Re: RotoCasting - a simple way to make Bellows and other hollow parts

well, I didn't want to enter too many details, I just outlined the process
and mentioned some advantages and potential risks.
Why making the "extra" inner shell ... ? it's fairy simple:
RTV silicone rubber is heavy, expensive and when you make thick walls it's
flexibility becomes relative.
By making that inner shell:
1) you save a lot of money
2) you save a considerable amount of weight (the outer shell can be made
in several ways, including casting light-filled resins)
3) you get a "removable" shell, the actual mold, which allows you to
reproduce even the deep undercuts without parting the mold too much.
4) the inner shell could be removed (because wore out, broken or for any
other reason), a new one poured in place and you get a new mold.
5) the use of the more expensive RTV silicone for our purposes is
considered for some reasons, as it allows us to not consider drafts,
and it freely demolds from any material with very little preparation.
6) the costs of the RTV silicone (an average product) is about the double
respect to a pure urethane resin. Not even comparable to a polyester,
and also not comparable to a filleld polyurethane.
7) the weight of the same mold, if poured in mass in RTV silicone rubber
could be as much as four to five times the same mold made like I suggest.

About the cost of the 2 axis rotational machines, well ... I didn't
investigate too much as I built my first one starting from an industrial
machine I got (just came out off production line from a friend's factory
where they use them to manufacture toys).
That one was an heavy one, fully mechanic (as it was designed to cycle
through an oven and a cooling water jet several times per day).
I lost that in an accidental fire which destroyed a customer's plant,
including all my equipments in the prototype workshop, few months ago.
I'm rebuilding it now, much lighter, based on standard aluminium frames
(Bosch like) with separate electronic controls over axis.
I think all of you, in this mail-list, can easily build one with very
little money, a surplus motor (mine one was a 3 phase 2HP but it's not
necessary
for these purposes) a frame and a few gears or pulleys. Gears should work
better for this purpose.
I remember to have seen some for sale over the internet, specially suited
for casting urethanes, but they were too pricey, around 2,500/3,000 USD.

>Message: 7
> Date: Thu, 3 May 2001 00:11:26 -0600
> From: "Smoke" <gordonr@...>
>Subject: Re: RotoCasting - a simple way to make Bellows and other hollow parts
>
>Well, I've looked at your site.
>
>I don't see the point in making an "extra" inner shell (filling the cavity
>produced by coating the model with clay or whatever). Why not just Cast the
>outer shell in two (or more for complex parts) over the original model and
>use that for the rotational casting? That would save a lot of extra work
>and material.
>
>How cheap are those double axis rotational casting machines?
>

About the problems I experienced with spray silicone, I specified that in
rotational casting you have some combined effects from the chemical,
thermal and
mechanical action, all together. The final results, when you're casting
resins with 15', 20' or even longer demolding time, could be that the
casting whashes
out the release agent from the mold walls and a mechanical adhesion occurs.
I think that an even longer demolding time would allow for a chemical
adhesion to start.
I tested a number of different release agents, the one usually supplied by
smooth-on is super. I discussed the problem with some chemical experts from
the
urethane suppliers and silicone was always excluded for roto-casting.

However, I'm specifically speaking of rigid molds, not about RTV silicone
molds. In fact, for some reasons, I had to use rigid urethane molds, with
the proper drafts,
in order to cast some housings. The walls were pretty deep .
I had to cast very fast RIM urethanes, with a very short pot-life (about
30") and about 5' to 7' demolding time. That short pot-life (mixing time)
and a very small
tolerance in mixing, lead me to build a pneumatic mixer (using the
disposable mixing nozzles), the only way to control the process since 2 or
3" more or less
during the mixing could produce an uncured part or an incomplete part.

About the casting material, you mentioned testing with RTV silicone. I
would not. Silicone is not the best for that use, there're so many flexible
urethanes, still less
expensive than silicone and with excellent properties, especially if
talking about corrosion resistance.
Flexible polyurethanes work perfectly if you need to enclose some
mechanical fittings inside the part (the mounting ends for example).

> From: "Smoke" <gordonr@...>
>Subject: Re: RotoCasting - a simple way to make Bellows and other hollow parts
>
>BTW....we use Spray silicone t school for all our mold making (RTV molds).
>It works very well for us...most of the time.
>
>However, some surfaces...such as stones...need to be sprayed with a coat or
>two of shellac prior to using the spray silicone. Also, a light coat of wax
>sometimes helps as well.

I hope I answered to your questions.
Bye

Paolo