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Old 01-19-2013, 11:44 PM   #1 (permalink)
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Vacuum Forming a mini agitated hopper - first step

In the spirit of half baked, unrealized mini-agitated loader projects, I present... half of a sportshot shell made via a vacuum former.

I was a little rough while pulling the plastic from the mold and it cracked.

I'm planning a fully realized agitated hopper sometime soon but I figured I'd start documenting my work.

While waiting for my ABS / Kydex sheets, casting materials, servos and other electronics to arrive I wanted to play with the thermoformer I just bought so I grabbed a nasty unmarked 12x12 Plexiglass sheet from Hobby Lobby and got to it.

My next few tests will be focused on putting together a female mold, integrating nylon spacers, testing electronics and battery configs.

Lessons learned:

- Thin plexi cracks really easily. 1/16 and 1/32 sheets of ABS and Kydex are on the way.
- Cooking plastic in the oven is kinda scary. I need to build a proper heater.
- I was using an old 2.5 HP shop vac and I don't think it produced enough suction to apply details effectively. I need to get my monster 6 HP unit back from my brother.
- Telling when the plastic is ready is a bit of an art. I may purchase an infrared temperature gun.
- Buy a small bandsaw to replace the Dremmel I used to clean up the edges.

More pics:

Here's a link to the video by a awesomely ridiculous guy that got me thinking about vacuum forming (crank the music up):

How To Vacuum Form - YouTube

Last edited by onecaribou; 01-19-2013 at 11:47 PM.
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Old 01-20-2013, 12:41 AM   #2 (permalink)
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Cool stuff. Are these duplications for practice or are you mod'ing the shell after you form them?
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Old 01-20-2013, 01:16 AM   #3 (permalink)
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Originally Posted by desertT1 View Post
Cool stuff. Are these duplications for practice or are you mod'ing the shell after you form them?
This one was for practice but I have a couple options going forward. The current plan is to take an existing hopper and build up the areas that will hold the electronics with clay, alumalite, etc. Then I'll cast a female mold out of plaster which is what ill use as the mold to vacuum form.

The point of the plaster cast is to allow me to add slots for nylon spacers (for the screws, strength) which should become part of the shell when the heated plastic is applied.
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Old 01-20-2013, 04:41 PM   #4 (permalink)
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Some things I learned with my setup..

Place the vacuum source as close to the form as possible. I found the best thing was to just take the vacuum out of an old push vacuum and mount it directly under my low volume vacuum box.

Heat from above. As the plastic heats and deforms it moves AWAY from the heat source. This makes for a very even heat distribution. I ended up using a $20 skillet from Walmart with my frame touching the edges of the skillet creating a very small volume area between skillet and plastic. It works wonderfully.

Cover the mold with talc or baking powder. It not only makes removing the part infinitely easier, but creates a very small air gap that improves vacuum near the top of the mold.

Spend a few minutes on a setup that allows you to transition from heat to vacuum as quickly as possible. My setup has a skillet directly above the vacuum box with aluminum angle that holds the frame aligned. There's a switch that automatically triggers the vacuum once the frame gets so low. It makes things SO much nicer. It also allows you to get better results and use thicker plastic.

Let gravity stretch the plastic first. Don't apply a flat sheet to a mold with a large depth. The air flowing out will cool the plastic.

I'll post some others as I remember them.
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Old 01-20-2013, 04:46 PM   #5 (permalink)
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all I have to say is clear plexi agitated sport shot?! YES PLEASE! would be a perfect pump/light speedball setup hopper.

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Old 01-20-2013, 07:32 PM   #6 (permalink)
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Looking good Eric.....
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Just smooth and easy like a toothless hooker!

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Old 01-20-2013, 07:50 PM   #7 (permalink)
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Looks fancy! I was just looking at my rotor and old halo guts thinking about mini hoppers
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Old 01-20-2013, 08:39 PM   #8 (permalink)
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Thanks for the support guys.

Originally Posted by Kilroy View Post
Some things I learned with my setup..
I really appreciate the advice. I used talc this time and I think it worked but the plastic got locked on the inside of the lid. I'm going to use a plug next time to avoid this. If my giant shop-vac doesn't work I may just spring for a small vacuum pump and reservoir. Once I have the hang of things I may just use the plaster cast for forming (if I can drill small enough holes in it) and forgo the "table".

Do you have any experience integrating threaded nylon standoffs into your designs?
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Old 01-20-2013, 10:56 PM   #9 (permalink)
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Here's what I recommend to save yourself some of the headaches I ran into...

Plaster makes the best mold. It's porous and once covered in talc, nothing adheres. This means you will get really great detail or alternatively need less vacuum. Additionally it will not deform with repeated castings like the hopper will.

Unfortunately plaster gets hot when curing so the hopper will deform if you try to cast it directly. Personally I would get some cheap silicone rubber "tin-cure" casting mix at an art shop. Pour inside a fully assembled hopper and let cure. Cut the silicone slightly past where the factory hopper's halves meet. This ensures the mating face of the hopper will be at right angles and you will not be cutting along a fillet.

Next make a plaster box similar to Aram's. Let the vacuum tube end in several locations. Ideally where you want the 'stand-offs' to be.

Since plaster is porous, the wooden box is actually what's creating the seal. If you look closely you can see that after his plaster hardened he removed the wood "bottom" and applied it to the "top". His plastic sheet creates the final 'lid' on the chamber. Be sure to glue the wood together as well as screw.

Plaster is also great because you can pre-heat it. This makes the vacuum stage even more forgiving, increases detail and reduces pressure requirements.

That hand vacuum pump of his probably produces a better pressure differential than any shop vac. I need to try that. By the way, try to minimize the dead volume between pump and mold as much as possible when using an impeller style pump. Vacuum cleaners don't produce much of a pressure differential as it is. They rely on flow and more of it is actually a bad thing because it decreases (by the square) the pressure drop. You often hear of people getting better results with a lower HP vacuum because of this.

Once the plaster cast is made, drill holes and insert wood dowels. The exposed end of these dowels should be rounded with a height just short of the hopper's half way point (slightly lower than the mold face). These will obviously act as screw bosses so size their OD accordingly. Possibly give them a tapper. Keep them near the edges of the mold so they remain short. I always coat them in a heavy oil like linseed.

I recommend other plastics like polypropylene or polystyrene. You actually got really good results. Polycarb can be a bich.

Best of luck.
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Old 01-20-2013, 11:16 PM   #10 (permalink)
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If you intend to work with polycarbonate, remember that it will absorb water, just sitting around. When you heat it to a pliable state, the water will turn to steam and cause air bubbles in the plastic, resulting in a very weak spot.

To "burn out" the water, you will need to heat the polycarb in an oven for an extended period of time. I seem to remember that for 3/16" thick material, we would set the oven to warm (~140-160*F) and place the piece on an aluminum foil covered cookie sheet. The piece would remain there for 3-6 hours.

I made an awful lot of combat robot armor from polycarb. It is a decent material to work with, but can be very unforgiving.


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