Machine Design

Thermally conductive polymers delete the heat

The conventional plastic shown on the left can't spread or dissipate heat, resulting in a localized hot spot. Conversely, the thermally conductive plastic, CoolPoly, (on the right) uniformly spreads heat, giving a more isothermal profile across the part. CoolPoly also dissipates the thermal energy to its surroundings, lowering maximum part temperature.

Designers at Apple Computer, Cupertino, Calif., realized, after the initial design was completed, that they needed a heat spreader to dissipate heat from the video processor in the PowerBook computer. But space was extremely limited. The unique design of the heat spreader was possible because the thermally conductive plastic was injection moldable. To make a metal or ceramic part to fit in the available volume and meet all component clearance requirements was impossible.

A new family of polymers from Cool Polymers Inc., Warwick, R.I., can replace metal, ceramics, and conventional plastics for heat-sensitive applications. The polymer family, called CoolPoly, is said to be 100 to 500 times more thermally conductive than standard plastics.

Conventional, nonthermally conductive polymers tend to experience localized heating that can degrade the polymer and lead to part failure. But the addition of nonmetallic fillers in commodity and engineering thermoplastics such as ABS, nylon, liquid-crystal polymers (LCP), and polyetheretherketone (PEEK) makes uniform heat dissipation possible.

Parts made from the CoolPoly polymers are also 40 to 50% lighter than equivalent aluminum and components. They can be processed similar to most engineering thermoplastics and find use in a wide range of applications including computers, automotive, electronics, motors, heating and cooling systems, housings, gaskets, and lighting.

CoolPoly composites have low coefficients of thermal expansions (CTEs), low mold shrinkage, and lower operating temperatures, that will help add life to components and devices that use them. This combination of properties produces parts that meet tight design tolerances with little distortion. According to Cool Polymer's Jim Miller, wall thicknesses of 0.020 in. and as molded tolerances to 0.0003 in. make CoolPoly a viable metal and ceramic alternative for dimensionally critical parts in medical, optical, mechanical, and microelectronics applications.

Thermally conductive plastics
The ability of a material to conduct heat is characterized by its thermal conductivity and measured in units of Watts/meter-Kelvin (W/mK). Conventional plastics are considered good thermal insulators (poor conductors). All unfilled plastics have a thermal conductivity around 0.2 W/mK. Styrofoam, a material engineered for its thermal resistance, has a conductivity of 0.02 W/mK or one order of magnitude less than the base polymer.

Historical attempts to modify plastics to improve thermal conductivity met with limited success. New composite materials and improved injection-molding processes now let plastics carry thermal conductivities at least 100 times that of the base resin. This thermal conductivity is similar to metals such as stainless steel and titanium — 15W/mK, but less than die-cast magnesium and aluminum alloys which range between 50 and 100 W/mK, or extruded aluminum at 150 W/mK.

Heat transfer is only partially dependent on material thermal conductivity, however. In many designs thermally conductive plastics can match or exceed the performance of metals and ceramics. The tendency for designers to focus only on material thermal conductivity may unnecessarily limit design choices. This is because total system thermal management is determined by a combination of three modes of heat transfer: conduction, convection, and radiation. Most applications have significant conductive and convective thermal resistances.

Heat sinks, for example, conduct heat to the surface then transfer it to the surrounding air by convection. The sum of these resistances determines how efficiently the heat sink cools the device. As a result, total heat transfer is not linearly dependent on material thermal conductivity. Besides, many applications don't require the highest conductivity material to maximize heat transfer.

Historically, heat-sink designs were not truly engineered because of limited material choices and design and analysis tools. For instance, a common material for heat sinks is aluminum because it has good thermal conductivity and is not as heavy as copper. But tools such as computational fluid dynamics (CFD) which calculate heat-transfer performance of any design and material choice. It lets engineers leverage the plastic's advantages, namely design flexibility, parts consolidation, and low weight, while meeting the heat-transfer requirements of the application.

Information for this article provided by Jim Miller, Cool Polymers Inc., 333 Strawberry Field Rd. Warwick, RI 02886, (888)811-3787,

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