3D Printing Mixer Extruder

The evolution of the Self-Replicating Machine – Taking the next step in the development of increasingly sophisticated low-cost 3D printing


An increasing choice of relatively low-cost 3D printers from many manufacturers is fast enabling the technology to become a key tool for prototyping in the design and development of electronic and electromechanical products. However, a major dynamic within the 3D printing arena is the self-replicating machine. Leveraging the open source movement, the Replicating Rapid Prototyper (RepRap) project is an initiative to develop a machine that can print the majority of its own components, thereby providing the potential to put 3D printing capability into the hands of everyone at relatively low cost.


RepRap Printers – Today’s Technology

RepRap printers – the latest of which is the Ormerod – employ Fused Filament Fabrication (FFF) technology, which is also known as Fused Deposition Modelling (FDM).  This creates 3D objects from extruded thermoplastics: essentially melting strands of plastic that are deposited in fine layers in an additive manufacturing process. More specifically, a spool of biodegradable PLA (polylactic acid) polyester-type material is fed into a heated chamber and squirted out via the small nozzle of a computer-controlled plastic-glue gun: this makes the first layer on a baseplate, which is then lowered by a very small amount ready for the second layer to be added on top of the first, and so on. An overview of the process used in the RepRap printers is shown below.

The RepRap 3D printer process (courtesy of reprap.org)


The Next Step – Mixing Materials

Almost all of today's low-cost 3D printers work with a single material.  A few can work with multiple materials, but they do so separately; for example, if there are two materials - red and blue - then you can print an object that has both red patches and blue patches.

The current RepRap Ormerod is a single-material or monochrome 3D printer that has been configured to work with one type of plastic at a time. But it is designed so that its next stage of development will leapfrog ordinary multi-material printers to give a material-mixing printer.  Fed with red and blue it will be able to print in red, in blue, or in any shade of purple in between.

But it will not mix just two materials, it will mix five.  This means that when fed with white, black, cyan, magenta and yellow it will be able to print in any colour.  Although, to set expectations at the right level, the FFF process used by the RepRap printers will not offer the instant sharp transitions in colour that can be achieved in a photograph produced by an inkjet printer.

But it doesn't end with colour.

While not all materials will mix well together, there are many mixable plastics that can be printed and that offer different properties: some being quite soft and flexible while others are hard and rigid. By changing the proportions of a mixture it is therefore possible to print an object that has any desired degree of stiffness at any geometrical position inside that object. Interestingly, the building of objects with graded mechanical properties is actually a trick used by living organisms, which have control at the microscopic scale and build upward from the cells. A human head for example has a rigid bone skull yet a bendable cartilage-based nose. Bone and cartilage are made from similar materials, but when mixed together in different proportions, they will provide parts with different mechanical properties.

To this end, RepRapPro Ltd is currently developing an extrusion head or nozzle that will allow the mixing of different materials together to achieve combined mechanical properties and colours. Building upon contributed research from students at the University of Bath (papers are available here), the RepRap prototype mixer extruder is capable of processing inputs from multiple PLA (polylactic acid) filaments to achieve materials that have additional colours and/or mechanical properties. Based on an existing RepRap printer, software and firmware has been adapted and written to meet the requirements of the new mixer extruder design. Demonstrator objects in many different colours as well as objects with gradually changing material properties have already been achieved with prototype mixer-head designs.

The development of this filament mixing capability means the RepRap project has made an important step forward, enabling the fast and easy production of full-colour objects as well as the building of much more advanced components that use both hard and soft plastics. The development also moves the project closer to fully exploiting the possibilities of fused filament fabrication as a 3D printing technology, and ultimately being able to combine very different materials, such as metal alloys or resins along with thermoplastics, in a single-shot production process.


The RepRap Project

The RepRap project was founded in 2004 by Adrian Bowyer when he was a senior academic in the Mechanical Engineering Department at Bath University.  He is now a Director of RepRapPro Ltd, which is one of the companies based on the RepRap project.

According to Bowyer, there is a symbiotic relationship between RepRap machines and people: the machines are like flowers that provide nectar for insects, except that they provide goods for people; and the people are like the insects that will help the flowers to replicate, except that they put the RepRap machine together. However, unlike flowers and insects, the self-replicating machine will not evolve by random mutation.  It will be be changed by selective breeding in the same way that we made dogs out of wolves.  The design of the machine will inevitably and deliberately be improved over time, making the printer more accurate or easier to build perhaps. Because the design files are made readily available via the RepRap community, the better designs will have greater reproductive fitness and so spread more widely.

Importantly, while a RepRap machine is able to make all of its plastic parts, the project specifies that all the remaining components required for the construction of a duplicate machine, such as the electric motors, electronics assemblies and various other materials like metal threaded rods, must have two properties: firstly, they must be relatively cheap and simple to obtain; and secondly they must be easily available to everyone everywhere in the world.