Magazine Archive
  November/December 1996
A Glass Act

It takes a lot of automation and quality control to produce glass. Here’s a look at one glassmaker who uses a touch of glass—that is, cullet—to make its new containers.

By Kristina Rundquist
Kristina Rundquist is an associate editor of  Scrap.

It’s a gray, blustery day in upstate New York—not the kind of day that inspires activity. But inside the Owens-Brockway Glass Containers plant in Auburn, life is bustling. Here, seven days a week, 24 hours a day, endless streams of amber bottles glide off conveyor belts to be packed into cartons and shipped to two of the largest U.S. breweries.
   These bottles contain, on average, more than a third recycled glass. The 75-acre Auburn plant started using cullet—defined as crushed recycled glass—in 1985 under the ownership of Miller Brewing Cos. Today, two years after Toledo, Ohio-based Owens-Brockway purchased the facility, up to 35 percent of its total glass tonnage is cullet-based. And the firm has increased its cullet use at many of its other plants nationwide, earning it the reputation as one of the most aggressive cullet consumers in the business.
   Though glass recycling is hardly new—U.S. manufacturers have used cullet to make new glass products since colonial Jamestown—it has gained momentum in the past few years, reaching 37 percent of total glass output in 1995. This growth can be attributed to increased collection through curbside recycling programs and stepped-up demand by companies such as Owens-Brockway. In the past five years, in fact, the use of cullet has increased an estimated 82 percent, with glass manufacturers melting more than 2.5 million tons of cullet in their furnaces in 1994.
   And why shouldn’t glass recycling be growing? Glass, after all, can be recycled time and time again without any loss of quality. Plus, cullet can offer glassmakers both financial and production savings over virgin feedstock.
   While that answers the “why” of glass recycling, how, precisely, is cullet recycled? To learn the answer, come take a tour of Owens-Brockway’s Auburn manufacturing plant.

Cullet What You Will
The glassmaking process starts far from the company’s furnaces and production lines, beginning, in fact, in underground deposits of silica sand, soda ash, and limestone. The majority of glass containers—around 63 percent—are made from these virgin materials, which are mined and transported via rail in about 95-ton loads. These materials are some of the Earth’s most abundant resources, so common that it’s not unusual for transportation to cost more than mining.
   Upon arriving at the Owens-Brockway plant, these powdery virgin materials are unloaded into underground storage areas before climbing to be deposited in enormous, 50-foot tall silos, courtesy of a six-story bucket elevator.
   Meanwhile, the other main ingredient in glass production—cullet—arrives by truck for processing in the Auburn plant’s recycling center. Cullet comes in two varieties: furnace-ready home scrap, such as rejected bottles from the production line, and foreign or ecological cullet, which arrives from outside vendors in either a raw form that requires further processing or as furnace-ready feedstock. Glass producers increasingly prefer the latter option, often contracting with third-party processors to prepare cullet to their quality specifications. As George Connally, Auburn plant manager, notes, “While we have the means for processing on-site, most producers are getting away from it because it’s turning into a secondary operation.”
   At the Auburn facility, processing cullet is a one-man job, done in a separate building that’s large enough to house several bunkers of raw, unprocessed cullet and a Grayson cullet processing system, the plant’s centerpiece. Processed foreign cullet is stored outside on a concrete pad. Though this material is supposedly furnace-ready, it too will take a trip through the processing system for one final quality check.
   The system, which can reportedly process 18 to 20 tons of cullet an hour, is a budding engineer’s dream—conveyor belts run up and down at all angles, hoppers and bins are strategically placed below, and massive, steel boxes—vacuums, it turns out—loom overhead.
   The Grayson system works like this: Crushed glass is fed into a hopper and dumped onto a conveyor belt. It first passes under a magnet that separates out the steel bottle caps and other ferrous items, which are subsequently sold to a scrap processor. The steel-free cullet then heads through a screening stage designed to extract aluminum caps and other oversized particles. Any glass that is screened out is collected and sold for use in such applications as reflective beads and glasphalt, an asphalt-cullet mix. Meanwhile, an overhead vacuum inhales the paper and fine particles the screen doesn’t catch. Next, the emerging cullet stream is rapidly ground to make the cullet smaller and more uniform, which improves its meltability.
   Because there’s still a chance that tiny aluminum particles are present, the cullet passes through two eddy-current nonferrous separators, which toss any residual metal out of the mix. What’s left is clean cullet, which is stored in silos.

Why Cullet?
The principal goal of cullet processing is to produce a secondary feedstock that is free from potential contaminants because, as any glassmaker will attest, even the slightest contaminant can cause serious damage. Aluminum particles, for instance, turn into a silicon oxide in the furnace and form a BB-like pellet. “It’s very destructive,” says Dante Garcia, batch, furnace, and recycling superintendent. “These pellets adhere to the side of the furnace and continually emit bubbles. When a pellet does come out, it can become lodged in a bottle and cause it to crack.”
   And aluminum isn’t the only concern. Corell and other heat-resistant glass types are among the worst contaminants with high melt temperatures. Lead crystal causes clear marbles to form. Ceramics harm the furnace’s feeder mechanism. Plastic rings and labels cause color and chemical changes. Copper and brass can drill holes in the furnace, and the list goes on. “Anything not consistent with the glass density will crack it,” Garcia notes.
   Even clean cullet can create problems due to its abrasive nature, which can cause equipment to wear out faster.
Considering all of these very real concerns, why use cullet in the first place?
   The answer? Because cullet melts at lower temperatures than virgin feedstock, it cuts the energy demand of glassmaking furnaces. Accordingly, furnace emissions are reduced and furnace life is prolonged. 
   Another advantage is that cullet “takes the place of virgin materials,” Garcia notes.
   It’s also easier to make glass from cullet, glassmakers say. When making glass from virgin feedstock, the proportions must be just right—about seven parts sand to one part each of soda ash and limestone, with water, gypsum, limited amounts of carbon, and cyanide rounding out the recipe.
   As these materials are mixed, some of the batch is lost through oxidation. In other words, if you start with 100 pounds of virgin materials, you’ll generally net about 85 pounds of molten glass. Cullet suffers no such oxidation loss. If you start with 100 pounds of cullet, you’ll end up with 100 pounds of molten glass.
   Thanks to these advantages—and despite the potential contamination problems cullet can introduce into the process—it’s still economical and beneficial to use it, Garcia says.

Batchmaker, Batchmaker, Make Me a Batch
Once adequate supplies of virgin and recycled raw material are on hand, Owens-Brockway can get down to its real business—making glass containers.
   From the storage silos, each feedstock is automatically weighed, discharged onto a conveyor, dumped into a holding bin, and then weighed again. If the weight is on target, the concoction heads on for five minutes of mixing. At this stage, water is added to the dry ingredients much as milk is added to cake batter. Its purpose is to make the mixture homogenous and facilitate cohesion in the furnace.
   From the mixer, it’s on to the batch cans, which are giant, conical, blue vats that hold the mixture—up to 10,300 pounds of it—until furnace time.
   The melting stage begins when the mixture is fed into a side-port furnace that gradually heats the mass to a toasty 2,820oF. Owens-Brockway’s A-furnace (glass producers assign letters to their furnaces in alphabetical order) has a dead-load capacity of 460 tons, while the smaller B-furnace can hold 420 tons of molten glass. Both are fueled by natural gas burners.
   These furnaces feed into the glass-forming machines, which are described in terms of their production capacity. The A-furnace, for instance, feeds into two 10-triple and one 8-triple forming machines, whereas the B-furnace is attached to one 10-triple and one 12-triple.
   What exactly do these “triple” terms mean? The answer is quite simple.
   Each furnace, or tank, is attached to several forming machines, or shops. Each line has a forming machine that encompasses 6, 8, 10, or 12 sections. Each section is capable of receiving one, two, three, or four molten glass gobs, thus producing the same number of bottles.
   So, a furnace with two 10-triple lines means the furnace is connected to two forming machines, each with 10 sections, with each section capable of producing three bottles at a time. Depending on the size of the bottle, a 10-triple machine can crank out more than 400 bottles a minute, while a 12-triple machine can produce more than 500 in the same time.
   From the batch cans, the mixture is fed through a charger into the furnace’s melting area, where it will remain for three to four hours until completely melted. The molten glass ebbs into the refiner, which stabilizes its temperature around 2,400oF, then flows into the forehearth, whose primary job is to ensure that the glass is a consistent 2,150oF throughout.
   Though the temperature is measured at each section and each side of the forehearth, including the alcove, “the front section is the critical area,” says Dean Harris, forming specialist. “If the glass there is not on temperature, you can’t make bottles. The bottle makeup is maintained in the forehearth.” To ensure that the temperature doesn’t digress from the required level, alarms are set to be triggered should the temperature vary by more than 10oF.
   After conditioning, the molten glass flows into a spout, which looks less like a spout and more like a large, rotating bowl with an outlet at the bottom. Here, a plunger forces the glass out through three holes to the waiting shears below, which slice the glass into gobs. These red-hot glass globules then enter a load funnel, falling into a scoop that loads the different forming sections of the machine. This is a critical stage. “We’re looking for even, cylindrical gob shapes,” notes Harris, “because the shape of the gob determines the shape of the bottle.”
   Gobs fly into the individual section’s blank side at a searing 2,100oF, and it’s here that the first-stage form, or parison, is made. A mold clamps itself around the gob and a plunger rises into the center, pressing it upside down into the pre-mold. Immediately, an invert arm swings the glowing, orange parison head over tail to the section’s front end. A blow tube swoops down onto the mold and expands the parison from the inside, while a vacuum pulls at the bottle’s exterior. The temperature is now between 1,000o and 1,200oF.
   Searing hot bottles slide onto the conveyor belt at timed intervals and are coated with a tin-based, hot-end mist to make them scratch-resistant. This, essentially, is the base-coat of glass coatings. If a problem is detected with a bottle at this stage, it is blown off the line, shunted to the basement, processed into cullet, and then recycled.
   Meanwhile, the bottles that pass inspection continue down the line and are “stacked”—that is, stood side-by-side—in the lehr oven for annealing, a process by which the stress that is inherent in newly formed glass is removed through uniform, controlled heating and cooling. “It’s important that the bottles are stacked well so that when they exit, the cold-end spray will be even,” notes Harris.
   During the next 25 minutes, the bottles are heated to above 1,000oF, then cooled to 700oF. When they emerge from the lehr oven, the bottles receive a cold-end, Duracoat spray that makes them even more scratch-resistant and ensures they will glide easily along the conveyors. “The spray makes the bottles slippery and glistening,” says Harris.
   Like windup tin soldiers, the bottles make their way down the conveyor belt, getting siphoned and split off at regular intervals to be packed into open-topped cartons.
   The cartons are then sent down a ramp where a machine called a palletizer automatically stacks them in a specific load pattern and places them on a pallet. “A different pattern is made for every tier so that it has stability,” Connally says. “It’s similar to brickwork.” The load is compressed, strapped, covered by a paper cap sheet that will protect the open bottles from foreign objects, and placed in one of the plant’s four warehouses to await shipping. Bar-coded load tags are placed on every pallet to tell the employee loading the truck exactly what’s there and where it’s going.
   Prior to loading, parts of the straps are cut, says Connally. “We fill parts of the truck with air bags and map the carrier’s route to avoid rough roads and any sharp, uphill climbs. Still, our biggest problem is getting the load delivered in good shape.”

A Quality Endeavor
As this production process indicates, making glass containers requires stringent quality control before, during, and after the containers are formed.
   At Owens-Brockway, for instance, every bottle is marked with a code that indicates not only the date, but also the 15-minute interval in which it was formed.
  Other checks are done as well. Employees verify the accuracy of the scales daily, temperatures are continuously monitored by machine, and both infrared and incandescent beams check bottles for flaws as they pass by.
   Additionally, Owens-Brockway uses a hot-end check. Here, bottles are randomly plucked from the line and machines measure their height, lean, wall thickness, and thread dimensions, then feed this data into a computer. If any bottle is out of round or has an uneven finish, that portion of the line can be diverted and recycled back into the system as in-plant cullet. 
   Meanwhile, in the quality lab, polariscopes are used to check the annealing process, transmission checks are conducted to verify color, special meters measure the Duracoat shell and hot-end mist application, and each sample bottle is checked for capacity and density.
   The final inspection comes as the bottles are loaded onto the trucks. Pallets are examined for load packing and to make sure that the cartons are in good shape.
   From there, they’re off, heading toward beverage filling lines and eventually to the restaurants, bars, and grocery stores nearest you. With any luck, they’ll end up again at your curbside, nestled amidst other recyclables in the bin, beginning their journey back to a cullet processor—and so on and so on.
   Let the cycle go unbroken.