Molding + casting

"In this week, a lot of stuff goes wrong." - Henk

Saco notes

Saco took us through our Thursday lesson. His push bike wheels are a great example of what can be done with molding and casting.

Below are some useful resources on molding and casting.

Additional resources

• Here's a really nice mold making video
• Epoxy vs Urethane
• Kochi student: Jofin Thomas
• Michelle's week
• And, of course, Nadieh's week

Group project

• review the safety data sheets for each of your molding and casting materials
• then make and compare test casts with each of them
• compare mold making processes

I was in charge of testing the following:

TYPE	POT LIFE	CURE TIME	STRENGTH
Clear Flex 30 (urethane rubber)	15 mins	16 hrs	Shore A: 30
Ure-bond II (urethane adhesive)	5 mins	24 hrs	Shore A: 85
Ure-bond 90	8 mins	16 hrs	Shore D: 90

For the group project I tested the most toxic three materials. When I read the data sheets, I judged their dangerousness relatively. Meaning, I didn't take the precautions seriously enough.

Henk urged me to be more cautious. And I really realized how much worse my chemicals were after read the specs of the materials Sam had tested, which had almost no safety warnings.

Here are the safety warnings of my materials:

Clear Flex 30, safety specs:

Ure-bond II, safety specs:

Ure-bond 90, safety specs:

I decided to ask AI for more information.

For instance, the Urethane agents, why are they toxic?

The toxicity is mainly due to the presence of isocyanates and other reactive chemicals in the mix. Isocyanates are known for causing respiratory irritation, sensitization.

• When inhaled, isocyanates irritate and inflame the lungs, causing coughing and breathing difficulty.
• They can trigger immune sensitization, leading to occupational asthma—even at very low exposures.
• Repeated exposure may cause chronic bronchitis, lung damage, or hypersensitivity pneumonitis.
• Severe reactions can include airway narrowing and potentially life-threatening asthma attacks.
• Some isocyanates are suspected carcinogens, increasing long-term health risks.

That scared me enough to be more careful.

Group project results

Wax bricking

Melt the wax chips:

Stir the pot:

Pour into the mold from the corner.

NB. Pour once and then don't add more! The wax begins to cool when it's in the mold and adding more from the pot, later on, creates bubbles and bulges.

Individual project

For this week's individual project I am having a bit of trouble deciding what to make.

For my final project, I believe a non-slip / textured mat could be useful for creating friction -- hold things in place -- that might help with final project. It felt a bit too 2D, though.

I decided to make a sphere - so that I can make a paper weight to cast the local orange in.

Mold design process

This is an great guide on: How to make a two part mold in Fusion.

For CNC wax molds, make sure to make a positive mold. The first mold I designed was a fail because I designed a negative mold.

Sigh.

I learned some great steps for designing a two part mold in Fusion, though.

Lekker.

How to lower the opacity of a body:

Lowering the opacity is useful so that I can see inside the mold box.

This next step is where I learned a nice trick, but what ultimately ended up being incorrect. I used the combine tool to cut the sphere out of the box. A nice tool, indeed, but I needed to make a positive mold.

You can also use the combine function to make pins for lining up the molds.

The split function is useful for making two part molds where the two sides of the mold have different shapes. In my project both sides would be identical, so this step wasn't necessary, but good to know regardless.

Here I extrude small cylinders and use another new tool, the pattern tool, to neatly arrange pegs that I can use to line up the molds later on.

Final-ish mold design

I followed the steps above to make a positive. This was the final outcome.

Final design file for mold (CNC).

Mill settings

Reminder: Feed rate is the speed at which the cutting tool moves through the material.

Plunge rate is the speed at which the cutting tool moves vertically down into the material.

Milling process

• No vacuum - the offcuts are collected and melted again into new bricks.
• Use double sided tape and woodblocks to clamp down wax bricks
• Flatten edges for Z-leveling
• Adjust spindle speed to 6000rpm

Milling results

CNC issues

My 3D model depth was wrong and the block was milled all the way to the double sided tape.

Thankfully, the Waag team made me a new block for my birthday.

Then the block came loose during the first layer of my roughing cut. I stuck it down and clamped it harder and the rest of the roughing cut went fine.

However, after the first pass of my finishing cut, the mill took a huge chunk out of the far edge of the mold.

Henk came to check my VCarve settings, but noticed that the wax block had come loose again.

I redid the clamping and added small wedges under the pieces of wood on the perpendicular sides where the block kept coming free to increase clamping power on the bricks.

Silicon mold

One half was made with Dragon Skin 10 MEDIUM:

Shore hardness	Pot time	Cure time
10A	20 minutes	5 hours

NB. To check how much silicon is needed, fill a mold with water, pour it into a cup.

Mixing the equal parts of Dragon Skin 10:

In my design, the edge of the sphere went to the very top of my mold walls. Irja had an issue with the width of her silicon mold being too thin. Learning from that, I duct taped the edges of my wax mold, to make the silicon mold deeper.

The other half was made with SORTA-Clear 18:

Shore hardness	Pot time	Cure time
18A	60 minutes	24 hours

This is the orange that I planned to cast in resin.

Mold outcome

It's not the outcome I had hoped for, unfortunately.

For the sphere to look good, the surface needs to be pretty much perfect.

Having to fit the sphere within the height of the wax mold also meant that the resin mold was a bit too small.

Lastly, the resin I used wasn't very clear. There are epoxy resins which take longer to set that are clearer. I would try future projects with different epoxy.

Molding / casting questions

What design / physics to consider when making the sprue and air valve?

I felt like this wasn't obvious and my classmate Irja was also wondering. Luckily (hopefully), in my case, the hole at the top of my mold, aka the sprue, should be wide enough that extra air valves aren't necessary.

Here's a tutorial from a mold making page that talks about when air valves are necessary and how to use them.

How to make sure the orange doesn't create bubbles when the resin is poured in?

Like in this video, coat the orange in acrylic spray or paint / pour on a resin layer.

For time's sake, I decided to spray the orange in acrylic.

NB. Resin pouring tips: get resin to the correct temperature for optimal mixing (read datasheet for what that amount is)

Will the orange float or sink? How to get the orange to float / not to float?

It floated. Unfortunately, in my rush, I didn't take care of this properly.

What will the CNC will be able to cut from my design? Are the corners too tight?

I rounded the inner edges of the mold and left the rest.