More than fifty years ago, a Midwestern food company started producing TV-dinner trays. The plant evolved into a dedicated food container producer and continues to push the envelope of food packaging innovation by producing the aluminum and plastic thermoform food containers used by delis, bakeries, catering companies and small food producers globally.
The plant produces 420 different items in plastic and an additional 200 items in foil. It produces three types of plastic containers: corn-based, compostable polylactic acid (PLA); biaxially oriented polystyrene; and polyethylene terephthalate (PET) – each with its own production needs. Coordinating such a degree of variance requires tightly controlled production processes.
Moreover, to stay competitive, keep prices low and meet sustainability goals, plant operators are continuously looking to reduce waste. To do so, they must repeatedly reduce scrap and materials consumed, and create lighter-weight, higher-strength products. Improving manufacturing processes to meet these goals requires timely knowledge of the production process. Only with a transparent, real-time view of what is happening at each step in the process can operators make the best decisions for the business.
Resin pellets serve as the base for all of the company's plastic containers. At the company's primary plant, 10 large silos, each 18 feet in diameter, hold up to 40,000 pounds of pellets. Resin pellets are constantly drained from silos and pulled into a blender where they are mixed with additives. The mixture is then sent through an extruder and unwound into thermoforming machines where plastic sheets are pressed, stretched, shaped and cut into containers like those used to hold cookies at your local bakery. Once the plastic containers are cut out of sheets, the remaining web of unused plastic is sent back into the system. This plastic is reheated and reused in a continuous cycle.
For the system to function, the extruders need a constant feed of plastic. While the rate of production can be slowed or sped-up, it cannot stop. If the extruders stopped feeding plastic, the metal rollers would make direct contact with each other. This contact can damage the machine, resulting in several days of downtime. “The Rockwell Automation integrated control and monitoring solution gives us scalability, reliability and serviceability,” said the plant manager.
To supply the continuous feed, the silos must not run out of resin pellets. However, because the silos are made of metal, the resin levels are not physically visible. To keep track of the amount of resin in each silo, plant staff employed a manual process utilizing a plumb-bob. A plumb-bob is a weight suspended from a string used as a vertical reference line, or plumb line. This basic technology has been in use since ancient Egyptian times.
An operator would travel to each silo and press a button that initiated an automatic, yo-yo-type plumb-bob. The plumb-bob would be lowered down from the top of the silo, and once it made contact with the surface of the resin pellets, after anywhere from 10 seconds to two minutes, it would be pulled back up. An operator records the time of plumb-bob travel, goes back to the work station and plugs that metric into an algorithm. The algorithm translates time into the number of feet the plumb-bob dropped to determine how much resin is in the silo. Operators can then determine if more pellets need to be added to keep up with current and upcoming production demands.
Unfortunately, this process is error prone. When pellets are first added to the silo from the top, the material forms a cone in the center. Pellets are drained from the bottom of the silo and form a downward shaped cone. Pellets are almost never at a uniform or “plumb” level across the silo. Additionally, the plumb-bob doesn't always strike material in the same spot each time it's lowered. All these deviances taken together can throw off the operator's estimation of the amount of resin in each silo by several thousand pounds.
Miscalculations of resin levels using this system caused major outages for the plant on its PET and PLA extruders on more than one occasion. Downtime on just one extruder can cost more than $450 per hour and make hundreds of pounds of plastic unusable. In a best case scenario, cleaning out a stalled extruder requires at least two hours, after which seven workers must devote four to six hours to get the line back up and running. In a worst case scenario, a broken extruder can mean days of downtime. Additionally, the company runs a lean facility, always running “just-in-time” to meet customer orders. Any downtime can delay order deliveries and strain customer relationships.
With a day of downtime in the plant costing in excess of $10,000, breakdowns are simply unacceptable. When a new plant manager arrived in 2009, one of his first tasks was to find a way to prevent such breakdowns. After attending the annual Automation Fair® event hosted by Rockwell Automation, the plant manager saw potential solutions in action and realized he could get all the software, hardware and support he was looking for from one company.
The plant manager and his team worked with Rockwell Automation Global Solutions to develop a silo monitoring system that provides continuous, real-time information on resin pellet levels in each silo. At the heart of the system is an Allen-Bradley® ControlLogix™ programmable automation controller (PAC), which provides a flexible, scalable and industry-proven control platform.
Laser-array sensors installed within the silos stream real-time information on resin pellet levels to the PAC. Allen-Bradley PanelView™ human-machine interfaces (HMIs) utilize FactoryTalk® View Site Edition (SE) visualization software to take data and turn it into an easy-to-read, resin-level indicator displayed on the operator screens. The software issues alarms when resin levels drop below predetermined production requirements. FactoryTalk ViewPoint software allows operators to access the system from an Internet browser to monitor operations remotely from any location where Internet access is available.
“The Rockwell Automation integrated control and monitoring solution gives us scalability, reliability and serviceability,” said the plant manager. “I swap out old components and controls, updating with Rockwell Automation products as upgrades are necessary. From a technician standpoint, standardizing expands serviceability. If all systems and machines are running on ladder logic, we gain significant advantages in ease-of-use and maintenance.”
The system tracks the status and sequence of all silo events and allows operators to monitor parameters and status information in real-time to easily identify and diagnose problems. The old plumb-bob system now functions as a backup system to verify data from the new system.
A Rockwell Automation engineer worked closely with the plant team to develop different operator screen displays and was on site at the plant for commissioning and deployment of the new system. This on-site expertise helped the company save time and costs in the system rollout. Since installing the system, the plant hasn't seen any extruder breakdowns, translating into more uptime and more profits.
The next step for the plant is to expand the integrated control and visualization system further down the line, and potentially into additional facilities. “Our operators like one-touch control on their screens,” said the plant manager. “For plastics production, extruder operators need to see exactly what's happening at their machines this moment, but they also need to understand chill water temperatures, material loads, air compressor pump pressure and more from across the facility. They need all this live, in real-time to react quickly to any issues. The whole process of increasing output, reducing waste and improving quality comes down to giving our operators the information they need to react more quickly.”
“We are going to spend the balance of this year collecting data from our new system and develop an ROI database where we can equate the time and downtime savings from the new system into a dollar value,” said the plant manager. “From there, we can relatively easily scale this system to our other manufacturing facilities. We've already got the screens and code developed. The control system, computer server and IT network are all in place. I can't wait to have the purchasing manager for all facilities watch in real-time as each resin silo is filled at plants across the country. Then his buying could truly be just-in-time.”
The results mentioned above are specific to this customer's use of Rockwell Automation products and services in conjunction with other products. Specific results may vary for other customers.