Silicone Rubber Sheet Bonding: What You Need to Know

Picture this: You work for a contract packaging company that is building out a new facility, complete with lines for handling pouches, bags, and other flexible packaging, as well as cup and tray sealing. Veterans in the packing industry, your company already understands the added value silicone tooling has for producing consistent, high-quality results. Now you’re tasked with determining what kind of silicone sheet material would best work for the heat seal bars, jaws, and heads that comprise the packaging machinery.

What to Consider When Selecting Silicone Material

As an engineer in the packaging industry, you know that modern heat seal packaging machinery needs the right combination of time, temperature, and pressure to create reliable seals with low scrap rates. You also know that such machinery tends to run fast and hot with little tolerance for downtime.

You’re looking for silicone that is at once thermally conductive and heat resistant, meaning you can utilize the silicone for heat sealing while maintaining a consistent (and not too hot) application of heat. You also want it to be durable and strong but flexible, so you can apply even pressure across surfaces without needing to replace or repair the silicone surface.

In particular, you want silicone rubber that will effectively adhere to your metal tooling. You might be chalking that up to whatever glue or other adhesive you apply, but the silicone sheet material you choose matters.

How to Attach High Temperature Silicone Rubber to Sealing Surfaces

Of course, you could always opt to purchase metal bonded silicone sheets, forgoing the need to apply adhesive yourself. But you choose instead to buy unbonded silicone sheets to save money and be able to utilize existing metal tooling.

What options do you have for attaching unbonded sheets to the machinery’s seal heads and seal bars? High temperature heat conductive silicone sheets come fully cured via vulcanization. Try glueing a rubber band to any surface, and you’ll find it’s nearly impossible. Fully cured silicone is even harder to adhere to surfaces.

So how is it that metal bonded silicone sheets even exist? Before delivering the silicone sheets to your door, silicone manufacturing companies often utilize compression molding, and a chemical bond, to adhere the silicone to the metal sheet. Using a combination of heat and pressure, vulcanization cures the silicone, setting the bond between silicone and metal.

Why Adhesives Struggle to Work

But wait, aren’t there silicone adhesive products on the market today? No, we’re not saying that companies that promise glues for silicone sheets are selling snake oil. There’s a lot to consider, including what material you’re adhering the silicone to and for what purpose. However, it is a scientific fact that fully cured silicone has a low surface energy, which is a fancy way of saying it’s “non-stick.” But that doesn’t mean sticking is impossible.

Certain super glues may work well enough for some use cases but fail when subjected to high heat. The pressures and temperatures of the high-speed heat seal process cause the adhesive to liquify and pool. The adhesive tries to pull the silicone rubber along with it, creating warps and unevenness in the silicone sheet surface. When it’s no longer flat, a sealing surface can’t provide the uniform pressure necessary to create reliable, leak-free seals.

Silicone sealants have become popular because they adhere to both metal and silicone, but application can be tricky. The integrity of the seal may be affected by contaminants on the metal, leading to poor or uneven application, which can in turn affect the machinery’s performance. Silicone sealants can also be temperamental during the curing process, reacting to changes in temperature and humidity such that the seal becomes unreliable.

Why Compression Molding is Better

As a hypothetical engineer at a packing company, you know that heat seal applications require sealing surfaces to be as parallel and flat as possible in order to reliably produce leak-free seals. The temperatures, pressures, and short cycle times of modern heat seal applications are simply too demanding for imperfect solutions.

Compression molding of uncured silicone to metal acts on the materials at the molecular level, evenly distributing the silicone and its physical properties across the surface of the tool. This also ensures full contact between the silicone, the adhesive-free chemical bond layer, as well as the metal base. The result is a chemical bond between the silicon rubber and the metal backing that’s as close to permanent as you can get.

The simple reality is that the silicone metal connection will never be so strong as when it’s forged pre-curing. And while bonded silicone sheets may cost more upfront, their reliability is priceless. After all, scraping a batch of sealed products due to machine malfunction and paying to replace damaged parts will cost more in the long run.

Flat vs. Curved: How to Tailor Your Needs

Many silicone manufacturers will take the bonded sheet material and precision grind it for flatness. If flat is what you’re looking for, right away you have the ideal solution. However, due to the demands of contract packaging, some systems need to be able to apply seals on curved surfaces. That’s where your choice of metal comes in.

Stainless Steel and half-hard aluminum offer firm, warp-resistant solutions. Softer metals like dead soft aluminum are more pliable, making them easy to shape into curved or crowned heat seal tooling. Thanks to the chemical bonding, manipulating the material in this way won’t impact the integrity of the silicone coated sheet.

The Takeaway

Time to make your final decision before the facility goes live.

You tell the higher-ups that fully cured silicone rubber is devilishly difficult to attach to any other material using glues or other adhesives. Heat seal requirements are demanding and unfriendly to adhesives, resulting in unreliable seals, leaks, and unsustainable scrap rates. Sure, a bonded sheet might cost more up front but it’s the more cost-effective option in the long run. Now you have a solution that’s not only thermally conductive and consistent in its results, but also durable, reliable, and ready when you are.

« Go back to the blog homepage