The BaySystems polyurethane systems house in Otterup, Denmark, has joined forces with Isotherm AG in Uetendorf, Switzerland, to develop a cost-effective process for the production of housings and the protection of sensitive electronic components. It is based on polyurethane systems from the Baydur E and Bayflex E ranges, which are processed in a single step using reaction injection molding (RIM) technology.
Polyurethane systems outperform other materials and material combinations for this application, because they can be formulated precisely to meet individual requirements - from soft, solid elastomers to semi-rigid and impact-resistant integral skin foams. In the past, different materials were needed to meet the individual requirements - with varying degrees of success. Now there is a single material that can be customized to suit virtually any application.
Material properties can be made to measure
In the RIM process, the two main components - isocyanate and polyol - are introduced in liquid form into a countercurrent mixing head via a dosing system. These are then homogenized and injected into molds at low pressures of up to 6 bar. Here, the material reacts rapidly to produce polyurethane. Finished parts can usually be removed from the mold in less than two minutes. This leads to a very cost-effective method of combining housing production and electronic encapsulation in a single production line. The polyurethane's properties can be varied by selecting different starting materials. The low viscosity of the polyurethane reaction mixture ensures that even areas of the electronic assemblies that are difficult to reach are enclosed effectively. The low internal pressure in the mold means that RIM technology can also be used with inexpensive synthetic resin and aluminum molds.
"A further benefit of manufacturing housings with polyurethane is that the RIM process facilitates the production of molded parts with wide variations in wall thickness. For example, it would also be possible to produce thin housing shells that are rigid and lightweight - with fully integrated reinforcing ribs, ventilation slits, spring locks and metallic design elements," explains Gerd Viertel, an expert in polyurethane encapsulation of electronic components at BaySystems. "These kinds of developments demonstrate the expertise and customer-focused activities of our global BaySystems systems house network."
To meet the requirements of electronic components, it is important that the polyurethane reaction proceeds very quickly, gently and with little shrinkage, as required. Curing takes place at a maximum temperature of 120 °C. Subsequent annealing is not necessary. The thermal conductivity of the polyurethanes, which is essential for the heat dissipation of electronic assemblies, can be raised by adding fillers and is largely independent of the material's hardness.
Clear-cut benefits for many applications
Motor vehicles in particular are being equipped with an ever-increasing number of electronic applications. The delicate pin contacts of components and assemblies require reliable protection against oxidation caused by weathering. The connecting cables are exposed to constant vibrations, and their insulation must not be damaged in the event of contact with fixed parts. "These and other requirements place exacting demands on both the material and the processes used in its production," explains Daniel Lüthi, Managing Director of the process engineering company Isotherm. "Using polyurethane materials and modifying the RIM technology makes it possible to fill even the smallest spaces via, in some cases, very narrow flow paths. To this end, we have developed machines with low discharge rates of less than 15 g/s (or 10 ccm/s)."
Rigid polyurethane systems exhibit high dimensional stability and excellent abrasion resistance. The good adhesion of polyurethanes to many other materials ensures the effective long-term protection of electronic components, cables and connectors. Depending on the individual application requirements, shots of as little as 2 g are possible using specially developed metering equipment. What's more, increasing flame retardancy requirements, e.g. UL 94 V-0, are necessitating the use of greater quantities of fillers. Isotherm offers filler-resistant piston metering machines for cable and connector encapsulation for this purpose.