3D printing a submarine hull with a Hangprinter

We are Southampton University Human Powered Submarine Society (SUHPS), an engineering team from the University of Southampton.

We design, test and race human-powered submarines at international competitions; our next competition is the European International Submarine Races which will be held in the UK in the summer of 2022.

Each year we are required to design and build a new submarine and are encouraged to push boundaries with innovative designs. This year we plan to make parts of our modular submarine hull using fused deposition modelling. To do this, we will need to build a large format 3D printer, and we have decided to make it based on the open-source hangprinter created by Torbjørn Ludvigsen.

www.hangprinter.org

Due to covid, we don't have access to the facilities to build it, so we have started pre-assembling components to speed up the build as soon as we are allowed to.

The Hangprinter is similar to conventional "delta" 3D printers, but it uses a system of cables in tension to position the printing head, which is attached to triangular support (effector).

Goal

Up until now, we have raced with submarines built out of fibreglass composites. This approach, however, is limiting in that once the hull is built, modifying it becomes impossible, which limits our ability to improve subsystems such as the controls for the rudders.

We aim to print parts of the hull of our next submarine.

Ceiling plate

All the electronic components are mounted on a wooden board attached to the ceiling above the printing surface. It holds:

• The power supply units
• One Arduino Mega equipped with a RAMPS shield allowing it to control the stepper motors
• Four sets of stepper motors and spools to position the triangle that supports the printing head.
• A level converter to convert logic signals between 3.3 V and 5 V.

Since we have access to part of Southampton University's workshops, we will use a laser cutter to engrave the precise location of the components on the board based on the original Hangprinter documentation.

Aluminium extrusions and mounts will allow us to take off the ceiling plate and put it back on very quickly as well as position it precisely.

The spools, controlled by stepper motors, wind or unwind high-strength, low-elongation cables to move the effector. The steppers, which are normally open loop will be equipped with Mechaduinos to provide them with feedback, thus ensuring precise movements without slip.

Effector

The effector is the triangular structure that holds the hot end. It is connected to the electronics on the ceiling plate via ribbon cables chosen due to its lightweight and flexibility. It consists of three 400 mm sections of 12.5 mm aluminium square tube joined by 3D printed parts. A high-flow 80 W hot end with a dedicated power supply will allow us to keep printing times as low as possible. Typical desktop 3D printers use nozzles with an extrusion area of 0.13 mm². In contrast, we expect to print the final parts with 1.4 mm nozzles, which results in a nozzle extrusion area twelve times larger.

Printing surface

A circular MDF board will serve as the printing surface. It will be coated in PVA glue to increase adhesion due to the large sizes of the parts which increase the risk of curling. The printing surface will rest on triangular support made of Unistrut and custom waterjet cut aluminium plates. Screws on each vertex will facilitate levelling the surface should we have to move it.

Future development

Due to the experimental nature of the Hangprinter, we will iterate and improve on the design to adapt it to our needs. For instance, due to the large dimensions of our parts, we might have to find a way to heat the printing surface to avoid thermal stresses due to the temperature gradient.

The hangprinter will be controlled via a dedicated Raspberry Pi 4B coupled to a relay.

A full list of components and assembly instructions can be found on the Hangprinter website.