High-speed molding presses use chillers with centrifugal compressors to provide the tight temperature control needed for injection molding.
Billions of cell phone cases, laundry baskets, plastic totes and storage bins are produced via plastic injection-molding. That is why there is a lucrative, multi-billion-dollar plastics injection-molding industry in the United States — and why companies in that industry are facing stiff competition from overseas producers. One enterprising manufacturer found a way to reenergize its domestic plastics manufacturing with the help of energy-efficient centrifugal compressors.
“In a plastic injection-molding machine, it is important to cool the mold properly to get maximum productivity,” says Mike Clavelli, West Coast regional manager for Niles, Ill.-based Thermal Care, Intertech’s process cooling system provider. As a manufacturer of process cooling equipment and systems, Thermal Care installed two centrifugal chillers using variable-speed compressors from Danfoss Turbocor at injection-molding company Intertech Plastics, when the Denver-based company expanded its operations.
“When Intertech added two new Husky 1,100-ton high-speed molding presses, we implemented the chiller installation using an array of variable-speed technologies. This solution delivers variable cooling capacity that is precise, reliable and efficient, which is giving Intertech a huge domestic and global competitive advantage,” Clavelli says.
Handling the Heat of Plastic Injection-Molding
An injection-molding machine employs two basic components:
- A clamp unit to hold the mold.
- An injection unit where plastic is heated and mixed before being injected into the mold.
Time, speed, pressure and temperature all must be controlled to ensure proper operation and maximum throughput.
Depending on the resins, mold temperatures can range as high as 180°F (82°C). At the beginning of each production cycle, the clamp unit closes the mold, and a screw in the injection unit pushes forward to shoot molten material into the mold. Continual pressure is applied to hold the plastic in place until it solidifies.
While the material in the mold cools, the screw rotates and retracts to draw in new material for the next shot. When the cooling part reaches the precise temperature setpoint, the clamp opens to eject the finished piece and start the next cycle. Depending on the part size and type of plastic, the entire cycle can take anywhere from a few seconds to more than a minute.
“Productivity depends on cooling that mold properly,” says Clavelli. “The difference in throughput can be huge. For example, with good temperature control, you might be able to make 20 parts per hour instead of just three.”
Energy-Efficient Cooling for Injection-Molding Machines
For efficient cooling, Thermal Care designed and manufactured a plant-wide central chiller cooling system to handle the requirements of Intertech Plastics’ two injection-molding presses.
The system comprises two centrifugal chillers, a cooling tower and condenser and evaporator circuits using variable-frequency drive pumps. The piping was designed to scale up to add another chiller if needed.
To maximize the chillers’ efficiency, Clavelli applied Danfoss Turbocor centrifugal compressors with an oil-free, magnetic-bearing design instead of rotary screw compressors commonly used in the industry.
“We’ve been successful using Danfoss Turbocor centrifugal compressors in injection-molding cooling for nearly a decade,” says Clavelli. “In this case, a 140-ton chiller employs two Danfoss Turbocor TT300 compressors, and a 70-ton chiller uses one TT300 compressor.”
In this installation, the chillers supply a 1,000-gal thermal storage tank with chilled water at 50°F (10°C). From there, a liquid cooler pump control circulates water to the injection process to cool the molds as needed. The tool and mold sections can range in temperature from 150 to 180°F (66 to 82°C). Depending on the temperature requirements for the part, a mold-temperature-control unit may be used to mix warmer water with the 50°F supply water to provide the exact degree of cooling required. The temperature of the water that returns from the process is approximately 60°F (15.5°C).
Because the amount of supplied chilled water varies, this application proved well suited for the Danfoss Turbocor compressor’s variable-speed capability.
“A rotary screw compressor may use stepless speed control or slide valve control to vary capacity, but those will be inefficient in certain conditions,” says Clavelli. “Danfoss Turbocor compressors vary compressor speed to deliver the needed cooling capacity.”
When full cooling capacity is not needed — which is about 95 percent of the time — the Danfoss Turbocor design reduces the centrifugal compressors’ rotations per minute to produce the amount of compressed refrigerant gas needed to match the cooling load. In a centrifugal compressor, electrical consumption is directly proportional to motor-shaft speed. Therefore, reducing the number of rotations per minute reduces the amount of electricity consumed linearly: Running at 50 percent speed reduces the compressor’s electricity consumption by about 50 percent. At the same time, the coefficient of performance (COP) — a measure of energy efficiency calculated by dividing the amount of cooling by the amount of power supplied to the compressor — is much better as the cooling load is reduced.
For example, at 40 percent capacity, the COP of a centrifugal compressor from Danfoss Turbocor can be as much as twice that of a rotary screw compressor, according to the company.
Integrates with a Free Cooling Configuration
Clavelli appreciates the inherent energy efficiency of the centrifugal compressors, but he also values their ability to use colder cooling tower water.
The low outdoor ambient temperatures in the Mile High City mean that Intertech can allow the ambient conditions to cool the water in the cooling tower. That cool water in the cooling tower can be used to reduce the condenser water temperature rather than producing cool water via the chillers, Clavelli explains. “Whenever you can do that, it means less work for the compressor.”
Often called free cooling, using ambient conditions to deliver process cooling typically provides an energy savings over other more active means of cooling.
Unfortunately, conventional centrifugal chillers cannot always run with cooling tower water below a 65°F (18°C) temperature limit. Below that temperature, the oil in the refrigerant cannot circulate properly to ensure compressor lubrication. Therefore, for those systems, 65°F becomes the minimum entering condenser water temperature.
“We don’t have oil circulation or oil management issues with Danfoss Turbocor compressors…because the Danfoss Turbocor design uses magnetic bearings that do not need oil,” Clavelli notes. “The impeller shaft spins in a friction-free magnetic field. Because there is no oil to clog the system, a condenser water reset option can be used to enhance chiller efficiency to reduce energy consumption.
“That makes it is easy to use a free-cool circuit with variable-frequency drive pumps for both the cooling tower and chilled water loops,” he continues. “This configuration lets us take full advantage of off-design outdoor temperatures to avoid mechanical compression as much as possible. Variable-speed capacity control lets us scale cooling capacity up and down as needed.”
The result: compared to the chillers it replaced at Intertech’s facility, the Thermal Care system with Danfoss Turbocor compressors has cut energy costs dramatically. Instead of paying $38 per hour for cooling, costs are $3.40 per hour.
The increased energy efficiency by operating in free cooling mode means that cool days at Intertech are welcome. Clavelli recalls a recent report from Intertech: On a cool Denver morning, the engineer stopped by the chiller system. The total compressor demand was at 3 percent — meaning the company was taking advantage of 97 percent free cooling plant-wide.
Taking Advantage of Compressor’s Features
Clavelli also notes that the new chillers have a 50 percent smaller footprint than the chillers they replaced, freeing up space in the mechanical area. The magnetic-bearing design eliminates the bearing race-support structures and oil system, reducing actual operating weight to 265 pounds per compressor. (By contrast, a conventional screw compressor can weigh more than 1,000 pounds.)
Additionally, with a peak acoustic level of 71 dBA, the new chillers are quieter than a typical screw compressor. Clavelli notes that for Intertech, low sound levels were critical to this project. “It is nice to be able to stand beside the chiller while the compressors are running and hold a conversation without ear protection.”
Another critical benefit is controllability and reliability. Among the features integrated into the system by Thermal Care are on-board programmable logic controllers (PLCs) on the chillers to provide a user interface. With its inherent digital operation, each Danfoss Turbocor compressor provides nearly 80 points of diagnostic information: temperatures, pressures, volts, amp draws, log and save data, adjust setpoints or control parameters, alarm history, faults and demand profile, including kilowatt usage.
“Access to that level of information ensures us and the customer that things are running right,” Clavelli says, “and that we could take prompt corrective action if needed, with the option to access that information remotely over the Internet.”
Another advantage for Intertech is the total cost of ownership for the new chiller system. The savings the chiller plant affords helps make Intertech more competitive in plastics injection-molding market.
By cutting cooling costs by an order of magnitude — from $38 to $3.40 — the new chillers will save Intertech about $42,000 in annual energy costs at the current process load compared to rotary screw chillers. Just as important, the new cooling system has improved the manufacturing operations in key ways:
- Cycle-time efficiency has improved by 5 percent.
- Plant yield has increased by 9 percent.
- Scrap has been reduced by $194,000.
Also, the installation qualified for a five-figure rebate from the local utility, Xcel Energy.
Clavelli also notes that the oil-free centrifugal compressors lower maintenance costs. He points to the compressor’s magnetic-bearing design, which he says cuts Intertech’s total cost of ownership. “By enabling a chiller solution that enhances the manufacturing operation, we’re giving Intertech a significant competitive advantage. In fact, they were able to attract a large customer to reshore 70 percent of their business, which added 25 percent more jobs,” Clavelli says.
To learn more about magnetic-bearing compressors from Danfoss Turbocor Compressors Inc., Tallahassee, Fla., call 850-504-4800 or visit www.turbocor.com. To learn more about process cooling equipment and systems from Thermal Care, Niles, Ill., call 847-966-2260 or visit www.thermalcare.com. To learn more about Intertech Plastics, visit www.intertechplastics.com