Additive Manufacturing 101: Part II

By Terry Wohlers, President, Wohlers Associates

The "Wohlers" column is authored by Terry Wohlers for Time Compression.
This column was published in the March/April 2010 issue.

[Editor’s note: This is a continuation of a series started in the last issue of Time Compression that provides information on the various types of additive manufacturing processes.]

Laser sintering (LS) is a process that has grown in popularity for making strong plastic parts. Major players in the North American market for LS are machines from EOS (eos.info) and 3D Systems (3Dsystems.com). Aspect Inc. (Japan), Beijing Longyuan Automated Fabrication Systems Co., Ltd. (China), Trump Precision Machinery Co., Ltd. (China), and Wuhan Binhu Mechanical & Electrical Co., Ltd. (China) also offer LS systems, but they have had little commercial impact and are not available in the U.S. or Europe.

LS produces objects from powdered materials using one or more lasers to selectively fuse the particles at the surface of a powder bed, layer by layer. “LS” typically refers to the processing of polymers. It uses liquid-phase sintering that involves the melting of the plastic particles. The word “sintering” is a historical term in the industry and a misnomer, as the process typically involves full or partial melting. Some LS systems can process metals and other materials such as sand for making sand molds and cores. I will cover LS metals and applications when covering the metal-based AM systems in a future column.

Complete melting of an LS thermoplastic results in a nearly fully dense part, while partial melting results in a part with some porosity. This depends on both the processing conditions that the user controls, as well as the polymer being processed. While plastic parts from LS can exhibit some porosity, this is usually not a problem for most applications. If sealing the surface is necessary, common methods are available to accomplish this.

The non-sintered powder that surrounds the parts during the build process serve as support material, holding the parts and overhanging features of a part while each layer is being produced. When the build is finished, the parts are fully embedded in the powder bed. The powder must be removed completely, but not until the powder has cooled, which can take hours. Large parts can take up to 40 hours just to cool. Removing the parts prematurely can cause them to warp, so a gradual cool down is important. Tools and subsystems are available to ease the process of removing and containing the loose powder when removing the parts, although much of the effort is manual.

The most popular class of thermoplastic for LS systems is polyamide. Parts made in this material are strong and can include thin walls and fine features. Typical applications are for fit and function prototypes. Companies in aerospace, automotive, and consumer electronics use LS to prototype housings and parts that go into complex, functional assemblies.

LS parts can be coated and finished to a glossy surface, but it can require a lot of work to achieve it. Photopolymer-based AM processes generally work better for producing a polished and painted surface. The same is true for using LS parts as patterns for silicone rubber molds. It is possible, but parts from systems that produce parts in photopolymer are best suited for these types of patterns. They are relatively easy to sand, polish, and coat.

A growing number of companies are using LS for actual part production. When building multiple parts simultaneously, throughput can be very good compared to some other AM processes. It’s not as fast as injection molding, but you don’t have to wait for the mold to be produced or an injection molding company to produce the parts. With LS, manufacturing can begin as soon as the CAD data is ready. This means that hundreds of plastic parts can be manufactured before tooling is available. Consequently, LS can serve as “bridge” manufacturing while tooling is being produced. If production runs are relatively small, LS can serve as the sole method of manufacturing, which eliminates tooling all together. And, the parts can be highly complex—shapes that you could not otherwise make in one piece using conventional methods of manufacturing.

Material cost is a consideration because LS materials, like most other AM materials, cost much more than injection molding materials. If production runs are low, LS can still be more advantageous given that there is no need to produce tooling and because production can commence much more rapidly. And if a design change is necessary after manufacturing has begun, the change is relatively painless because you don’t have to rework tooling. Even with all of these benefits, LS material cost is still a consideration, especially with large parts that consume a lot of material.

It is possible to recycle plastic LS powders, although not 100% when running polyamide. Between each build, it is necessary to mix in pure virgin material with the used material. If you do not, the part quality declines. The amount can vary from a low of about 30% new material to not more than about 50% new. About 30-35% is typical for non-filled polyamide. This means you are sacrificing this amount of the loose powder between each build. At $130 per kilogram (2.2 lbs.), this can add up to a significant amount of money.

Most plastic-based LS systems sold in the U.S. and Europe are priced from about $225,000 to $1-million. The base price of an LS system from EOS that is capable of producing parts in PEEK is about $1.3 million.

Although systems from Solidscape (solid-scape.com) use thermoplastics, the machines and materials are entirely different from laser sintering. They employ inkjet printing technology to jet a thermoplastic-based wax for the part being built. They also deposit a soluble support material that dissolves when submerged in a special liquid solution. The systems use a single inkjet nozzle for the build material and a second nozzle for the support material. The resolution and surface finish is excellent, and arguably the best available among AM systems, but the process is slow. Consequently, most organizations use them for making small, intricate parts.

The most popular application for the Solidscape systems is making wax patterns for investment castings. The vast majority of the systems are used in the jewelry industry to manufacture rings and other types of fine jewelry. The systems are being used increasingly for dental, medical, and special niche applications. Solidscape typically does not compete with fused deposition modeling (FDM) or LS systems, although it does compete some with PolyJet from Objet (objet.com) and high-resolution stereolithography. Both of these systems use photopolymer. The systems from Solidscape range in price from about $35,000 to $55,000, making them much less expensive than LS systems.

Solido (solido3d.com) offers a system that uses polyvinyl chloride (PVC), a thermoplastic, for its build and support material. The PVC is fed into the build area of the machine from a continuous spool of sheet material. An X-Y plotter system operates over the build area and cuts the periphery of each cross section of the part being produced. Each layer of plastic is bonded to the previous layer using a special “glue,” in combination with an “anti-glue” to prevent bonding in areas of the layer where they should not be joined.

The sheet material that surrounds the parts being built serves as the support material. After the build is complete, this material is removed manually. Solido has streamlined the process to make it easier than it otherwise would be, but it can still take some time and effort, depending on the features and size of the part. The amount of waste can be considerable, although Solido accepts it back for recycling and covers the cost of shipping.

Most of the parts produced by the machine are in white or amber-tinted clear material. The clear material allows for semi-transparent parts in the X-Y build direction. Relatively thin walls and fine features are possible to produce with the latest generation of the machine. The system is being used to produce a wide range of models and prototypes for consumer products and consumer electronics. It is priced from $2,950 to $9,950 in the U.S., depending on which purchasing package the customer chooses. Note that the $2,950 system price requires the purchase of supplies that brings the total to $14,950.

This concludes the primary AM systems that used thermoplastic materials. Part III will discuss the systems that build parts in photopolymer. TC

[Author’s note: Thanks to Tim Gornet, Brent Stucker, and Jim Williams for their input.]