week 12

Molding & Casting

ASSIGNMENTS:

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

making the design

I wanted to make a unit that can be used as a screen module in screen. I saw some works in Aarhus School of Architecture similar to that and planned to make it similar to that.

scripting and designing the piece

I scripted one module in Rhino- Grasshopper and modeled the base for the molding with it. The parameters can be seen in the GIF:

The geometry was exported to a STL mesh and taken in Partworks3D to prepare it to mill in ShopBot.

Basic dimensioning and orientation.

Giving safe material offset and setting origin point.

Creating the roughing toolpath using 1/4th inch 4 flute flat end up-cut bit at 5500 rpm. Feed and Plunge both kept to 1.5 inch/sec. Time estimated: 33minutes

Creating the finishin toolpath using 1/4th inch 2 flute ball nose up-cut bit at 12000 rpm. Feed and Plunge both kept to 2 inch/sec. Time estimated: 59 minutes

Creating the finishin toolpath using 1/4th inch 2 flute ball nose up-cut bit at 12000 rpm. Feed and Plunge both kept to 2 inch/sec. Time estimated: 59 minutes

Roughing toolpath preview.

Finishing toolpath preview.

milling the mold

Now the pieces are supposed to milled from MDF. So I took some MDF lying around in the lab and glued them overnight to make a solid block. and now it is ready for milling.

The first milling was complete and ready to process but due to no greasing the rubber mold did not come off the MDF brick.  Next time it did not get milled for machine fault and in the 3rd time it came out perfectly.

Next step was to prepare this block to be made into the PU Rubber mold.

I screwed the supports on to the base block so that I can simply unscrew and release the rubber mold. All other parts were cut in Shopbot CNC for maximum precision.

With help of Chandni I made a greasing compound to use. It has 1 portion each of Machine  Grease, Handwash & Dishwash soap and Mustard oil. It was absorbed into the mold a lot. Further, I polished the mold with grease.

making the rubber mold

First thing was to zero the scale using the glass I would use for the mixture. I have used  Vytaflex 40 Urethane Rubber compound to make the rubber negative mold.

Now I have mixed Part A and Part B of the compound by weight at 1:1 Ratio.

It was mixed for 3 minute before pouring into the mold.

It was kept still for 16 hours before opening the mold.

Now its required to put the Liquid Plastic to cast a part from it. This time I have used Smooth Cast 300 Liquid plastic. It should be used as 100A : 90B.

But by mistake I had mixed 1:1 ratio and it solidified in the glass while pouring.

CAUTION: It is an exothermic process. So one need to be careful while mixing the liquids. It generate a high amount of heat.

Then I corrected the proportions and poured again.

It is amazing to see the transparent liquids reacting with each other and plasticizes and hardens.

I kept the mold as it is for 15 mins and removed the part out of it.

The Rhino Grasshopper file and STL can be downloaded from here.