FA 15 - Molding and Casting

Molding and Casting is the process of creating a mold from a 3D model or a physical object, and then casting it with certain material to make copies of it, or to reply the model. I found this definition from the guys of Smooth-On that defines it pretty well:

Molding or Moldmaking is the act of creating the cavity / form that carries a negative or reverse impression of an original model. Molds can be made of a rigid material, such as plaster or plastic resin or more commonly, a flexible material such as rubber. The material to use should be chosen considering the material of the model, the material to be used to make castings, and whether there are any undercuts.

Casting is the act of pouring liquid material into the cavity of a mold. After a period of time, this liquid will cure via chemical reaction or cooling. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay.

2 - Object to create:

Here are some references of the "Rubber Foot" I want to make to be used for my final project.

Rubber foot:

I wanted to make something would be useful for my final project, so I decided to create some rubber foot to add under the wood box, so that it gets more adhesion to the surface and to protect it from scratching it. I want to test doing it maybe with different styles of molds, and also with different materials or biomaterials.

"Commonly called rubber bumpers or bumpons, rubber feet are generally used on all forms of equipment. They are usually of the screw on type with an internal metal washer. Commonly, they also come with an adhesive back to avoid using screws. The rubber is of a special material to prevent the equipment from skidding. These also help in stabilizing the enclosure and in protecting the finish." West Florida Components definition.

3 - Creating the 3D mold

Here is the basic process of how I created the mold in Fusion 360.

First I created a sketch of what would be a side cut of half of the piece I wanted to cast. I made this keeping in mind that my piece would have a radio of 20mm, since I wanted the whole piece to have a diameter of 40mm, and a minimum height of 5mm in the middle, where the bolts would go, and a maximum hight of 10mm.
I created a sketch square of 50 x 50 mm that would be the inner part of the mold. I knew that I wanted to leave at least a 5mm distance between the outer side of the piece, and the inner side of the mold so I could easily release it.
I revolved 360º the first sketch I did so that it would create the shape I wanted for the rubber foot.
I extruded the square sketch 15mm to the top, for it to be 5mm above the highest part of the object I wanted to create to give some body to the mold, and created a shell inside of the box. I gave an exterior wall thickness of 10mm to give some sturdiness to the mold. The size of the wall is not going to affect much when the mold is milled in wax, but if you 3D print it, it will give make it more stable.

FILE:
Patas_de_caucho_FINAL.stl

4 - Molding in Wax

Here you will find the process of creating the mold in Wax, creating the file, and milling it in an SRM-20.

First you will need to cast a wax block that has at least the dimensions of the 3D mold you´ve just created or bigger. I did this by melting wax in a pot, and then pouring it in a 5mm acrylic mold they already had in the FabLab and let it cool down until the next day and un-mold it.

You could then create the files for the SRM in fabmodules, mods or in the SRM native software. I created mine in fabmodules and is the process I documented.

NOTE: In my first try to mill the file I created, I tested it in the air (with the Z set 20mm in the air) to check where it was going to cut and it was cutting outside of the mold. I checked everything again and realized that the problem was that the ORIGIN of my STL file was not in a corner, but it was in the middle instead, so I had to adjust the file by setting the origin to a corner and do the fabmodules process again.

In Fab modules you first select the input file to read, and select your STL mesh file.

NOTE: If you don´t see your mesh loading in the screen, it could be because of the view the file was created with. So you might need to adjust the view angle in fabmodules, but to know if you have the right view, since the top and bottom view look the same in the mesh, you will need to calculate it. If you only see a flat image, you might be looking at the mold from under.

You select Roland Mill in as the Output Format. For the process, we are going to do first a Rough Cut in which the machine does a first pass removing material from the outside to the inside, layer by layer with a 50% overlaps.

ROUGH CUT FILE:
Patas_de_caucho_origen_ROUGH_CUT.rml

We will need to create another file for a Finish Cut, that is a two pass more detailed horizontal and vertical removal, with a 90% overlaps.

FINISH CUT FILE:
Patas_de_caucho_origen_FINISH_CUT.rml

The drill we use for this is a 1/8 inch drill (3.175mm), and you could also change the drill between this to do the rough cut with a flat one and the finish cut with a round one if you want more detail.

In my case, since my object didn't needed that much detail, I did it all with a flat one.

INPUT SETTINGS:

- Set units to 25.4 (relation between mm and inch).

- Change the view angle if needed.

- Scale the mesh to 1.

- Set the resolution to 400dpi if your mold does not need much detail.

OUTPUT SETTINGS:

- Select SRM-20 as the machine.

- Set speed to 20.

- Set the Z axis move to home to high above the border of your mold, instead you have done a leveling of the mold first.

NOTE: Molds are higher in the outer walls than in the middle, you could do a leveling by sending a flat file to mill to correct this, or mill in the middle, checking carefully that no wall is going to hit the drill while it moves.

PROCESS SETTINGS:

- Set tool diameter to 3.17mm (1/8 inch).

- Change tool overlap depending of the quality wanted. This setting changes from 50% in rough cut to 90% in finish cut.

5 - 3D Printed Mold:

I also created a 3D Printed version of my mold to check the difference in quality with each process. For this I created a mold containing the piece I wanted to cast.

- I used a 0.4 Noozle.
- A resolution of 0.2.
- And an Infill of 10%.
- Printed in PLA.
- In a Creality Ender 3 Pro.

6 - Silicon Mold:

Process of silicon mixing and molding.

For the Silicon Mold I used the Easyl 940-FDA -Silicona Alimentaria which comes in two different pots that you have to mix in equal parts of 1-1. You should always check for the datasheet to see the specifications of the mix, because it could be in a proportion of volume or weight. In the datasheet you can also see the whole process of preparation and the indications you should have in mind.

To measure the amount of silicon needed, a good tip was to fill the mold with water, and pour it into a measuring container to check the volume. That will be the volume needed of silicon (half of the volume of each one of the mixes).

Then you should clean and dry the mold very well, and add some Release Agent in spray. You should do this outside since it is a hard chemical, from a distance of 20 centimeters from the mold, and try not to inhale the spray.

Poor the two parts of the silicon and mix it very well. In the datasheet is specifies the time you have to work with the silicon, which in my case was 30 minutes, and the time it will take to dry and get hard. You could also put it into an oven, but in my case I let it dry for 5 hours and it un-molded pretty well.

7 - Final Result:

After experimenting with the wax and 3D printed molds, I wanted to experiment a bit more so I did the pieces in silicon and added some food colorant to give them color and transparency.

It was risky to cast silicon with silicon, but it worked pretty well. You just need to be sure that you add enough release agent so that it doesn't sticks together, and for the food colorant, I just added one drop and mixed it pretty well before casting it.

Here are some pictures of the final results. I lost my phone with the pictures of the pouring process of the rubber foot, but I'm going to show some pictures of how the final feet look in my box when they are already installed.

I liked a lot the result of the mold made in the 3D printer, since it gave the silicon a pretty nice texture of the layers, actually I could even say that I like more the 3D printed version than the wax one because of this layer textures that it gives to the finished part.

Other considerations taken from Tue's documentation:

- The thicker the walls of the negative mold, the better: leave more spaces from the highest point as well as the edges of the model.
- Make the thinnest thread possible while pouring material, and design the pouring holes and the air vents as big as possible.
- In order to minimize bubbles, pour silicone-upon-silicone, layer-by-layer instead of pouring the mixture all over the surface of the wax mold.
- For casting double-sided objects, harder silicones and casting materials with lower consistency are recommended.

8 - Group Assignment:

For this group assignment, we needed to read the material datasheet and understand it and also trying to cast with different materials and comparing them. In the end, I used the EASYL 940 FDA SILICON for my project, because I wanted this rubbery soft - elastic finish in the rubber feet. Anyway, here is some information about some other materials used.

Machinable wax is harder and has a higher melting temperature than most other waxes, so it can be machined, cut, or shaped using standard CNC machines. High-quality surface detail and dimensional accuracy are possible.

Dimensions: 146 x 88 x 36mm
Hardness: 52D
Melting point: 115°C
Flash point (the lower the flash point, the easier it is to ignite the material): 240°C
Respiratory, hand, and eye protection: none required under normal product handling conditions.

EASYL 940-FDA is a platinum silicone that has very good resistance to heat (220°C) and negligible shrinkage. The difference between tin-cure (condensation cure) and platinum-cure (addition cure) silicones lies mainly in the shrinkage of the cured rubber over the short and long term: tin-cure silicone molds generally exhibit higher shrinkage over time. EASYL 940-FDA has a very low viscosity which makes it flow very well while casting and that there are practically no bubbles.

Hardness: 25A (the Shore A scale is used for softer rubbers while the Shore D scale is used for harder plastics)
Viscosity: 6,000mPas
Working time (time period for mixing and pouring the liquid before it starts being cured): 30 mins
Cure time: 3-5hrs
Tensile strength: 8mPas
High temperature resistance: 200°C

- Exceptional fluidity, easy to mix and remove from the mold.
- Releasing agent is not required (but recommended).
- Excellent air bubble release, vacuum degassing is not required.
- Wear safety glasses, long sleeves and gloves.
- Gram scale not required for mixing parts (1A:1B by volume).
- The liquid rubber should level off at least 1.3cm over the highest point of the model surface.

Hardness: 15A
Viscosity: 3,000mPas
Working time: 30-60 mins
Cure time: 3-6hrs
Tensile strength: 4.5mPas
Releasing agent is required

- Wear safety glasses, long sleeves and gloves.
- Gram scale not required for mixing parts (1A:1B by volume).
- The liquid rubber should level off at least 1.3cm over the highest point of the model surface.

Most of Smooth-On's clear resins are quite toxic and may cause skin and eye irritation, and can be harmful if inhaled. Below is the information in the technical bulletin and safety data sheet:

Hardness: 72D
Viscosity: 100cPs
Working time: 10-20 mins
Cure time: 2-4hrs
Tensile strength: 3,170psi
Flexural strength: 3,100psi
Compressive strength: 3,080psi

- Gram scale not required for mixing parts (1A:1B by volume).
- Releasing agent is required.
- Wear safety glasses, mask, long sleeves and rubber gloves.
- Wash skin thoroughly after handling.
- Avoid breathing dust/fume/gas/mist/vapors/spray.
- Do not eat, drink or smoke when using.
- Do not use foam or glass mixing containers since the resin generates heat.

Acrystal Prima is an easy-to-use, solvent-free acrylic resin composed of an acrylic emulsion (part B) and a crystal-based powder (part A). It is non-toxic, odorless, and has minimal exotherm.

Hardness: 83D
Mixed density: 1,730 kg/m3
Dry density: 1,730 kg/m3
Working time: 8-10 mins
Cure time: 1.5hrs

- Retardant is recommended to increase the working time.
- Wear safety glasses, long sleeves and gloves.
- Gram scale required for mixing parts (2.5A:1B by weight)

Aa per its name, it is expanding water-blown polyurethane foam which is easy-to-use and cures rigid and strong. FOAM‑iT! 4 means the foam will expand approximately 14 times its original volume.

Mixed viscosity: 300cPs
Expansion: 14 times
Working time: 1.5 mins
Cure time: 2hrs

- Releasing agent is strongly required.
- Wear safety glasses, long sleeves and gloves.
- Gram scale not required for mixing parts (1A:1B by volume)

Material Comparison:

For this experiment, we compared 2 different kind of silicones, and 3 types of hard materials.

I was really impressed with the food-safe mold. It was stiff and stable with minimized gaps between 2 halves of the mold. Details of the food-safe mold also looked a lot sharper, while the normal mold had poorer quality. However, the EASYPLAT 00-40 silicone has a lower viscosity and excellent air bubble release that made the mixing process faster and easier.

The 3 finished objects were all lovely. Under good lighting conditions, the air bubbles of the urethane resin object were easier to be seen, but the clear amber color was nice. It best represented my original intention. I also really liked the surface of the acrylic resin object, it would be great if I can cast the full double-sided object with this material. I was looking forward to a more rigid result, so foam casting probably wasn't my goal this week.

Polyurethane resins are the best options for casting small, delicate or highly detailed forms. Acrylic resin is a better option for casting bulkier forms or faster hardening time. Acrylic resin is also considered extremely weatherproof and this makes it ideal for outdoor applications. Acrylic resin didn't yellow when exposed to sunlight, while the surface of the foam was destroyed and the polyurethane resin yellowed quickly.

Mobirise page builder - Click for more

Fab Academy 15

Molding and Casting

INDEX:

Here you will find an Index with links to easily navigate through the section content.

1

2

3

4

5

6

7

8

1 - Molding and Casting:

What is the difference between molding and casting?Source: Smooth-On