Parameters of the impact

Basis concept: Reminder
The impacts surface treatment is based on the principle of a continuous grains projection which strikes on a more or less strong way a part, generating different induced effects. In this process, each grain has at the time of the impact, a kinetic energy function of its speed and its mass.
E = 1/2 mv2
The initial speed of the grain produced by the projection system varies according to the principle used (conducting vector or centrifugation) and the residual speed with the impact varies according to the air braking, in which the grain shape and density intervene. It is thus important to take into account the parameters which follow and vary according to the considered application.

The impact intensity
It is directly related to both basis values above and it is proportional to the grain size and the square of its speed. A small variation of one of these two parameters (in particular speed) can modify lightly the impact intensity. This intensity will be function of the work to be achieved, the hardness of the layer to be removed, the desired surface finish or the imposed "Almen" intensity.

Granulometry
The grain size conditions not only their weight (variable according the density of the product used), but also their speed and finally the number of grains to the unit of weight. This number increases in a spectacular way when the size decreases. It means that with equal working conditions, a fine grain works more quickly than a big grain because of the infinitely larger impacts number per unit of weight or time. In practice, the finest grain is always the best compromise between the output and the consumption.
Covering rate: Cn = 1- ( 1 - C1 )n

Speed
This is the preponderant element in the impact intensity, but it cannot be measured directly. As a matter of fact, under equal conditions, the grains speed is all the more high since their size is small and as a consequence, the strike force is higher. On one hand, it means that for a fine grain, the initial energy is better used, with a faster covering capacity, but on the other hand, wearing or fragmentation is accelerated. It will be necessary to find the good compromise between the efficiency and the consumption in order to obtain the best quality/ price ratio always present in mind.

Parameters of the projection

Outflow
This is the number of grains thrown with by unit of time.
With a free shot, it is function of:
- the nozzle diameter (pressure system)
- the injector diameter (suction system)
- the use pressure
- the granulometry of the thrown product
- the nozzles number or guns used.
With a blast wheel, it is function of:
- the wheel diameter
- the blades dimension
- the rotation speed
- the granulometry of the thrown product
- the blast wheels number.
This is the important element because it affects the work speed directly and therefore the labour cost.

The projection distance
The distance between the nozzle or the blast wheel and the surface to be treated affects directly the speed of the grain when it arrives on the target (brake created by the air) and therefore the impact intensity. As a matter of fact, the distance affects thus the work speed but also the consumption.
Some preliminary tests will allow finding the ideal compromise, taking into account that a longer distance gives a faster and more uniform covering of the surface. This because of the increase in the opening angle of the nozzle shot or the blast wheel recovery area.

The projection angle
This angle, existing between the perpendicular of the treated surface and the throw direction, affects at the same time the consumption and the efficiency of the impact. With equal working conditions, it is the 90° throw perpendicularly to the surface that will cause the higher consumption because of the bounding off of a great number of grains which while rebounding make a shielding effect and are self-destructed.
The experience shown that the more efficient and the more economical work is obtained with a projection angle ranging between 45 et 60°. Under these conditions, the consumption can be a 30% lower than the one of a perpendicular shot and the output inversely proportional.
For the shot peening operations, the intensity of the impact imposed can be obtained only by increasing the projection angle until 80°, or even 85°, without ever reaching 90°.

The exposure
This is the time during which the surface to be treated is subjected to the grains bombardment. From its duration will follow a surface finish more or less clean or mechanically treated.
Time has a direct incidence on the cost, thus one will endeavour to reduce it by increasing the outflow and by the implementation of movements/ part or movements/ shot as efficient as possible in order to obtain the best cover possible.