Ana Cabral

Week 12

Molding and Casting

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The assignment

This week's assignment was to design a 3D mold, machine it, and cast parts from it.



Design the 3D mold

Making a 3 steps molds was quite confusing. I started by making a sketch of the intended final part and kept on drawing by hand (perspective and section drawings) in order to structure the digital file.

I decided to mold a cast a module of the final project - not for a specific reason, but just to explore the shape in other materials and who knows if new ideas would come out of it...

So I used Grasshopper,using the parametric file to re-size the shape - this was the first time I was molding and casting... I thought a smaller mold would be wise in terms of time and use of material.

I baked the shape and worked with it in Rhino, using mainly the trim function. I had troubles working with the 3D modelling in Rhino, the solid booleans (union, subtract, etc) were not working the way I expected... I've worked many years with AutoCAD, so I exported the file as a .dwg.

In AutoCAD, I added a pyramid in the center of the module, extruded all parts and made use of boolean functions to add and subtract volumes.

When I was done, I saved it as a .STL, knowing this format was compatible with V-Carve.



Generating toolpath

To generate the toolpath for the large CNC milling machine, I used V-Carve (like in week 7).

I opened a new document and set up the job... Material size, Z zero and origin. I then imported the .STL file and oriented it.

I created 2 toolpaths, a 3D roughing and a 3d finishing.

For the 3d roughing I chose the 1/4 inch ball nose bit.

For the 3d roughing I chose the 1/8 inch ball nose bit.

The tool choice was a compromise… some of the details (holes) were smaller than the tool diameter. In order to get them machined, I would need to pick a smaller bit for the finishing or run an extra finishing with a smaller tool (1/16 o 1/32 inch). For a first test, I decided to use only the 1/4 and 1/8. If I decide to integrate this weeks work in the final project, I’ll then work further with smaller bits and have all details in place.

I find it quite comforting to make use of the preview function.

I saved both toolpaths in the same file, on a usb, and I was ready to machine!



Machinning

So I took the usb to the computer conneced to the large CNC milling machine, transfered the files and removed the usb again to avoid any interference.

I started by fixing the wax, using double sided tape and 2 pieces of wood that are screwed to the sacrificial layer.

I zeroed X, Y and Z axis and proceeded with cutting the part. I only had to follow instructions on the screen, the program stops itself and guides the user to change tool and Z zero according to new milling bit.

I didn't turn on the exhaust on purpose, so I could collect the wax chips and re-use them.

IMPORTANT: tighten well the milling bit!! And, since the exhaust is not on, I figured it was helpful to stop the cutting here and there to clean up (using space key, so it could resume without losing position), otherwise the chips force the milling bit to work harder.

I haven't done it this time, but considering to reduce the feed rate and/or the pass depth for the 1/8 ball nose... this tool is thin and long and even though wax is a semi soft material, both me and Ola had issues when using this bit. (break, slide down, etc.)

The results were not entirely satisfying... for the first attempt, the pc was not plugged, so it stop a tiny bit before finishing…. otherwise it went fine… SO REMEMBER TO ALWAYS make sure the computer connected to milling machine is connected to the plug and with the right settings that avoid it going in stand by!!!!

On the second attempt, maybe the milling bit was not well tighten or the collet not well posiotend… but the milling bit slided down and broke on the top. I you take a look at the picture below, you see traces of the milling bit sliding down early in the finishing process...(see squared volume in the centre, inside the red circle).

When the jobs were done, I cleaned up, saving most of the wax chips in plastic bags to be melted and re-used. The rest, I vacuum cleaned.

I’ll use first result to proceed. I would like to machine it again, if time allows it, but my fellow student also needs access to the machine and most likely there will be a lot of mistakes and learning with casting as well, so I want to go fast through the whole process and eventually repeat it, the second time with a better understanding and better results.



First casting - silicone

So I took a look at the different materials we had in the lab for casting, and decided to use the Oomoo 25, mainly because it has a short curing time (75min) and I could see some other students from previous years have been using it and with good results. It is a mix of 2 parts, ratio 1:1 (volume), instructions are written in the bucket, and a nice Material Safety Data Sheet came with it, reminding to use protective eye wear, gloves and clothing - it's very likely to cause eye and skin irritation before cured.

Also, after stired, has a pot life of 15min. So everything needs to be prepared to make the casting process fast.

I started by opening part B. As you can see in the pictures (and they also recommend on the label in the bucket), it needs to be stirred before mixed with the other part.

I was about to do the same with part A, but when I opened the bucket, I found it weird that the content was very stiff... I did a quick search on youtube, and as you can see in my video and the one from Oomoo, there was something wrong with the one we got. (we were using it for the first time, and we ordered it in February, in the beginning of the FabAcademy cycle)

So looked into other kinds of rubber we had in the lab, ended up using SORTA Clear 37.

Also 2 parts, ratio 1:1, takes 4 hours to cure and it's food safe! :)

Once more I looked into the MSDS, special focus on personal safety and disposal/cleaning up.

The pot life here is a little longer, 25 min. Anyways, I prepared everything before hand.

The parts have to be stired individually and then again after put together. Stiring has to be done slowly and caefully, to avoid air bubbles.

After casting, both me and Ola left the lab, leaving a big safety warning next to our work.

And after 4 hours....



Second casting - liquid plastic

I took a look again at what we had in the lab. I decided to cast the final part with acrylic plastic. Since all these products cannot last long on the shelf after opened, I prefer to use the 1 pint products. So I'll be using liquid plastic, Smooth-Cast 305. Like I did for the silicone, I read the MSDS, everything was very similar. One must use protective eye wear, gloves and clothing, ratio 1:1 in volume, short pot life (7min) and 30min cure.

The instructions also recommened to use a release agent and I could see we had a universal mold release spray, so I have used it according to the instructions in the spray can.

Measure the volume by using these small cups next to each other.

And I was ready to cast.

After 30 min... :)

I was really impressed with the good looking and the level of detailing of the final result!! Also, liquid plastic has a nice texture and consistency. It's quite flexible, deformable, but still strong. It takes only 30min to cast and can be mixed with pigments (get colored!!!!). I can see some potencial in integrating it in the final project, as a way to better integrate/protect electronics.



Files

All files regarding this week can be found here. To download the files, right click with the mouse and chose “save as”.

2D_GRASSHOPPER and 2D_RHINO

3D_AUTOCAD and 3D_STL

TOOLPATH_VCARVE and TOOLPATH_SHOPBOT