By James D. Miller
Edited by Jean M. Hoffman
Conventional plastics, on the other hand, although lightweight and easily molded, are good thermal insulators and therefore poor conductors. Thus, conventional plastic components overheat often developing localized "hot spots" because they can't dissipate or spread heat effectively. These hot spots degrade the mechanical performance of the part which eventually may fail.
The recent development of thermally conductive plastics now lets designers specify injection-moldable polymers that carry the same heat-transfer capacity of metals and ceramics. Through parts consolidation and more design freedom, these injection-moldable resins often form parts half the weight of their aluminum counterparts.
Thermally conductive plastics also have good chemical resistance and give up heat faster during molding than conventional plastics which drops cycle times 20 to 50%. An inherently low coefficient of thermal expansion (CTE) boosts dimensional stability and lowers shrink rates. This lets them replace metals and ceramics in dimensionally critical parts for microelectronic, optical, mechanical, and medical applications.
HOW THEY WORK
Thermally conductive plastics come from both engineering and commodity thermoplastics such as polypropylene (PP), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), nylon (PA), liquid-crystal polymers (LCP), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK). These base resins are compounded with nonmetallic, thermally conductive reinforcements. These reinforcements boost thermal conductivities 100-fold and have little affect on the base polymer's existing manufacturing processes.
Conductivities for thermally conductive polymers typically range from 1 to 20 W/mK but a few grades conduct up to 500 W/mK. This is 5 to 100 times greater than conventional plastics which have conductivities on the order of 0.2 W/mK. Thermally conductive plastics also challenge metals such as stainless steel which has a conductivity of 15 W/mK and some aluminum casting alloys with conductivities which range from 50 to 100 W/mK.
Thermally conductive polymers have higher flexural and tensile stiffness and lower impact strengths than conventional plastics. The reinforcements expand the effective temperature range and reduce flammability of the base polymers by lowering application and "hot spot" temperatures. The reinforced polymers can also be either electrically insulative or electrically conductive — providing EMI/RFI shielding properties.
One of the most effective ways to design with thermally conductive plastics is to begin with thermal modeling and prototyping. Design tools such as computational fluid dynamics (CFD) calculate heat-transfer performance of any design and material choice. This saves time and money up front, and ensures confidence when proceeding to manufacturing.
Designers at Intellegent Motion Systems, Marlborough, Conn., for example, used die-cast aluminum to build a combination enclosure and heat sink for a stepper motor with integrated electronic controls. Not satisfied with aesthetics, tolerances, and cost, IMS used CFD modeling to help pinpoint a thermally conductive plastic alternative. The CFD helped identify a material with a lower thermal conductivity than die-cast aluminum. In spite of lower thermal conductivity, the plastic part produced almost identical thermal performance as the aluminum. Both materials kept the housing temperature under 60°C with the motor running at top speed and torque.
The fact that the plastic and the metal transfer heat at similar rates explains a critical design point. Total heat transfer involves not just conduction, but also convection, and radiation. In the stepper-motor design, heat transfer is limited by how well the housing dissipates heat via convection which is constrained by surface area of the housing as well as the airflow around it. The big plus for thermally conductive plastic is that it halves the total manufacturing cost and lets IMS meet tolerance goals of ±0.002 in.
OTHER DESIGN TIPS
Although thermally conductive plastics have similar processing properties to those of the base resins, pay attention to these design details:
Wall thickness: A few thermally conductive plastics have higher viscosities than their conventional plastic counterparts. Therefore, thin walls over larger areas can be more difficult to mold. The thickness-to-flow ratio is better than die casting, but is not equivalent to that of the base resin. A recommendation is to use ribs as flow leaders. Ribs help offset filling difficulties in some parts while improving strength and heat transfer.
Tight tolerances: High fill rates tend to lower mold shrinkage. Typical shrinkage in the direction of flow for plastics with thermal conductivities of 15 W/mK can be as low as 0.0001 to 0.001 in./in. While shrinkage in the transverse direction of flow ranges from 0.001 to 0.002 in./in.
Corners: Thermally conductive plastics are stiffer than conventional plastics. All internal corners should have a radii to help reduce stress and increase strength.
Special features: Delicate structures such as fins and ribs should be reinforced due to the stiffness of these plastics. More generous radii also helps. Finally, the increased stiffness of the thermally conductive plastics also may limit the size of features.
WHY PLASTICS CAN MANAGE HEAT LIKE METALS
In many applications, convection rather than conduction limits the total heat transfer. Heat transfer is determined by a combination of three modes: conduction, convection, and radiation. Because heat often moves through a part faster than it can be removed from its surface, heat transfer and thermal conductivity are not linearly related.
The following theoretical analysis of heat transfer across a flat plate helps illustrate that in many applications thermally conductive plastics can manage thermal energy as well as metals and ceramics.
Conductive heat transfer (T1 – T2) through the one-dimensional, (1.5 × 1.5-in.) flat plate is the movement of heat from the heat source to the opposite wall of the plate through the plate's thickness (l). Convective heat flow is the temperature difference between T2 and the surrounding ambient air temperature, Ta (25°C).
In the example, the temperature differential ΔT due to conduction (T1 –T2) scales with the conductivity of the material. For every tenfold increase in conductivity the ΔT drops by a factor of 10. However, the absolute temperature drop (Δ) is not linear with conductivity. For example, an increase in conductivity from 0.2 to 2 W/mK results in a drop in temperature across the plate of 200°C. Yet, a similar tenfold increase in conductivity from 20 to 200 W/mK results in only lowering the temperature 2°C across the plate. Thus, there are diminishing returns on the system temperature differential (the effectiveness of the heat transfer) with increasing thermal conductivity.
Additionally, on the convective side, the ΔT (T2–Ta) is independent of the material thermal conductivity. The convective ΔT depends only on the surface area of the plate exposed and the heat-transfer coefficient (h). Note that the radiation mode of heat transfer is also independent of the material thermal conductivity.
Finally, the lower the sum of the conductive and convective ΔTs (ΔTtotal) the more efficient the heat transfer. In this example, the ΔTtotal decreases with increasing thermal conductivity up to about 20 W/mK. Above 20 W/mK, the convective ΔT overwhelms the conductive ΔT and there is no further significant decrease in the ΔTtotal.