Typewriter Talk / Durometer

**Posted by OregonJim** 04-2-2023 03:43:15

I'm looking to purchase a durometer to measure platen hardness - which is the proper scale to get - SHORE Type A or SHORE Type D?

Thanks for any insights.

**Posted by Pete E.** 04-2-2023 10:47:53

For typewriter rollers, Shore A is used.

Shore A (SHDA)- for soft to hard materials such as soft rubber, elastomers, neoprene, silicone, etc.
Shore D (SHDD) - for medium to extra hard materials such as hard rubber, epoxies, hard plastics, PVC, acrylic, etc. "

**Posted by OregonJim** 04-2-2023 12:08:04

Thanks, Pete. I guess once a platen reaches 100 SHORE A, there's no point in measuring further as it needs to be resurfaced. Is that reasonable thinking?

**Posted by Pete E.** 04-2-2023 13:37:18

Jim,

I think even a Shore D meter still reads out from 0-100.

I never bothered with a meter. If my thumb nail can leave an indent in the rubber, that is good.

I have many platens that are rock-hard. I do not worry...as I use an Avery sheet of laminating plastic for a backing sheet to my typing paper on all of my machine...

**Posted by Uwe** 04-2-2023 15:04:26

I use a Shore A durometer, which is what you'll need to test the hardness of typewriter platens and rollers. It's a very useful tool. Whenever I receive a new machine I typically test and record the platen's hardness, and then after cleaning the platen and rollers with rubber rejuvenator, I'll test again to determine what effect the rejuvenator had.

A good platen/roller should test in the 90 to 92 Shore A range. I've found that rejuvenator will rescue a platen that's in the 94-96, but it won't help much with those that are rock hard and off the scale (although it can give extra grip - never sand a platen).

The importance of using a Shore Type A (ASTM D2240) durometer is that it matches the device's spring tension and its indentor (pin) with the material being tested. The type A indentor is a 1.4 mm, 35º truncated cone, which explains why the less scientific test is to push a pencil point into the rubber at the far end of a platen.

**Posted by robmck** 04-2-2023 21:11:21

I've been using a Shore A meter. When I started searching, I found that vintage mechanical meters made by Shore could be had for the same price as digital ones, so I went with the older ones. (A friend of mine who's a machinist got me into this habit when he pointed out that he's found the quality of a vintage Starrett is better than a recently made Starrett, and similarly for other brands).

Mine came with a nice wooden case and calibration block. $48 including shipping & tax.

**Posted by CoronaJoe** 04-2-2023 21:12:27

All Shore scales go 0-100. The difference is the ranges of hardness over a variety of materials and as Uwe points out it also how the tool is calibrated and force needed. For example a Shore A durometer uses a 1.8 spring whereas a Shore D durometer uses a 10 lb spring and all three scales OO, A, D, have regions of overlap. A 90 Shore A is comparable to a 40-45 Shore D. I suspect the Shore A was used as the A scale tool had more utility and availability than say the D scale that covered tires to hard plastics such as for pipe and hard hats.

**Posted by skywatcher** 04-2-2023 22:02:41

Hi Jim

The mechanical Shore-A tire durometer from Amazon is adequate for the needs of most of us typewriter collectors and users. Talking with Peter Short of J.J. Short Associates, Peter says that new platen hardness is 88 to 90 Shore-A for a manual typewriter and 95 Shore-A for a Selectric. As Uwe says, the pin of a Shore-A durometer will only make a very small dent in the platen which will rebound if the platen is 95 Shore-A or lower. If the platen is 96 or harder, the pin can't penetrate deep enough to leave a dent. A Shore-D durometer has almost a sharp point that will leave a small hole in the platen.

A platen that reads 96 or higher using a Shore-A durometer is basically too hard for a manual typewriter. Using a Shore-D durometer isn't going to give you any useful information, so is more or less a moot point unless you are just curious as to how over hard the platen is, if that makes sense.

**Posted by OregonJim** 04-2-2023 22:31:32

Thanks guys. My main question was whether or not Shore A would sufficiently cover the whole range of possible platen hardness. Sounds like it will, at least for my purposes. It's just a curiosity, really, and a tool to help me better assess these things by "feel", once I know what the feel corresponds to, number-wise.

**Posted by CoronaJoe** 05-2-2023 01:36:59

If you can afford it, go for it as you can learn is my criteria for accessing instruments Next thing you know you'll be checking the difference between snow tires and regular tires etc. I have the real deal Shore instrument from an estate clean out that was used by a sales engineer in the rubber biz so it was very affordable. I disagree with Short's platen hardness. 90-92 is what is in the repair manuals for most typewriters such as what Ames published before it closed and what I got from them. 88 doesn't seem much of a difference but it is noticeable. The last platen I had done by another firm used 88 and while it was way quieter there was embossing in the paper with perforation by punctuation type. Short's main business is products for industrial , agricultural, and commercial printing. The typewriter platens are a new side gig for them and I've come across a couple of unhappy shops. However that they picked up the ball where Ames left off is amazing so take care and whatever is sent keep the variables and chrome ends protected.