In the future, thin-film heating will allow plastic parts to be produced with greatly improved surface quality. Researchers at the Fraunhofer Institute for Mechanics of Materials IWM, Freiburg, Germany, say they have found a way to make the whole process more energy efficient.
For example, in making waffles, the iron must be heated to just the correct temperature. The same principle, they say, applies to the manufacture of plastics parts, such as displays, facings, covers and instrument panels, using injection molding techniques.
A liquid plastic melt is injected into a steel forming tool, which is heated like a waffle iron. The point is to produce a perfect cast of the tool’s surface, which may be smooth or feature a functional structure. Using injection molding to realize the desired structural and functional surface qualities of plastic parts is intended as a one-step process without the need for any extensive finishing work. The process also has to be economic and energy efficient.
To manufacture plastic parts with high-end surfaces, the entire forming tool is heated to around 110°C using a technique known as variothermic tempering. Thermoplastic materials, such as polycarbonate, are processed at similar temperatures. In order to get the finished plastic part out of the mold without damaging it, the mold must be cooled by around 20° to 30°C. This has to be done for every production cycle before the whole process can begin again, which consumes a considerable amount of energy, they explain.
Working to improve the situation, the researchers teamed up with the Kunststoff-Zentrum in Leipzig to develop a new kind of tempering technique that, depending on the product, is up to 90 percent more energy efficient than other techniques used. The trick, they say, is to avoid heating up the entire tool, which can weigh half a ton or even more depending on the plastic part being produced. All that must be heated is the surface of the tool that actually comes into contact with the plastic melt.
This is possible due to thin-film heating. Researchers coat the wall of the forming tool using a vacuum-based coating technique known as sputtering. The sputtered material is deposited onto the surface of the forming tool in layers only a few micrometers thick. Not only can this extremely thin coating be used to heat the forming tool surface to the desired temperature, but it can also withstand the thermodynamic stresses that occur during injection molding.
Electrical insulation is provided by a ceramic layer, which shields the conductive heating layer from the steel tool underneath. The layer that actually gets heated is made of a specially designed conductive hard material. In this case, the challenge of sputtering lies not only in producing a perfect insulating layer so as to avoid any short circuits, but also in integrating a sensor into the thin-film heating layer. A sensor placed here can measure the temperature of the tool wall and be used to regulate the manufacturing process.
To achieve this, the researchers set about integrating incredibly fine thermocouples, made from nickel or nickel-chrome alloy and each just a few hundred nanometers thick. Thermocouples can be produced using thin-film technology and incorporated into the insulation layer. Due to their extremely low mass, thermocouples react incredibly fast to temperature changes and make it possible to directly measure the temperature of the tool wall. Through a series of laboratory experiments, the researchers were able to demonstrate that thin-film heating can be used to achieve the desired tool wall temperature very quickly indeed.
The researchers are now looking for industry partners to help prepare the process for use in series manufacture.