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Vaccuum forming problem - 1/17/2008 12:12:41 AM   
Campy



Posts: 3375
Joined: 1/2/2002
From: Baltic, CT, USA
Status: offline 		I am having problems pulling the windshield for a scratch built.

The plug is about 6" wide, 4" high and 2 1/2" thick (Plus a 3/8" ply base). Due to the size of the plug I made it out of hard balsa with Hobbico balsa filler to take care of any gouges/imperfections. I put 3 - 4 coats of waterbase poly on the plug (sanded between coats ) to harden the balsa up and provide an extremely smooth finish.

I am using PETG for my windshield material. Due to the size of the windshield I have tried .040 and .060 PETG. The .040 windshield is BARELY stiff enough.

The problems.

1. The windshield sticks to the plug and is almost impossible to get the plug out. I have put screw eyes into the ply to try to get the plug out and have straightened the screw eyes in the process. At present I am using a cutoff wheel on a dremel to split the mold (on a section that will be scrap) to get the windshield off the plug, but it damages the plug.

2. Even though I let the poly dry for 4 or 5 days, I get small bubbles in the center of the windshield. The bubbles are noticeably larger (and more of them) when I try to use the .060 PETG.

I have made wheel pants, tail cones and cowls without these problems. The plugs for the tail cones and one of the cowls were also done in balsa using the Hobbico filler and sealed with the waterbase poly - no problems provided I let the plug cool down for 1/2 hour between pulling parts.

Is it due to the size of the plug, the waterbase poly, or ?????

What can I do to correct these problems ?

Thanks
       Post #: 1

RE: Vaccuum forming problem - 1/17/2008 4:07:45 PM   
rcalfred



Posts: 391
Joined: 12/7/2001
From: Elk Grove Village, IL, USA
Status: offline 		Could it be that the poly is slightly melting due to the heat of the PETG? A suggestion.....try pulling another canopy, but leave it on the form. Then, sand it with about 400 grit paper until it has a uniform surface. (It helps to smooth out any other lumps and bumps.) Then try another forming. It has worked for me. Also, others have suggested that the bubbles that form are from water that the plastic has absorbed. Try "heat soaking" the plastic, on your pulling frame, for about an hour at a temperature below that used for the vacuum forming. That would dry out the plastic. Then try another pull. Worth a try anyway. Regards.


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Al Sortwell
AMA 15663

(in reply to Campy)
       Post #: 2

RE: Vaccuum forming problem - 1/17/2008 4:09:06 PM   
seanreit



Posts: 6633
Joined: 4/17/2002
From: Cedar Park, TX, USA
Status: offline 		Would pva work?

(in reply to rcalfred)
       Post #: 3

RE: Vaccuum forming problem - 1/17/2008 5:04:20 PM   
Campy



Posts: 3375
Joined: 1/2/2002
From: Baltic, CT, USA
Status: offline
quote:

ORIGINAL: rcalfred

Could it be that the poly is slightly melting due to the heat of the PETG? A suggestion.....try pulling another canopy, but leave it on the form. Then, sand it with about 400 grit paper until it has a uniform surface. (It helps to smooth out any other lumps and bumps.) Then try another forming. It has worked for me. Also, others have suggested that the bubbles that form are from water that the plastic has absorbed. Try "heat soaking" the plastic, on your pulling frame, for about an hour at a temperature below that used for the vacuum forming. That would dry out the plastic. Then try another pull. Worth a try anyway. Regards.



Thanks.

I will try the heat soaking 1st. If that doesn't work I will try pulling a windshield with .020 or .030 PETG and then sand any bumps/lumps out before trying another pull with .060.

Question on this 2nd method - How do you keep the second pull from attaching itself to the 1st pull from the heat ? You joggged my memory (seems it needs a good swift kick these days to get it working ) Awhile back I read something about dusting the 1st pull with talcum powder to prevent the 2 from sticking together. With parts that are going to be painted I see no problem, but what about with a windshield/canopy ? Will the talcum powder mess up the clarity of the 2nd pull ?

(in reply to rcalfred)
       Post #: 4

RE: Vaccuum forming problem - 1/17/2008 7:36:30 PM   
Larry S



Posts: 443
Joined: 3/3/2004
From: Manteno, IL, USA
Status: offline 		I should have posted my problem here instead in another area but here goes. I am trying to make a canopy and I am using a plug made of plastic of paris from the old canopy. Yesterday was my first attemp at vacuum forming one. I made up a frame to hold the 020" PETG that I had and heated that in the over till the plastic was drooping. I then took the hot plastic out of the oven and pressed it on the plug with the vacuum running. What I got was a piece that had folded in several places on the front of the plug. I tried a heat gun with no luck. Second time I tried it, I heated the plastic till it was drooping more then the first time and tried that again, and again it folded in places on the front of the plug. What am I doing wrong? Should I be heating the plug first? Or should I have the plug up on an angle so that the first thing the hot plastic goes on is the curve at the top front of the plug??
Larry

(in reply to Campy)
       Post #: 5

RE: Vaccuum forming problem - 1/19/2008 4:59:10 AM   
Campy



Posts: 3375
Joined: 1/2/2002
From: Baltic, CT, USA
Status: offline
quote:

ORIGINAL: Larry S

I should have posted my problem here instead in another area but here goes. I am trying to make a canopy and I am using a plug made of plastic of paris from the old canopy. Yesterday was my first attemp at vacuum forming one. I made up a frame to hold the 020" PETG that I had and heated that in the over till the plastic was drooping. I then took the hot plastic out of the oven and pressed it on the plug with the vacuum running. What I got was a piece that had folded in several places on the front of the plug. I tried a heat gun with no luck. Second time I tried it, I heated the plastic till it was drooping more then the first time and tried that again, and again it folded in places on the front of the plug. What am I doing wrong? Should I be heating the plug first? Or should I have the plug up on an angle so that the first thing the hot plastic goes on is the curve at the top front of the plug??
Larry


You are getting the plastic TOO HOT. Reduce the amount of sag to about 2/3 of the 1st attempt and try again. Judging the amount of "sag" is probably the hardest part of vaccuum forming. It usually takes several attempts to get the right amount of "sag" for a given thickness and type of plastic. Take the plastic out of the oven (off the heat ) and do the following all at the same time: Turn it over so the sag is now on top and press firmly STRAIGHT DOWN on the plug. Again, it takes a couple of times to get it down pat.

FWIW- My box has one hole 1 1/4" in the center of it and I put my plug on some scrap 1/32" ply pieces. I made a frame to go around a couple of hot plates and lined it with aluminum foil.
I have my plastic (in its frame) supported about 3 1/2" above the heating coils on the hot plate. I put a handle on each side of my plastic frame so I could easily flip it over. The heating unit and vaccuum box are set up next to each other in the basement. (The wife would have a GIANT fit if I used the oven for heating the plastic ) I also keep my heat gun plugged in and ready to use in case the plastic doesn't pull all the way down. With the vaccuum running, I heat the offending area(s) until they pull into the plug. If you have not done so yet, MOUNT THE PLUG ON A 3/8" or 1/2" PLYWOOD BASE (just CA it to the base. The base should be about 1/32" - 1/16" larger than the plug. The reason for this is that a lot of times the plug gets "stuck" in the finished part. You need a way to get the plug out without damaging it or the part. Put some screw eyes into the base and now you have something to grab on to or use with a screwdriver for leverage to get the part out,

I get my plastic from US Plastics. http://www.usplastic.com/ A 24" x 48" sheet of .040 PETG is about $10.50.

Hope this helps

< Message edited by Campy -- 1/19/2008 5:07:46 AM >

(in reply to Larry S)
       Post #: 6

RE: Vaccuum forming problem - 1/21/2008 3:49:38 PM   
Larry S



Posts: 443
Joined: 3/3/2004
From: Manteno, IL, USA
Status: offline 		Thanks for the info,
I'll give that a try. Yes, I may have had the plastic to hot, wasn't sure how much sag I should have had. What I have here is a oven in the basement we don't use all that much so that is perfect for what I'm doing. When I made my plug I added some material all around the edges so that if I needed I could cut the finished canopy off the plug and still have an edge to trim. Everything seemed to work except for the folding of the hot plastic around the curve of the plug. I also get my plastic from the same supplier but I am going with 020" plastic instead thinking that would be easier to form. This time I have enough plastic for a few tries. Unfortunately, it may be a few days before I'll have time to try again. Other work is taking priority. I'll let ya know how it comes out.

Larry

(in reply to Campy)
       Post #: 7

RE: Vaccuum forming problem - 2/21/2008 7:43:25 PM   
garyshy


 

Posts: 21
Joined: 12/10/2007
From: Livingston, TX, USA
Status: offline 		Campy,

I'am building a b-24j 144" liberator and the tail turret that was sent is for a 101". Guess they don"t make one for 144" so I am now having to make one from plastic. I have done vacuum forming before but a long time ago. What type of plastic and what size (thick) would you use on a cylinder approx. 4 1/2" high and 3 1/2". Thanks in advance. I am up a creek and your forum post has helped greatly.

Gary

(in reply to Larry S)
       Post #: 8

RE: Vaccuum forming problem - 2/21/2008 9:04:54 PM   
Ed Smith


 

Posts: 2820
Joined: 12/29/2001
From: Brantford, ON, CANADA
Status: offline 		When vacuum forming cockpit canopies there is a tendency for the plastic to wrinkle/fold at the narrow and of the canopy. what is happening is too much plastic and not enough shrinkage. I is a bit of trial and error but place some small blocks at the front end about 1/2" away from the plug. properly placed these blocks stretch the plastic away from the plug and prevent the folding. Sometimes.

Ed S

(in reply to Campy)
       Post #: 9

RE: Vaccuum forming problem - 2/25/2008 12:49:16 AM   
Campy



Posts: 3375
Joined: 1/2/2002
From: Baltic, CT, USA
Status: offline
quote:

ORIGINAL: garyshy

Campy,

I'am building a b-24j 144" liberator and the tail turret that was sent is for a 101". Guess they don"t make one for 144" so I am now having to make one from plastic. I have done vacuum forming before but a long time ago. What type of plastic and what size (thick) would you use on a cylinder approx. 4 1/2" high and 3 1/2". Thanks in advance. I am up a creek and your forum post has helped greatly.

Gary


I have had excellent luck using PETG. If you check the earlier posts you will find the source I use for my plastic (they also have other types - lexan, ABS, etc ). For something that size I suggest .030 (perhaps .040 depending on how well it will be fastened and the amount of vibration that it will be subjected to). I am a relatively new user with the vaccuum forming, so others may have better suggestions as to the material type and thickness.

(in reply to garyshy)
       Post #: 10

RE: Vaccuum forming problem - 2/25/2008 1:35:15 AM   
tony-howard


 

Posts: 280
Joined: 9/28/2004
From: Monroe, WA, USA
Status: offline 		PETG normally doesn't absorb much moisture. Not like ABS. But the heated material will release any moisture in the mold. My experience with plaster is that it takes several days to dry - longer the more mass the mold has. It may feel dry, but it's not. And that will cause blisters.

With plaster molds I like to use ABS as a slip sheet under the PETG since the two will not stick because its deformation temperature is higher for the ABS. I think it's easier and for sure faster than finishing the plaster. If you do enough pulls, the ABS will 'wear out" and need to be replaced - no big deal.

I find a compressor and an air gun is often very effective at blowing the part off the mold. Just be sure to be ready to catch it - don't ask me how I know!

Recently I had a chance encounter with a guy who vacuum forms parts for full scale boats for a living. He made an interesting comment to the effect that vacuum forming is more an art than science and pointed out that new pulls often take a fair number of tries before they get it right. And others go right the first time. That's sure been my experience.

So I now keep notes when a part comes out right just in case I need to do another.

So keep at it.



< Message edited by tony-howard -- 2/25/2008 4:07:30 PM >

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Tony
pacific-scaled-aircraft.com

(in reply to rcalfred)
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RE: Vaccuum forming problem - 2/25/2008 7:25:06 AM   
joey_snaproll


 

Posts: 149
Joined: 11/17/2003
From: Kelowna, BC, CANADA
Status: online 		You Said it Tony,
Take notes It's the only way you'll avoid doing the same mistake over and over. More so if you only do it a one day here ,a month later do some more. Sure sucks to throw away canopy after canopy with folded plastic, vacuum seal breakes ,air pockets and more,keep notes.

Joe

(in reply to tony-howard)
       Post #: 12

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