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

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[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.

[drcrash]

Thanks to both Doug and Drcrash for the responses.

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.

One suggestion would be to use vignetting to control stretch of the plastic between the parts. You need a strip of metal or something to shield a strip of plastic from IR, between each pair of adjacent parts. If that strip of plastic doesn't heat up enough to stretch much, it will act like a custom piece of frame there.

(I learned this from DC, a.k.a. "One of Many", here.)

Do you have a screening rack where you could put a strip of aluminum foil or something?

[elmosxyls]

Thanks for the responses.

I will try to set up a hot box to dry out the ABS...blistering has not been too much of a problem, more of something I have noticed from time to time. I had always assumed that I was overheating the sheets. It does add to the scrap pile though.

I will also look at a custom blocking screen. It might help for me to determine the best spacing for multiple part plugs (ones that will go together as finished parts) and set them up on their own jig. That would make producing a custom blocking frame easier.

My big issue at the moment is platen and oven size...I could only make the 24x24 when I considered power requirements during construction. I do plan on making a 24x48 machine at some point.

Also, I want to thank Doug for a nice design and for good customer support.

I will be making a few parts sometime this weekend and I will report the results.

Thanks,

Ryk

[kayaker43]

Ryk,.. thanks for the kind words. I was surprised you didn't call me sooner. I rely on the feedback and interaction to learn and improve the machines.

The shading trick can work well, but mostly on shallow parts that don't need much sag. The mask has to be fairly close to the plastic and you can imagine how the shading pattern could go all wrong when the plastic droops away from the mask. You can also force plastic down between the parts physically with a grid system, but its tough to get it in place fast enough.

Keep us posted.......

Doug

[elmosxyls]

Note to everyone out there...this is not a sales pitch, just my opinion and observations.

It is possible to construct a system for less than I did and I will say that buying the plans and all the recommended equipment is somewhat expensive. However, I am very pleased with the system that I constructed from the Proto-form plans and I recommend them. For your information, I bought my vacuum pump and oven elements from Doug but as the plans indicate, the items can be purchased elsewhere. I chose to buy from Doug as he not only had the necessary parts available and in stock, but he has had experience with these items and with the vacuum forming process. The only issue that I had with the oven elements was with a single element collar and Doug was quick to ship me a replacement part. Good customer service.

All the other issues that I have had problems with exist with any vacuum forming system and are based mainly on the materials and user knowledge, as the responses in this thread have shown.

I appreciate the persons who have written responses to my questions and I also want to express my gratitude to the persons who set up and maintain cnczone and to all of us in this group who contribute and benefit from the knowledge contained herein. I would like to add that I built a router based on Joe's excellent plans, also from cnczone.

I have a busy day planned tomorrow but I intend to make some parts and comment on my (hopeful) success. I'll let everyone know how it goes.

Once again, thanks to all for this community.

Ryk

[coonhunter]

Elmosxyls
Justed read some of the responses to your ?'s , My exp. with abs is your blistering is caused mostly by moisture in the material. Dry in a oven for 24 hrs , your webs can be several things, material to hot,mold not coming up tru rings far enough, tool design,vacuum kicking on to early and also your material. You may need to build a plug. Also if you have a lot of problems with thin areas and webing you may want to run your mold on top .

[elmosxyls]

Coonhunter, thanks for the reply and sorry to everyone for not getting back to this issue sooner...lots going on and all. I am very grateful for your advice though.

I have been working with the designer of the vac-former on several issues, not the least of which is a problem vac pump. I don't know if he wants his name mentioned, but I will say that he not only designed a good machine, but has been very helpful in advising and is top-notch in customer service. I am working through the webbing issues and hope to have successful results to post soon.

As to the blistering issues, it seems to be random, but that could be from mixing my stock of ABS. The supplier assured me that the 1/8" material is virgin grade but that a random sheet might be older stock and therefore more likely to have higher moisture levels. I agree that it is a good idea to pre-dry my stock of ABS.

Thanks,

Ryk

[fencer]

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?



Ryk

AIR CONVECTION OVENS
This is an often-neglected type of heat source for thermoforming plastic sheet. The normal mind-set is that you need some kind of a machine with a heating source to form plastic sheet. This is not necessarily so. All you really need is a box with a uniform heat source and a method of hanging the sheet in the box. Next you set the ‘oven’ at the normal forming temperature of the sheet, put it into the ‘oven’ for ten to fifteen minutes, depending on the thickness, pull it out of the ‘oven’ and form it over a mold connected to
a vacuum line. There are certain parts that can be hand formed better than any machine could ever do. The largest thermoforming job I have ever witnessed was done by hand forming with multiple molds.
The benefit here is a uniform gentle heat. Because the heat is all convection heat, it is very easy to keep gloss levels low and consistent. It is also a great method of prototyping.
It certainly is not preferred over most machine operations but it is an option. Some things such as foam core materials are best formed via this type of heat. If you have wet stock, it will have the best chance of running this way. How often do you here of moisture blistering out of an air convection oven? It can happen but it is rare.

Thanks,

[elmosxyls]

Fencer...thank you for the response and info. This is an old thread and I'm surprised that you found it. I have not been keeping up with things lately, as usual life gets in the way of fun.

My job deals with many issues of cnc and vac forming and I still do a fair bit at home. I appreciate all the advice and will definitely try your suggestions, especially for larger parts that won't fit on the platen of my vac former.

Thanks,

Ryk