Unfilled siliconeHardness comes from the compound, not from filler

If your part needs 70 Shore A, we buy 70 Shore A silicone. We do not buy a 50 and add filler to it until the durometer reads 70.

Our material purity policy

Purity here is a measurement, not an adjective. Ash content is the direct measure of how much non-rubber is sitting in a cured compound, and we run it on our own muffle furnace.

1. We buy silicone base at the hardness the job needs. We do not buy a softer base and add filler to raise it.
2. We add carbon black to EPDM and nitrile because those compounds need it for structural strength. We do not add chalk, china clay, talc or any other cheapening mineral filler to any compound we make, unless the application demands it.
3. Ask, and we will state the ash content on your order.

Until and unless something is there to give the compound a property it actually needs, we are not putting it in.

The durometer will not tell you

Silicone base comes in different hardnesses and we buy the one the job needs. If somebody wants 70 Shore A, we buy 70 Shore A. If somebody wants 30, we buy 30. We do not buy one hardness and turn it into another.

The cheaper way is to buy a softer base and add mineral filler until the hardness reads what the drawing says. And it works, in the sense that the number is correct. But the filler that raised the hardness has taken elongation, tear strength and compression set recovery away with it. The part will pass your incoming inspection. Then it goes hard in service and stays hard, and by that time it is already sitting inside your product.

Rubber is sold by weight. You use it by length.

You buy rubber by the kilogram. You use it by the metre. Those are not the same thing, and what sits between them is density.

Filler is mineral, and mineral is heavier than the rubber it is replacing. So every kilogram of a filled compound has less rubber in it and more stone. It is denser, and you get fewer metres out of it and fewer pieces off the same mould.

Our silicone is light because we have not put anything in it to make it heavy. Same kilogram, more metres. This is why a cheaper price per kilogram is very often not a cheaper price per metre, and the filled part does not last as long on top of that.

And for extrusions we usually do not sell by the kilogram at all. We quote by the metre. So the density question is not yours to worry about. You know what a metre costs you before you order, and that is the number you need when you are costing the part.

Ask us for the number

Ash content tells you how much filler is in a cured compound. You burn the rubber off and weigh what is left behind. We have a muffle furnace here and we run the test ourselves. If you want the figure stated on your order, ask us and we will put it there. We are not asking you to take our word for this.

Compound is mixed here from over 800 formulations, every batch goes on the rheometer before it runs, and finished parts are checked against IS, BS and ASTM methods. The whole sequence is on the process page.

EPDM and nitrile need carbon black. They do not need chalk.

In EPDM and nitrile we do have to add carbon black. That is not filler in the cheapening sense, it is what gives those compounds their structural strength, and you cannot make them without it. Anybody telling you otherwise is selling you something.

What we still do not add, in EPDM or in nitrile, is cheapening mineral filler. So those compounds also come out light and the properties stay much higher than what you will get from a filled one. Until and unless something is there to give the compound a property it actually needs, we are not putting it in. The materials guide compares all six base polymers side by side.

Filler, hardness and ash content