Short description:
In the most common configuration, the material is sealed between a matrix of the desired shape and a flat fixed steel plate coated with brass or aluminum liner. The molded electrode is also usually made from a brass strip one or two inches high, the same as the desired seal, and is attached to a plate mounted on a press. Of course, the type and size of the press, shaped electrode and bottom plate depend on the required application.
General consideration:
Print quality is determined by three factors:
- Heating Current (RF Power)
- Heating time
- pressure
To some extent, these factors are independent of each other, for example, more current or more pressure does not necessarily reduce the sealing time. These factors determine the type and thickness of the material, as well as the total number of seals.
Heating time:
When you turn on the power, the material heats up and its temperature increases, naturally, as the temperature rises, heat is connected through the dies and air until the heat balance value is reached. At this point, the amount of heat released in the plastic material remains constant. This temperature, indicating a peculiar equilibrium condition between the heat generated and heat loss to compaction, should be above the melting point of the plastic.
This is the time required (measures in seconds or fractions of it) to reach this melting point, defined as "heating time".
Heat losses are naturally higher with thinner materials and less with thicker materials. Indeed, very thin materials (less than 0.004 inches) lose heat so quickly that it becomes very difficult to seal them. It was found that some bad heat conductors can be used as buffers, which do not melt, easily destroyed by high frequency.
A typical heating period is between one and four seconds. To reduce failures, we propose to set a timer that determines the heating cycle, slightly above the minimum time required for a good seal.
Pressure:
The electrodes provide heating current to melt the material and pressure to fuse it. As a rule, the lower the pressure, the worse the seal. Conversely, a higher pressure usually creates a better seal. However, too much pressure will lead to excessive thinning of the plastic and unwanted extrusion along the sides of the seal. A fire can be caused by the fact that the two electrodes move closer to each other, because they damage the plastic, the buffer and / or, possibly, the matrix.
In order to obtain high pressure and yet avoid the aforementioned drawbacks, the “stop” on the press keeps the moving cube in its motion. This is configured to prevent the matrices from completely closing when there is no material between them. This also prevails the stamp from the complete cutting of the material and at the same time gives a seal of a given thickness. When using the type of stamping seal stops are not installed on the press, as required thinning of the region of the lacrimal seal.
To ensure a uniform seal, it is necessary to ensure proper pressure at all points of the seal. To insure this, they grind dies, completely flat and held parallel to each other in the press. They must also rigidly design stamps to prevent deformation under pressure.
Power:
The power required for good compaction is directly proportional to the compaction area. In addition, thicker materials require less energy than thinner materials, because thinner materials lose heat faster for dies. Our tightness calculator shows the maximum compaction area available for each device. However, keep in mind that these numbers are calculated for concentrated areas. The sealing area will be smaller for long thin seals and for some materials that are difficult to seal.
Adjustment of power, time and pressure:
When setting up a new sealing task, the first test should be carried out with minimum power, moderate time and average pressure. If the compaction is weak, you should gradually increase power. For maximum freedom from burning or arc, power should be kept as low as possible, in accordance with good sealing.
Seals should be held in parallel to ensure uniform pressure at all sites. If there is too much extrusion or if the seal is too thin, a “stop” press seal should be used. To set the stop, place half the total thickness of the material to be sealed on the bottom plate. Close the press and adjust the lock nut. Then insert the entire thickness of the material into the press and print. Check the result and lower or raise the "stop" as needed.
If the seal is weak in certain places, then the stamps are not even. Check and adjust the adjustment screws. If these adjustments are still unsatisfactory, the stamp may need to be superficial.
After you have done a lot of seals, the stamps are then heated essentially, and time and power may require re-adjustment after a few hours of work. To eliminate readjustment, they equip many machines with heated top plates for preheating to working environment temperature. The use of heated rollers is desirable when using tear-off seals.
Arc:
If you do not make various adjustments correctly, arcing through the material may occur. Fire may also occur when the material to be sealed has a different thickness in different parts of the seal or where the matrix overlaps the edge of the material. In these cases, arcing may occur in the air gaps between the material and the die. Increasing power can sometimes fix this.
Ignition can also occur due to dirt or foreign objects on the material or stamps. To avoid this, it is necessary to ensure that the material and the machine are clean.
Sharp angles and edges on arrays can also cause arcs. The edges of the matrix should always be rounded and smooth. When sparking occurs, it is necessary to thoroughly clean and smooth the matrix with a fine emery cloth. Never attempt to seal a material that has previously been shorted.
Arc suppression:
Since they now make the sealing electrodes larger and more complex, it is important that there is no arc damage on the crystal. Although stamps are being repaired, the loss of production time for repair can be prohibitively high.
We supply all Thermatron equipment with arc suppression devices. The function of this device is to feel the possibility of an arc and then turn off the RF power before a damaging arc can occur. Before performing full production cycles, sensitivity control is usually set (which can be set for various applications and areas of compaction). The device does not prevent arcing, but perceives an arc, and then turns off the power that precedes the damage to the matrix.
As an option, an arc suppressor tester can be added to the block, which checks the arc suppressor before each cycle to ensure proper operation.
Buffers:
As a rule, high-frequency heating is improved by a thin layer of insulating material called a buffer. You attach it to one or two dies to isolate the material to be sealed from the die. This does several things: it reduces heat loss from materials to dies; it compensates for the small irregularities of the surface of the matrix and can help make a good seal, even if the matrix is not completely flat; it reduces the tendency to arc when using too much time or pressure. In general, it makes a better print with a smaller arc. Buffer materials must have good heat resistance and high voltage. Of the many materials used (bakelite, paper, glass, Teflon, Mylar glass, silicone, fiberglass, etc.). Bakelite (class xxx thickness from 0.010 to 0.030 inch) can be successfully used in most cases. A strip of cellulose or acetate tape attached to a molded matrix can be used with highly efficient results.
