A dilema in vacuum forming

[Ringo]

I swear I'm gonna kill my PC I did this big reply ,hit spell check and poof!!! Gone. Don't use a heat gun for your main heating source Get more room between the oven and plastic !1.5 is to close and explains what it is doing. Find a way to move the oven Higher around 5 inches total is a good starting point .Different elements have different patterns so you are going to have to experiment on the height before bolting or welding. I know you are getting frustrated but trust me when you make your first good pull you will do a happy dance. Just gota get the bugs out. I don't use PETG that much but I think If you don't stretch it to far or poke any holes you can reuse it Don't try to pull it if it's not heated properly If it does the same thing it has been add more space (oven to plastic)

[screenzzzz]

Thanks for for the positive attitude, I'll put on some yee ha dance music and pratice a few steps. Being that my up and down is air assisted and my heat hood is a constant I will go to manual and use a couple of blocks.

Also, as I have an exhaust ean close by I'll cover it up so that it doesn' pull air from under the heat panel. do you think I should attach a foil skirt around the hood?

Ah, it's another day in paradise! i couldn't locate a temperature gauge to double check consistancy of temperature under the hood, will pop over to graingers on Monday.
Thanks

[Ringo]

The room your in will be like a tanning bed if you don't use some kind of cover around the oven that and the temp will be inconsistent .And you don't want that heat loss. Foil should work while your experimenting with oven height but once you get close build something more permanent .Steel sheeting .Galvanized will work stainless would be better. then build some sort of housing around the out side to help insulate it. I hope I'm not confusing you I'm not the best at explaining things .One,DC,To tall,All are giving you good advise. I have read threw some of there threads and they are pretty impressive . One last thing you should add is some kind of switch to shut the oven off when the frame is in the down position

[screenzzzz]

I with you on the heat loss, My NEW dilema is as you can see in the pics my unit wasn't made for a skirt around the heat unit (maybe it's a boy) also the old heat rods were two inches into the unit, the new heat system is flush. I will experiment with the height Monday, after I 've finished with the tec for the temp controller.

Getting back to the skirt, I will have to bend the back board to allow a skirt to hang, which would be okay as it would help shield the electric box.

I can set up an on off system so that when the hood comes forward it will turn the power off, then as it back it will switch it back on. Great

I agree One, dc, and Totall had a lot of great advise, but, so did you mate! It just took me a while to understand the logic and language. All you blokes have been bloody fantasic and I would have been at a loss without you all, so thanks for being there.

Catch you next week

[drcrash]

Plain aluminum flashing works great for reflective sidewalls, and it's dirt cheap. (Something like $7 for a roll ten inches wide and 10 feet long, or $15 for a roll 20 inches wide and 10 feet long. It may be called "valley roll" in the roofing dept. of your hardware store.)

Aluminum is more reflective than steel in the infrared. Flashing is not glassy smooth, but it's smooth enough that you get blurry "mirror images" of the heating elements that make it appear to the plastic that the heating elements continue indefinitely past the walls.

Just don't bash the stuff around... it's thin. (And don't cut yourself on it.)

[drcrash]

BTW, if you have reflective sidewalls, you can't control the heat by varying the distance of the plastic from the heating elements. If you move the plastic down, the IR that would miss the plastic without a reflective wall in the way just hits a sidewall, mostly bounces off, and still hits the plastic on the bounce.

If you don't have reflective sidewalls, controlling the heat by moving the plastic away from the heating elements is still a bad idea. It changes the heat distribution, so you heat the plastic unevenly.

To properly control heat, you need to control the heating elements, either reducing the voltage or turning them off part of the time.

[screenzzzz]

Actually guys you are both correct! I %$%%$* screwed up.

By me getting the new heat unit installed in the hood, I've taken away the area for the heat to mix.

The bottom pic shows how the heat elements radiate, by taking away the couple of inches under the elements, by placing the new ones flush there is no mixing area!

So, sides are needed to recreate the heat mixing area, I think what you are both saying is that, by finding a comfortable heat mixing distance with the use of aluminium foil, then replace it with a permanent mini skirt.

The other very important factor is temperature control. That I will resolve Monday.

My choices here are:

a. Replace heat element with original type.
b. Change height for air lift, make a skirt and cut away part of the back board, shoot!

Thanks guys

[Ringo]

Dr. Brought up a good point about oven height I wasn't so much suggesting changing the oven height to change the temp. By experimenting with oven height temperature isn't the major concern . What you are accomplishing is finding the sweet spot for you elements to be efficient in giving a uniform heat to the plastic. You already know what happens when you are to close . You will also notice when you are to far away .It will give strip gaps threw out the sheet You are looking for just before the floor of the element pattern .But don't go crazy on this just start at 4 or 5 inches and try moving it up and down and see how it's heating the plastic so you can see when it heats it evenly (No hard spots) .. To be honest My Machines all have side skirts I have never tried with out them Safety tip make sure your wires are shielded from the oven

[Ringo]

I'm not a big fan of having the oven moving that much while forming on a small machine I have never used one like that but I would think it would be kinda dangerous if it broke loose . You have another choice making the oven fixed and having the frame move up to it then down to the platen .Just thinking out loud here

[One of Many]

Sorry for dropping out for a while. Power has been out for 4 days and last report was maybe by the end of this week.

I'll catch up and then post more suggestions if I can.

For now you are in well experienced company and gaining a wealth of your own!

DC

[screenzzzz]

Hey there One, sorry to hear about the power, You were missed! Hope your power people get it together asap.

Ringo The combination of your response and dr's painted a very clear picture for meand what I got was basically what you just described, sometimes it's not what you read but how you read it. I definitly needed that combo of opinions to see both sides of the coin.

I don't think the oven will break loose, only, I am goin to try as much as possible to keep it more in the stationary forward position. The reason being more of your last comment (Safety tip make sure your wires are shielded from the oven).

I have just rec'd new temp controller, will let you know how it works out.
Have a g'day mate

[screenzzzz]

Hey All hows the power situation up in your neck of the woods, I hope that they've been able to fix it.
well guys she works good, consistant temp control, within a couple of degrees, suck like a __________ I'll let you fill that in. I took your suggestion all and used thin washers undeer the brass die. I going to pick up a sheet of aluminium for the sides of the hood tomorrow, as per dc and Ringo. i also have a couple of rolls of soft pvc coming in, any of you blokes worked with it?
Merry christmas everyone
GAry

[One of Many]

Power is still out and reports of another wind storm are being tossed around so to speak. I guess rural living has it's down side, but nothing would make me move the 13 miles back into the city where I grew up. Several stores were looted when the power went out. Yet the city officials cannot make the connection to all that section 8 housing and massive apartment complexes they allowed to be built and destined to turn a neat little town into a rough neck slum!


One snippet I caught in your description of the plastic going crystalline?

I am not sure what you mean there. If you are getting bubbles, this could be moisture boiling in the sheet during heating. I do not recall if PETG was a hygroscopic material, but a couple days in 130-150deg heat box should take care of that. Many materials can retain moisture and that moisture will come out slowly at low temps or attempt to come out rapidly at high temps.

The thin washers do not just support the die. Where I had just put washers at the corners of large dies, the plywood would flex enough around the bung hole to seal off all vacuum to the die. The parts would form about half way then stop cold. A few more washers(siliconed in place) to keep the tool and the ply seperate was pretty simple.

DC

[drcrash]

One snippet I caught in your description of the plastic going crystalline?

I am not sure what you mean there. If you are getting bubbles, this could be moisture boiling in the sheet during heating. I do not recall if PETG was a hygroscopic material, but a couple days in 130-150deg heat box should take care of that.

My impression is that PETG is slightly hygroscopic, but so little that it doesn't need pre-drying like (say) acrylic---usually. YMMV, etc.

My first guess would be heating it too fast, so that something in it vaporizes and bubbles rather than staying in solution or outgassing slowly.

I'd turn the heat down (if only by flipping the power on and off) until it takes several minutes to heat the plastic. If it still bubbles, I'd try pre-drying it. But if you're set up for pre-drying, it couldn't hurt to do that right away.

Paul

[Ringo]

I run into moister problems once in a while using Lexan It will look like tiny air bubbles.I'm sure One and DC know more about PETG then I do But I kind of think having it that close to the elements would stripe between them and melt directly under the elements

[Ringo]

I'm going to try a experiment and wondered if you guys ever tried this . I dug out my Protoform cause it has the most open area so I can see . Any way I want to make a dome shape with out any mold at all. The Proforma has a frame that moves up to the oven and down to the platen .Instead of evaking I'm going to fill the tanks with Psi. air. I'm going to use a 2'X4' sheet of Lexan ,Heat it normally Maybe a little longer on the sag time and I'm not going to preheat the platen. but I am going to heat the middle of it with a heat gun My theory is with the platen cool around the edge it will seal fast while I blast the air out of the platen thus making one big air bubble .It has 2, 40 gallon tanks so it should be able to hold the plastic up until it cools (Reverse sag so to speak)making a big dome if all goes right! Plan B is to just let the plastic sag shut down the oven bring the frame down about half way and just letting it cool that way Which now that I think about it will try that first.Any thoughts ? Have you ever tried this or did I just have one beer to many when I came up with this

[drcrash]

My guess is you'd be better off just heating the plastic with the Protoform.

I've never done it, but my understanding is that you get pretty cool domes with acrylic by just sagging and/or sucking it into a circular hole in a box. You want the heat to be very even so that it sags into the same shape all the way around.

Not sure about doing it with polycarbonates, though.

If you're going for a circular dome, like a hemisphere, I'd think 2 x 4 sheets would be very wasteful. You could probably just make a 2 x 2 square box and attach the plastic right to it, if you put it up close enough to the heating elements. (You could even keep heating while it's sagging into the box.)

[screenzzzz]

So All, you've become a country boy eh, I'm somewhat envious, living closer to nature as anything else in life has it's ups and downs. Snow adds a real pretty look to the enviroment. a couple of years ago I went up into the Catskills in my suv with new tires, decided to see how good it was. I Drove in deep then slid into deeper snow, didn't get out till next day (I always have camping gear in the back), It was real nice getting up in the morning throwing on the coffee, Whoa this is a long story so I'll stop here.

Hey Guy's, I haven't seen bubbles lately, guess she doesn't like the heat! my presumption is bassically the same as dc.
a. No heat control.
b. 1" away from the heat unit.
I was waiting for the out side edges to soften, and the centre was halfway between glass and liquid.

For me the washers were to raise the brass die higher, the original mesh was very fine with very little lift. I thought a little more space would give a better
Whoosh of suction.

Coming from very little experience, also having seen some 10/12" domes, If one blows air up one will get a web effect. If one can blow heat into the inside of the dome, who know, you could probably blow yourself a place to go when the wife gets her knickers in a twist.

In my last thread I mentioned soft PVC! Anyone??

[One of Many]

POWER.......has returned.......Hooray!


Funny that you mention that Ringo!.

Actually, I did do this with a piece of 4' square 1/4" lexan(polycarbonate). We just let it droop, but there was a lot of weight in that sag to help. It was a dome for a friends Hot Tub Gazebo. I built a lumber frame with shrinkage compensation of what I expected it to be as finished size with a flange for fastening it to the structure. Not like it needed to be precision, but it turned out very nice.

We did do bubble prestretch pretty often. If you have ever seen a snap back box forming, it is the opposite of this. The box opening is used as the seal edge. It is just not very practical to expect perfect dimensional repeatability if you are controlling the buldge manually with a gate valve. It works well for prestretch prior to actual forming on very tall dies. When the tall die is on the upper platten, it is timed such that the die pokes into the bubble and basically rolls the bubble on as the die descends. After the die base meets the snap back box, then the vac takes over to pull the material against the die. After a cooling cycle, the die and the snap back box retract from the material leaving the cooled part in the frame.

What are you making, a sky lights or cool bubble windows for a chevy van?

DC

[One of Many]

These are the foothills to the Cascades, so snow can be part of its charm, but also another seasonal inconvenience for commuting. I'll suffer with the Deer, Rabbits and the occasional Fox rather than dodge bullets in my old stomping grounds, Heheh!


DR had some good points on heat distance and distribution. As I have posted in another thread, you can string some wire lines to make a screen tray under your heater. With different screen densities, you can mask off heat in hot spots by placing the screen on top of the wire lines. This allows you some control over time, temp and radiant intensity, where you need it and where you don't. You are gaining some experience on how to read the materials condition. Let that guide you toward making the heat consistant throughout the sheet(I.E. Cause and Effect).

You can blow the bubble up, but keeping the air from leaking while it cools without it collapsing on itself could be a challenge.


We did a lot of PVC, but it was either natural ivory, black or gray. I would not say it was soft. The smell was awefull!
There was a keyboard cover/skin we did for a while, but I don't recall the material designation off hand. I want to say it was EVA or PVA or some thing rubbery.

DC