At this stage I'm building my first CNC router machine and it will probably take me sometime to finish it. I found a cheap and easy solution to make my plaque pattern. I used 8mm MDF routing it all around. As for the letters, I got childrens sticky foam letters in a local supermarket (very cheap). After marking the pencil lines for the letters all that was needed was to stick them into place. As past experience tought me MDF is not the best material for a pattern so this time I gave it a few coats of clear varnish and that did the trick.
After the pattern was ready I constructed a new flask from some scrap timber. Flasks dimentions: W.600mm H.170mm and the depth is only 80mm (40mm cope 40mm drag) because the pattern is only 12mm in thickness.
I started making the sand mould and I was hoping that the gaps I left between the letters and the overall thickness of the letters would be sufficient enough to prevent the sand from sticking between the gaps when lifting the pattern out of the sand.I don't know if I mentioned this before, but if the sand is being rammed too hard it will stick to the pattern when lifting the cope part.
If the sand is rammed to lightly the mould might break and the casting appearance will be damaged.
The trick is to find the right ramming strength balance.
Sometimes a bit of trial and error is needed ;-) .
In this casting I didn't use a gating system for the sprue and riser. I only used the sprue and riser feeder extension (pressure head) to prevent aluminum shrinkage. The aluminum feeders really work, I made two identical test castings of aluminum eagles. For the first one I used a sprue riser extension and the other without. The eagle casting without the feeder had shrinkage in a few spots. The eagle casting with the sprue and riser feeder was perfect.
In the images you can see three aluminum spikes on the back of the plaque, those are the locations of the venting holes to allow gases out preventing casting defects.
The plaque turned out very well without any shrinkage or major defects. Plaque size: 495x75x12mm. All the extra aluminum was casted back into ingots. I used again my new buffing wheel with the brown buffing compound. In the image of the cut sprue and riser you can see they did their job by "feeding" the aluminum casting.
It's a bit hard to see in the images the shiny finish of the letters but I can see my reflection in them.
I took this image after painting the aluminum plaque with copper paint.
