CA1152391A - Ultrasonic smoothing method and apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE A method and apparatus for smoothing and levelling a thermoplastic coating on a substrate by a utilization of ultra-sonic energy. The coated substrate, which may for example be wax coated paperboard, is conveyed past an ultrasonic generator which directs ultrasonic vibratory energy onto the thermoplastic coating. The ultrasonic energy heats and works the coating thereby resulting in its levelling and smoothing. The coated substrate is conveyed over an idler roller located immediately below the ultrasonic generator which functions to precisely position the substrate relative to the generator.

Description

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ULTRASONIC SMOOTHING METHOD AND APPARATUS
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates, in general, to the treat-ment of a subs-trate on which a thermoplastic coating is applied and, more particularly, pertains to the treatment of the thermo-plastic coating with ultrasonic energy to result in substantial levelling and smoothing thereof.
Wax coated paperboard is frequently utilized in the manu-facture of many articles of commerce such as containers for food products as cereals, crackers or the like. In the manufacture of these items, printing is normally applied to the product after the application of the wax coating. Unless the coating is uni-formly applied to the paper substrate, problems are often encoun-~ tered in the printing operation which result in the finished ; product having an unsightly and aesthetically unpleasing appear-ance. This is particularly a problem when the aesthetic qualities , of the resulting container are relied upon for customer appeal in the marketplace.

2. Discussion of the Prior Art In the prior technology a wax coating has been applied to a paperboard substrate, and has then been smoothed and levelled by passing the coated substrate under a stationary or rotating heated metal wiper bar. Unfortunately, this prior art approach has not always proved to be satisfactory as it often resulted in streaks in the finished product~ Further, if the level of the heated wiper bar was not accurately adjusted by the operator, the resulting product often had significant variations in con-~, tinuity. Also in this arrangement the paperboard web is in con-';

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stant contact with the heated smoothing bar, and i~ the equip-ment is shut down for any period of time, the heated smoo-thing bar would often damage the web surface in contact with the bar.
; Tsuchiya et al U.S. Patent 3,956,046 discloses a method and apparatus for ultrasonically welding a number of composite sheets in an overlapped relationship. More particularly, this reference discloses ultrasonically welding overlapped paper sheets, having coatings thereon of a thermoplastic resin, to form a con-tainer such as a milk carton. In this arrangement the sheets to be connected or welded are placed between a working table and an ultrasonic horn, and ultrasonic vibratory energy is uniformly applied to the seam of the overlapped sheets. The ultrasonic vibratory energy results in a melting of the thermoplastic coatings and a resultant joinder of the sheets. This patent is typical of many prior art arrangements wherein ultrasonic equipment is util-ized to weld thermoplastic materials in either a spot or seam weld. The requisite ultrasonic equipment is readily available commercially, and generally includes at least a power supply, a converter and a horn. The power supply generates a high frequen-cy electrical signal, typically at 20,000 hz. The converter receives the electrical signal from the high frequency power supply, and converts it into a mechanical vibration. Generally, the converter is one of two types, either piezoelectric or mag-netostrictive. Piezoelectric converters utilize a polycrystalline element such as lead zirconate-titanate which exhibits electro-strictive behavior. In these converters the piezoelectric ele-; ment is excited by a high frequency electric current which causes it to mechanically vibrate at the frequency of the electrical signal. Magnetostrictive converters employ a ferromagnetic

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material which exhibits magnetostrictive behavior, chanying di-mensions when magnetized. In these converters a high frequency alternating current is applied to an exeitation coil, developing an alternating magnetic flux field, and the magnetostrictive core mechanieally vibrates at the frequency of the field. The ultra-sonie horn transfers the vibratory energy from the eonverter to the work pieee. Ultrasonic horns are available in a varie-ty of different shapes sueh as eatenoidal, exponential, eireular, reet-angular with rounded or square corners, and many other suitable shapes which may be utilized depending upon the particular com-mercial application.
In ultrasonic welding, a thermoplastic material is typ-ieally subjected to ultrasonie vibrations produced by equipment of the type previously described, and causes a combination of surface and intermolecular frictions in the interfaced area.
The ultrasonic vibrations cause the thermoplastic materials to melt, resulting in a chemical bond and uniform weld. A wide variety of available thermoplastic materials are recognized in the prior art as being suitable for ultrasonic welding applications.
; 20 Ultrasonic equipment is also utilized in the prior art to perform scan or continuous welding in which parts to be welded are nested on a conveyor system and transported beneath one or more vibrating horns. Intense heat is immediately gen-erated thereby resulting in welding of the parts, which are thereafter removed from the conveyor system.
Ultrasonie equipment is also utilized in the prior art in a proeess known as ultrasonie insertion in whieh a hole is pre-molded slightly smaller in diameter than an insert whieh it is to receive. U1trasonie energy travels through the plastic until it meets the joining area between the metal insert and plastic, ',~''~;

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creating heat by the plastic vibrating agains-t the me-tal, there-by melting the plastic and permitting the insert to be driven into place.
Furthermore, ultrasonic equipment is recognized in the art as having other industrial applications including ultrasonic sewing for cutting, sealing, sewing and quilting fabric and film, ultrasonic soldering for fluxlesssoldering and tinning of non-ferrous metals, ultrasonic machining for dri~ling, threading, grinding and milling of hard brittle materials, and ultrasonic metal welding for producing lap joints.
Although ultrasonic equipment has been utilized in the prior art in a number of industrial applications, it has not been utilized in an ultrasonic smoothing technique as described herein.
_MMARY OF THE INVENTION
In order to obviate or ameliorate the shortcomings and limitations encountered by the prior art processes for smoothing a thermoplastic coating on a substrate, it would be desirable to have a system in which the power requirements are less than those of current systems, a more uniform wax continuity is achieved by more effective levelling of the wax, fewer production adjustments are required in operation of the system, and the resultant wax coating has a superior gloss and is thereby more aesthetically pleasing.
Accordingly, in one disclosed embodiment of the inven-tion, thermoplastic coating applied to a substrate is levelled and smoothed by the application of ultrasonic energy thereto from an ultrasonic generator. The ultrasonic energy heats and works the thermoplastic coating, thereby resulting in substantial levelling and smoothing -thereof. Further, a conveyor system is utilized to ' ~15%~9~

convey the substrate with its thermoplastic coating past the ultrasonic generator at a uniform rate. In a preferred embodi-ment an idler roller located immediately below the ultrasonic generator is utilized -to precisely position the substrate and coating relative to the generator.
Accordingly, it is a primary object of the present in-vention to produce a substrate having a thermoplastic coatiny thereon which is applied uniformly and smoothly over the substrate surface.
Another object of the present invention is to produce a uniform coating on a substrate via the application of ultrasonic energy thereto, resulting in melting and working of the coating and a substantially uniform levelling and smoothing thereof the substrate.
Further, another object of the present invention is to produce a substrate having a thermoplastic coating thereon which is smoothed by a technique using less power than arrangements currently utilized in the art, in which fewer control production adjustments are required, and further wherein the thermoplastic coating has a superior finish.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects of the present invention for an ultrasonic smoothing method and apparatus may be more readily understood by one skilled in the art with reference to the follow-ing detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings wherein like ref-erence numerals are utilized to refer to identical components throughout the several views, and in which:
Figure 1 illustrates schematically a prior art approach ~5239~
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Figure 2 is a schematic illustration of a preferred embodiment of the present invention wherein ultrasonic energy is utilized to achieve a uniformly level coating of thermoplastic material on a substrate; and Figure 3 illustrates a second embodiment of the present invention similar in concept to Figure 2 but utilizing a rectangular horn having rounded corners.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring now i~ detail to the drawings, Figure 1 illus-trates a prior art arrangement for smoothing and levelling a thermo-plastic coating 10 on a substrate 12, with the combination thereof forming a composite web 14. In this arrangement, a heated smooth-ing bar in the form of a roller 16 is positioned immediately above the thermoplastic coating 12 on the web 14 which is being conveyed in the direction of arrow 18 past the relatively stationary smooth-ing bar at a substantially constant rate. The heated smoothing bar produces a combination of pressure and heat on the thermoplastic coating to result in smoothing and levelling thereof, but may also result in streaks in the finished product, particularly if the height of the bar above the web is not properly adjusted. Further-- more when the apparatus encounters downtime for whatever reason, the heated smoothing bar is in continuous contact with a small por-tion of the web, and thereby often results in damage thereto. In other prior art arrangements similar in concept to tha-t shown in Figure 1, the heated smoothing bar may be nonrotatively mounted in the apparatus. In either arrangement, the bar may be suitably heated as by hot water, steam, electrical coils, etc.

Figure 2 illustrates a method and apparatus for smoothing -~

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a thermoplas-tic coating 10 on a substrate 12 pursuant to the teachings of the present invention wherein the web 14 is conveyed, as by a typical roller conveying system, under a suitable ultra-sonic generating system 20. In the illustrated arrangement the . ultrasonic generating system 20 includes a high frequency power supply 22 for generating a high frequency electrical signal at a suitable frequency, for instance 20 kilohertz, a converter 24 which is basically a transducer for converting the high frequency electrical signal to a mechanical vibration at the same frequency, and a horn 26 which conducts the mechanical vibration -to its trans-mitting end 28 whereas the mechanical vibrations are converted to an ultrasonic acoustical signal which is directed onto the thermo-plastic coating 10 to cause heating thereof. Horn 26 is shown as a slotted bar horn which extends across the full width of web 14.
. The substrate is supported adjacent to the rectangular transmitting end 28 of the horn by an idler roller 30 positioned beneath the ;: ultrasonic generator and the function of which is to maintain a ` precise position of the web relative to that of the ul-trasonic generator. The transmitted ultrasonic energy heats and works -the thermoplastic layer, pushing the coating uniformly over the sub-strate and causing a substantial levelling and smoothing thereof.
Levelling and smoothing of the coating in this manner results in .- substantial continuity of the thermoplastic coating, thereby caus-ing the resultant layer to have a superior gloss to finishes ob-tained with prior art arrangements as illustrated in Figure 1. The power supply 22, converter 24, and slotted bar horn 24 are commer-cially available items, obtainable for instance from Branson Sonic Power Company, a division of Branson Ultrasonics Corporation, Danby, Connecticut.

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The ultrasonic smoothing appara-tus is precisely con-trolled in frequency and power input to control the generated temperature and pressure electronically, thereby resul-ting in -the elimination of errors caused in the prior art by imprecise human adjustments. Further, the continuity of the wax coating should be superior to that achieved by prior art arrangements as illustrated in Figure 1, and also should result in the achievement of a superior gloss on the wax coating. Fur-ther, a minimal pressure is applied to the wax coating while its continuity is being worked and pushed by the ultrasonic acoustical energy.
Figure 3, illustrates a second embodiment of the presen-t invention, similar in concept to the arrangement shown in Figure 2, but wherein the horn is a rectangular horn 32 having rounded corners, - which should be suitable in the practice of the present invention, and which is also commercially available from Branson Sonic Power Company.
The present invention was developed to level and smooth a thermoplastic wax coating on a paperboard substra-te, with -the finished composite product being suitable for the manufac-ture of containers for food produc-ts such as cereals, crackers or the li]ce.
However, it should be apparent that the teachings of the present invention have applicability to other suitable substrates and thermoplastic coatings also.
While several varia-tions of one embodiment of the pre-sent invention have been described in detail, it should be apparent that many modifications and variations thereof are possible within the ordinary level of expertise of one skilled in the art.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for smoothing a thermoplastic coating which has been applied to the surface of a substrate, comprising:
a. an ultrasonic generator positioned adjacent to said substrate for directing ultrasonic energy onto the thermoplas-tic coating to cause heating and working thereof;
b. means for moving the relative positions of the sub-strate and the ultrasonic generator, thereby resulting in a uniform levelling and smoothing of the coating on the substrate.

2. Apparatus for smoothing a thermoplastic coating applied to a substrate as claimed in claim 1, said ultrasonic gen-erator being arranged to straddle one surface of said substrate, said moving means including an idler roller positioned adjacent to the ultrasonic generator at the opposite surface of said sub-strate to precisely position the coating and substrate relative to said ultrasonic generator.

3. Apparatus for smoothing a thermoplastic coating applied to a substrate as claimed in claim 1, wherein said ultra-sonic generator includes an ultrasonic horn.

4. A method for smoothing a thermoplastic coating ap-plied to the surface of a substrate, comprising:
a. directing ultrasonic onto the thermoplastic coating to heat the coating;
b. and moving the substrate and the ultrasonic gener-ator relative to each other so as to thereby produce a uniform levell-ing and smoothing of the coating on the substrate.

5. A method of smoothing a thermoplastic coating ap-plied to a substrate as claimed in claim 4, wherein said moving step includes utilizing an idler roller positioned adjacent to the ultra-sonic generator on the opposite side of the substrate to precisely position the coating and substrate relative to said ultrasonic generator.

6. A method of smoothing a thermoplastic coating ap-plied to a substrate as claimed in claim 4, wherein an ultrasonic horn is utilized to direct ultrasonic energy onto the thermoplastic coating.