11. Molding and casting¶
Week assignment
- Review the safety data sheets for molding and casting materials, then make and compare test casts with each of them (Group)
- Design a mold, mill it and use it to cast parts (individually)
Review the safety data sheets and test¶
We tested several materials available at the lab with Edu. We learned security practices and then we checked the data sheets for each material. Adrien collected all this information in this table and shared it with the group.
| Product | TINSIL 70-60 | FOAM-IT 4 | PLASTIL FS-20 | EasyPlast 00-40 | Acrylcast |
|---|---|---|---|---|---|
| Producer | Polytek | Smooth-on | Feroca | Feroca | AcrylCast |
| Specificity | Good for metal | 14 times its size | quality/fast curing | Easy to use | Stone like |
| Shore Hardness | A60 | N/A | A20 | A40 | Hard |
| Shrinking | -0.8% | 1400% | NO | <-0.1% | - |
| Pot Time | 30mn | 1.5mn | 8mn | 30-60mn | - |
| Curing Time | 24hs/2hs-100ºC) | 20mn | 25mn | 3-6hrs/20mn-120°C | - |
| Mixing | 5A/100B | 1/1 vol | 1A:1B | 1:1 vol | - |
| Degassing | YES | NO | NO | Not compulsory | - |
| Release Agent | Powder | N/A | Pol Ease 2500 silicon-free | N/A | - |
Testing¶
Machinable waxes come in blocks of different colors and different sizes. In the FabLab they are recycled using the machining remains of other waxes. Generally, the darker a block is, the stronger it is. But if it is recycled, the color can be given by the mixture of waxes. The most important thing is to know at what temperature you can put it in the oven. If it is a new block, it says so on the box. For the lab blocks, temperatures above 100 degrees are not recommended.
Mixing and degassing¶
It is important to respect the proportions of the technical sheets of each product. Especially for high ratio mixing (ex: 5/100). With the TINSIL 70-60 we had a problem and it started curing before we could pour it into the mold.
When the mixtures have a short working time it is important to set a timer to calculate the times to mix and pour the material.
During the mixing process, the mixture fills with bubbles. To achieve a bubble-free surface, a degassing process can be done. Degassing can be done in various ways. Mechanically tapping gently the silicone mixture container on the table or tapping the mold once the material was poured into it. It can also be done with a vacuum pump, but this process is for silicones with longer working time. Reducing pressure will increase the size of the bubbles and the buoyancy will do the rest, making the bubbles go to the surface.
Demolding¶
Release agents will help to make demolding easier but it’s important to check the following:
- If the mold is made of silicon you need a silicon-free agent
- Olive oil or similar can be use and are cheap.
- Powder for metal casting (Always check if it’s compatible with your mold)
- Petroleum based spray
- Blowing compressed air in the inside corner of the mold after the silicon is completely cured.
Safety¶
It is always important to follow the security information on the material data sheet, but these are some basic security measures:
- Wear protective glasses
- Use 2 gloves to be able to remove one in case it has been stained and you need to touch something.
- Do not mix materials from different products.
- If any product touches your skin, wash it as soon as possible.
- If you are usinf toxic products, make the mixtures in a ventilated place.
Designing a mold and milling it¶
For this assignment, I wanted to try something simple but with some level of detail. I started by modeling my shape with Blender as it’s easier to make more organic shapes and I didn’t have any specific requirements. I made a cubic dead octopus. Then I saved it as an .obj to be able to import it in Tinkercad.
Following Josep’s guide I used Tinkercad to scale the model. My model was already cut by half to be able to create a one-side mold. So, I created the box walls. But before I measure the block of wax that I was going to mill to make sure that my box was the right size. The size of the wax was 148 x 88 x 37 mm.
When I finished the design I exported the file as an .stl to import it into fabmodules to create the CNC path.
I followed these steps:
- Clicked on input format and selected mesh(.stl)
- Selected my model in .stl
- Configured the following parameters:
- Units/in: 25.4
- View scale: 1
- dpi: 500
- Clicked on calculate height map
After the map was calculated, I followed this steps to create the file for the first cut:
- Clicked on output format and selected Rolland mill(.rml)
- Clicked on process and selected wax rough cut (1/8)
-
Changed the following output parameters:
- Machine: SRM-20
- x0 (mm): 0
- y0 (mm): 0
- z0 (mm): 0
- zjog (mm): 5
-
I checked the values of the process but didn’t changed any parameters as I was going to use a 1/8 end-mill.
Then I clicked on Calculate and it started su simulate the cut.
After it was finished I clicked on save and got my first file.
I took the wax block, put on it 3 stripes of double-sided tape and put it inside the Rolland SRM-20 machine. Then, set the origin XYZ following the same steps as on the PCB production week.
I selected the file and when it started cutting the machine made a strante sound, so Edu set the speed to 70%. Then, I lower it to 50% as the cut was still too agressive.
After checking that everything was fine I went back to the software and prepare the finishing file.
- I changed the end-mill to wax finish cut (1/8) then checked that all parameters were fine. I noticed that the tool ovelaped parameter changed from 50% to 90%, which makes sense as it should improve the surface finishing. The onlything I changed was the type. I choose to use a ball end instead of a flat end.
These are the end-mills I used 1/8 4-flute flat-end for the first cut and 1/8 4-flute ball-end for the finish-cut as my shape had round edges.
While I was waiting I checked which silicon I wanted to use and decided to use Easy Crystal. So, I accessed the website and checked the instructions.
| Product | Work time | Work time | Curing time | Shore hardness | Mix Viscosity | Color |
|---|---|---|---|---|---|---|
| EASY CRYSTAL | 1A:1B vol | 45 min | 4 - 5 hours | A25 | 500 +- 100 cps | transparent |
I wanted something faster so I checked the Platsil FS-20
| Product | Work time | Work time | Curing time | Shore hardness | Mix Viscosity | Color |
|---|---|---|---|---|---|---|
| PlatSil FS-20 | 1A:1B vol or weight | 8 min | 25 min. | A20 | 3.800 cP. | translucid |
The data sheet states that there is almost no contraction.
So, when the milling was finished. I took my piece and made sure that the wax removed from the block will remain in the collection tray of the machine.
I filled my mold with water, and poured the water into a plastic glass to measure the volume. I used this glass as a reference to measure the silicon mix. I took another plastic cup and put it on a digital scale. As, my mix needed to be equal parts of A and B, I poured first A and weight it (70gr). Then, I added B until I got to 170gr.
Nil set the timer for 8 minutes, and mix for 2 minutes. It looked very bubbly but it was homogeneously mixed. I poured the silicone into the mold from one end and little by little so that it filled all the cavities. Now I had to wait 25 minutes for curing time.
I came back for my mold 30 minutes later and it was done. I had forgotten to place the mold-release agent but this silicone is quite flexible and my mold did not have any complex shape. So, it came out very easily.
Casting the octopus¶
To test the mold I decided to make some soaps. I had to chop up some soap, put it in a glass jar, and add some cream to it. Then I put it in a bain-marie.
Toilet soap is not like pure glycerin, it is creamier and solidifies incredibly fast. It was quite complex working with it. Anyway, I managed to make about 4 octopus soaps.
Find the final files here: