A dilema in vacuum forming

[drcrash]

If the seal around the platen edge is pretty good, it shouldn't take a big tank to do this.

A tank basically does two things:

1. It sucks most of the air out from under the platen in a hurry, getting the plastic to more or less form around the buck, and greatly reducing the volume of space between the plastic and the platen. (In doing so, the tank gets polluted by the air sucked in during the initial "pulldown." If the tank isn't several times bigger than the volume of air being sucked out, it will significantly weaken the vacuum in the tank. (Or you can use a 2-stage system where you use a small tank for that, shut it off, and open another tank that's not polluted.)

2. It sucks most of the remaining air out, pulling the plastic down really hard to get good detail, and stays ahead of air leaking in around the edge of the platen.

In this case, #1 is very easy. The mold is basically flat, so just draping the plastic over it will not trap a lot of air under the plastic, as it would with a tall, skinny buck.

So what the (pump and) tank need to do is mainly to stay ahead of leaks, to build up a good vacuum under the plastic---or upper plate, if you go that route---and hold it until the plastic has cooled below its thermoforming range.

If there's a lot of leakage around the edge of the platen, a small tank will fill up with air fairly rapidly, weakening the vacuum substantially before the plastic has cooled below its thermoforming range. This will gradually release the pressure, maybe too soon.

So if you don't have a big tank, it's important to have a good seal. You can make a pretty good gasket out of weatherstrip by

1. mitering the corners without cutting all the way through it---cut a triangular notch on the inside edge where you want a corner, and bend the weatherstrip until it meets itself. Glue that seam shut with some kind of rubbery glue. (Contact cement or silicone caulk.) And...

2. putting a little petrolum jelly on the top of the gasket to help it seal against the plastic or plate or frame or whatever it seals against.

[screenzzzz]

I tired the weather strip only, it kept getting stuck to the petg, I replaced it with some silicone rubber strips that I had laying around. I then siliconed them to the base. Tomorrow I will lay a piece of cold petg on the border of silicone strips, let the vac max out then see if I have any leaks.

I have some real fine 44x44 stainless steel mesh, I will open up the holes under the plate and place a piece of that down.

One of the things I noticed today was, that the sides of the [etg are not getting saggy, I tried a hot gun to little avail. I measured the heating unit, hot area is 16x18, that in itself makes a huge difference. The original hot plate for the unit had 39" heat rods in it, any idea where I can find them?

After and during the pull I usually have beween 10-15lbs of pressure or about 3 tons.

I didn't think of the mitering and have already cut the silicone rubber.

Looking forward to your version of a hard days nite Ringo

[One of Many]

Forming temp will be around 250-320 deg.

A couple potential problems contributing to your dilema:

The terminology you use(Mother, Father plate and lens) is adding some confusuion to the process. The brass plate is a die, a mold or a tool, the material is just that.....PETG material being turned into a lens.

The material will NOT be an exact copy of the die due to shrinkage of roughly .007/inch. Which will equate to a slight pitch loss in your lens after cooling. Over all the part will end up .084" short compared to the die original dimension of 12" x 12". The materials extruded direction(AKA grain) can play a part in the shrink also. Shrinking more in one direction than the other. While it may not be a true dimensional copy, it will pick up any surface imperfections.

Of course, I do not really know if you need a specific feature/detail outside of the top ribbed surface besides a flat ribbed lens after forming.

Given a glimps of the equipment being used. If the top surface was my only concern. I would support the die 1" above the frame it is setting in now(vacuum box riser?), and add the ring(or incorporate it into the riser) as I posted previously. I could get away with using a much smaller piece of material(if I had an adjustable frame). Maybe 15x15 or 16x16 which includes 1/2-3/4" all the way around to clamp on. The frame will absorb some of the radiant heat, so some excess is required since that area will not be quite as hot.

The holes in the plywood deck, outside of the die do not serve much purpose unless you truely need material over each end of the die to hang below the finished ribbed portion. I am confident raising the die, installing the ring, sealing off the vacuum to only the area within the ring, and letting that hot material seal on the added ring will yield a formable part within the ribbed die surface. The mesh under the die allows the vacuum free flow. Although as the material pulls into the .007 grooves nearer the ring, won't get perfectly tight. The parts trim line should be inside this anyways. An oversize blank of material is no big deal. If you duct tape where the die sets on the plywood, that would be good enough.

Informing you of the extruded lenticular sheet was just an idea that you might not need to form your own at all.

Yes, even as Ringo posts. The die must be near forming temp. You do not want the material to freeze in place on first contact with the die BEFORE it has a chance to form. He knows that gaining the experience to READ the reaction of the material as it forms(or in this case doesn't form) does take time.

DC

[screenzzzz]

Morning All,
DC, I like your piece on shrinkage as there are two parts to my process that I will be Forming for! One part the shrinkage will not be a problem. this is the part we are discussing now, I will look for an adjustable clamp, I would like to reduce size petg needed.

Taking into consideration all the great info supplied, I am going to:
a. Replace my multi hole base to the single hole.
b. As my mesh is very fine! 2 layers to a point 1/4" around the edge of my plate.
c. Bring in my silicone rubber ring to 1/4" from the mold.
d. Miter corners
e. Keep the lifters (aluminium strips) at either end of the parallel lines.
f. Raise the plate above the frame 1".
g. Lay a board on the ring and do a leakage test, I guess the best way is the wet finger method.
h. Hotgun the die.

The Second part, where I need to be as precise as possible. Correct me if I am wrong, I will use the hot platen method, where the petg is is pressed into the die. My theory being the longer the die and petg are one (petg sticks to the die), as they cool down to the lowest temperature (unknown at the moment) then the shrinkage will be less?

Does anyone know of a conversion or attachment for a reduceable clamp.

Have a great day all, GAry

[drcrash]

I'm not sure what you mean by "plate."

Are you talking about the lower die, which forms the grooves, or an upper plate like I was suggesting?

If you use an oversize upper plate, and have a nice piece of silicone to use as a gasket, you don't need to miter the corners. (It would be a shame to cut a nice piece of silicone, and silicone is hard to glue together once you do.) Just round the corners of the gasket if the silicone is flexible enough. You'll lose a few square inches of area under vacuum, but that won't be a big deal.

[screenzzzz]

Actually I am going to try both ways,
a. as you mentioned! I have a noce pc of slightly convex 1/2" plywood.
b. Heat up the die to approx 200/220f
c. Heat the plywood.
d. lay the petg on the die.
e. suck like crazy.

2.
a. Heat up the die to approx 200/220f
b. heat up the petg and bring that down.

Prior to both applications I will reset up the area as mentioned in the earlier thread.

Pic from the studio, ah it's a beautiful day hear, mind you I'd rather be in the mountains camping.

[Ringo]

Just looking at your pictures . When the frame that holds the plastic is down and you are looking at it horizontally the platen should be higher also if the corners of the platen are rounded helps too I built a platen while back using a wood base on top I laid a heavy wire mesh and then covered that with aluminum sheating using a rubber mallet I slowley worked the aluminum around the sides and nailing it on the bottom .I used a good silicone around the edge of the aluminum to seal it the corners are rounded so it takes a little time . drill the holes in the aluminum not the wood. the wire mesh provides the space for the vacuum then I drilled two holes 1 1/4" threw the wood on the bottom of the wood to attach to the tanks The frame is steel and is lined with fine sand paper .when the frame is down the outer edge of the platen seals great .I suck at explaining things but I'll take some pictures. One last thing is to preheat every thing including the frame! when you are forming.230 degrees is not hot enough you need to be closer to 300 on the plastic .The mold should be around 200

[screenzzzz]

I took the unit apart and rebuilt it, (pics) I will speak with temperature controller tech tomorrow and get that on line. Do you think I need to raise the die any higher or does anyone see any immediate changes. the base board is 1/2' higher than the clamp.

Air pressure very good holds at 25lb for about 2 mins and then very very slowly recedes.

[One of Many]

25lb or 25 inches of mercury(hg)?

If the ambient pressure acting on your skin is 14-15psi and 0psi is 29-30 inches of mercury. We can remove the ambient pressure on one side of the plastic(vacuum) to let the 14-15psi on the opposite side to do the work for us.

With that in mind. Consider what will happen during forming with the void under those thin aluminum plates.

How big is the hole under the die to port vacuum?

A 1" hole or several 1/4" holes should work fine. The screen under the die need not be a full sheet. I've used a few 5/16" flat washers to do the same thing. Strips of aluminum, whatever.....Just enough to support the die and leave room for vac flow.

Since you won't/can't use a total sealing ring up close to the die. At minimum I would trim the shim you have under the die to net width along each side and double back tape some solid bars to the plywood deck(same height as the die top) in place of the thin side sheets. The vacuum will still pull between the bar and the die, but support the material enough to keep from sealing off the grooves until fully formed.

The silicon rubber strips seem to me to be more of a nuisance than any asset to the forming process. Again unless you need some feature or detail inside the space between the silicon and the die, I am clueless how it will benefit the part. The material outside of the strips will not fully form and will be scrap anyways.

That large of a blank (" x " sheet of .02 PETG) may sag around 4"+ at forming temp. For smaller blanks, you would need to fab your own sub frame pieces to fit into the master frame you already have.

If you have not already seen these. Here is a link to some video's of actual forming on a machine similar to yours.

Vacuum Forming How-to Video's

DC

[Ringo]

A Shop vac??? yikes LOL Actually My first little formen used one of those a wopping 8 LBS. His frame looks good is yours like that? I agree with One on the silicon strips you shouldn't need them That's one reason to have the platen up a little higher then the frame so it seals around the out side edge of the platen itself. It looks like the guy in the movie had his heating elements spread out evenly (a good thing) but if he had stainless of even galvanized sheet behind the elements the heat exchange would be a lot better and also hotter.Dont purge the tank like he did in the movie. Time it at frame down just before or just after. If you do it to soon you will lose that hard pull . The tank pulls hardest when you first open it .With a vacuum cleaner it dosen't matter (no tank ) but for what you are doing that wont cut it.. When you first heat the plastic it will wrinkle a little and even sag then it will tighten up and then sag again .That's when you watch how far the midle sags and when it get's to that point it's ready to form If you do it on the first sag the plastic isn't near hot enough if you get it to hot then you can get webbing or thin sides from stretching. I dont really spell this bad two weeks ago I spilled a soda on my keyboard That and I need to proof read before I post sorry bout that

[drcrash]

I don't really understand the setup in the pictures. What kind of machine is that?

Is there a regular platen under the plywood sheet? Can you use it?

It looks like a commercial machine. Most commercial machines either have a fixed, raised platen on top, with a rounded metal edge that the hot plastic forms around and seals to directly, or they have an open-topped box with a platen that can be raised and lowered inside the box. (And the plastic seals to the edge of the box.)

If you don't have either, I can see why you'd jury-rig a sheet of plywood as a platen. And if so, I can see why you'd need the silicone gasket---the plastic isn't going to seal very well to the rough edge of a sheet of plywood, as it would to a rounded metal platen edge.

Please do explain what's going on. What's under there?

Also, where is the heater? Is the green thing stacked on top the heater, or just a lid, or what?

The angled frame looks like the hinged frame for a flip-over former, where the oven and platen sit next to each other; you heat the plastic over an oven, then flip it over onto the platen. But there doesn't seem to be an oven on the other side of the hinge, unless the plate sticking up in back is part of a funny hinge system... so I'm not at all sure how the machine is supposed to work.

Could you post some pictures that show the whole machine, and explain its usual operation?

[screenzzzz]

dr,
The machine is an Auto-vac, The way it works is
a. The brass die rests on the stationary base under the die is the mesh
b. The base has a 1 1/4" hole in it.
c. yes the heater is the green thing above the base.
d. The angled frame is the clamp, it goes up to the heater then slams down over the die.
e. yes, the silicone square around the die is due to the fact that my heater is 17x17 and the base is 20.5x20.5.

The process I am useing is,
a. Heat die, by manually resting it under the heater.
b. Placing die in shown location,
d. placing a pc of petg in the clamp
e. Raising the clamp under heater.
f. dropping hot plastic (clamp unit) over die.
g. Vacuum plastic.

You all mentioned the silicone strips, as i mentioned above they are there to reduce the vacuum area, the petg does not get heated up on the edges.

The two strips at either end of the die are to hold open the valleys, so that the petg does not seal on the edges of the die.

Not shown is the fact that the clamp hold the petg 1/2" below the base.
The vacuum holds well at 25? for a couple of minutes then, very slowly recedes.

i could not find a pic defintion of the steel ring effect is it possible that is what I am creating with the silicone border?

The petg does sag and sometime ends up spidering, I presume this is caused by too much sag?