Hi Steve, Well, before I delve into the world of RP, let me first say
that it has been my experience that people who use CNC to make models
tend to make one model of a design and then have a standard rubber
type mold made from the model (correct me if I am wrong here guys).
The Rubber molds are used to reproduce the design in wax. I don’t
know of anyone that would make say 100 CNC parts of the same thing in
wax just because they needed 100 castings. Normally, due to the time
required to Mill a CNC part, it is not economical, and there is much
more waste doing it that way than shooting wax into a mold.
Ok, onto the RP (Rapid Prototyping) World:
For my 9-5 job, I work as a mild mannered Design and Project
Engineer for the cosmetics packaging world. In this job I have had
to have many of my virtual 3D models turned into RP parts. Most of
the RP methods do not have a good enough surface finish to be used in
jewelry, and there would be substantial cleanup after the fact.
However, STL are very capable of producing almost any shape, even
complicated undercuts and such. Another downfall of RP is cost. The
last job I did was using the STL process. I was making a new type of
roll-on applicator for the makeup word. The model was to be nothing
fancy, just the outside shape that was about the size of a small egg.
Cost for a normal STL was $175ea, the cost for a high definition STL
was $265. To tell the truth, I’ll be damned if I could tell the
difference between them when they arrived. I guess the detail was
very slightly better on the more expensive one. This is an ok price
for the cosmetics company to pay because they stand to make millions
of $ in sales on the finished item. However, this is too much for me
to prototype a new ring if I am not assured of an end profit.
There are several methods that are used. Parts done by the “STL”
(Stereo Lithography) process are probably the most common. These
parts are made of a medium hard plastic material. The STL parts are
not that great when it comes to detail (by jeweler standards). They
have a hard time with fine lines and small letters/details. With
any RP process, the bigger your detail, the better your end results.
Also, the dimensions of the finished piece are only good to within
B1.010" in most cases, thought they claim better. Let me quote from
one supplier’s website: “Stereo lithography (SLA) is often considered
the pioneer of the Rapid Prototyping industry with the first
commercial system introduced in 1988 by 3D Systems. The system
consists of an Ultra-Violet Laser, a vat of photo-curable liquid
resin, and a controlling system. A platform is lowered into the resin
(via an elevator system), such that the surface of the platform is a
layer-thickness below the surface of the resin. The laser beam then
traces the boundaries and fills in a two-dimensional cross section of
the model, solidifying the resin wherever it touches. Once a layer is
complete, the platform descends a layer thickness, resin flows over
the first layer, and the next layer is built. This process continues
until the model is complete. Once the model is complete, the platform
rises out of the vat and the excess resin is drained. The model is
then removed from the platform, washed of excess resin, and then
placed in a UV oven for a final curing. The model is then finished by
smoothing the “stair-steps.””
Next comes FDM (Fused Deposition Modeling)
Forget it for our industry. These things are made from some rigid
plastic and they look really terrible, hold no detail, and don’t cost
much less than the STLs. Not to mention that the plastic they are
made from is horrendous to do anything with. Think about what it
would be like to try to make a detailed model from Styrofoam and you
might get the idea.
"Stratasys Inc. introduced fused Deposition Modeling (FDM) in 1990.
FDM is a solid-based rapid prototyping method that extrudes
materials, layer-by-layer, to build a model. The system consists of a
build platform, extrusion nozzle, and control system. The build
material, production quality thermoplastics, is melted and then
extruded through a specially designed head onto a platform to create
a two-dimensional cross section of the model. The cross section
quickly solidifies, and the platform descends where the next layer is
extruded upon the previous. This continues until the model is
complete, where it is then removed from the build chamber and cleaned
for shipping. "
Last for tonight is SLS (Selective Laser Sintering):
When made in plastic, this in on par with STL, though I think it can
get some finer details. However, there is at least one supplier
that I have run across that uses a Metal Version of this process
which looked quite feasible for our industry. The metal version
seemed to be able to hold almost perfect details and should be fine
for duplicating an original. Problem is again cost. I had asked for
a quote on the metal process for two 1" diameter pendents. Nothing
spectacular, just something like a recessed Yin-Yang symbol. The
quote was $345 for the pair + s&h. This is not a bargain in my
opinion. Perhaps if I was going to make and sell 100s of them, but
for the little guy this is just not practical
"DTM Corporation introduced Selective Laser Sintering (SLS) to the
commercial world in 1992. SLS uses a laser to sinter powder based
materials together, layer-by-layer, to form a solid model. The system
consists of a laser, part chamber, and control system. The part
chamber consists of a build platform, powder cartridge, and leveling
roller. A thin layer of build material is spread across the platform
where the laser traces a two-dimensional cross section of the part
sintering the material together. The platform then descends a layer
thickness and the leveling roller pushes material from the powder
cartridge across the build platform, where the next cross section is
sintered to the previous. This continues until the part is completed.
Once the model is complete, it is removed from the part chamber and
finished by removing any loose material and smoothing the surfaces.
Now, I belive that all the plastic materials above can hold a heat up
to about 280B0F before they melt, which is good enough to shoot wax
into, if you were to RP a mold with these processes; However, as I
have pointed out already, since the detail of a 3D model in these
materials is not great, why would you even want to consider making a
model mold with this process. Also, if a part costs this much $,
imagine how much more a mold for the part would cost. Perhaps if I
was going into full production and needed a metal mold I would
consider metal SLS; but I think rather I would have the Metal Part
made from the SLS and then send that out for silicon molds.
The nicest things that I have found about the RP parts are that:
They are very quick to get done
They tend to be less $ than having a regular machine shop do it
They can be of better quality than CNC parts, but not always.
They can handle very complicated shapes that might be impossible
or impractical to do via machining.
Did I mention that they were quick? Some places can turn them
out overnight for you.
Now that I have expelled my wind for tonight and have completely
the question that started this all. I hope I have answered at least a
little of what was needed.