i am

but i have access to an industrial grade pump so i think it should go some what well i hope.
Might I suggest a negative vacuum form then?
This is a 3mm (1/8") Perspex dome using reverse vacuum forming. Basically I placed the sheet on the top of my vacuum chamber, got a seal and began heating the plastic to 200 degrees C. I had created a suction in the chamber to about -15" and as soon as the plastic became ply-able, it began to sag in. I watched the guage and as it dipped to -10", turned the pump back on to take it back to -15" and then shut it off again.
I will be running more tests on this method soon where I belive the molds can be made from a hard silicone with a shore of A40+. There is also an A65 available as well. The silicone is supposed to take temps up to 350 degrees C and because it is rubber, it will A, make a perfect copy of the master part and B, always give a good seal.
Having used both positive and negative, I see more advantages using negative vacuum forming over positive.
Some of those are -
1. time where you have just seconds to evacuate all the air with a positive pull. Neg pulls can be progressively increased as needed.
2. heat where you have just seconds to get the plastic down before the plastic begins to cool. Neg pulls can be heated during the process or re-heated if required.
3. part size where the temp and the plastic types/thickness affects the final part size normally resulting in parts that are slightly over-sized. When making a buck for positive vacuum forming, you much take this size increase into consideration. With Negative vacuum forming, the part is the exact size of the mold very time.
4. Details are often lost in postive vacuum forming unless you are making paper thin pulls. With negative vacuum forming, it is possible to have sharper lines or use thicker plastic on deeper pulls. Even though this part of the Alien is a long dome, it is a tall part due to the length.
5. Material wastage is one negative about positive vacuum forming. You need a massive sheet of of plastic that is at least 40% larger that the footprint of the part you are making a pull from. So you also need a massive platen and heater plus suction to make this work. I believe it is possible to make parts like this from way smaller gear using reverse or negative vacuum forming. The dome in my photo was just 450mm x 450mm x 3mm. If this could be done in a positive vacuum form, I would suggest that the sheet of plastic would need to be at least 1200mm x 1200mm.
6. Ease of use. At that size (1200mm x 1200mm), it is now a 2 man job just to set it up, not to mention the energy needed to push the plastic down in the first place (unless you have access to a motorized pro version of that size). With negative forming, once you get a seal, you can start the heating slowly.
7. Fails due to plastic burning is common with DIY positive vacuum forming because you are on a time line. You need to get as much heat into the plastic as you can to allow the plastic to form properly. Sometimes you won't get enough at the edges and as a result, have sloping sides. Look how tight my negative vacuum form is. The only thing I did wrong with that dome was I did not use a frame and as the plastic heated and stretched and was pulled down, it also warped and eventually broke the seal. If I had used a good frame and clamping system, I would have been able to have a nice flat surround.
8 Positive vacuum forming is noisy. The vacuum must be on the whole time. With negative forming, once the vacuum is started (and assuming a good seal is made), you can shut the pump off only turning it on to increase vacuum when needed.
Some things to think about anyway.
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