Machine Vision: End User Solutions
Applications: Plastics
Article courtesy of Joe Gochar, Logical Systems Inc.
Problem - Plastic Cap Inspection
The production of bottle caps, or closures as they are referred to, is an ideal application of an industrial vision system. With production rates of 500 to 1000+ parts per minute, manual inspection is just not feasible.
Plastic closures are made in an almost infinite variety of sizes, shapes, colors, and types of lining material. However, a typical production run could be in the hundreds of thousands, if not millions of parts. A typical production line would be dedicated to a particular size and shape closure providing an ideal situation for machine vision.
Inspection Parameters
The types of defects inspected are as follows:
- Missing liner
- Mis-cut liners (moon cuts)
- Off center liners
- Not properly seated liners
- Pull tabs cut too deep
- Standing pull tabs
- Missing tabs (on a tri tab cap)
- Out of round caps
- Short or broken caps
- Molding flash
- Hot melt glue on the threads
- Splice tap on the liner
Solution
This vision system uses Sherlock along with a PCVisionPlus framegrabber connected to a single camera positioned directly above the cap, with the cap being back lighted with an LED strobe. This allows a detailed inspection of each cap (even white caps with white liners) and reliably rejects caps judged to have a defect.
The inspection sequence occurs as follows:
- Leading edge of a cap breaks a photoelectric switch to trigger the framegrabber.
- The framegrabber resets the progressive scan camera and sends out the strobe pulse.
- A strobe system fires the backlight LED and the picture is taken.
- Sherlock processes the image and generates a reject pulse if required.
- When the cap exits the PE switch, if a reject is necessary, a high-speed reject actuator kicks the cap off the incline into a reject chute. Good caps continue through the system.
The mark of a good vision application is to reliably identify and reject faulty parts and to accept only good parts. When a million caps per day can be processed through a system, a very low false reject rate must be maintained.
Integrating the vision inspection to an existing production line requires the close cooperation of the vision integrator and the factory engineering staff. A successful application is more readily achieved when the factory staff is an integral part of planning, implementing, and maintaining a system and it is up to the integrator to provide the necessary training and documentation.
