RF Sealing Process Overview

R.F. heat sealing is accomplished by sending a high frequency (heating) current through two or more layers of thermoplastic material placed between two sealing dies. The dies are machined as the final product outer and or inner shapes per drawing. The dies are mounted onto a (servo, pneumatic or hydraulic) press to close the dies and apply pressure for sealing the materials. The RF energy at 27.12 MHz frequency excites the molecules of the materials, and bonds them together by melting the material under the dies. Some products require sealing of other objects between the layers which requires switching of power from one location to another location of the dies this is performed using an RF Switch. After the seal cycle the RF power is disconnected and dies remain closed to cool the sealed area and after the cooling period, the materials become joined together at the point of the seal.

Basic RF Sealing process requires similar setup shown below:

1. RF Generator
2. Auto-Tuner
3. RF Switch
4. Sealing Dies
5. Press (Air or Servo)
6. Die temperature control system

The Quality of a seal depends on four factors:

1. R.F. Power
2. Sealing time
3. Die Clamping Pressure
4. Die temperature

Sealing Time
”Sealing Time” is defined as time required to elevating the material temperature to a melting point to bond the number of material layers under a set RF power when two dies are closed.

As the power is turned on, the material heats up and its temperature rises. Naturally, as the temperature rises, heat is conducted through the dies and the air until a state of heat balance is reached. At this point, the amount of heat generated within the plastic material remains constant. This temperature, indicating a sort of equilibrium condition between generated heat and heat loss in order to seal must be above the melting plastic. The heat loss is without a doubt greater with thinner material and less with thicker material. Indeed, very thin materials (less than .004”) loose heat so rapidly that it becomes very difficult to seal them. (see paragraph on Buffers). The usual sealing period ranges from one to four seconds. To minimize failures, it is suggested that the timer determining the sealing cycle should be set slightly above the minimum time found necessary for a good seal.

Pressure
The electrodes provide the sealing current to melt the materials and the pressure to fuse it. Generally the lower the pressure the poorer the seal. Conversely, a higher pressure will usually produce a better seal. However, too much pressure will result in an undue thinning out of the plastic material and in an objectionable extrusion along the sides of the seal. as a result of the two electrodes moving closer to each other, arcing may be caused, damaging the plastic, the buffer, and possibly the die.

To obtain high pressure and yet avoid the above disadvantages, the moving die is restrained in its motion by a “stop” on the press which is set to prevent the dies from closing completely when there is no material between them. This prevents the die from cutting completely through the material, and at the same time gives a seal of predetermined thickness. When a tear seal type of die is used, the stops are not set on the press, since a thinning of the tear seal area is desired.

To insure a uniform seal, the proper pressure must be obtained at all points of the seal. To effect this, the dies are made or ground perfectly flat, and held parallel to each other in the press. The dies must also be rigidly constructed to prevent warping under pressure.

Power

The amount of power required for a good seal is directly proportional to the area of the seal. Moreover, thicker materials require less power than thinner materials. Our Sealing Area Chart shows the maximum area of seal obtainable with each unit. However, it must be kept in mind that these figures are for long thin seal, and for certain materials that are hard to seal.

Adjusting Power, Time, and Pressure

When setting up a new sealing job the first test should be with minimum power, moderate time and around 80 PSI pressure. If the seal is weak, power should be increased gradually. For greatest freedom from burning or arcing, the power should be kept as high as possible, consistent with good sealing.

The dies must be held parallel to produce even pressure at all sections. If there is too much extrusion or if the seal is too thin, the press sealing stop should be used. To set the stop, place half the total thickness of the material to be sealed on the lower plate. Close the press and adjust the stop-nut finger tight. Then insert the full thickness of material in the press and make a seal. Check the result and lower or raise the stop as required.

If the seal is weak at certain spots, the dies are not level. The leveling screws should be checked and adjusted. If these adjustments are still unsatisfactory, the die can be shimmed up at the problem areas or the die may have to be surface ground.

After several RF seal cycles the dies warm up and adjustment of system parameters (time and power) may require re-adjustment after several hours of operation. To eliminate parameter adjustment the dies are designed with heated upper platens to pre-warm dies to operating temperatures. This process is desirable when performing tear seal applications.

Pre Seal time and a heated platen can also change these factors
Heated Platens also maintain consistency.

Materials Sealability Chart

 Sealing Area Chart

Total Sealing Area in Square Inches

With average conditions, good grade vinyl, quality electrode dies, timing & pressure settings. For tear seal and special applications ask our technical staff

Common formula to calculate the wattage is 3 square inches = 1 KW

(3KRF solid state generators may require 30-50% less power)

Arcing & Arc Prevention   

Arcing

If the various adjustments are not made correctly, arcing through the material may occur. Arcing may also occur when the material to be sealed has different thickness at various parts of the seal or where the die overlaps the edge of the material. In these cases there can be arcing in the air gaps between the material and the die. Sometimes this can be remedied by increasing the pressure or decreasing the power.

Arcing may also occur because of dirt or foreign matter on the material or dies. To avoid this, care must be taken to keep the material and the machine clean.

Sharp corners and edges on dies may also cause arcing. The die edges should always be rounded and smooth. When arcing occurs, the dies must be carefully cleaned and smooth with emery cloth and solvent.

Warning: Never seal material with dies that had just arced without cleaning.

Surface Flash

This is sometimes confused with arcing. A flash that occurs on the surface of the material during the sealing cycle, smoke and or black carbon layer is left. Clean all traces of carbon off. This is caused by any combination of these:

Press pressure too low or press stop set too high.
Power set too high.
Die too cold.

The arc suppressor usually will not stop this unless it burns all the way thru.

Arc Suppression

Since electrodes are now being made larger and more complex, it is essential that no damage due to arcing occur on the die. Although dies are repairable, the loss of production time for repairs is prohibitive.

Most sealing equipment is supplied with arc suppression devices. The function of this device is to sense the possibility of an arc and then turn off the R.F. power before a damaging arc can occur. A sensing control which can be set for various applications and sealing areas is easily pre-set before full production runs are made. The device does not prevent arcing but senses the arc, then shuts off power which prevents damage to the die.

As an option, an Arc Suppressor Tester (Self Test) can be added to the unit, which tests the arc suppressor before each cycle to insure proper operation.

Buffers

In many cases sealing is improved by a thin layer of insulating material called “buffer”. This is attached to one or both dies to insulate the material to be sealed from the die. This does several things: It lowers the heat loss from the materials to the dies; it compensates for small irregularities in the die surface and may help to make a good seal even if the die is not perfectly flat; it decreases the tendency to arc when too much time or pressure is used. In general, it makes a better seal with less arcing. Buffer material should have good heat resistance and high voltage breakdown. Many materials used - bakelite, paper, glassine, Teflon, glass, Mylar, silicone fiberglass, etc. Bakelite grade XXX about .010 to .030 inches thick can be used successfully in most cases. A strip of “Scotch” cellulose or acetate tape adhered to the shaped die is sometimes used to this advantage

• Ultrasonic

Ultrasonic horns are designed to resonate at 15, 20, 30, 35, or 40 kHz, but since every application is unique, custom manufacturing assures that all special requirements are accommodated.

• Spin Welding
  Spin welding is a process of rotating symmetric round parts, creating friction between the two parts, melting them together.