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MMD > Archives > February 2007 > 2007.02.19 > 07Prev  Next

Hot Melt Adhesives
By John A. Tuttle

Hi All,  In response to Bill Mackin's posting about the air motor
I'm rebuilding, the bellows were still air-tight.  However, they were
getting stiff (see Note 1), and there were numerous leaks in the
block and sliding valves (see Note 2).  So, the motor had to be
completely dismantled.

(Note 1: Even though the cloth was air-tight, it had taken (what I
call) a 'set' because the instrument sat in storage for over ten years.)

(Note 2: Sealing all but one chamber at a time, I sucked on the supply
flange to see how much leakage there was.  A couple of the sliding
valves were worse than the others, but as the wear patterns on the
block also showed, none of the sliding valves were making a good seal.
After checking each chamber individually, I sealed all of them and
sucked on the flange again.  There was still leakage, indicating that
there were leaks in the block itself.)

As soon as I started taking off the first pneumatic (see Note 3),
I realized it was glued to the block with hot-melt glue.  And,
remembering the damage I had done to other air motors that were
assembled with hot-melt glue, I decided to try something different.

(Normally, I would do one of two things.  One, I would use a very thin
knife that I heated with a propane torch until the blade just started
turning red -- which always burned the wood to some degree.  Two,
I would cut the cloth all the way around -- except at the hinged end --
and use an iron to heat the wood until it got hot enough that the
bellow could be pulled off -- which always 'baked' the wood to some

(Note 3: Normally I use a chisel (that's about the same width as the
bellows) and a relatively small hammer to break the bellows off the
block.  And, there's always an initial "cracking" sound which indicates
that the hot hide glue seal has been broken.  When the unit is put
together with hot-melt glue, the chisel will go into the seam --
between the block and the bellows -- but the bellows will simply start
to bend.)

This time, I used a microwave oven, and kept heating the block and
bellows for 30 seconds at a time on 'Hi' until they started feeling
loose.  The I gave it one more 30 second run, and I was able to pull
all the bellows off in under 15 seconds.  However, by the time the were
all off, the glue holding the cloth had cooled enough that the cloth
didn't want to come off easily.  So, I put each bellows in the
microwave, and used 15 second intervals until the glue was soft enough
that the cloth pulled off with relative ease.

I think that answers Bill's question...  More later as things progress.
Actually, the block has been prepared for final assembly and two of the
bellows are ready to be recovered.  _But,_ it was no cakewalk, and
something happened that I didn't expect when I sealed the vacuum
chamber in the block.  What a mess!  I'll leave you guessing at
this point.  Sorry, I have to get back to work...

By the way, I've taken 41 pictures so far, including ones of the only
real mistake I've made so far.  Everything I've written so far and all
of the pictures will be part of the web pages about the rebuilding
process from start to finish.   I'm not in any hurry -- the customer
doesn't need the motor back until early summer...

John A Tuttle
Brick, New Jersey, USA

(Message sent Mon 19 Feb 2007, 23:41:03 GMT, from time zone GMT-0500.)

Key Words in Subject:  Adhesives, Hot, Melt

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