1/4" mat'l vac form questions

[elmosxyls]

Hello,

I have a 24" x 24" vac former that I built using Doug Walsh's plans (top oven). The equipment works very well for 1/8" and thinner materials (mostly ABS and styrene) but I am having issues with thicker materials. I have been told to heat the material in cycles, (apply heat, pause, apply heat) in order to heat the material throughout without blistering the top. I have not been able to pull parts successfully in thicker materials and hope that you experts can lend some advice.

I have a two stage setup (5 gallon tank and rotary vane 10cfm pump) to pull the full vacuum after opening and then closing the main valve.

It could be that I'm just not being patient enough with the heat cycling...any ideas?


Thanks,

Ryk

[drcrash]

If you're blistering the top, it sounds like you're probably heating it too fast and need to turn the heat down one way or another. How long are you heating it?

You should allow a few minutes per millimeter for the heat to soak through.
You can heat somewhat faster at the beginning, and then slower while the initial pulse of heat soaks though.

1/4" plastic is doable with single-sided heating.

Cycling the power on and off by hand does work.

You can cut the heat in half with an inexpensive heat control made from a big power diode (a.k.a. "rectifier"). Here's a thread about that over on www.tk560.com:

http://www.tk560.com/phpBB2/viewtopic.php?t=606

If you cut the power in half, you'll not only change the intensity of the IR, but the frequency distribution. That might actually be a good thing.

Doug Walsh designs his machines to run at an "efficient" element temperature (around 1350 or 1400 F IIRC) where the central hump of the IR emission distribution coincides with an IR absorption spectrum peak for most plastics.

(That is, the elements put out IR that plastics are especially good at absorbing.)

A different emission spectrum might work better, allowing more of the IR to penetrate further into the plastic, rather than burning the surface.

(A significant fraction can go right through, depending on the plastic, but with an aluminum reflector on the opposite side, you can get a double-sided heating effect to some extent. On a small top-heating machine, I've used an aluminum cookie sheet to bounce IR back up to/through clear acrylic. Seems to work fine; if the elements & plastic are pretty much surrounded by reflective walls, the IR will bounce around until most of it is absorbed by the plastic.)

[elmosxyls]

Drcrash, thanks for the reply.

To answer your question, I don't time the process, just judge by the sag in the material and the feel from underneath by poking the material and noting how quickly it resumes it's now curved shape. I usually think it's ready when the material has sagged below the holding frame and pokes fall back out quickly. If I had to guess, I would say it's approximately 3-4 minutes for 1/8" ABS. I have noticed that I get better pulls on subsequent runs and I am assuming that is due to the frame being hot.

Recently I successfully re-heated a bad pull to finish a prototype (didn't seal well on the first go). I had blistering on that one, but I expected it since I was re-heating a now very complex-ly curved surface with some areas much closer to the elements.

I am also thinking of raising the platen by 1/4 to 1/2" to get a better initial seal.


Thanks again

[joakim]

maybe you could try to put your plastic in the "baking oven" first?
that way you would get heating from both sides.

[elmosxyls]

Joakim, thanks for the reply.

The vac former is in the garage and the oven is upstairs. Besides, I don't think the 24"x24" blanks would fit in the oven.

I have thought of heating the plug side first, not till it sags, just a little to get it hot. Then lowering the frame, flipping the blank, and then finish heating.

Any thoughts?


Thanks,

Ryk

[drcrash]

Joakim, thanks for the reply.

The vac former is in the garage and the oven is upstairs. Besides, I don't think the 24"x24" blanks would fit in the oven.

I have thought of heating the plug side first, not till it sags, just a little to get it hot. Then lowering the frame, flipping the blank, and then finish heating.

Any thoughts?

I've done that, and it helped. I was heating 1/4" Sintra (which has a low thermoforming range, and is a reasonably good thermal insulator because it's slightly foamed) on a bottom-heating oven that was a bit on the hot side, and a bit uneven. I flipped the plastic two or three times, both side-for-side and end-for-end, to avoid burning either side or either end. That was 12" x 18" sheets, though, and I was flipping them by hand.

I don't know how easy that would be on a 2 x 2 foot Proto Form, though. (I'm not clear on how the clamping frames attach to the lift lever mechanism.)

[elmosxyls]

Drcrash, thanks for the reply.

To give you an idea of how the frame is attached to the lift carriage, I am attaching three photos. The first is an overall shot from the side and the next two are close ups showing the top frame resting on the bottom frame and the top frame lifted off of the bottom frame.

Forgive the green paint, it's all I had, and I haven't gotten around to painting the oven cover yet.


Thanks,

Ryk

[kayaker43]

Ryk

The Proto-Form is my design so I'll try to help.

I pretty much discourage messing with the temperature, as drcrash mentioned, it really is fairly well matched to the absorption characteristics of the plastic, and plastics as a group are pretty close to the same. You can just move the plastic a little further away from the elements, this doesn't affect the wavelength, but does reduce the intensity. However, I think your blistering is not so much heat related anyway.

I'll take a guess and say you are having problems with ABS? This plastic is notorious for having moisture problems and this usually shows up as blistering or more accurately lots of tiny bubbles in the sheet cause by moisture expanding. I doubt you will have any problem at all with styrene, but I always expect moisture trouble with ABS and polycarbonate. It just seems like overheating or scorching, because it helps when you heat slower, but this is actually just giving the moisture a little chance to dry out.

Fresh virgin ABS can be heated pretty aggressively. ABS is available as "wide spec" which means some amount of recycled content, and "virgin" with new resin only. The virgin grade costed 4-5 times as much. You have to ask for virgin grade if you want it. Some of the utility or "wide spec" grades are unusable for vacuum forming, while another batch may form wonderful. If you want to produce an ABS part in quantity, you can save a lot of money with utility grade if you're lucky, but run the risk of lots of scrap. Pre drying is costly and still can't save a really bad piece of plastic. If you want repeatability, you have to factor in the cost of fresh virgin material and even then have to deal with proper storage or pre-drying. As you can tell utility grade ABS is not my favorite stuff. I suggest you switch vendors until you get some better material.

Here's some other tips for thick plastics. Pay attention to heat loss on the bottom side. Even a slightly drafty or cool room will make a difference. I once had all kinds of problems at a trade show until I realized the air movement was causing problems. Some times a simple barrier around the machine helps tremendously to keep it running in still air. Also I've heard of people rigging up a reflector on drawer slides to go underneath. and another shining heat lamps at the bottom side?

I've done styrene and PETG very easily at 1/4 in thick, and I've done lots of 3/16 ABS once I found some fresh plastic. I've also done 1/4 ABS with great results, then tried another time with an old sheet and it bubbled up like crazy. Take a sample of the blistered plastic and cut through the affected area, then carve it with a razor knife to get a good clean edge, It will probably be full of holes like a sponge? That would be from the water expanding.

Hope this helps

Doug Walsh

[drcrash]

Joakim, thanks for the reply.

The vac former is in the garage and the oven is upstairs. Besides, I don't think the 24"x24" blanks would fit in the oven.

[...]

I have thought of heating the plug side first, not till it sags, just a little to get it hot. Then lowering the frame, flipping the blank, and then finish heating.

Any thoughts?

You can make a simple "hot box" for drying out of plywood, with a regular incandescent light bulb at the bottom to warm it. (A light bulb puts out several times more heat than light, so it makes a good low-power heater that won't set things on fire.)

I used to have a link to plans for that, but it doesn't work anymore.

You can leave a bunch of sheets in the box for a day or two, to slowly bake out the moisture.

I'm told it works, but haven't done it myself yet.

IIRC the plans I saw had the sheets standing vertically, with a little air space in between and around them, and a few small vent holes drilled in the bottom and top of the box, so that air would gradually flow up through the box by natural convection and carry the moisture away.

[kayaker43]

I built an incubator with a light bulb in a box when I was a kid and ended up with cooked eggs. It was very hard to control the slowly changing temperature, so plan on some kind of thermostat. A wood box should be fine and since the sheets won't reach softening temperature, they can be racked standing up with air space between them. Vent holes will be necessary of course.

You will need enough bulbs or other heat source to get up to 250 degrees for drying polycarbonate, but every other type will be lower. Call your supplier for specifics on the sheet you are using but I think ABS falls in the range of 175-200 ?

I have only been able to rescue sheets with mild problems. Severe moisture issues mean scrapping the plastic. Just because you bought it yesterday doesn't mean its not old plastic. My local distributer just keeps stacking fresh plastic on top of the old stuff. The bottom of the pile can be several years old, and I've seen ABS sheets stacked in the parking lot in the rain.

If you were manufacturing a product, you would get fresh dry virgin sheets directly from the extruder and have no issues.

[elmosxyls]

Thanks to both Doug and Drcrash for the responses.

I do plan on running production parts both from ABS and from Kydex or Spartech (they have an impact resistant material that is similar to Kydex). I plan on running 1/8" and 3/16" for the most part, and some possible 1/4" as well.

I am assuming that the ABS that I get, relatively cheaply, from my supplier is not virgin grade so I'll look into it. Unfortunately, I have several quotes out there based on the price of the ABS from my supplier so I hope the cost of the virgin material won't be too much higher.

Any thoughts about spacing for various height parts to avoid webbing? Is there a basic calculation that you might recommend for parts of varying heights? I've been doing trial and error so far with mixed results and lots of scrap.


Thanks,

Ryk

[kayaker43]

Expect to pay a lot more for virgin ABS material, similar to Kydex prices I would think,.. or build a drying oven and expect some scrap rate.

Keep in mind the old 1:1 rule, A mold should be at as far from the edge of the platen as it is tall. For multiple molds, double that spacing between parts. That's a good starting point but mold shape factors in a lot. A bunch of dome shapes could almost be touching eachother at the bases, but cubes would need at least 2x their height, and would probably still web?

Sheet properties and thickness also matter, so best to experiment.