Double-seal overmoulding on aluminium: when the solution seemed impossible

How we developed a seal in 4 weeks that others had failed to produce in three years

When they called us, the project was nearly dead.

Three years of attempts, different suppliers, resources invested. And the problem was still there, unsolved. They were seriously considering giving up on the entire project.

We sat down with their technical team and listened. Not just to the stated problem — but to everything: the actual operating conditions, the system constraints, what they had already tried and why it had not worked.

The problem

The component was a machined die-cast aluminium valve with very tight tolerances and a stainless steel pin integrated into the upper section. The requirement was to overmould two sealing gaskets simultaneously — one at each end of the valve, 180 degrees apart.

But that was not all. During the discussion, a critical requirement emerged that further complicated matters: the compound had to ensure low friction under operating conditions. The part operates in a system where actuation force must be minimal — yet the seal must be reliable and consistent over time.

Two requirements pulling in opposite directions. And which explains, at least in part, why others had failed.

Why aluminium is a challenging substrate

In our line of  work, we know this all too well: aluminium is one of the most complex substrates to work with. It is not like steel. Its surface oxidises naturally, with chemical properties that vary depending on the alloy, the surface treatment, and even the storage conditions of the component.

Achieving a durable structural bond between rubber and aluminium requires: specific and controlled surface preparation, primers and bonding agents compatible with that specificalloy and the selected compound, process parameters optimised to the last detail; and a mould designed to manage the flow of the compound on both sides simultaneously.

There are no shortcuts. Every variable matters.

Three years of failed attempts

The customer — a multinational industrial group — had already faced the problem head on. For almost three years, various suppliers had attempted to develop a solution. These were large companies, with in-house resources and laboratories. The result was always the same: an inability to guarantee stable, long-lasting adhesion, or sealing issues under real operating conditions.

The project was on the verge of being abandoned.

What we did

We worked on three fronts simultaneously: selecting and optimising the compound to meet the required tribological characteristics, developing a surface treatment process specifically tailored to that aluminium alloy, and designing the mould for the simultaneous overmoulding of the two gaskets at 180 degrees, with the stainless steel pin as the central element to be managed.

It was not straightforward. But it is exactly the sort of problem we are used to dealing with.

The result

Four weeks after our first meeting, we delivered the samples in the smallest size.

I will never forget the technical team’s reaction. After three years of failed attempts, they did not expect to see those samples so soon. And above all, they did not expect them to work.

They worked.

We currently manufacture four sizes of that component using 4-cavity and 8-cavity moulds. We have been in production for over four years, maintaining consistent quality and with no issues encountered in the field.

If you are facing a similar problem

Overmoulding on aluminium is not impossible. It is complex, requires specific expertise in materials, surface treatments and process, and leaves no room for approximation.

If you have a sealing challenge that others have not been able to solve — on aluminium, special alloys, engineering plastics, or stainless steel — get in touch. The first thing we do is listen.

Foresti Alexander