Archive for the ‘Laser’ Category

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

05/20/2009

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.

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Direct Metal Laser Sintering (DMLS)

05/20/2009

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.

Laminated Object Manufacturing (LOM)

05/20/2009

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)

05/20/2009

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)

05/20/2009

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).