Vacuum form problem
 

Hi, this is about clear canopy vacuum forming.

I'm building a Katana T30 2.3m and have some trouble with the canopy.
My usual supplier/ vacuum former cannot pull it because it's too big so I build my own
"machine".
Now all over this forum you will be able to read how simple vacuum forming is and YouTube is full
of vacuum formers showing it's a piece of cake. It may be but I'm not very successfull to pull a clear
canopy of sufficient quality to put it on the model.
The problem I encounter is wave forming at the top of the canopy as if the top plastic isn't pressed against the mould.
I use PETG of 1.5mm (0.06"). As vaccuum supplier a 1500W (ca 2hp)vacuum cleaner that gives me 280mbar vacuum.
The frame is 550x720mm (21.6"x28.3")and I let the PETG sag 7cm (ca 3") before I push the mould into the plastic.
The mould is a polyester over foam and final sanded with 320grid (the very smooth finish 1000grid didn't work either)
The plastic wraps around the mould clean except at the top where it doesn't touch it and forms big wavy bubbles.
As you can see from the pictures, my set-up is a bit unusual because the oven is at the bottom and the mould on top.
I do this for maximum efficiency of the oven and I can push the mould into the allready stretched plastic which
eliminates some other waves I got when working the other way round.
I think my problem is that the top side is isolated from the vacuum, but how come, vacuum form specialists,
that's my question.

RE: Vacuum form problem
 

The problem may be that the crown contacts the soft plastic before the vacuum seals. I do vacuum forming at work. We use female molds. We control the time the material drapes into the mold.

By allowing the hot plastic to contact the cool mold before suction is applied you can trap air. Our molds are also aluminum that are heated to 90C. This may not be practical for a home set up, but going to a female mold may help.

RE: Vacuum form problem
 

Thanks for reply, trapped air might very well be the problem alltough I don't understand the mechanism of this.
Negative mold would include airdrainig holes at the bottom of the mold which will lead to visible dots at the top of the canopy if I got it right.
I was thinking of making such holes at the top of the male mold. Perhaps I allso can play with the speed of vacuum suction.
I guess it's a matter of experimenting and wasting lots of € on plastic.
Vacuum forming is one of those things, when it's working fine one cannot understand why it wouldn't and when it's not one cannot understand why it shouldned.
Nothing is simple in this world.

RE: Vacuum form problem
 

You would not necessarily need suction holes at the bottom of a female mold cavity, though it would help. In the cases where we have them in that location we sometimes have a problem with a "fingerprint" like pattern forming around the suction holes. On smooth canopies like you show, suction holes would defiantly show. This wouldn't be a problem on a scale canopy where the holes can be hidden on the edge of "framing".

Perhaps if you lowered the heaters a few inches and allow the plastic to drape far enough that when you drop the form into the plastic clamping frame there is no contact until the vacuum draws it up.

As far as speed of applying suction, you can make the suction holes bigger, or put more of them. More holes is better, but more work too. We run a vacuum pump/receiver tank at about 20-25 inhg and a 1/2"NPT solenoid valve to apply suction.

RE: Vacuum form problem
 

I've been told that anything over 220 grit sandpaper actually makes pulling nice parts more difficult.

RE: Vacuum form problem
 

Maybe the plastic is sagging to much when you put the mold to is creating an air pocket. I have done in home forming and found it was best to let the plastic be pushed a little further by the mold and letting my shop vacuum pull the restaround. I was pulling the plastic down on the mold and not inverted as you are doing.

RE: Vacuum form problem
 

In high volume production setups allowing the plastic to droop almost to the point of contact with a female mold is pretty standard. Female molds can be difficult to make for a home setup, especially if they are made from aluminum as ours are. The OP's machine which drops a male mold into drooping plastic is probably the second best way of doing it. I have found that the amount of "droop" (we call it drape) is fairly critical. Too little and the material can get stretch marks. To much and you can get wrinkles. If you go with minimal droop, then stretch the material over the mold, it will cool more quickly at the crown and stop stretching in the middle, and the bulk of the stretch will happen at the sides. This will give a pull that is thicker in the middle, and thinner around the edge. This may be OK for your canopy, depending on how big a difference.

One thing cuwaert needs to take into account is that when I am doing this, the material droops away from the heat source, and the intensity of heat applied to the middle of the material drops off. The edges stay warmer. This gives us a situation where we have slightly thinner side of our parts. You will have the opposite situation, where you risk over heating the center of the sheet.

I had a huge headache controlling thickness until we tried one of these heaters. http://www.infraredheaters.com/solar-f.html . You will need a power supply for it. We got one from here http://www.scrpower.com/ . They're not real cheep, the heater we got, 12"X18" was about $250US. I forgot how much the PS was, and you may not need such a nice one.

RE: Vacuum form problem
 

Thanks for replies.
I know it's hard to solve my problems 5000miles from here. So many parameters are involved.
I have a fairly good result just by draping the plastic over the mold in the classic way( mold under the plastic). I have the strong
impression that the plastic was too hot because in my last result I accedentally pulled the plastic "too" cold and the result was
better.
I intend to continue in that direction because the last attempt was a bit chaotic and yet the result was the best until now.

RE: Vacuum form problem
 

maybe try less sag and let the mold push the last bit of plastic to elimiinate the air pocket? and yes a 320 grit or less on the mold allows the vacuum to pull better


just a thought

RE: Vacuum form problem
 

I have a canopy male mold that is about 20in long. We put some suction hole near the top of the canopy where the frame will be so they don't show in the finished canopy.

My mold is fiber glass cloth covered and our first tries the grain of the cloth shows very slightly so you need enough epoxy resin on the outside so the cloth does not show through.

RE: Vacuum form problem
 

It's the age-old problem off the oven being too hot in the middle! You might try a (removable) heat deflector in the center of your oven to create a cooler spot in the center. Might require a little experimentation but it will be cheap, easy, and reversible. My opinion (as others have said) is that the center of your plastic is too hot compared to the rest which results in more stretch before it is pulled to the mold, causing it to bunch up a little.

Interesting set up you have there, it looks like my machine turned upside down! :D

Sid, how is that Starfire coming along? Oh how I want a Starfire B.

RE: Vacuum form problem
 

I was hoping to fly the F-94C Starfire in June by have been sidelined by health problem in my family. In fact I am at the hospital as I type. I think I need about 3 good weeks work to attempt the first flight.

RE: Vacuum form problem
 

Gday,
We have begun to cut short kits here in Glen Allen VA and we are seeking some one local that can produce our canopies.We have original canopies that we need to reproduce.
We are also seeking some one with a large enough machine to reproduce our cowls.
Any help would be appreciated.
VAMODELBUILDERS

RE: Vacuum form problem
 

I don't have any experience with this, but it does seem that pulling up would be a problem. The warm plastic in the center would remain closer to the heat source and gravity would have its way.

However, your setup looks very nice and close to what I've seen for large, successful setups on YouTube. The one I remember flipped the plastic from an under heater like yours onto a top down pull. Seems like your setup might easily convert to that format, and I'll bet that solves your problem?

Can you tell us more details on yours? What did you use for heating elements and power supply?

RE: Vacuum form problem
 

I was looking at the frame you are using to hold the plastic sheeting while heating. It looks a bit on the thin side which is alright if you are using spring clamps with a small gap between each. But if you are screwing it down, at the spacing you have, you might be getting some material slip which might account for the rippling pattern. Did you ever consider placing a metal plate over the heating element to diffuse the heat and not have hot zones over the coil and cooler zones in the spacing? Just a couple ideas to consider.