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There are several reasons why metal still has a strong position in a wide range of automotive applications: good mechanical properties such as stiffness and impact strength are two of the most commonly cited. Until now, the challenges to shift from metal to plastics were primarily focused on these mechanical properties. But now increasingly E&E systems are being introduced which have to meet different requirements such as heat resistance, electrical conductivity/shielding and especially thermal conductivity. One example is high brightness LED lighting, whose performance strongly depends on temperature. Another example is power electronics where high currents and power are generating heat. Finally batteries, where good performance depends critically on temperature. For these applications, aluminium is currently predominant. But is this the only valid candidate? And what level of thermal conductivity is really needed?
ThermaTech headlight (photos: PolyOne)
Polymer solutions provider PolyOne has taken the thermal conductivity challenge and worked on developing formulations to reach high thermal conductivity values to compete with aluminium, while taking other properties into account.
Five times less thermal conductivity than aluminium. But what is necessary?
The thermal conductivity of the Therma-Tech thermally conductive formulations from PolyOne can reach up to 20W/mK in-plane, which is 50-100 times greater than standard thermoplastics, but only 1/5th that of cast aluminium.
In order to determine the levels of thermal conductivity which have to be reached, PolyOne carried out an analysis, which has shown that materials with thermal conductivity >1W/mK already begin to significantly reduce the heat build-up in the part compared to nonconductive materials. But this analysis also revealed that a thermal conductivity of 10W/mK can provide performance in an application that is comparable to aluminium, even though it has a higher conductivity (about 100W/mK). This is due to the fact that for solid bodies, convection is the main driving factor in thermal transfer, together with their shapes. The use of aluminium can therefore appear to be “over-engineered” for most heat sensitive applications, and it was determined that a material with about 10W/mK, such as thermally conductive thermoplastics, could do the job as well.
Solution case studies in hot spot reduction
The thermally conductive Therma-Tech solution from PolyOne has been employed successfully in automotive lighting applications where high temperatures are present. For example, the material was employed to reduce a hot spot in an automotive lighting system mounted on a premium German car. The customer had a hot spot issue in the headlamp, and was willing to go to high heat-resistant plastics or aluminium, which would have necessitated addition of a painting process step. Instead of resisting the heat, PolyOne proposed to dissipate it with thermally conductive thermoplastics. As a result the customer was able to use a PA66-based thermally conductive formulation instead of aluminium, resulting in a 37% weight saving, eliminating the painting process stage, and resulting in a 35% cost savings per part.
Another example is the use of the thermally conductive thermoplastic in a lamp containing LEDs. Because of their complex nature, LED lighting systems are prone to localised hot spots and related thermal management issues. Die cast aluminium was first chosen for the heat sink, but there was a need to cut costs, whilst offering a greater design freedom. According to PolyOne, the use of two formulations of thermally conductive Therma-Tech grades could solve the customer’s challenges, with 33% less weight, and the standard injection moulding processing meant less scrap, resulting in a saving of 13% on the final price.
In general, Therma-Tech conductive solutions are said to enable a manufacturer to continue using an already purchased injection mould, thus avoiding the additional costs for mould modifications which would be required to switch to an expensive high temperature thermoplastic. Compared to standard thermoplastics (for instance PA66 GF30), thermally conductive materials quickly solidify when injected into the mould. To overcome this problem, the temperature has to be raised slightly to ensure the mould cavity is completely filled. The switchover point, the point where the injection phase switches over to the holding pressure phase, takes place later, and because of this the switchover pressure also increases. Due to thermal conductivity, the cycle time can be reduced by up to 50%. The end result is a component with better dimensional stability with less anisotropy.
Therma-Tech heat sink for LED lights
Freedom of design, and guidelines for designers
In general, the use of thermoplastics offers a greater freedom of design than aluminium. However, design guidelines should be considered: cooling surfaces need to be adjusted in regard to the thermal conductivity of the material. The higher the thermal conductivity, the smaller the base surface has to be. However, the higher the thermal conductivity, the longer the cooling ribs need to be. It is also possible and recommended to look for hybrid solutions when heat sources bring high power, in order to get the best out of the two materials while keeping the design freedom and compactness of the system.
Linear LED module from the company Kruunutekniikka
Summary – metal replacement in heat sensitive automotive applications
Automotive customers face challenges with heat management in advanced lighting systems, HEV/EV components, HVAC and fluid handling systems. Metals and aluminium are not the only options, and thermally conductive plastics can be considered as an alternative in many cases.
With a typical thermal in-plane conductivity of 20.0W/mK, thermally conductive thermoplastics improve cooling and reduce temperatures at the heat source. And analysis showed that using a metal heat sink with a higher thermal conductivity would have only a minor impact on temperatures and heat management, but would have a major influence on cost and weight.
www.polyone.com
