Mold cooling: should you try a more direct approach?

A though the vast majority of molders cool molds with equipment that controls the temperature of a circulating fluid, a few suppliers of mold-cooling systems have taken the approach of monitoring and controlling the temperature of the mold steel itself. Intuitively, this alternative strategy appears to make sense because it seems to be a more direct approach to the goal.

At least two commercially available mold-temperature-control systems are designed around this direct mold-sensing principle. Proponents claim these alternative systems result in less scrap, tighter part tolerances, and shorter cycles. Few major cooling equipment suppliers agree, although some do offer direct mold-temperature sensing as an option.

HOW THE SYSTEMS WORK

A conventional mold-temperature-control unit is based on the principle of continuously circulating a fluid stream (normally water) through the mold at a constant temperature. The fluid is heated and cooled to maintain its setpoint temperature.

In a departure from this approach, two commercial systems attempt to control the temperature of the moldcavity surface--not the entire mold. They substitute a short pulse of very cold water during part of the cycle for the continuous circulation of warmer water in a conventional system.

Both unconventional systems are fed by a central chiller or cooling tower, which reportedly provide enough pressure to give turbulent flow without the need for extra pumps.

The MoldMonitor system has been offered by Cito Products Inc. since the late 1970s. Cito president Horst Wieder estimates that the company has sold units to about 1300 customers. Originally, most units were on small presses, although Wieder claims an increasing proportion of installations are on larger presses of up to 3000 tons. Largepart applications include refrigerator panels, washing-machine inserts, automotive door panels, and truck bumpers.

The MoldMonitor consists of three basic components: a single-zone controller, heat sensor, and solenoid valves. The Cito system divides a mold into zones depending on the mold's geometry and the distribution of heat load. Each zone has its own MoldMonitor unit. The Cito system places a high-resolution thermistor sensor within 0.100 to 0.500 in. of the part surface to sense the temperature increase as plastic enters the mold.

The microprocessor in the MoldMonitor unit uses this information to start and stop a burst of cold water immediately after the mold has been filled. The pulse of water is triggered by the hot melt raising the cavity temperature near a thermistor beyond a threshold set for the type of plastic being molded.

Tuning of the pulse is based on the amount of BTUs to be removed from the part. The cooling pulse is never throttled; if more or less cooling is needed, the controller adjusts the length of the cooling pulse. After the pulse ends, the remaining heat in the part is allowed to dissipate through the mass of mold steel.

The second system was introduced about five years ago by REPS Ltd. in the U.K., and has been offered here for about a year by Injection Molding Industries. More than 200 units have been installed worldwide. Although none have yet been sold in the U.S., multinational companies such as Apple Computer, Ford Motor Co., and a major razor-blade manufacturer reportedly are using REPS units in overseas molding operations. The range of applications includes auto bumpers, grilles, lenses, as well as electrical parts, computer housings, plumbing, dunnage, and personal-care products.

Basic components of the REPS system are a controller with up to four zones of control, heat sensors, an induction-heating coil, and solenoid valves. Sensors are not placed as close to the cavity as in the Cito system--no closer than 0.500 in., according to Donn Seres, president of Injection Molding Industries.

The cooling pulse is triggered from the molding press (not the heat sensors) at the end of the injection phase of the cycle. No time is lost waiting for the melt's heat to travel through the steel to reach a sensor, says Seres. The controller analyzes the mold-temperature profile over the last several cycles and calculates the length of the cooling pulse needed for the next cycle.

At the start of cooling, all water valves are turned on. As the temperature of one zone reaches its setpoint, the valve controlling that zone is turned off to provide additional water flow to those zones that still require cooling.

The induction coil installed in the mold surrounding the core/cavity area allows heating and cooling to occur separately within the same molding cycle. The coil is turned on after cooling to maintain a minimum cavity temperature. The coil can be also used for mold preheating at start-up.

WHAT'S THE BENEFIT?

These suppliers claim that their pulsed-cooling systems result in lower costs. faster cycles. and higher-quality parts. Cycles are said to be faster because the mold runs warmer during the filling phase, easing melt flow. Better part quality reportedly results from less molded-in stress, owing to more uniform temperature across the cavity surface.

Seres acknowledges that the REPS system may have a higher initial cost than conventional systems. However, the price includes four zones of closed-loop cooling, two zones of heating control, and two zones of open-loop proportional cooling control. Installation of the sensors and induction coil in the mold generally accounts for one-third of system cost.

Wieder estimates that the Cito system is generally one-third less expensive than conventional units. While the MoldMonitor eliminates expensive pumps, power lines, and heaters, it does require insertion of sensors in the mold.

Both suppliers claim much lower operating costs, particularly in regard to energy usage. A recent test by a U.K. molder of nylon electrical connectors resulted in a 22% cycle-time improvement with the REPS system over a conventional cooling system, says Seres. Energy usage by the REPS system was less than one-third that of the conventional unit's pump alone (not including the heaters).

Tests conducted by Dicten & Masch, a custom molder in Nashotah, Wis., showed cycle-time reductions of up to 18.5% with the Cito system as compared to a conventional unit without increasing energy consumption.

Wieder adds that pulsed-cooling systems are more compact, taking up no floor space and reducing clutter around presses.

Both suppliers of pulsed systems say start-up time is relatively short. Because it can use the induction coil for mold preheating, the REPS unit can usually produce good parts within three to seven shots, according to Seres. Cito, which does not use an induction coil, recommends heating up the mold by running the melt 5-10 [degrees] F warmer than normal on an accelerated cycle. Startup is fast because only the surface of the mold needs to be heated, says Wieder.

One small custom molder in Wisconsin says the Cito mold-temperature controllers on its 20 presses (35 to 150 tons) keep molds within a narrower temperature range than is provided by conventional cooling systems and has helped to eliminate temperature swings and variations, which results in higher quality parts. This molder also likes the pulsed system because it eliminates the maintenance requirements of pumps and heaters.

SKEPTICS REMAIN

Suppliers of conventional cooling systems have not rushed to imitate the two mavericks. Wayne Lange, industrial market sales manager at Sterlco, concedes the lower initial cost and energy use of the pulsed systems, but he counters that monitoring the temperature of the fluid gives a truer reading of the overall process. He cautions that the location of the probe may not give a true indication of the entire temperature profile in the mold.

Cito's Wieder replies that probe location is a question of common sense. Usually, it is placed midway between the gate and end of fill. For complex parts, it may be placed near a hard-to-fill area. On the REPS unit, Seres says that triggering the cooling from the injection press makes the location of the heat sensor somewhat less critical.

Meanwhile, Pat Oza, president of Cooling Technology, believes that direct mold-temperature sensing requires constant opening and closing of the valves and puts a strain on the system. In his view, there are fewer problems with hot spots in the mold when using the continuous flow of water in a conventional system.

Wieder denies that there are hot or cold spots because his system allows the water to "soak" in the mold channels after the end of the cooling pulse, dissipating heat through the mold mass.

Al Fosco, v.p. sales of Conair Tempro, believes that turbulent flow is a more critical factor than water temperature in providing uniform cooling. He says a conventional unit with its booster pumps is likely to provide a higher pressure differential and therefore more turbulent flow and better cooling than a pulsed system with colder water. He also believes that conventional hot-water circulation units are better equipped to process high-temperature engineering resins.

John Fitzgerald, regional v.p. of AEC/Application Engineering, cautions that the thermal and mechanical "inertia" of the process may make it difficult to match the BTUs from the melt by opening and closing a water valve.

DIRECT SENSING AS AN OPINION

Some suppliers of conventional mold-temperature-control systems offer direct mold-temperature sensing as an option. Logic Devices, for example, supplies sensors in the mold for some of its customers. According to president Paul Allen, sensing the temperature directly at the tool may help to compensate for changes in other parameters, such as molding cycle speed or ambient air temperature. "With a thermocouple in the tool, you are closer to what you want to control," he says.

Other suppliers, like Advantage Engineering, tried applying direct mold sensing to their conventional systems but saw no clear benefit.

Most suppliers of conventional mold-cooling systems see only a very limited market for pulsed-cooling systems. One major chiller supplier says that fewer than 1% of its customers even bring up the idea of pulsed cooling. "We've looked at the possibility of developing a pulsed-cooling system, and decided there was no market for it," says an executive of this firm. He acknowledges that pulsed cooling with direct sensing may have advantages in very tight-tolerance applications, but says putting a thermocouple in the mold is not necessary in the majority of processes.