Manufacture of platens and Durometer values and Durometer Types

The manufacture of typewriter platens is described in an article in the June 10, 1919 issue of The Rubber Age.

The article is written by and for those deeply familiar with the rubber manufacturing industry, it is filled with jargon and technical terms. Typewriter-related, it explains the function of rubber as the platen material.

A bit further on, after delving into manufacturing, the desired hardnesses of the platens is mentioned too.

The mentioned readings of between 65 and 70 on a durometer are surprising.

When Shore Durometer values for typewriter platens are mentioned today (or yesterday, for that matter), the values are generally in the 87 to 97 range. Those are Shore (A) values, this A is not always added, but to be unambiguous it should be added. There is a wide variety of different Shore scales and the letter is needed to identify on what scale the value should be interpreted, or pur another way; to identify what instrument was used to read the value.

The actual hardness needed in 1919 is surely not different from the hardness needed on a typewriter today; so the article must be meaning different durometer values than the default, usual Type A Durometer.

There are additional hints in early references to Shore Durometers and their use. When introduced in 1915, there is no mention of different scales or types. The dial simply says "Hardness"  without any extra Type indication. The original Durometer is most suitable for soft rubbers, such as for automobile tyres of the period. Very early on however, there are references to a soft rubber durometer and a hard rubber durometer.

Readings with the original tester on hard rubber, i.e. with values over 90, were advised to be unreliable. Another method, or instrument, would be needed to measure hard rubber.  The solution that was taken by Shore was to create a hard rubber tester by changing the flat-tipped indenter-pin of the Durometer for a pointed pin. This sharper, pointed pin was already in production for the Elastometer, so this would have been a simple, obvious step to take that required almost no effort or costs.

In literature into the 1930s there is mention of a Shore soft rubber durometer and a Shore hard rubber durometer, suggesting this is what these types were originally called. This would fit with/explain that the box of this ~1941 Type A Durometer has 'soft rubber' scribbled inside the box lid.

These two types, soft and hard rubber, were (later?) defined as the Type A and the Type B by the company. The Type letters were added to the dials by at least 1930, probably earlier.

The Type A in the picture above would have been called a soft rubber Durometer (or simply Durometer) in 1919. The ~1958 Type B in the picture below would have been called a Shore hard rubber Durometer back then:

The dial on this instrument is marked as Type "B" and it has the sharp, conical pin. The spring and everything else is identical to the original Shore Durometer, i.e. the Shore Type A Durometer.

When making a new platen for the Blickensderfer a while ago, several hardnesses of TPU were tried. The platen made with TPU that was sold as having a hardness of Shore 95 A was experimentally rated as best. With an old sector-type Durometer however, readings below 20 or above 90 are not reliable, so the actual hardness of the finished platen could not be checked.

Testing this newly made platen with a Shore hard rubber durometer, i.e. with a Shore Type B Durometer instrument, it shows a value between 65 and 70! When slowly rolling over the platen (method for testing cylindrical parts), the value reaches close to 70 at its maximum.

The thickness of the rubber on the platen core is about 3mm - a bit too thin for a proper measurement. Nevertheless, this result is good enough to confirm that the durometer values in the 1919 article are indeed meant to be read as Type B, i.e hard rubber durometer values.

Another conclusion from this is that within a few years from the introduction of the Shore Durometer, already a second type of Shore Durometer was introduced. (For such a common and even standardized instrument, not all that much of its history is available - perhaps not surprising, a deeply niche subject :-)

The Shore Type A Durometer is the original instrument of 1915, the Type B is a second, hard rubber instrument that was in use already in 1919.