Thursday, March 25, 2010

Fastners for 3D Printing: Screw it, we'll use science. Part 2 of n

Previously on I Heart Robotics the internet blog, we used the power of science to test wood screws as fasteners for 3D printed parts. Now that our plastic thread forming screws have arrived and we have a few moments, we will use the power of science to test these as well.

The primary problems we saw with using wood screws was that the flat head required a countersink and they have a tendency to split the screw boss at high torques. The plastic thread forming screws we have sourced are available with a pan head and Torx® drive. The Torx® drive allows for better torque transfer and less chance of stripping the screw head.

The screws used in this test are Camcar® PT® Thread-Forming Screws for Thermoplastic from Acument® Global Technologies. On the datasheet they are listed as K40-1.79x10TXP P/T or as part number 3BT-P8009-00.
In New York City, you can order them from Century Fasteners.


For each experiment two plastic parts were printed. One part had a series of square screw bosses and the other part was a plate with a series of holes. The assembled parts were placed in a vice and a Ryobi cordless drill model P205 with adjustable torque was used to tighten each screw until failure.

The torque settings on the drill produce an unknown torque and have an unknown error and unknown repeatability. However for the purpose of comparing the relative holding strength of each fastening method, these issues should not prevent a rough estimate from being made.

The parts were printed in ABS on a Stratasys FDM Machine with the holes oriented upwards. This is the same material as Shapeways Grey Robust.
As noted, some holes are printed at the desired size and some were printed as pilot holes and drilled to the required size.

Failure Modes

Stripped threads are by far the most common failure mode. This failure indicates that the threads are not capable of holding a higher load.

This photo shows a cracked screw boss which is caused by the hole being too small, forcing more material to be displaced by the screw. It is also a limitation of the 3D printing process due to the inter layer bonding.

Split bosses are caused when the holding force of the screw thread is greater than the structural strength of the plastic. Using an aluminum outer screw plate would allow greater torques to be reached but the usefulness of that may be limited.

Here stress marks on the side of the part indicate the location of possible failures.

Experimental Results

Each screw was tested 6 times and the results have been uploaded to Google Docs. The updated information is available on the sheet named "Plastic Thread Forming Screws".


These screws are great for 3D printed parts. For the specific screws used in this test, printing a 2.8mm diameter hole will work great. If you need greater holding strength then print pilot holes that have a 1mm diameter and drill them out to 3.1mm. One possible disadvantage of these screws is that they require a Torx driver which you or your customers may not have. At some point in the future we will be testing additional fasteners.


Loren said...

This is of interest for building things. Do you know of anyone who has performed load tests on printed parts in general compared to molded or extruded parts?

The big question would be how much heavier a printed part would have to be to match the strength of the other.

I Heart Robotics said...

That is actually a really complicated question.

A material like aluminum or injection molded plastic will generally have a uniform density and attributes such as the modulus of elasticity and yield strength do not change with respect to orientation.

3D printed parts however are anisotropic and the maximum tensile load in the Z axis could be several orders of magnitude less than if you applied the same load along the X or Y axis. The Z axis in this case would be the vertical axis through the extruder head.

The problem with trying to scientifically measure the mechanical properties is that most of the test equipment is designed to apply much larger loads so the measurements are often within the noise margins of the tensile test machine. If you are really interested I can try to find the test data I have, though I'm not sure how much insight can be gained from it.

Looking at it qualitatively instead of quantitatively he biggest problem fused filament fabrication machines such as the MakerBot, RepRap and StrataSys Prodigy Plus have for producing robust 3D printed parts is interlayer bonding. Different process such as <a href=">SLS</a> may be able to produce more isotropic parts.