12. Molding and casting¶

Assignment¶

• Group assignment
  • review the safety data sheets for each of your molding and casting materials, then make and compare test casts with each of them
• Individual assignment
  • design a mold around the stock and tooling that you’ll be using, mill it (rough cut + (at least) three-axis finish cut), and use it to cast parts

Molding and casting class¶

I started the week watching the molding and casting class to learn more about the different materials and processes to start the activities.

I acquire some of the most cited materials in the molding processes for sculptures, surface finishes etc.

My first experiment was to make a key mold with gray plastic adhesive mass.

After reading the instructions and characteristics in the data sheet of the product, I started measuring the amounts I would need, in the proportions of 100g of plastic mass to 1g (20drops) of catalyst. Due to its toxicity, it was necessary to use a mask with filter against organic vapors, nitrile gloves and safety glasses.

For this test I measured 63.56g, so the amount of catalyst used was approximately 13 drops. The plastic mass is very easy to mix with the catalyst (component responsible for accelerating the stiffening process), so I used a plastic spatula to make the mixing process. Then I poured it into a small box to get a rectangular shape and pressed a key against the surface of the plastic putty.

My second experiment was to make the same key mold, but now using silicone rubber.

I repeated the same process of analysis of the data sheet of the product and measured the quantities that would be needed, in the proportions of 100 parts of silicone rubber to 5 parts of catalyst. I used 60g of silicone rubber and 12 drops of catalyst. Due to its toxicity, it was necessary to use the same protective equipment mentioned above for the handling of the plastic mass. For the process of mixing the silicone rubber with the catalyst, I used a metallic stick available in the laboratory. I poured it into a small box to get a rectangular shape and pressed a key against the surface of the silicone rubber.

After letting the materials rest, I removed the key and analyzed the result of the molds obtained. The mold made with plastic adhesive did not obtain a good result. After the curing time, the material became very rigid and it was necessary to break it to remove the used key.

In contrast, the mold made with silicone rubber obtained an excellent result. After the curing time, the material proved to be very malleable and elastic, allowing for easy removal of the key.

Based on the results obtained, I chose to use silicone rubber in the manufacture of the mold for the final design of the activity.

3D milling¶

I spent a few days thinking about building a 3D model that represented something geek that I like, and thus being able to enjoy its construction over the weekend. However, most of my ideas would take a long time to model. While browsing the thingiverse website I found this design.

I had a little difficulty understanding how the mold manufacturing process worked, especially the concepts of positive wax, negative mold and as a result the final piece. Since this is a simple setup, I left out the most robust modeling software and used tinkercad to make the necessary adjustments to the 3D model to create the machining paths for the wax. The first step was the creation of a solid box with the dimensions of the wax block that will be used in the activity (170x115x24mm)

Then I use the “duplicate and repeat” tool, located by the sheets of paper in the upper corner of the screen. Once this is done, an identical box will be created on the previous one. I designed the new box according to the work area where the 3D model will be machined (70x70x20mm). I select the “align” tool and center the two boxes. From there I select the inner box and then the option “hole”.

And then, I just import the 3D model I found on the thingiverse.

I used the “align” tool again and centered the imported model within the area of the inner box.

Mold Manufacturing¶

The material used for machining was sculpture wax, widely used in dentistry to create molds for dental prostheses.

However, its shape and individual size were not suitable for machining, so it was necessary to melt the wax to acquire a block shape, as shown below.

To melt the wax I used a microwave, applying the maximum power every 4 min in 2 series to two portions of wax. As a result I obtained the block shown below, with dimensions 170x115x24mm

Sculpture wax is not toxic, however, when heated, the wax releases a very strong odor that can suffocate during a prolonged period of exposure. Because of this, I used a mask with an air filter during the entire process of casting the wax.

The Machine¶

The machine used for this activity is the Monofab Roland SRM-20 described in the Electronics Production activity (week 5).

Exporting the File¶

The process of exporting the file was very simple. After opening the Modela Player program, I selected the file I made on tinkercad and determined its dimensions and orientation.

From this I determine the type of milling that will be done on the part.

Then I create the limit machining area and the machining paths. I used dimensions for the machining area close to the dimensions of the wax. In this same stage, I determine the type of material that will be worked. As in the software configurations there is no wax material, I selected chemical wood because it has a similar texture and resistance.

I analyzed the quality of the machining using the “preview results” tool.

After analyzing the machining quality, I saved the file and started working on the machine and adjusting the machining speed to 70%.

Milling¶

My first machining test failed, as I used only two strips of rubberized tape to secure the wax block to the work table. As a result, during the deepest cuts, the block broke free and broke into several pieces. Thanks to the machine’s emergency stop system, work was stopped and there was no accident or damage.

After the event, I cleaned the machine, put all the wax pieces together and fused it again to create a new block.

Now, again using rubberized 3M tape, I fixed the wax block on the working table of the milling machine with 4 strips, and fortified it with adhesive tape on the sides between the block and the table.

With everything ready I started the process on the machine, using 70% of the speed.

After milling, the wax sculpture suffered a break in its lower left corner. This incident was due to the wax not being compacted during the casting process, which ended up generating a small air gap in the region that broke.

However, the piece was not completely lost. I decided to choose to continue with it for the next step, since it would take a long time to melt the wax and mill it again. I placed the wax piece in an expanded polyethylene foam box and started to prepare the materials for creating the mold.

At first I measured 304.37g of silicone rubber and used approximately 61 drops of catalyst.

However, this amount was not enough to completely cover the piece of wax. Then I added another 100g of silicone rubber with 20 drops of catalyst and poured the mixture over the piece, covering it completely.

It took approximately 18 hours until the end of the curing time of the material, however this time may vary depending on the amount of catalyst added.

With the help of a spatula and scissors I started to remove the part of the mold and trim some spare burrs, and so I got the result below.

We can see that the ruptured area was completely filled by silicone rubber, which was already expected.

Casting Polyester Resin¶

After manufacturing the mold, I analyzed the technical sheet of polyester resin available in the laboratory. This is a material widely used in the manufacture of sculptures, tables, floors and other purposes, as it guarantees a hardness and resistance capable of replacing metallic and plastic materials in general.

Due to its high toxicity, I followed the recommendations for protective measures described in the technical sheet using goggles, rubber gloves, closed shoes and apron, avoiding the contact between the materials and my skin as much as possible. Using all the appropriate PPE, I did a first test preparing a mixture in the proportions of 100g resin for 20 drops of catalyst. Then 250 g of crystal polyester resin was added together with 50 drops of catalyst and I poured the mixture into the silicone rubber mold.

After 12 hours, I remove the mold part with the aid of a metal clamp, and below is the result obtained.

As I did not have a vacuum chamber in the laboratory, my piece had a significant amount of bubbles on the back. I did a second test using the same proportions, but trying to mix with less speed to prevent less air from entering the mixture, and after pouring the mixture into the mold I had the idea of add some green and blue LEDs to the piece to turn it into some type of lamp decorative.

As expected, mixing at a lower speed resulted in considerably less bubbles. And with the addition of LEDs, I got the following result for my molded part.

Adding the LEDS¶

To add the LEDs, I thought of a circuit to be used and how to power it. At first I thought about using a 18650 lithium battery with 3.7V supply voltage. Once the form of supply was decided, the capacity of the battery to safely supply the circuit was checked.

It is necessary to check 2 properties: voltage and current. First, I looked in the manufacturer’s data sheets for the voltage and current values of the LEDs used, and it was found that the LEDs have operating values very similar to others that I have already used, 15 mA and 2.2 V, therefore, calculating the resistance value required for them and using the selected battery, we have:

Then, the circuit containing only the resistor with the LED was tested, thus obtaining:

The values were quite different from those measured, so it is necessary to check the dissipated power, as the datasheets indicate a maximum power of 60 mW.

LED color	Verified Voltage	Verified Current	Verified Power
Green	2.85V	8.5mA	24.225mW
Blue	2.87V	8.2mA	23.52mW

As 13 LEDs will be used and all in parallel, the operating current is expected to be the sum of each of the LEDs.

Both verified powers can be considered safe, so the next step is to check if the battery is capable of supplying current for the entire circuit, when looking in the datasheets, it was found that the maximum current supplied by the battery would be 2.6 A, so it is safe and possible to apply the circuit.

Assembly¶

For the assembly, there was no board specifically for this activity, but a scheme designed to connect all LEDs in parallel.

Video1 - Blinking

Round object¶

I had a meeting with my international evaluator and we have a very nice chat how to improve this week. The main point is to make a round object. So because of that I chose the Captain America’s Shield, that has more rounded surfaces.

Chosen the object, the next step was to prepare the settings on SRP Player software to mill the wax sculpture just as the last time. I also decided to increase the curvature to make the details clearer after the process since the 3D model wasn’t looking so rounded.

After that, it was necessary to finish the settings to produce the file to mill the wax, following all the steps used before on the last try.

After preparing the file, the next step was to prepare the wax in the machine and mill it. A process was carried out to analyze the product’s technical file and measure the quantities of material that would be needed, in the proportions of 100 parts of silicone rubber to 5 parts of catalyst. I used 20g of silicone rubber and 4 drops of catalyst. Due to its toxicity, it was necessary to use protective equipment for handling silicone rubber. For the process of mixing the silicone rubber with the catalyst, I used a metal stick available in the lab. So, after the mixtures were made, I poured it into the sculpting wax mold.

After 2 days, the silicone rubber was removed, it was seen that at the bottom of the mold there was no full curing of the silicone rubber. So, to solve this problem, which may have been due to the fact that the amount of catalyst was not enough to make the full curing of the silicone, so I used a new blend of silicone rubber with 20 drops of catalyst that I made.

So because of the bad result of the molding a chande to Polyester Resin casting

After the mold was made, I analyzed the polyester resin technical sheet availabe in the lab. It is a material widely used in the manufacture os sculptures, tables, floors and other purposes, as it guarantess resistance and hardness capable of replacing metallic and plastic materials in general. Due to its high toxicity, I followed the recommendations for protective measures described in the technical sheet using googles, rubber gloves, closed shoes and apron, avoiding as much as possible the contact of the materials with my skin. Using the appropriate EPI, I did a test by preparing a mixture in the proportion of 100g of resin to 20 drops of catalyst. Then 20g of crystal polyester resin was added along with 5 drops of catalyst. A cardboard support was created so that when the resin was poured on top of the rubber mold, it would be totally mixed and not to spread out in the process.

Below a small video showing the shield

Video2 - Round object (Captain America’s shield)

Conclusion¶

This week was possible to learn the importance of moulding and the advantages of this process.

• All the files done and datasheets in this week assignment can be find in the repository

Files