16. Molding and Casting

idea 1 - Toyota MR2 casting

This week I first wanted to make a epoxy cast of one of my cars, a Toyota MR2.

For this, I start by reusing an old design I have on an old computer.

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I’ve made remesh and stl extract a few years ago and I will reuse them.

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But there is a big problem : it’s extremely difficult to mill this design with only a 3 axis CNC mainly because of side mirrors and rear spoiler. Cut in 2 is a idea but there no plane I can use to only cut in 2. I know I can slice it in more parts on Z axis or cut on many axis but it’s time consuming and start by doing it too big for the first try increases risks to fail.

So I will made a mold of the side for chocolate casting.

I import my stl to fusion 360

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To work with mesh in fusion 360, you first have to deactivate design history

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Verify you have enabled mesh workspace in preferences

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Now you have Mesh visible on UI

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I go in mesh mode and I made a plane cut.

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The fill function don’t work with this model, maybe a error on the body but I have my side.

I decide to restart from the stl, i try some repairing on meshlab and meshmixer but result wasn’t good so I go back to blender and simplify directly my model. Also I want to be sure that my model is close.

I remove brakes & caliper and fill wheel arches. I also rework on side mirror to make it millable on 3 axis.

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Filling side vent

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filling other side

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filling wheels

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working under the car to fill holes and reduces gap

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modifying side mirrors, here difference between normal (left) and modified for CNC (right)

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cut testing

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I first go to meshlab to clean stl but it crashes many times so I move to meshmixer. In the meantime, I change size.

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After many mix of using blender/fusion/meshmixer/meshlab, I finally import the side of the model in fusion360 and convert mesh to brep. Then I create a sketch and

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to project the silhouette of car on the sketch.

but shape is not fully closed in projection, so after repairing edges, finally I put some lines to find where shape is open

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when zooming to the max, I found 2 points not coincident, I let you see the distance…

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finally, I clean the sketch, I export an stl and go back to meshlab to clean model again.

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and the challenger is freecad but tools in it doesn’t deal better with my model than others software.

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Not finished yet. Still a lot of work!

idea 2 - Subaru Impreza WRX casting

In the same time, I work on another project. I’ve made this first for 3D printing. Finally, due to lack of time to finish the MR2 project, I fallback to this.

I start first from a blueprint of a Subaru Impreza WRX 06-07 found with google images on internet.

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There are some errors & artifact, so I start to clean image and remove details with paint.net

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Then I switch to Inkscape, I made a vector from my raster image

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Then I save my vector a svg file and I go to fusion 360. I made a sketch and import my SVG file. Here I add some line to separate outline to inner lines because I will extrude at 2 different heights. Outline will cut the shape of cookie, so it will be higher than inner lines that just make trace on cookie. I also double outlines to make my cutter more stiffer.

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A lot of work to finally go to this result. I have continue to remove details but I keep them in the sketch if I want to add them later. Here a render of the 3D object.

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Finally because I use this to make a mold for a mold. I make a derive to another project. In this new project, I add a sketch with 2 rectangles surrounding the car

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And then I extrude to make a box

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I will make paths and mill when I go back to fablab because I have to measure endmill before.

idea 3 - Cat casting

Back to fablab, I want to go fast and catch up my late so I make a simpler design to mill it. Later when I have more time I will try cars but I know I have to work on 3D models.

I take a picture of a cat from internet

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The svg is available here. I import i directly on fusion 360 and change the scale.

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Now I rework some details and make 2 different extrusions.

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Now I smooth edges with fillet, having a error message, i have to rework on the mustache before make fillet.

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Now I sketch a surrounding box and extrude it.

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Luc says to add chamber on outside to unmold more easily

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Then I go to manufacturing, I create my setup

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I define size of the material and position of my object to mill.

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I start with a 3D pocket clearing

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I create an endmill, here i use a 4mm 2 flutes endmill.

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I set max depth to 6mm, wax is soft and easy to mill. I verify toolpath with simulation. All is green! I let 0.5mm stock to finish with an smaller endmill.

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With the same endmill, I add a ramp and leave no stock. I don’t take care of yellow lines but it was a bad idea, many many moves en Z axis, many small moves on X & Y. It very stressfull for the structure of the machine. Don’t try this with a cheap CNC.

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Then I add a face, bad idea…

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Even selecting the bottom, I don’t look on good point of view when simulating but face just mill to the top to surface material. And I my case, with this shape and because I force my 0 en Z axis to be lower, it draw lines on the top (I’ve let boundaries to center of endmill).

I add a new step, that normally replace face, it’s a parralel strategy :

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The same in another point of view, to be sure this time !

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Now i add a finishing path with a 1mm endmill.

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Here my final strategies used on the wax.

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The face part is useless, it works only on top. And because I manually force Z axis a little bit lower, result is worse with this.

Finally I use a parallel to have smooth finish on the flat bottom (T2 parallel 2)

Milling the mold of the mold

Then I use the big CNC of AgriLab.

Because I don’t want to glue or screw the wax, I create a low profile quick release wax holder :

Due to his low profile, this holder permit a few error when using CNC, nothing is higher than the top of material in the travel tools holder <> material. I have also at least 7cm on each directions lower than the top of material. With this trick, if I make error on my axis, I have time to stop the machine if I see it goes in wrong way.

Other interesting part, not used here, if I want to mill several wax, it takes only a few second to change material!

Here the end result, nothing move when milling :

Casting the mold

I will made my mold with RTV 3428 A and B from : https://espritcomposite.com/rtv-alimentaire/40-silicone-contact-alimentaire-3428.html

Specifications : It’s a silicone for food. White color. Proportion 100g of A for 10g of B Timing for the full curing : 16h at 20°C or 40 minutes at 60°C Maximum timing too pour into the mold : 90 minutes at 23°C

specification document in french : rtv3428_a_b_ft.pdf

I’ve found security notice on last year Luc page because supplier doesn’t give us one: RTV3428-MSDS.pdf

Because there’s still no vacuum pump at AgriLab, I create another machine to have a better result on silicone casting. I know 2 way to remove bubbles on casting: vacuum or vibrate (I use this technique when I make concrete for building things like terrace or house). Better than complain of unavailable vacuum pump, I build my own machine to vibrate silicone in the mold.

To find correct volume of silicone, I fill mold with water.

Then I pour water in translucent glass jar and I mark the level.

I empty the jar and put it on a scale and tare. Now scale is at 0.

Now I fill the jar up with silicone (A part) to the level mark. Weight is 91.6g.

Now, I have to add the B part @ 10%, so I add 9g. I’m now at 100g. I have an extra 10% security volume because silicone stick everywhere so I lost a few with each object touching it (the jar, the wood stick I use to mix A+B, etc.). I fill the mold with the mix and put all on the machine I have created to add vibration.

And a little video of the system :

16h later, time to unmold, here the result, all is perfect molded, bubble free, no default :

I just cut a little the side with a cutter to clean object :

Casting final object in chocolate

Disclaimer: following technique may be not the perfect one but remember context: 2020, COVID-19, big shutdown, I can’t go everywhere to find all the stuff.

At home, I try to cast my final object. I try first with dessert chocolate but it doesn’t work.

Here is the second attempt with another chocolate, I usually use this in cookies. I melt it in a bowl in a water-bath @ 60°C. I have no specification on this chocolate to melt it and I can’t set lower than 60°C on this hot plate.

Chocolate is melt, time to pour with a spatula :

I fill mold with chocolate using the spatula. I press to fill everywhere and avoid bubble, at the end I slam the mold with chocolate to remove bubbles (vibration).

Someone gave me a old vacuum & seal machine, so I put mold + chocolate on a bag, vacuum (also for removing bubbles) and seal it. Now I can put it in the freezer.

2 hours later, I unmold my chocolate.

Attachments

RTV3428 spec

RTV3428 security notice

caMold.f3d I’m sorry but manufacture part isn’t included in file due to a crash of my computer and it wasn’t saved. Never trust online software!