Field Testing the Viability of 3D-printed Oyster Farm Equipment

Material and tool suitability

    The common small-scale 3D printers originally envisioned for this project were not suitable even for small numbers of prototypes. The plastic filaments determined by project partners to be suitable for marine use required higher operating temperatures and printed much slower than more commonly used materials. After initial testing on available equipment, nylon filament was calculated to take 60 hours per bag with regular intervention required by the operator.  Fabrication using this method would require far more labor than was budgeted for in our proposal and would tie up equipment and resources at partner organizations and schools. It would not be cost effective even for small numbers of prototypes.

    Laser cutting was performed on PETG and a sample of UHMW polyethylene (a plastic related to VHMW polyethylene with longer hydrocarbon chains) by cooperating educator Paul Meinersmann (Technology & Makerspace Director St. George Municipal School Unit). A single PETG bag was cut and assembled, though we ruled out constructing more bags using this method and material. Cutting time alone took 4.5 hours and produced residues that would be harmful to the tool with continued use. The UHMW polyethylene took multiple passes for the laser to cut at full power (60W), and the material was also ruled out as a viable option.

    With this realization we pivoted to exclusively using CNC routing with sheet plastics at Haystack Mountain School of Craft's specialized fabrication lab (FabLab).

    The additional workload at Haystack had to be fit in around existing programming at the facility, which set back our field trial. Our extension into the 2023 field season allowed us to collect a full dataset included wear, damage, and fouling caused by overwintering. 

Wear and damage in the field

Damage was measured in several ways

• by material
• by location on bag
• by position on farm

    There were strong differences in durability between materials. The PETG bags were far more fragile than HDPE and VHMW polyethylene. PETG was more than twice as likely to be damaged than HDPE (the standard material used in aquaculture) when built with the same construction methods (3.4 instances of damage per bag for PETG compared to 1.6 instances of damage per bag for HDPE). VHMW was even more durable, averaging only 1 instance of damage per bag.

    These differences were even more pronounced when examining the number of bags that experienced some form of structural failure (requiring repair/reconstruction before being able to perform the basic function of retaining oysters). 43% of PETG bags had a structural failure over the course of the experiment, compared to 13% of HDPE bags and 0% of VHMW bags.

    PETG was much more susceptible to cracking and tearing than the other materials, but was less likely to register chafing or erosion. It is unclear whether the the susceptibility to cracking was a product of the material, or the thinner sheeting used (though fabricators advised that thicker sheeting would be less flexible and even more brittle). VHMW saw less damage from chafing, cracking, or tearing than HDPE, but saw more discoloration. This could, however, be due to the fact we used a white variant of this material that made staining from fouling organisms more apparent. We did not see the expected sun-bleaching discoloration in any of the materials tested.

    Most of the consequential damage observed on bags occurred on folds, corners, and bag closures. This damage was found on split or cracked folded-edges, on welds, or most often- on broken tabs. Though chaffing/erosion was recorded, this damage was largely superficial. There was no structural damage associated with discoloration- (which was connected to with staining from contact with fouling organisms instead of sun damage). 

    Floating bags and bags deployed in the top bay of oyster-cages saw more damage than bags in the bottom bay of oyster cages. Given the types of major damage observed in the project (broken tabs, seams and closures), this seem likely due to mechanical stressors instead of light exposure (which would most affect the sun-facing panels). The top bays of oyster cages at Winnegance are tighter than bottom bays (pontoon fasteners and structural reinforcement take up space). Tabs and edges of bags rubbed and caught-up on the edges of cages- particularly when bags were removed for handling. Floating bags faced the stress of holding oysters without the added structure of a cage- putting extra strain on weak point like scored seams, welds, and tabs.

Other damage

A protective film was found on some of the PETG sheets. While most of it was removeable, some bags had sections where the film was fused to the sheeting. Over time portions of this fused film began delaminating.

On bags with 4mm holes, incomplete routing left some small circles partially attached to the plastic sheeting. With time and handling, some of these plastic circles were shed into the water.

These material and fabrication flaws presented plastic-pollution sources that should be eliminated in future prototypes.

The lone laser-cut PETG bag was constructed of smaller panels of plastic which were welded together. These welds were a weak point and failed during a large storm early in the project.

Fouling

The two field sites were situated in dissimilar environments and saw very different communities of fouling organisms.

Fouling at Winnegance

    Winnegance Oyster Farm- located on an inshore bay experienced an abnormally high fouling year in 2022. Bryozoans, mussels, sea vases, and barnacles were the most prevalent fouling invertebrates.

    VHMW had the highest peak coverage of the most difficult to manage fouling species- mussels (7.5%) and sea vases (5.3%) - though only slightly higher than observed in HDPE (mussels 5.4%, sea vases 4.25%). VHMW also saw the highest peak fouling of crust-forming bryozoans (8.4%). HDPE saw the highest incidence of barnacles (4%)  and coral-like bryozoans. Despite being deployed for the shortest time of all materials in the experiment, PETG bags had the highest peak of feathery-bryozoan fouling.

Filamentous ulva fouling was similar across materials until the final set of measurements in 2023 when much higher levels were found on VHMW (37%) compared to HDPE (18.5%) and PETG (7.5%). VHMW also saw fouling from filamentous red algae (7.5%) in September of 2022. It is unclear whether the higher fouling on VHMW sheeting was a product of the material or its color. 

Fouling at Hurricane Island

Hurricane Island's offshore field site saw a different collection of fouling species. Tunicates and jingle-shells accounted for the majority of invertebrate fouling at this site.

At peak, HDPE saw more fouling by both orange tunicates (15.5%) and red tunicates (7.5%)  than observed on VHMW (5%/0.8%) and PETG (0%/0%). Jingle shell fouling was most prevalent on PETG (4.5% at peak) than on HDPE (2.4%) and VHMW (0.3%)

Algal fouling at Hurricane Island was dominated by brown algae (recorded as a group, with the exception of kelp which was recorded separately). Ulva lactuca (sea lettuce) was also abundant on PETG during October 2022 measurements.

Handling

    The experimental gear had some immediate benefits. The flat, drilled surfaces of CNC routed bags (versus the woven texture found on commercial gear) prevented oysters from clumping and from getting stuck in bag corners. Flat surfaces were also far easier to effectively clean with a paint-scraper compared to woven-texture commercial bags. This easy manual cleaning made air-drying even more effective.

    VHMW bags were exceptionally rigid for their thickness, making them easier to handle when holding a heavy load. The rigidity of this material could allow thinner sheets to be used, which could potentially reduce both costs and fabrication time.

    Clear PETG sheeting made it easy to observe oysters in bags without having to handle them (though this benefit may diminish if bags discolor over time).

    The experimental bags had some major handling drawbacks as well. Bags were heavier than standard commercially available equipment and were much more slippery.  Bag closures required additional fasteners and were difficult to use in the field.  The tabbed construction protruded from the edge of bags and caught on the wire mesh of oyster cages. Making removal from cages difficult and caused damage to tabs.