RF heat sealing also known as dielectric sealing is accomplished by sending 27.12MHz High Frequency power through two or more layers of thermoplastic (polar) material that has been placed between two electrodes (RF sealing dies).

In the simplest terms, RF welding systems are microwave ovens that cooks plastic instead of food. The difference between the two is that in a microwave oven, you put the food between two electrodes that do not move, in RF welding systems the electrodes compress the material. The material melts under the RF energy creating the product shape and form needed.

The dies (electrodes) are manufactured using brass, aluminum and/or other types of metal. They come machined with the shape of the final product. The two standard types of dies are:

1. Single cycle - to perform flat and tear seals in one seal cycle

2. Dual Cycle - to perform port seal and perimeter seal in two cycles applying RF power to different locations of the product through an RF Polarity Switch.

The dies are mounted onto a pneumatic or servo operated press bed and Ram. When the press closes, the dies apply pressure. When sufficient RF energy is present the different layers of material are sealed together. When properly welded the material will tear before the seal. It is important to remember to allow sufficient cooling time after stopping the energy flow to solidify the molten material.

The RF energy at 27.12 MHz frequency is connected to one side of the die and the other side is grounded. When RF is energized it excites the polar molecules of the materials compressed between the dies and fuses the layers together.

Some products require various components to be sealed on the material surface or between the layers of material. This process may require uneven power consumption for the component seal versus the perimeter seal. To control the RF energy transfer onto a specific area of the product, the sealing die is constructed from isolated modules. When sealing occurs the RF power is distributed to each module through a RF polarity switch. 

After the seal cycle is complete, the RF power is turned off, but the dies remain closed to cool the product.  After cooling, the layers are homogeneously bonded together.

Each product and process have a unique predefined control parameters that dictate the seal quality. The seal thickness is controlled by hard stops or pressure that limits the die travel. Hard stops are better, making the process repeatable.

1. Die compression
2. Pre Seal Time
3. RF Power and Process Tuning
4. Sealing Time
5. Cooling Time
6. Die surface finish and parallelism
7. Die temperature
8. Buffer Material
9. RF power connection
10. Ground connection

When RF welding tougher materials, electrode pressure is critical for achieving a good bond.

Electrical effect: low pressure will cause arcing or surface flashes.

Mechanical effect: low pressure will cause poor seals.

When RF welding some materials, too much pressure will "crash" the plastic before it reaches the melting point causing uneven or bad seals.

When too much pressure is applied the material will rip apart and the die surfaces will touch creating a short or arc. To prevent this the press stroke should be controlled by a stroke adjustment or hard stops built into the die. In most applications hard stops are used to prevent the dies from closing more than is required.

To ensure a uniform seal, the proper pressure must be obtained at all points around the seal area of the dies. The dies must be made or ground perfectly flat, and held parallel in the press. They must also be rigidly constructed to prevent warping under pressure.

Pre-Seal Time is defined as the time required to close the press and build enough pressure to apply RF power. The time is required to eliminate air gaps between the layers of materials. Air gaps cause arcing and/ or flashing to occur.

Old vacuum tube RF sealers have ZERO pre-seal power. ONEX RF systems are capable of sensing the flow of the press cylinder. The material can be excited using low or high power RF energy, tuning the process before the main seal power is applied.

The amount of power required for a good seal is directly proportional to the seal area of the product. When the RF power is turned on the molecules start vibrating and the material heats up(by changing polarity of the molecules in the different layers of polar material at 27.12 MHz). When too much energy is applied, the material will burn. When not enough energy is applied, the material layers will not bond. When the correct power level is acheived, the material temperature will rise and a state of equilibrium will be achieved between the generated heat and heat loss through the contact surfaces of the dies. Heat loss is greater with thinner materials. When sealing ultra thin materials (less than 0.004”) heat loss is so great that it becomes very difficult to seal the material layers together without the use of buffer or heated dies.

The sealing chart below shows the amount of energy required to seal the product based on the size of the seal area. Besides the seal area and material thickness, the material type or types also plays a big role in setting the RF power.

Main Seal Time is defined as the time required to reach the material melting point to bond the number of material layers under a set RF power when the dies are closed.

Cooling Time is defined as the time required to cool the plastic after turning the RF power off and letting the material flow and fuse before opening the press.

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.

The dies must be held parallel to produce even pressure around the seal area. If there is too much extrusion or if the seal is too thin, the press sealing stops need to be adjusted. To set the stops, adjust the press to half the total thickness of the material. Close the press and adjust the stops finger tight. Then insert the full thickness of material in the press and make a seal. Check the results and lower or raise the stops as required. Lock the stops in place.

If the seal is weak at a particular area, the dies are not level. The leveling screws should be checked and adjusted. If these adjustments are still unsatisfactory, the use of a shim may be required or the dies must be surface ground.

After several RF seal cycles the dies warm up and system parameters (time and power) may require re-adjustment. To eliminate parameter adjustments, some dies are designed with heated upper platens that pre-heat them to the proper operating temperature. This process is used in tear seal applications.

Pre-Seal time and heated platens can change these factors. Heated platens help maintain consistency.

Materials Sealability Chart

(x) = Response of the materials in the 20 to 30 Mc/sec range

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

(ONEX RF technology require 30-50% less power)

Arcing

Arcing may occur in several cases:

• When material layers have different thicknesses in the seal area.
• When the die overlaps the edge of the material.
• Too much power.
• Bent Mandrel or too close to one side of the die when port sealing.
• When the port seal did not melt the material to the correct thickness
• Dirty materials or carbon build up on the die surface
• Sharp corners on the dies

In summary an air gap, unparalleled dies and foreign material in the seal area can cause arcing. 

The die edges should always be rounded and smooth.

When an arc occurs, the dies must be carefully cleaned with an emery cloth and solvent. In ONEX RF systems die damage is virtually elimated, but it's always recommended to clean the dies to avoid arcing and prevent die damage.

Surface Flash

This is sometimes confused with arcing. A flash occurs on the surface of the material during the sealing cycle. It leaves smoke and or a layer of black carbon on the die. Clean all traces of carbon off. Surfacr flash is caused by:

• Press pressure too low or press stops set too high
• Power set too high
• Die too cold

The arc suppressor usually does not stop the flash unless it burns the material before the cycle ends. ONEX RF Systems are designed to detune the welder via The Match network, since during the flash the material temperature goes so high it almost reaches a plasma level and that changes the circuit impedance.

Arc Suppression

Most dies are complex and expensive not counting the fact that down time can reach hours if die damage occurs, it is essential to protect the dies from arc damage. Although dies are repairable, the loss of production time for repairs is extremely expensive.

Most sealing equipment comes with an arc suppression device. The function of this device is to sense the possibility of an arc and then turn off the RF power before the arc can damage the dies. ONEX RF Arc detect circuit has the fastest response time in the industry. The electronics use 5vdc power and Op Amp circuit to detect voltage drop when current surges occur due to an arc. It automatically shut down the RF power output. In parallel to shutting the RF Power, the Match Network tries to detune the process. No sensitivity adjustment is required with ONEX RF systems unlike other systems in the field.

ONEX RF's Arc Shut Down device does not prevent arcing but rather senses the arc and instantly shuts off power which prevents damage to the die. Note: The Arc-Detect circuit functionality can be tested by shortening the upper and lower dies without presence of RF power. If the Arc detected message appears on the screen then it functions properly.

Buffer Materials

When the material is heated between the two electrodes, heat loss occurs through the metal surface of the dies. In order to minimize the loss an insulating material, knows as “Buffer Material” is used to isolate the article from die metal surface.

In most cases sealing is improved by using a thin layer of Buffer between one or both layers top and bottom dies and article.

The Buffer Material does several things:

• It lowers the heat loss from the materials to the dies
• Compensates for small dents in the die surface
• Decreases the tendency to arc when thin material is used.
• In general, it makes a better seal with less arcing.

Buffer Material should have good heat resistance and high voltage breakdown such as:

• Mylar (one of the best and most used Buffer material)
• Bakelite (Bakelite grade XXX about .010 to .030” is used quiet often)
• Teflon (Because it is least RF conductive material can cause issues)
• Silicone fiberglass
• Glassine