by: Andy Oliner
Radio frequency (RF) heat sealing is the most common approach for sealing vinyl products – especially for loose leaf binders. This article will discuss some of the basic principles of RF heat sealing; why buffers are important in the process; and the most common applications.
Exactly what is heat sealing?
When you are fabricating an item from two or more pieces of plastic, you need a way of welding these pieces together. When you need to weld sheet materials together in an overlapping fashion, the preferred method is heat sealing. There are three types of heat sealing. In order of increasing cost, they are impulse sealing, RF sealing, and ultrasonic sealing. As the cost increases, the sealing process cycle time reduces. Most small and mid-size manufacturers have found RF sealing to be a good compromise between the cost in machinery and the benefit of increased efficiency and throughput.
What is RF heat sealing?
RF sealing works in much the same way as a microwave oven, where radio frequency energy is beamed throughout the interior of the oven. The frequency of the RF energy in a microwave oven is such that it excites the water molecules in the food that is in the oven. The water molecules start moving around a lot faster, which in turn, heats the food from the inside. Likewise, certain kinds of plastics have polar molecules. That means that they can be excited by RF energy, just as water molecules are excited by a microwave oven’s RF energy. An RF heat sealer is just like a microwave oven, except it is not enclosed and the RF energy is tuned to a different frequency – one that excites the plastic molecules instead of water molecules.
Heat sealing generally requires a solid, flat lower platen (on which the materials to be sealed are placed) and a heat sealing die above the lower platen. During the heat sealing process, the die comes down to the platen holding the material to be sealed and the die is energized with RF energy. This energy, along with the downward pressure that the head of the press is exerting, seals the material together in the desired shape.
Why use RF heat sealing to generate heat vs. applying heat energy direct?
Both methods accomplish the same end result, which is to get the plastic hot enough to weld together. The direct application of heat is referred to as “impulse sealing”. However, just as your microwave heats a lot faster than your regular oven, a RF heat sealer heats the material more quickly than an impulse sealer. Consequently, the rate of production increases significantly. In addition, plastic heated with RF energy cools down more quickly than plastic heated by conventional methods, which means it is safer to handle, comes off the machine more quickly, and keeps its shape better immediately after the sealing process.
Why is it important to use a buffer with RF heat sealing?
Another difference between sealing with conventional heat versus RF energy is that when using the RF method, a buffer is needed beneath the material to be sealed. Why? Believe it or not, the purpose of the buffer is to require you to use more RF energy to seal. As you seal, the plastic is melting and reducing in thickness, because the die is pressing down on nearly molten plastic. The RF energy is busy exciting the 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 the sealing die. Without a buffer, as soon as the plastic is thin enough in one place, all the generated RF energy would flood through that one weak spot to the bottom platen and burn a hole in the product that is being sealed.
When you use a buffer, the additional dielectric resistance that the buffer provides means that even as the sealing area of your product thins, you continue to have a substantial electrical resistance to the RF energy. This keeps the RF energy diffused throughout the surface of the sealing die and stops it from breaking through one part of your product and concentrating in that spot. It increases the window between incomplete sealing (too little power) and burn-through (too much power). In addition, the buffer accomplishes the following:
- Provides the right balance between sticking and releasing the material that is sealed.
- Serves as a landing place for the sealing die that’s softer than the steel of the bottom platen.
- Cushions the plastic being sealed to absorb inconsistencies in the material.
A balance between sticking and releasing is an important ingredient for successful RF heat sealing. The single most common buffer problem is sticking, with the plastic material adhering to the buffer and not coming away easily (special buffers are made to eliminate this problem). However, too much release also can be bad, especially if there is a multi-step sealing process. You do not want the product sliding all over the place as it moves from station to station. To achieve a good registration, the product needs to stay in place on the buffer belt. A certain amount of sticking also is helpful in that too much release can cause the product to stick to the sealing die instead of staying down on the buffer where it belongs.
What plastics and types of products are most commonly RF heat sealed?
The most common plastic utilized in RF heat sealing is PVC (vinyl), but other plastics also are being utilized, such as urethane, PETG, certain kinds of nylon and styrene, and polyolefins with EVA. The products that utilize RF heat sealing are more numerous than one might think. Some common products include the following:
- Loose leaf binders
- Pocket protectors
- Sheet protectors
- Blow-up toys
- Shower curtains
- Intravenous bags
- Seat cushions
- Automobile tops, doors, and sun visors
- Tarps, tents, and awnings
Andy Oliner is the President of Oliner Fibre. Oliner offers a large variety of materials specifically engineered as Heat Seal Buffers. For more information, contact Oliner at (800) 654-6373 or visit www.heatseal.com.