11. Molding & Casting¶
Group assignment¶
For the group assignment we conducted several tests with the products available at the lab. We grouped the information collected in the following table. It was the occasion for us to understand the specificity of each product and see how one or the other would match the need of the design we want to do for this week assignment.
| Products | TINSIL 70-60 | FOAM-IT 4 | PLASTIL FS-20 | EasyPlast 00-40 | Acrylcast |
|---|---|---|---|---|---|
| Picture |  |
 |
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 |
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| Producer | Polytek | Smooth-on | Feroca | Feroca | AcrylCast |
| Specificity | Good for metal casting | High expansion and light | For regular modling, good quality but fast curing | Easy to use. Good pot time for beginners | Stone like |
| Shore Hardness | A60 | N/A | A20 | A40 | Hard to the contact |
| Shrinking | -0.8% | 1400% | NO | <-0.1% | |
| Pot Time | 30mn | 1.5mn | 8mn | 30-60mn | |
| Curing Time | 24hrs (or 2hrs at 100ºC) | 20mn | 25mn | 3-6hrs (can be reduce at 120°C to 20mn) | |
| Mixing | 5A/100B | 1/1 volume | 1A:1B | 1:1 vol | |
| Degassing | YES | NO | NO | Not compulsory | |
| Release Agent | Powder | N/A | Pol Ease 2500 - Do not use silicone based release agent | N/A |
General information¶
Choosing your wax¶
The machinable waxes come in blocks of different colors. Usually the darker the stronger. For some application a specific hardness of the block might be desirable so one has to choose accordingly. At Barcelona Fablab, the team tries to recollect the milling dust of the wax blocks to reuse them as much as possible. In that sense the mold creation is pretty environmentally friendly. So with ti,e the wax looses a little bit its original color because they get mixed. So make a test if needed before choosing your block (if you need to cure in a oven for example).
Mixing¶
When preparing your mixture, really take care of the proportion. Especially for the high ratio mixing (ex : 5/100). With the TINSIL 70-60 we had an issue and it cured during what was the expected pot time. It resulted in an impossible to pour mixture.
Degassing¶
When creating a mold, one want to have the smoothest surface (or at least a surface representative of the wax block). To do so, we want to avoid bubbles at the contact surface between the silicone and the mold. To do so some silicone producer advise to proceed to degassing. Degassing can be done in several way. Either mechanically by hitting in a gentle way the glass of silicone on the table to propels bubbles toward the surface. It can also be done using a vacuum pump. The pressure reduction will increase the size of the bubbles and buoyancy will do the rest by bringing the bubbles to the top.
This picture is the perfect counter example. NO degassing was applied to the silicone and it produced a rough surface due to the presence of bubbles during the vulcanization process.
Demolding¶
Some products can be used to make demolding easier.
- Olive oil !
- Some powder for metal casting (but check it is compatible with your products)
- Petroleum based spray
If your silicone is not compatible with the products mentioned above you can always try to blow a bit a compressed air in the corner of the mold a the demolding time.
Pouring¶
Pouring the silicone is not an easy part either. Indeed one wants to avoid pouring it directly in the center of the mold otherwise air bubbles might get trapped. The technic consists in pouring a thin cascade of silicone in a corner of the tilted mold and waiting for the product to reach the top.
Examples of application¶
- Silicons We got to see different silicone already cured fro, the hardest (shore 60) to the elastic ones (shore 20).
- Expandable foam Expandable foams require to think well the curing process and how to maintain the mol. Indeed as the expand a lot the will try to wide open the mold. One has to find the good balance between holding the mold and not applying to much pressure on it.
- Technical mold This mold is a nice piece of work which allowed its creator to integrate a resistance within a button. Also to win time the wax mold has different level of finishing to focus high quality finish on the technical surface to save time.
- Metal casting Using TINSIL 70-60 we created a temperature resistant mold that can receive melted metal. The process is a bit tricky as unlike silicone, metal does not support several casting layers (better to do everything at once). We learned that wood can be used for this purpose because the carbonized layer will protect the rest of the wood from the heat.
Safety¶
All the above examples are nice, but before starting using those products a compulsory step is to check the risk of using the products. Below I present the analysis and links of the safety data-sheet of some of our group assignment silicons.
The safety data-sheets refer to the GHS of classification and labeling of chemicals. Deeper information can be found on the wikipedia pages in case of doubt on a classification.
TINSIL 70-60¶
In case the links get broken with time due to a change in Polytek website, one can go the Polytek official website : https://polytek.com/products/tinsil-70-60-silicone-rubber
- 2.Hazard identification
As one can see below there is a lot of warnings related to its toxicity and flammability. As a consequence several rules of use are given such as avoid working close to flames and hot surfaces, not breathing vapors, avoid releasing in the environment.
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- First aid measure Another consequence of the hazardness of the product is the recommendations for first aid measure in case the rules of use were not respected. The producer gives information in case of eye contact, skin contact, inhalation etc…
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- Fire-fighting measure The product being flammable, some information come to help the user in case a fire starts due to this product. In this case the recommended fire extinguisher are CO2, dry chemicals, foams or water.
The part B does not require GHS (Globally Harmonized System of classification) label due to its low toxicity. The producer just says it can be mildly irritating.
The First aid and fire measures are similar to part A.
“FOAM-IT 4”¶
The product is considered as harmful and and hazardous to the human health if we look at the pictograms of the GHS.
It is interesting to see that in case of ingestion it is important not to induce vomiting.
Conclusion¶
After seeing all these tests I decided to use EasyPlast 00-40 for my personal assignment because it seems easier to use for a first time. It has a long pot time and does not require a specific treatment. It is a bit long to cure but I prefer that to a low pot time.
Personal assignment¶
Molding and Casting Process¶
In this chapter I will present the process I applied to create my mold. The idea is to create a simple mouse toy. To do so several steps have to be respected that I try to represent in the following diagram. During the process I made some small mistakes that I use to learn the correct way for my next mold and remember the good practices.
My diagram
Part design¶
For the design the two first things to know are :
- What will I design
- The size of the wax block
To do a design that fits on my wax block I simply added the wax block in my design.
Here is the mouse design. One can see that I tried to make forms that are compatible with the machine used to produce the mold. Indeed the eyes and hears have, at worst, vertical surface and no close angles (like a sphere) to be sure the whole thing will be milled properly.
Now within my design I add the wax block and check that the proportions are fine
and that I will be able to mill it.
Last step is to choose the tool I will use for my design. Below is the selection of end mills we have at our disposition.
I want to mill fast with a reasonable quality. For the roughing I want to go really fast so I check that the biggest end mill we have fits between the walls of my mold and the mouse.
Danger
My 3D design has some issue that one would like to avoid (I realized after milling)
- The walls of the mold should be tilted to ease demolding
- The distance between the top of the mold and the highest point of the model should be a least 5mm and I had 3mm 0
Mold Milling¶
Once the design is ready we can start milling the mold. The mold milling starts with planning the milling strategy on a software called MODELA.
File preparation¶
My milling strategy consist of 3 phases described in precision in below’s tab. I want to do the obvious roughing to remove the material around the shape of the mouse. Then I will do a different finishing on each half of the mouse:
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Flat Using a flat end mill I will be able to properly mill the 90° angles but the finishing quality for round surface is not the best
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Round Using a round end-mill allows a nice finishing for round surface but it does not access narrow angles as the flat end mill does.
I also planned a little bit of overlap of the top of the model to see the combination of flat and round end mill. It seems that the best result are obtained using flat end mill follow by the round end mill.
The roughing has to be executed over all the area of the mold. The final 3D form should be not to different from the finishing needs to avoid high quantity of material to be removed during the finishing which is slow.
The tool is a 1/8int end mill with 4 flutes. It allows to remove high quantity of material and is thin enough to pass between the walls and the 3D model. The scan lines are chosen because it goes a bit faster (win 50mn instead of 1h with X direction scan lines)
The Finishing flat will be applied only on the left part of the 3D model. It will not give the best result but is a way for me to see the effect in the final model.
The parameters are set in consequence.
The Finishing round is applied only on the other part of the 3D model. It will give a better effect but not all the angles will be milled.
The paramaters are set in consequence.
Below one can see the simulation done by MODELA of the milling strategy. We will see how realistic it is after the real milling.
Milling¶
Roughing¶
The roughing process lasted 1h for not such a big model. It is important to check that the first 10mn are going as expected before leaving the machine work on its own.
At the end of the roughing a lot of chips have been created. It is important to remove them to avoid overheating during the finishing process. Leaving them would decrease the finishing quality due to possible melting.
Finally the roughed 3D model appears and looks a lot like the simulation. There is no undesired left part. We are ready for finishing.
Finishing¶
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Flat finishing
The flat finishing went as expected because the surface is not so neat but the angles between the lower part of the model and the model are fully cut.
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Round finishing - failed
The first round finishing failed because I went too quick and forgot to change the end mill. So I did a round end mill with a flat end mill. The problem is that the software thinks it can go closer to the limit (round end mills normally are further from the surface for a given tip position) so it removed more material than expected. We can see the upper part with a rough/lower surface.
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Round finishing
To correct that and have an example of round finish I “round finished” the part where did the flat finished (the first one). In the picture below we can see the obvious difference.
As a consequence; a good strategy is to do roughing in two steps, starting with flat and end mill and finishing with the round one.
Molding¶
Now comes the time to prepare my silicone and pour it the mold.
Preparing the silicon¶
To know the quantity of mixture to prepare I filled the mold with water and measured the volume +20% (some of the mixture stays in the post and tools).
After a good manual mixing I had to degas the silicone as recommended in the product data-sheet. The pot time being from 30mn to 1h I have time to do it. The weather was pretty cold the day of the mixing so I think I had around 45mn to do the preparation.
Pouring¶
Now that the mixture is ready, it is time to pour it taking care of :
- Wear eye protection, gloves, appropriate clothes
- Pour slowly in a tilted mold starting from a corner
Here is the video of the process.
Note
The editing is not amazing but this was a 300mb video that I converted to an 8mb video ! I added comments etc … using ffmpeg. I could do better with more time but wanted to test ffmpeg capacities. You can find my recommendation for this tool in my trick page.
Here is the result right after pouring the silicone and 2hrs later.
Mold Result¶
After a whole week-end I had the occasion to unmold my creation (well … a step to my creation).
The silicone got a bit whiter as expected and the touch feeling confirmed that it was cured, the surface at least. As the mold is pretty small and has a thin wall I went really carefully while removing it from the wax. I blowed a little bit of compressed air on the side of the mold to help the process.
Here is the final result, we can see the difference of finishing of the wax mold printed on the silicon mold. I will see on the final piece how it impacts.
Casting¶
Colofonia¶
For the casting process I chose to use a product colofonia which is the result of processing pine resine. It has a reasonable melting temperature (approx 120ºC) and is not too dangerous.
Warning
Even though it is not too toxic, some recomandation of use.
- Wear gloves made for high temperature. It sticks to the skin like caramel.
- DO NOT wear plastic gloves !
- Melt it in a well aerated place to avoid fumes that could be produced by over heating.
And here is the video of the pouring. One can see it is pretty delicate because it solidify quickly, it’s hot and sticky.
After pouring I put the mold to cooldown
While demolding I had a bad surprise, the material is extremely britle and I broke the tail. The final result is not so shiny as I expected because each and every tiny milling detail can be seen on the surface. That might require polishing to get a perfect result.
Acrylic¶
I made a second test with Acrylic and green tint to see how well they mix and the final effect. The tint requirement is high see properly the color. It is more adapted to “pastel” light colors.
Here is the picture of both the Colofonia and Acrylic casting
Lecture¶
Files¶
3D files
MODELA files