Deerfield, FL -The notion of an all-plastic heat exchanger may seem a bit contradictory. After all, plastics insulate while heat exchangers need just the opposite-a material that conducts heat. But the wrong thermal properties couldn't stop Alma Coats. In her job as a polymer chemist for Peregrine Industries, a manufacturer of heat pumps for pools, she recently helped develop a new type of heat exchanger made entirely from a liquid crystal polymer (LCP) modified to promote thermal conductivity.
Lighter and more corrosion-resistant than the brazed copper and aluminum models currently used by Peregrine in its pump systems, this prototype LCP heat exchanger completely shuns metal, according to Coats. "Most so-called plastic heat exchangers consist of copper tubes surrounded by plastic," she explains. The Peregrine heat exchanger, by contrast, doesn't sport even a bit of metal, not even for the fittings.
Of all the plastics available in thermally conductive formulations (see sidebar), LCP provided the best balance of properties for Peregrine's application, which calls for both corrosion resistance and good mechanical properties. According to Coats, LCP is inert to swimming pool chemicals and refrigerant. "We deal with a very corrosive environment," she says. "Many plastics would have a hole eaten through them in a heartbeat." LCP also offered the mechanical properties needed to withstand the pump system's operating pressures, which can exceed 4,000 psi.
At the same time, LCP offers a high viscosity, enabling extremely high loadings of the thermally conductive additive package-a proprietary graphite filler and other materials to aid processing. Coats says Peregrine has injection molded heat exchanger parts with filler loadings as high as 68% by volume. At those levels, "the LCP mostly acts as a matrix for the conductive filler," she says. LCP also has the barrier properties to prevent any migration of refrigerant, adds Scott Hay, an engineer whose product development company, 3DE, helped Peregrine model the heat exchanger in PTC's Pro/Engineer.
To put all its material requirements together in one formulation, Peregrine worked with RTP Company (Winona, MN) to compound a material with a thermal conductivity of 25 W/m-K, a tensile strength of 10,000 psi, and a heat-deformation temperature of 440-500F at 264 psi.
Peregrine will ultimately license this patent-pending material technology for use in other applications, and Coats notes that the amount of thermally conductive filler can vary to suit other thermal-management applications. "We can go from heat sink levels to in excess of 25 W/m-K," she says. What's more, the technology is compatible with other functional requirements, including impact modification, lubrication, static dissipation, and UV weathering. "The plastics can be engineered to meet a range of customer requirements," she says.
Geometry counts too. As much as the materials matter, Peregrine's heat exchanger also relies on a new design that departs from traditional brazed units. "The material and design work together," says Coats. Peregrine's design consists of a series of injection-molded LCP plates joined into a stack with a structural adhesive and overmolding. The plates vary in size according to heat exchanger capacity. Each plate features a pattern of thin ribs that define two sets of channels-one for water and the other for a Freon refrigerant. "It's not just the material, but also the thinness of the ribs that lets the heat pass through efficiently," she says, noting that the ribs are only 0.024 to 0.062 inches thick. What's more, the channels have been designed to promote a turbulent water flow, upping the efficiency ante even more, according to Coats.
Before it molded even a single plate for its plastic heat exchanger, Peregrine went through loads of upfront engineering work. The company hired a consultant to perform a computer simulation of substantial difference in plastic's thermal conductivity in the through- and in-plane directions. Peregrine also employed 3 Dimensional Engineering (Pompano Beach, FL) to tweak the design for moldability and help design the tooling. And the company worked with Ralph S. Alberts Co. (Montoursville, PA), an injection molder that specializes in rapid prototyping, to produce these difficult plastic parts in a hurry (see sidebar).
With its heat exchanger now in the prototype stage, Peregrine still has some work to do before it goes into production-most of it to optimize the design for manufacturing and assembly. For example, given LCP's broad chemical resistance, finding the right adhesives has proven to be a challenge. "There are hundreds of thousands of adhesives on the market," says Coats. "A large percentage of them would not work for our application." So Coats is currently investigating a wide variety of methods to join the plastic plates-from vibrational welding to heat staking to new, marine-grade structural adhesives.
Before it goes commercial, Peregrine also has to finish testing its heat exchanger to see how well the plastic works as part of a whole system. After all the hard work, the Peregrine heat exchanger may never offer the heat-transfer capabilities of a steel heat exchanger. But then again, it doesn't have to match metal to be successful. "We don't need the optimum DT as long as we have the corrosion resistance," Coats says. "Sometimes steel is overkill."
Epoxy tooling takes the heat-and the pressure too
Time waits for no product-not even for radical ones. So when Peregrine Industries moved ahead with its revolutionary all-plastic heat exchanger, it looked for a fast turn-around from injection molder and rapid prototyper Ralph S. Alberts Co. (Montoursville, PA). Working from a stereolithography master of the heat exchanger plates, Alberts created prototype tooling from a tough, new castable epoxy from Ciba Specialty Chemicals (East Lansing, MI).
According to Mike Downs, Albert's prototyping manager, the whole art-to-part process took about four and a half weeks. The bulk of that time, however, wasn't devoted to producing the tool-which is made completely from epoxy except for a few aluminum inserts on the parting line. Instead, it took time to learn how to mold Peregrine's material, a highly-filled LCP that it did not easily flow into the thinwall heat-exchanger plates. With a 68% filler, melt temperature of 760F, and injection pressures of 18,500 psi, "it's not an easy material to work with," Downs notes. "You can't cheat on temperature and pressure."
Though it's a so-called prototype tool and runs a difficult material under high temperatures and pressures, the mold has held up remarkably well. So far, Downs has gotten more than 400 parts from it. "And the ribs are holding up too," he says.
Downs believes this tooling material opens up the door to all sorts of high-temperature, high-pressure engineering plastics-such as Ultem or PEEK-that had been difficult to mold in cast epoxy tools in the past. As a material for bridge tooling that spans the gap between prototype and production volumes, "this epoxy may be the Holy Grail, "Downs says.
More heat, less
Thermally conductive plastics are hardly new, but in years
past, sky-high prices limited their use. "The fillers were expensive,"
says Brian Ruhland, an engineer for RTP Corp. (Winona, MN), a compounding
firm whose products include thermally conductive plastics. Today, however,
the cost-benefit picture has started to shift with newer fillers that cost
less than yesterday's thermally conductive additives while promising
better thermal and mechanical performance.
RTP, for one, will this year offer new thermally conductive compounds that depart from the expensive carbon-fiber technology as well as from metallic or ceramic fillers that could compromise properties. Scott Koberna, product manager for thermal compounds, estimates the newer forms of graphite filler will be roughly 50% the cost of carbon fibers. These newer compounds will join RTP's current line-up of thermally conductive compounds in LCP, PEEK, PC, nylon, PPA, PPA, and other engineering materials. Other companies, including LNP Engineering Plastics (Exton, PA) and M.A. Hanna Engineered Plastics (Norcross, GA), offer thermally conductive materials of their own.
Promising applications for the most thermally conductive compounds include a variety of automotive and industrial heat exchange applications. "There are tons of applications that use metal only for thermal conductivity," Koberna says, and he believes many of these applications are ripe for replacement with lighter, corrosion resistant plastics. Consumer electronics, where packaging requirements sometimes favor small, free form parts, could also benefit from injection-molded plastic heat sinks. "Thermally conductive plastics give the OEM an alternative to metal in many applications," Koberna says.