Printing Technologies

So there are currently ten posts covering eleven different printing processes. In order to learn them better I decided to summarize them all in my own words, which I hope are easy enough to understand (feedback is greatly appreciated, both conceptually and grammatically). Here is a list of all of the types I have covered and their appropriate links. I have also categorized them so that you can see some of the underlying technologies/similarities/differences.

SLA – Stereolithography
SLS – Selective Laser Sintering
FDM – Fused Deposition Modeling
LOM – Laminated Object Manufacturing
3DP – 3-Dimensional Printing
Inkjet and MultiJet Printing
SGC – Solid Ground Curing
JP – Jetted Photopolymer
DMLS – Direct Metal Laser Sintering
LENS – Laser Engineered Net Shaping & DMD – Direct Metal Deposition

The following websites were extremely helpful in researching:

http://www.custompartnet.com/wu/additive-fabrication
http://www.efunda.com/processes/rapid_prototyping/intro.cfm
http://home.att.net/~castleisland/home.htm
http://home.utah.edu/~asn8200/rapid.html

…as well as manufacturers of these technologies, whose pages are linked within individual posts.

What I have found in doing this research is that many of the processes are very similar, but deal with slightly different materials. They vary in speed and quality, and it seems that if you are familiar with all of these technologies, there will always be one that stands out for a specific application. I will be constructing a map soon that I hope will shed light on these subtle differences and help to analyze cost and speed in addition to accuracy, tolerance, etc.

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Laser Engineered Net Shaping (LENS) / Direct Metal Deposition (DMD)

LENS and DMD are the same technology. LENS/DMD is used to print parts out of metal using a print head (as opposed to the DMLS process). The print head moves in all three axes. A laser is focused through the print head and metal powder is injected into it. The powder is sintered as it exits the head and is put down on the model.

An inert shroud gas is used inside of the print head to shield the metal from oxygen (so that it sinters correctly and can be controlled more accurately).

from http://home.att.net/~castleisland/lens.htm:

These technologies have been utilized to fabricate and repair injection molding machines, and to create specialized parts for aerospace applications.

LENS/DMD is limited at the moment because support structures would have to be made out of the same material as the model, which makes them difficult to remove afterwards.

Advantages?
• The printed objects usually have desirable metallurgical properties and are completely dense.
• Can be used not only to fabricate but to repair parts (something DMLS is not capable of doing).

Disadvantages?
• Severe overhangs are an issue because of a lack of a different material for support structures.
• Objects usually require some post-print machining.

Direct Metal Laser Sintering (DMLS)

DMLS is identical to SLS, except that the machine sinters metal powder instead of plastic powder. The model does not require a finishing stage.

According to custompart.net, DMLS is capable of printing steel alloys, stainless steel, tool steel, aluminum, bronze, cobalt-chrome, titanium, and ceramics.

One of the most common uses for DMLS is “rapid tooling,” which is the production of specialized tools that go in machines; these tools may be specific to one application in one machine and therefore aren’t produced in mass quantities. The cost of traditionally producing one of these pieces (by CNCing, casting, etc.) is extremely expensive and wasteful in comparison to printing it. DMLS has proven successful for making these sorts of parts.

Moldmakingtechnology.com has an excellent article on a 2003 collaboration between two companies (Morris Technologies and Extreme Tool & Engineering) testing the pros and cons of using DMLS to make molds. They were somewhat disappointed with the results.

Advantages?
• No waste is generated and little energy is used, as compared to traditionally machining an object. This is great for prototypes and one-offs.

Disadvantages?
• In Morris and ET&E’s test, the mold they printed had a considerable amount of warpage and required additional machining, as well as polishing. Bottom line: not a good solution for moldmaking.

Solid Ground Curing (SGC)

SGC was a process developed by Cubital Inc. of Israel. The technology is no longer being produced, as Cubital Inc. no longer exists (though Objet Geometries Ltd. of Israel retains intellectual property of the process). I have included it here because it is an interesting and unique approach to printing.

SGC uses photosensitive liquid in a layer by layer process, however the main difference is that it exposes an entire layer at one time.

Before the process can begin, a series of plates must be printed. Software splits the CAD model up into thin layers, and each layer is printed (2-dimensionally) onto a plate. The plate acts as a mask; any model cross-section is transparent while the rest of the plate is opaque.

The machine first sprays a layer of photopolymer into the working area. The first printed plate is loaded right below a UV lamp; a shutter opens and the entire plate is exposed to the photopolymer at once. The cross-section that was exposed hardens the photopolymer, and afterwards the uncured photopolymer is sucked up by a vacuum.

Next a layer of wax is put down over the unexposed area (evening out the bed) and the entire layer is milled so that it is completely flat (all of the excess material from the milling stage is vacuumed up). Another layer of photopolymer is sprayed on, the next plate is loaded, and the process continues.

At the end the model is contained within a block of wax, which gets melted away. No other post processing is necessary.

efunda.com has a nice illustration of the process (it is fairly difficult to explain):

This method failed because of high acquisition and operating/maintenence costs.

Advantages (while it existed)?
• Very fast and decently accurate (though not as accurate as SLA).

Disadvantages?
• Produces a lot of waste.
• Expensive in comparison to other methods (for both material and operation).

Jetted Photopolymer (JP)

JP printing is identical to InkJet printing, but instead of using thermoplastic, it dispenses photosensitive liquid. The print head contains a UV light; after a layer is put down, it is cured with the UV light. This eliminates the need for a separate curing process and is very accurate.

Advantages and disadvantages are similar to Inkjet printing.

Inkjet and MultiJet Printing

Inkjet printing is almost identical to FDM, however the plastic is held in a liquid state inside of the machine before being dispensed. As soon as the print head dispenses the liquid plastic, it cools and solidifies on the model. This process makes for very smooth finishes, however the print time is slow.

The print head on most inkjet printing machines consists of two dispensers; one for the thermoplastic, and the other for wax that acts as a support material.

Another feature typically included in this technology is a plane milling stage. Between each layer a plane is rolled over the model, cutting extraneous material off of the top layer. This ensures that the layer is precisely flat. This has something to do with the slow print speed.

This method sees some of the smallest layer thicknesses of any of the technologies: as small as 0.0005 inches per layer.

MultiJet is indentical to inkjet, but with many print heads simultaneously dispensing plastic. 3D Systems manufactures MultiJet machines (though they call them Pro-Jet) which have several hundred nozzles.

Advantages?
• Very accurate.
• Smooth surface finish.
• Supports are wax and can therefore be melted away.
• Quick print time for MultiJet.
• Milling stage means better accuracy.

Disadvantages?
• Slow print time for Inkjet.
• Supports must be melted– a separate process in itself.

Three Dimensional Printing (3DP)

The name is a bit confusing, since it doesn’t give any insight into the process, but 3DP is the term used to describe  Z Corp’s technology, which they employ in their line of ZPrinters.

The machine uses a bed of powder and a print head. The print head dispenses a plastic resin which binds to the powder and solidifies it. It spreads on a layer of powder with each new print layer.

Like SLS, the unsolidified powder stays in the workable area so support structures are unnecessary.

Z Corp’s claim to fame on their printers, though, is their ability to color the resin as the machine prints. This means that one model can be printed in any combination of colors; it is even precise enough to print text into an object. Taken from the Z Corp website:

In addition, it can print moving parts the same way that FDM can. The quality of the finished models is rough compared to other technologies, though the parts can be machined afterwards.

Advantages?
• 24-bit color system…
• Prints moving parts.
• No supports necessary.
• Relatively fast printing time.

Disadvantages?
• Rough finish.

Fused Deposition Modeling (FDM)

FDM is gaining ground in the printing world. It is arguably the most “printer-like” of all methods; it consists of a moving bed (Z-axis) and a print head (X-axis and Y-axis). The print head has a heating element in it; thermoplastic is forced into the print head, melts, and is squeezed out, not unlike toothpaste. The bed is usually cooled so that the plastic hardens soon after being placed down. Again, this is a layer by layer process; some systems can print layers as small as 0.178 mm.

From custompart.net:

Since it is squeezing out spools of plastic filament, there are different types of plastic available to print with. ABS is the most popular; if you are not familiar, ABS plastic is what LEGOs are made of. It can also print PPSF and PC plastics.

Stratasys, the developer and leading manufacturer of FDM technology, offers FDM systems that not only print parts as large as 36x24x36, but print moving parts. The machine can differentiate between separate objects within a CAD file, allowing you to print, for example, a system of gears that works right out of the machine. You can print objects within other objects, etc.

Support structures are necessary, but the print head contains a second element for extruding support material. Stratasys’ FORTUS systems (expensive) contain a bed of water soluble liquid that you place the models in after they print; the liquid disintegrates all support material from the model.

Advantages?
• ABS plastic means durable and functional models.
• Several different material options.
• Prints moving parts.
• Separate support material with removal system.

Disadvantages?
• Slow print time compared to some other methods.
• Rougher surface finish than SLA.

Laminated Object Manufacturing (LOM)

LOM uses a laser, but the material and process greatly differ from SLA or SLS. It is based on the idea of laminating material; that is, building up a model sheet by sheet with adhesive in between each sheet.

On the side of the bed is a roll of paper with adhesive on one side; at the start of each layer, paper is rolled over the whole bed and heated so that it adheres to the previous layers. A laser traces over the cross section of the model and then cross hatches over all of the extraneous material so that it can be removed later.

At the end of the process the model is contained within a block and you must brush off all of the cross hatched parts (which break off in cubes as a result of the cross hatching… see the picture below). The finished material is described by most as “wood-like.” It can be further machined like wood, though the model has to be sealed through whatever means in order to prevent moisture damage.

AZoM.com has a nice visual description of the process:

LOM machines have also been created that laminate plastic or metal sheets using the same process.

Advantages?
• Cheap materials.
• No support structures are necessary.
• Larger working area than most RP technologies.

Disadvantages?
• The excess material can be difficult and time consuming to remove.
• AZoM.com says that it has problems producing good bonds between layers, and that it has difficulty producing hollow parts.
• However cheap the material may be, it generates a lot of waste compared to other methods.

Selective Laser Sintering (SLS)

Selective laser sintering is similar to SLA. It uses a laser, but instead of photosensitive liquid, the laser heats a bed of thermoplastic powder. It sinters the powder, fusing it into larger chunks, again in a layer by layer process.

A roller first rolls a thin layer of powder onto the bed; the laser traces a cross section of the model, and when it finishes the bed moves down one layer and the roller rolls another layer of powder on.

From custompart.net:

To speed up the sintering process, the whole workable area inside the machine is heated up to just below the melting point of the plastic. This way the laser does not need to be as powerful and can move quicker through each layer.

Advantages?
• This method uses material similar to thermoplastic, so the models are rigid upon completion.
• A major advantage is that support structures are unnecessary; since the unsintered powder isn’t removed until the model is finished, it provides support.

Disadvantages?
• This process is not as accurate as SLA. Since it is difficult to control exactly how much powder gets sintered, models often come out grainy or with excess plastic on them.
• The models are also porous, so some sort of varnish is necessary to seal and strengthen them.
• The workable area must be cooled down when the model is finished, which, according to some companies that use SLS technology, can take up to two days.

Stereolithography (SLA)

Stereolithography is the oldest and one of the most common methods for printing. It prints by using either one or two lasers focused at a pool of photosensitive liquid. When the laser(s) focus on one point at the top of the pool, the liquid solidifies. This happens a layer at a time, each layer being typically about 0.1mm thick. Each time the machine goes to print a new layer, the bed moves down and more photosensitive liquid is poured in.

When the model is done printing it goes into another machine to be cured under an ultraviolet light.

From efunda.com:

So what are the advantages?
• The machine is very accurate and the vertical step size is small. This is good if you are looking for exact specifications.

Disadvantages?
• The models can’t be handled straight out of the printer, as they need to be cured first.
• Support structures must be printed depending on the specifications of the model, but because of the nature of the machine the supports must be made out of the same material as the model. This means that you have to cut them off manually after curing, which can leave artifacts (I have a first hand experience with this one).