Assembly in Action: Sensors Error-Proof Cap Assembly
April 22, 2009
Miniature Precision Components Inc. (MPC-Prairie du Chien, WI) molds and assembles a number of different plastic products for the automotive industry, including oil caps, PCV valves, thermostat housings and quick-connect ports for emission control systems. It employs about 450 people and operates 41 molding machines, ranging in size from 25 to 550 tons.
Recently, MPC upgraded its oil-cap assembly machine by substituting a machine-vision inspection system to take the place of the traditional tool and sensor technology it had been using previously.
“We achieve quality through automation, and machine vision has been a key component of our automation strategy for the last seven years,” says MPC manufacturing engineering manager Shane Harsha, explaining the reasons for the upgrade.
In addition to the vision system, the machine employs a pair of vibratory bowls-one to feed the caps, the other to feed the O-rings-and a rotary indexing table to move the caps through a number of processing stations.
In operation, an O-ring is first loaded onto the dial fixture and then indexed to a second station, where an in-line conveyor feeds the blank caps from the other vibratory bowl and a pick-and-place arm presses the caps on top of the loaded seals.
Throughout this process, it is critical that the O-ring and cap remain correctly oriented to ensure that the seal installed in the finished oil cap will function as intended. The cap must also be correctly positioned for subsequent pad printing.
To ensure all these operations are performed correctly, MPC installed a trio of Checker 202 vision sensors from Cognex Corp. (Natick, MA). The first checks to see if there are any inverted O-rings between the vibratory feeder bowl and inline conveyor. The second checks to see that each O-ring is positioned correctly before it is installed in a cap. The third ensures that each cap is oriented correctly prior to assembly and printing.
The inverted, or “flipped,” O-ring inspection station is especially important, because each O-ring has a sealing bead on only one side, which must be placed face down if it is to function correctly. In the past, MPC relied on the feeder bowl’s mechanical tooling to prevent inverted O-rings from entering the system. However, the tooling was unreliable, and O-rings that were slightly warped or upside down occasionally made it past, causing the machine to shut down.
“Having the operator flip these seals and restart the machine was really eating into our efficiency,” Harsha says. “If the production rate dropped from 360 to 200 caps per hour, it cost us about $20,000 a year in downtime.”
To detect the presence of any inverted O-rings, the vision camera looks for the pattern of the sealing bead on each ring and then signals whether or not it has been detected. If the sealing bead is missing, the vision sensor sends an output through an optic coupler to a pneumatic solenoid that blows the inverted O-ring off of the line and back into the feeder bowl.
In addition to being effective, the system was easy to implement, taking less than an hour to set up.
“The small size, built-in lighting, variable working distance, ladder logic and free-running capability make these devices very simple to install,” says MPC automation and tooling engineer Brian Champion. “Unlike with the vision systems that we’ve used in the past, I am able to set up the entire inspection with a Checker in just a few minutes.”
“The vision sensors have helped us achieve zero-defect rates in the manufacturing process-while lowering scrap,” Harsha adds. “They are the perfect solution for many of our inspection and error-proofing applications.”
For more on machine vision, visit www.cognex.com or call 508-650-3000.