12. Molding and casting

This week assignment focuses on working on our molding and casting skills, the closest experience related to molding was when I made chocolate molds, understanding how to generate the mold generates a bit of confusion, but I am sure that I will achieve the objective.

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.

Group assignment

How to read a Safety Data Sheet (SDS), for beginners

A safety data sheet (SDS) is an important document that the US Occupational Safety and Health Administration (OSHA) requires employers to make available to employees for any potentially harmful substances that may be handled in the workplace. Your shop should have the SDS printed and readily available in an SDS binder (usually a large, bright yellow or bright white binder, ask your shop manager where it is if you are unsure), for any materials that you might be using in class. If you bring outside materials into the shop, you should first obtain and read the SDS for those materials, then print a copy and deliver it to your shop manager for inclusion in the aforementioned binder.

We must read the SDS of each molding and casting materials that we are going to use:

Brief description of the materials

Mold-max 25

Mold Max™ Silicones are tin-cured silicone rubber compounds that have exceptional working properties and library life. Mold Max™ Silicones feature Smooth-On’s exclusive “Libra™” catalyst for long library life. Mold Max™ silicones can be pigmented with Silc Pig™ silicone colorants. Pot life for the Mold Max™ 25 is 60 minutes and cures overnight at room temperature. Vacuum degassing mixed material using a vacuum pump and chamber to remove entrapped air is recommended.

Features

  • Compared to the other Mold Max™ products, Mold Max™ 25 has a lower viscosity Part A, making the rubber easier to mix and easier to vacuum de-gas.
  • The working time for the Mold Max™ 25 is an extra-long 60 minutes for large mold pours or for pouring several molds from a single batch of rubber.
  • Mold Max™ Silicones will reproduce the finest detail and are suitable for a variety of industrial and art related applications including making molds for reproducing prototypes, furniture, sculpture and architectural elements.
  • Mold Max™ 25 can be thickened with THI-VEX™ additive for brush-on applications.
  • Mold Max™ Silicones can be used to cast a variety of materials including wax, gypsum, low melt alloys/metals and urethane, epoxy or polyester resins (without using a release agent).

Oomoo 25

No Vacuuming – No Scale – Easy To Use . . . OOMOO™ 25 is an easy to use tin cure silicone rubber compound that features a convenient one-to-one by volume mix ratio (no scale necessary). It has a low viscosity for easy mixing and pouring . . . vacuum degassing is not necessary. This product cures at room temperature with negligible shrinkage. OOMOO™ 25 has a 15-minute pot life and 75-minute cure time. For The Novice Mold Maker - OOMOO™ silicones do not have great tear strength. They are good for making simple one or two-piece block molds. If you require a high tear strength silicone, Mold Max™ silicones are recommended. OOMOO™ 25 is suitable for a variety of art-related and industrial applications including making one and two-piece block molds for sculpture and prototype reproduction, casting plaster, resins and wax. OOMOO™ silicones are also suitable for electrical potting and encapsulation applications.

Features

  • Convenient one-to-one by volume mix ratio (no scale necessary)
  • Low viscosity for easy mixing and pouring . . . vacuum degassing is not necessary

Smooth Cast 305

The Smooth-Cast™ 300 Series of liquid plastics are ultra-low viscosity casting resins that yield castings that are bright white and virtually bubble free. Vacuum degassing is not necessary. They offer the convenience of a 1A:1B by volume or 100A:90B by weight mix ratio. The differences between them are pot life and demold time. These resins readily accept fillers (such as URE-FIL™ 3 from Smooth-On) and can be colored with SO-Strong™ or Ignite™ color tints (Smooth-Cast™ 325 series accepts pigments better than the Smooth Cast™ 300 series). Fully cured castings are tough, durable, machinable and paintable. They resist moisture and mild solvents. Applications for Smooth-Cast™ 300 Series Liquid Plastics include reproducing small to medium size sculptures, making prototype models, special effect props and decorative jewelry.

From theory to practice

Unfortunately most of the compounds were in poor condition, in a desperate attempt to test the Oomoo I decided to mix it with my own hands (using gloves) and I managed to get a viscous mixture that I could apply in my mold, the same happened with the Mold-max It was expired and it was not possible to apply it to test, last but not least the resin worked correctly as you can be see in the glass located on the left, it cured in less than 30 minutes and we were thus able to test what materials we can use. I’m sorry for the mess in the photo, since most of the compounds were out of date I managed to run a quick test.

Well the first test did not go as expected, so that material is discarded, now I will use Mold-max and Silicone rubber RTV, until I can get a mold in which I can pour my mixture.

As you can see, the best result was obtained when using a product whose expiration date was not yet met, the presence of bubbles in the mold is almost zero, unlike the Moldmax mixture, these bubbles were transferred to our cast and ended up being shaped in our token, but with Silicone rubber RTV we got the best result, almost perfect I would say.

Individual assignment

In order not to complicate myself, I decided to make a coin, being more specific a Bitcoin token, initially I planned to model something more complex, due to the good resolution of our milling machine (Monofab SRM-20), but I decided to do something simple due to the short time with we have to perform tests and allow the material to cure correctly, because of that I will be able to test different materials in our mold.

I decided to use Tinkercad because the shape is provided by the .stl file that we downloaded from Thingiverse, it is only necessary to create a mold, first we define the final measurements of the coin, in this case it will be 50mm in diameter and 10mm thick. So the first thing I have to do is create 2 identical pieces that when joined will form the coin in its entirety and assign a thickness of 5mm to each one, so when both pieces are put together they will add a total of 10mm.

Now we must focus on creating the mold that will contain our tokens, since this will be a 2-sided model we must also consider adding holes to be able to pour the molding material and the most important thing is to add some guides that fit on both sides to be able to fix correctly both parts of the mold and prevent it from moving while our token is curing.

Finally, with the mold already finished, we can export it in .stl format and load it into Fabmodules, we must select the appropriate parameters for both the rough cut and the finish cut, taking into consideration the diameter of our endmills for each type of work, generated both toolpaths it is only necessary to load it to the monofab, prepare the material and start with the milling process.

For the milling process, both for the rough cut and finish cut I decided to use a 1/8 SE 2FL endmill, because my mold did not have very complicated details and I followed the suggestions of my instructor, as you can see the finish result was very good and due to the size of the endmill the milling time was considerably reduced.

Unfortunately at the time of starting the milling process, it began to execute a previous job, when I realized it I stopped the milling process but it was too late, because I only had a block of wax I decided to continue with the process with the hope that it did not directly affect my part, after the process I confirmed my suspicion and there was damage to the part, luckily it was minimal. So I decided to use plasticine and repair the parts that suffered damage, luckily my model did not have many details and with a lot of patience I managed to fix all the damaged parts.

Molding and casting procces using Mold-Max 60

Molding procces

Due to the fact that all the compounds were expired, I decided to use Mold-Max 60 for the moment (Expiration date December 2019), at the time of mixing I could verify that the viscosity was not the same and that there would be problems with bubbles, I mixed part A with 3% part B using the scale to define the exact quantity. I continued with the procces and test the final result, I was right and in the end the mold ended up with bubbles in some details.

Casting procces

For the casting process I decided to use resin since according to the previous tests it seemed to work correctly, so I made the mixture of 2 equal parts with the help of the scale and secure my mold using tape to prevent leakage and rubber bands to keep the mold tightly pressed. The resin cured in 30 minutes as indicated by the manufacturer (change from clear to white color) and I decided to leave it for 1 hour more to be sure that everything would go well, finally, separate the mold and the final result came out much better than what I expected.

Molding and casting procces using Silicone rubber RTV

Molding procces

For this second attempt, I did a search for compounds available in the local market and I was able to get RTV silicone rubber, they told me that it was the correct one to make molds so I bought it. This mixture worked in a different way since it used a catalyst that had to be measured by a specific number of drops, performed the necessary math operations and defined the amount of mixture, elaborated the mixture and poured it into the wax matrix to make the mold, as you can see this mold was much better that the first and the details managed to stay mostly.

Casting procces

For the casting process I decided to use resin since according to the previous tests it seemed to work correctly, so I made the mixture of 2 equal parts with the help of the scale and secure my mold using tape to prevent leakage and rubber bands to keep the mold tightly pressed. The resin cured in 30 minutes as indicated by the manufacturer (change from clear to white color) and I decided to leave it for 1 hour more to be sure that everything would go well, finally, separate the mold and the final result came out much better than what I expected.

You can download the files below:

Week Assessment

During this week, have I :

  • [ ] Linked to the group assignment page ? Yes, I did.

  • [ ] Reviewed the safety data sheets for each of our molding and casting materials, then made and compared test casts with each of them? I was able to get it to work because most of the compounds were expired.

  • [ ] Documented how I’ve designed my 3D mould and created my rough and finish toolpaths for machining, including machine settings? I detailed the whole process so that someone else can use it as a guide.

  • [ ] Shown hoy I made my mould and cast the parts? I took photos illustrating the whole process.

  • [ ] Described problems and how I fixed them ? We are never safe from mistakes, this time my piece of wax suffered some extra passes from the mill, but I was able to patch it up with plasticine.

  • [ ] Included my design files and “hero shots” of the mould and the final object? As a first attempt I am satisfied with the result