What does durometer mean regarding rubber stamps?

A common issue that comes-up when CNCROi.com makes custom rubber stamps that are out of the ordinary self-inking Trodat range are questions relating to durometer. Luckily, many of my customers spec that but I’ve found that people outside of the rubber world don’t know what it means so let’s take a look!

The durometer is simply a scale of material hardness, which comes in handy for rubber and silicone dies but not very useful when it comes to things like wood though I’m sure that there is an application where it’s relevant, I just haven’t come across it yet. So, let’s focus durometer here specifically relating to rubber (silicone) stamp production.

The durometer range we actively play with here at CNCROi.com is 60 through 90 where the higher the number, the harder the rubber will be. Keep in mind that having a harder rubber isn’t necessarily better or worse than a softer one, it depends what you are using it for and stamping details.

For instance, for hot presses where the rubber is adhered to 1/4″ or 6 mm aluminium plate, the best durometer would be 60-70 though if you have lots of fine details, going up a bit more ads resolution capabilities and holds heat better for a more consistent result.

For a real-world comparison, the hardness of the pencil eraser you have on your desk is probably around 60 durometer, it comes apart pretty easily (which is what you want for this application) and generally lasts as long as the pencil it’s adhered to. What would happen if that pencil had an eraser with a durometer of 90? Well, remember when those rubber erasers would dry out and smudge or rip the paper? Yep, the durometer went up causing less then stellar erasing abilities.

CNCROi.com has a hot press machine by the way, here’s a pic of it above. What I want you to focus on though is the action, it’s straight down. Unlike a pencil eraser, there is no shearing forces involved, it’s up and down using PRESSURE and HEAT. So, higher durometer means more consistent heat distribution across the rubber die and more tonnage force capabilities downwards.

This is why most heat transfer dies are around 70 to 80 durometer, depending on what it’s working ON, as it gives a good time to pressure consistency for transferring of images at a good resolution.When you start to heat these rubber dies up, they act a bit differently and having something mushy under foil isn’t a good idea as it tends to spread out, which is what you don’t want to have happen under pressure.

So, you may be wondering what 90 durometer, which is very hard, would be good for? Well, some materials required a lot more pressure, far more than what you see with the t-shirt vendors at the mall. Think of branding aluminium products where you require extremely fine detailed images or text. Not only that, it needs to be consistent, well, higher durometer is how it’s done.

Now, there are certain combination durometers out there as well, for instance, dual durometer contains a 90 durometer top surface with a softer 60-70 durometer underlay. This gives you “best of both worlds” depending on the application you intend to use this for.

How do you know which is which?

Some manufacturers label their rubber with the durometer right on it after testing while others go with a color range. For instance, one of our suppliers provides rubber in orange (60 durometer) and it goes darker as the hardness increases to a brown (90 durometer).

There are also half durometers, for instance, we got a sheet in the other day and it was 65 durometer (sold as 65) so the numbers can be specified within a range or specific number.

How thick is rubber?

Rubber can be bought in endless thickness variances, for instance, most of the Trodat Professional and Printy Mobile stamps are made using rubber that’s 1/8th (3 mm) thick but I have some 3/4″ (18 mm) rubber in the shop that’s about 70 durometer as well as some far thinner stock.

Thickness of rubber and the durometer of the rubber have a direct role towards getting the results you need for  your custom application. Contact CNCROi.com right now and we’ll work with you for the optimal solution to your production issues!