On Tue, 14 Sep 2004, John Ferrell wrote:

> A home vacuum cleaner arrangement is not really up to the task. Besides, my
> earlier experiments with home vacuums lead me to believe a lot of air FLOW
> is essential to avoid toasting the motors.

That is true, but you do not need absolute vacuum for pulling the plastic 
so an adjustable leak in the vacuum line will let in enough air for the 
motor to cool itself. Also it pays to look at rpm limiting the motor. I 
have destroyed 1 motor by over-revving it in vacuum (not by overheating). 
I estimate that the rpm at least doubled when the vacuum side was properly 
sealed. Nomimnal was about 35,000 rpm at 0.3at vacuum. If the rpm had 
doubled it would have overloaded the outer parts of the armature to 4 
times nominal mechanical load. In my case the armature became unbalanced 
and touched the stator, and then welded itself to the stator when some 
plastic parts melted. The impulse caused the motor, which was not bolted 
down, to start rolling on the floor. I have also heard of overdriven 
vacuum cleaner motors causing the aluminium turbines to explode and shred 
holes in the turbine housing.

I believe that using pressure instead of vacuum is easier even if more 
labor is required (hold down dogs or screws for the frame). It may also be 
more dangerous, pressure could cause explosive atmosphere with solvents 
from the plastic or cause structural failure in the mold and a small 
explosion.

Peter
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