Additive manufacturing processes are classified into seven areas on the basis of:

• Type of materials used
• Deposition technique, and
• The way the material is fused or solidified

These classifications have been developed by the ASTM International Technical Committee F42 on additive manufacturing technologies. The work of this Committee focuses on the promotion of knowledge, stimulation of research, and implementation of technology through the development of standards.

The seven major additive manufacturing processes as classified per ASTM F42 are:

1. Photopolymerization
2. Material jetting
3. Binder jetting
4. Material extrusion
5. Powder Bed Fusion
6. Sheet Lamination
7. Direct Energy Deposition

The end-user generally first chooses an additive manufacturing process that best meets his needs, and then the most appropriate material for the process and application.

As far as polymers are concerned, the most popular additive manufacturing processes are photopolymerization, material jetting, powder bed fusion, and material extrusion. The materials used in these processes can be in the form of liquid, powder, or solid (formed materials such as polymer film or filament).

The method of consolidation and applicable additive manufacturing process is illustrated in the figure below.




The specific chemical types and forms of the polymer materials that are used in each process are identified later in this guide. Print materials made of plastics and polymers are defined by the parameters of their parent 3D printing processes.

Common to all additive manufacturing processes is the use of a computer, 3D modeling software, layering material, and a manufacturing machine. The layering material can be almost anything, but polymers, both in solid and liquid form, have generally been used because of their available forms, formability, and end-use properties.

3D printable models may be created with a computer aided design (CAD) package, via a 3D scanner or by a plain digital camera and photogrammetric software. 3D printed models created with CAD result in reduced errors and can be corrected before printing, allowing verification in the design of the object before it is printed.

Now let's see the various types of polymer materials used in additive manufacturing process and the method by which they are consolidated into a three-dimensional shape.  For a more detailed description of the technologies behind computer technology and the specific equipment used, you can refer to other references.^2,3

Polymers Used in Additive Manufacturing

The specific chemical types and forms of the polymer materials that are used in each process are identified in the table below. Although at first glance, this may seem to be an abundance of materials, there is a growing need to develop and process a much greater variety of materials than currently possible.

The general feeling is that the materials that now exist for additive processing processes do not meet the requirements of the majority of industrial products, and materials need to be developed specifically for additive manufacturing processes and end-user applications.

Polymeric Materials for Building 3D Parts	Photo- polymerization	Material Jetting	Binder Jetting	Material Extrusion	Powder  Bed Fusion	Selective Heat Sintering	Sheet Lamination
Liquids:
Epoxy resin	X	X	X
Acrylic resin	X	X	X
Binder / powder hybrids		X
Powder:
PA 12					X	X
PA 11						X
PC				X	X	X
PS				X	X	X
ABS				X
ABS – PC blend				X
PP						X
PPSU				X
Starch		X	X
Elastomer / cellulose		X	X
PLA		X	X
TPU		X	X	X
HDPE				X
PEEK				X		X
PEI				X
Solid Sheet:
Polyester film							X
Polyolefin film							X
Polyvinyl copolymer film							X
Other thermoplastic film							X
Other thermosetting film							X
Melt (Molten Liquid):
ABS				X
ABS – PC blend				X
PPS				X

After understanding, additive manufacturing basics, now let's discuss seven major additive manufacturing processes one by one in detail...

Photopolymerization

In the photopolymerization process (also known as stereolithography) a pre-deposited liquid photopolymer in a vat is selectively cured by light activated polymerization (Figure). It is one of the earliest and most widely used rapid prototyping technology.

Materials Used in Photopolymerization process

The polymeric materials used in the photopolymerization process are mainly radiation curing acrylics and acrylic hybrids.

• The light activated polymer quickly solidifies wherever the beam strikes the surface of the liquid. 
• Once one layer is traced, it is lowered a small distance into the vat and a second layer is traced on top of the first layer.

The self-adhesive property of the photopolymer causes the layers to bond to one another, and eventually a complete three-dimensional object is fully deposited and hardened. Designs are then immersed in a chemical bath in order to remove any excess resin and post-cured in an ultraviolet oven. It is also possible to print objects "bottom up" by using a vat with a somewhat flexible, transparent bottom, and focusing the UV upward through the bottom of the vat.

Photopolymerization - General Characteristics

Photopolymerization generally provides the greatest accuracy and best surface finish of any AM prototyping technology. Over the years, a wide range of materials with properties mimicking those of engineering thermoplastics have been developed. Other characteristics of the photopolymerization process include:

• Support structures are required during build
• Post-processing is required to wash and post cure parts
• Advantages: high resolution and accuracy, ability to produce complex parts, smooth surface, accommodates large build areas
• Weaknesses: parts are not as durable as those manufactured with other AM processes
• Major applications: prototyping, consumer toys, electronics, guides and fixtures.

Although photopolymerization can be used to produce virtually any design, it is often costly. The cost of resin and stereolithography machines was once very high. Recently, interest in 3D printable products has inspired the design of several models of 3D printers which feature drastically reduced prices (less than $10,000 for an industrial sized printer. Several companies are now producing photopolymerizable resins at prices as low as $80 per liter.

Photopolymers used in 3D imaging processes must be designed to have a low volume shrinkage on polymerization in order to avoid distortion of the solid object.

• Common monomers utilized include multifunctional acrylates and methacrylates combined with a non-polymeric component in order to reduce volume shrinkage. 
• A competing composite mixture of epoxide resins with cationic photoinitiators is becoming increasingly used since their volume shrinkage upon ring-opening polymerization is significantly below those of acrylates and methacrylates Free-radical and cationic polymerizations composed of both epoxide and acrylate monomers have also been employed, gaining the high rate of polymerization from the acrylic monomer and better mechanical properties from the epoxy matrix. 

Continuous Liquid Interface Production (CLIP)

Although stereolithography was originally touted as a fast process for building prototype models, it is not fast enough for most full-production manufacturing. Conventional 3D printing processes are in reality only two-dimensional printing that is done over and over again. Full parts may take many hours or even days to produce. Very recently, a new photopolymerizaton technology, called Continuous Liquid Interface Production (CLIP), was introduced that claims speeds 25-100 times faster than traditional 3D printing.⁴


The CLIP process works by carefully balancing the interaction of UV light (which initiates photopolymerization) and oxygen (which inhibits the reaction). Part production is achieved with an oxygen-permeable window below the UV image projection plane. This creates a “dead-zone” where photopolymerization is inhibited between the window and the elevating polymerizing part (Figure). In this way parts that usually take hours to manufacture can be made in minutes.

Material Jetting

Material jetting creates objects in a similar method to a two dimensional ink jet printer.

• Material is jetted onto a build surface or platform, where it solidifies, and the model is built layer by layer. 
• The material is deposited from a nozzle which moves horizontally across the build platform. 

Machines vary in complexity and in their methods of controlling the deposition of material. The material layers are then cured or hardened using ultraviolet (UV) light.

As material must be deposited in drops, the number of materials available to use is limited.

Materials Used in Material Jetting Process

Photopolymerizable resins are suitable and commonly used due to their viscous nature and ability to form drops. However, molten polymers can also be used with an elevated temperature printing head, and the molten polymers then solidify at ambient temperature.

One distinct advantage of this process is that it allows changing of product material during a build. In this way graded material properties are possible. Other characteristics of the material jetting process are:

• Major sub-classification is “3D printing” using low viscosity ink.
• Light curable materials are generally used; however, molten thermoplastic materials, polymer solutions, and dispersions can also be used.
• Wax is often used as a support.
• Advantages: good surface finish, high resolution, allows full color parts, enables multiple materials.
• Weaknesses: Part may have low strength and durability, low viscosity prevents fast build-up.
• Major applications: high resolution prototypes, circuit boards and other electronics, consumer products, tooling.

Binder Jetting

In binder jetting, a thin layer of powder (polymer, metal, or ceramic) is rolled across the building platform.

• A printer head then sprays a liquid adhesive binder to fuse the powder particles together. 
• The binder is applied only in the places specified in the digital computer program. 
• This process repeats until the three-dimensional object is formed and the excess powder that supported the object during the build is removed and saved for later use.

Materials Used in Binder Jetting Process

The binder jetting process uses two materials:

  » A powder for part build-up and
  » A binder to consolidate the powder

The binder is usually in liquid form. Binder jetting is capable of printing a variety of materials including metals, sands, and ceramics.

Some materials, like sand, require no additional processing. Other materials are typically postured, sintered, or sometimes infiltrated with another material depending on the application and final density requirements for the part.

Binder jetting is unique in that it does not necessarily employ heat during the build process

Other additive techniques utilize a heat source which can create residual stresses in the parts. These stresses must be relieved in a secondary post-processing operation.

Additionally, the parts produced via binder jetting are supported by the loose powder, thus eliminating the need for a build-plate. Spreading speeds for binder jetting outperform other processes. Binder jetting has the ability to print large parts and is often more cost-effective than other additive manufacturing methods. Other notable characteristics of the binder jetting process are:

• Uses fine polymer powders (100 mm) and jetting of a liquid polymeric binder to bind the powder, subsequent infiltration is possible
• Advantages: low waste, relatively fast and simple process, allows for full color, uses a wide range of materials
• Weaknesses: rough surfaces
• Major applications: prototyping, tooling.

Material Extrusion

Material extrusion is becoming one of the most prominent additive manufacturing processes. In this process the part is made by depositing an extruded material layer by layer. Generally, a thermoplastic filament is unwound from a coil and supplies material to an extrusion nozzle.

• The nozzle is heated to heat the filament and provide for a semi-molten polymer to be extruded in either horizontal or vertical directions. 
• The plastic hardens immediately after being extruded and bonds to the layer below as shown in Figure.
• The entire system is contained within a chamber which is held at a temperature slightly below the melting point of the plastic. 

Materials Used in Material Extrusion

Several materials are available for this process. ABS is the most widely used polymer, but other polymers have also be used such as:

• Polycarbonate
• High density polyethylene
• Polycarbonate/ABS blends
• Polyphenylsulfone, and
• High impact polystyrene 

In addition parts have been produced from biopolymers such as polylactic acid and from processed plastic waste (See table below).

Material extrusion is somewhat restricted in the variations of shapes that can be fabricated. Other notable characteristics include:

• Material extrusion additive manufacturing is also known as fused deposition modeling (FDM) or fused filament fabrication (FFF)
• Thermoplastic filament (3mm in diameter) has become a commonly available build material
• Multiple materials can be used for both the product build and support
• Advantages: multiple materials and colors can be used, availability of equipment, parts have good structural properties, inexpensive and economical
• Weaknesses: surface quality may require post processing, relatively slow build times, requires strong filament and high processing temperatures
• Major applications: prototyping, tooling, office manufacturing.

Materials extrusion is one of the simplest and least expensive additive manufacturing process. In fact a toy 3D printer⁵ including software that will be on the market in the fall of 2016 for a price of $299.

FDM printers use two kinds of materials:

• A modeling material, which constitutes the finished object, and 
• A support material, which acts as a scaffolding to support the object as it is being printed. Support materials are usually water-soluble wax or brittle thermoplastics, like polyphenylsulfone (PPSF). 

Because thermoplastics are environmentally stable, part accuracy (or tolerance) doesn’t change with ambient conditions or time. This enables FDM parts to be among the most dimensionally accurate.

Once an object comes off the FDM printer, its support materials are removed either by soaking the object in a water and detergent solution or, in the case of thermoplastic supports, snapping the support material off by hand. Objects may also be sanded, milled, painted or plated to improve their function and appearance.

Materials and their Characteristics

Acrylonitrile Butadiene Styrene (ABS) and ABS Blends An ABS prototype has up to 80% of the strength of injection molded; this means that ABS printed products using FDM is extremely suitable for functional applications. ABSi is an ABS type with high impact strength. The semi-translucent material is USP Class VI approved. It has a good blend of mechanical and aesthetic properties. ABS-M30 is 25-75% stronger than the standard ABS material and provides realistic functional test results along with smoother parts with finer feature details. It is a biocompatible (ISO 10993 and an ideal material for medical, pharmaceutical and food packaging industries. It is sterilizable using gamma radiation or ethylene oxide (EtO) sterilization methods. ABS-ESD7 is a durable and electrostatic dissipative material suited for electronic products, industrial equipment and jigs and fixtures for assembly of electronic components.

Polylactide (PLA) Polylactide, or PLA, is a popular plant-based thermoplastic material. It is a both light and strong.  The wide range of available colors and translucencies and glossy feel often attract those who print for display or small household uses.  When properly cooled, PLA seems to have higher maximum printing speeds, lower layer heights, and sharper printed corners.  Combining this with low warping on parts make it a popular plastic for home printers, hobbyists, and schools.

Polycarbonate (PC) and PC Blends Polycarbonate is the most widely used industrial thermoplastic. In FDM products it is accurate, durable, and stable for strong parts. PC has superior mechanical properties, heat resistance, and high tensile strength PC-ABS is a blend of polycarbonate and ABS plastic which combines the strength of PC with the flexibility of ABS. It has Superior mechanical properties and heat resistance of PC, excellent feature definition and surface appeal of ABS, and high impact strength. PC-ISO is a strong, heat-resistant engineering plastic commonly used in food and drug packaging and medical device manufacturing. It is biocompatible, gamma and EtO sterilizable and complies with ISO 10993 and USP Class VI. The material gets its name from being a polycarbonate (PC) material with ISO certification.

Polyimide (PI) ULTEM 9085 is a high flow polyetherimide blend that is strong, lightweight and flame retardant (UL 94-V0 rated). It was developed primarily for the aerospace industry and also has applications in other niche industries. ULTEM 9085 is an ideal candidate for functional prototyping and end use parts applications. in the aerospace. It has a high strength to weight ratio and a high heat deflection temperature (160°C).

Polyphenylsulfone (PPSU) PPSU is a thermoplastic with the highest heat and chemical resistance of all FDM Materials. It has great strength and is sterilizable by all processes. PPSU is ideal for applications in caustic and high heat environments.

Powder Bed Fusion

Powder fusion is similar to binder jetting, except the layers of powder are fused together using a heat source, such as a laser or election beam. This process is also known as selective laser melting (SLM) or electron beam melting (EBM) when using metal powder. An alternative method to liquefying the powder by heat is to use sintering.


All processes involve the spreading of the powder material over previous layers usually with a roller or a blade as shown in Figure below. A hopper or a reservoir below of aside the bed provides fresh material supply.


Some other additive manufacturing processes, such as stereolithography (SLA) and fused deposition modeling (FDM), often require special support structures to fabricate overhanging designs. While SHS does not need a separate feeder for support material because the part being constructed is surrounded by unsintered powder at all times, this allows for the construction of previously impossible geometries.

Materials Used in Powder Bed Fusion

Polymer powders used in powder bed fusion processes can be either amorphous or crystalline thermoplastic particles. Typical polymers are:

• Polyamide
• Glass filled polyamide
• Polyetheretherketone
• and Polystyrene

Polyamide 12, either pure or blended is the major option. The utilization of polyamide 11, polyamide 6, and elastomeric polymers such as TPE and TPU are growing. Thermosetting powders such as epoxy have also been used to produce pure plastic parts or as a binder to use with metal or ceramic particles. Kruth⁶ , et. al., provides an excellent dissertation on the consolidation of polymer powders by selective laser sintering, and Schmid⁷, et. al., provides information regarding the combination of intrinsic and extrinsic polymer properties necessary to generate a polymer powder likely for SLS application.⁷

Polymeric powders are commonly produced by ball milling. However, most SLS machines use two-component powders, typically either coated powder or a powder mixture. In single-component powders, the laser melts only the outer surface of the particles (surface melting), fusing the solid non-melted cores to each other and to the previous layer.

Shape and surface of the particles determine the behavior of the resulting powder to a great extent. In case of SLS powders the particles should be at least as feasible formed spherical. This is in order to induce an almost free flowing behavior on the part bed of an SLS machine. A certain particle size and distribution is necessary to be processable on SLS equipment. This distribution is favorably between 20 μm and 80 μm for commercial systems.





However, a major disadvantage has been the limited spectrum of suitable materials due to the high cost for cryogenic processing of powder and the available powder properties (size, molecular weight, melt flow, etc.) required for manufacturing. Other notable characteristics of the powder bed fusion process are:

• Part is embedded in a block of unsintered powder which acts as a support that must be removed.
• Part can be produced in a vacuum to reduce porosity.
• Various thermal energy sources can be used, and as a result there are several variations of this process: Direct Metal Laser Sintering (DMLS), Electron Beam Melting (EBM), Selective Heat Sintering (SHS), Selective Laser Melting (SLM), Selective Laser Sintering (SLS).
• Advantages: low waste, relatively fast, complex structures are possible, wide range of materials, no support required, high heat and chemical resistant materials
• Weakness: high equipment cost.
• Major applications: Aerospace, Automotive, Medical products, Tooling, Dental implants.

Also Read: How Plastics Can Boost 3D Printing of Medical Devices

Sheet Lamination

In the sheet lamination additive manufacturing process, thin sheets of material are bonded together using adhesives or a heat source to form a three-dimensional product. The sheet lamination processes are also known as:

• Ultrasonic additive manufacturing (UAM) when ultrasonic bonding is used to laminate thermoplastic sheets together, and 
• Laminated object manufacturing (LOM) when adhesives are used for lamination

Materials Used in Sheet Lamination

Polymers are often used but paper or metal foils are also typically processed and find application in cases where heat sensitive materials cannot be used and low costs must be realized. Almost any polymer can be used as long as it is available in thin sheet form and can be bonded by either adhesives or heat.

The main advantages of sheet lamination are:

• Low materials cost
• Many substrates are available (e.g., paper, film, foil)
• Process does not require a closed environment 
• High volumetric build rates 
• Allows for combination of materials and embedding components

The primary disadvantages are that complex geometries are difficult to produce, and it can be less accurate than other additive manufacturing processes. Other characteristics of this process are:

• Uses binding materials such as adhesives or energy (e.g., ultrasonic welding)
• Relatively large parts can be produced
• Possibility to use low cost, easily available building materials such as paper, plastic film, or metal foil
• Bonding equipment can be simple (even by hand) or automated
• Major applications: large parts, tooling

Directed Energy Deposition

Directed energy deposition processes generally do not use polymeric materials but employ metal wire or powder. High energy heating sources such as a laser are directed at the material to melt it and build-up the product.

Directed energy deposition is considered to be a more complex and expensive additive manufacturing process, but it is commonly used to repair or add additional materials to existing components. Other characteristics of directed energy deposition include:

• Similar to powder bed fusion except material is first injected into an energy field
• Common substrates are metal, metal wire, glass, and ceramics

Strengths	Limitations
Can operate in open air Multiple materials can be used Large parts are possible High single point deposition rates Not limited by direction or axis	Expensive equipment lower resolutions and reduced ability to manufacture complex parts Final machining is often required
Major applications: Repair or build-up of high volume parts

The table below provides a quick recap and description of these processes...

Process	Description	Technology
Photopolymerization	A vat of liquid photopolymer resin is cured through selective exposure to light (via a laser or projector). This then initiates polymerization and converts the exposed areas to a solid part.	Stereolithography (SLA) Digital Light Processing (DLP) Continuous Liquid Interphase Production (CLIP) Scan, Spin, and Selectively Photocure (3SP)
Material Jetting	Droplets of material are deposited layer by layer to make parts. Common varieties include jetting a photo-curable resin and curing it with UV light, as well as jetting thermally molten materials that then solidify at ambient temperature. This process was the origin for the term “3D Printing”.	3D Printing (3DP) Multi-Jet Modeling (MJM) Drop on Demand (DOD)
Binder Jetting	Liquid bonding agents are selectively applied onto thin layers of powdered material to build up parts layer by layer. The binders include organic and inorganic materials. Metal or ceramic powdered parts are typically fired in a furnace after they are printed.	Drop on Powder (DOP) Powder Bed printing
Material Extrusion	Material is extruded through a nozzle or orifice in tracks or beads, which are then combined into multi-layer models. Common varieties include heated thermoplastic extrusion (similar to a hot glue gun) and syringe dispensing.	Fused Deposition Modeling (FDM) Fused Filament Fabrication (FFF)
Powder Bed Fusion	Powdered materials are selectively consolidated by melting them together using a heat source such as a laser or electron beam. The powder surrounding the consolidated part acts as support material for overhanging features.	Selective Heat Sintering (SHS) Direct Metal Laser Sintering (DMLS) Electron Beam Melting (EBM) Selective Laser Melting (SLM) Selective Laser Sintering (SLS).
Sheet Lamination	Sheets of material are stacked and laminated together to form an object. The lamination method can be adhesives, ultrasonic welding, or brazing (metals). Unneeded regions are cut out layer by layer and removed after the object is built.	Selective Deposition Lamination (SDL) Laminated Object Manufacturing (LOM) Ultrasonic Additive Manufacturing (UAM)
Direct Energy Deposition	Metal powder or wire is fed into a melt pool which has been generated on the surface of the part where it adheres to the underlying part or layer. The energy source is usually a laser or electron beam. This process is essentially a form of automated build-up welding.	Laser Metal Deposition (LMD) Electron Beam Free-Form Fabrication (EBF3) Direct metal deposition (DMD) Laser Engineered Net Shaping (LENS

Additive Manufacturing Application Trends

The development of innovative, advanced additive manufacturing technologies has progressed quickly yielding broader and high value applications. This accelerating trend has been due to the benefits of additive manufacturing compared to more conventional manufacturing processes. Some of these benefits are:

1. Lower energy consumption
2. Less waste
3. Less dedicated tooling
4. Reduced development costs and time to market
5. Innovative designs and geometries
6. Part consolidation (fewer parts with more complex design)
7. Customization of parts (e.g., for medical implants, specialty repair parts, parts where other manufacturing facilities are not available such as on ships or in space.

The industries that will benefit most in the immediate future from additive manufacturing and the value provided are:

Industry	Applications and Value
	Aerospace and Defense Concept modeling and prototyping Manufacturing low-volume complex parts (electronics, engine parts, etc.) Manufacturing replacement parts anywhere Manufacturing structures using lightweight, high strength materials
	Automotive Testing part design to verify correctness and completeness Parts for race vehicles, luxury sports cars, antique cars, etc. Replacement of parts that are defective or cannot be purchased Manufacturing structures using lightweight, high strength materials
<	Electronics Embedding Radio Frequency Identification (RFID) devices embedded inside solid materials Short lead time electronic products Polymer based, three-dimensional micro-electromechanical systems Microwave circuits fabricated on paper substrates
	Tool and Mold Making Universal tool holders with standardized pocket sizes Die casting forms Injection molding tooling Tooling for prototyping of short lead time surgical devices
	Medical Design and modeling methods for customized implants and medical devices Processes for fabrication of “smart scaffolds” and for construction of 3D biological and tissue models

Properties of Polymeric Materials for Additive Manufacturing

The penetration of these industries is still limited, and this limitation has much to do with the types of materials that are available. The properties of new materials must be compatible with the deposition tool as well as the application. Some of the properties for new, sought-after polymeric materials include:

• Mechanical Stability: The material should maintain its form during processing including the support of subsequent layers. High mechanical stability of the final part allows it to be handled quickly and provides property imitation of conventionally processed materials (e.g. by injection molding).


• Chemical Stability: The material has to have a consistent chemical structure and it must be inert when in contact with other materials during and after processing. This will allow possible combination with other materials without undesirable reactions.


• Thermal Stability: The material should have properties (melt flow, particle size, adhesion, etc.) that are required for the additive manufacturing processes chosen. It should also have thermal properties (glass transition temperature, creep resistance, low and high temperature strength, etc.) required for the end-use application.


• Biocompatibility: Biocompatibility will become important in AM parts that are manufactured for biological applications such as bodily implants and orthodontics. It will also become significant in parts that must be recycled or deposited in a waste facility. The parts should have low or no toxic effect on the environment and be biodegradable when necessary.

As is the case with any newly developing industry, adequate information and its dissemination to the material and machine suppliers and end-users are paramount. Especially needed in the additive manufacturing industry is the:

1. Development of a shared, standardized third-party data repository that contains material property data that leads to the proper choice of materials and
2. Standards and protocol for part manufacture independent of region or time and standards for material and part testing.

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