CROSS-REFERENCE TO RELATED APPLICATION(S)
BACKGROUND OF THE INVENTION
Applicant claims the priority date of U.S. Provisional Application No. 60/276,312, filed Mar. 16, 2001.
The present invention relates to laser beam ablation. In particular, the present invention relates to a laser ablation technique which increases the quality of ablation on polymeric substrates, especially on polymeric substrates having a thickness of 2 mils (0.002 inches) or less.
Typically in plastics, ablation is either by vaporization or degradation of the polymeric 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.
- BRIEF SUMMARY OF THE INVENTION
In the case of plastics, and more preferably polymeric materials including polyester, a gas laser is used to vaporize and ablate the polyester substrate material. Preferably, the gas used as a lasing medium comprises carbon dioxide. When using carbon dioxide as a lasing medium, the natural output wavelength of the laser beam is approximately 10.6 microns. A 10.6 micron wavelength corresponds to an absorption depth of a few thousandths of an inch in polyester. However, when the substrate material has a thickness of about 2 mils (0.002 inches) or less, and a high-precision ablation is needed, the aforementioned absorption depth is too deep, and the plume and heat affected zone deform the surface edge of the kerf beyond tolerable limits. It is therefore desirous to minimize both the plume formed and the heat affected zone to increase the precision of the ablation, and improve quality characteristics of the ablation upon the polymeric substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention includes a method of ablating a polymeric material with a laser. The present inventive method preserves the molecular structure of the polymeric material along a portion adjacent to a kerf formed by the laser. The method comprises the steps of selecting the polymeric material for ablation, selecting a laser having an output wavelength corresponding to a wavelength exhibiting the greatest absorption in the polymeric material, and lasing the polymeric material with the laser. The output power and focal spot of the laser adjusted such that at least 70% of an incident power of the laser absorbs within about 0.001 inches of the polymeric material.
FIG. 1 is a side view of a laser ablating a substrate material.
FIG. 2 is a side view of an ablated substrate with a deformation at a surface edge of the kerf and other plume debris.
FIG. 3 is a side view of a polymeric material ablated by techniques of the present invention wherein deformations along the kerf edge have been eliminated.
The present inventive method is directed at increasing the precision and the overall quality of a laser ablation on a plastic substrate. Preferably, the plastic substrate is a polymeric substrate, and more preferably, the polymeric substrate has a thickness of less than 2 mils (0.002 inches). However, it is within the scope of the present invention that the present inventive method may be applied to all plastic substrates regardless of their thickness. For purposes of this description, overall quality of the laser ablation may be defined by the appearance of the ablation, including discoloration (if any) of the plastic, breakdown of the molecular structure along the ablation which produces the discoloration and shrinkage of the substrate, and any type of “roll-over” effects whereupon debris or material from the ablation migrates or settles upon a surface of the substrate, as illustrated in FIGS. 1 and 2. Also 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.
A 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.
FIG. 1 illustrates a kerf 10. The kerf 10 formed by the laser ablating the substrate includes a plume 12. Surrounding the plume 12 is a heat affected zone 14, which includes a region of molten fluid-like material 16, and a region of the substrate 18 which intimately contacts the molten region 16. The plume 12 is a plasma-like substance comprising reacted chemical by-products, molecular fragments, free electrons and ions. The plume 12 is formed when material vaporizes beneath a surface of the material substrate and is not allowed to immediately exit from the kerf 10; a resultant from having too deep of an absorption depth. The plasma-like plume 12 will optically absorb and scatter the incident laser beam, and can also condense upon the surface of the material substrate immediately surrounding the kerf, 10 as illustrated in FIG. 2. 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 melted material present during ablation, if not eradicating the presence of the melt altogether.
The molten region 16 forms about the plume 12, and consists of substrate material having a temperature greater than a molecular degradation temperature but less than the vaporization temperature. Energy contained within the molten region 16 conductively transfers to the surrounding region 18, which in turn raises the temperature of the surrounding region 18. If the temperature of the surrounding region 18 rises above the degradation temperature of the substrate, ill effects occur. Such ill effects include discoloration of the substrate material, along with shrinkage of the material. This shrinkage of the substrate material results in deformation of the surface of ablation, and contributes to the “rollback” effect. The negative effects of discoloration and rollback are both time and temperature sensitive. Thus, it has been discovered that by reducing both the temperature of region 18 surrounding the molten material 16, and reducing the amount of time the molten 16 material is in contact with the surrounding region 18, the quality of the ablation is greatly improved.
The present inventive method takes into account three parameters that each aid in reducing the heat affected zone of the kerf. The parameters include the output wavelength of the laser, the output power of the laser, and the focal spot of the laser upon the substrate.
Taking the laser beam wavelength into account, it is preferable that a wavelength be chosen that maximizes the absorption of the laser. A wavelength having the least transmissivity through the polymeric substrate should be chosen to maximize the absorption of the laser to minimize absorption depth. By minimizing absorption depth, thermal conduction to other areas of a kerf (as illustrated in FIG. 1) 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. This is best illustrated in FIG. 2, wherein the rollover and deposition of debris from the plume are prevented from occurring.
Another secondary side effect of thermal conduction upon the surrounding kerf area is that of the heat affected zone. 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, the kerf walls being 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 solidifies, contributing to the rollover effects.
Additionally, by not confining the heat affected zone, thermal conduction raises the temperature of walls along the outer boundaries of the kerf, which may lead to a breakdown of the molecular structure of the polymeric material. This molecular breakdown leads to discoloration of the polymeric material, and shrinkage of the polymeric material. In plastic substrates there are two temperate regions that define when and to what degree discoloration and molecular breakdown occur. These temperate regions are the glass-transitional phase and the degradation temperature. At or above the glass-transitional temperature, the solid substrate begins to exhibit fluid properties. The substrate becomes less rigid as structural bonds between molecules begin to break. However, upon cooling the substrate, these bonds readily reattach and little discoloration, if any, occurs.
When the temperature of the substrate approaches the degradation temperature, irreversible effects to the molecular structure begin to occur within the substrate. Along with the bonds between the polymers breaking, the molecular structure of the polymers begins to erode. Upon cooling, many of these polymeric molecules are “shortened” which produces the shrinking effect within the substrate. Also, discoloration and even blackening of the substrate material occur as the substrate temperature approaches and exceeds the degradation temperature of the material.
Other examples of ill-effects produced by an over-broad heat affected zone exist. In the situation where laser ablation is used to selectively ablate layers of a laminate comprising more than one substrate layer, a precise heat affected zone is advantageous for several reasons. Containing the heat affected zone to a minimum reduces damage done to an underlying adjacent layer of the laminate not being ablated. Also, when several passes of a laser beam occur relatively near one another, each pass selectively ablating different laminate layers, the heat affected zone of each pass may overlap one another, and either remove unwanted substrate material or deform an edge of a substrate layer with the molten material, as described above, contacting a layer adjacent the ablation point. Thus, 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.
Additionally, in the laminate example above comprising several layers of polymeric material wherein a laser beam selectively ablates at least one layer but fewer than all the layers, minimizing and confining the heat affected zone also minimizes any thermal conduction damage to an adjacent unablated layer. This increases the preciseness of the ablation, which may be of particular interest depending on the thickness of each layer, and the final application of the polymeric laminate.
Experiments were conducted to achieve the following objectives: reduce absorption depth to more precisely target a smaller portion of substrate material; reduce the heat affected zone; increase the amount of energy directed at the plume; reduce the amount of time high temperature material interacts with the heat affected zone; and reduce the time that the heat affected zone interacts with the surrounding substrate region. The polymeric substrate chosen was a polyester. The polyester had a glass-transitional phase temperature range between 212° F.-400° F., preferably about 300° F., and a degradation temperature of about 500° F.
For the purposes of this analysis, it was convenient to use percentage of the incident energy from the laser absorbed per 0.1 mil of thickness. It should be noted that of the total output energy of the laser, there is some energy reflected from the surface of the material, typically 5% to 10%, but as this does not absorb and add to the effects, it was assumed to be constant and did not affect the quality of the cut edge.
It was discovered that a formula to determine the effect and/or thickness to absorb a specified percentage per 0.1 mil of substrate material is as follows:
A=Total absorption in the thickness specified
a=absorption in a 0.1 mil thickness of material
n=thickness desired to calculate multiplied by 10,000
This formula was based upon simultaneously adjusting the total power output and the focal spot of the laser to achieve an irradiance of about 106 Watts/cm2. At this irradiance, at least 70% of the incident radiation would have to be absorbed within a few thousandths of an inch of substrate material to achieve the overall qualities of the ablation. Less percentages were found to leave discoloration along the cut surface edge of the polymeric substrate, and would also show signs of rollback.
Using the above formula, and rounding off to the nearest 0.1 mils, the thickness needed for 70% of incident laser within a polyester substrate was calculated at various wavelengths:
| || |
| || |
| ||Wavelength (microns) ||Thickness of Substrate (mils) |
| || |
| ||10.60 ||1.5 |
| ||9.40 ||0.9 |
| ||9.24 ||0.4 |
| ||8.99 || 0.13 |
| || |
At an output wavelength of about 9.24 microns, about 70% of the incident power from the laser was absorbed within 0.4 mils of the substrate material. The size of the plume was constrained such that temperatures within the plume were well above 1000° F., but existed for such a very short period of time (a few miliseconds) that temperatures within the heat affected zone were well within the glass transitional phase and did not approach the degradation temperature. FIG. 4 plots wavelength versus transmissivity through polyester. The graph has a 3% margin of error.
The 8.99 micron wavelength corresponds with the wavelength that exhibits the least transmissivity (i.e., the wavelength exhibiting the greatest absorption) through the polyester substrate. At 8.99 microns, it was experimentally determined that polyester has about a 0.1% transmissivity, which corresponds to an absorption of about 99.9%. Applying the same irradiance as with the 9.24 micron wavelength, 80% of the incident power should be absorbed with in 0.5 mils.
Thus, the present inventive method can be applied to other polymeric materials, including, but not limited to, polyethylene and polyamide. By determining the wavelength exhibiting a transmissivity of about 0.1%, and more preferably at 0.01%, within the selected polymeric material, irradiating the material with a laser at that wavelength and at an irradiance of about 106 W/cm2, at least 70% of the incident power absorbs within about the first 0.5 mils, resulting in a high quality cut showing minimal signs of discoloration, if any, and no rollback.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.