Groundwork · specifying the material
Hardness is the one number customers ask for by name, but it is best decided last. It is a balance between holding its shape and releasing the part, so the right answer depends as much on who is demoulding it as on how it looks.
Inputs
There are four things to answer, and none is a matter of preference. Once you have answered them, the hardness follows.
If your part has undercuts, deep relief or re-entrant features, the mould needs to flex to release it, so it must use a softer material. A part that lifts straight out needs no flex, allowing the mould to be harder and last longer.
A heavy, dense cast can bow a soft mould, leaving the part thicker on one side because the cast material's weight pushes harder. Delicate castings that break on release exert less force and push more softly.
A mould that works all shift needs to cope with being handled by someone in a hurry. Softer moulds can tear at thin corners. Harder moulds resist tearing, but they are less forgiving for the person demoulding them.
Softer silicone captures fine detail beautifully, but does not retain it as well over time. Harder silicone keeps an arris, or sharp edge, crisp for longer, but is less able to capture fine detail in the first place.
Why no chart
It might be tempting to add a table matching hardness values to applications. However, that guidance would be wrong often enough to cause real problems, so we have chosen not to include one.
That is because hardness never acts alone. Wall thickness affects a mould's behaviour just as much as hardness does. In practice, a thick, soft mould can be stiffer than a thin, hard one. Tear strength matters at least as much, and it does not track hardness neatly. So, two moulds with the same hardness but made from different grades can have very different working lives. Cavity geometry changes things again. A soft mould with a deep, narrow feature will grip harder than a firm mould with an open one.
There is also the human factor, which no chart can capture. The same mould can have two very different lifespans: one in the hands of a maker who owns the piece, and another with a production operator at the end of a long shift. Specifying the mould for the first situation but delivering it for the second is one of the most common reasons moulds fail early.
When a previous supplier's hardness value worked for them, it's natural to ask for the same one. We first ask what your mould needs to do rather than simply matching the number. Often, the original number was right. Sometimes it was inherited from a completely different job, and repeating it only repeats the problem. If your mould fails at the same corner every time, hardness is one of the things we will look at, alongside the geometry and how the mould is being used. See design for manufacture for the rest of that conversation.
Checking
Once cured, silicone can still change over time. Heat cycles can slowly stiffen it, chemical attack can soften it, and the mixing ratio, cure conditions and age can all slightly affect the finished hardness.
We measure it rather than assume it. Hardness is read on the cured material, and the grade and reading are recorded against your order. This helps your replacement mould behave like the one it replaces, rather than simply being made from the same product name.
It also gives us a useful comparison point if a mould comes back having gone hard, become tacky or torn early. If it has stiffened over a season, that tells us something about the conditions it was working in. That insight usually helps shape the specification for its replacement.
That same record makes replacement moulds a straightforward conversation rather than a fresh development.
How we get there
Tell us about your part, what goes into the mould, how often it is pulled and who pulls it. We will recommend a grade and hardness, explain the trade-offs, and adjust our recommendation if you share any details about the duty that we did not know.
If your job sits on a boundary, a trial mould is often the sensible next step before you finalise the production specification. That's where prototype tooling helps. Finding out on a disposable mould costs less than discovering an issue with a matched set.
For hot-duty and food-contact work, the silicone grade determines the available hardness, rather than the other way round. See high-temperature moulds and food-contact moulds.
At a glance
Silicone hardness in everyday terms
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From idea to finished mould
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Ready to start?
Send us a drawing, photograph or sample. We'll come back to you with a mould strategy, a material recommendation and a price.
Real people, a real workshop
No call centre and no middlemen. When you contact TCI Mouldings, you speak to the team who design, build and check your mould.
Thomas LaneManaging Director
George PrestonDesign & Project Manager
Kelly BallingerHead of Partnerships
Jim AndertonProduction Manager
TheodoreEmployee Welfare