells: Walnut shell abrasive is a hard naturally occurring material made from crushed walnut shells. It is the harder of the soft abrasives, available in a variety of sizes for blast cleaning and polishing softer surfaces that could incur damage from harsher abrasives.
- Corn Cobs: Corn cob abrasive is a granular abrasive manufactured from crushing the dense woody ring of a corn cob into various grit sizes. It is the softer of the naturally occurring abrasives making it ideal for cleaning, deburring, burnishing and de-flashing applications.