Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-01-31
2003-05-27
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S222100, C356S236000
Reexamination Certificate
active
06570177
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to a method and apparatus for material inspection, and more particularly, to a method and apparatus for detecting defects in transparent or translucent material particles and for separating a batch of material particles into portions of like grade or quality.
2. Background of the Invention
Material inspection is an essential part of any manufacturing process. Manufacturers must ensure that the raw materials used in manufacture meet or exceed standards of quality, such as size, color, and purity. Inferior raw materials degrade the quality of the final product and reduce the manufacturer's sales and profits. Thus, to maintain strict product specifications and satisfied customers, manufacturers demand that raw materials adhere to a minimum quality or grade.
Sensitive to such concerns, suppliers of raw materials routinely conduct inspections with objectives such as identifying and removing flawed material, assessing the overall quality of a batch of material, and separating a batch of material into portions of like size, color, purity, or grade. In the plastics industry, for example, suppliers inspect raw polymer pellets to identify manufacturing defects and to grade a certain lot of pellets for purity. Based on the grading information, suppliers can price lots according to their quality and can offer to manufacturers grades of pellets that meet minimum manufacturing requirements. In this manner, manufacturers do not have to pay for unnecessary purity and, in turn, can maintain competitive product pricing.
When inspecting material, suppliers and manufacturers look for a variety of defects, depending upon the type of material. In food products, for example, defects include foreign matter, uncooked portions, unprocessed or clumped portions, and contaminants from pests such as insects or rodents. In plastic pellets, defects generally include foreign matter, charred raw material, contaminants from unmelted base constituents of the polymer material (often referred to as gels), incorrectly sized or colored pellets, broken pellets, and pellets that are stuck to each other. In addition, manufacturers sometimes measure the amount of fines (small chips or thread-like pieces that can break away from the pellets during manufacturing and transportation).
Traditionally, material inspection has been a slow, labor-intensive process limited to testing small samples instead of all material that is incorporated into the final product. Thus, theoretically, a sample might not be representative of the defects present in the rest of the material. Although the following discussion of the traditional methods of inspection is in the context of the plastics industry, the methods and their associated drawbacks apply equally to other material inspections, e.g., food processing. In the plastics industry, the current methods for inspecting raw plastic material include: 1) visual inspection of pellets by a person; 2) inspection of polymer ribbons formed from pellets; 3) inspection of molten polymer; and 4) automated inspection of the pellets. It is important to note that these methods are typically suitable for base or raw materials that are transparent or translucent. Generally, however, this requirement is not a problem because coloring is usually added late in the manufacturing process.
Visual inspection of pellet material by a person is the most common method of material inspection. It is generally conducted in a quality control laboratory separate from the manufacturing process. The visual inspection method typically involves spreading a sample of particles on top of a light table (e.g., a glass or Plexiglas™ table with a light source below its top) or other white or light-colored surface, and examining each particle for a defect. If the size of a possible defect is small, the inspectors must strain their eyes to observe the defect or perhaps use a magnifying glass to focus on each particle. Although using the light table or light-colored surface enhances the defects, the process is only as reliable as the eyes and concentration of the human inspector. In addition to human error, using human inspectors increases labor costs and significantly reduces speed at which material is analyzed.
Inspection of polymer ribbons involves melting raw material pellets into a molten form, extruding the molten material into thin, ribbon-like shapes, and inspecting the ribbons for defects. The ribbon shapes are flatter than pellets, which eases handling and presents a larger viewable surface area. This ribbon inspection technique can be incorporated into manual (visual inspection by a person) and automated methods of inspection. Despite the advantages in handling and viewable surface area, the ribbon inspection technique suffers from the added time and expense of melting the raw material pellets. The equipment and manpower needed to accomplish this extra step add significantly to the overall cost of material inspection.
In addition to analyzing ribbons, some inspection techniques analyze the molten polymer itself, in a device known as a flow cell. The flow cell is a chamber with a conduit viewable through a window. U.S. Pat. No. 4,910,403 to Kilham and LeBlon discloses a flow cell typically used for the molten polymer inspection technique. The molten polymer is channeled through the conduit, illuminated, and inspected as it passes under the window. This inspection technique can analyze the molten material either manually or with an automated device. Although this method can identify defective portions of the molten polymer, the method cannot separate those defective portions from the remaining acceptable portions. Thus, the method is suitable for grading the molten polymer or monitoring a manufacturing process for quality control, but not for removing defective portions and improving the quality of the molten polymer. In addition, the flow cell and the equipment necessary to convey the molten polymer introduce additional costs and complexities to the inspection process.
Generally, automated inspection of polymer pellets involves passing the material in front of a device that detects defects using technologies such as photography, x-rays, and digital line scanners. For the plastics industry, the typical automated inspection method, which does not include the burdensome step of melting, takes a picture of a pellet as it passes in front of an illuminated background.
FIG. 1
shows an example setup of this technique. A camera
100
takes a picture of a pellet
102
as it passes in front of a light source
104
. Typically, light source
104
is an illumination source such as the fiber optic backlight disclosed in U.S. Pat. No. 5,187,765. A beam of light
106
illuminates the center of pellet
102
and reaches camera
100
. For purposes of explanation and comparison, this application will refer to this automated inspection as the backlighting method.
Although the backlighting method can speed the inspection process, persons skilled in the art recognize that the method is not as accurate as the visual inspection method described above. Principally, the reduction in accuracy is due to the lighting of the round or almost round pellet. Because of the round surface, a clear polymer pellet exhibits a lensing effect that refracts light around the edges of the pellet, in much the same way that images are distorted around the perimeter of a crystal ball or marble.
FIG. 2
shows how the light
200
is refracted or redirected as it passes through pellet
102
. The pellet refracts light
200
so that the pellet edges appear to be illuminated only by the dull light around light source
104
instead of by the bright light originating from light source
104
. As a result of the light refraction, pellet
102
appears darker at its edges than at its center.
FIG. 3
illustrates an example of this lensing effect and the shadows that result along the edge of the pellet image. As evident in
FIG. 3
, if a defect exists at the edge of the pellet, the dark edges caused by the refr
Shyy Yeu-Hwa
Struckhoff Andy
Allen Stephone B.
DCS Corporation
Shaw Pittman LLP
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