Brief Description:
In the most coarse set up, the material is sealed between a die of the desired shape and a flat stationary steel plate covered with a brass or aluminum liner. The shaped electrode, too, is normally made of a brass strip one or two inches high, as thick as the seal wanted and fastened to a plate mounted on the press ram. The type and size of press, shaped electrode and lower platen will, of course, depend upon the required application.
General Consideration:
Three factors decide the quality of a seal:
Heating Current (R.F. Power) Heating Time Pressure
To some extent these factors are independent of one another, for example, a larger current or more pressure does not necessarily sell out the sealing time. The type and thickness of material and the total are of the seal decide these factors.
Heating Time:
As you turn on the power, the material heats up and its temperature rises, naturally, as the temperature rises, heat is conducted off through the dies and the air until a stat 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 balance condition between the heat generated and the heat loss to the seal must be above the melting point of the plastic.
It is the time required (measures in seconds or fractions of this) to reach this melting point defined as the "heating time".
The heat loss is naturally greater with thinner material and less with thicker material. Indeed, very thin materials (less than 0.004") lose heat so rapidly that it becomes very difficult to seal them. From this we can see that, overall, thicker materials wish more heating time and less power than thinner materials. Furthermore, it was found that distinct poor heat conductors that do not melt of deteriorate verily under the impact of high frequency can be used as buffers. Bakelite, Mylar, silicone glass and Teflon, for example, are exquisite in enhancing the seal.
The usual heating period ranges from one to four seconds. To sell out failures, we propose that the timer determining the heating cycle should be set slightly above the minimum time found needful for a good seal.
Pressure:
The electrodes supply the heating current to melt the material and the pressure to fuse it. Generally, the lower the pressure the poorer the seal. Conversely, a higher pressure will normally furnish a good seal. However, too much pressure will supervene in undue thinning out of the plastic material and in an objectionable extrusion along the sides of the seal. Arcing may be caused because of the two electrodes attractive closer to each other thus damaging the plastic, the buffer and / or possibly the die.
To obtain high pressure and yet avoid the above disadvantages, s "stop" on the press restrains the attractive die in its motion. This is set to forestall the dies from conclusion fully when there is no material between them. This also prevents the die from cutting fully 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 wanted.
To insure a uniform seal, the allowable pressure must be obtained at all points of the seal. To insure this, they grind the dies perfectly flat and held parallel to each other in the press. They must also rigidly make the dies to forestall warping under pressure.
Power:
Power required for a good seal is directly proportional to the area of the seal. Moreover, thicker materials wish less power than thinner materials because thinner materials lose heat to the dies more rapidly. Our sealability calculator shows the maximum area of the seal obtainable with each unit. However, bear in mind that these figures are calculated for concentrated areas. The sealable area will be less for long thin seals and for distinct 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 medium pressure. If the seal is weak, you should increase power gradually. For greatest leisure from burning or arcing, the power should be kept as low as possible, consistent with good sealing.
The dies must be held parallel to furnish 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 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 supervene and lower or raise the "stop" as required.
If the seal is weak at distinct spots, the dies are not level. The leveling screws should be checked and adjusted. If these adjustments are still unsatisfactory, the die may have to be outside ground.
After making many seals, the dies then warm up substantially and the time and power may wish readjustment after several hours of operation. To eliminate readjustment, they equip many machines with heated upper platens to pre-warm dies to operating temperatures. Use of heated platens is desirable when doing tear seals applications.
Arcing:
If you do not make the various adjustments 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. Expanding the power can sometimes remedy this.
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 engine clean.
Sharp corners and edges on dies may also cause arcing. The die edges should all the time be rounded and smooth. When arcing occurs, the dies must be considered cleaned and smoothed with fine emery cloth. Never try to seal material that has previously been arced.
Arc Suppression:
Since they are now making sealing electrodes larger and more complex, it is needful that no damage due to arcing occurs on the die. Although dies are repairable, the loss of production time for repairs can be prohibitive.
We supply all Thermatron tool with arc suppression devices. The function of this expedient is to sense the possibility of an arc and then turn off the R.F. Power before a damaging arc can occur. Before full production runs are made, normally a sensing operate (which can be set for various applications and sealing areas) is preset. The expedient does not forestall arcing but senses the arc, then shuts off the power that prevents damage to the die.
As an option, an Arc Suppressor Tester can be added to the unit, which tests the arc suppressor before each cycle to insure allowable operation.
Buffers:
Typically rf heating is improved by a thin layer of insulating material called a Buffer. You attach this 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 outside 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. Overall, it makes a good seal with less arcing. Buffer materials should have a good heat resistance and high voltage breakdown. Of the many materials used (Bakelite, paper, glassine, Teflon, glass Mylar, silicone, fiberglass, etc.). Bakelite (grade xxx about 0.010 to 0.030 inches thick) can be used successfully in most cases. A strip of cellulose or acetate tape adhered to the shaped die can be used with extremely sufficient results.
Electronic Sealing
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