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Here are some 3D printing facts that one should keep in mind when reading an article on the topic from media sources that don't really understand the technology.

3D printing is slow. It can take hours to print the parts for even a very simple prototype.
3D printed parts are not suitable for any sort of load-bearing application. They are not structurally sound. The posted article is very short on facts, but I can say with absolute certainty that they did not 3D print the actual engine. There has been some experimentation with 3D printing metal powder, which is then cast. I think this is what was used for the 3D printed gun. But this technology is still very experimental.
3D printing can be tricky to get right, and has problems with parts that have certain geometric characteristics (for example, it doesn't do parts with sharp overhangs).

So given all that, no, 3D printing is not even remotely close to creating the "Eventual Doom of UAW". In fact, if robotic assembly didn't do that, it's hard to see how 3D printing will.

3D printing is great for prototyping small parts. For example, at my job we 3D printed a 3D model of a populated circuit board so that we could check that the board when installed in the server had adequate clearance and wouldn't hang up on anything when the server was inserted or removed from the rack.

3D printing is not yet a substitute for more traditional methods of manufacturing such as subtractive methods (milling, machining), casting, etc.

All that you say is true. However, imagine a day not too far off when every home has a 3-D printer. Buying the composite material , the non-printable components and paying the license fee for the design (or pulling one from public domain) means I can get a 2nd car or a commuter vehicle built in my own garage for the investment of a few weekends. That makes the 44 hour print time essentially a non-issue.

Not everyone will want to build their own car just as some people don’t like mowing their own lawn but it could easily be possible and could be very competitive.

3D printing, robotics, Cad/cam = end of UAW. Small shops can make custom cars all over now, no need for big manufacturing plants. A guy with a big garage can make custom engines with automatic 5 axis milling machines, etc. Yes the paradigm is changing allowing for decentralization of assembly of automobiles.

Where I see this having a near future payoff is where they print the battery for the electrics into the body as they print the vehicle.

3D printing is also useful for automotive restorations.

Let’s say I have a broken front headlight bracket for a 1941 car that I cannot find anywhere. Through 3D printing I can scan and reverse a driver-side part and mirror image it for the passenger-side part.

3D printing is not yet a substitute for more traditional methods of manufacturing such as subtractive methods (milling, machining), casting, etc.

You'd better get on the horn and let SpaceX know about this, as they are now using 3D printed rocket engines.

"Compared with a traditionally cast part, a printed valve body has superior strength, ductility, and fracture resistance, with a lower variability in materials properties. The MOV body was printed in less than two days, compared with a typical castings cycle measured in months. The valve’s extensive test program – including a rigorous series of engine firings, component level qualification testing and materials testing – has since qualified the printed MOV body to fly interchangeably with cast parts on all Falcon 9 flights going forward. "

http://tinyurl.com/pgjup4j

A few comments:

3D printing is slow - true for most plastics printers though they are getting faster. The laser printers appear to have a greater spead depending on the wattage of the laser

Load-bearing ... depends on the load. Some 3d printers can print in ceramic, concrete, metal etc. Some FDM 3d printers (most home plastics printers) can print in polycarbonate which is a fairly strong plastic. SLS laser printing “welds” the metal powder and directly forms the part. Further, 3d printers can be used to print molds that are then cast. So this can really cover a lot of the manufacturing process.

A point that also needs to be brought up is the accuracy requirement for a high efficiency modern engine. The thousands of an inch requirement means that, for now, the only real 3d technology able to print engine parts is SLS which is more expensive than the car.

Sharp overhangs in plastics can be accomodated with support structures that are removed in the post production finishing. There is even a water solulable extrusion that can be printed as the support material. This enables a part to be printed, then soaked in water to remove the support structures after the part has been printed. Again, such systems are beyond the price range for most home users at this time.