Casting of intake pipes by silicone rubber moulding.

Brajlih, T. ; Drstvensek, I. ; Balic, J. 等

Abstract: During the process of modifying and optimizing the intake system of a two-stroke motorcycle engine the original position and orientation of a carburettor were changed. Therefore a new design of an intake pipe with different functional and constructional demands was needed. In order to quickly produce a functional new pipe model for engine testing, some rapid manufacturing procedures were used. Mould patterns were manufactured with rapid prototyping technology and the functional parts were produced with silicon rubber moulds. Additionally a small batch production of intake pipes using the silicone rubber moulding was investigated. It proved that silicone rubber moulding could be a rational solution for series of up to hundred pieces if the mould consists of multiple nests.

Key words: Rapid manufacturing, Rapid prototyping, Rapid tooling, Silicon rubber moulding.

1. INTRODUCTION

Optimization of a two-stroke motorcycle engine has resulted in a modified carburettor position. Therefore a new design of an intake pipe was needed. In order to quickly test the new engine layout, several functional models of the pipe had to be produced in a relatively short time. It was decided, that rapid manufacturing procedures will be used in production of this series of pipe models.

2. BUILDING A PROTOTYPE

For pipe's prototype production the PolyJet[TM] rapid prototyping procedure has been chosen (Objet, 2005). Therefore a three dimensional CAD model of the pipe was drawn according to the functional and constructional demands of the modified intake system. Then the CAD model was uploaded into the rapid prototyping machine software package where the model orientation in the workspace and the support structure was defined.

[FIGURE 1 OMITTED]

When the prototype's building was completed the support material was removed with water jet. Then, the prototype was installed in the engine's intake system, in order to test the constructional properties. Functional properties on a working engine were tested also, but the prototype could not be used in a long-term testing, because the working temperature around the pipe is around 60-70[degrees]C and the prototype's material softens considerably after being exposed to such temperatures for some time.

3. PREPARING THE MOULDS

For the production of the functional intake pipe vacuum casting of the two-component resin with the temperature resistance of 135[degrees]C was used. Mould was made from silicon rubber. Because the pipe is hollow, the mould with an inserted core had to be used, so the model can be demould after the curing process is completed (Rapid tooling, 2005). Then the inserted core must be removed from inside the pipe. In the case of a straight pipe removing the core would not be a problem. But in our case the pipe is curved, therefore the core can not be pulled out. Instead the core must be melted out. So it is very important for the core to be built from an appropriate material. The core must remain solid up to the preheating temperature of the silicon rubber mould (65[degrees]C). In addition the core material must be completely melted at a temperature lower than the temperature resistance of the cast material of the pipe (135[degrees]C), so it can be removed without any influence on the shape and the structure of the pipe. Considering this demands, the eutectic, low melting point (69[degrees]C) alloy MCP69[TM]was chosen for the core (MCP, 2005). The pipe's core had to be cast separately in a different silicon rubber mould. Because of the chosen lost core casting procedure the original prototype could not be used as a mould pattern. Instead the rapid prototyping procedure was used to build two different patterns needed for the moulds. First pattern was the pipe's core with two positioning elements and the second pattern was the pipe together with the inserted core (and positioning elements). Both patterns were built sliced in half, so the parting plane could be added easily. After the prototype building was completed, both patterns were polished, in order to improve the surface. After the inlets and raisers were added, patterns were positioned inside wooden boxes and the silicon rubber was poured around them. Then both moulds were put into a vacuum chamber and decompressed in order to remove air and water from the silicon rubber. When the silicon rubber vulcanized, both moulds were cut and patterns removed (Drstvensek, 2004).

4. CASTING THE FUNCTIONAL PIPE

[FIGURE 2 OMITTED]

Core was cast with the low melting point alloy that was melted and poured into the silicon rubber mould. The mould was preheated on 70[degrees]C to prevent the premature solidification of the alloy. After the cooling phase the core was extracted from its mould and inserted into the pipe's mould

[FIGURE 3 OMITTED]

Closed pipe's mould was preheated to 65[degrees]C (which is lower than the melting point of the core's alloy (69[degrees])). Then the mould was put into a vacuum chamber. Both components of the resin were poured into two cups (each for one component) in appropriate mixture ratio. The temperature of the curing process of the resin has been set at 65[degrees]C for 45 minutes. Then the pipe (with the metal core still inside) was demould.

For the core to be removed the part must be warmed above the melting point of the core's alloy (69[degrees]) but still in the limits of the temperature resistance of the resin that the pipe was cast from (135[degrees]C). After the core's melting out, the inner surface of the pipe could be further cleaned with hot water (70-80[degrees]). Finished pipe was then mounted on the engine which could now undergo more long-term testing than with the rapid prototype of the pipe.

[FIGURE 4 OMITTED]

5. PRODUCTION OF SERIES

After successful testing of the pipe, demand was made for production of 100 pieces series. At this point the main weakness of the silicone rubber moulding has presented. Due to the complexity of our model the existing two moulds would be unusable after casting app. 20 pieces. The simplest solution would be to prepare new moulds after the previous moulds become unusable. But producing 100 pieces in such a manner would not be time/cost efficient. Therefore it was decided to prepare moulds with several nests. Simple calculation was made to estimate the most cost efficient number of nest in a mould. It was taken into considerations that to prepare a mould with more nests more patterns are needed and have to be build with a RP machine.

[FIGURE 5 OMITTED]

Secondly the minimum number of moulds required to produce a series of 100 pieces was estimated (presuming that one mould would endure 20 casts).

[FIGURE 6 OMITTED]

Figure 6 shows that common costs of production are decreasing until the mould with 7 nests. Further increase to the number of nests results in raise of production costs due to the higher number of patterns required. However, regarding the small difference between 4 and 7 nests mould (app. 2[euro] on individual piece) and the technical difficulties in making the mould with seven nests the four nests mould version was chosen for series production.

6. CONCLUSION

This project has presented a mayor weakness of the rapid prototyping techniques. Because of the RP material's properties the prototype can not always be functionally tested. It was shown that vacuum casting into silicon rubber moulds with a wide array of different resins with various properties available, can nicely solve such problems. While the RP pipe was not suitable for engine testing the RP procedure is still necessary for the quick manufacture of the mould patterns. In a case of larger series of the parts (from the same resin) is needed, the mould can be made with several nests and the costs of part production can be considerably lowered.

7. REFERENCES

Drstvensek, I. (2004), Layered Technologies, Fakulteta za strojnistvo, Maribor

MCP Rapid tooling homepage available at: http://www.mcp-group.de/, Accessed: 2005 5 20

Object geometries homepage available at: http://www.2objet.com/home.asp, Accessed: 2005 5 22

Rapid tooling survey page available at http://home.att.net/~castleisland/tl_c.htm, Accessed: 2005 5 9