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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]
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.
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
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?)
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
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.
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?
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
Learn cause and effect through experience. Mastering those relationships is the "Common Sense" ability within the art of any trade.
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?
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.
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
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
Learn cause and effect through experience. Mastering those relationships is the "Common Sense" ability within the art of any trade.
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