United States Patent Office P, , „ 4 3fB8,,8j;40
Patented Apr. 15, 1969
This invention relates generally to the joining of materials with high frequency radial vibrations and more particularly to an improved method and apparatus for fabricating containers from similar or dissimilar materials. 25
In recent years, there has been a great increase in the use of thermoplastic film or thin sheet as a packaging material and as a liner in containers. The great increase in the use of these materials for packaging is due to the fact that the package can be made fluid-tight to thus en- 30 close the packaged article to prevent the entrance or exit of moisture from the package or container. This fluidtight feature is a result of the fact that certain thermoplastic films are inherently moisture-tight, and in view of their thermoplastic nature, can be ultrasonically sealed to make 35 a moisture-tight seal. In view of these marked advantages of thermoplastic film over other packaging material, there has been great activity in the art of sealing thermoplastic film material with the use of ultrasonic energy.
Plastic sealing with ultrasonic energy has been growing 40 rapidly in importance, not only in the packaging field but also in other areas as well. The art of sealing two plastic surfaces together by applying ultrasonic vibrations perpendicular to those two surfaces by means of an applied longitudinally vibrating tool is by now well known. In 45 fact, it must be appreciated that sheer vibrations are readily absorbed by thermoplastic materials and hence will not transmit well. This is the basic reason for employing compressional waves to do the sealing when the vibrations must travel through the thermoplastic material 50 in order to get to the desired sealing location.
Most existing ultrasonic joining or sealing devices generally comprise a generator producing an alternating current with a frequency that is in the ultrasonic range, a transducer preferably of the magnetostrictive type, which converts the high frequency electrical output of the generator into a high frequency longitudinal mechanical vibration or compressional wave, and a member or tool connected to an end of the transducer and being operative to transmit the vibrations longitudinally therethrough. The tool is vibrated in the direction of the longitudinal axis of the transducer, and the sheet material to be sealed is positioned between and in contact with the working surface of the tool and a hard surface or anvil positioned in a plane perpendicular to the longitudinal axis of the tool.
In the bonding or joining together of thermoplastic sheet material through the use of devices of the type described above, the tool is vibrated perpendicular to the planes of the two films or sheets being joined, that is, in the direction of the longitudinal axis of the transducer, "° to obtain the repeated compressing of the thermoplastic material at the indicated ultrasonic or high frequency and
thereby generate heat internally within the thermoplastic materials so that fusion therebetween results without affecting substantial areas of the sheet materials adjacent with the joint. Although the above method and ultrasonic equipment has found acceptance and use for sealing of thermoplastic materials, it is substantially limited to the sealing of flat surfaces.
A major disadvantage of the prior art systems for ultrasonic joining is that they are limited for use in operations where it is not required that any substantial arc or curvature in the plane of the longitudinal vibrations is required. If a seal requiring a substantial curvilinear distance and wherein said linear distance is in the plane of the longitudinal vibrations, it has been heretofore impossible to obtain said seal without rotation or relative movement between the tool and the surfaces to be joined.
The primary object of the present invention is improved methods and apparatus for the sealing or joining of materials.
A further object of the invention is to provide improved methods and apparatus for the joining of materials utilizing radially directed ultrasonic vibrations.
Yet, another object of this invention is to provide an improved method of affecting the joinder of relatively thin sheet material by the application of high frequency radially directed vibrations.
An additional object of this invention is to provide improved apparatus to permanently joining thermoplastic materials having a predetermined shape.
In accordance with an important aspect of the present invention, it is possible to seal surfaces that have a substantial curvilinear arc in the plane normal to the longitudinal vibrations. For small curvatures, it is sufficient to curve the output of the blade sealing element, because the angle of incidence of the compressional waves into the plastic will not become so great as to cause considerable sheer energy to be introduced at the plastic surface. As soon as such sheer energy becomes significant, the surface of the plastic, which is not to be sealed, overheats and undesirable distortion results, and the ability to get the ultrasonic energy to the sealing site itself diminishes.
When the curvature of the surfaces to be sealed is large, we can no longer rely on longitudinal vibrations, except in the special case when the longitudinal vibrator output is reduced to a very small area, approximately a point. Then, of course, the tool may be made to traverse any path while being oriented so as to provide compressional waves in the plastic wherever said tool is applied. But, this invention foresees these difficulties and removes them all, by providing a curvilinear surface, every point of which in the sealing region desired vibrates substantially perpendicular to the tangent to the surface at that point. It will be appreciated at once that such motion is not possible through simple longitudinal vibrations. Instead recourse must be had to a characteristic mode of vibration of the tool being used, such that the above perpendicularity criterion of vibration is maintained. One simple example of this kind of tool is one shaped in the form of an annular disk. Such a tool is described in Patent No. 3,093,937, assigned to the present assignee, and wherein the present applicant is a co-inventor. The tool may be readily vibrated by means of a longitudinal vibrator attached thereto, in its fundamental radial vibration mode. In this case, every point on the outer and inner circumference of the annular tool will execute high frequency vibrations which at each circumferential point are radially directed (i.e., perpendicular to the tangent to the circumference at that point).
In accordance with the present invention, the materials to be joined are supported in contact with each other in the area of the joint and ultrasonic energy applied thereto. Although the vibratory energy is applied substantially
