US2908320A - Apparatus for heat sealing - Google Patents

Description

Oct. 13, 1959 H. A. ROHDIN 2,908,320

APPARATUS FOR HEAT SEALING Filed Dec. 8, 1954 2 Sheets-Sheet l IIOWARD A- Roma/Al INVEN TOR.

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Oct. 13, 1959 H. A. ROHDIN 2,908,320

APPARATUS FOR HEAT SEALING Filed Dec. 8, 1954 2 Sheets-Sheet 2 Ha WARD A. MO/N INVENTOR.

United States Patent 1,903,320 APPARATUS FOR HEAT SEALING Howard A. Rohdin, Glen Ridge, NJ. Application December 8, 1954, Serial No. 473,866

4 Claims. (Cl. 15442) It is an object of this invention to provide a method and apparatus for heat sealing which will have all of the advantages of the so-called impulse sealing while avoiding many of the disadvantages of that method as currently practiced.

It is a further object of this invention to provide a method and apparatus as aforesaid which, while accurately adjustable over a wide range of conditions may nevertheless be produced at minimum cost.

The above and other objects will be made clear from the following detailed description taken in connection with the annexed drawings in which:

Fig. 1 is a schematic diagram of the wiring of the improved apparatus;

Fig. 2 is a schematic diagram of the hydraulic circuit of the improved apparatus;

Fig. 3 is an elevation, partly in section of the sealing mechanism; and

Fig. 4 is a section through the bleeder valve shown in Fig. 2.

It was recognized more than thirty years ago that heat seals would be improved if the parts were rapidly cooled following the initial heating and pressing step. Thus applicants prior Patent No. 2,392,695 showed the desirability of conducting the cooling operation without any separation of the surfaces engaged with the material to be sealed until the cooling was complete. Recently these principles have been applied to the so-called impulse sealing in which a metallic member of small mass is pressed into contact with the plies to be sealed. The metallic member is then subjected to a surge of electric current of very short duration. This surge is sufficient to bring the metallic member up to the requisite sealing temperature and also to transmit suflicient heat to the plies to bring these also to scaling temperature. When the current is cut off, the metallic member, due to its low mass, cools rapidly along with the plies which have been sealed. When the cooling is complete, the pressure is relieved.

The sealing cycle just described is subject to certain objections. The first of these is that the entire temperature rise of the heated member occurs while the member is in pressure contact with the material to be sealed. This rise may be from two to four hundred degrees F. above room temperature and must take place within a fraction of a second. This requires extremely high current and produces a great instantaneous expansion of the heating member which expansion is resisted by the pressure grip of the jaws. Such resistance sometimes induces stresses within the member exceeding its elastic limit. When this is the case the stresses, acting in compression, tend to wrinkle or buckle the member with resultant imperfections in the seal. A second and serious defect arises out of the conditions just described. A current value sufiiciently highto raise the temperature of the member several hundred degrees in a fraction of a second is sufiiciently high to bring the member to incandescence if permitted to flow for too long a time. The period of current flow, therefore, not only must be short, but must be regulated with the utmost accuracy, :since excessive temperature would be destructive of the work with which the member, during the heating stage, is in pressure contact. Such precise timing adds quite disproportionately to the cost of the apparatus. A concrete example will make this clear: the usual heating period in an impulse sealer runs from 0.1 to 0.2 second. A timer which is accurate within 0.01 second is in itself an expensive item, but even such a timer has an accuracy only within 5% of an 0.2 second cycle and its accuracy is only within 10% of an 0.1 second cycle. Since the cost of timers increases geometrically with the limits of accuracy, effective timing of an impulse sealer is extraordinarily costly.

The present invention overcomes the defects above noted. The temperature rise is accomplished while the metallic heating element is out of contact with the work, and the heating is discontinued after the member is in pressure contact with the work to be sealed. In order to minimize the mass of the member, whereby to accelerate the cooling step, provision is made for a predetermined time lag between the attainment of full pressure and the cutting off of the current. While this involves a timing step, the timing does not require pinpoint accuracy, since it is carried out under conditions of constant or slightly decreasing temperature and under such conditions there is absolutely no danger of damage to the work to be sealed.

Referring now to Fig. 2 there is shown a heating ele ment 10 positioned between a lower pressure jaw 12 and an upper pressure jaw 14. A rod 16 connects the upper jaw 14 with a piston 18 slidable within a cylinder 20. A spring 22 opposes downward motion of the piston 18. A pressure line 24 connects the cylinder 28 to a valve 26 containing a conventional, slidable core 28 for connecting the line 24 alternately to a pressure source 30 and to an exhaust line 32. A solenoid M, when energized, draws the core 28 to the pressure position shown in Fig. 2.

A special needle valve 34 has pressure connection with the line 24 and is in turn connected by a line 36 with a pilot valve 38. The arrangement is such that when sufficient pressure is applied to the pilot valve 38, the valve will draw the core 28 to its opposite, or exhaust position. A conventional pressure switch P is also connected to the line 24- for a purpose to be described later.

Referring now to Figs. 1 and 2, and assuming that the core 28 of Fig. 2 is in its leftward position, a cycle is initiated by closing a manually operated switch 50 to energize the solenoid M thereby to draw the core 28 to its rightward position as shown in Fig. 2. The switch 50 may be push button or treadle operated as convenience may dictate, or, in a fully automatic operation, as in a bag making machine, it may be cam operated. In any case, it should be of the snap contact type. The valve 26 is of the balanced pressure type so that only momentary energization of the solenoid M is required to initiate the cycle.

At the moment when a cycle is initiated, a normally closed relay T will be in closed position establishing a circuit between power lines L1 and L2 through a fixed resistance R1 in parallel with a rheostat R2. A third fixed resistance R3 will be in parallel with the fixed resistance R1 and the rheostat R2 through the pole 52 of a normally closed two pole relay 54 and the current of all three resistances will flow from L1 through the heating element 10 thence through the normally closed relay T to the line L2. Since, in a parallel hook-up, the total current'is equal to the sum of the currents flowing 3 through the several resistances, the condition just described produces maximum current fiow through the heating element 10.

As will appear hereinafter, the maximum current condition just described can occur only after the pressure jaws 12 and 14 are separated. With the three resistances in parallel the current through heating element 10 is sufii oient, very quickly, to raise its temperature to the desired maximum.

At least one end of the heating element 10 is secured to a drum 56 formed of Micarta or other suitable insulating material. The drum 56 is spring biased in a counter-clockwise direction to place the element 10 under tension. As the element 10 heats up it expands permitting a slight rotation of the drum 56 which carries or approaches a contact arm 58. Upon rotation of the drum 56 contact arm 58 establishes a circuit from the line L2 through the closed contacts of the relay T, through a line 60, through the coil 62 of the relay 54' thence through the resistance R3 to the line L1. The coil 62, being energized, draws the pole 52 out of contact and draws a second pole 64 into contact to close a circuit comprising a line 66 which runs through the normally closed contacts of the relay T to the line L2 and which thus places the coil 62 and the resistance R3 in series back to the line L1, thus locking the relay 54 and removing the resistance R3 from any relationship to the heating element 10 which thereafter is subject only to the sum of the currents flowing through R1 and R2. The lock-out feature of the relay 54 is provided in order to avoid any hunting condition of the drum 56 and its arm 58 and thereby to avoid any risk of overheating the element 10 while it is in pressure contact with the work.

The relay T is of the time delay type preferably adjustable for a lag of from to 0.5 second between the time it is energized and the time it breaks the circuit. For the purpose of this disclosure, energization of the normally closed relay T is made responsive to the attainment of a predetermined pressure in the line 24 (Fig. 2). It could, however, be made responsive to a predetermined mutual approach of the pressure jaws 12 and 14. If the work to be sealed is very light the lag of relay T may well be 0, but if the work is thick, heavy and heat resistant, a lag of as much as 0.5 second may be required. The alternative to the lag would be to increase the mass and thereby the heat capacity of the element 10, but such an increase would slow down the cooling step and it is this step which occupies most of the time during which the jaws are in pressure contact.

With the energization of solenoid M the core 28 is drawn to the position shown in Fig. 2, thereby connecting the line 24 to the pressure source 30. This drives the piston 18 downwardly to close the jaws 12 and 14. Contact between the jaws 12 and 14 brings about a sharp pressure rise in the line 24 which actuates the pressure switch P. The switch P momentarily closes the circuit through the coil 70 of a two pole solenoid 72. This is a normally open relay and both of its poles 74 and 76 are drawn to closed position when the coil 70 is energized. The pole 74 closes a circuit through the normally closed limit switch L and serves to keep the coil 70 in energized condition. The upper pole 76 closes a circuit to energize the time delay relay T which, after a predetermined lag, breaks all of the circuits through the heating element 10.

Referring now to Figs. 2 and 4-, the valve 34 is shown to have a connection 34' to the line 24. The valve is made up of a shell 80 Within which is slidably mounted a piston 82 having a central, conical bore 84 surrounding a needle 86. The needle has threaded engagement with the shell 80 and is adjustable by means of a knurled head 88. A duct 90 connects with the line 36 running to the pilot valve 38. The piston 82 has an upwardly projecting portion 92 ending in a concave valve seat 94 which engages with a mating formation formed in the shell 80. An aperture 96 is formed in the shell 80 to establish communication between the atmosphere and the space above the piston 82.

Initially the piston 82 is at the bottom of the shell 80. The sharp pressure rise incident to the jaws 12 and 14 coming into contact lifts the piston 82 to the position shown in Fig. 4 with the channel 84 narrowly limited by the needle 86. Thereupon there results a relatively slow build-up of pressure in the valve 38 until this pressure becomes sufiicient to cause the pilot valve 38 to shift the core 28 to its leftward position as shown in Fig. 2, thus cutting off the pressure source 30 and connecting the line 24 to the exhaust line 32. This results in a sharp pressure drop in the line 24. The piston 18 and the jaw 14 are lifted by the spring 22. The drop of pressure in line 24 permits the piston 82 to drop to the bottom of the shell 80 thereby giving the duct 00 direct communication with the aperture 96 for prompt unloading of the pilot valve 38.

As the jaw 14 rises it momentarily opens the limit switch L thereby deenergizing the coil 70 and permitting both poles 74 and 76 to return to their normal open position. This in turn breaks the circuit of time delay relay T which closes the circuit through heating element relay 54 immediately reconditioned the circuit of the one end of the heating element 10.

fixed resistance R3 to operate in parallel with the resistances R1 and R2.

While a wide variety of specific mountings of the heating element 10 could be devised, the arrangement shown in Fig. 3 is suggested. Here frame 100 has legs 102 and 104 slidably mounted in amain frame element 106. A spring 108 supports the leg 102 while a spring 110 supports the leg 104. Above the leg 104 an arm 112 terminates in an insulated clamping jaw 114 in which is secured Above the leg 102 is an arm 116 in which the roller 56 is mounted and spring biased in the direction of the arrow 118. An arm 58 is secured to the member 116 and upon rotation of the roll 56 incident to expansion of the element 10 the arm 58 contacts the element 10 and thereby establishes a circuit through the line 60 as shown in Fig. 1.

When the jaw 14 descends under pressure applied by the piston 18 it carries the heating element 10 with it into contact with the jaw 12, the work being interposed, preferably, between the element 10 and the jaw 14. The springs 108 and permit the frame 100 to sink with the element 10.

A roller 120 is journaled in a bracket 122 secured to the upper jaw 14 for the purpose of contacting and actuating the limit switch L. It is clear, therefore, that there can be no resumption of heating until the jaw 14 has risen a predetermined distance above the jaw 12, and thereby, through the roller 120 has opened the limit switch L.

From the foregoing it will be clear that the requirements of absolute accuracy of timing have been minimized. Ample time is provided for the heating in the period during which the jaws are out of contact. The accelerated heating is automatically discontinued before the jaws come into contact and when contact is established the rate of heating is only suflicient to maintain a predetermined maximum temperature. For this reason, the time lag between the establishment of pressure contact and the discontinuance of heating is not sharply criti-.

cal since no ill effect will result from a moderately excessive lag. Damage to the work is completely avoided due to the fact that maximum temperature is attained and assured before the heating element contacts the work.

It is not intended to limit this invention to the precise details disclosed which obviously are susceptible of considerable modification.

I claim:

1. Apparatus for heat sealing superimposed plies of heat scalable material comprising: a pair of opposed jaws; means to move said jaws into opposed pressure engagement with said superimposed plies therebetween; a member having a predetermined heat capacity interposed be tween at least one of said jaws and said superimposed plies; means exerting yielding lengthwise tension on said member; means for heating said member to a predetermined elevated temperature; means responsive to the engagement of said jaws to discontinue said heating; and means responsive to the separation of said jaws for re suming said heating.

2. Apparatus according to claim 1 in which discontinuance of the heating of said member is responsive to the development of a predetermined pressure between the jaws and resumption of heating is responsive to relief of said pressure.

3. Apparatus according to claim 1 in which the means for heating said member to a predetermined elevated temperature include means for accelerating said heating,

means for discontinuing such acceleration upon attainment of said predetermined temperature, and means operative thereafter for maintaining such temperature.

4. Apparatus according to claim 1 in which discontinuance of the heating of said member is responsive to the development of a predetermined pressure between the jaws and resumption of heating is responsive to a predetermined separation of said jaws.

References Cited in the file of this patent UNITED STATES PATENTS 2,460,460 Langer Feb. 1, 1949 2,675,054 Langer- Apr. 13, 1954 2,697,773 Gordon Dec. 21, 1954 2,759,524 Davis Aug. 21, 1956 2,762,420 Stanton Sept. 11, 1956 2,766,809 Parham Oct. 16, 1956 2,780,275 Rusch et a1. Feb. 5, 1957 2,800,162 Rohdin July 23, 1957

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