An automotive manufacturer requested an investigation to manufacture this product using alternative methods. These are normally used by means of forging the rings. The alternative method selected was the investment casting route. Superior materials could be used with this near net shape manufacturing method, so that one of the normal heat treatment procedures could be eliminated. Several synchron rings, seven, were to be made if the first trail was successful. The rings were of different sizes and the geometry varied slightly.
This was a very interesting project. The manufacturing process starts with the wax injection moulding process. The wax patterns, synchron rings, are then assembled to tree. Several rings are then made on one tree. The wax tree is then covered with fine sand to obtain a smooth surface finish on the final castings. Several coatings with slurry and sand allow a shell to be build up around the wax patterns. The wax is then removed so that the cavity is hollow. A TORVAK flash type furnace is normally used for this process. The shell is then baked and hardened. The shell is removed from the casting and the metal casting cleaned. A hand fettling process then removes gate and feeding systems. Final machining then takes place only on the required surfaces.
Only 2D manufacturing drawings were available, so 3D CAD models had to be made. Using the 3D model data, the mould was designed. A toolmaker was selected to manufacture the wax injection mould tool. The selected 3D geometry was converted into the appropriate format so that the toolmaker could generate the CNC toolpathes required to machine the electrodes.
A trail mould was initially made. This mould was used to determine the actual component shrinkage and the repeatability of the manufacturing process. Making several test pieces we were able to gather sufficient data. The test pieces were measured at a metrology laboratory. It was not so simple to measure these items as the gating system caused the rings to shrink somewhat triangular. It was necessary to shape these rings round again by using a press. This data was then used to produce the final mould for this particular ring to achieve a casting within specification. The same shrinkage was used to manufacture another three different moulds for another three different synchron rings.
Accuracy played a big role in this manufacturing process. Part of the quality requirement is the metallurgical aspects. The design of the gating and feeding system was critical. The ideal gating system could not be used as it complicated the fettling procedure. We decided to use a three star type feeding system. The gates needed to be designed correctly in order to avoid porosity in the ring. Porosity occurred in the ring and also closed to the gates. It was clear that the gate geometry needed to be changed. The final changes resulted in porosity free synchron rings.
Design, Product Development and Tooling
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