Taking the heat (away) with pulsed cooling

You wouldn't guess it based on the relatively few molders who actually use it, but pulsed cooling has been around for more than 20 years. However, it's been just in the last few years that technology has made this method of mold temperature control more attractive to molders.

Look on the Internet at injection molding forums, and you often see a molder asking about the fundamentals of pulsed cooling. Most have heard of it, a few have seen it, several are curious: What is it? How does it work? Does it work? Is it worth it? How much does it cost? What do I do to get into it?

Pulsed cooling is one of those technologies that requires you embrace a relatively different concept, in the process setting aside some of the mold temperature control tactics you've used for so long. The physics that describes pulsed cooling is probably voluminous enough to make for a good master's thesis. But even the basics are worth exploring.

Conventional Control
Conventional mold temperature control, say pulsed cooling advocates, is inefficient because it attempts to keep the mold at a set temperature throughout the cycle, whether the cycle is in the fill or pack and hold stage. This creates what's called a thermal barrier. It's a gradient band of temperate mold steel that starts at the cooling channels and emanates outward. Eventually these gradients merge, segregating the core and cavity from the rest of the mold (Figure 1). As a result, the steel on the shop-side of the mold is unused for cooling and can sweat if its temperature falls below the dew point.

Further, while coolant enters the mold at a set temperature, it absorbs heat as it travels through the mold. Therefore, cavities near the inlet of circulating fluid tend to run cooler than the cavities near the outlet. The proponents of pulsed cooling believe a mold kept at one temperature throughout the cycle can restrict the flow of resin entering the mold; as soon as melt hits the cavity, it starts to cool and solidify, even before the part is full. This, combined with cool and warm spots, can lead to molded-in stress, uneven cooling, warpage, and out-of-spec parts.

Complicating this configuration is that most mold temperature controllers measure the temperature of the circulating fluid, not the temperature of the mold steel itself. While the fluid in the cooling channels absorbs heat from the mold steel, the temperature of the fluid is always "behind" the temperature of the mold and will rise and fall only after the temperature of the mold steel rises and falls. This delay makes it difficult for mold temperature control systems to adjust quickly to changing temperature conditions.

Pulsed Cooling Control
Simply put, pulsed cooling is a closed loop system that measures mold surface temperatures and pulses cool or ambient water to the mold to quickly dissipate heat created by the addition of hot melted resin. The idea is to add and remove heat from the mold in natural peaks and valleys in the molding cycle (Figure 2). "It's a cyclic action," says Horst Wieder, president of Cito Products (Watertown, WI), a pulsed cooling systems manufacturer. "It's not extrusion. It should be treated cyclically."

First, pulsed cooling, despite the relatively sophisticated name, requires little in the way of additional hardware. To make it work, all you need are at least two mold temperature sensors, a regulating valve between the press and the coolant source, and a controller. Beyond that, it draws from the same chillers and coolant systems you have right now. That's the easy part. Where most people get hung up is in understanding just how and why it works.

Pulsed cooling relies on three basic principles that make it different from traditional cooling:

Measure and control the temperature of the mold steel. This concept is at the core of pulsed cooling. Instead of measuring the temperature of the coolant, molders install temperature sensors in the mold itself, at least one for each half of the mold (Figure 3). Holes are drilled between the cavity and the cooling channels and any competent molder can do the job himself. The sensors are then connected to a pulsed cooling controller, a relatively simple device that tracks temperatures and modulates the flow of coolant.

Each sensor in the mold is called a zone. At least two zones are recommended for effective cooling. You can go up to four, using eight or more zones for larger and multi-cavity molds. You can use pulsed cooling without temperature sensors, but such a configuration strays greatly from the fundamentals of the process.

Don't inhibit the flow of resin during fill by circulating coolant. This is where most molders start to balk at pulsed cooling. The idea is, during the fill stage of the cycle, to cease the flow of coolant, letting it sit in the channels. This is done by the pulsed cooling controller, which shuts off the valve that regulates the flow of coolant from the chiller.

When the hot resin hits the mold, it flows freely into the mold. Rather than immediately trying to lower the temperature in the cavity, heat is allowed to build naturally, unabated. Flow, in theory, is relatively even and uninhibited, producing a stronger, more robust part. Heat generated, meanwhile, transfers from the part to the mold steel to the static coolant in the channels. The heat can also move past the channels to the steel on the shop side of the mold, warming it and preventing sweating.

After fill, pulse cool water through the mold to remove heat. Let's say you've installed the temperature sensors, hooked up the controller, and connected the regulating valve between the press and the coolant source. You decide that at ejection, you want the mold steel to be 150F--the temperature that produces good parts at the best cycle. So, you start a cycle, coolant stops flowing, resin is injected, and heat starts building. Then, at the end of fill, or at a set maximum temperature--say 220F in this case--the controller opens the regulating valve and cool water pulses through the mold, removing heat and quickly bringing the mold back down to 150F before part ejection.

How does this work? Letting heat build naturally is not as drastic as it might sound. Remember, the fill stage for most cycles doesn't last more than a second or so, depending on the size of the part, melt pressure, and rate of injection. And even on large parts with multi-second fills, it's still a minority fraction of the cycle.

So if the mold is 150F at the start of the cycle, a pulsed cooling system might let the temperature go up to 220F, as opposed to a continuously circulating system that might hold the temperature to 180F. The question is how can pulsed cooling bring the mold back down 70 deg F (from 220F to 150F) faster than the continuously circulating system, which has only 30 deg F to go?

It relies on a basic premise of thermal conductivity: As the temperature difference increases between the two media (coolant and mold steel), the rate of heat transfer also increases. Remember, you're pulsing cool water, maybe 60F or so. That means the temperature difference (ÿt) is 160 deg F. This difference means heat quickly and readily transfers from the part to the mold steel to the coolant. The ÿt in a conventional system, where the coolant might be 130F, is not as high. The rate of cooling, therefore, is reduced. The result, most often, is a faster fill (no resistance) and faster pack with repeatable hold.

Who Supplies It
Currently, there are two primary suppliers of pulsed cooling systems. One is Wieder's Cito Products. The other is RE Promotional Services (REPS), based in Cannock, England. Systems from each suppliefollow the same cooling philosophy, but there are some differences imechanics and execution.

For instance, Rowland Evans, owner of REPS, interfaces his controller with the machine's and starts coolant pulse when it reads the end-of-fill signal.

The coolant then runs for a calculated time to remove the heat input from the polymer and bring the mold back to ejection temperature. Wieder says his systems circulate the coolant when the sensors reach a certain maximum temperature, usually at the end of fill. Then, at a set minimum temperature, circulation is stopped.

Evans says most molders can grasp the concept of pulsed cooling. What many don't like, he notes, is the thought of putting a temperature sensor in the mold. "I think it's a lack of understanding of how very important the sensor is," he says. "It's just a reluctance to do something they're not used to doing." For many molders and shop floor managers, he says, drilling a hole in the mold falls outside of standard operating procedure. It contradicts what they've done and have known to work for many years. Still, knowing the temperature of the mold metal is the centerpiece of pulsed cooling. "Molders talk about mold temperature, but most of the time they're talking about water temperature," Evans says. "And it's fatal."

Both Evans and Wieder say pulsed cooling can be used on any injection molding application, regardless of material, cycle length, or heating and cooling requirements. The two also admit most of their new business comes from molders who've hit the wall with a particularly troublesome high-heat material that's chronically out-of-spec (see sidebar) but emphasize that any molding application can benefit from the technology. Evans says pulsed cooling is widely used on molds with molding surface temperatures between 50F and 140F. Pulsed heating and cooling is used between 86F and 380F without the use of heated fluids. Wieder says he has a system installed on a 5-second cycle mold as well as on molds with cycles that are minutes long. "There really is no limit," he says.

Experimental Results
Theory and philosophy aside, there is empirical evidence demonstrating the effectiveness of pulsed cooling. Most notable is a paper authored by Amanda Yelsh and Peter Zuber, technical development engineers at GE Plastics. The paper documents the results of a series of tests Yelsh and Zuber conducted while molding with pulsed cooling systems. The part used in the tests was a laptop computer bezel. Yelsh and Zuber installed four temperature sensors, one in the A half and three in the B half of the mold.

Three different materials were processed in this mold with different mold-temperature-at-ejection and melt-temperature setpoints (see Table 1). GE used a Cito Products pulsed cooling system for the tests and also installed strip heaters in the mold to help it reach operating temperatures faster on cold starts. All three materials were processed with a traditional circulating mold temperature control system and the pulsed cooling system. Yelsh and Zuber measured mold surface temperatures at part ejection with a pyrometer and recorded cycle times for each material and cooling configuration to obtain results.

For cycle time, Yelsh and Zuber measured a 14 percent reduction between conventional and pulsed cooling systems for the PC/ABS and a 10 percent cycle time reduction for the PC. The PBT could not be measured because significant shrinkage in the part prevented the discovery of a good benchmark to identify minimum cycle time.

Next, they measured the mold surface temperature upon part ejection. On average, Yelsh and Zuber discovered that pulsed cooling held average surface temperatures at ejection to within ±3.2 deg F in the three zones measured. Conversely, conventional cooling, on average, held surface temperatures to within ±23.7 deg F (see Table 2). Not only did the pulsed cooling system hold temperature better, but in the PC/ABS test, each pulse averaged .2 gal of water per cycle. This compares to the 3.8 gal average used in a continuous-flow system.

The Cost
As with most systems designed for use with molding machines, the cost of a pulsed cooling system is dependent on a lot of things. For a ballpark estimate, Evans says he sells a basic four-zone system for as little as $6000, but a typical system runs about $10,000. For an eight-zone configuration, you can expect to spend $18,000 to $20,000. Wieder says his systems generally run about $2000 a zone, including sensors, valves, controllers, and other ancillary equipment. A basic two-zone configuration will run $3600 to $4000, he says.

Return on investment also "depends," but among savings and improvements Evans and Wieder cite a 6 to 10 percent cycle time reduction, reduced water use, less molded-in stress, reduced scrap, and reduced energy use.

Wieder and Evans both say installation and setup is relatively simple and can be done by the molder.