Re: [CAD_CAM_EDM_DRO] Making bellows

I would suggest a simple method to make your own bellows:
Rotational moulding is the solution, it's cheap and quick.
And it produces professional results.

Don't worry, you don't need an oven and a cooler, just a rotational molding
"machine" (arm),
a proper mould and the necessary quantity of an elastomeric urethane resin.
Cold rotational molding is not limited to urethanes, any thermosetting
resin or even a natural latex would work.
But for bellows, a good flexible polyurethane (see www.smooth-on.com for
example
or http://www.freemansupply.com/oldsite/ciba3.htm ) would work best.

You can get closed shapes from one easy mould and, if you smartly design
the mold parting plane,
you can obtain two open bellows (C shaped) by simply cutting the produced
part along the parting line (better
if you make a small undercut in that part of the mold).

I successfully used different kind of resins, both in flexible (RTV) and
rigid moulds for prototypes and short runs.
Be careful about the release agent (silicone or whatever else) as the
chemical (aggessive) action of urethane compounds
combine together with the mechanical action inside the mould, during the
process. This can - often - remove the
separator from the mold walls and may cause a mechanical union between the
mold and the part.
You should still be able to remove the part, but it will be a real pain.

The ratio between the two rotational axis is, usually, set to 4:1 and the
rotating speed is quite slow, in the range of few hundred
turns per minute. It may depends on the mold shape and size, as well as the
characteristics of the material being molded.

The whole process can take from 3/4 minutes for very fast (RIM type)
urethanes to 15/20' for the slow ones.
I would suggest you to try to use faster ones, in the range of 6-8' as the
longer demolding time increases the risks for
the above mentioned separating issues.
However, when using flexible resins and - even better - RTV molds, the
problem will disappear.

Finally, if you're going to use flexible molds (both urethane and/or RTV),
pay a good attention to properly
support the walls, especially deep and thiny walls as they may deform
during the process due to mechanical stress.
A good idea (also to save costs and materials) would be to pour a thinner
layer of RTV silicone inside a recess
between the part and the outer rigid shell. The RTV skin could be as thick
as 1/2", self releasing, and easily removable if needed.
The rigid (outer) shell can be made out of any inexpensive material
(plastic or plaster) allowing for a safe fastening to the
rotating plate and ensuring the necessary stability to the whole mold.

It will not take too long to get impressive results from this very simple
molding technique.
You can mold very big hollow parts with a very little money.

Have fun

Paolo



A

>Message: 10
> Date: Wed, 2 May 2001 00:12:55 -0600
> From: "Smoke" <gordonr@...>
>Subject: Re: way covers
>
>Wonder if there's an easy/cheap way to make a round bellows from RTV, latex
>rubber or whatever?
>
>Anyone have any ideas?