SLA: Bad surface finish on flat surface with holes

From: Stahlhut, Todd A. (
Date: Tue Sep 09 1997 - 17:46:00 EEST

Please excuse my interruption with a RP related topic....

I thought I would ask the SLAers out there about their thoughts about a
problem I see on our SLA 250/50. When we are building a large flat with
 holes, or raised bosses or text - we get a bad surface finish on the
top surface. You notice raised lines in the surface even with the top
or bottom edges of the protrusions and holes. It is very noticeable and
unsightly on equipment panels and covers. My suspicions is that it is
due to the drawing method on the SLA 250. Imagine a flat plate with
two holes in the middle.

                  | |
                  | |
                  | _ _ _ _ _ _ _ _ |
                  | * * |
                  | * * * * |
                  | * ------- * -------- |
                  | |
                  | |

As the laser draws it moves from the left edge to the right edge of the
part starting at the bottom. As it reaches the bottom tangency point of
the first hole, it draws to the top tangency point from the left part
edge to the right edge of that hole. It then skips to drawing inbetween
the two holes, then from the right edge of the right hole to the right
edge of the part. After that it starts drawing again from left part
edge to right part edge. This is obviously simplified due to the change
in draw directions and start points to avoid shrinkage - but hopefully
you get my point. Where I notice these raised edges is along areas
where it stops drawing one section by the holes over to the next hole.
In my example this would be along the top and bottom horizontal tangency
lines to the holes The dashed lines illustrate where I see the effect.
What I think causes this is the post curing that occurs while it skips
to draw the next section between the holes. Being a home handyman, I
kind of liken this to keeping a wet edge when you paint - if you don't -
you see a darker edge.

We are using the epoxy resin which seems to make this more noticeable
than the older acrylates. Does anyone else see this and more
importantly - how can you avoid it or at least minimize it? It seems
like if 3D would change the drawing method it might help. If rather
than drawing between each hole/protrusion you just skipped across the
open areas, completing a straight across scan - it would minimize the
cure time between subsequent passes. Many of my customers are after a
asthetically pleasing look, and this effect definitely detracts from
mockups of front panels.

Your thoughts ????

Regards, Todd Stahlhut

This archive was generated by hypermail 2.1.2 : Tue Jun 05 2001 - 22:40:23 EEST