Abstract:
Sealed synthetic resin containers are spin welded by bringing opposed, generally complimentary horizontal mating surfaces of the lid and bowl into contact under a predetermined axial pressure and rapidly accelerating rotation of the lid relative to the bowl while maintaining substantially the axial pressure to produce melting of the mating surfaces until a desired torque value is attained. Thereafter, the rotation of the lid is immediately decelerated to terminate the rotation and allow the mating surfaces to bond. Generally, the torque is monitored repeatedly during the accelerating rotation until a plateau is reached and the accelerating step is continued for at least 50 milliseconds thereafter.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the sealing of plastic lids or closures to plastic containers and, more particularly, to the spin welding of lids and containers. 
     The process of spin welding is one in which the friction between relatively rotating lids and containers such as bowls and cups causes the synthetic resin at abutting surfaces to melt and bond the two surfaces upon cooling. Generally, many spin welding processes have involved spinning the lid or cover at a relatively high speed and then bringing it into contact with the container which produces the melting friction as the lid rotation is slowed and then stops. Exemplary of such a process is U.S. Pat. No. 3,297,504 to Brown et al. 
     The industry has been seeking ever shorter spin welding cycles to expedite the filling and sealing process, while obtaining a reliable weld over the entire circumference to ensure a good seal. 
     Hofuis et al U.S. Pat. No. 5,772,103 describes an alternate process in which the lid and container are first brought into contact under vertical load and then rotational force is applied for a preset small number of rotations to effect the desired fusion. 
     In any spin welding process, there is a problem in dealing with variables such as variations in surface characteristics and configuration, contamination of the abutting surfaces by the contents of the container during filling, etc. 
     Accordingly, it is an object of the present invention to provide a novel method of spin welding lids and containers which will accommodate surface variations and produce well sealed containers. 
     It is also an object to provide such a method which may be practiced easily and rapidly in high speed filling lines. 
     Another object is to provide novel apparatus for spin welding lids and containers which accommodates surface variations and produces well sealed containers. 
     SUMMARY OF THE INVENTION 
     It has now been found that the foregoing and related objects may be readily attained in a method of making sealed synthetic resin containers which comprises molding thermoplastic synthetic resin into bowls and lids which have substantially complimentary mating surfaces extending about the periphery thereof. One of the bowls is filled with product, and the opposed, generally horizontal mating surfaces of the lid and bowl are brought into contact under a predetermined axial pressure while gripping the bowl and lid securely. The lid is rotated to the bowl with rapidly accelerating rotation while maintaining substantially the axial pressure to produce melting of the mating surfaces until a desired torque value is attained. The rotation of the lid is then immediately decelerated to terminate the rotation and allow the mating surfaces to bond. 
     The rotation is effected at a high degree of acceleration and the termination of rotation is effected at a high degree of deceleration. The torque is monitored repeatedly during the step of accelerating rotation until a plateau is reached, and the rotation is continued for at least 50 milliseconds thereafter. The monitoring of the torque is conveniently effected by monitoring the power being drawn by a motor used for producing the accelerating rotation. 
     The predetermined axial pressure is preferably 200-500 p.s.i., and the acceleration and termination of rotation is effected in less than four relative rotations of the lid relative to the bowl. Both the acceleration and deceleration are at a rate of 35,000-250,000 rpm/sec. 
     Desirably, there is included the step of directing steam onto the mating surface of the bowl prior to bringing the lid into contact therewith, and nitrogen is preferably admixed with the steam. As a result, a partial vacuum is produced in the sealed container. 
     The apparatus for producing the spin welded containers comprises a holder for firmly gripping a bowl, a holder for firmly gripping a complimentary lid, and means for moving the holders relative to each other to bring mating surfaces into contact. Pressure applying means is provided to apply predetermined axial pressure on the abutting surfaces, and acceleration and deceleration means is provided for rapidly accelerating rotation of the holders relative to each other and for rapidly decelerating the relative rotation while maintaining substantially the predetermined axial pressure. Torque sensing means is included to sense the torque generated at the mating surfaces which is operative upon the acceleration and deceleration means to determine when the acceleration has reached a desired value indicative of the desired melting of the mating surfaces and then to effect the deceleration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevational view of the lid spinning assembly of a spin welder embodying the present invention; 
     FIG. 2 is a side elevational view thereof and also fragmentarily illustrates the bowl holding subassembly; 
     FIG. 3 is a rear elevational view of the lid spinning assembly of FIG. 1; 
     FIG. 4 is a sectional view of the chuck subassembly drawn to an enlarged scale; 
     FIGS. 5A-5D are graphic representations respectively, of applied pressure, torque, velocity and rotational distance for a cup/lid assembly of different samples to a reach predetermined torque set point; 
     FIGS. 6A-6D are graphic representations illustrating the relationships, respectively, of applied pressure, torque, velocity and rotational distance; 
     FIG. 7 is a flow chart of the logic for the spin weld cycle; 
     FIG. 8 is an enlarged fragmentary cross sectional view of a bowl holder, bowl and lid prior to the spin welding thereof; 
     FIG. 9 a  is a bottom view of the lid holder; 
     FIG. 9 b  is a cross sectional view of the lid holder; 
     FIG. 10 is a fragmentary cross sectional view of the lid holder blade in the section designated by the broken line circle of FIG.  9  and drawn to an enlarged scale; 
     FIG. 11 is an enlarged fragmentary illustration of the blade holder and lid engaged thereby; 
     FIG. 12 is an enlarged fragmentary cross sectional view of the lid and bowl following spin welding; 
     FIG. 13 is a partially diagrammatic perspective illustration of a production unit for spin welding containers; 
     FIG. 14 is a diagrammatic plan view of the production unit of FIG. 13; and 
     FIG. 15 is a graphic representation of velocity and torque during the spin welding operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning first to FIGS. 1-3, therein illustrated is a single station spin welding machine embodying the present invention and which employs a lid supporting and spinning assembly generally designated by the numeral  10  that mounts the lid holder assembly generally designated by the numeral  12 . The chuck subassembly  14  of the lid holder assembly  12  includes at its lower end the lid holder  20  in which is seated a lid  32 . Rotation of the chuck subassembly  14  and lid holder  20  are effected by the servomotor  16  through the gear box  30 , and the servomotor  16  and lid holder assembly  12  are vertically movable by the air cylinder/piston  28  on slides  34 . 
     As seen in FIG. 2, a fragmentarily illustrated bowl or cup carrier generally designated by the numeral  24  has a cup holder  26  thereon in which is seated the cup  22  in alignment with the lid  32 . The cup holder  26  has a multiplicity of projections or teeth (seen in FIG. 8) about its periphery to grip the cup  22  firmly therein against relative rotation. 
     Torque monitoring and power supply means to the servomotor  16  are indicated by the numeral  6  and control and power means for the air cylinder  28  are indicated by the numeral  8 . 
     Turning next to FIG. 4, the chuck subassembly  14  is seen to include a linear bearing  44  in which the shaft  38  of the servomotor  16  is slidably seated. The shaft  38  acts upon a compression spring  40  so that, when the servomotor  16  moves the lid holder assembly  12  downwardly against the cup carrier  24 , the spring  40  is compressed while the chuck  14  slides upwardly relative thereto. 
     As seen in FIGS. 9-11, the lid holder  20  has a multiplicity of teeth or gripper blades  46  spaced thereabout which are driven into the lid  32  by the downward movement to prevent its rotation therein. 
     As seen in FIG. 8, the cup holder  26  has teeth or gripper blades  48  spaced thereabout which are driven into the cup  22  by the pressure exerted on the upper surface thereof as the lid holder assembly  12  moves downwardly. Thus, the lid  32  and cup  22  are both restrained against rotation relative to their respective holders  20 ,  26 . 
     As also seen in FIG. 8, the peripheral portions of the lid  32  and cup  22  are configured to provide a cooperating circular groove or recess  70  and a projecting circular rib  72  which extends thereinto. The cup  22  also has a circumferential flange  74  which extends outwardly of the rib  72  and cooperates with a depending circumferential flange  76  on the lid  32 . 
     The assembled container is illustrated in FIG.  12  and it can be seen that the resin in the adjacent surfaces of the lid  32  and cup  22  about the recess  70  and rib  72 , and about the flange  74  and flange  76  has fused to provide a relatively large area of bonding and sealing. Any contamination on those adjacent surfaces is expressed circumferentially and any flash is blended between the opposing flanges  74 ,  76 . 
     In operation of this apparatus, a cup  22  is loaded into the cup holder  26 , and the lid  32  is loaded into the lid holder  20 . The piston/cylinder  28  is actuated by the control  8  and moves the lid spinning assembly  10  downwardly against the cup  22  in the carrier  24 . As this downward movement takes place, the gripper blades  46 ,  48  are embedded into the lid  32  and cup  22 , and the spring  40  is compressed to provide the desired biasing force. 
     The servomotor  16  is actuated by the control  6  to effect relative rotation of the lid  32  relative to the cup  22  until the desired torque is reached at which time the control  6  rapidly decelerates the servomotor  16  and relative rotation to produce the desired welding of the opposing surfaces of the lid  32  and cup  22 . 
     The desired parameters of the process will be discussed in detail hereinafter. 
     Although the apparatus described hereinbefore is suitable for low volume productions and testing of materials and configuration, FIGS. 13 and 14 diagrammatically illustrate a high speed packaging installation provided by a cooperating pair of belt drive units generally designated by the numerals  80 ,  82 . The unit  82  has a single drive belt  84  which has cup holder elements  86  thereon providing a semi-circular recess  88 . The unit  82  has a first drive belt  90  with cooperatively configured cup holder elements  86  thereon, and the belts  84 ,  90  are synchronously driven by a gear (not shown). As can be seen, the drive paths of the belts  84 ,  90  are elongated with arcuate ends. The belts  84 ,  90  initially converge from the intake end to bring the cup holder elements  86  into abutting relationship, travel parallel through the central spin welding area, and then diverge at the unloading end. 
     A conveyor belt  94  extends between the units  80 ,  82  below the cup holder elements  86  and conveys the cups  22  into the space between the cup holder elements  86  on the belts  84 ,  90 . As the belts  84 ,  90  converge, the holder elements  86  firmly grip the cups  22  between them, and the conveyor belt  94  then desirably descends slightly so that the cups  22  are supported only in the holder elements  86  as they pass through the welding station. Thereafter, the conveyor belt  94  ascends to support the welded containers which are released by the holder elements as the belts  84 ,  90  diverge towards the discharge end of the installation. 
     The unit  82  has a turret  96  with a second drive belt assembly  98  which is synchronously driven with the belts  84 ,  90 , and it carries the brackets  100  upon which are mounted the servomotors  102  and which are slidable on the vertical posts  104  by action of cams (not shown) at the entrance and exit ends of the welding area. The lids (not shown) are inserted into the lid holders (not shown) on the side of the belt path opposite the welding area. 
     As in the prior single station embodiment, the motor  102 , clutch and lid holder containing the lid (all not shown) are moved downwardly by a cam (not shown) at the beginning of the welding area to load the spring and embed the gripper blades in the cup and lid, and a short burst of rotation of the motor  102  produces the spin weld of the opposing surfaces. 
     As is conventional, the cups or bowls are filled with the product prior to movement on the conveyor belt into the spin welding installation. 
     As indicated hereinbefore, the method of the present invention is one which involves the rapid acceleration of relative rotation of the lid and disc after they have been brought into contact and are under axial pressure which is maintained during the spin welding operation. The torque generated (indicated by the power being drawn) by the motor producing the rotation is monitored to determine when it reaches a plateau and the rotation continues for a short time thereafter, following which the motor and rotation are rapidly decelerated. As will be appreciated, this torque will increase until the opposing surfaces melt and reduce the frictional drag. This ensures that the spin weld cycle lasts only for the time required to obtain sufficient melting to produce a good bond. 
     As can be seen in FIG. 15, there will be an initial spike as the motor overcomes inertia after which the power requirement drops down and thereafter climbs to a plateau as the resin melts. The rotation is continued for a short period after reaching the plateau, usually at least 50 milliseconds, to ensure thorough melting and expression of any contamination on, the opposing surfaces. Continuation time periods greater than 100 milliseconds really provide no significant benefit and may be detrimental. 
     During the spin weld cycle, the axial pressure on the opposing surfaces is maintained by the compression spring as the surfaces melt and flow, and this ensures the desired rapid and substantially uniform fusion. This axial pressure should be at least 100 p.s.i. and is preferably in the range of 200-500 p.s.i. 
     The motor and its controller must effect rapid acceleration and deceleration at a rate of 35,000-250,000 rpm/sec. This can be effected by the motor directly or through appropriate gear drive. 
     With the present process using axial pressure as described above, contamination on the surfaces to be bonded will be expressed outwardly, and flash is preferably confined by providing a pair of cooperating circumferential flanges as illustrated. 
     FIGS. 5A-5D illustrate the variation in revolutions for three different specimens to reach the desired plateau in torque while applying the same pressure and rate of acceleration and deceleration. 
     FIGS. 6A-6D illustrate the relationship between applied pressure, torque, velocity, acceleration, deceleration and rotational distance for a specimen cup and lid having a diameter of 3{fraction (7/16)} inches and fabricated from a polypropylene/polyethylene resin and having a configuration substantially as seen in FIG.  8 . 
     Although either the lid or the cup may be rotated, the lid is preferred because of lesser inertia and the tendency for the contents of the cup to be violently agitated by the rapid acceleration and deceleration 
     The term “cup” as used herein include cups, bowls, bottles and other containers of generally circular cross section. 
     The computer control for the spin welding assembly easily effects and synchronizes the several motions and cycles. Monitoring of the power demand is easily effected by an ammeter in the circuit to the motor, and the biasing force can be measured by a load cell. Movement of the lid assembly towards and away from the cup can be effected by hydraulic or pneumatic piston/cylinder units, or by mechanical action using cams and the like. 
     If so desired, the spin welding installation may include a series of nozzles to direct steam about the filled cup prior to the welding station so that steam fills the headspace in the container when the lid is sealed. When the steam condenses, a vacuum is formed in the container. 
     For some applications, it may be desirable to provide a controlled atmosphere in the headspace. As is known, providing a gas in the headspace will reduce the time required to cook the contents of the container after the lid is sealed. In a rotating cooker, this bubble will also agitate the contents to reduce the time required. By using steam and nitrogen injection prior to sealing, nitrogen can be included in the headspace, and it is inert to the contents. 
     Thus, it can be seen from the foregoing detailed description and attached drawings that the novel spin welding process of the present invention is rapid, efficient and accommodating of variations in surface characteristics of the mating surfaces and of contamination thereof. The method can be practiced in high speed lines which can be fabricated and operated at reasonable cost.