In CNC machining dimensions and tolerances are very important, but other factors are important – especially from the customer's perspective. Very often requirements include a high-quality surface finish, without any tool marks or chips.
To deliver such product machining factories often use a vast variety of finishing processes. One of them is cleaning the surface (copper, aluminum, steel) with specialized abrasive material in the stream of high-pressure air. Most often used abrasive agents are sand, special glass balls, ground nut shells, and even semolina.
What is cleaning the surface with glass blasting?
The surface after glass blasting becomes smooth and clean for a long time.
The reason for that is simple – during that process increases hardness of the surface and its resistance to accumulation of dirt and dust. Glass blasting closes micropores in material and smoothes the surface.
To perform that process requires a specialized blasting cabinet and well well-selected abrasive agent. Such selection is dependent on the process's material and expected surface quality.
Parts subjected to that process are closed in special chambers where they are cleaned with high-pressure air with added agents.
What types of abrasive agents are used in glass blasting?
In the process of selecting the best-suited abrasive agent operators often use the Mohs scale. It is a 10-grade scale that describes material hardness based on their susceptibility to scratch harder material by softer ones.
For example – if we want to glass blast brick (Mohs scale – 6) we should use a softer abrasive agent, for example, baking soda (Mohs scale – 2.5).
Using harder agents can lead to removing parts from the surface and can lead to damage in the process.
Among the most popular abrasive agents used for glass blasting, it is worth mentioning:
- baking soda (Mohs scale – 2.5) – it is a very soft abrasive agent, used often to clean lacquered surfaces,
- lime flour (Mohs scale – 3) – used often for relics renovation,
- plastic abrasive (Mohs scale – 3.5) – soft abrasive used in automotive and airplane industries to sandblast many types of steel (stainless, carbon steel) mainly due to the marginal possibility of causing sparking,
- quartz sand (Mohs scale – 5 to 7) – the cheapest abrasive on the market, used to sandblast almost any material,
- glass microspheres (Mohs scale – 5 to 7) – often used in metal manufacturing industries. Smoothens and shines the surface. Most often used in preparation of the aluminum surface before anodization or powder painting,
- ceramic microspheres (Mohs scale – 7 to 8) – the most expensive out of popular abrasives, used mainly to remove surface stress after the plastic machining or welding,
- synthetic corundum (Mohs scale 9) – a type of aluminum oxide; the hardest abrasive used to rough the surface, often used to process acid-resistant steel.
What are the advantages of glass blasting?
Practical applications of the bead blasting process are focused on removing impurities and small damages from the detailed surface.
That includes remains of oils, stains, and discolorations.
On top of that glass blasting helps to achieve a shining surface – especially when performed on the aluminum alloy.
That process is often used by shops and factories that offer galvanic services – like oxidation of the surface (eg. aluminum anodization) or saturation with elements (eg. nickel coating).
Glass blasting has also a preventive role – can preserve a wide range of details from dirt.
Glass blasting and dimensions of the part
Poor selection of abrasive agents can cause changes in the dimensions of processed parts. Such removal of even a small part of the top layer of material can lead to deviation from the specification, especially in high-demanding applications. Good examples are parts made out of copper.
To avoid such problems, it is good to perform additional tests of selected blast media on metal surfaces and measure the final effect. For example – the edges of copper parts after glass bead blasting can deform. Measurements performed with a contourographer show the radius on the edge – such change can cause problems with further installation of delivered parts.
Such testing can help to identify too hard abrasive media before mass-processing and select the best-suited blasting technique for the material and client requirements.
The simplest solution would be a selection of a softer agent, a series of tests, and measurements. Thanks to such tests, the industry learned that the semolina – available in almost any grocery – is a very efficient (although: rarely used) medium for abrasive blasting. Copper parts can be efficiently blasted with such an agent, without the risk of damaging edges of the material.
What factors can impact surface quality after glass blasting?
Glass blasting is a process that leads to achieving the expected quality of the surface. Among many, the factors that can impact such results are:
- quality of the abrasive agent – the better, the less unwanted dust, and such material can be reused or used more efficiently,
- structure and shape – that is why it is crucial to avoid unwanted mixing of abrasive agents – the same dimensions or similar in appearance (eg. glass beads media of different dimensions),
- nozzle operating pressure – it should be set and calibrated for a particular surface, agent, and expected result. It should be lower if the aesthetics are the most important; if glass blasting should remove unwanted material from the surface (eg. lacquer or rust) – the pressure should be higher,
- the angle of the air stream – is a very important factor in maintaining the dimensions of parts,
- experience and skills of the blaster operator – that allow to achieve high repeatability of the process (also in situations when blasting is performed on automatized devices – then such knowledge is crucial for the right setting up the blasting procedure).
In the context of well-performed blasting, it is important to remember the risk management, selection of the right agent, and nozzle operating pressure.
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