Laser ablation of multiple layers

Details Laser ablation of multiple layers Laser ablation of multiple layers

2002-03-18

2003-04-01

Staicovici, Stefan (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Laser ablative shaping or piercing

C264S139000, C264S482000, C219S121680, C219S121690, C219S121850

Reexamination Certificate

active

06540952

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to laser beam ablation. In particular, the present invention relates to a laser ablation technique in which a corner portion of a laminate is ablated in a step-like manner.
For purposes of the present application, ablation is defined as the removal of material from a plastic polymeric substrate with the aid of incident light from a laser beam. Typically in plastics, ablation is either by vaporization or degradation of the substrate. During degradation, laser radiation is used to break the polymeric bonds between the monomers thus forming a gas or a liquid at the focus of the beam. The material is usually either vaporized and ejected by the vapor pressure of the gas or the vapor pressure of the gas is enough to remove any liquid material. A gas jet may be used to assist this process but typically the vaporization of the material provides the majority of the energy to remove the material. The existing technology that provides quality ablation usually requires a laser operating the in ultra- violet using either excimer lasers or Tripled Nd; YAG lasers operating a wavelength of 0.355 microns.
Nd:YAG lasers and excimers have drawbacks when degrading polymeric materials because of the low wattage available (9 watts is a very high power in this class of laser) and the fact these lasers are typically pulsed providing limitations to the speed of removal of material. Industrial processing of polymers requires a much higher power is typically ranging from 25 watts to 200 watts and even up to 2 kilowatts for some applications. Thus the Nd:YAG and excimer lasers are not preferable choices of processing plastics in volume because of the limitations in speed and high cost for even the lower power available.
As mentioned, an alternative technique for laser ablation is vaporization. During vaporization, the substrate material absorbs energy delivered by the laser beam. The absorbed laser energy is converted to thermal energy, and at a certain temperature, dependent upon the characteristics of the substrate material being used, vaporization occurs. These characteristics of the substrate material include absorption depth and heat of vaporization. Because the ability to absorb laser energy is dependent upon the material used, the characteristics of the substrate material also limit the depth at which useful ablation can occur. The depth of the laser ablation is also determined by the laser beam pulse duration, the laser beam energy density, and the laser beam wavelength.
Taking the laser beam wavelength into account, it is preferable that a wavelength be chosen to minimize absorption depth. By minimizing absorption depth, thermal conduction to other areas of a kerf formed by the laser beam are also minimized. This is advantageous in high-precision laser ablation because the kerf is constrained to a more precise area. By constraining the kerf to a precise area, secondary effects of thermal conduction upon the area surrounding the kerf are minimized. These secondary effects include the formation of a plume and a heat affected zone.
Referring to
FIG. 1
, the plume is a plasma-like substance comprising reacted chemical by-products, molecular fragments, free electrons and ions. The plume is formed when material vaporizes beneath a surface of the material substrate and is not allowed to immediately exit from the kerf; a resultant from having too deep of an absorption depth. The plasma-like plume will optically absorb and scatter the incident laser beam, and can also condense upon the surface of the material substrate immediately surrounding the kerf. This effect leads to deformations on the surface of the substrate, which in most situations is not desirable and not acceptable. By choosing an appropriate wavelength of the substrate material, the absorption depth is minimized which also minimizes the amount of plume present during ablation, if not eradicating the presence of the plume altogether.
Another secondary side effect of thermal conduction upon the surrounding kerf area is that of the heat affected zone. The heat affected zone is defined as the edges of the kerf area immediately following the laser beam where molten material solidifies. The breadth of the heat affected zone is dependent upon the thermal properties of the substrate material. The higher the thermal diffusivity of the substrate material, the greater the extent of the heat affected zone. And as with the plume, absorption depth of the substrate material is also an important factor to take into account. If the absorptive depth of the substrate material is too deep, the vaporized material will not be able to exit the kerf immediately, and the surrounding area will degrade to the extent that thermal conduction through the substrate material allows the energy to pass therethrough. This leads to abnormalities or deformities along the edge of the kerf, including the surface edges of the kerf becoming molten and deforming as molten material within the kerf provide compressive forces along the kerf walls, wherein the kerf walls are defined by the interface between the molten and solidified substrate material. The molten surface edge of the kerf tends to rise above the surrounding surface of the substrate material, then solidify. This produces a deformation or “roll-over,” which is disadvantageous for certain applications of the substrate material. An example where it is disadvantageous occurs when the substrate material is a laminate sheet that is placed upon another laminate which was also ablated. The deformation on the surface edge of the kerf prevents full and intimate contact of an adjoining laminate sheet.
Also, when several passes of a laser beam occur relatively near one another, each pass selectively ablating different laminate layers, the heat affected zones of each pass may overlap one another, and either remove unwanted substrate material or deform an edge of a substrate layer proximate the heat affected zone when the molten material, as described above, contacts a layer proximate the ablation point. This deleterious effect becomes more prominent when the layers of the laminate are thin, especially under 2 mils (0.002 inches). Thus, as best illustrated in
FIG. 3
, upon making several passes within close proximity to one another, the ablation of one pass affects the preceding pass, and an unwanted notch or divot occurs in the polymeric laminate at the ablation edge, or kerf edge.
BRIEF SUMMARY OF THE INVENTION
The present invention includes a polymeric laminate having a generally arcuate, step-like corner portion and method of forming same. The laser ablation technique prevents divots from forming when selectively ablating corner portions of polymeric laminates in a step-like manner. Each layer of the laminate is ablated in a generally arcuate path. The path of a first layer neither overlays nor underlies the arcuate path ablated in any other layer proximate the first layer.


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