CAD: Methods for Model Building

Engineerblogger
May 10, 2012

Credit: Conduit

Prototyping systems can move a design concept from CAD data to a 3D model rapidly and accurately when using the appropriate tool for the task.

Rapid prototyping, the construction of physical objects using additive manufacturing technologies, entered the R&D field in the late 1980s. The process typically starts with a 3D design generated from CAD software. A machine reads and translates the data and deposits layers of liquid, powder, or metal, building a model in a series of cross sections that correspond to a CAD drawing. The cross sections are fused to create a prototype, allowing developers to physically view and interact with the product design.

In comparison to other manufacturing methods such as injection molding, additive manufacturing systems can typically produce models from various materials in a few hours, rather than a few days, depending upon the size of the prototype.

Competing technologies—stereolithography, laser sintering, fused deposition modeling, 3D printing, laminated object manufacturing, and electron beam melting—offer options in the types of materials used and the method of building layers.

Building with light

Stereolithography, which is based on photosensitive chemistry, was developed in 1986 by Chuck Hull of 3D Systems Corp., Valencia, Calif. Commonly known as SLA, the process “utilizes a UV laser and scanning technology to accurately cure layers of liquid UV-curable photopolymer resin,” says Greg Elfering, director of sales, 3Dphotopart/3D Systems. Guided by a design generated in 3D CAD software, a UV laser beam traces a layer of the cross-section pattern on the surface of liquid resin. The exposure to the laser light solidifies the pattern traced on the resin and adheres a new layer to the layer below. The process continues until the prototype is complete. Excess resins and any support structures are removed to finish the prototype.

Currently, there are 10 different types of SLA machines on the market suited for use in lab and office environments. As the first additive method, stereolithography "has the most mature range of materials and applications," says Elfering. The materials used allow for SLA prototypes to mimic mainstream engineering-grade thermoplastics.

And although there are many benefits associated with SLA, the technology can be expensive. Prices for stereolithography machines run from $100,000 to more than $500,000 and the photo-curable resin material can cost $300 to $800 per gallon. Researchers who want to test their products in prototype format but can’t afford the capital and operating costs can turn to additive manufacturing services providers and get prototypes made to order.

Building with heat

Selective laser sintering (SLS)—also known as plastic laser sintering—was developed in 1989 at the University of Texas by Carl Decker under the sponsorship of the Defense Advanced Research Projects Agency (DARPA) and commercialized by DTM Corp. (now 3D Systems Corp).

In parallel, Hans Langer, the founder and CEO of EOS, Munich, Germany, developed a laser sintering technology in Europe under a R&D budget that was supplied by BMW, says Andy Snow, regional sales director, EOS North America, Novi, Mich.

The SLS process uses the heat of a high-power (typically carbon dioxide) laser to sinter or melt powdered thermoplastic materials in layers and can produce parts with tolerances and detail similar to SLA, but with an added advantage of strength. While SLA resin materials have proven stronger now than in the past, they lack the durability and stability of thermoplastics, which do not lose shape, post-cure, or become brittle over time, according to Snow.

SLS has become the leading process for those in the aerospace and medical industries because the technology offers "freedom of creation, where they are able to make geometries that aren't necessarily made on other traditional methods, such as CNC machining, casting, injection molding, and so on," says Snow.

Direct metal laser sintering (DMLS) is a related prototyping technique also developed by EOS. Instead of using thermoplastic material, the technology relies on metal powder, such as titanium, Inconel, aluminum, and cobalt chrome.

DMLS uses a high-powered Yb-fiber optic laser to fuse metal powder into a solid part by melting. Parts are built up additively layer by layer—typically in 20 µm layers. Highly complex geometries can be created directly from 3D CAD data in hours, without any tooling, says Snow.

DMLS offers benefits over other conventional manufacturing techniques and SLA, including: speed, since no tooling is required and parts are built much faster compared to conventional manufacturing; more rigorous testing of DMLS parts for manufacturing applications; more design freedom; more efficient designs in technical applications; and, the use of many alloys.

Fusing layers together

Fused deposition modeling (FDM) technology was developed by S. Scott Crump, current CEO of Stratasys, Inc., Eden Prairie, Minn., in the late 1980s and was commercialized in 1991. FDM uses proprietary Stratasys software, which processes an STL or stereolithography CAD file, "mathematically slicing and orienting the model for the build process," says Joe Heimenez, technical communications and public relations manager, Stratasys.

If required, support structures are automatically generated. The system dispenses two materials, one for the model and one for a disposable support structure. The system follows a tool-path defined by the CAD file, and liquefies and deposits thermoplastics through an extrusion head in 0.005 in layers from the bottom up.

Binding force of 3D printing

Another popular technique, 3D printing, was developed at Massachusetts Institute of Technology in 1993. Z Corp., Burlington, Mass., "launched in 1995, licensed the technology and packaged it into the first 3D printer—the Z402—in 1996," says Joe Titlow, vice president for products at Z Corp. The company developed three generations of ZPrinters, including five different models from the ZPrinter 150 to the ZPrinter 650.

Known for its speed and its affordability, 3D printing is also "the only technique that allows models to be constructed in multi-color," says Titlow.

The process works by selectively binding powder particles together, layer by layer, using a high-resolution inkjet printhead. A thin layer of powder is spread across the build area, creating layers. A printhead flies over the powder, depositing droplets of binder where solid parts will be located. This binder binds the powder particles together.

While the technology is the most affordable, the prototypes produced do not have the quality and detail that come with laser sintering.

Other techniques

Laminated object manufacturing (LOM), developed by Helisys Inc.—now Cubic Technologies, Torrance, Calif.—uses layers of adhesive-coated paper, plastic, or metal laminates that are successively glued together and cut to shape with a knife or laser cutter.

Electron beam melting (EBM) technology, developed by Arcam AB, Sweden, manufactures parts by melting powder layer by layer with an electron beam in a high vacuum. Unlike DLMS, the parts developed are fully dense, void-free, and relatively strong.

Source: R& D Magazine