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Liquid Polymer Systems for Additive Manufacturing

The first commercial system was the 3D Systems Stereolithography process based on liquid photopolymers. A large portion of systems in use today are, in fact, not just liquid polymer systems but more specifically liquid photopolymer systems. However, this classification should not be restricted to just photopolymers, since a number of experimental systems are using hydrogels that would also fit into this category. Furthermore, the Fab@home system developed at Cornell University in the USA and the Reprap systems originating from Bath University in the UK also use liquid polymers with curing techniques other than UV or other wavelength optical curing methods.

Using this material and a 1D channel or 2 1D channel scanning method the best option is to use a laser like in the Stereolithography process. Droplet deposition of polymers using an array of 1D channels can simplify the curing process to a floodlight (for photopolymers) or similar method. This approach is used with machines made by the Israeli company Objet who use printer technology to print fine droplets of photopolymer “ink”. One unique feature of the Objet system is the ability to vary the material properties within a single part. Parts can have softfeel, rubber-like features combined with more solid resins to achieve an overmolding effect.

Controlling the area to be exposed using digital micro-mirror devices (DMD) or other high-resolution display technology obviates the need for any scanning at all, thus increasing throughput and reducing the number of moving parts. DMDs are generally applied to micron-scale additive approaches, like those used by Microtec in Germany. For normal-scale systems Envisiontec uses high-resolution DMD displays to cure photopolymer resin in their low-cost AM machines. The 3D Systems V-Flash process is also a variation on this approach, exposing thin sheets of polymer spread onto a build surface.

(Gibson I., Rosen D. W., Stucker B., Additive manufacturing technologies: Rapid prototyping to direct digital manufacturing, p. 29)

Discrete Particle Systems for Additive Manufacturing

Discrete particles are normally powders that are generally graded into a relatively uniform size and shape and narrow distribution. The finer the particles the better, but there will be problems if the dimensions get too small in terms of controlling the distribution and dispersion. Again, the conventional 1D channel approach is to use a laser, this time to produce thermal energy in a controlled manner and, therefore, raise the temperature sufficiently to melt the powder. Polymer powders must therefore exhibit thermoplastic behavior so that they can be melted and re-melted to permit bonding of one layer to another. There are a wide variety of such systems that generally differ in terms of the material that can be processed. The two main polymer-based systems commercially available are the Selective Laser Sintering (SLS) technology marketed by 3D Systems and the EOSint processes developed by the German company EOS.

Application of printer technology to powder beds resulted in the 3D Printing (3DP) process. This technique was originally developed by researchers at MIT in the USA. Printing technology is used to print a binder, or glue, onto a powder bed. The glue sticks the powder particles together to form a 3D structure. This basic technique has been developed for different applications dependent on the type of powder and binder combination. The most successful approaches use low-cost, starch- and plaster-based powders with inexpensive glues, as commercialized by ZCorp, USA. Ceramic powders and appropriate binders as similarly used in the Direct Shell Production Casting (DSPC) process used by Soligen to create shells for casting of metal parts. Alternatively, if the binder were to contain an amount of drug, 3DP can be used to create controlled delivery-rate drugs like in the process developed by the US company Therics. Neither of these last two processes has proven to be as successful as that licensed by ZCorp. One particular advantage of the ZCorp technology is that the binders can be jetted from multinozzle printheads. Binders coming from different nozzles can be different and, therefore, subtle variations can be incorporated into the resulting part. The most obvious of these is the color that can be incorporated into ZCorp parts.

(Gibson I., Rosen D. W., Stucker B., Additive manufacturing technologies: Rapid prototyping to direct digital manufacturing, p. 30)

Molten Material Systems for Additive Manufacturing

Molten material systems are characterized by a pre-heating chamber that raises the material temperature to melting point so that it can flow through a delivery system. The most well- nown method for doing this is the Fused Deposition Modeling system developed by the US company Stratasys. This approach uses an extrusion technique to deliver the material through a nozzle in a controlled manner. Two extrusion heads are often used so that support structures can be fabricated from a different material to facilitate part cleanup and removal.

Printer technology has also been adapted to suit this material delivery approach. One technique, developed initially as the Sanders prototyping machine, that later became Solidscape, USA, is a 1D channel system. A single jet piezoelectric deposition head lays down wax material. Another head lays down a second wax material with a lower melting temperature that is used for support structures. The droplets from these print heads are very small so the resulting parts are fine in detail. To further maintain the part precision, a planar cutting process is used to level each layer once the printing has been completed. Supports are removed by inserting the complete part into a temperature-controlled bath that melts the support material away, leaving the part material intact. The precision of Solidscape machines makes this approach ideal for precision casting applications like jewelry, medical devices, and dental castings. Few machines are sold outside of these niche areas.

The 1D channel approach, however, is very slow in comparison with other methods and applying a parallel element does significantly improve throughput. The Thermojet from 3D Systems also deposits a wax material through dropletbased printing heads. The use of parallel printheads as an array of 1D channels effectively multiplies the deposition rate. The Thermojet approach, however, is not widely used because wax materials are difficult and fragile when handled. Thermojet machines are no longer being made, although existing machines are commonly used for investment casting patterns.

(Gibson I., Rosen D. W., Stucker B., Additive manufacturing technologies: Rapid prototyping to direct digital manufacturing, p. 30)

Solid Sheet Systems for Additive Manufacturing

One of the earliest AM technologies was the Laminated Object Manufacturing (LOM) system from Helisys, USA. This technology used a laser to cut out profiles from sheet paper, supplied from a continuous roll, which formed the layers of the final part. Layers were bonded together using a heat-activated resin that was coated on one surface of the paper. Once all the layers were bonded together the result was very like a wooden block. A hatch pattern cut into the excess material allowed the user to separate away waste material and reveal the part.

A similar approach was used by the Japanese company Kira, in their Solid Center machine, and by the Israeli company Solidimension with their Solido machine. The major difference is that both these machines cut out the part profile using a blade similar to those found in vinyl sign- aking machines, driven using a 2D plotter drive. The Kira machine used a heat-activated adhesive applied using laser printing technology to bond the paper layers together. The

Solido machine uses the plotter drive to draw adhesive to bond the layers and separate materials to ensure key features and boundaries are not bonded. Solido parts are made from polymeric sheet material that results in much stronger final parts.

(Gibson I., Rosen D. W., Stucker B., Additive manufacturing technologies: Rapid prototyping to direct digital manufacturing, p. 31)

Nonplanar Systems

There have been a few attempts at developing AM technology that doesn’t use stratified, planar layers. The most notable projects are Shaped Deposition Manufacture (SDM), Ballistic Particle Manufacture (BPM), and Curved Laminated Object Manufacture (Curved LOM). The Curved LOM process in particular aims at using fiber-reinforced composite materials, sandwiched together for the purposes of making tough shelled components like nose cones for aircraft using carbon fiber and armored clothing using Kevlar. To work properly, the layers of material must conform to the shape of the part being designed. If edges of laminates are exposed then they can easily come loose by applying shear forces. The Curved LOM process demonstrated feasibility but also quickly became a very complex system that required conformable robotic handling equipment and high powered laser cutting for the laminates.

It is possible to use short fibers mixed with polymer resins in FDM. Fibers can be extruded so long as the diameter and length of the fibers are small enough to prevent clogging of the nozzles. Like Curved LOM, it is somewhat pointless to use such a material in FDM if the layers are aligned with the build plane. However, if the layers were aligned according to the outer layer of the part, then it may be useful. Parts cannot be built using a flat layer approaching, in this case, and thus process planning for complex geometries becomes problematic. However, certain parts that require surface toughness can benefit from this non-planar approach.

(Gibson I., Rosen D. W., Stucker B., Additive manufacturing technologies: Rapid prototyping to direct digital manufacturing, p. 165)