Gus Breiland, Customer Service Engineer
Manager, Proto Labs
CNC machining and injection molding have both been around for a long time, but until relatively recently, neither was a viable option for prototyping of
plastic parts. An injection mold could crank
out parts by the thousand, but setting up
to produce that first part could take weeks
or months and cost tens of thousands of
dollars. Similarly, CNC machining could
produce the same part over and over, but
not until the toolpaths had been created,
and those too took lots of time and
manpower. The setup costs for either
could be amortized if the number of
parts was large enough, but prototyping
is all about small numbers of parts produced quickly and inexpensively, to be
examined briefly, and then set aside.
Until relatively recently, the only way
to make prototypes was working by
hand from prints. Unfortunately, it was
a laborious and occasionally error-prone,
but necessary, process.
3D Printed, CNC Machined,
or Molded Prototypes:
Which and When?
When it comes to component fabrication for production ready parts, designers typically have a good idea
which process they’d like to specify. However, when it comes to prototyping, they may not be as clear on the
best process for their needs. This article looks at three common fabrication processes—3D printing, machining,
and molding—and clarifies which to use when for prototype parts.
The first practical technology for automated prototyping of plastic parts was
3D printing, an additive method that was
invented in the 1980s and commercialized
in the ‘90s. 3D printing was the child of
Autodesk CAD software and the computer
printer. Autodesk and other CAD packages,
first in two dimensions and then in three,
allowed designers to create, in the “mind”
of a computer, a virtual model—fully defining a solid object. The printer, meanwhile,
could lay down a two-dimensional image
that came from that same electronic brain.
The replacement of ink with either a liquid
that could be solidified or a fusible powder, and the stacking of “two dimensional”
layers upon one another, was a logical,
if technologically challenging, next step.
Suddenly, designers could create a 3D CAD
model at the desktop and have a 3D part
in hand in hours or days. The part, at least
in overall form, duplicated the CAD model.
Human error in translation from plan to
part was eliminated and designers no longer had to try to imagine how a paper design would look and feel in physical form.
3D printing quickly became the method of
choice for plastic prototyping.
3D printing technologies have continued
to grow in scope and capabilities, but they
have remained limited in both the range of
materials they can use and the structural
strength of their products. Machining, on
the other hand, being a subtractive process,
has long been able to produce solid objects
in any of hundreds of materials (Figure 1).
Realizing machining’s potential as a practical prototyping method, however, was
Machining did have one advantage.
Unlike 3D printing, which required new
production technologies, CNC machining
already had the equipment in place. The
challenge was developing software for
converting CAD models to toolpaths. 3D
printing’s process of slicing a solid into layers was relatively straightforward. Completely automated conversion of CAD models to
machine-tool motions in three axes, along
with automatic fixture generation, was more
complex, and the goal was not reached until
2007. Now that the software exists, CNC
Figure 1: FDM or fused deposition modeling
part sample—FDM process builds parts from
the bottom up through the use of a computer
controlled print head.
The first practical
technology for automated
prototyping of plastic parts
was 3D printing...