RE: Void Normal Direction

From: Blasch, Larry (
Date: Thu Nov 13 2003 - 22:33:51 EET


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

Jonathan Chertok. Principal
Austin, Texas [512] 407 9628

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