3. What Materials are Used for Bead Blasting?
Most commonly, lead free soda-lime glass beads are the media of choice for bead blasting. These can be supplied in a wide range of bead sizes, to select the coarseness of the surface finish desired. This glass does not contain free silica molecules, which are a hazard when inhaled.
It’s considered environment friendly in its manufacture and it is inert matter, i.e., it has virtually no chemical reactions in the normal environment which is part of the reason that it will not attach to (or colonize) the blasted surface. Since it is a tough glass, produced by a stress relieving process, beads can be re-used 20-40 times before an excessive number of beads will shatter and influence the surface with sharp points, making it very cost effective.
While very effective in peening softer materials (plastics, Aluminum etc.), it is not suited to peening harder or tougher surfaces such as Titanium and steel. Its low cost makes it ideal, however, as a cleaning medium for harder materials, when no surface effects are desired, other than contaminant removal.
Steel shot is an important bead type, manufactured (similarly to most bead types) by spraying molten steel into an inert gas stream and then sorting the resulting range of particle sizes by grading/sieving. Sizes range from 0.125mm diameter (S70) to 3mm diameter (S930) and harnesses from 40 to 62 Rockwell C values. This medium is virtually indestructible in the normal process of bead blasting, so some suppliers recommend screening to remove detritus and re-use indefinitely, other suggest changing steel shot beads at 100 uses. Durability is related to size and hardness – smaller, softer beads will not break whereas larger harder ones will eventually.
Steel shot is widely employed for a range of degrees of aggression in surface preparation of Iron, steel and other harder materials such as ceramics. This can vary from cleaning or removing corrosion through to deep and visible shot peening, for surface hardening by ‘work hardening’ and imposing residual compressive stress in the ‘skin’.
Aluminum Oxide particles are not generally beads but fractured ceramic shards which fracture further and blunt in use. This crosses the line from bead blasting to grit/sand blasting and is generally used for surface abrasion rather than reformation.
Aluminum Oxide is a long way up the hardness scale (approaching diamond hardness) and this makes it ideal for surface alteration of nearly all materials, where a sharp and angular micropattern is desired whose point scale depends on the particle sizes used and material removal is the target. This can even work well in surface refinishing hardened steel components and is often used as a way to smooth machined and cast surfaces and to prepare for painting – though it isn’t suitable for soft materials like plastics and wood, unless significant material removal is desired.
Aluminum oxide blasting of surfaces results in very uniform, matte surface finish and it is, for example, the precursor stage for matte anodizing of Aluminum. Care must be taken, as Aluminum oxide is generally brown and this can cause some surface staining, which is addressed by a chemical cleaning stage in the preparation for satin anodizing, for example.
- Nylon, polystyrene, acrylic, polycarbonate and others – soft and durable, used for deflashing and deburring and surface preparation of delicate and high value parts.
- Urea formaldehyde particles – slightly harder, used for deburring and deflashing but can also be used for paint and corrosion removal from delicate surfaces.
- Melamine formaldehyde particles – same applications as Type II but a little harder and more aggressive, where deeper abrasion is required.
Being lightweight and elastic beads, they can be used for precisely defining the degree of surface effect that is achieved, removing brittle and weak adherent materials like oxides and pait, while having no effect on the substrate. Careful selection and use can even allow deflashing of plastic moldings without influencing the part finish. Plastic media are unsuitable for creating surface finishes, as they are simply too soft to peen any but the very softest materials.
Chopped Wire Media
There is other media available, though less commonly used; chopped strands of fine wire, often 304 stainless steel, high Carbon steel and Zinc alloys. These media are used to provide narrowly specified patterns/sharpnesses of surface finish on selected materials, particularly where a very uniform particle size is required – and they tend to preserve their size and shape for longer than common bead types. They are also smoother, as they contain no pits or splits and make very low dust contamination of blasted surfaces.
- Stainless steel (ferric) chopped wire is used in applications where ferrous contamination and residues cannot be tolerated.
- High Carbon steel chopped wire is used because it does not fracture and leave residues, but very slowly wears to spherical shape.
- Zinc chopped wire is also used where ferrous contamination cannot be tolerated.