Printing Mega-Benchy - A Comparative Evaluation of Extrusion Limitations

Experiments, MDPH2, Pellet Head Extruder -

Printing Mega-Benchy - A Comparative Evaluation of Extrusion Limitations

Large format printers possess capabilities that have allowed us to overcome what were seen as previously insurmountable barriers in the world of 3D printing- but that is not to say they do not have their own limitations. Lest we fall prey to the “bigger hammer” fallacy, it is important to recognize that as with any tool, large format printers excel at certain tasks while actually underperforming smaller printers at others. In broad strokes, we celebrate large format printers not only for their larger build capacities but also for their speed. Even if you put a filament-driven print head on a 4 foot square build envelope, the polymer output rate is going to mean that a 4 foot tall print is going to take an exorbitant amount of time. The same print on a pellet head capable of pushing out extrusions through larger nozzles will take a fraction of the time. However, the generally accepted shortcoming of these fast, high output prints is that they lack the resolution and fine detail of the same print accomplished with a sub 1mm nozzle. 

There are a number of other problems to overcome while printing with large diameter nozzles. Larger diameter extrusions have significantly more mass and this can be seen any time you ask a print to resist gravity. For example, a 2.5 mm extrusion is going to sag when bridging more severely than a 0.4 mm extrusion. A similar effect can be observed while printing overhangs, although the slope (or rise over run) will be the same between a 2.5 mm nozzle printing 1.25 mm layer heights and a 0.4mm nozzle printing .2mm layer heights, the additional mass of that leading edge is more likely to roll over to the outside of the print with the large extrusion. Additional cooling efforts are required as well, to keep heat pockets from building up in areas of deposition that give gravity a longer window to enact changes on the geometry. 

Even if you are brand new to the world of 3D printing, you are likely familiar with Benchy. Arguably the most popular single print of all time, it was created as a “stress test” for printers and various print settings or configurations. Different parts of the geometry are designed to challenge the printer, and dimensions can be checked with a caliper against the model reference for accuracy. For example, the roof of the cabin and the tops of the porthole windows provide overhangs, and the outside of the hull is a large, smooth expanse to reveal any irregularities in surface finish. The original Benchy is 60 mm long, and at a low infill prints in just an hour or two on most machines. Most of you have likely printed a Benchy, and so have we- but it occurred to us recently that we had never printed a Mega-Benchy.

It doesn’t require an actual test to see how a large format printer equipped with a 3mm nozzle would do trying to print the original scale Benchy. Forget about capturing the detail of the wheel in the cabin - all of the walls of the cabin itself are too small to be captured by that large of a line width. The slicer software preview is all you need to see to know it won’t work. But how well can a 3mm nozzle perform on a scaled up Mega-Benchy? We thought it was worth trying to have a comparison to see how some of these challenges stack up when scaled. 

Mega-Benchy is 20” long, 15” tall, and weighs in at just under 11 pounds. It is about nine times larger than the standard dimensioned Benchy. A standard nozzle for a filament-driven printer is 0.4 mm, so to keep scaling 1:1 we would have had to use a 3.6mm nozzle. We cheated a bit and used a 3.0mm nozzle. Sliced with 1.5mm layer heights and 20 percent infill the print time was around eight hours. It was printed from LX-175 (PLA) on the MDPH2. We actually thought it printed pretty well! The surface finish on the hull was quite smooth and consistent, the whole print was symmetrical, and there wasn’t any warping. The overhangs of the hull printed fine, and the portholes and windows/doorframes of the cabin printed decently, though they had imperfections at the very top of the circles. The cabin roof was, quite frankly, a mess on the inside- though it did manage to level out by the time it was printing the top surfaces. This showcases the extra difficulty presented by bridging with large extrusions. The chimney was mostly smooth, but had a fairly ugly z-seam on one side. We were surprised at how well some of the details came out, such as the steering wheel- however, the tiny text on the back of the boat is illegible. Overall there was a lot of stringing on the print due to using undried polymer, and about halfway up the cabin walls the rechopped pellets got clogged in the feed throat for a minute and the extrusion petered out, though it impressively got back on track and finished printing! 

It is somewhat common knowledge that despite being a boat, Benchy doesn’t really float. There is not enough ballast in the hull to offset the weight of the tall cabin, so it flops over and then takes on water, sinking. Despite having seen this happen before, we couldn’t help but try to float our Mega-Benchy down the stream that runs next to our warehouse. You won’t be surprised at what happened… it sank. Not all experiments succeed! Perhaps a bigger, better Benchy is next, or the same size but a smaller nozzle. We were able to find a few examples on the internet of even more mega Mega-Benchys than what we printed! Have you printed any unusual Benchy models?  We’d love to hear about them, feel free to share them in the comments!

 

Below are some photos from the experiments:

1. S.S. Mega-Benchy

2. Hull surface finish

3. Porthole and front window overhangs

4. Door overhang

5. Chimney and roof steps

6. Steering wheel


2 comments

  • Massive Dimension Team

    Cameron, we documented Mega-Benchy’s maiden voyage down the culvert stream outside of the warehouse. It is not much action, but there is some.

    Watch here: https://www.youtube.com/watch?v=McqGBM3QvoA

    If you mean the action that comprised the actual printing of Mega-Benchy I’m afraid that though it was live-streamed the recording was deleted from our Twitch account before we transferred it to Youtube.

  • Cameron Pauline

    Can we see it in action?

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