What is Heat Sealing?

This page contains a pretty detailed and somewhat lengthy explanation of what RF Heat Sealing is and where our SealGuard Buffers come in. I tried to keep it pretty straightforward, but there is a lot of it.

If you’re not up for the whole thing, you can skip around with the bookmark links below, or just figure you need the Buffers anyway, why bother reading this stuff, and just skip this whole page. Or you could print it out, and read it at your leisure. You don’t need to know the theory to be able to use our Buffers, but I thought a lot of people would like a little explanation of the basics.



What is Heat Sealing?

When you need to fabricate an item from two or more pieces of plastic, you need a way of welding them together. When you need to weld sheet materials together in an overlapping fashion, the preferred method is Heat Sealing.

In Heat Sealing, the two or more pieces of sheet plastic to be welded together are placed in a Heat Sealing Machine. The Machine looks like a large punch press with Heat Sealing Die where the punching die would usually be.

The Heat Sealing Die generally consists of a solid, flat lower platen on which the materials to be sealed are placed. The Heat Sealing Die is above the lower platen. The Heat Sealing Die consists of one or more bars, usually brass, which are shaped in the fashion in which the seal is desired. For example, if you want a straight seal, you use a straight bar. If you want a seal in a circular shape, you use a die with a circular bar.

When the plastic is in place and the machine is activated, the die comes down to the platen holding the material to be sealed, and the die is energized – either with Heat Energy or with Radio Frequency Energy. This heats the material and, along with the downward pressure that the head of the press is exerting, seals the material together in the desired shape. After the seal is completed (the time necessary to seal is set by a timer on the machine; generally it’s a few seconds or so), the machine moves the die back up, and the sealed product can be removed and new material can be put the platen on to be sealed.

Keep in mind that I’ve described a very plain vanilla Heat Sealing Machine. There’s a thousand variations –

  • Machines with turntables to allow the use of different operators to lay out the material to be sealed, and to remove the finished product.
  • Machines which seal on belts which are themselves the Buffer material (most people use our HiSeal®).
  • Machines which allow multi-step sealing operations, either with a belt of Buffer or a turntable carrying the item from sealing station to sealing station.
  • Machines which integrate RF Heat Sealing with other manufacturing operations like insertion, cutting or punching, again usually on a belt of Buffer to carry the item from station to station.

And many others; your mileage may vary.

What is Radio Frequency Heat Sealing?

Ok, so far, so good, but where does this Radio Frequency stuff come in? You may recall a little while ago, I mentioned that the die required energy to heat the plastic sheets to seal them together (it also uses pressure on the heated plastic). That makes sense; something’s got to weld this plastic together. I said that either Heat energy or Radio Frequency (RF) energy could be used. Well, Heat energy is pretty much self-explanatory – you just melt the plastic together as if you left on a radiator too long.

But RF sealing takes a little more explaining. Do you know how a Microwave Oven works? When you turn it on, it beams RF energy throughout the interior of the Oven.

You may know that Radio Waves have a frequency. (If this is too elementary for you, I’m sorry, but I want everyone to get it). When you change your radio from one station to another, you’re changing the frequency selection of the tuner, so that it receives radio waves of a slightly different frequency.

RF energy in your Microwave Oven is similar to the RF energy that your radio station puts out, except that it’s a lot more powerful (because your food is right next to the transmitter; actually, your radio station puts out a ton more power, but by the time it gets to you it’s pretty diffused). Also, the Microwave Oven is emitting RF energy on a completely different frequency.

The frequency of the RF energy in your Microwave Oven is such that it excites the Water Molecules in the food that’s in the Oven. The water molecules start moving around a lot faster. You may recall from high school physics that when molecules get moving faster, they create heat. Well, that’s what happens to the food in your Microwave – the fast moving Water Molecules heat it up from the inside.

Well, that was a nice lengthy explanation, and now you understand your Microwave Oven a whole lot better, but what’s this all got to do with welding pieces of plastic together? I’m glad you asked, because here comes the payoff (finally!). Certain kinds of plastic have polar molecules. That means that they can be excited by RF energy, just like water molecules are excited by a Microwave Oven’s RF energy. An RF Heat Sealer is just like a Microwave Oven, except it isn’t enclosed, and the RF energy is tuned to a different frequency, one that excites the Plastic Molecules instead of exciting Water Molecules.

Why use Radio Frequency to generate heat instead of just applying Heat energy directly?

They both accomplish the same thing, to get the plastic hot enough to weld together, but just as your Microwave heats a lot faster than your regular Oven, an RF Heat Sealer heats the material more quickly than a plain Heat Sealer. It takes less time to make a seal, so your production rate goes way up. In addition, (again just like a Microwave Oven) plastic heated with RF energy cools down more quickly than if it was conventionally heated, meaning it will be safer to handle, come off the machine more quickly, and keep its shape better immediately after the sealing.

What does a Buffer do?

Now we get to the fun part, at least for me, because this is where what we sell comes into the picture. One other difference between sealing using conventional heat and sealing using RF is that when you seal with RF, you need a Buffer beneath the material that you’re sealing.

Why? Believe it or not, the purpose of the Buffer is to require you to use more RF energy to seal! As you seal, you are melting your plastic and reducing the thickness, because the die is pressing down on nearly molten plastic. The RF energy is busy exciting your plastic, but like any electrical field, it is looking to complete the circuit in the easiest way possible. The nearest Ground is the bottom platen of your sealing die. Without a Buffer, as soon as the plastic is thin enough in one place, all the RF energy your machine generates would flood through that one weak spot to the bottom platen, and burn a hole in the product that you’re sealing.

When you use a Buffer, even as the sealing area of your product thins, the Buffer provides you with substantial electrical resistance to the RF energy, keeping it diffused throughout the surface of the sealing die, and not allowing it to break through one part of your product and concentrate in that one spot. It increases the window between incomplete sealing (too little power) and burn-through (too much power).

The Buffer also does other things:

  • It can give you the right balance between sticking and releasing the material that you seal.
  • It can serve as a landing place for the sealing die that’s softer than the steel of the bottom platen.
  • It can cushion the plastic you’re sealing to absorb inconsistencies in the material.

While I’ve got you here, let’s spend another minute on that first item. A balance between sticking and releasing? The single most common Buffer problem that people have is sticking, the plastic material adhering to the Buffer and not coming away easily (and we have special Buffers to eliminate Sticking that no one else has). But too much release is bad too, especially if you’re using a multi-step sealing process. You don’t just want your product sliding all over the place; you want to be able to achieve a good registration from step to step.

A certain amount of sticking is also helpful in that too much release can cause your product to stick to the sealing die instead of staying down on the Buffer where it belongs.

What kinds of plastics are RF Heat Sealed?

The most common is PVC (vinyl), but we have customers sealing Urethane, PETG, some kinds of Nylon, some kinds of Styrene, and Polyolefins with EVA. There are others, but some of them can get pretty exotic.

What kinds of products are RF Heat Sealed?

Lots more than you might think. RF Heat Sealing is a technology with very wide applications. Some common ones are:

  • Loose-leaf Binders
  • Dust Filters
  • Bladders in Footballs
  • Pocket protectors
  • Blow-up Toys
  • Inflatable Rafts
  • Intravenous Bags
  • Tarps, Tents and Awnings
  • All sorts of packaging
  • Sheet Protectors
  • Shower Curtains
  • Automobile tops, doors, and sun visors
  • Seat Cushions

I could go on and on . . . (In fact, I already have!). Sorry if I inadvertently left out your product.

For more information about our SealGuard Buffers, click here.

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I gratefully acknowledge the assistance of Art Livingston of Heat Seal Services in putting together this page. Most of the facts are his, any errors are mine.