Re: Void Normal Direction

From: Ian Gibson (
Date: Tue Nov 11 2003 - 08:21:36 EET


You could build the balloon-like structure with FDM, assuming the thickness
was that of the extruded material or greater and that there was no
significant overhang. There would be no support material inside and it
would be fully enclosed. The software should therefore be capable enough to
understand that.


At 06:14 AM 11/9/2003, you wrote:
>Why bother creating a pattern that has an enclosed void? Think about it
>for a moment. Until that void has a path to the outside, the object should
>be considered a solid anyway. Here's my point:
> Take a balloon and inflate it. Tie off the neck so no air
> escapes. We all know that the balloon is hollow. It's filled with air,
> and the actual structure of the balloon is a skin that is stretched
> around this air.
> Truth is, that balloon is a solid object. Philosophically
> speaking, of course.
> Once you tied off the neck, creating a border between the outside
> air and the air trapped inside, the inside air became part of the
> structure of that balloon. If you took a cross-section of this new solid
> object, it would have a surface of rubber a few mils thick surrounding a
> core of pressurized air.
> Now take that balloon, attach it to the bottom of a bucket and
> pour in plaster of paris. After the plaster sets, pop the balloon. Then
> remove the rubber from the cavity you just created.
> Next, fill the cavity with molten metal. After the metal cools,
> break away the plaster and study the resulting object.
> It's a very heavy, solid ball of metal. Did you really expect it
> to be a thin shelled, hollow ball of metal?
> The point is, if that void has no exit to the outside of the
> object, then as far as the software is concerned, this is a solid object,
> and it won't bother with building the object as such. It may be deleting
> the void as a modeling error.
> When you cut the quarter-section out of the model and it builds
> properly, I'll bet you are penetrating to the void, giving it a
> connection to the outside of the object. Therefore, it is no longer an
> irrelevant void, so the software retains it as part of the model.
> If your intent was to build the object and then drill into the
> void or cut it in half, why didn't you simply design the model with the
> drill hole (or at least a pilot hole) already there, or already in two pieces?
> Assuming that there is a very valid reason for having a fully
> contained void in the model -- how are you going to clean out the support
> material/structures from inside this void? The object may very well end
> up being a solid, anyway.
> - Bill
> Stress is when you wake up screaming and then you realize
> you haven't fallen asleep yet.
>On Nov 7, 2003, at 10:52 AM, Jonathan Chertok wrote:
>>I have a shape that has an enclosed void, which seems to want to print as
>>a solid. If I take a quarter section out of the model it prints fine,
>>however as a fully enclosed volume it prints as a solid.
>>I am wondering if Magics Communicator Lite has a "direction" command that
>>would show the normal directions for its X, Y and Z sections - as the
>>model appears to be formed appropriately.
>>I also have the demo version of STL Editor that works great and would be
>>interested in knowing if there is a better way to inspect this model with
>>STL Editor as well.
>>Rhino seems to show the model with the void, but does not show any
>>interior normal directions.
>>Is there something tricky about this process that I should know?
>>Universal Joint: Buildings + Digital Design
>>Jonathan Chertok. Principal
>>Austin, Texas [512] 407 9628
>>Full Service Design and Construction
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Dr. Ian Gibson

Currently on study leave at
National University of Singapore,
Dept. Mechanical Engineering
9 Engineering Drive 1
Singapore 117576
Tel: +65 6874 1917
Mob: +65 9087 3512

"Everything really is stupidly simple, and yet all around is utter confusion,
don't look around to find the sound that's right beneath your feet"

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