Bead Blasting Finish: The Complete Guide in 2022
Updated: December 03, 2023
Introduction
Bead blasting is a critically important surface finish process for CNC machining metal parts. It can provide consistent and appealing surface finish on a wide range of materials and convert surfaces from coarse and irregular to smooth and attractive, provide excellent keying for paints and lift overall product quality dramatically.
This article is a detailed introduction to the principles of bead blasting, covering media, applications, limitations and benefits, in a short but comprehensive introduction.
1. What is Bead Blasting?
Bead blasting introduces a stream of particles or beads into a high-pressure air nozzle, propelling the beads at high speed towards a surface. This can be to remove material contaminants such as corrosion on steel or pollution residues on stone. If more aggressive in application, it can be used to alter the surface properties of material by peening.
When similar techniques are used to abrade material away, this generally involves silica sand (sand blasting) and not smooth beads. This is quite different in practice from bead blasting, which aims to achieve a cosmetic surface finish, or prepare for surface coatings such as paints or chemical treatments, without influencing the dimensions of the surface being blasted.
Bead blasting is therefore a widely used process for the finishing of, or preparation of many product surfaces in many materials. It can be the last operation on the surface of a product, to render it clean and free of detritus. It can also be the final preparation before plating or painting.
At its most aggressive, bead blasting acts as shot peening, which is the cold working of a part, generating an outer surface of residual stress that modifies the characteristics of some materials. In steels, for example, this can ‘work harden’ the outer skin.
2. How Does Bead Blasting Work?
Bead blasting involves projecting beads of glass, glasslike and metallic particles onto a surface being addressed. This generally leads to a bead dimpled and clean surface, as each bead impact point becomes a micro feature in the treated surface.
This can result in any level of surface alteration from; clean and otherwise unmarked; through slight, fine marking to make a ‘satin’ look; to deep beed pits that are often referred to as ‘shot peened’.
Bead blasting can be used to mask surface defects in components and can achieve very uniform results between components. It tends to make brighter and more attractive surfaces.
The beads are delivered at high speed in a pressurized air stream, either from a handheld gun or from an automated nozzle. Generally, these are referred to as siphon or suction blast guns, and the beads are pulled into the main air streat of a standard air nozzle from a side vent that is connected to the bead reservoir.
The suction results from the constriction at the throat of the nozzle which induces acceleration and reduced pressure (bernoulli’s effect) which pulls an air stream from the bead reservoir, carrying the beads to the main air stream. A second type of gun uses a hopper to drop beads into the air stream, but this approach is less widely used as it limits the bead supply available to that which is carried.
Reference Post: Bead Blasting vs Sand Blasting: Two Metal Surface Finish Types Compared
3. What Materials are Used for Bead Blasting?
Glass Bead
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
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
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.
Plastic Media
- 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.
4. Applications of Bead Blast Finish
Bead blasting is a process that is suited to the range of surface quality improvements from the most delicate of operations such as deflashing plastic moldings, to more aggressive approaches such as corrosion removal from CNC milling metal parts, through to altering the physical properties of the surface of metal parts by stress relief/work hardening.
- Surgical component and implant surface preparation; implants that must key to bone and integrate into new bone growth work better when they have a textured surface. This is commonly achieved by various types and degrees of bead blasting. Medial implant media must be biologically inert, so that any trapped contaminants do not interfere in healing – so glass and Aluminum oxide beads re preferred.
- Plastic mold tools become encrusted and contaminated with scorched polymer residues, in heavy use. It is normal to clean tool components by bead blasting, generally with non aggressive media. This is particularly true of hot runner nozzles that can become constricted by such buildup, severely affecting production capacity.
- Cleaning of delicate products is similarly important, often involving walnut shell blasting media as a method of applying very low abrasion while removing contaminants such as chemical residues and oils.
- Iron castings are aggressively surface modified by peening with steel shot, to render surfaces uniform and remove any burrs and surface irregularities. This is widely used for cast iron cookware and machine parts,
- Electronic components can have manufacture residues removed by bead blasting with plastic beads of various grades, for example to remove conformal coatings in PCB repair.
- Many levels of peening are applied to products to achieve desired qualities of surface finish, removing machining and casting features.
- Aluminum parts are Aluminum oxide bead blasted prior to anodizing, to produce a desirable satin anodized finish.
- Preparing surface for keying to external media or paints. Bead blasting with sharp media can massively improve the adhesion of materials to the part surface quality.
5. Tips to help to get the best Bead Blasting Finish
There are many and varied applications for bead blasting, and each media type, each substrate and each desired surface finish has its own specialist requirements, so a comprehensive list is too extensive for an introductory document like this, but some general rules for best results can be outlined easily.
- Use low air pressure. This makes for a more controllable blasting process, especially with aggressive media such as Aluminum oxide. A start points of 50 PSI nozzle pressure is widely recommended for siphon blasters, as it extends bead life while giving good results. In particular, high-pressure results in more fractured beads, which coats the good beads in finer particles which alter the surface properties and can become embedded in the blasted part.
- Start with softer media and work up the grades, to avoid damaging parts by excessively aggressive attack. The softer the parts, the more important this can be, as the degree of damage to, for example, high value aviation composite components is significant.
- For fast, fine etching use Silicon carbide media, this allows close control and low pressure use, but aggressive etching of ceramic and stone materials.
- Keep the blast cabinet in good order – well sealed to prevent media and contaminant leakage and with a clear window that is protected with a removable/disposable cover film so you can see what’s going on.
- Keep the media clean by filtering out dust, part beads and removed oxides/paint/contaminants.
6. Summary
Overall, bead blasting is an important industrial and restoration process that is widely used across all branches of manufacturing. It is often a manual process for high value components. It can equally be automated to allow its use in higher volume, lower value applications. Where the component needs this type of surface modification, there are few good alternatives. Bead blasting of various types is a critical tool in the manufacturing solutions options.
For help with this and other related matters, please contact us and we will be pleased to discuss your needs.
