Kform just returned from the SME RAPID 2007 conference. The RAPID (2007) conference is the largest annual rapid manufacturing event in North America. A great deal of information is available in the form of lecture series, panel discussions, and product exhibitors from industry leaders. This presentation serves to debrief some of information obtained at the SME RAPID event to current and potential Kform clients. The different types of additive fabrication and rapid manufacturing technologies will be explained along with adjunctive processes and services, such as 3D scanning Metrology.

Additive Fabrication

Additive fabrication refers to a group of technologies used for building physical models, prototypes, tooling components, and finished production parts—all from 3D CAD data, medical scans, or data from 3D object scanning systems. Unlike machining processes, which are subtractive in nature, additive systems join together liquid, powder, or sheet materials to form parts. Shapes that may be difficult or impossible to manufacture by other methods can be produced by additive systems. Based on thin, horizontal cross sections taken from a 3D computer model, systems produce plastic, metal, ceramic, or composite parts, layer by layer.

Rapid Prototyping

There are a myriad of terms, Rapid Prototyping, Solid Freeform Fabrication, Rapid Manufacturing, Desktop Manufacturing, Direct Manufacturing, or Layered Manufacturing, etc, all essentially referring to the same group of technologies. Arguably, there are slight differences between rapid prototyping and additive fabrication. Rapid Prototyping (RP) refers to a manufacture directly from 3D data. Additive technologies would be considered rapid in this respect although not all rapid technologies are additive (several companies make subtractive, milling-like rapid machines, but the applicability is questionable). The term rapid prototyping is somewhat of a misnomer. The build time for a part may be 3 to 72 hours and the technologies are progressing away from concept to functional, “near net” parts. The RP market can be easily divided into several key segments along process lines: 3D printers, SLA, FDM, and SLS. Each process technology has a unique set of advantages and disadvantages making some more attractive in different markets.

3D Printers

3D printers refer to the low end of the RP market spectrum. There is significant growth in this area of the market. The product offerings in this segment have a distinct price advantage which has made them very attractive to startup companies and engineering groups in need of concept models. The products are, in general, very office friendly being clean, easy, and quiet. The end result product of this process is well-suited for concept modeling and engineering discussions. Several systems even have color printing capabilities allowing for a physical representation of FEA analysis. There are many drawbacks to the implementation of these systems. The material selections are extremely limited. The available materials result in very brittle parts. The dimension tolerance is not considerably accurate. Due to the small, office-oriented machine the build size is correlatively small.

SLA

Stereolithography is an additive fabrication process utilizing a vat of liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part cross-section pattern on the surface of the liquid resin. Exposure to the UV laser light cures, or, solidifies the pattern traced on the resin and adheres it to the layer below.

After a pattern has been traced, the SLA's elevator platform descends by a single layer thickness, typically 0.05 mm to 0.15 mm (0.002" to 0.006"). Then, a resin-filled blade sweeps across the part cross-section, recoating it with fresh material. On this new liquid surface the subsequent layer pattern is traced, adhering to the previous layer. A complete 3D part is formed by this process. After building, parts are cleaned of excess resin by immersion in a chemical bath and then cured in a UV oven.

Stereolithography requires the use of support structures to attach the part to the elevator platform and to prevent certain geometry from deflecting due to gravity. Supports are generated automatically during the preparation of 3-D CAD models for use on the stereolithography machine, although they may be manipulated manually. Supports must be removed from the finished product manually (can be by hand or using a waterworks dissolving process); this is not true for all rapid prototyping technologies. (SLA, 2007)

All layer-based manufacturing technologies have a stair-step effect that occurs along Z-axis curves due to layer thickness. SLA, in general, has the best surface finish in layered manufacturing. There are several major disadvantages to SLA. The material is not easily or cost-effectively swapped. The photo-reactive material is in the form of a very costly resin, which does not lend to change. The material properties of photopolymers degrade over time. Six months after a part is created with an SLA process, the part will be extremely brittle.

SLS

“Selective Laser Sintering (SLS, a registered trademark of 3D Systems, Inc.) is an additive rapid manufacturing technique that uses a high power laser (for example, a carbon dioxide laser) to fuse small particles of plastic, metal, or ceramic powders into a mass representing a desired 3-dimensional object. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (e.g. from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.

“Compared to other rapid manufacturing methods, SLS can produce parts from a relatively wide range of commercially available powder materials, including polymers (nylon, also glass-filled or with other fillers, and polystyrene), metals (steel, titanium, alloy mixtures, and composites) and green sand. The physical process can be full melting, partial melting, or liquid-phase sintering. And, depending on the material, up to 100% density can be achieved with material properties comparable to those from conventional manufacturing methods. In many cases large numbers of parts can be packed within the powder bed, allowing very high productivity.” (SLS, 2007)

SLS is seemly in direction of the mid to high-end market. The trend is toward normal, non-niche applications in the mainstream. As mentioned, there are a wide variety of materials available. The technology can produce functional, “near net” work (near net meaning finished; surface finishing and additional machining may still be required at this point). Parts can be stacked vertically using a sandwich-like technique in the build chamber, as the powdered material supports the Z-axis. Of the various RP types, SLS is the most cost effective in the high-end due to build capabilities, material cost/reuse, and application.

Fused Deposition Modeling

Fused Deposition Modeling is a rapid prototyping technology that utilizes a spooled plastic filament fed through a heated rod to an extruder nozzle to additively create parts. The key advantage to the FDM approach is the capablity to employ real, standard materials. Materials, such as polycarbonate (PC) and Acrylonitrile butadiene styrene (ABS), are used to form real end-use parts. Other rapid prototyping processes have materials only approaching the properties of traditional materials, such as PP-like, ABS-like, etc. The machines are fairly clean running and are well suited for office environments. FDM machines and materials are priced considerably lower than SLS and SLA. The market growth and success of this product is understandable when consnidering the applicability coupled with the lower cost.

3D Scanning and Metrology

3D scanning is a complex field of different product technologies allowing the capture of digital dimensional data for the use in CAD/CAM software. The application of scan data allows digital validation and QC of complex machined (or RP) parts. This process would be similar to the implementation of the Virtex LaserQC system Kform currently operates. A second application of 3D measurement technologies is reverse engineering. The combination of an accurate metrology system and rapid prototyping allows an organization to replicate almost any part without costly engineering or tooling design. Both applications require a product with versatility and accuracy.