Plastic Automobile Windows
Process Technology. Plastic is finding increasing use as a replacement for glass in automobile windows. An extremely dynamic market is developing, but one that places special requirements on materials, designs and production techniques. Success depends on an efficient logistical chain that addresses the product-specific requirements and the complex manufacturing processes.
Injection Compression Moulding Techniques Set the Scene
Currently, there are basically three different injection compression moulding techniques in use. Most closely related to the classical injection compression technique employed, for instance, for thinwalled articles is the so-called long-stroke compression technique utilising the clamping unit. Engel Austria GmbH, Schwertberg/Austria, demonstrated this process at the K 2004 in a two-component variation with two horizontal injection units and a stack mould with a rotary center section. In the longstroke compression technique, the approximately 4 mm thick and almost 1000 mm wide cavity of the injection compression mould is opened by an amount about equal to the part thickness; as a result, the pressure required during filling drops drastically. Subsequently, the clamping unit of the machine closes the enlarged cavity and the compression motion distributes the melt. Because of the large part dimensions and the edge gating required for windows, asymmetrical loads that must be counteracted are created in the mould during the filling and compression phases. With the technology described here, this is accomplished by means of platen parallelism control for the moving platen that regulates the pressure in the hydraulic cylinders on the four tiebars individually.
Another variation of injection compression moulding that is employed to produce windows with low moulded-in stress is the expansion compression technique offered by Krauss-Maffei Kunststofftechnik GmbH, Munich/Germany. The mould is completely closed at the start of injection; it is also possible to close the cavity to below the final part thickness desired. The benefit of this measure: it addresses the jetting phenomenon that causes problems with edge gating. After the cavity is filled completely, the part thickness is increased by enlarging the cavity, under controlled pressure, by the amount of the compression stroke through the introduction of additional melt. Once the desired compression stroke is reached, the gate is closed' and the compression force is applied and counteracts the shrinkage of the plastic like a holding pressure distributed over a large area. For precise control of part formation, the stroke of the clamping unit is monitored very carefully. Using this technique, Freeglass GmbH & Co. KG, Schwaikheim/Germany, is already producing stationary side windows and roof modules in series.
For long flow paths, Battenfeld GmbH, Meinerzhagen/Germany, in cooperation with Summerer Technologies, Rimsting/Germany, has developed the "IMPmore" process, which it demonstrated in public for the first time at the NPE 2003. The basic idea here is likewise a phased enlarging of the flow cross-section that, in this case, is provided by a wedgeshaped opening of the cavity. This is achieved by tilting the moving platen of the clamping unit with the cavity surface by a few degrees prior to closing of the mould. Here, too, closing of the machine platens is delayed until after injection has started. By narrowing the wedge-shaped compression gap, the cavity fills at very low pressure. As shown at the K 2004 with an approximately 1m² demonstration window, this process permits injection moulding of even large windows with little moulded-in stress. In conjunction with the edge gating, closing of the wedge-shaped compression gap also creates asymmetrical opening forces in the mould. These are counteracted with the aid of hydraulic cylinders incorporated into the mould, so that the machine is subjected to an apparently uniform opening force. Especially good access for robotic removal of the moulded part is provided as the machine tiebars retract from the stationary platen during motion by the mould.
Quelle: Kunststoffe 2/2005 Christian Hopmann, Sebastian Göbel, Jan Forster Plastic Automobile Windows 18.02.2005