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'[EE]: Queries re heat sealing polythene sheet'
2004\06\16@222239 by Russell McMahon

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Summary: Looking for information on heat sealing polythene sheets together:

I wish to heat join polythene (polyethylene) flexible sheet - similar to
standard plastic bags but thicker.
My application is the (very) experimental fabrication of an
air-solar-heating envelope ** using clear plastic sheet as the top layer and
black plastic as the bottom. I'm using 250 micron plastic, which is
considerably stiffer than standard plastic bags (which are perhaps 50 to 100
micron) but is still very flexible.

I want to seal the edges of a rectangular "bag" completely reliably so that
it neither leaks or separates accidentally.
My trial bag is 1.5m x 4m and my final target bags are about 4m x 5m.

You can buy small thermal sealers for a few dollars which are meant for heat
sealing plastic bags for eg food storage. I believe and am told that these
are too low powered for what I want to do. They have a heated element on one
side and two ?Teflon (= PTFE) coated jaws which grip the plastic as you
slide the sealer across the bag. I do not have one to hand at present but
recall that they work OK once you establish a considerable knack for best
speed of travel. Factors include plastic thickness and exact makeup.

Commercial "pulse" units are not useful as they are intended for a fixed
width of plastic - eg a relatively small bag.

You can buy commercial continuous units which would probably work well.
Typical units use hot air applied at a number of points, rollers or feed
belts and multiple PID temperature controllers.They melt a zone of plastic
which is then compressed and cooled. Such a machine would probably be ideal
but I have no access to one and the price is extremely unattractive for
"playing".

My experiments with a soldering iron at various temperatures are not
promising. One can achieve a good weld but with cutting of the sheet or only
partial (or no) sealing. I'm probably going to have to go to something which
at least crudely approximates the commercial machines operation. Perhaps a
small sealer with a heater element running on speed*.

I could attempt gluing/sealing (maybe silicon rubber) but this is an
unattractive alternative and polythene is an extremely difficult plastic to
deal with.

I'd welcome any comment from people who have had experience in this area.




       Russell McMahon




* cf Niven-Pournells "Legacy of Heorot" - marvellous story.

** I'm aware of the issues of mechanical strength, UV lifetime etc.

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2004\06\16@223521 by rixy04

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I have a rather large hot wire sealing machine that I used to shrink wrap LP's
(33 1/3rpm). Since I no longer use it for that purpose, I now use it to seal
parts bags for shipping components. It gives a real professional appearance. I
save and collect all kinds of bags now that companies ship equipment in. Some
are pretty thick bags and it does do a good job sealing them. I suppose they
still sell them for bag sealing. I got mine about 15 years ago for about
US$350.00 Don't know what they go for now. Here's a sample of other commercial
units:
www.uline.com/Group_46.asp?pricode=wf426
Rick

Russell McMahon wrote:

> Summary: Looking for information on heat sealing polythene sheets together:
>
>
> I'd welcome any comment from people who have had experience in this area.
>
>         Russell McMahon
>

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2004\06\17@105007 by Spehro Pefhany

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At 02:22 PM 6/17/2004 +1200, you wrote:
{Quote hidden}

I've worked on a pulse controller... it's fairly simple, but can't really
discuss the specifics in public.

{Quote hidden}

Yes, I doubt this will work very well. Polyethylene has to be treated
(eg. plasma) before gluing, and even then..

>I'd welcome any comment from people who have had experience in this area.

What about a travelling "zone" sealer that could wheel across the sheet?
Perhaps you could use a heated ball bearing (the whole bearing, not just
the ball) with the lubricant replaced by a high temperature type?
Just a thought. You'll probably need to control the temperature, pressure
and maybe the rate of travel too (automatically) to get consistent results.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2004\06\21@124555 by Sergio Masci

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What about building some kind of ultrasonic welding tool?

You could build something with a small tip and surrounded by a collar. The tip
would stick out only a very short way past the collar so that when you press
down on two sheets the tip would only be able to dig in a short way. The collar
would stop it going in too deep and cutting the sheet. It would only melt where
you supply adiquate pressure between the tip and the platic and the pressure
would drop off as the tip digs into the platic and the collar comes into play.

Regards
Sergio Masci

http://www.xcprod.com/titan/XCSB - optimising PIC compiler


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