July 3

July 3, 2005

21 plastic booobs

It took a heck of a lot of trial and error and experimentation to learn the nuances of vacuum forming PETG plastic, but I think I have it. Check this baby out:

Here's a better shot (better because this one does not include a view up my nostrils):

Yes, it's Garrie Hill's Varna canopy bubble. Well, I made it from a fiberglass mold that I pulled from one of Garries bubbles that he so kindly sent me for the Rocket. I built the fiberglass mold because I wanted a small mold that was representative of the MDF fairing mold that is currently being CNC machined for me to practice on.

The final result after many, many, many learning attempts is pretty good - possibly good enough for the fairing shell. There are small bumps where the vacuum holes are, but I think I can make the vacuum holes smaller and hopefully eliminate those small marks. Garrie says for 1/16" thick PETG, the holes should be .3*1/16" dia and the holes I drilled are a bit bigger.

When I make the fiberglass mat mold from one of Garries bubbles, I put a layer of fiberglass fabric on the inside of the mold so that I would be smoother. When the mold cured, there was a fine weave pattern left from the glass cloth. This weave pattern shows up slightly on the PETG plastic, but if I sand it down (sand it on the mold, not the plastic), with 150 paper, then the pattern does not show up on the plastic. This is good because I do NOT want the exact texture of the mold surface imprinting onto the plastic. The reason is that after hand sanding the MDF fairing mold, there will be a slightly rough-ish texture to it. The fairing shell finish has to be very smooth. I learned that there is a trick to keeping the mold surface finish from imprinting onto your hot plastic. Since there are no secrets here, following are the steps to vacuum forming PETG:

1. Heat source = 4000 watt, 220 volt quartz infrared lamps positioned 8" above 1/16" thick PETG plastic.

2. Female cavity mold = smooth to the touch with a surface finish of cardboard. Drill many .0208" holes in the bottom of the mold. The more the better. I drilled 12 holes that were evenly spread out over the surface of the mold.

3. I found the best way to provide a vacuum to the holes in the mold was to cover the bottom of the mold with breather fabric, then air-tight cover that with a sheet of poly using the gummy tape used for vacuum bagging.

4. Spray and clean the PETG plastic sheet with a non-static spray so dust doesn't still to it. Then clamp it onto the top of your mold.

5. Swing the lamp back and forth to ensure that the sheet is evenly heated.

6. As heating progresses, start to slowly apply vacuum pressure using a ball valve in your vacuum line. When I first discovered this, I simply let gravity make the hot plastic sag. With the breather blanket set-up, there is not enough air movement between the plastic sheet and the mold surface to allow the hot PETG to sag naturally, so you have to start sucking the air out using your vacuum pump. In fact, if you don't start using the pump slowly, the air trapped between the plastic and the mold bottom heats up, expands and can push a hot PETG bubble up over 12" high!! Then it bursts and you have a bunch of plastic bag material in your mold. The SLOOOOW vacuum process should last for about 4 minutes. Continue to use the ball valve to pull the plastic down to within about 1/2 inch of the mold surface. DO NOT let the hot plastic touch the mold yet!

7. As soon as the plastic sags to within about 1/2" of the mold surface (or closer, but NOT touching), immediately pull the mold away from the heat source and let it cool and harden. You can leave the vacuum setting where it was because it will hold the wet plastic in place until it cools. This would probably be the best place to stop the entire process because the plastic has not touched the surface of anything, and therefore is still very smooth and optically clear. I think that one could make a SERIOUSLY great fairing or canopy bubble if a shape within a half inch of the mold shape was good enough (which is probably the case 60% of the time). In this case, I need the fairing to be the exact shape of the mold, so stopping now is not an option.

8. The slow vacuum pull encourages the plastic to maintain it's thickness all the way to the bottom of the mold. If you pull the vacuum too fast, the bottom (part closes to the bottom of the mold cavity) of your plastic part could be paper thin.

9. After the PETG has cooled and hardened, apply FULL vacuum. Now turn the heaters back on and wait a minute or two until the plastic has touched the mold surface. As soon as the plastic touches the bottom of the mold, pull the mold away from the heat and turn the heat off. Continue to apply vacuum until the plastic is hard and only warm to the touch. This step will bring the plastic down to your mold surface, but shouldn't leave too many surface imperfections on the plastic due to an imperfect or slightly rough mold finish, or other mold issues like chill marks, mark-off or dust specs, etc.

Here is the reason I think this method works - As soon as the plastic starts to re-heat (under vacuum), the top layer of the plastic melts first because it is closer to the heat source. As soon as the plastic sheet gets hot enough to be sucked down into your mold by the vacuum pressure, it doesn't mean that the bottom surface of the plastic has melted yet. It may be flexible enough to form into your mold without being completely melted all the way through. If the bottom layer is still semi-hard by the time it touches the mold, AND you immediately remove the heat, then the smooth bottom layer of the plastic is preserved, yet your plastic part is the exact shape of the mold cavity.

Now, PETG only comes in 8' long sheets and my fairing is 10 feet long. The next challenge, is to use the small bubble mold to figure out how to join two pieces of PETG plastic together to vacuum form one entire 10 foot long fairing.

Here is the photo log of the last week:

To start with, I used a 1000 watt, 12" quartz bulb, but it wasn't hot enough
The mold is a stainless steel bowl with a hole drilled in the bottom.

This shot shows a piece of plastic vacuum formed into the bowl. The x marks on the bowl were attempts to get the vacuum to spread throughout the mold without having to drill additional holes.

These are two 4000 watt, 5 foot lamps that were joined together and suspended from the ceiling with a steel cable.

I can raise or lower this as required - AND swing it back and forth for even heating.

1 plastic bowl
Here are all of my various experiments.

With a single hole at the bottom of the mold, I discovered that the plastic wanted to be sucked directly down to that hole. To spread out the vacuum over the surface of the mold, I tried various breather blankets like felt and peel ply. I also tries scoring the mold bottom. All attempts aside from drilling more vacuum holes left patterns in the plastic.

Once I figured out how to make a decent plastic bowl, I covered my Garrie Hill PETG canopy bubble with fiberglass mat and polyester resin. The layer directly next to the plastic bubble is one layer of fiberglass fabric.
This shows the surface finish of the mold cavity. The fiberglass fabric pattern can be seen and felt - smooth, but slightly bumpy.
To provide a vacuum to all of the holes that I drilled in the bottom of the mood, I tried to bond these 1/4 plastic tubes to the glass mold.
If I wear this on my head, I can communicate with my alien friends.

Actually - it's an attempt at a vacuum manifold, but the yellow sticky vacuum gum melted away as soon as I started the heat lamps. I couldn't get it to work at all.

So - I ripped all the tubes off, and covered the whole thing with a soft breather blanket, then vacuum bag film. The vacuum hose fits onto the bag with a suction cup and the breather blanket allows air to circulate over the surface of the mold. I could easily drill any number of holes in the mold this way.

The 1/16" PETG sheet sits on a neoprene weather stripping gasket and is held in place with a wood flange and clamps.
This shot shows the plastic melted down into the mold


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