In some previous posts I have discussed the pros of 3D printing of fountain pens, but alongside those advantages there are some important downsides. This post discusses them and the strategies to mitigate them that have been used in the design and manufacture of Platypus pens.
In order to understand the difficulties for fountain pen making that come with the pen being predominantly 3D-printed, we need to have a clear idea of the 3D printing process.
Platypus fountain pens are made using a type of 3D printer that extrudes a thin stream of molten plastic—a type of printer that is usually described as FDM for ‘fused deposition modelling’. The extrusion head of the printer moves around in a programmed manner in a 2D plane to deposit a thin pattern of molten plastic which rapidly cools and solidifies. Layers of plastic are built up one upon the other to form a 3D shape from 2D layers.
If you are interested in learning more about 3D printing then you can read some of my other blog posts, and I recommend the video series about the basics of 3D printing created by YouTuber Tom Sanladerer.
The layer by layer process intrinsic to 3D printers allows for a remarkable range of object shapes and designs to be formed, but that flexibility comes with a downside: 3D printers are quite slow. The components for one Patypus fountain pen take about four hours to print altogether. (And then more time is taken to assemble and finish the pen, but that is not the topic of this blog post.)
Rapid prototyping and development
Despite their slowness 3D printers are widely used in the process called ‘rapid prototyping and design’. A 3D print might takes hours to complete which stands in contrast to a few seconds to injection mould an object from plastic, but the 3D printer is ‘rapid’ because the tooling for the injection moulding process might take weeks or months to make or procure. It’s the total turnaround time from design to inspection and testing that is rapid with a 3D printer, and that short turnaround facilitates design iteration and optimisation prior to the product being manufactured by more conventional methods.
The 3D printers that sparked the development of consumer-level 3D printers were called RepRap in recognition of that role: the “Rap” part is short for rapid prototyping. The “Rep” part of RepRap is short for self-replicating. The first RepRap machine was able to print many of the components of itself, which were then used to make a second RepRap machine which printed the parts for a third, and so on. My second 3D printer—the one on which I print the Platypus pens—is constructed with parts printed by my first, and the first was repaired and improved with new parts printed by the second: a nice RepRap circle.
It may not need to be said, but Platypus pens are the product of rapid prototyping and iterative design.
Limitations of FDM printing for pen production
The layer by layer process of FDM 3D printing brings some important limitations to the objects that can be printed, and those limitations are relevant to fountain pen production. Here is a list of some. Each will be dealt with in turn below.
• Surfaces are not smooth
• The dreaded seam and pimples
• Weakness at the layer lines
• Threads are difficult
Surfaces are not smooth
The surface of an object printed with a 3D printer will display some sort of evidence of the layer by layer printing. With the FDM printing used for Platypus pens the layers present as crests separated by valleys as a result of how the filament is extruded from the printing nozzle onto the layer below. Using finer layers (i.e. layers that are less thick) gives a smoother finish because the crests and valleys are small, but there are more of them. The diagram here represents cross-sections through a wall of an object printed with thick layers (say, 0.3mm) and with finer layers (0.15mm).
The roughness of layers is often considered to be a limitation of 3D printing and a deficiency in the finished objects. However, Platypus pens are designed so that the layer lines contribute positively to the quality of the resulting pens: the pens would not be produced with completely smooth surfaces even if it was possible. To a large extent, the unusual and eye-catching character of Platypus fountain pens comes from the layer lines. See the blog post Layers of Sheen.
Even the un-patterned grip section of these pens benefits from the layer lines as they make the grip more ‘grippy’ and mean that even sweaty hands can hold a Platypus fountain pen without slipperiness.
The dreaded seams and pimples
Any time an object is being printed layer after layer the printer nozzle has to move from the end of one layer to the beginning of the next. Sometimes that movement is a simple vertical jump, but most often it involves horizontal movement as well. The hot nozzle contains molten plastic and so inevitably some plastic leaks out during that movement and is deposited into and onto the object being printed.
That problem is typically combatted by having the filament retract suddenly just before the layer change and then un-retract when the nozzle reaches the start of the next layer. The retractions and un-retractions almost always leave visible defects on the surface of the object, defects that are called seams where they are vertically aligned and pimples where they are randomly scattered over the surface.
Platypus pens are designed to almost entirely eliminate this problem. Most of the components are printed in a single layer and so there is no seam or pimples. Well, not exactly one layer, but the extrusion path is a single helix. Single wall helical printing is often called ‘vase mode’ printing, and it eliminates seams and pimples by removing the need for filament retractions and un-retractions.
There is one component in the Platypus pens that is printed in the conventional layer after layer mode: the section inner. It does have a seam at the thread that connects to the pen body. You may need a magnifying glass to see it because the part was printed with a high quality printer fitted with a direct drive extruder and the printing settings are optimised to minimise the seam.
Weakness at layer lines
The layer by layer nature of 3D printing usually leads to uneven strength. The plastic is strong in the direction of its layer plane because it consists of a continuous strand of extruded plastic, but the object can be weak at the layer junctions because the plastic of adjacent layers is incompletely welded together.
That inter-layer weakness is not much of a problem for thick-walled objects and for solid objects, but for a long thin-walled thing like a fountain pen it can be critical. Furthermore any weakness is especially problematical when the object is printed with a single wall, as are most parts of a Platypus pen! What to do? Well, the solution to this problem is conceptually easy: make an epoxy-bonded sandwich of nested components. The strength of the sandwich is much, much higher than the strength of the two printed layers.
The cap of a Platypus pen has an outer layer made with shiny, colourful filament and has an interesting surface pattern and the plastic. That layer, by itself, is pretty weak, but that doesn’t matter because it is sandwiched together with a liner made of a different plastic selected to suit its structural and functional roles.
The body is made of three parts glued together with epoxy: the outer thread and band which slip into the end of the body proper, and the inner threaded liner that extends almost to the end of the body. After being sandwiched together those components make structures that are much stronger than the sum of their individual properties would suggest.
The section has a single wall helically printed coloured grip on the outside and a conventionally printed inner. The only part of the pen that is entirely one piece is the section thread that goes into the body. It is made of a modified PLA that has strong inter-layer adhesion and is not brittle. It is relatively thick wall where it meets the grip and is more than strong enough when the pen is fully assembled.
Threads are Difficult
A process of rapid prototyping and iterative design was used to come up with the several strategies employed to help make robust and effective threads for the Platypus pens.
Strategy 1: the male and female parts of the thread have different layer heights. That reduces the possibility of the threads interlocking on the layer lines and improves the feel of the threads in use as well as reducing wear.
Strategy 2: use a thread cross section that ‘knows’ that the thread is being 3D printed. My usual 3D model design software is the open-source CAD program OpenSCAD, and there is a very useful thread library available for it that includes a huge range of official thread designs. They work well as long as the thread size is large relative to the layer height and precision of the 3D printer. Unfortunately the threads of standard nib units are very fine and so I had to develop my own software for threads. (Yes, it involved a process of rapid prototyping and iterative design!) On a platypus pen only the male section to body thread is a standard design (M10x1 for Model 1 and M10.5×1 for Model 10).
Strategy 3: reduce thread wear by ensuring that the male and female thread parts are made of different plastics. The layer by layer construction of 3D printed threads means that they are never perfectly smooth and so a thread with close tolerance is going to the prone to both galling (where two surfaces freeze together from friction induced ‘cold welding’) and wear. In the Platypus pens the threads are made of dissimilar plastics, one harder and one softer. In use the harder plastic distorts the softer during the initial running in of the thread and the result is a good fit with minimal ongoing wear. Using a softer PLA in the cap liner not only helps prevent cap thread wear, but also minimises the possibility of the cap scratching the body surface when the cap is posted and un-posted.
Strategy 4: minimise the risk of thread damage from cross-threading. The cap liner is shaped with an inner taper that guides the section during capping so that the cap and body threads are aligned before they come together. It is only possible to cross thread the cap and body if you cap the pen without the section installed, and why would you do that? The body to section thread can be cross threaded if you are inattentive—as could that thread on any pen—but it’s a fairly hard to do because the section has an unthreaded lead-in that goes into the body before the thread engages.
Perfection!
So, given how effectively the difficulties and disadvantages of 3D printing have been overcome or minimised, Platypus pens mut be perfect, right? No, not right. If you look closely enough you will be able to see some minor flaws in any Platypus fountain pen. Those flaws come from the nature of 3D printing which cannot compete with the perfection of computer-controlled lathes and modern injection moulding for precision and repeatability. Squeezing molten plastic out of a nozzle like toothpaste out of a tube necessarily involves some imprecision and the occasional minor glitch.
Platypus pens are skilfully designed and carefully hand made to a high standard and you can expect them to be very comfortable, eye-catching, unusual, and, above all, to write well.
