Sally Williams @ Fab Academy 2015

Assignment: 

• Design a 3D mold, machine it, and cast parts from it.


Design

I decided to design, mould and cast a shallow dish which could be used to serve olives, incorporating an out of sight stone disposal pocket. My original design was quite curved across several planes, including a spherical form which created an overhang challenge which would require a complex multii-part mould.  Therefore, as a first mould and cast project I decided to simplified the design. I tool the opportunity again to explore CAD in Rhino and SolidWorks.

  




For my first attempt, I simplified the design to have vertical walls on outside (preserving the spherical base interior, but removing the spherical overhang of the form in the middle of the dish):

  

Despite having been a significant learning curve for me to get to grips with Solidworks, I did appreciate all the dimensioning flexibility its parametric design tools enabled along the way.  The design and modelling process has given me an in-road into Solidworks.

The model was saved as an .STL (file size: 478kb), ready to be opened in Partworks, which is the interface to the Shopbot CNC machine. 

Preparing the materials for making the form

Machinable wax was remelted in cake tins in the oven at around 220˚C. This took around 45 minutes. I think its worth making sure that all the wax has completely melted throughout, otherwise I suspect weaknesses develop in the material which can show up during milling stresses. Also, impurities may cause some unpredictable results as well as imperfect surfaces. Once melted, the wax has to be allowed to cool down. I cooled the block outside (temp approx 12˚C), which I deemed to be not too cold but perhaps this was too sudden? I filed down the rasied edges around the block (consequence of heating and shrinking from edge as it cooled down) so that I had a flatter, more stable surface with which to screw down onto a mobile base for securing to the sacrificial bed of the Shopbot.




Setting up the shopbot and toolpaths to mill the form

 

Previews of rough and finishing toolpaths milling:




The image below demonstrates both the work of the rough (stepped mill path - using flat end mill), next to the finishing path using the ball-nosed mill finishing path (crossing at 45 degrees). Also visible is the fracture which caused a piece of the inner ellipsical wall to break away.




I had also selected the 'extra passes' option, but as it milled, I could not detect any improvement in surface smoothness or finish - perhaps the stepover setting needed to be decreased, so as not to create a 'micro-ribbing' in the surfce. As this was taking extra time with no apparent gain, I decided to abort the completion of the extra passes toolpath.



I used multi purpose plaster to fill the fracture:

     

Measuring the volume of the form to estimate quantities required:

 

Prepartion of Liquid Silicone Rubber to make mould:

Instructions:

 

 


Methods to try and remove trapped air bubbles within body of silicone:

Placed on a vibrating machine (extractor motor in this instance on left) and a vacuum chamber (right):

   

The images below show that some areas that are still very tacky to touch - this is a sign that the hardener solution had not been thoroughly mixed. I remember noti as I mixed, noticing the mixture begin to harden and became aware that 'flow' time was limited. In retrospect I remember the voice of my tutor "when you think it is ready, you probably need to mix for a little longer..."

  


However, if air bubbles had not been trapped within the silicon fluid upon pouring, the actual quality of the mould would have been quite good.

Preapartion of Crystacast - a product that is similar to Drystone (but competitively available in UK market).

 


I had forgotten to stop pouring the plaster into the mould to the right height! Therefore I inherited the skirting around the base. Although the material is strong, it was quite easy to shear, I was therfore able to use a strong pair of scissors to shear away the unwanted edges for my prototype.

 

First prototype of table dish for olives shown below (using grapes which look huge in this scaled down version).





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