From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Re: Sealing a vacuum in a bag?
Date: Mon, 02 Nov 1998 01:05:02 GMT

On Fri, 30 Oct 1998 17:27:36 -0500, Glenn Ashmore
<gashmore@mindspring.com> wrote:

>OK, I know this is not a metalwork question, but you guys have more
>mechainical inginuity than most places on the web.
>
>I need to heat seal a plastic bag after evacuating it to a moderately
>high vacuum.  The official way is to do it in a $50,000 vacuum chamber
>with fancy electro actuators.  That is obviously out.
>
>I was thinking about heat sealing the bag except for a small gap at one
>corner and inserting a tube, but how do you seal around the tube and
>still be able to slide it out enough to heat seal the gap?

My nephew has one of these gadgets in his meat store so I took a peak.

I has a vacuum chamber about 2 x 2 x .5 ( in feet). The top is a 1
inch thick piece of plastic so one can watch what is happening. There
is a wire type heating element which seals the bag at the appropriate
time.

He puts the food in the bag and places the bag in the vacuum chamber
with the open end of the bag across the heating element. First the
unit blows air in the bag to ensure that the bag comes down evenly on
the product. (His explanation) then it starts pumping air out and the
bag shrink wraps the product.

There is no need to have any tubes sticking in the bag because when
the vacuum is created in that chamber the air will come out of the
bag. Once it is sucked down to  the machines satisfaction an
insulating bar comes down so that the bag is squeezed between the
insulating bar and the heating element. At his point the juice is
applied, the bag is sealed, the pressure is released, and the lid pops
open.

I suppose one pays dearly for that if you are in the meat business and
buy new but he picked his up at an auction for a few bucks.

Now my wife has a vacuum press in her picture framing shop. This one
is used to e.g. get the bubbles out when you mount a large poster on a
piece of foam core. With out going and measuring I would guess it will
take 3 x 5 foot panels. The top is plate glass which opens for the
access. The bottom is a flexible membrane which I presume is made from
a rubberized sheet. The vacuum pump is nothing to rave about but the
equipment does remove bubbles.

I am under the impression that you plan to use this for refrigerator
insulation. Back in the days when I was gainfully employed with one of
the larger refrigerator manufacturers this idea passed through the
research laboratory as an improvement for improved consumer market
refrigerators.

A lot of money was spent by some pretty talented research types over
quite a few years with access to almost unlimited resources. The
obvious goal was tantalizing. The project was finally abandoned as
impractical.

The damned things just weren't dependable to hold the vacuum and when
they failed as they always did you ended up with a one inch thick
refrigerator wall which was a lousy insulator.

With all due respect  to your abilities as a researcher and
experimenter do take into account the possibility that when you launch
that beast you may be cooling your beer by throwing a whole bunch of
negative BTUs at it while it is contained in a box that has lost
almost all of its insulating properties.