Differences Between Plastic and Metal Additive Manufacturing

History
As freeform fabrication technologies continue to develop and mature, more industries and individuals have experience with them. Additive manufacturing (AM) was first introduced in the late 1980s, primarily for prototyping. Users of this revolutionary technology have continued to push for more aggressive, high-value applications. Plastic materials, either in the form of filament, powder or liquid photopolymer, were the first to be offered since they were easier to process to an acceptable result. Digital manufacturing, in any of its various incarnations, names or catchy acronyms, has historically been limited to tooling and pattern applications. A select few have been able to drive these technologies into true production, but largely remain the exception and not the rule.

Early adopters should be applauded for their creativity and daring in this new field. They challenged the status quo, validated the impact of additive processes and helped drive machine developers to cross into the realm of metals, thus opening an entire class of materials to drive new and even higher value applications.

Figure 1. Example of metal AM-produced component. Source: SLM Solutions

To cross the line from prototype to production is significant, both in reward and risk. Mistakes made in prototyping are expected and hailed as a positive step toward product refinement. In production, mistakes are no longer acceptable and performance is characterized by consistency, repeatability and dependability, while coupled with processes to ensure that the material quality meets minimum standards.

Considering the potential payoff, many companies believe the rewards now exceed the risks given the current state of the technologies. Many are pivoting from their experience with plastic-based technologies to hasten the adoption of additive metals.

Similarities and Differences in the Plastic and Metal Process
Both plastic and metal AM build a part layer-by-layer. Because of this, much of the design process and other aspects of manufacturing are the same.

To achieve the desired accuracy, plastic and metal parts use a similar approach to design modification. Typically, the main issues with accuracy involve the contraction of thermally-processed materials as they cool to room temperature and the potential for distortion if cooling is uneven across the part geometry. Accounting for these can be handled in a computer-aided design (CAD) package in both types of AM processes.

As with other manufacturing methods, the building process must be considered during the design phase. While AM has the capability to make parts that computer numeric control (CNC) and casting cannot — such as negative draft angles — it still requires some considerations. These design considerations are similar for both plastic and metal and include supplemental geometry, part shrinkage and post-processing needs, as well as others.

While these two material processes do share many things in common, the details highlight significant differences. Metal requires tighter machine tolerances and adjustments, which vary depending on what is being manufactured. To enable production it may be necessary to have access to many more machine parameters. While important, it is also inherently more complex and requires a greater degree of training due to the interplay and impact that a parameter change may have on the overall part quality.

Supplemental Geometry
Both metal and plastic AM use a common language for the supplemental geometry that may be added to a part to facilitate the build process. Supports, or support structures as they are commonly called, may look similar for both plastic and metal, but they serve very different purposes.

For plastics, the supports generally provide the function for which they are named: a support scaffolding upon which overhanging features of a part geometry can be built. These are typical for plastic-filament and liquid-based processes.

Metal also uses supports, and they may look similar to those used in plastic processes, but they serve a very different purpose. Due to the stresses that accumulate in parts made via laser powder bed (LPB) processes, supplemental geometric features tend to serve more of an anchoring function. In other words, they are holding the part down and counteracting the tendency for curl.

Designing supports for both metal and plastic processes has been reduced to little more than a few clicks of a mouse button. STL files are imported into the preferred setup software where a parameter-based evaluation of the part determines where supports are needed. In many cases this approach yields a greater number of supports than are actually needed, but to edit or delete them prior to the build process is a relatively simple process.

The objective for a good support strategy is to balance the requirements of the process, material, part geometry and quality expectations while simplifying the cleanup of the part after it's built. This is most often taught by machine vendors as part of the instruction an operator receives when the machine is purchased. Often, modifications to build strategy or part orientation may be the simplest means to reduce supports. Many processes do not require supports for part features that are created at approximately 45 degrees or greater from the horizontal plane. Additionally, internal features like holes or passages can be redesigned, if permissible, from round or square to a teardrop shape that will also reduce or eliminate the need for supports. This is particularly valuable when the path of internal passages would make it difficult or impossible to manually remove the supports.

However, it is important to note that the powder-based build media used in certain processes negates the need for supports altogether. Inkjet-based processes also use a gel-like media for support that is ultimately washed away with water or a mild sodium hydroxide solution.

Part Shrinkage Challenges
In both metal and plastic AM, part shrinkage must be considered. Despite being dealt with in similar ways, the materials have different characteristics. In many cases software can automatically compensate for shrinkage in plastic since the manufacturing temperature is lower. For metal, shrinkage needs to be tackled on a case-by-case basis. Compensations can be made in the CAD model or by adjusting scaling parameters in the machine software. Generally, the accuracy of an AM-created metal part will fall within the tolerance range of a traditional casting process.

Figure 2. Powder bed of metal material. Source: SLM Solutions

What Material Options are Available?
Early AM materials were polyamides, polycarbonate and casting wax. In the early days of laser sintering, nylon was the exception but is now often the go-to material. Acrylonitrile butadiene styrene (ABS) as well as other plastics — such as polyethylene terephthalate (PETG), which is routinely used to make beverage, food and other containers — are excellent for FDM.

Metal has advantages in strength and heat resistance. Metals such as aluminum and cobalt alloys are popular. Other metals such as titanium alloys and steel are also available. Each material has its own properties that need to be considered for project use and design. Once the material is chosen, it is equally important to match the machine parameters to the material and design.

Comparison of Post-Processing Techniques
Post-processing prepares AM parts for use. In plastic, post-processing includes removing support structures and extra powder. This can be done with a variety of equipment, sometimes as simple as a knife. Other post-processing techniques such as vapor smoothing and sanding can further refine a part.

Post-processing in metal may also involve removing support structures but often requires dedicated equipment due to the material's strength. In some cases grinding, sanding and burnishing can achieve the desired finish. If necessary, metal parts can be further processed with CNC equipment.

Qualifying Parts
Qualifying parts for production is critical. Each part must be qualified through the process and with the materials in use. If a second machine is used, the machines must be similar enough so the qualification can translate between machines. Customers may use test coupons around the build to make sure the part continues to meet the specifications during production.

Once the part is qualified and approved for production, nothing can change within acceptable tolerance. The materials must be the same and the machine must be locked down. If the machine is upgraded or changed, the part must then be requalified.

Machine Attributes
Performance and safety are about repeatability, speed, throughput and maintaining a high integrity process. Multiple lasers are an example. If one machine can act like two or more as far as throughput, it is an obvious advantage. Those who have previous metal experience can also benefit from a machine from SLM Solutions, a provider of metal AM manufacturing technology

Closed-loop powder handling is another important factor to consider for metal AM. Avoiding contaminants is important because foreign materials will impact the material properties; even a hair could make a difference. SLM Solution's machines are closed loop, decreasing the chances of contamination.

Material reuse requires testing to ensure that what you build will be the same with virgin versus recycled materials. The answer will depend upon the materials and process; as an example, titanium might not be as reusable as other materials. Reuse of material when applicable can save on costs and waste.

Figure 3. Overview of the metal AM process. Source: SLM Solutions

Moving to Metal
Working with plastic AM will give users a good background to work with metal, although metal AM is inherently more complex. While there are many similarities such as the way things are made and the use of supplemental geometry, there are also many differences. One area that new users of metal will find challenging is the many parameters they can set up in the machine. While plastic machines have some parameters, the users typically have fewer choices and plastic AM is a more forgiving process. This leads to plastic AM being more of a push-button process that doesn't require much user interaction.

For metal, having an open parameter set to adjust the machine is important due to the wide variety of materials and design options. Producing a quality part will require fine tuning the parameters to match the material and geometry. While this tuning is necessary, it also means that users are more likely to make errors by using the wrong parameters. It is at this point that having the correct resources will become a key aspect of production — this is one of the main benefits of partnering with SLM solutions. The company has the required people and support to help the customer when they need it. Customers can discuss problems such as porosity, temperature and strength with SLM solutions and receive the guidance to make the machine work at a critical time.

Production vs. Prototype
Plastic AM is often used for prototyping, although as the technology has matured it is beginning to be used more often for limited production runs. Like metal, plastic AM has to compete with casting. The process and industry of metal and plastic casting is well established. For plastic, when large production runs are required, casting is often the better option. For short production runs, the tooling cost will often be too high and AM is the better choice.

A metal machine from SLM Solutions is designed with production in mind and is designed to be in the critical path of production where downtime can cost money. The machines have the tolerances, repeatability and speed required. At the same time, they produce parts that can be compared to casting.

Safety
According to Underwriters Laboratories, SLM Solutions offers the safest AM metal technologies on the market today. Once a customer expresses interest in bringing an AM technology into a new site, one of the first meetings will be with a safety team. SLM Solutions' equipment will meet safety requirements with few to no modifications.

Conclusion
Metal AM is the evolution of manufacturing that sits on the shoulders of the plastic AM process. While metal has many similarities, it also has many differences that will require users to go beyond their current knowledge. Similarities such as the layer process and supporting geometry are important to recognize. The technologies also contrast with each other in important ways, such as the reason for scaffolding or the way a machine can be adjusted. Since metal AM is often used to make a finished product, it requires other considerations such as qualifying parts and machine throughput and downtime.

When it comes to AM, there are many design and machine parameters for a user to consider. With metal, this is even truer than with plastic, as changes in the settings can impact the finished product in terms of porosity and strength. Having partners at SLM Solutions who can identify issues and know how to solve them, often even before the customer knows there is a problem, can really make the difference.