RE: Void Normal Direction

From: Ian Gibson (
Date: Fri Nov 14 2003 - 03:44:15 EET


I was wondering whether you or anyone else can verify (or has already done
so) this through a simple experiment/test?

I'm not quite so convinced that the problem comes from the CAD. I was
thinking that maybe it could also be a bug in the support generation
software. Or indeed it could be deliberate to avoid weird postcure effects
when the part is removed and placed in the oven.

I don't have an SLA machine else I would try it myself.


At 04:33 AM 11/14/2003, you wrote:
>Yes, you can build a cube of Swiss cheese with an SLA machine. The totally
>enclosed "Bubbles" within the block would build as a enclosed volume of
>liquid resin.
>I believe the problem you are experiencing stems from your modeling software
>not the RP method.
>When you attempt to model a theoretical construct that contains only a
>single twisted surface, you are creating the problem. Single surface objects
>don't exist in nature unless they are a solid shape like a sphere. Even a
>soap bubble has a thickness, and therefore has an inside and an outside
>If the twisted surface that you are attempting to create wraps back onto
>itself, most solid modeling software, by default, will assume the trapped
>volume is a solid.
>When exporting the data for RP, the CAD modeler will follow the rules of STL
>file generation and export the STL surface data as a set of triangles, each
>with a single normal facing in only one direction.
>See if you can create your surface with a thickness of some small amount.
>The modeler can then export the STL data as a pair of surfaces "back to
>back" and may let you build the object as you envision it.
>Larry Blasch
>Design Engineer
>CAE Systems Administrator
>OPW Fueling Components
>P.O. Box 405003
>Cincinnati, OH 45240-5003 USA
>Voice: (513) 870-3356
>Fax: (513) 870-3275
>-----Original Message-----
>From: Jonathan Chertok []
>Sent: Thursday, November 13, 2003 11:35 AM
>Subject: RE: Void Normal Direction
>After a little thought on the responses to this question I am wondering the
>Could you build essentially a cube of swiss cheese using SLA RP for
>I understand that there is an issue with the fact that the voids should have
>no draft type angles in the downward build direction that would trap
>But would you need to put tiny holes through all the voids in the "cheese"
>in order to get it to build?
>I'm curious about this,
>Universal Joint: Buildings + Digital Design
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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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