As the power is turned on, 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 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 heat generated and heat loss in order to seal must be above the melting plastic. It is time required (measured in seconds or fraction thereof) to reach this melting point that is defined as the “sealing time”. 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.
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 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.
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 seals 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 making a number of seals, the dies then warm up somewhat and the time and power may require re-adjustment after several hours of operation. To eliminate re-adjustment, many machines are equipped with heated upper platens to pre-warm dies to operating temperatures. Use of heated platens is desirable when doing tear seal applications.
Pre Seal time and a heated platen can also change these factors.
Heated Platens also maintain consistency.