Gecko Investment Casts

I spent some time on the gecko project today. I've been making some single sided wall hangings along with the free standing lizards.

From gecko

This is a wax imprint taken from the top half of the two piece mould seen in earlier posts. Its located in a makeshift formwork cut with the foam saw (Had to use it for something) Glue gunned together and dipped in hot wax (around 65 degrees celcius max) and sealed with a fine clay.
The next step is to mix up the plaster investment. Hopefully I will be able to get a couple of aluminium casts out of this mould if I'm careful. The investment mix is 1 part fast setting plaster of Paris, 1 part silica dioxide (Silica flour from pottery suppliers) and 1 part cold water by volume. The water is measured into a bucket, the the silica is poured in gently and finally the plaster.
If you use a bucket you should get a small hill of plaster sitting above the surface of the water.let the bucket sit for 5 minutes until the plaster at the surface looks waterlogged. This stops lumps forming in the plaster.
When you mix it, don't stir, just put your hand in to the bottom of the bucket and squish the mix between your fingers until it's thoroughly mixed. It kinda feels like soft mud while your doing this. Mix in a couple of handfuls of glass fibre chopped into short lengths, this reinforces the mould and stops it flying apart if something goes wrong when you pour metal into it.

From Holding Tank

Then pour the mix into the formwork starting from the lowest point. Keep drizzling the mix in slowly to a single spot. This lets the mould fill evenly and expels air bubbles as it goes. Then give the formwork a couple of sharp taps and walk away. Don't go near for at least 90 minutes. Vibration now while it cures will weaken the mould and may cause failure. After 90 minutes steam out the wax. I've been using a modified wallpaper steamer at school and it does the trick. Then as soon as the wax is steamed out transfer the mould to a kiln and ramp the temperature 50 degrees an hour to 650 degrees celcius, hold an hour and ramp down to 400 before removing and pouring. This ensures all the wax is burnt off and minimises thermal shock when the metal is poured in.
I tend to bury the mould in sand before pouring to stifle any leaks and offer an extra layer of safety in case the mould explodes. I like to plug the sprue of the mould with clay while I bury it and pull the clay plug out just prior to casting. This keeps any shit from falling in the mould. Obviously an open mould like this one can't be plugged but needs to be dusted out prior to casting.