The surfaces preparation

Cleanness and roughness
The main aim of impact surface treatment is generally the cleaning of a part in order to remove the rust, the calamine, an old paint or any other contaminated agent, creating simultaneously, if necessary, a fixing relief for the adhesion of the future coating.
One must consider the two distinct but complementary following notions:
- the cleanness which is a chemical state
- the roughness which is a physical state
These notions are often mixed up.
One must admit that cleanness, with its reference to the Swedish or French standards is one thing, and roughness measured for example by comparison with a rugotest is another. A great cleanness can be obtained with micro-beads or vegetal abrasives without any guarantee of adhesion. Conversely, a great roughness (comparable to the coarse N° 18 of the rugotest) can be obtained with an aluminium oxide without reaching a medium cleanness (type SA2). (see heading qualities). The main work consists in always finding the right compromise between these two contradictory requirements and thus obtaining the best cost price, cleanness, the required roughness according to the rule book.

Evolution
The companies reasoning in a spirit of saving know that a good protection against corrosion is cheaper than the replacement of the constitutive parts damaged by rust. Today, for this reason, we try to use coatings of better quality instead of the cheap paints used formerly. If the surface is treated beforehand on a proper way, these coatings last much more. If the residues of calamine, rust or former paint are not totally eliminated, it is of no relevance to use a better quality coating because the corrosion process will go on under the coat and will destroy the fixing base.
If it is properly done, the impact surface treatment is "the most economical method" to obtain the necessary fixing so that the coating last longer. Nowadays, all the coatings manufacturers advocate very precise directives for the previous surfaces treatment which are intended to receive their products. It is advisable to respect them.

The other applications

The impact effects
As explained above, the choice of the kind of abrasive is preponderant in the result obtained.
Indeed, the projectile plays the major role. Therefore, "one must handle this tool".
With a hard and angular abrasive, the part surface is attacked with each impact like a blow of a pick.
With a hard and spherical abrasive, the part surface is hammered with each impact like a blow of a hammer.
In the first case, the result obtained on the surface is an increase in roughness with the opening of the surface structure. It gives a more or less tortured and dull appearance.
In the second case, the result obtained is the crushing of the surface. The direct consequence of this is the closing of the surface micro pores that modifies the surface roughness and gives it a gloss satiny appearance.
According to the shape of the abrasive grain used, we notice that different results can be obtained and direct the "impacts treatment" towards:
- to the surface preparation,
- or the surface cleaning and finishing,
- or the surface mechanical treatment.
The effect induced by the impact of the abrasive generates different techniques.

The abrasion effects
Two main actions stand out:
1) The support deep attack in order to remove a contaminated agent which sticks a lot and which has a certain plasticity.
This is the stripping.
2) The creation, on the surface, of a more or less pronounced roughness in order to:
- increase the contact surface to obtain a better adhesion of the future coating.
- expose possibly the active centers of a metal for a better absorption of a deposit.
This is the surface preparation.
3) Besides the use of hard abrasives which modify appreciably the surface of the treated parts, it is possible to use angular but very soft abrasives (vegetal or plastic) which make it possible to clean the parts without attacking their surface (glue, varnish or any other residue removing).
This is the cleaning.

The hammering effects
The hammering action is required for several consequences:
1) The removal of contaminated agents in some very precise cases (i.e.: founding or forging):
This process is used either to eliminate the sand covering some products.
This is the desanding.
2) Either to break the resistant calamine sheets before a traditional shot-blasting treatment.
This is the decarbonizing.
3) Modification of the surface geometrical profile by modeling and without material removing. The material must have plasticity qualities. With a low intensity, the effect will be purely aesthetic.
This is the satin finishing.
4) Modification of the part geometry, always by the material move, very visible on the fragile parts, such as sharp angles, threads, tools marks or rough patches.
This is the flattening.
5) When the material will be a metal with certain elasticity characteristics, the bent surface, because it will have exceeded the tensile yield strength will enclose a subjacent coat of compressive metal. This metal will not have exceeded its tensile yield strength and will enclose a force that wants to be released. This will be the positive element of a better resistance to the fatigue of the parts concerned.
This is the shot-peening (see heading shot-peening).
6) The partial release of this force will cause the product strain if this force acts quite close to a neutral fiber, thus forming a couple. All the products with low sections (sheet metals, strips, wires) for which the strain is feared are concerned. However, if it is well controlled.
This is the peen forming (see heading shot-peening).
De facto, the application fields of the impacts treatment are vast and varied. They meet very precise industrial needs and are not limited to the only examples mentioned below. Any solid product can be projected and generate a different result to satisfy as well the metal industry as the jewellery, the glass-making, art, the wood, the plastic and the composites industry or any other (see heading compatibilities).