3d Model Resources

Where to Find 3-Models for Printing

Thingiverse – When you say 3-D printing files, this is the site that most people think of it. Easy to search, and almost every file posted is designed and optimized for fabrication via 3-D printing.

GrabCAD – GrabCAD is Thingiverse’s older, PhD holding brother. Designed as a site for engineers to share CAD files, GrabCAD models are generally capable of being used for 3-D printing, but additional optimization or repair might be needed.

Modification and Repair

-If you’ve got a file, and it’s not in a shape that you can use, there’s a few options.

NetFABB – A handy tool for chopping apart, inverting and resizing models. It also can convert files from one format to another, which is useful if you need to get a .STL file from another format. It’s available as freeware. Paid versions provide even more usability.

Microsoft STL Repair – If you have a file that won’t slice properly, or isn’t watertight, and causes issues during printing, give it a run through this free tool that NetFabb and MS developed together. This program is super useful since a lot of freely available 3d models aren’t always designed with printing in mind.

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Acetal Hypothesis…Confirmed!

As you might have read in a recent post, I had a gut feeling that the reason Acetal filament fails so catastrophically compared to Nylon might be a function of the thermal conductivity of the print bed. The standard aluminum bed of the printrbot simple, while great for a heated setup, works as a heatsink for materials. Acetal/POM, which has a low thermal conductivity easily shifts its heat over to the printbed, and cools down too quickly.

However, that’s just a guess. The only way to confirm this would be a trial with a less thermally conductive bed. Thanks to the wonderful folks over at the Metrix Create:Space over on Capitol Hill (in Seattle), I was able to get some acrylic sheet and composite board cut to replicate the bed of my simple[1].

After attaching the bed to the simple, I ran some trials with PLA to get everything calibrated in and setup.

Then I began running the Acetal.


I think the white acrylic is pretty slick looking.

Results were interesting. The material initially printed and laid down well, and was actually beginning to look quite great. As is the case with POM/Acetal, the filament was extruded as a clear plastic, and stayed clear for some-time. It began to cool down, and return to its natural white color.

Unfortunately, the print began to curl at its edges, and at about the 7th layer, it curled up enough to catch the print head and get pulled off the bed.

The “finished” piece is below:


So yes, there was a failure, but there was also forward progress. This is the farthest I’ve gotten in a POM Print, and it confirms that one of the limiting factors in getting accurate prints with this material is its ability to control the rate at which this material cools.

Next steps, are to begin sourcing parts and materials for a heated bed, and put together a heated bed setup. Also, to get a large rubbermaid container to place the printer in during prints, to try and limit the amount of formaldehyde produced from printing this material at high temperatures.

[1] eps file for printbed 

[2] Printed at 245C for First Layer, 235C for Subsequent Layers. Print layer height of .4 MM

Thermal Conductivity

As part of my interest in printing with POM, I’ve noted before that the largest issue with the material is the incredibly rapid rate at which it cools down. It dumps all of its heat into the environment around it so quickly that it audibly pops off the bed.

Thermal conductivity, is a measure of how quickly something absorbs and transmits heat. If you look at a simple table (such as this one), you can see at a glance, the values for different materials.

Looking at PLA, thermal conductivity is a mere .13 W/m-K, as borne out from the experience any of us have had grabbing a fresh print off the bed without allowing it to cool. It absorbs heat slowly, but also releases it slowly.

Looking at Acetal/POM, it’s only a mere .23, Nylon is .26 and something like Phenolic Resin (a common choice for high temp applications) is only .15

Air itself is only .024. Which, by the way, is the reason insulating gear/clothing for the outdoors is built around having fibers that trap air. Your body heat gets the material “up to temp” and then it takes a very long time for it to cool back down.

So, given all of that, it raises the question of where the heat present in the POM that’s being printed is going. The answer to that is in the bed. Aluminum has a thermal conductivity of 205!

What that means then, is that the aluminum bed allows prints to rapidly cool down, by serving as a heat-sink for the print that you’re making. Now, normally, this wouldn’t be much of an issue. PLA works just fine afterall, and aluminum printbeds are a standard for the ease at which they can be made into heated beds.

However, my hypothesis is, that by using a bed substrate which has low thermal conductivity, it might be able to just barely push the needle to make printing in POM work without adding a heated bed.

Looking at the list, Balsa wood stands out. It’s cheap, it’s the lowest thermal conductivity of any wood, and it’s easy to laser-cut. Acrylic also stands out for similar reasons. The advantage that I believe Balsa wood, or any wood presents is that wood has greater porosity, which means more sites and locations for the extruded plastic to form mechanical bonds with the substrate.

Which means that my next steps are:

  1. Create a pattern based on the current Printrbot Aluminum Bed I have.
  2. Purchase Acrylic and Balsa Sheet at a size near to the current bed.
  3. Have the materials laser cut per the pattern I create. (Seattle has just such a space, and I’ve been meaning to check it out anyways)

Expect to see a post in a week or two outlining how this all goes for me!

Build a Prosthetic Hand – At Home!

Building off my prior post, if you’re at all interested in working with the good folks of E-Nable, I can’t recommend enough that you sign up, and then join the Google+ group.

They are doing some amazing work on that site, and it deserves as much recognition and help as possible.

But say you just want to make the hand? Maybe you need a decoration, or just want to show off the capabilities of your printer.

Check out the Raptor Hand by E-Nable then. Designed to be made almost entirely out of 3D printed parts, it’s a really fancy looking piece of kit, and a great test of your skills.

http://enablingthefuture.org/upper-limb-prosthetics/the-raptor-hand/

It’s particularly a good test of how your printer handles tolerances, and how calibrated it is. If things aren’t correct, the pins which snap the flanges and fingers together simply won’t work right (either falling out or not going in), and the “ligaments” won’t thread correctly. If you want to be even more futuristic, take a look at the limbitless arm, which is the only 3D printable open-source myoelectric arm that I can think of.