A dilema in vacuum forming

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    Default A dilema in vacuum forming

    I hav e an old 22x22 Vacuum unit w/overhead heat and a 10 gallon tank, pump sucks in at about 25lbs.
    I am using .020/.040 petg.
    I am trying to form a lens, ( pic attached ) the depth of the valleys between the lines of the lens is about .007".
    I am getting the petg to form around the lens plate, but not the lens themselves. i.e the petg is not forming in the valleys, I get a flat inner surface. I am very open to idea's does anyone have any.
    Thanks Gary[/img]
    Well I guess I'm not dropping a pic!
    the plate is 12x12" it has 30 convex lines per inch. my vacuum table has holes every 2"[/list]

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    If there is no venting at the bottom of the valleys the plastic will chill off long before enough vacuum will be pulled in this area. With 40 mil matrerial you could get by with .032 drill bits. but with it being a lense construct you would probably want to vent with a drill in the 70 to 80 regions. Another thing to consider is that pet likes hot molds. If you want super detail you are going to have to pressure form.

    JohnD.



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    Default A dilema in vacuum forming

    Drilling is out of the question! I thought of pressure forming, I picked up a t-shirt heat transfer machine! It works great around the sides but, the petg's flat as a pancake in the centre ( the hot platern is slightly concave ) I agree with you John it does need pressure, thanks mate



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    What kind of "lens" is this? How fine is the detail that you'd need for the optical properties you want? (E.g., do you need 1/30" ridges to show up a little, or do you need the planes between them to be distinct and flat?)



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    Default A dilema in vacuum forming

    Actually, there are no planes only valleys with convex ridges and they have to be as precise as possible to allow the lens to focus. I have tried in the past the method with the t-shirt machine. It was not the greatest, there were quite a few imperfections + I had wiped on silicone to the plate and a silicone sheet under the petg which affected the clarity as the silicone left a dull film. Also i had to wait quite a while for the brass plate to cool down before removing the petg.
    If you have any idea's for vacuum forming other than drilling the plate or, any other process, or idea's I'm all ears.
    Thanks for your response GAry



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    For the kind of fine detail and precise angles you need for a focusing Fresnel, I wouldn't think vacuum forming would be the way to go. A silicone mold and clear resin casting would seem much better.



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    And then there's the issue of refractive index... if you make the copy out of something with a different refractive index than the original, it will not focus the same way even if you do get the shape right. Depending on what kind of image you need, it might more or less work with a different focal length, but it's likely to be useless.

    What kind of lens are we talking about? I'm not asking for any proprietary detail, just general parameters. Is it something like the Fresnel in an overhead projector?



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    I take it the die is a dish shape? Are the ribs toward or away from the die? Does the vac plate close off the holes outside the blanks 12x12 area?

    Even if you pressure form it, just as in vacuum, the dish will trap air if no way for it to escape. one small hole if .010-.015 would not be noticable if you can get your pressure or vacuum time and heat under control.

    Another method is compression forming after heating. For a lens like yours, I would line the upper die with felt to keep from distorting the ribbed surfaces while id forces the material to conform into the lower die.

    With such a fine rib, the heat can level out those ribs too?

    DC



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    Firstly, the plate is for a lenticular lens, Dr you are quite right the female has to be a total duplicate of the male as well as a specific thickness. The plate is flat DC, and there is no upper die.
    The input I am getting is great, I'm from the school that, it's not what I read, it's how I read into it.
    I'm going to try two methods.

    a. I am going to place (in pic) a pc of aluminium over the two ends and coveing a line of holes, my hopes are that I will have more pull from that area. Rolling Stones, You can't always get what you want but, if you try some time you just might find you get what you need, I hope.

    b. Attaching the plate to the bottom of the heat transfer platen (this will cover the convex curvature of the platern) and not use the silicone, I'll try a teflon sheet under the petg. I am hoping that, if the male plate being brass will allow a release of the petg when it has cooled down
    Any thoughts?

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    Interesting problem.

    The lenticular lens itself is curved overall? It's not one of those flat ones with all the grooves straight and parallel?

    For a flat one, you could try using your vacuum former as a pressure former, by putting an oversized rigid plate over the hot plastic, and sucking that down to squeeze the plastic. All of the force from sucking down the whole plate would be transferred to the plastic. (If your rig isn't sturdy enough, though, you could break your platen.) You'd want to make sure the middle got squeezed a bit sooner than the edges, to push the air outward through the grooves as the plate comes down.



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    The lenticular lens does have straight and parallel grooves only they are not like triangles, they have convex tops.
    Okay, let's take that into consideration!Let's say I place this plate on top of the hot plastic acoule of points have to be taken into consideration.
    a. The plate will cool down the plastic fast, if so, will that effect the forming.
    b. I have a problem vacuuming the top plate as the vacuum holes do not lay outside the holding frame.
    What I have found that is giving me a better pull into the orifices of the lens, is the laying and sealing of the aluminium strips at either end of the father plate.
    I am going to try blocking a few more holes around the plate, this will concentrate more suction into the top of the plate. I am also going to try your suggestion by adding a heavy plate with a wooden base (won't be a cold as metal)
    As far as being sturdy enough, I believe this baby was made in the 30's solid iron, it takes 4 blokes to lift it.
    GAry



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    I understand there is no upper die now. That would be in compression forming the lens as an option.

    As I read it, the die is a concave/convex depression and that you have no vac holes inside this concave area. I take that to mean, once the material seals off around the rim, it is much less likely the material will pull down into the concave.

    If any of the holes in your platten suck air during the forming process, you won't get full vacuum under the lens. In a sealed cavity, vacuum, just like pressure is equal on all surfaces. You cannot get the vacuum to suck harder in one particular area.

    Unless you are trying to prevent leakage in the die area by using the bars you plan to add to give the sheet something to seal against. The platten with the vac holes in it should still be sealed off with the exception of what is directly under the forming area. Any external leakage is the path to least resistance, leaving losses if nothing pulling on the plastic itself.

    There is also a method that utilizes a seal ring. Pressing on the material, just outside the trim line to hold the material to the tool. Nothing more than a non cutting cookie cutter that stomps down on top of the material sealing it against the die prior to applying vacuum. It prevents leakage at the material edge if the material is not held tight in a clamping frame. This precludes that vacuum must be ported to the die cavity only.

    I'd need to see the die, your process and forming machine to offer anything of further use to help resolve your dilema.

    DC



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    All of the platen holes outside the area the plastic covers should be sealed off---or, if you're using a bigger plate, there should be a compressible gasket of some sort to seal to the platen around the edge. Any platen air holes further out should be taped over or otherwise sealed off, to keep the vacuum in.

    A more detailed description of the shape would be very helpful. Is it flat or domed overall? If it's domed, how much is it domed, over what distance?

    I suggested a rigid metal plate just to distribute the force evenly over the plastic during forming. That amounts to a flat die for the non-grooved side. I'm guessing you want that surface smooth; a wooden plate might be a lot of work to get smooth enough.

    I wouldn't be surprised if a steel plate worked fine; if the PETG or whatever plastic you're forming is thick enough, it will insulate the grooved side of the plastic long enough to form. (This should all happen very quickly.) On the other hand, maybe not, and maybe it'd be bad to cool the plastic that fast on one side. You could cover the flat die (plate) with a flat piece of some other plastic, preferably with a higher melting point. For a metal plate, that would provide some insulation. For a wooden one, it'd provide a smooth surface.

    This is harder if your shape is domed, of course. If so, you can could cast a domed adapter to match, off the non-grooved side of the lens you're copying.

    Either way, I'm wondering if you can get good enough detail for a lenticular lens. Even assuming you squeeze the air out well through the grooves (and maybe some tiny holes), I'd guess you'll get enough rounding of the edges of the grooves (the edges of the flat refracting strips) that it will blend the images it's supposed to keep separate, at least somewhat, and likely have other optical flaws.

    What kind of lenticular lens is this? Is it for a 2 image thing where you see one image if you're off-center one way, and another if you're centered or off-center the other way?

    Paul



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    Also, how many of these things are you making? If it's only a few, would you be better off casting it in resin?

    You may need to cast it anyway, to get a buck you can drill vent holes in. And if you're only making a few, it probably makes more sense to just cast the parts conventionally.



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    I have enclosed a pic of lens.
    I take that to mean, once the material seals off around the rim, it is much less likely the material will pull down into the concave. .
    Hmmm Not necessarily and I'll try and cut to the chase.If when the petg is brought down it is held above the lens, then sucked onto the lens, which have valleys (also the middle sinks faster than the sides). Then logically it sb pulled down centre first.??
    I agree You cannot get the vacuum to suck harder in one particular area. But, what I can get is a longer stronger pull in a smaller area.
    Now I tried Dr's suggestion of adding weight only, I did it after the vacuum, now what I am getting from you is, to add the weight prior to or as close as possible to suction OK
    Also when I tried the weight on after vacuum it had very little effect as the petg had already cooled.
    As I mentioned earlier I am half way into the valley.
    I'll get pics of machine tomorrow.
    GAry

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    Quote Originally Posted by screenzzzz View Post
    If when the petg is brought down it is held above the lens, then sucked onto the lens, which have valleys (also the middle sinks faster than the sides). Then logically it sb pulled down centre first.??
    I'll look forward to the picture.

    So I think it is becoming clear.....


    You are expecting the die with ribs in it to form the properties of a lenticular lens. I would think you will end up with a corregated sheet of PETG instead of a lens, but that is pure conjecture on my part. I do not see the back side remaining optically flat. You might have better luck displacing the material similar to roll forming threads or knurling, I. E. Embossing with a hydraulic press between two heated plates. I am not sure what would result since shrinkage in the plastic can make it a challenge to keep it flat after removing it from the grooved die plate.

    Just to see if you could get it to pull tight. I'd try forming directly on the grooved die plate itself. The only change I would make would be to place a snug fitting square ring around the 12x12 die, the same hieght as the die. Seal the outside of the ring to your table so that vacuum is only pulled through the a very small gap between the outside of the die and inside of the ring. If you had to, you could place the die on top of thin screen, shims etc to prevent restrictions to the vacuum holes below the die. You could also cut small grooves into the side faces of the ring or die to port vacuum to each of the grooves on your die plate. This should create a natural path so vacuum will pull the material into the grooves of the die plate with minimal effort if the material is hot enough. If need be use a heat gun to get and keep the die hot so it does not chill off the plastic before it has a chance to form.

    Once the material is done heating(preferably in a frame), drape it completely over the ring and die, then hit the vacuum. This must be done quickly. Fine detail is best obtained where the material is pulled tight as a product of proper heat in both the material itself and the die forming temp, then minimum time from heating to point of forming seal and then vacuum. This can be very critical on thin material, but there will always be some limitation based on material characteristics and thickness, not to mention equipment. As the material forms, the surface area the vacuum is acting on gets smaller, it cannot pull it tighter besides the fact it is returning to its solid form rapidly.

    Once it is formed on the die, you can turn a fan on it and let it cool for a few minutes, but not too long. When the sheet returns to rigid should be adequate, but it still may be hot to bare skin.



    Do you realize that you can buy PETG in sheet with the lenticular aspect extruded in. Spartech Lenticular Lens Sheet

    Sorry for the confusion. For some strange reason, I thought you were trying to form this as pre-made lens sheet to a specific shape on a female die.

    DC

    Last edited by One of Many; 12-08-2006 at 03:30 PM.


  17. #17
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    Default What about?

    Does it have to be Petg? Have you tried Lexan?. I went threw the thread and I'm a little cornfused on what you are forming but I only use Petg for testing because it's cheap . If I could see a picture of your machine it would help. I think you said 25 lbs which is ok but how big is the tank? Also are you heating the mold? and how far are you saging? and why am I asking so many questions? LOL



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    Hey Ringo keep asking, the more questions creates more potential for answers!
    Well, are you ready for this one. Even though I replaced the Temp controller and the thermo couple, I am having problems with getting above 230f. (I borrowed a temperature gauge) Now if that's the case, is it possible that even though the petg is sagging, and feels hot to the touch it's not soft enough to form such fine lines.

    if so, now that I added the aluminium strips to the ends of the father plate, and I am getting half the depth of the lens forming on the petg, It is a possibilty that with more heat I can get this to work.
    My dilema on this is that the bloke who was helping me set the controller told me to make a few adjustments, let it run for 20 minutes and then call him back. When I called he had left for the weekend grrr.

    2nd method with heat transfer machine, I had manufacturer get me a me a new hot plate, it was perfectly flat, (made my day) my problem with this set up is I have to place something between the hot platen and the petg, I used a silicone sheet, then a telon sheet. I did get a good emboss only the sheet became somewhat opaque. I'm not sure if it was the petg and heat or the after effects from using the sheets, anyone have any thoughts?

    Ringo
    It does not have to be petg, I have some pvc coming in next week.
    Pics attached.
    I am not preheating the mold!
    As far as the sagging, did I tell you about this girl I knew. No really about 1"-2".

    DC
    You too mention heating the mold also, we both agree on a tighter area around the mold. I like the mesh under the die idea. As far as the grooves I believe that has been accomplished by laying the alluminium strips along the end, the alluminium lays on top of the lens leaving the valleys open for the pull. Heat gun hmm yes.
    Yes I do know that there are sheets out there only, as Paul mentioned the mother plate (petg) must be a perfect reverse of the father plate (mold).

    Hey Paul, I am making a couple of dozen some in hard plastic and some in soft pvc. I do not know how to cast, I am always open to learning only, time is of the essence.

    I am sorry I cannot be explicit! I am working on a patent and you blokes are being real helpful, I really appreciate your interest help and patience

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    When I talked about a plate over the top, I wasn't suggesting a heavy plate. I was suggesting a plate to broaden the area that the vacuum former sucks over, as well as to provide a flat die for the non-grooved side.

    With a 21 x 21 inch platen, you have 441 square inches to work with. If your vacuum pulls, say, 12 PSI, that amounts to over two tons of suckage---the vacuum can pull a plate down much harder than you can press down with the weight of anything you'd want to lift onto the platen.

    So if you put a rigid plate over the plastic, and have a compressible gasket around the edge, your vacuum will suck the plate down with a couple of tons of force. If the PETG is what it rests on, it will press the PETG down with two tons of force, about three times what you'd get if over a 12" square sheet alone.

    You can probably use regular weatherstripping from the hardware store as a gasket---say, 3/8" x 3/4" sticky-on-one-side foam rubber weatherstripping. If that's not tall enough to meet the plate, put a strip of something down and put the weatherstrip down on top of it.

    If there's any flex in your "rigid" plate, it'll tend to push the plastic down around the edges, creating a seal (more or less) and squeeze the air toward the middle. That's the opposite of what you'd want. You'd rather it pushed down first in the middle, and squeezed the air out through the grooves, toward the edges. So you may want a plate that's slighty convex. To get that, you could put a few sheets of paper under the plate, of different widths, so that they add up to slightly thicker in the middle than toward the edges. If you put something slightly flexible under the paper, such as a sheet of acrylic, that will smooth out the stairsteps of the progressive thicknesses of paper, as well insulate the PETG from the thermal conductivity of the rigid plate.

    Ideally you'd want a "rigid" plate that was very, very slightly flexible, so that this setup would push down hard in the middle, forming the PETG into the grooves there, but then slightly bending the plate so that it flattens and the pressure moves outward and squeezes the air out through the grooves, as the PETG is formed into the grooves in an outward wave.

    The built-up-layers of paper in the middle and the flexure of the plate under two tons of pressure should both be about the same as the depth of the grooves you need to form.

    Last edited by drcrash; 12-09-2006 at 12:04 AM. Reason: fixed typo


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    Paul
    I want to absorb this slowly, You did not mention heat! Then this is pressure form, and if @ 12 lb I have over 2 tons then @ 25 lbs wow, I like it. Convex sounds logical. Wow, the insanity I've put myself through, to be given a possible answer that's really quite simplistic. Thanks Paul I'll give it a whirl in the morning



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A dilema in vacuum forming

A dilema in vacuum forming