JEC COMPOSITES MAGAZINE - Issue #66 - July/August 2011 - 37

Fig. 8: The SpriForm process

subsequent assembly injection moulding (see Figure 9). Good bond strength optimizes the overall mechanical properties of the finished component. The left part of Figure 9 clearly shows a good bond strength between the injected material and the thermoplastic matrix material on the exposed organic sheet fibres following a rib stripping test on the sample manufactured with the SpriForm process. The bond was so strong that the matrix material was stripped from the fibres. On the right, the figure shows a sample that failed the rib stripping test because its ribs were cold-moulded between the injected and matrix material. In the next step, the organic sheet is transferred to the tool using the handling system. Special devices keep the organic sheet in shape in the partially open tool. The grippers which, up until this point, had kept the organic sheet in place, open and withdraw from the tool in order to complete the transfer of the preheated preform to the SpriForm tool. The gripper is now free to pick up a new preform which is being preheated in the furnace. This results in an optimized process with primary cycle synchronization similar to an inline process. The preform is then pulled and draped over the tool die using the tool’s closing function. This draping process moulds the organic sheet into its final form. Heating the organic sheet to a sufficiently high temperature guarantees good formability. Finally, a reinforcing structure (ribs) is created inside the closed tool using back injection of thermoplastic material. Here, too, a uniform positive material bond is required. As previously mentioned, the quality of the bond is largely determined by the temperature of the bond counterparts (organic sheet and injection moulding compound) at the effective interface.

The first step is to preheat the organic sheet outside the tool. Follow-up steps are synchronized with the primary cycle time in order to prevent the preheating period from determining the cycle. Infrared heat is used to heat up the organic sheet, making it malleable. The preheated organic sheet is removed from the heating zone by needle grippers mounted on an appropriate handling system (linear or 6-axis robot), and then transported to the tool by the handling system. Handling time is a

Fig. 9: Left: SpriForm process; right: standard process

process-relevant parameter, and therefore decisive for component quality. It must be kept to a minimum. In order to optimize the moulding of the organic sheet, the adapted heating curve is calculated during preliminary tests, allowing for heat losses. Optimal temperature at the point of insertion ensures good organic sheet moulding and (according to the compatibility principles) good bond strength between the rib structure (injected material) and the organic sheet (matrix material) during the

No66 July - August 2011 / jec composites magazine

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JEC COMPOSITES MAGAZINE - Issue #66 - July/August 2011

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