A vacuum chamber is used for vacuum degassing and other small experiments that require a vacuum environment. Vacuum degassing is the process of using vacuum to remove gases from compounds which become entrapped in the mixture when mixing the components. It's usually used when casting or molding with silicone and resin.
This instructable demonstrates how to use the vacuum chamber located in the mold-making room at Pier9 workshop Autodesk. The vacuum chamber is very easy to use, and very easy to break also. So it's high recommended that you go through this instructable before jumping on it directly.
Step 1: Determine If Degassing Is Necessary
Whatever material you're working with, it's important to read the material datasheet to find the following information:
High viscosity materials require degassing while low viscosity materials don't. You can easily judge the viscosity of a material. High viscosity material feels dense, flows slowly, and once you mix A & B, you'll see a ton of bubbles in the mixture. On the contrary, low viscosity material feels very liquidy, and even when you mix vigorously you won't see much visible bubbles at all.
You can surely degas low viscosity materials, and you will see bubbles coming out. But any liquid has some air in it, so the bubbles you see are just the small neglect-able air content in your mixture, and once degassing stops, they'll go back to your mixture again.
2. Pot Life / Working Time
Technically pot life and working time are not the same. But they are close enough to not make the distinction here. It means the amount of time you have after starting mixing your materials until the viscosity of the mixture gets so high it doesn't flow anymore ( become unworkable).
It's very important to know the pot life of your material before mixing. Once mixing starts, you're losing time. Degassing is extra time. If you're too slow in this process, you might end up wasting your material when it starts setting in your mixing container. This is also why it's better to mix small amounts instead of a big volume, because mixing and degassing for a bigger volume can drain your working time very fast.
3. Demold Time, Cure Time, Post Cure
These information are not directly related to degassing, but it's important to know as well.
Demold Time is the amount of time for your casting to cure to the point where it can be removed from the mold without distortion. It might not be fully cured at this time.
Cure Time is the amount of time your casting takes to reach its full properties.
Post Cure is often used to stabilize and strengthen your casting. It often involves heating the casting for a short period of time.
Step 2: ALWAYS Do a Test Run
The vacuum chamber we have doesn't always work so well. So it's highly recommended that you do a test run before putting your project in. If you put your project in directly, you run the risk of wasting your precious working time if the chamber doesn't work.
Follow the following steps in the EXACT order:
1. Check the oil level on the vacuum chamber. It should be anywhere between the min. and the max. marks.
2. The nut on the air opening (where air goes back into the vacuumed chamber) should be all the way out.
3. Wipe the red rubber seal clean. This makes sure that the chamber will be sealed properly.
4. Start the pump. The switch is at the back of the pump.
5. Turn on the valve. This valve opens the airway between the pump and the chamber, and the pump will start to suck air out of the chamber.
6. Observe the pressure meter. It should go all the way to somewhere between 25-30. If it doesn't go above 25, it means the chamber is not properly sealed. You should stop the whole thing, tighten all the nuts on the chamber, and start the test again.
7. Turn off the valve. Then stop the pump.
This is the most important step if you don't want to break the vacuum chamber. If you stop the pump before turning off the valve, oil in the pump will be sucked into the chamber through the airway. This will ruin your material in the chamber and also create a big mess.
8. Adjust the nut on the air opening to let air back in, until the pressure drops to zero and you can lift the lid up.
Step 3: Degassing: Operations & Observations
The best result is achieved by degassing two times. First degas in the mixing container, pour the mixture into your mold, then degas a second time in your mold. However, this second degassing is not always necessarily, especially when the geometry of the mold doesn't support degassing inside of it. (because bubbles will rise much higher than the original mixture level while degassing)
Always put a piece of paper on the bottom of the chamber. This is to make sure that if your material spills out during degassing, it won't make a big mess inside the chamber.
Usually degassing takes about 3-5 minutes. But it's better to observe the change of your material than relying on a fixed time. When degassing starts, bubbles will rise up very quickly. If you're using a paper cup as your mixing container, you should only have your material up to 1/4 of the cup's height, because the bubbles rise up so high they'll spill out of the cup very easily. The bubbles will reach a high point, then start to slowly go down. They will eventually stop at a level lower than the highest point, but still much higher than the original level. When the height of the bubbles no longer change, and the bubbles become bigger and move around slowly, your degassing is done. Warning: if you observe huge bubbles and your material becomes super sticky, this means you're running out of your working time. You should stop degassing and pour your material ASAP if you don't want to waste it.
Turn off the Valve, then stop the pump. Slowly adjust the nut on the air opening. If you do it too quickly, the air will go back into your mixture.
Step 4: Cleaning Up
1. Label your work
The mold making room is the least regulated space of the workshop, and a lot of people go there to do things from project to laundry. It's highly recommended to label your work and protect it from being moved around or having things dropped in it, etc. It's also helpful to have a warning sign saying that your material is curing in process, because cured and uncured material is hard to distinguish, especially for people not experienced in casting and mold making.
2. Always clean up after you're done with your project. Uncured materials, especially resin, can cause severe skin allergies. Uncured silicone is very difficult to remove if you get on your clothes. It's important to clean up the work station so that whoever's after you in using the mold making room won't be exposed to potential hazard.
3. If oil leak happens
Oil leak happens when you stop the pump before closing the valve. If it happens, wipe clean the chamber and put a new piece of paper inside. Never leave your mess to the next person. Do a test run again, following the steps in the exact order. If oil leak still happens, notify a shop staff.
Step 5: Additional Information on Silicone Products
RTV silicone is very useful in mold making and there's a wide variety of them to choose from based on your specific application. Always think about the properties you need before deciding which kind of silicone to use. A few things to consider:
1. Hardness / Flexibility (measured in Shore)
Do you want your silicone part to be rigid enough to hold a shape without distortion? Or do you actually desire the softness / flexibility of the silicone in applications like soft robotics?
( Tap Plastics PlatInum RTV Silicone has some other great properties such as: low viscosity which means it doesn't require degassing, and super fast cure time.)
Does your silicone mold has weird geometries so that having mold transparency can be very helpful?
3. Food Safe Applications
Most silicone products are not food safe. For food applications, you need to choose your silicone product and mold release more carefully.
Silicone can be easily colored using pigments.
5. Quick & Dirty Molding without Mold Boxes
Some silicone product comes in putty form. Wrap the putty on whatever you want to get a cast of, and it sets very fast. It's the easiest casting process for sure.
Recommendation: Smooth-on Equinox