Electric heating – Metal heating – By arc
Reexamination Certificate
2001-02-20
2003-07-29
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121830, C219S121850
Reexamination Certificate
active
06600129
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a device and a process for sintering a powder with a laser beam. A device and a process for powder with a laser beam are already generally know from EP 0731743 B1, as show in the upper area of FIG.
2
.
DESCRIPTION OF THE RELATED ART
In the field of design and prototyping, and the in particular in industrial model construction, the development of new types of more rapid production methods are continuously being sought. One category of this type of rapid production methods is referred to as the rapid prototyping process. This category includes selective laser sintering (SLS). Laser
Laser sintering is a process in which material in the form of a powder is at least partially melted by increasing the temperature, so that the individual particles of powder sinter or adhere to each other. In SLS this increase in temperature is achieved by irradiation of the powder with a laser.
The production of a part using SLS occurs in layers. A laser beam with a, substantially constant output is directed in sequential lines over a powder bed and joins the powder particles where it strikes them. In areas in which no sintering is to be achieved, the laser is switched off. As soon as one layer has been completely worked over, the powder bed is lowered and a new layer of powder of is applied.over the preceding—now partially sintered—layer. The just described sequence of process steps are repeated for the present and further applied layers until the, entire construction component has been completely produced.
However, it SLS with substantially constant laser output produces parts with uneven material layer characteristics. It sometimes happens that an area of the powder bed completely melts. This results in grooves or hollows. It likewise sometimes occurs that an area of the powder bed does not sufficiently melt and thus not all particles adhere to each other. This type of uneven sintering leads to non-uniformity in the production process and to poor material characteristics.
For compensation of this problem it has been proposed, in EP 0731743 B1, to detect the temperature of the powder in a moveable detection manner in the area of the sintering position and to control the output of the laser depending upon the detected temperature, in order to homogenize the temperature balance of the part during the manufacturing process. For this, the radially symmetric emitted temperature radiated from the sinter position is directed over the same scanner-mirror as the laser beam, thereafter is decoupled by means of a dichromatic beam splitter and focused upon a IR-sensor via a lens.
Compensating in accordance with EP 0731743 B1 improves the construction quality; however, non-homogeneities nevertheless occur, which cannot be tolerated in applications with high quality requirements, and thus commercial applications of this technology are limit.
In U.S. Pat. No. 5393482 and in the corresponding U.S. Pat. No. 5508489 processes and a device are described for sintering a powder with two superimposed laser beams—one focused and one defocused laser beam—as well as with multiple temperature detection areas.
SUMMARY OF THIS INVENTION
The task of the present invention is comprised of providing a device and a process for sintering a powder with a laser beam including a temperature detecting surface, which are capable of further reducing the occurrence of.this type of non-homogeneity.
A task is inventively solved with respect to the device to be provided in that the device for sintering a powder with a laser beam comprises
a device for production of a laser beam, which beam impinges upon the upper surface of a powder along a moveable sinter position,
a device for detecting, along a moveable detection position, the temperature of the powder in the area of the sinter position,
a device for regulating the laser beam, which regulates the output of the laser beam depending upon a signal from the device for detecting the temperature of the powder,
and further
a device for compensation for position-dependent errors in temperature detection, which device for compensation takes into consideration at least one of the following sources of error depending upon the position of the detection area on the upper surface of the powder:
the geometric changes of the moveable detection area, which depend upon the position of the detection area on the upper surface of the powder bed,
the chromatic error, which is brought about by a variable strength of the bundling effect of the employed optical system at the various wavelengths of the laser beam and the thermal radiation,
the correction errors, which occur almost unavoidably in practice during the co-axial adjustment of both beams.
A constant surface relationship between the laser spot and the thermal radiating detection .area is essential for correct temperature detection.
If the focusing of the laser beam occurs via a simple lens or a lens system, then this is conventionally optimized for the center point within the construction field. For the outer or peripheral areas of the construction field, there results thus an enlargement and a dissipation of the laser spot and therewith the sinter position, since the laser beam widens outside of its focus point and beside this impinges upon the upper surface of the construction field with a tilt angle. For the attainment of a constant sinter quality over the entire construction field, a suitable optical system is thus used for compensating for the widening of the laser beam and the therefrom resulting changing of the geometry of the laser spot and the sinter position, via which optical system the focus of the laser beam is controlled.
A similar position dependency occurs in the detection of the IR-radiation emitted upwards from the moveable detection area, wherefrom—without taking this into consideration—errors in the temperature detection result.
When the laser beam and the temperature signal are directed over the same optical system, then the geometric changes of the sinter position and the detection area should remain almost constant (as long as the relative position of the two beams does not change).
The position dependency of the geometry of the detection area can be better understood by the example set forth in the following text and in FIG.
1
:
Assuming that in the starting position the laser beam impinges at sinter position Si perpendicularly upon.the upper surface of the powder bed. The detection area E
1
is likewise detected from a perpendicular perspective. The thermal radiation emitted from the upper surface of the powder bed, as proposed in EP 0731743 B1 is directed over the same scanner mirror as the laser beam, then is however decoupled from the laser optical system and focused via its own lens system. The lens system is so designed, that it forms a circular observation window; in accordance therewith the detection area in the starting position likewise appears circular. During the sinter process the laser beam is then guided to sinter position S
2
, there it umpinges with a particular tilt angle upon the upper surface of the powder bed. The observation window now as before is circular, due to the tilt relative to the upper surface there results now however a distorted detection area E
2
, which exhibits an elliptical shape and due to the larger distance of the upper surface to the focus system, which is optimized to the distance to the construction field sinter, E
2
now is significantly larger than E
1
.
The laser beam is moved along.lines. It is of importance which direction this line of movement has relative to the main axis of the ,ellipse of the detection area. The portion of a hot sinter line L on a detection area E is maximal (assuming equal size of the detection area) when the sinter line and the main axis of the ellipse coincide (L
3
:E
3
); it is minimal, when they .are perpendicular to each other (L
4
:E
4
). Even when the laser and the thermal radiation are directed, through the same optical system, the surface relationship from the laser spot to the temperature detection
Shen Jialin
Steinberger Jürgen
Daimler-Chrysler AG
Evans Geoffrey S.
Pendorf & Cutliff
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