Vacuum Casting – The Process
First, the silicon mould is made. The SLA master patten is suspended in a box by the gate, runners and risers. The box is then filled with silicon rubber, which completely encapsulates it. When fully cured, the mould halves are seperated by cutting into the silicone to meet the patten. It is a skillful process and the geometry of the part will determine the mould-making technique.
The master patten is removed from the mould. Fine details such as logos and surface textures are reproduced exactly.The split line is continued across the open central area by a thin film, which was attached to the patten prior to casting in silicon. This technique produces a split mould, which has incorporated gate, runners and risers. The silicon is flexible and so will typically last for around 20 cycles before it has to be remade.
It is taped together to stop any movement during the casting pipes are attached which will deliver the liquid polyurethane (PU).
The two-seperate parts of the PU are stored at 40 °C. Mixing them together causes a catalytic and exothermic reaction to take place and the PU warms up to between 65 –70 °C.
The casting process takes place under vacuum where liquid PU is drawn into the mould. This ensures that the material has no porosity, will flow through the mould cavity and will not be restricted by air pressure. When it is poured in, it flows down the gate and into the runner system. The PU is held high above the mould, so as it poured into the mould and runs underneath, gravity forces it to rise upwards. The runner is designed to supply a curtain of liquid PU. It flows through the mould and up the risers. There are many risers to ensure that the mould is evenlyand completely filled.
After a few minutes, the mould cavity is filled and the vacuum equalised. The mould is left closed in an oven at 40 °C until the PU has fully cured.
The ejection process begins by blowing pressurized air into the risers. This helps to release the casting from the mould. The two halves of the mould are seperated to reveal the casting. It is removed with the risers intact, which are removed by hand.
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