8.3 The Business Case: Definitions and Considerations

AM Cost Model

Although the business case considerations are much the same as any traditional methods, calculating the recurring costs, more specifically, the part costs of an AM part are specific to the AM process and material. To determine the AM part cost, a cost model can be applied. The AM cost model is specific to each AM process and generally is made up of printing costs, material costs, and post-processing costs. A simplified cost model created by Dr. Tim Simpson of Penn State is shown below. This cost model describes the Metal Laser Powder Bed Fusion process for a single laser, but the same type of thought process can be applied to any AM process to generate an equivalent cost model.

User-defined inputs:

1. Material (see Table 8.1)
2. Part Volume (mm³), including supports
3. Max part height (mm)
4. Machine operating cost ($10-30/hr)
5. Pre-Processing/Post-Processing Cost (30%-40%)

Outputs:

$\text{Material Costs (\$)}=\text{Part Volume}\cdot \text{Material Density}\cdot \text{Powder Cost}$

$\text{Build Time (hr)}=\frac{\left(\right(\text{Part Volume})/(\text{Build Rate}\cdot \mathrm{80\%}\left)\right)+\left(\right(\text{Max Part Height})/\text{Layer Height})\cdot \text{(Recoat Time)}}{3600}$

$\text{Total AM Part Cost (\$)}=\frac{\text{(Build Time)}\cdot \text{(Machine Operating Cost)}+\text{Material Cost}}{(1–\text{Pre-Processing/Post-Processing Cost \%})}$

Starting with material, you must consider that the metallic powder feedstock is 5-10x more expensive per kilogram than the equivalent bar stock of the same material. If you were to consider the cost of material alone, you would have to reduce material by 80-90% to have a good business case for AM with a part-for-part substitution. This is achievable only in rare cases where you have significant processing and associated material waste from the original stock material.

Next the hourly cost of the machine is calculated by estimating the depreciation per hour over some assumed useful life. Typically, this is 5-7 years at 4000-6000 printing hours since this technology is changing so rapidly.

Printing speed is where this cost model should be calibrated to a specific machine to be more accurate, and certainly depends on the number of lasers and even the part geometry as parts with extremely complex or thin wall features can take longer to scan. For a first pass estimate, a single laser build rate is considered around 8-30 cm3/hr. for most single-laser PBF systems. Unfortunately, this is more like an entitlement number, so it can be assumed that you’d achieve on average about 80% of this build rate consistently. To account for this, the build rate is divided by 80% in the cost model formula. Printing time is not only a function of laser speed, but also needs to account for the recoating time where fresh powder is spread over the build box to get ready for the next layer of printing. This is called the recoat time and is typically 6-10 seconds on current Laser PBF machines. The total time spent recoating is based on part height and layer thickness and this per layer recoat time as relayed in the formula above.

The final part of the AM cost model is the post-processing. The next section dives into why the post-processing costs vary by AM process, and they also vary by application since it is very design driven. For this simplified cost model, the post-processing costs can be considered between 30-40% of the total part costs. (We typically use a source such as the Wohler’s Report to attain baseline costs.) The total part costs can then be calculated by scaling the AM printing costs + material costs by this 40% markup to account for final post-processing.

While this cost model is specific to Laser Powder Bed Fusion with a single laser, it illustrates how you could build a process specific (even machine specific) cost model for your organization by including factors such as material costs, build time, and post-processing.

AM Technology

One of the first steps in building a business case for AM is selecting the process that will be used for manufacturing. This decision is driven by a thorough analysis of the application and selecting the best AM process to meet the requirements. Just as parts made with additive manufacturing should be intentionally designed for the AM process versus the traditional process. More granularly, AM applications should be designed for the specific AM process selected, and sometimes even the specific machine. Selecting the right 3D printer requires careful study of the requirements and the end to end AM process. At a high level, each of the 7 ASTM classifications has its own considerations and value chain as determined by the process and post-process steps required to realize a fully finished part. These steps in turn feed the business case. Each of the processes introduced in Chapter 2 Core AM Technologies and Supporting Processes have different business case considerations including printing, post-processing, and feedstock.

Laser Powder Bed Fusion is tailored towards small, detailed parts, and can transition from thin to thick sections fairly well. At some point, there is a tradeoff between achievable detailed features and size of the part due to the residual stresses that are built up in a large part.

Binder Jet is capable of detailed features in small parts where sintering effects are less of a concern. It is not ideal for larger parts in general, and there is a space where detailed features may not be achievable alongside thick sections due to sintering distortion and the effects of gravity while the part is going through densification in the vacuum furnace.

Figure 8.5 An ExOne Binder Jet machine uses a radiant heater to cure a layer of binder deposited into the metal powder bed. (credit: Modification of ExOne Binder Jet machine by Oak Ridge National Laboratory/Flickr, CC BY 2.0)

Sheet Lamination is a hybrid process that only makes sense to select for very detailed parts that take advantage of the in-situ machining element of the process due to its low productivity. The low productivity makes it an impractical process for large or simple parts.

Material Extrusion (direct metal) and DED will also require post-processing to achieve details and are ideal for larger parts. The DED process can make the largest parts since often it works with a robotic arm that is not limited to a defined machine bed size.

There are specific designs which are a best fit for a particular process, and some designs which may overlap into 2 or more processes. These overlapping sections are perhaps the only limited cases where it may be possible to trade off costs of one AM process versus another. However, the most important takeaway is that part design drives the process selection, which in turn has business case implications. The printing cost is driven by the time the part occupies the printer, which is a factor of design and machine productivity. The other cost drivers include feedstock and post-processing, which are both also process specific.

In AM, the material input is typically in the form of powder or filament, except in sheet lamination where it is a thin foil. Powder, wire and foil have additional processing done to them in order to get them into this form, so the price for these materials can be higher than common mill products or bulk plastics. However, it is important to note that the process is doing more of the “work” so the material input for AM is typically the minority cost element.

For most metal AM processes, printing represents the single largest cost element. For the most part, this is the depreciation of the machine represented in a dollar per hour basis. The less time on the printer, the less cost is added.

Lastly, post-processing can be a large cost driver, and more so in some AM technologies versus others.

Laser Powder Bed Fusion

Printing time is the largest cost driver for the Powder Bed Fusion (PBF) process. As mentioned above, part design and productivity are the factors that contribute to the printing costs. Because productivity is mostly influenced by the machine makers, the focus for the AM user should be first on part design. Thoughtful integration of design for additive manufacturing into part design will drastically affect total costs. Other factors that drive printing time for L-PBF include number of lasers and layer thickness. These are further explored later in this chapter, but at a high level more lasers and thicker layer thickness yield higher productivity but must be traded off with cost of equipment to add more lasers and part density for thicker layers.

The other cost drivers of PBF are feedstock and post-processing. PBF feedstock is expensive as compared to traditional manufacturing feedstock and as compared to most other AM technologies. This is due to the process yield and strict size requirements which is further explained in the subsequent section.

Finally, post-processing is also a major cost driver in PBF parts and includes the cost to depowder, stress relieve, remove parts from the build plate, and perform typical machining for interface points and tight tolerances.

Directed Energy Deposition

Directed energy deposition (DED) is a broad range of technology that uses a focused energy source to melt and apply a feedstock material. The various types of DED can have very different economic implications depending on their energy source and feedstock. In this section, the financial considerations of the following DED technologies will be explore further: DED Powder, DED Wire, and DED Cold Spray.

Directed Energy Deposition with powder is one of the highest cost processes second only to Laser PBF. This is likely because they both use a similar laser, have a small puddle size (2mm diameter or less) and create similar productivity. Due to the low productivity and high capital costs of these machines, the printing cost is the biggest cost driver. Post-processing for powder DED always includes stress relief because of the heat input to the part in the welding process. Although this process can create nearly final (near-net) shapes, it’s still often necessary to final-machine these parts for the fully finished part. Finally, the powder feedstock, although still expensive due to its low process yield, is the smallest cost driver for this technology.

Unlike DED with powder, Directed Energy deposition with wire is very efficient at printing. It will always require heat treatment for residual stresses and machining since the large (>4mm diameter) puddle inhibits the ability to produce fine details, making the post-processing the highest cost driver for this technology. Next in line is the feedstock, since wire can be an expensive product form.

Cold spray has a very similar cost breakdown as DED with wire and is very fast and affordable. It does not make near net shapes so additional machining will almost always be required, again making the post-processing the highest cost driver. In some cases, a heat treatment could be necessary as well. The powder cost for cold spray can vary wildly depending on the chemistry or alloy from very inexpensive to more expensive than laser powder bed fusion, specifically in titanium.

Sheet Lamination

Sheet lamination is the third-highest-cost AM process after L-PBF and DED with Powder, again driven by machine productivity. The machine does more, therefore costs more. Sheet lamination has minimal printing costs partly because the print speed is higher but also because the material input is expensive, and the process provides details at the expense of material wastage. The printing and machining are coupled leaving post-processing to traditional metal working and final finishes.

Metal Binder Jetting

Metal binder jetting is also a metal powder bed process but has a very different solution space as compared to other metal AM processes. Not surprisingly, it also has very different cost drivers with post-processing being the main driver followed by feedstock and finally printing costs.

As metal binder jetting is probably one of the fastest methods of AM, it’s easy to assume it’s also the cheapest. The costliest portion of metal binder jetting however, is actually in the post-processing. The printing process of binder jetting produces a “green state” part, which must be cured and sintered before a finished part is ready for final finishing such as machining or surface finishing. Both curing and sintering are lengthy processes and use additional equipment, and sintering furnaces can be quite expensive.

The second cost driver of metal binder jetting is the feedstock. The powder for metal binder jetting is typically less expensive than other powder bed AM feedstocks such as laser PBF. This is because the binder jet process is built off the legacy Metal Injection Molding process which is mature and used for extremely high-volume applications in automotive and other heavy industries that have driven down feedstock costs over decades.

Finally, the printing cost of metal binder jetting is the lowest cost driver due to this technology’s high productivity and lower cost of machines. The machines can be 2x or even lower cost compared to laser PBF machines. This is because they are made up of relatively simple and commoditized hardware such as inkjet printheads instead of lasers. The technology is also inherently faster than laser PBF because and among the fastest of AM processes. Like inkjet printing, there is a consistent layer speed parameter defined that is independent of how much part vs. empty space is on that layer. Binder is jetted only where that layer requires solid material, but the amount of solid material is irrelevant to speed of the jetting pass.

Figure 8.6 Autodesk setup showing 3D nested parts stacked together in the same build box. As you have probably seen in other images, this image is just the start of how complex these nesting approaches can become. (credit: Modification of “Autodesk meshmaker layout packing” by Creative Tools/Flickr, CC BY 2.0)

In addition to efficient nesting, the parts also print faster because they do not require supports for printing. Sometimes supports are required for sintering, which reemphasizes the high cost and difficulty of post-processing binder jet parts. In summary, metal binder jet printing is a fast process which has great potential for relatively niche design space given today’s technology maturity, however the full process must be considered to validate the AM business case.

Direct Extrusion

Direct Extrusion is quite fast and not very expensive. It is an efficient method for building with larger amounts of material. For metal extrusion, the feedstock is more like bar stock, therefore readily available and inexpensive. It does require similar post-processing to DED with wire due to its inability to make near-net shapes, which is why the post-processing is the highest cost driver for this technology.

Polymer Material Extrusion

Most of the focus of this section has been on metal 3D printing, but similar concepts and cost drivers apply to the polymer processes. For Polymer Material Extrusion, the most widespread of any of the AM technologies, it is hard to categorize the cost-drivers due to the wide variety of available machines in the market. In general, this is a slow process, so printing time is high, but the printers are often commoditized and cheaper than the other process machines. The feedstock cost also varies widely and can be anything from hobby level, low end plastics, to high strength, flame resistant, polymers made for industrial end use parts. Typically, filaments are a more expensive version of the polymer versus pellets common to the injection molding industry, with some of the larger machines using pellet feedstock for precisely that reason. Finally, post-processing is still required to remove supports, paint or finish surfaces, but it typically is lower cost to finish plastic parts as compared to metal.

Photopolymerization

VAT Photo-polymerization is the oldest and most mature of the AM process technologies, yet still very costly as compared to other AM polymer processes. The original process uses a similar laser tool path tracing method as Laser PBF and has similar low productivity. The machines are also quite expensive, making the printing category a high cost driver. The feedstock is a resin-based polymer and can be an expensive form of the material. The printed parts require washing and post-curing, but after that is complete, the parts produced have excellent surface finish and detailed features eliminating the need for further processing. A more recent iteration of the Photo-polymerization process called Continuous Liquid Interface Production (CLIP) is significantly higher productivity, claiming 25 to 100x faster than the original photo-polymerization methods. It works by continuously curing the 3d parts instead of tracing each 2D layer’s blueprint. This method would still have similar feedstock and post-processing considerations but significantly reduced printing costs.

Material Jetting

Material Jetting is the process in which droplets of build material are selectively deposited by a horizontally traversing nozzle onto a build surface where they solidify. The process has a similar setup to binder jetting and shares the same level of productivity and common inkjet industry hardware. The material is the highest cost driver since photopolymerizable resins and molten polymers are an expensive form of polymers as compared to filament or pellets. This technology uniquely allows for varying materials to be deposited during the build, allowing for dissolvable supports (which are always required) or different colors. This benefit minimizes the post-processing steps necessary after the build and makes the post-processing a small contribution to cost. Despite the low cost of post-processing and the processes’ high productivity, this form of printing is still among the costliest polymer printing processes.