Porous steel takes cooling to the part

As most molders know, a lot of the time in any cycle is spent cooling the part. Therefore, the faster and better a part is cooled, the faster the cycle. And on the press, time saved is money earned. The problem with traditional cooling is that transfer of heat away from the molded part depends on the thermal conductivity of the mold material, water temperature, the water flow rate, and the position of the water channel in the mold. All of these variables, taken together, can make for inconsistent or unstable cooling, leading to hot spots and thermal stress, which can prolong the cycle and cause part rejects.

Aga Gas, with a U.S. base in Cleveland, has come up with a way to transfer heat directly from the surface of the part, bypassing the traditional water channel configuration. The technology that makes this possible is called Toolvac. It uses cores and cavities made of a sintered microporous tool steel combined with liquid CO2 gas. The tool surface temperature is controlled by evaporating the liquid CO2 and transporting it through the steel's pores directly to the part. This speeds cooling, which speeds the cycle.

At Electrolux's Plastcenter plant in Motala, Sweden, the Toolvac system helped knock 12 seconds off a cycle molding leveling feet for refrigerators. Plastcenter molds some 4 million of these feet, made of a 25 percent glass-reinforced polyamide, every year. With traditional water cooling, the cycle on the part was 32 seconds. After the introduction of the Toolvac technique, the cycle dropped to 20 seconds. Plastcenter sales and marketing manager Jerry Martinsson says the system not only improved productivity, but part quality as well. "Previously we had problems with air enclosures that created burn marks on the parts so that they had to be rejected," he says. "Now the air escapes through the microporous Toolvac material, and it has been possible to guarantee a steady and high-quality output of the leveling feet."

Also in Sweden, at molder Autoliv, the Toolvac system was used to take 15 seconds off a cycle molding a bobbin for a seatbelt inertia reel. This part, molded from polypropylene, must meet extremely tight tolerances at high productivity with low cost. The 2+2-cavity tool produces 15,000 pairs of bobbins a week, but the cycle was limited by thick-to-thin sections in the core that prolonged cooling. Toolvac microporous steel was placed in the core and CO2 lines were installed to help speed cooling. As a result, a cycle of 55 seconds using traditional water cooling was reduced to 40 seconds with the Toolvac system. (See diagram.)

The operating principal of the Toolvac relies on this scientific premise of heat transfer: The greater the temperature difference between the material and the cooling medium, the smaller the distance between the heat and cooling sources, and the greater the contact surface, the more heat energy can be removed per unit of time. By this premise, heat removed by conventional water cooling is less than efficient as there is little difference between the mold and cooling water temperatures; also, the heat must travel a greater distance to the drilled water channel, which has a limited surface area.

The liquid CO2 used by the Toolvac can reach temperatures as low as Ð109F, which provides the large temperature differential. By vaporizing and moving through the microporous steel, the CO2 comes in direct contact with the molded part, eliminating the distance between the heat source and the cooling medium. Finally, the CO2 expands throughout the steel, covering the entire surface of the steel/ plastic interface, providing maximum surface area cooling.

The liquid CO2 - again, Ð109F - enters the mold by way of "capillary" tubes, measuring about .020 inch outside diameter. It passes through the capillary tubes into expansion chambers machined into the cavity and/or cores. There the liquid CO2 vaporizes before heading to the molded part via the micropores in the steel. After contacting the steel/plastic interface, the CO2 exits the cavity and/or core by way of evacuation channels machined into the steel. This flow of cold gas from the expansion chamber to the evacuation channel provides an efficient and effective exchange of heat.

David Cogar, product manager, plastic applications for Aga, says that in most applications the Toolvac system is used only in the core. Also, the Toolvac system is not intended to replace water cooling. Cogar says that by maintaining water cooling in conjunction with Toolvac, the operating temperature of the mold is reduced and stabilized, giving the CO2 less extreme temperatures to overcome to cool the part. "What you're doing is trying to make the best use of the CO2," he says.

The Toolvac system includes an Aga controller that interfaces with the machine's controller. The Gas Liquid Injection (GLI) control system consists of a PLC controller, the capillary tubes, solenoid valves to control the liquid CO2 flow at the mold, and the CO2 storage system. The controller can handle up to four molds and lets you control individual cavities and/or cores. Installation of the system, Cogar says, usually runs about $40,000. This includes the controller and installation of a CO2 tank, a concrete pad for the tank, and a pump station. Use of the tank and pump are leased by the molder, the rate of which depends upon the number of Toolvac molds installed and the molder's net CO2 use throughout the year.

The microporous tool steel itself is proprietary and manufactured by Aga in its headquarters in Sweden; for larger molds a third party manufacturer provides the steel. Cogar says Aga had contracted with six joint-venture toolmakers throughout the world, who are licensed to build molds from the special steel. In the U.S., there is one such toolmaker, Marland Mold in Pittsfield, MA. There are three licensed toolmakers in Sweden and two in Germany, any of which can produce molds for U.S. customers, Cogar says. Tool costs, he says, run anywhere from 10 to 25 percent more than comparable tools from traditional steel.

The steel does have some limits that Cogar says should be noted. Materials with additives of low molecular weight can leach into the pores of the steel, eventually clogging them. Such additives, reports Cogar, usually include UV stabilizers, antistatic stabilizers, and some colorants. However, Cogar says Aga thoroughly tests each mold and material before delivery. If leaching is discovered, Aga recommends a thin coating of nickel over the cavity and/or core to keep the material in the mold. Such plating, he says, does not interfere with the CO2's ability to reach the steel/plastic interface and provide adequate cooling. Otherwise, he says, "for most homopolymer materials, it's not an issue."

As a bonus, the Toolvac system can also be used to help eject parts from the mold. As the mold opens, a small amount of liquid gas is injected into the core. The pressure increase at the surface releases the part from the wall of the mold. This method, Aga reports, eliminates marks left on parts by ejector pins. Similarly, as described in the Electrolux application, trapped air can vent through the microporous steel.