Silicone Rubber Sheet Bonding: What You Need to Know
Last updated on October 11, 2019
Bumping up against the heat seal requirements ceiling
Where to start when you need high temperature silicone sheets?Dick and Jane had a problem. They work for a contract packaging company that was building out a new facility. Because they work for a contract packager, the new facility would have lines for handling pouches, bags and other flexible packaging as well as cup and tray sealing.
The project was winding down, but there was yet one more piece of the puzzle to work out. Dick and Jane’s team was tasked with choosing and preparing the right silicone sheet material for the heat seal bars, jaws and heads of the packaging machinery.
Why the decision to use United Silicone’s sheets?As engineers, they know that modern heat seal packaging machinery needs the right combination of time, temperature and pressure to create good seals. Good seals mean no leakers and the lowest scrap rates. They also know that the most efficient heat seal packaging lines run fast and hot. And frequent downtime is not acceptable.
Selecting United Silicone's high temperature silicone rubber sheets was the perfect choice:
- Highly heat tolerant
- Thermally conductive
- Durable, resilient and reliable
What about attaching the high temperature silicone rubber to the sealing surfaces?Knowing you need high performance silicone rubber for heat seal applications is only half the battle. The real puzzler is, how to attach the silicone sheeting to the seal jaws, seal heads, seal bars and other heat seal tooling.
If Dick and Jane had bought unbonded sheets to get some savings off the cost of metal bonded sheets, they would have had to work out how to attach the silicone sheets with a glue or adhesive.
Isn’t there some kind of adhesive that would have worked?What options did they have for attaching unbonded silicone sheets to the machinery’s seal heads and seal bars? High temperature heat conductive silicone sheets come fully vulcanized and cured. Dick and Jane quickly learned that trying to glue fully cured silicone to any surface is like trying to glue a rubber band onto something. It’s pretty much impossible to do.
Why not try adhesives anyway?Fully cured silicone is even more difficult than a rubber band to attach to another material. It is nearly as slippery as Teflon™. Dick and Jane knew they needed reliable performance to meet the requirements of quick cycle times, high pressures and high temperatures.
Silicone glued to metal with an adhesive turned out to not be reliable enough. Why is this?
Why don’t adhesives work for silicone-to-metal bonding?When an adhesive is applied to a flat sheet, it tends to pool and create uneven pockets. In use, the pressures and temperatures of high-speed heat seal processes cause adhesives to move around and pool up again. As this happens, an adhesive will try to pull the silicone rubber along with it. This creates warps and unevenness in the silicone sheet surface.
When it is no longer flat, a sealing surface cannot provide the uniform pressure needed to create reliable leak-free seals.
What you need to know about heat seal bars, jaws and headsDick and Jane knew their basic requirements. These include the fact that heat seal applications require sealing surfaces that are 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.
So, if silicone sheets are so notoriously difficult to attach to anything, what made United Silicone sheets the way to go? The answer is their silicone-to-metal bonded sheets.
What’s the trick to getting silicone rubber to stick to metal?Once fully cured, it can be extremely difficult to get silicone rubber to stick to surfaces. What other option is there? United Silicone's proprietary method chemically bonds the silicone sheets to a metal backing during the vulcanization process.
Why is the United Silicone solution better?This answer is not merely the use of a uniquely sticky adhesive or glue. Instead, the United Silicone manufacturing process creates a molecular level chemical bond between the silicone rubber and the metal backing. The result is as close to a permanent bond as you can get.
United Silicone bonded sheet material can be sliced and diced to create squares, pads or any other shapes needed for heat seal tooling or support fixtures.
Making the flatness of silicone-to-metal bonded rubber sheets even betterAfter the manufacturing process, standard United Silicone bonded sheet material is precision ground for flatness. A reliable silicone-to-metal bond and a completely flat surface eliminated two of the greatest challenges Dick and Jane faced in setting up reliable packaging lines.
What about heat sealing lines that need curved tooling?Because of the demands of contract packaging, some systems need to be able to apply seals on a curved surface. Dick and Jane did not need to go any further for a solution to this challenge.
In addition to stainless steel and half-hard aluminum, United Silicone also makes heat seal sheet material with dead soft aluminum metal backing. Dead soft aluminum backed silicone sheets are easy to shape and work for making curved or crowned heat seal tooling.
What are the key takeaways?Fully cured and vulcanized silicone rubber is devilishly difficult to attach to any other material using adhesives or glues. Heat seal requirements are demanding. Using adhesives in heat sealing pressures and heat conditions results in unreliable seals, leakers and unsustainable scrap rates. The fix for this is United Silicone rubber-to-metal bonded silicone sheets that are thermally conductive, highly heat tolerant, durable and resilient.
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This technical data sheet provides details on the United Silicone Ultrasil®, Thermosil®, FDA and FDA-3A heat seal silicone rubber formulations. These include mechanical as well as other properties.