Hot pressing of complex and near-net-shape 3-D components
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The company tfc and the Fraunhofer IGP cooperated in a joint research project on a hot-press technique for processing of prepreg materials. In this article the partners demonstrate how the production of three-dimensional FRC components benefits from the advantages of the curing technology.
New Hot-pressing Process
With respect to laminate quality and mechanical properties, prepreg technology is one of the most important processing techniques for structures made of fiber-reinforced composites (FRC). After depositing the prepregs — flat reinforcement fibers pre-impregnated with a thermoset resin — in the forming tool, the laminates can be cured in autoclaves, conventional convection ovens or hot presses .
The two techniques based on convection (heat transmission) of air (autoclave, oven) require long cycle times for heating and cooling of components and tools. Therefore, they are only suitable for low production numbers. Since, usually, one-sided tools are employed, high-quality surfaces and good size accuracies can be realized only on one side of the piece. The special vacuum construction — necessitated by the open tool — requires a large amount of manual work, as well as a high proportion of disposables (vacuum film, peel ply, etc.). Automation of the vacuum film application has not been achieved yet [1, 2].
Hot presses use efficient conduction (heat transfer) by means of closed tools. Because of short cycle times, they are suitable for production of larger quantities. By use of two-sided molds, high-quality surfaces and close component tolerances can be realized on both sides. High degree of automation and precise control of curing cycles make possible the production of high-quality and reproducible components [1, 2].
Structure and Operating Principle of the Hot Press
The two heating plates have a connected load of 4800 W each and a maximum operating temperature of 200 °C. They are controlled separately by two microprocessor controllers and two Pt100 resistance thermometers, their maximum heating rate is 5 °C/min. Two insulation plates prevent heat flow into the machine table and the pressure plate. The cooling channels integrated in the heating plates can be connected to a water pipe by means of a tube system. The water cooling is regulated by controlling the volumetric flow; hence the maximum cooling rate depends strongly on the mass of the forming tool. The hot press can be regulated continuously up to a maximum press capacity of 24 t. The specific pressure applied in the tool thus depends on the pressing area.
In order to study the hot-pressing process, a forming tool made of aluminum precision blanks is used for the production of two-dimensional laminates. The function surfaces of the tool are polished after milling to enable the production of very high-quality surfaces. The two tool parts are equipped with temperature sensors for process monitoring and more precise process control.
The instant of time when the maximum pressure is applied is an important parameter of the process control. The curing pressure should be applied just before the increasing temperature causes the declining resin viscosity of the top ply to become sufficiently low for resin flow to occur. If the pressure is applied too early, there may be an undesirably large resin loss. If, on the other hand, the pressure is applied too late, the resin flow may be insufficient due to increasing resin viscosity. Therefore, the timing of the pressure application depends on the heating rate .
The effective heating rate of the forming tool, slowed down by its net mass, for maximum heating of the heating plates (5 °C/min), is found by means of the temperature sensors. The result is about 4 °C/min. Due to the resulting longer cycle times, lower heating rates are not considered. Subsequently, oscillation tests are carried out on a rotational rheometer in order to study the flow behavior of the utilised prepreg material. Several plies are sheared in a well-defined measuring gap between two plates at the effective heating rate of 4 °C/min. The resulting resin viscosity curve displays a minimum at about 141 °C. Therefore, the application of the maximum pressure is set at a tool temperature of 130 °C measured by the temperature sensors. The maximum average cooling rate of heating plates and tool realized by the water cooling is about 7 °C/min.
Identification of Optimal Process Parameters
The parameter analyzes focus on the most important process variables, pressure and temperature, whose magnitude and duration significantly affect the quality and performance of the final products .
The hot press can be equipped with forming tools for production of two- or three-dimensional components.
To examine the optical and mechanical properties of the laminates, the pressure of the hot press is varied when manufacturing components approximately 2 mm thick from eleven prepreg plies. Independently of the pressing area, five different pressing forces of the apparatus between 0.5 and 4 t are tested. The focus is on these rather low forces with the aim to keep low the future investment and operating costs of a press machine for serial production. Heating and cooling rate (4 °C/min and 7 °C/min) as well as the instant of applying maximum pressure (130 °C) are kept constant.
The instant of time when the maximum pressure is applied is an important parameter of the process control.
Interim evacuation of the trapped air when positioning a single prepreg ply is an alternative method for reducing the pore content. However, this requires specially adapted tools (seal) and pressing equipment. Applying a vacuum during consolidation or curing has no beneficial effects .
After optical evaluation, laminates are measured in five places. Subsequently, samples are taken for thermal calcination according to DIN EN ISO 1172 to determine the fiber volume fraction (FVF), as well as for dynamical-mechanical analysis (DMA) according to DIN EN ISO 6721 and for the three-point-bending test (3PB) according to DIN EN ISO 14125. The results of the tests show a clear trend. The component thickness decreases and the fiber volume fraction increases with increasing pressing force, because more resin is pressed out of the mold, Figure 5 (a). FVF values up to 59 % can be realized, comparable to the FVF of 60 % that is considered an industry standard for aerospace applications . Higher fiber contents lead to higher stiffness and strength, Figure 5 (b). Storage moduli at 30 °C, determined by means of DMA, increase from 0.5 to 4 t by about 6 %, bending moduli by over 17 %. Maximum bending stress also increases significantly by about 7 %. Better stiffness and strength accompany a decrease in maximum outer fiber strain by about 9 %.
Production of a 3-D Demonstrator Component
One additional prepreg ply (n = 9) in the mold gap leads to an increase of the fiber volume fraction to about 68 %, two additional plies even increase it to 76 %. These values cannot be realized with the hot press, not even when producing two-dimensional laminates.
Due to the considerably increased mass of the tool, an effective heating rate of only about 3.3 °C/min is reached during the production of the demonstrator components. Additional measurements of the prepreg material on the rheometer, however, only result in a minor shift of the viscosity minimum to lower temperatures. Therefore, the instant of applying the maximum pressure does not have to be adjusted. Cooling is also significantly slower at about 4 °C/min.
The parameter analyzes focus on the process variables pressure and temperature.
The precise, polished forming tool in combination with accurate cutting and draping ensures a high surface quality, exact contours and very low post-processing effort. One of several reproducibly manufactured half suitcase shells is illustrated in Figure 1.
Integration of Inserts
The introduction of functional elements such as inserts should be integrated into the production of FRC components in order to avoid time-consuming and expensive post-processing steps . To verify the suitability of the hot-pressing technique for the integration of inserts, the first forming tool is modified slightly. Drill holes are put into the upper tool part to allow the embedding of so-called composite threaded bushes in the laminate.
Summary and Outlook
The hot-press technique is a curing technology for prepregs with great potential for serial production of FRC components. The hot press developed here enables the production of simple two-dimensional structures, as well as complex three-dimensional ones. High fiber volume fractions and accordingly good mechanical properties can be realized with sufficiently high pressure. Time consuming and expensive post-processing steps are no longer required due to near-net-shape production as well as the possibility of integrating functional elements into the process.
For transfer to serial production, the technique developed here requires a further decrease in cycle times. This goal can be achieved through greater automation of the individual process steps.
In this context, the deposition and draping of prepregs are of primary concern. During the present study, the forming tool still was loaded manually. For production of two-dimensional laminates, the associated effort remains acceptable. But complex three-dimensional geometries such as the half suitcase shell require distinctly more extensive and time-consuming manual working steps. Furthermore, only accurate work guarantees the reproduction of high-quality components. For this reason, development and testing of concepts for automatic draping and preforming are required.
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