- Assignment 09
Molding and Casting
To learn the technique of molding and casting I made simple stackable checkers.
I used Fusion 360 to make the drawing as it already has CAM functions integrated. The shape of the checkers is very simple and easy to realize as rotational solid.
After I cloned it with a rectangular pattern and added the joints and a mouth. Finally I added the walls and the centrelines, this for both halves.
To mill the master I chose to use two tools, a ball end mill and a flat end mill.
Ball end mill
With the Ball end mill I have created two toolpaths, one for roughing and one for surface finishing.
Flat end mill
With the flat tip I have reworked all contours on the contact surface.
To export the toolpath I used the Post Process feature and selected Roland ISO as Post Configuration. This I did separately for both tools so I have two files, one each.
I chose polyurethane foam to make the master because it is very easy to mill and even in case of accidental collisions I would minimize damage. So I took a sufficiently large piece and with some double-sided adhesive I glued it to a wooden panel that I then fixed to the table with some office clips.
Ball end mill
Before starting the roughing procedure I reset the User Coordinate Systems to zero at a stable location using the method for electronic production. This will be used for tool change.
Therefore I placed the tip in the lower left corner of the top surface of the workpiece and reset the G54 coordinate system to zero. I chose that coordinate system because it is the one used by the gcode generated from the previous step.
Here are some photos during the milling process (after pausing and aspirating the already milled foam).
Flat end mill
Once the ball end mill work was finished I inserted the flat end mill and put in contact with the copper base as before, but instead of zeroing I marked the value of the Z in User Coordinate System.
Then from the G54 coordinate system I increased Z to the same value I marked before and set it as new Z Origin.
This is the final result of the milling
The material I chose for the mould is a white RTV BL30 silicone rubber with a blue catalyst. This also serves as a dye to help you understand if you have mixed the two compounds uniformly.
I poured the mix into a corner of the master to reduce the formation of air bubbles and waited two hours that catalyzes completely.
In total I used 4 glasses of about 150g each with 5% catalyst added. Since the balance I used has an error of 1g I always kept a bit abundant, always approximating to excess.
After the catalysis I easily removed the mold from the master but I damaged it. So I had to wash and dry the mold before I proceeded with the casting.
To get a satisfying result I had to make some tries with different mixes.
The first attempt at casting was made with a two-component plastic to join the equal parts in volume.
Since the silicone is very soft I had to clamp it in the middle of two pieces of mdf to hold it in place. Then I proceeded with the casting.
Unfortunately, it has not catalysed correctly, probably because of the old age of the components.
The second attempt was made with a gypsum powder to be added with water.
For the doses I looked at the mix for good density, but I put not much water and didn't fit well into all the pawns. In addition, there were many air bubbles, even large ones.
So I made one last attempt to dilute the gypsum more and the result was quite satisfactory even though some small air bubbles remained.
This is the end result after sanding the joints.
More info on the Opendot group assignment page.