NO336577B1 - Procedures and apparatus for surface inspection - Google Patents

Description

The present invention relates to a method and an apparatus for inspecting surfaces. One embodiment of the invention relates to the inspection of elongated objects with defects that are most likely to be oriented in the object's longitudinal direction. In another embodiment of the invention, it relates to the inspection of surfaces that are in motion and where elongated defects are most likely to be oriented in the direction of movement. The invention can also be used to detect the general shape of an object and also to provide a quantification of the curvature of the cross-section.

Quality control of the surface of products, such as extruded products, is usually done by visual inspection. For example, such control can be done for products coming out of an extrusion machine to ensure that the extrusion process is operated within certain tolerances. Furthermore, control can also be done of extruded and also surface-treated products before they are packed and sent to a customer destination.

For many manufacturing processes such as in e.g. production of extruded products for building use, the visual quality of the delivered products will very often be of great importance. In some applications, most sides of the product may be visible in conjunction with other components in the final assembly. On the other hand, in some other applications it will be the case that at least one side of the product is hidden.

Visual inspection is time-consuming and it is also difficult to be able to inspect all sides of a moving object at the same time. Furthermore, with manual inspection, there may be some variations with respect to established quality tolerances due to varying lighting conditions, the persons carrying out the inspection may assess surface defects in different ways, etc.

It follows from this that it is important to provide an automated method and an apparatus that can carry out surface inspection of objects that move quickly, at the same time that all sides of such objects are inspected.

The working principle of the present invention is primarily based on dark field (dark field) illumination of the surface to be inspected. However, the corresponding bright field and gray field (grey field, near the bright field area, somewhere between the bright field and dark field areas), can also provide valuable information about the object's surface. One

image digitizing device (camera) is used to create images of dark field, gray field and light field illuminated surface. The dark field information is analyzed using a computer or similar, where the purpose of the analysis is to evaluate whether there are surface defects of a type and degree of severity that lead to the scrapping of the product, or simply determine a quality rating of the object. Dark field information captured by a camera can be used to obtain information related to the general geometry of the object in the same way as stated above, or to identify/recognize the type of object to be analyzed.

US 2008/0063246 A1 discloses an apparatus and method for detecting surface defects on a workpiece, such as an elongated metal bar, which is transported in one direction. It includes a lighting device placed near the elongated rod and at least one image digitizing device for recording images of the surface. The lighting device is arranged so that the surface of the rod in an illuminated state has a dark field and a light field. The lighting device illuminates the bar in a direction perpendicular to the direction of transport.

US 6,327,374 B1 shows a method and arrangement for automatic inspection of the surface of a moving object, where an area of the object's surface is illuminated from at least two different lighting directions. The object's illuminated surface area is imaged using a camera to provide information for analysis. The light sources illuminate the object's surface from different lighting directions at different times and the camera is a line scan camera. The arrangement also includes a controller for synchronized pulsing of the light sources and at least one line scan camera. The pulse frequency of the light sources should be above 1kHz. In SHk terms, the pulse frequency of the camera must be somewhat higher, corresponding to the number of light sources.

EP 0 898 162 Bl shows a method and apparatus for the automatic inspection of moving surfaces, such as steel strips, wood, leather or tiles. 1, a preferred embodiment comprises the solution at least three different lighting/observation channels. The solution is based on photometric stereo. Information about reflectivity, colour, gloss and the profile of the inspected surface is detected, for example, by an apparatus comprising a colour-based line scan camera and at least three distance-separated light sources with different spectral characteristics. The result of the image acquisition is registered images, such as R-, G-, B-images which in principle correspond to the lighting channels. The images are processed in several steps including estimation of surface irregularities, extraction of features and classification. The purpose of the system seems to be that by using three light sources with different spectral characteristics, the camera can collect three different images of the surface at the same time, which can improve the capacity and accuracy of the inspection. Alternatively, a light source can be used together with a plurality of sensor devices (cameras).

DE 195 11 534 Al relates to a method and an apparatus for detecting 3D defects, such as dents and steps in a flat surface, in applications for automatic surface inspection, based on the principle of photometric stereo. The surface being inspected is simultaneously illuminated with at least two lamps from different directions in dark-field conditions, where the light from the lamps has different colours. A color-based line scan camera is used for image acquisition and 3D defects are detected by analyzing the matte color values.

When an image-recording device is used together with a plurality of illumination sources, it appears from the above-mentioned state of the art that it is known to illuminate the illumination sources individually at a frequency which in sum corresponds to the recording frequency of the image-taker. In such cases, the acquisition frequency of the imager must be relatively high, at least when moving objects are inspected, to ensure sufficient input data from the surface.

Furthermore, it is also known from the above-mentioned state of the art to take pictures simultaneously at different observation/illumination channels, which will reduce the need for the high recording frequency at the picture taker.

GB 1407409 A shows a device for detecting defects such as blisters in a surface. The surface is illuminated by means of an arrangement comprising a laser and an optical system for directing the laser light towards a rotatable wheel equipped with a plurality of mirror facets. The axis of the rotatable wheel is situated parallel to the direction of movement of the surface to be illuminated, so that reflected light from the wheel, when rotating, sweeps over the surface in a transverse direction of its direction of movement in the form of an elongated strip of light. Light reflected from the surface is captured by a photomultiplier detection device, either directly or via a reflection system. Defects are detected by the fact that the light beam, which is normally reflected as an oblong light strip, changes when it hits a blister in the surface. According to the present invention, a sweeping light beam is not used to illuminate the surface of the object, nor rotatable, oblong mirror facets to illuminate the surface with a strip of light.

In accordance with the present invention, it is possible to inspect visible surfaces of a moving object, or by mutual movement between the apparatus and the object, where in one embodiment an inspection is made of all sides of the object at the same time. This will require that a sufficient number of cameras are available. The capacity and capability of the inspection equipment allows inspection of all visible sides of the object at a relatively high speed.

The invention is particularly suitable for the inspection of elongated products in a continuous or semi-continuous manner, and is suitable for the inspection of extruded products, with or without a treated surface. It is advantageous for the surface to have a certain directional reflection property, preferably of a glossy character, depending on the illumination source.

In particular, the invention is suitable for the detection of surface defects on products produced by extrusion, continuous or semi-continuous casting, rolling or other continuous or semi-continuous processes.

In accordance with the invention, the processing of images and the compilation of information from them can be done at a high speed and in a simplified manner.

Furthermore, it represents advantages with respect to a quantitative inspection of manufactured products.

The present invention thus relates to a method for inspecting at least one surface of an object which preferably has an elongated direction X where a lighting device is placed near the object, the method further comprising recording images of the surface by at least one image digitizing device where the lighting device comprises illuminators, or a non-discrete line of lights, which are placed either (a) along an axis, which axis is aligned obliquely with an angle a in relation to the direction X of the object, in a way where it illuminates a part of the surface of said object at angle a with respect to the direction X, or (b) around the object surrounding axes A', B\ which axes are aligned obliquely at angles a', p" with respect to the direction X of the object, in a manner where it illuminates parts of the surface of said object at angles a', p' with respect to the direction X, or (c) along a helical curve, which helix has an axis parallel to

the longitudinal direction X of the object,

so that in an illuminated state consisting of a dark field and a light field, defects in said surface can be visualized and imaged.

The present invention also relates to an apparatus for inspecting at least one surface of an object which is preferably elongated in a direction X where a lighting device is placed near the object, and at least one image digitizing device is placed for recording images of said surface, where the lighting device comprises illuminators, or a non-discreet line of lights, which is placed either

(a) along an axis A, which axis is oriented obliquely at an angle a with respect to the direction X of the object, in such a way as to illuminate part of the surface of said object at an angle a with respect to the direction X, or (b) around the object surrounding axes A', B', which axes are arranged obliquely with angles a', P' in relation to the direction X of the object, in a way where it illuminates parts of the surface of said object at angles a', p<* >with respect to the direction X, or (c) along a helical curve, which helix has an axis parallel to

the longitudinal direction X of the object,

so that in an illuminated state consisting of a dark field and a light field, defects in said surface can be visualized and imaged.

These and further advantages can be achieved by the invention as defined in the appended patent claims.

In what follows, the invention will be described in more detail by means of examples and figures where: Fig. 1 shows a basic layout for an embodiment in accordance with the present

invention, top view,

Fig. 2 shows the principle set-up as shown in Fig. 1, seen from one side and so on

shows an image digitizing device,

Fig. 3 shows a second embodiment of the invention, comprising an arrangement which

illuminates more than one side of the object to be inspected,

Fig. 4 shows a third embodiment of the invention, corresponding to what appears in Fig. 3,

but with a helix-shaped, non-discreet illumination,

Fig. 5 shows a sketch of the apparatus including image processing and

rejection device,

Fig. 6 shows an image of a surface that has blisters as a surface defect, Fig. 7 shows an image of a surface with longitudinal grooves (dielines), (a longitudinal

defect) as surface defect,

Fig. 8 shows a picture of a surface with color streaks as a defect,

Fig. 9 shows an image of a surface that has a dent as a defect,

Fig. 10 shows an image of a surface that has a wave as a defect.

As can be seen from Figs. 1 and 2, an object 2 has an elongated shape in the X direction, and is further placed for movement along this axis X. The object is illuminated with a lighting device (ID) which in this embodiment comprises three illuminators (la, lb, and lc). The illuminators can be of the LED type, ordinary light bulbs or similar. The wavelength of the emitted light should preferably be as short as possible to enable detection of small defects (eg, 5-6 µm or less).

The movement of the object can be done as part of the manufacturing process itself, by motorized rollers (21,22,23) or similar. The illuminators are placed above the surface of the object (2) and are positioned along an axis A (Fig. 1) which is tilted by an angle a with respect to the axis of movement X. In this way, a light field area (3) becomes visible with a corresponding angle of regard to the axis X, but also depending on how the topology of the surface (2) is.

At the peripheral areas of said light field area, the object (2) is illuminated in a dark field state in the areas (4,5). Located between the two mentioned dark field and light field areas, there will be a transitional area called the gray field area.

In this arrangement, the dark field area (4) is of particular interest, but the dark field area indicated as reference number (5) can also be used. This depends on the practical adaptation of the setup. An image digitizing device (20)

(camera or equivalent, see Fig. 2) is placed at a level above the object (2) and preferably perpendicular to the horizontal plane. In the images taken by the image digitizing device, information from the dark field area (4) is identified for further processing using an image processing system (described later). For example, images can be taken of a part of the dark field area with good dark field illumination, and further represented using an intensity profile based on line A<1>. Line A<1> is preferably oriented at a corresponding angle to line A, that is to say at the angle a<1>. There may be a deviation between these two angles depending on the topography or geometry of the surface being inspected, and may also be positioned slightly away from parallel to line A to avoid interference from the light field area.

Furthermore, under this setup one can first determine the light field area to define which part of the dark field area should be used to find information regarding the surface analysis, for example where to apply an intensity profile scan in the dark field area.

The light field area will appear as a "snake" of reflected light. The shape of the snake (curved or linear) contains information about the global surface deviation relative to a correct shape.

The type and shape of the object to be inspected, such as an extruded profile with a defined cross-sectional shape, can be determined with a camera or similar connected to a computer that images and analyzes the end region of the profile to define the profile type.

The result of the analysis can be compared with a set of predetermined profile types that are pre-stored in the computer. Then, in the subsequent surface analysis, a comparison can be made between the image analysis and predefined values to determine whether there are deviations that lie within predetermined tolerances or not.

When analyzing extruded objects, it should be understood that information regarding, for example, pressure lines (pressure lines) can be found in the light field area where this type of defect can produce a deviation in the form of a "snake". Other defects such as blisters or steps (dents) with an extent along the X-axis will be found by an analysis of the light field, gray field and/or dark field areas.

The distance between the lines A and A<1> in this version (Fig. 1) is chosen on the basis that the inspection of the surface can be done in optimal dark field conditions, and at a certain distance from the bright field illuminated area to avoid light disturbances from this area.

With the arrangement shown, defects indicated by reference numbers (6, 8 and 9) can be detected by the image digitizing device in the dark-field illuminated area. These defects can be local defects such as scratches running along the longitudinal axis of the object, as indicated by (6) or they can be continuous or semi-continuous generated by the production process of the object, and consist of valley-shaped defects (valleys),

pressure lines and ridges. With regard to extruded products, defects can be constituted by a commonly occurring defect called longitudinal grooves (dielines). Other variants of defects with regard to extrusion processes can be dents (8), pits or, for example, blisters (9) which have no specific direction of extension.

The present invention is particularly suitable for detecting defects that may occur in certain extrusion processes, especially in connection with aluminum extrusion processes, but not limited to this type of process.

In the arrangement shown, three illuminators appear. The principles of the present invention can alternatively be utilized by using a lighting device (ID) comprising two illuminators placed along an axis inclined with respect to the axis along which the object moves.

A main advantage of the arrangement shown here is that all the illuminators can be activated simultaneously and continuously because they are placed along an inclined axis with respect to the axis along which the object moves. In this way, part of the surface will be illuminated under dark field conditions from which the image digitizing device, preferably an area scan camera, obtains information. As the object is moved, a surface in accordance with its longitudinal direction will be illuminated, imaged and analyzed.

It is the mutual relative movement between the arrangement and the object that enables inspection of the object's surface along its entire length in this embodiment. Alternatively, the object may be stationary and the arrangement may be arranged for movement (not shown).

Figure 3 shows an apparatus which enables, in principle, the inspection of all visible surfaces of a moving object (2'). As in the previous example, the object (2<1>) is placed for movement in the X direction, in this case in the horizontal plane. The arrangement in this embodiment includes a ring (30) through which the object (2') passes. At the periphery of the circumferential or surrounding ring is placed an illumination device (ID) represented by illustrated illuminators (l'a - l'q) placed in a way where they completely enclose the object (2<r>) in such a way that it is illuminated. In this example, the plane of the ring is positioned at an inclined angle with respect to the normals of the object's axis of motion, so that dark-field illuminated areas are formed at least on some surfaces of the object in motion. In this arrangement, the main axes A', B' of the ring (30) are inclined at respective angles a', p' with respect to a plane normal (at right angles) to the axis X.

In the example shown, the main directions of the optimal dark field area are allocated along the lines A'<1>, B'<1>, inclined at angle a'<1> (surface 2' a) and correspondingly at an angle p"<1> (surface 2' bl. Said angles may be different with respect to the angles aP' of the lighting device (ID), due to the actual surface geometry of the object.

In another preferred example, the plane of the ring (30) is positioned perpendicular to the object's direction of movement X, but then a preferred location of the drawn illuminators (l'a-l'q) in the ring is such that they follow the curve of a helix, the that is, the ring is shaped like a cylinder shell where the illuminators are placed in a continuous, even thread-like pattern (helix).

This preferred solution makes it possible to illuminate all visible (non-enclosed) surfaces of the object under dark field conditions, where inspection can be carried out without light field interference from nearby light sources. This solution is not shown as such, but the lighting arrangement will in principle be similar to the design shown in Fig. 4 with the exception that the design in Fig. 4 shows a non-discrete lighting device.

Fig. 4 shows a similar arrangement as shown in Fig. 3, where an object (2") is surrounded by a lighting device comprising a lighting source 1" which can be a non-discrete light line, a fiber-based lighting source or the like. In the Figure, two image recording units or image digitizing devices (40,41) are shown. The light array is clearly indicated to be arranged in a substantially helix-shaped shape around the object to be inspected. In theory, one can replace this continuous line of lights with a corresponding helical structure, such as a helically shaped band or the like, which has assigned discrete lighting points.

In the above-mentioned embodiments, one or more image digitizing devices are placed at suitable positions. For example, four image digitizers may be arranged to take images along four principal axes that are mutually perpendicular to each other. In another embodiment (see Fig. 3), a first image digitizing device can be placed perpendicular to the surface (Ta) and a second perpendicular to the surface (2'b). while the other two image digitizing devices are placed in a similar manner with respect to the other two surfaces (Tc, 2'd) (not shown).

It may be preferred to use six cameras or image digitizing devices, which are placed along the periphery of a circle in a plane perpendicular to the object's direction of movement, possibly with a minor mutual adjustment in the X direction. It may also be preferred that the center of said circle coincides with the center of the object's cross-section. The sectors located between the cameras can preferably be 60°. Additional cameras can be used if the surface of the object to be inspected is complex. The number of cameras used will also be a matter of cost.

The above-mentioned embodiments related to a helix configuration of the lighting device can be called "full helix dark field" (FHDF).

In an extrusion process, a series of small longitudinal lines can be formed in the surface of an object. Small lines are normally acceptable, while larger lines produce longitudinal grooves (dielines) or sharp hill defects. Groups of small lines can give color streaks, which in some applications cannot be accepted.

The aforementioned defects are particularly challenging because they extend in the same direction as the extrusion itself, while the lighting should cover a large number of different professional designs. Generally, dark-field lighting sources must be built and optimized for a specific design, but in the case where a large number of varying profiles are to be inspected, the lighting, and consequently also the equipment, must have built-in flexibility. This flexibility can be achieved by a dark-field helix as described above.

It should be understood that in the embodiments related to Fig. 3 an arrangement of lighting sources in a cylinder shell device with or without placement of the lighting sources in a helix formation is mentioned. Other arrangements of lighting sources may also be possible within the scope of the invention. The main thing is to generate a dark-field illuminated area on selected parts of the moving object that can be used for image recording and analysis of surface quality, where disturbances from the light-field area are avoided.

For example, the illumination sources may also be arranged in a substantially square or rectangular frame which more or less surrounds the object, and where all sides are inclined at an angle a, p etc. with respect to the perpendiculars to the axis of motion so as to form a dark-field illuminated area at corresponding angles a', p<1>etc. on the surface of the object. This will be particularly useful for objects with a square or rectangular cross-section.

Fig. 5 shows a sketch of the apparatus including image processing and a de-icing device for a moving object.

Captured images from the camera(s) can be transferred to an image processing unit, here called sensor PC(s), which communicates with a Master PC. This unit can process images and extract data of importance for the relevant surface geometry and quality. Data coming from the image processing unit is compared with stored threshold data representative of determined values of acceptable surface quality. A PLC assisted robot can be controlled using signals from the processing unit and further be arranged to remove objects with rejected surface quality from the main transport path to the objects (not shown). A stroboscope control is provided to synchronize the illumination source(s) and the camera(s). A front end camera (not shown) can be used to determine the type of object to be inspected. The speed of the object can

determined using suitable equipment such as a encoder.

Fig. 6 shows blister defects photographed from the surface of an extruded object. The light field area can be seen from left to right somewhat near the center of the image, and a blister (blister) can be seen as a light transition where there is a dark spot above and below said transition. Immediately above and below the aforementioned blister, two further blisters can be seen as relatively light spots.

In Figure 7, an image of an extruded surface is shown and where defects of the longitudinal groove (dieline) type are evident. The two round spots in the image and another partially visible one at the right edge of the image are LED lighting sources placed in a helix configuration (slightly tilted in the image). These lighting sources illuminate the opposite side of the object. In the central part of the image, the illuminated part of the object is shown, where the light field area is the part of the surface that is clearly visible. The area illuminated by the light field is slightly tilted upwards to the right with respect to the object's longitudinal axis. At the upper and lower parts of the light-field illuminated area, both in the gray-field area and in the dark-field area, there are clearly visible lines, especially in the right-hand quarter of the image. These align longitudinal grooves (dielines) introduced into the surface during extrusion of the object.

In Fig. 8, so-called color streaks are shown which can be seen as vertical bands with an extension in the middle of the image of the object, and which are visible both in the dark field area, the gray field area and in the light field area. These defects can occur as a result of a change in the texture of the surface. Outside the object itself, three LED lighting sources are shown and partly a fourth, which illuminates the opposite side of the object.

Fig. 9 shows a dent in the surface of an object. This defect can be seen above the crossing light field area, and which partially "depresses" the light field "snake" in its left part. A dark spot appears above the depression. Such defects may be introduced during transport when the product leaves the extrusion press.

Fig. 10 shows a wave-shaped defect in an object, and is located above the light field area. Outside the object on its right side, an LED illumination source is shown which illuminates the opposite side of the object.

It should be understood that information from a light field area can also be collected and analyzed. In particular, 3D defects, pressure lines, dents, etc. i

extruded products are detected in this area.

It should further be understood that the present invention can have a number of applications. For example, inspection of products related to extrusion processes, rolling processes (plates and metal blanks) and casting processes such as block or bolt casting can be carried out by utilizing the present invention.

The invention can be used for the inspection of objects of different materials and surface treatment. For example, metals (steel, aluminum and aluminum alloys that can be surface treated, polished, painted or not treated on their surface, such as highly reflective), plastics, etc.

The invention can also be used in combination with other lighting setups, such as arrangements that can create an axial dark field and/or tangential dark field on the surface of the object. In this type of arrangement, the frequency of image recording performed by the image recorders and the on-off frequency of the various types of lighting sources must be synchronized to provide optimal results. Furthermore, the image processing must be coordinated with the various sources of imaging.

Claims (20)

1. Method for inspecting at least one surface of an object (2) which preferably has an elongated direction (X) where a lighting device (ID) is placed in the vicinity of the object, the method further comprising recording images of the surface by at least an image digitizing device (20), characterized in that the lighting device (ID) comprises illuminators, or a non-discrete line of lights, which is placed (a) along an axis (A), which axis is aligned at an angle (a) in relation to the direction (X) of the object, in such a way that it illuminates part of the surface of said object (2) at angle (a) with respect to the direction (X), or (b) around the object (2) surrounding axes (A', B'), which axes are aligned obliquely with angles (a', f<T>) in relation to the direction (X) of the object, in a way where it illuminates parts of the surface of said object (2) at angles (a' , P') with respect to the direction (X), or (c) along a helical curve, which helix has an axis which is substantially parallel to the longitudinal direction (X) of the object (2), so that in an illuminated state consisting of a dark field and a light field, defects in said surface can be visualized and imaged. 2. Method in accordance with claim 1, characterized in that the surface of the object (2) is illuminated in a dark field state in its full width and where corresponding images cover said width. 3. Method in accordance with claim 1, characterized in that the lighting device (ID) is positioned in such a way that it encircles the object (2) partially or completely, and that image digitalization devices correspondingly cover the illuminated area. 4. Method in accordance with claim 1, characterized in that the lighting device (ID) is placed in such a way that it mainly follows a helix-shaped curve. 5. Method in accordance with claim 1, characterized in that the lighting device (ID) comprises several lighting points which are activated simultaneously. 6. Method in accordance with claim 1, characterized in that the object is arranged for movement in its direction (X). 7. Apparatus for inspecting at least one surface of an object (2) which is preferably elongated in a direction (X) where an illumination device (ID) is placed near the object, and at least one image digitizing device (20) is placed for recording images of said surface, characterized by that the lighting device (ID) comprises illuminators, or a non-discrete line of lights, which are placed (a) along an axis (A), which axis is aligned obliquely with an angle (a) in relation to the direction (X) of the object, on a way in which it illuminates a part of the surface of said object (2) at an angle (a) with respect to the direction (X), or (b) around the object (2) surrounding axes ( A\ B'), which axes are aligned inclined at angles (<x\ p') with respect to the direction (X) of the object, in a manner where it illuminates parts of the surface of said object (2) at angles (a', p') with respect to the direction (X ), or (c) along a helical curve, which helix has an axis substantially parallel to the longitudinal direction (X) of the object (2), so that in an illuminated state consisting of a dark field and a light field, defects in said surface can be visualized and imaged. 8. Apparatus in accordance with claim 8, characterized in that the lighting device (ID) is positioned in accordance with a surrounding or peripheral frame with respect to the object (2). 9. Apparatus in accordance with claim 8, characterized in that the lighting device (ID) is mainly placed in accordance with a helix-shaped curve. 10. Apparatus in accordance with claim 8, characterized in that the lighting device (ID) comprises several illuminators which are preferably of the LED (light emitting diode) type. 11. Apparatus in accordance with claim 8, characterized in that the lighting device (ID) is represented by a non-discrete light row arrangement. 12. Apparatus in accordance with claim 8, characterized in that there are several image digitizing devices (20) or cameras positioned above the surface to be imaged. 13. Apparatus in accordance with claim 13, characterized in that there are preferably six image digitizing devices (20) placed in such a way that they surround the object, and are further placed with a mutual angular distance of 60°. 14. Apparatus in accordance with claim 8, characterized in that the images taken are transferred to an image processing unit. 15. Apparatus in accordance with claim 15, characterized in that data coming from the image processing unit is compared with stored threshold data representative of determined values for accepted surface quality. 16. Apparatus in accordance with claim 15, characterized in that a robot receives signals from said image processing unit and is further arranged to remove objects with rejected surface quality. 17. Apparatus in accordance with claim 8, characterized in that the object and/or the apparatus is arranged for movement in one direction (X). 18. Apparatus in accordance with claim 8, characterized in that the object is made of aluminum or an aluminum alloy. 19. Apparatus in accordance with claim 8, characterized in that the object is made of a plastic material. 20. Apparatus in accordance with claim 8, characterized in that the object is made by an extrusion process.