Principle and features

Foreword
The "laser" beam process cannot be classified in the previous categories because it does not use a gas, a liquid or a mechanical impulse as a vector but light impulses. It is a special process but it is included in the category of the impact treatments because of its ionic bombardment principle. It is a mechanical action of the light, which does not use and does not consume abrasive product. It is one of the original particularities of this system.

Historical background
The laser technique was created in the middle of the XX century. EINSTEIN had stated earlier, in 1920, the theory of the radiationstimulated emission which constitutes the laser basic principle. However, the first laser was created in 1959 only by an American physicist called MAIMAN.
The laser apparels were first a research tool in the fundamental physics laboratories and from the 70's were used as weappon in the military area and then in the industrial area as a tool used to pierce, cut, weld, guide, clean... It is at the end of the 80's only that the scientists studied the case and planned the development of a laser machine adapted for cleaning.

Physical principle
The laser cleaning is based on a photomechanical reaction induced by the interaction between the beam of light from a laser of high intensity short impulses and the polluting coat (greases, oils, oxides, paints, varnishes, cooking residue...).

Features
Transmission of the laser beam through optical fibre
Wavelenght (color) 1.06mm (close to infrared)
Impulse duration 10/12 nanosecond (thousand millionth of a second)
Impulse energy up to 1200 milliJoules
Energy density from 0.2 to 5 Joules/cm2
Impulses frequency from 1 to 120 Hertz
Homogeneous repartition of the energy on the whole surface of the beam.

Result
The light impulses emitted by the laser are absorbed by the superficial coat of the surface to treat. The high energy absorbed creates a plasma (ionized and unbalanced gas) highly compressed which is reduced in pressure generating a shock wave. This shock wave fragments and ejects the polluting coat in fine particles which are then sucked up through a suction system. The light impulse is short enough to avoid heat phenomena which could damage the surface.

Parameters

Energy intensity
In order to be efficient, it has to be adapted and adjusted between two thresholds:
- the cleaning threshold where the plasma is generated in the polluting coat,
- the damage threshold where the plasma begins to alter the substratum.
Each laser impulse removes a certain thickness of the polluting element.
If the coat is thick, several impulses are necessary to clean perfectly the substratum.
A very important effect of the combination of two thresholds is the cleaning autolimitation. The light impulses which energy density is higher than the first threshold will dig the dirt until the substratum. In this place, the energy density is inferior to the damage threshold of the substratum. No interaction is possible.

Nota bene:
This measuring out faculty (determined through previous tests) makes the laser treatment a spectral selective technique which makes it possible to attack on a same surface some products rather than others.

Important parameters
Laser power
The cleaning speed is proportional to the average power of the laser source. This corresponds to the energy brought in a second. Two adjustment parameters can be taken into account: the energy per impulse and the impulses repetition frequency. The average power is the product of both of these values.
Beam homogeneity
The cleaning homogeneity is directly linked to the repartiton of the light energy in the beam. A laser beam is generally bell-shaped like a "Gauss curve". It has much more energy density in the center than on the sides. The use of an optical fibre makes it possible to obtain a straight beam and its whole energy will be used in an optimal and homogeneous way.

Machines and applications

General points
The machines are generally made of:
- an energy generator with dimensions and power adapted to the treatment to make,
- a beam projection gun with shape and size adapted to the generator,
- a link optical cable between the generator and the gun for the energy transmission.
The surface treatment is done through a simple sweeping of the operator.
In some cases, it is possible to interface the projection gun with another gun (i.e. plasma projection) in order to reduce the operating times. In this case, both guns are set up on a robot which acquired and stored beforehand the geometry of the parts to treat.

Applications
- cleaning of stone, bricks, plaster, wood...
- cleaning of plasturgie moulds, elastomers, composite structures, glassware,
- surface preparation before pasting, heat coating, soldering,
- scouring of paint, vernish, solderingcordons,
- surface preparation before deposits,
- metals deoxidation,
- surface removal of grease,
- eliminating radioactive particles.

Advantages
Laser cleaning does not damage:
- no abrasive effect, (no abrasive)
- no mechanical contact,
- no heat effect,
Laser cleaning does not pollute:
- no material supply,
- no solvent,
- no polluted effluents,
- no fumes.
Laser cleaning does not generate photochemical effect.
The operator protection is reduced to a simple eye protection.