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 eliminated, but it's always recommended to clean the dies to avoid arcing and prevent die damage.
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. Surface 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.
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 shuts 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.
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 known as “Buffer Material” is used to isolate the article from the 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 materials)
- Bakelite (Bakelite grade XXX about .010 to .030” is used quite often)
- Teflon (Because it is the least RF conductive material, it can cause issues)
- Silicone fiberglass