You can already see one advantage of the additive over the subtractive process.
The additive process uses only the amount of material needed to achieve a
desired result. Subtractive methods discard material while creating an object. The
greater the cost of the material, the greater the advantage in minimizing its use.
The earliest use of additive manufacturing was in rapid prototyping [2]. Prototypes
serve as preliminary models of stuff (cars, planes, machines, parts thereof) which
allows the design to be viewed, tested, corrected, and later serve as a model for
the final product that is to come. Prototyping has been a part of engineering
design for a long time. Once computer-aided-design (CAD) software became
available, designers used it to create models. This digitized version of a model
was then sent to a machine which actually constructed the object.
Before the 1990’s the different processes of additive manufacturing went under
different names. By 1984, Chuck Hall had developed a process called
Stereolithography (SLA). The basic process works like this [3]:
• A CAD program is used to create a model of an object
• A piece of software (digital slicing)chops the CAD model into thin
layers, typically 5 – 10 layers/millimeter
• The three-dimensional printer contains a liquid photopolymer
(resin). An ultra-violet laser “paints” or “cures” one cross-section at
a time. On each layer, the laser beam traces a pattern.
• The printer platform drops down into the tank one layer thickness
(a fraction of a millimeter). More resin is introduced and, on this
new liquid surface, the laser retraces the pattern which now
adheres to the previous layer.
• The process repeats until the entire object is cured.
• At the end, the excess print material and any support structures
are removed. The finished object is cleaned of excess resin by
immersion in a chemical bath and then cured in an ultra-violet oven.
Stereolithography can print almost anything that can be designed in a CAD
program. The SLA process can produce objects with high detail and good
surface finish. On the down side, support structures are required to prevent certain
geometries from deflecting (or collapsing) due to gravity and to hold the 2D cross-
section in place to resist lateral pressure from the re-coater blade. The supports
are removed manually after printing. In addition, the machines need to be vented
because of the fumes from uncured materials can be toxic. Also, since both the
printers and the polymer resin are expensive, it is uncommon to see
stereolithography machines anywhere but in large companies.
Chuck Hall also developed digital slicing and the STL file format [4], all still in use