Twist-fill Bladder

As I mentioned in the previous post , the bladder is both the core and the Achilles heel of the early twist-fill fountain pens, at least from the perspective of today. The rubber tube or sack was not sufficiently robust to withstand the repeated stresses of being twisted and untwisted. The problem was compounded by the fact that the rubber would become stiff and embrittled over time.

Twist-fillers were not the only fountain pens that suffered from the failure of ageing rubber. Replacement of the rubber bladders, tubes, or diaphragms is an everyday occurrence for collectors of vintage fountain pens. Sometimes the replacement is easy (as I found with the bladders of old Conway Stewart pens), but sometimes it is very difficult (as with the Vacumatic Parker 51 that I broke…).

A Platypus Model 20 bladder being displayed by an expensive hand model. Notice the built-in flexure zones that minimise the stresses imposed on the bladder during use.

The Platypus Model 20 bladder

The bladder of the Platypus Model 20 will quite likely never need to be replaced. The material is very robust and strong.

The picture here shows a Platypus Model 20 bladder stretched by a dumbbell weighing nearly 10Kg. The bladder was stretched to nearly four times its normal length, and even though it did not return all the way to its initial length after the weight was removed it continued to spring back after being twisted.

A Platypus Model 20 bladder supporting nearly 10Kg (22lb)
A bladder undergoing a stress-test of 100,000 actuations.

The bladder shown in the video was actuated with a stepper motor (driven by a 3D printer control board) repeatedly for more than a day. After 100,000 actuations the bladder was still intact, still moved water in and out when twisted, and was totally undamaged by the experience.

It seems likely that the bladder would withstand far more than 100,000 actuations, but that is far more than any bladder would see in even a lifetime of continuous use.

Polyurethane (TPU)

The Platypus Model 20 bladder is made of polyurethane, a type of plastic that is often called TPU in the context of 3D-printing. Polyurethane is remarkably robust, chemically stable, and resistant to many solvents. Polyurethane’s flexibility and stability make it remarkably useful. It is widely used in many different roles, from stretch fabrics (Spandex, Lycra) to components of car suspension and skateboard wheels.

Polyurethane comes in many variants and different degrees of hardness. The TPU filament used for printing the Platypus Model 20 bladders is softer than many TPU filaments, with a technical Shore hardness rating of 82A, as compared to the most common 95A.  The particular filament used is Filaflex 82A, made by Recreus in Spain.

In contrast to rubber, polyurethane does not suffer from embrittlement much at all. It has even been reported that polyurethane becomes less brittle over time [1], albeit at elevated temperatures. It has also been reported that the presence of water hinders embrittlement from developing with ageing [2], a finding that might be relevant to polyurethane ink bladders.

I do have some limited long-term experience of 3D-printed polyurethane. I still have the tree frog that is the first ‘flexible’ print that I ever made. You can see in the photo that it is not a great print (I like to think that I could do much better now), but it sat on a windowsill for about 7 years and has proved itself both stable and an excellent dust collector.

The 3D-printed polyurethane tree frog is looking at some of the data collected to determine the rate of water loss through the walls of various bladders. Those data will be the subject of the next blog post in this series.

Polyurethane is not perfect

Not every property of polyurethane is perfect for its role as the bladder for the Platypus Model 20 fountain pen. Unfortunately polyurethane is slightly permeable to water. Not disastrously so, but even a little can lead to drying out of ink if it is left in the pen for long enough. The next blog post in this series will detail my studies of water loss from various bladder constructions and the ways that I have made the problem minor enough to be unproblematical for normal fountain pen use.


  1. Abbas Tcharkhtchi, Sedigheh Farzaneh, Sofiane Abdallah-Elhirtsi, B Esmaeillou, F Nony, et al.. Thermal Aging Effect on Mechanical Properties of Polyurethane. International Journal of Polymer Analysis and Characterization, 2014, 19 (7), pp.571-584. ff10.1080/1023666X.2014.932644ff. ffhal-01191410f
  2. Possart, W., Zimmer, B. Water in polyurethane networks: physical and chemical ageing effects and mechanical parameters. Continuum Mech. Thermodyn. (2022).

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