An Overview of Additive Manufacturing Considerations

Speed equates to cost, so faster prototyping and production mean lower costs and faster time to market. Although this statement seems to be fairly easy, there are some considerations to make when thinking of additive manufacturing, also called 3D printing, where a three-dimensional object is created by laying down successive layers of materials from a digital model. Rapid prototyping has become an all-encompassing term for the use of a class of technologies to construct physical models using computer-aided design (CAD) data. This is exactly why medical device companies are exploring the use of a variety of types of additive manufacturing to help cut prototyping and production costs. Here’s a look at some of the different additive processes available, their materials, applications, and some design rules to keep in mind when designing for manufacturability.

Processes

There are several additive processes all of which equate to certain needs, materials, and geometries, and all built directly from 3D CAD data.

The most recent technology is Direct Metal Laser Sintering (DMLS), an additive metal fabrication technology that fuses metal powder into a solid part by melting it using the focused laser beam. It can be used to build fully dense alloys within a matter of a few days. (See Figure 1)

After that, we have Stereolithography (SLA) and, although this is one of the first rapid prototyping processes available, it can not be used to build production grade materials. SLA requires the use of supporting structures that must be removed from the finished product manually. Although SLA can produce a wide variety of shapes, it is often expensive.

Another sintering process is Selective Laser Sintering (SLS) uses a high power usually pulsed laser to fuse small particles of plastic, metal, ceramic, or glass powders into a mass that has a desired three-dimensional shape. This technique can be used to build parts in Nylon-based materials that are production grade.

One of the most commonly used technologies is Stratasys’ Fused Deposition Modeling™ (FDM), which can be used to build production grade applications using acrylonitrile butadiene styrene (ABS) or polycarbonate (PC) materials. The technology is also known as fused filament fabrication. FDM is making a huge name for itself in the “do it yourself” market where retail consumers are purchasing small home-based systems for less than $1,500. Both FDM and SLS use melting or softening material to produce the layers, while SLA lays down liquid materials that are cured with different technologies.

Lastly, we have PolyJet™, a new photopolymer 3D printing process from Objet Ltd., that uses simultaneous jetting of multiple types of modeling materials to create a single piece 3D model. What is amazing about this process is the fact you can build multiple durometers on a single part thus mimicking overmold components. PolyJet technology allows the user to choose from more than 100 different build materials and can simultaneously build 14 different materials into a single model part.

Materials

Each additive manufacturing process uses its own materials, some of which can include hundreds of options. Rather than including every single material, this compressed list describes the most commonly used materials for each process.

DMLS: PH1 Stainless Steel 15-5 (high corrosion resistance, sterilizability, hardness and strength), GP1 Stainless Steel 17-4 (high corrosion resistance, sterilizability, high toughness and ductility), MP1 Cobalt Chrome (high mechanical properties in elevated temperatures and with good corrosion resistance), Ti64 (combination of high mechanical properties and low specific weight), IN718 (chemical resistance with an elevated temperature) and MS1 Tooling Steel (heavy duty injection molds and inserts for molding all standard thermoplastics using standard injection parameters, with achievable tool life of millions of parts).

SLA: Accura 25 (polypro-like), Accura 60 (polycarb-like), Somos 11122 (ABS-like and can be transparent) and Somos NeXt (durable with high feature resolution).

SLS: Nylon 12 (durable and flexible), GF Nylon (durable and rigid) and Duraform Flex (rubber like material).

FDM: ABS-M30i (biocompatible ABS (ISO 10993 USP Class VI certified) material), PC (most widely used industrial thermoplastic), PC-ABS (superior mechanical properties) and Ultem 9085 (FST, high heat, chemical resistance, highest tensile and flexural strength)

PolyJet: Vero White (rigid and white), Vero Black (rigid and black), Tango Plus (flexible 27a and amber in color) and Tango Black (flexible 61a and black in color). Objet Connex printers can also produce multiple durometers depending on the geometry and application.

Applications

Traditionally additive manufacturing processes have been used for rapid prototyping and product design stages within an OEM or design firm. However, users can now take additive manufacturing to the next level. Materials and processes lend themselves well to the orthopedic industry for a wide array of applications. The hottest trend is DMLS, where total knee replacement (TKR) surgeries can use femoral trials in a variety of sizes for both left and right side. Another hot application is patient-specific CT scan data custom drilling or cutting guides using the SLS process. (See Figure 2)

OEMs are now moving toward DMLS for their trials in the TKR because this not only saves money for the customer, but also saves a tremendous amount of time. Typical production for a set of 12 trials in the DMLS process can take about 10 days, whereas, traditionally it can take upwards of 12 weeks. The materials that are used in the DMLS process for TKR are strong enough and meet ASTM standards to be used within the surgical environment. (See Figure 3)

CT Scan data used to produce SLS nylon drill guides and cutting blocks have been the most talked about application for additive manufacturing in 2012. The reason is because of the speed to market, cost, and the process lends itself well to being effective for a disposable product.

There are now design firms using SLS nylon parts to create fully prosthetic limbs that are lightweight and extremely strong.

Additive manufacturing allows for part consolidation by combining multiple components into a single assembly. Processes like SLS and PolyJet lend themselves well to this application. Highly geometric designs make it easier for additive manufacturing to become another route or alternative to manufacturing. Now users can produce parts without the need for high tooling costs or super long lead times.

Design Rules

When do users know when to design a part for traditional manufacturing or for additive manufacturing? This will all depend on the material, quantity, and the design of the component. For instance, if it’s a part that requires hundreds or thousands of parts, most likely additive manufacturing is not the best route. If, on the other hand, the part is designed so that it cannot be manufactured with traditional methods, then maybe additive manufacturing is the best way to go. There are also parts that can’t even be built using additive manufacturing. Let’s examine some of those design ‘rules’.

First, if a part has a lot of undercuts, and the quantity is fairly small, then most likely it cannot be made using traditional techniques. However, will those undercuts work well with the additive process?

Some processes require support structures so this will, of course, depend on the material that is being used for the component. For instance, if the part has an undercut that you cannot reach to remove the support structure, then this part will not lend itself well to the DMLS process but might work well with SLS or PolyJet if the material suits itself to the project.

Design rules pretty much run the full gambit when it comes to the following: wall thickness, feature resolution, channels, and knife edges. Typically, designers want to keep wall thickness greater than .020" and in some cases closer to the .040" range. Each additive manufacturing system has its own limitations, so designers should work closely with their service bureau project manager in order to design the part correctly. Users can also check for internal voids where support material may be trapped. They may need to add a hole in the part to drain away some material. Be sure to save the file in the correct dimensions and let the service bureau know what size the part is so there is no confusion. In addition, always save the file at the highest resolution possible. Anything in that 3D CAD file will show up in the build.

A common question for additive manufacturing is, what are the tolerances? Typical tolerances will be .005" for the first inch and .002" inch per inch, thereafter. With these tolerances in mind, it is best to design the part with a clearance between parts. We like to see around .008" to .020" of clearance.

Conclusion

Additive manufacturing is taking a turn for the best with today’s economy. It has been said that the industry will reach $5 billion dollars by 2015. Service bureaus have been seeing increases in business, which means that firms are now innovating and working towards new products that will soon hit the market. Rapid prototyping has been around for more than 30 years, however, parts can now be created and received in just a matter of days, which allows faster design to market.

It’s exciting to see where the industry is heading. GPI as a company has made huge advancements in additive manufacturing and has hired more than 15 people in the past three years. Keeping manufacturing in the US is a company priority.

This article was written by Tim Ruffner, Vice President of New Business Development/Marketing, GPI Prototype & Manufacturing Services, Inc., Lake Bluff, IL. For more information, Click Here 

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