Week 10. Molding and Casting

Go to:
  1. Group Work
  2. Model
  3. Positive mold
  4. Negative mold
  5. Casting
  6. Results of the Week

1. Group work

For the group part of work we tried a few different materials for molding and casting. We used positive and negative molds left from previous years. For molding we tried two similar materials Oomoo 30 and Smooth-Sil 940.
Safety [safety datasheets: Oomoo 30, Smooth-Sil]

Making the negative mold from a positive mold

We used a positive mold made last year by Mikko Toivonen. It is a positive mold designed to make a two-part negative mold. So, one part we made from Oomoo 30 and another one with Smooth-Sil 940. Both, Oomoo 30 and Smooth-Sil 940, consist of two parts (A and B). So the first step is to mix components in a correct proportion. We had not any nice cups to measure out parts by volume, but we had scales to do it by weight. We made about 120 grams of Oomoo30 but about 40 left after we pured it into the mold. So when we were mixing Smooth-Sil we made about 85 grams and it was a perfect volume.

Cleaning the mold (on the photo is the second mold which we did not use) with compressed air. Lets call it pre-one step.

100A:130B for Oomoo30 100A:10B for Smooth-Sil 940

Then comes the vacuum chamber for 3-4 minutes to pull out air bubbles. At the beginning of this process the material goes up = air goes out, and in the end it starts to go down = almost no air inside. (after mixing Oomo 30 has violet color and Smooth-Sil is pink)

Then we gently poured materials into the mold. The pot life time for both materials is 30 min.

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Oomoo 30 was a little bit more dense Pouring Smooth-Sil 940 was a absolute pleasure

Cure time is 6 hours for Oomoo 30 and 24 hours for the Smooth-Sil

Some time later...

Casted from Oomoo 30 part has minor imperfections, but I call it success

And casted from Smooth-Sil part looks great!

Casting

We used Smooth-Cast 300 to make a couple of test parts. We used open mold, and a couple of two part molds from the previous year (thanks to Mikko and Kati). The Pot life of this material is only 3 minutes. But it does not require vacuuming. Cure time is 10 min. Proportion to mix by weight is 100A:90B.

How many engineers do we need to open the Smooth-Cast container?

Mikko's two part mold. One of the walls was too thin and it caused some defects in the result.

Two parts of the mold fixed with the duct tape.

Open mold.

Barbaric usage of the injection mold.

Because of the thin wall the result looks like this

Some minor inperfections, but ok

Fine result, but the second part of the mold pushed some air into the model so it has two dents on the back side.

Dents

Fullmetal Fail

We casted a test piece from metal as well. But! We did not have the talc powder and the result was a lot of a failure

The silicon mold can handle tempreture about 250 C. It makes it possible to cast from metals with low melting tempreture. We had bismuth-based alloy with melting tempreture about 240 C

But, as I mentioned earlier, we didn't have any talc powder which creates an air layer under the metal and helps it reach the bottom of the mold.

2. Model

For the individual assignment I designed a constant-width object. I took the idea from this video, Maker's Muse youtube channel. In the video you can find more about the shape and how to draw it in Fusion360.

I used Polygon and Circle tools to sketch the basic shape.

Rotate 180 dergees

It seems you are using an outdated browser that does not support canvas :-(

The shape

3. Positive mold

I created a box around my model. I created two pins for better alignment, pouring channel and air tube. To feel more secure during the milling process, the walls have a 12 degree angle.

I created Roughing and Finishing processes in Modela Player.

Create a new process.

Roughing

Next

Choose milling bit for Roughing (I used 1/8 inch Flat)

Next

Scan lines X

Settings according to the material.

Name the process and press Finish

Process created. Now we can simulate it (button in the lowe rigth corner, next to Cut button)

Roughing process simulated in Modela Player.

Finishing

Milling bit for Finishing process (I used 1/8inch_Ball)

Scan lines X+Y.

Then I saved a .prn file both for Roughing and Finishing processes.

The whole milling process took about 3.5 hours for me. You can run the milling process directly from Modela Player, but I would say, that it is better to save .prn file for both processes and run them fom V-panel.

To secure the wax stock in the machine I put masking tape on the bottom of the brick and on the building plane in the machine. Then, using hot glue gun, I secured the wax stock inside. This system worked better then doublesided tape we used before. Using V-panel I set zero X,Y,Z. And started the milling process. For the milling I used 1/8 SE 4FL for roughing and 1/16 SE 2FL BN — for finising.

Milling bits I used

During the milling you can't see what is going on. So, for a few times, I paused the process and moved the model to the View position. I liked the process very much.

This is how the surface looks like during the roughing process.

And this is the finished one. The surface feels great.

After milling the mold, I looked at my pouring and air shafts, and, it seemed to me, that they are too narrow. For the silicone mold it shouldn't be a big problem. I think, if there will be any problems I'll just make them bigger with a modeling knife.

4. Negative Mold

From my wax mold I decided to make a food-safe silicone mold from Smooth-Sil 940. To make it food safe you need to cure it for 24 hours.

As in the group work part, I mixed the components by weight: 100A:10B. I estimated that I need about 180 g for my mold. I took 160 g of component A and 16 g of component B. Here I made a mistake, which spoiled a little bit the result. We had an old pack and a new pack. From the old pack about 40 g of component A left. So I used them and the rest I took from the new pack. I think that it was a bad Idea. But I'll come back to it later.

So I mixed the components (160A:16B), put them in the vacuum chamber for a few minutes and poured the mix to the mold.

The mix in the vacuum chamber

The mix in the mold. The surface was very nice and smooth.

24 hours later...

I took one of the parts off rom the mold and at the very edges it still was not cured. I put it back and left in the mold for another night. But the damage was done and one of the parts has some imperfections now.

Part on the left is not perfect.

So, 36 hours later my silicone mold was ready. I left it in the oven on 60°C (140°F) to make it a little bit harder, cut the shafts a little bit bigger, and the mold was ready to pour something inside. Here I saw the second mistake I made: instead of two pins it is better to have a slot on every side of the mold for better alignment.

5. Casting

I had a dark chocolate bar, silicone mold and a kitchen just for me (my wife went for a research visit to France, so I could do whatever I wanted to). And I wanted a chocolate in constant width shape.

To melt the chocolate I put it my hiking cup with chocolate in a pot with water. So the chocolate won't hit up more then 99°C.

Part on the left is not perfect.

I secured two parts of the mold with a few rubber bands and poured the chocolate inside. To make the curing process faster I put the mold with chocolate in the freezer. And repeated the process until I got bored (lucky, that it happened in the end of the chocolate bar).

The result was not perfect, but good enough. I enjoyed the process a lot. Smell of the chocolate in the kitchen was a nice bonus.

Metal casting

Lucas made a very nice mold this week. And with our local FabLab tutor Ari we casted a very nice medalion in metal. Wax and silicone molds are designed and made by Lucas. I participated in the casting process and I processed it after.

Wax mold Made by Lucas

Silicone mold Made by Lucas. Tou can see talk powder on top of the mold.

Medalion, casted by Lucas, processed by me.

6. The Results of the week