Re: Raster is faster

From: M. Burns ( ((
Date: Mon Dec 04 1995 - 11:26:13 EET

Hi Folks,

     Sorry, I must disagree with Ulrich and Terry. Raster is not
necessarily faster. It depends on the geometry. And in three dimensions,
that dependence is more pronounced than in two, so the history of 2-D
plotters is not a good analog here. Some of the relative advantages of the
two scanning methods are:

     -- Raster: Easier and less expensive to adapt to multiple jets. This
makes it faster for shapes that cover wide swaths of space in each layer.

     -- Vector: Can skip between regions that need processing, without
processing all of the empty space in between (except for one tool motion
between the regions). This makes it faster for shapes with wide open
spaces in many layers. BPM takes advantage of this characteristic by
always making only the shell of an object, so that every shape has this
characteristic. This is kind-of cheating, and they pay by having dismally
fragile models.

     -- Vector: For deposition processes (such as BPM and FDM, but not
SLA, SLS, or LOM, which also do vector scanning), easier and less
expensive to adapt to aimed (instead of just downward) deposition. The BPM
fabricator is the only one that does this so far, with its two rotational
freedoms in the jets. The advantages of aimed deposition over downward are
(a) possibility of reducing dependence on support structures, which has a
whole set of further advantages, and (b) freedom, when the appropriate
software is developed, from today's constraint to building in flat layers.

     -- Vector: The resolution of fabrication is limited by the resolution
of the motion control systems, not by the resolution of the raster
pattern, so it can be better if the raster pattern is a constant of the
hardware, as it will usually be for a multi-jet head.

     I wouldn't write off vector deposition processes, like FDM and BPM,
just yet! Counterarguments welcome.

     (This is an independent technical opinion. Ennex Fabrication is not
engaged by either Stratasys or BPM. Copyright (c) 1995, Ennex Corp.)

Best regards,
Marshall Burns

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