Using a contoured head for sealing lidding stock

A method and plate structure for peelably sealing lidding stock to a sealing flange of a container. The plate structure has a face portion that is configured in cross section to contact the lidding stock and force the same against the flange surface without cutting the sealing layer of the stock and in a manner that indents the stock and flange, and retains sealing material in a sealing area to effect a good but peelable seal between the lidding stock and flange. The face portion includes an indentor made of a material of high thermal conductivity relative to the material of the remainder of the plate structure.

BACKGROUND OF THE INVENTION 
The present invention relates to the sealing of lidding stock to the flange 
of an open ended container in a manner that permits peeling of the stock 
from the flange. (Lidding stock is generally a multi-layered sheet of 
material provided with a sealing coating, layer or film.) More 
particularly, the invention is directed to a head or plate structure for 
heat sealing lidding stock to a container flange. 
Lidding stock is sealed to a container flange by a plate or ring means that 
applies heat and contact pressure to the lidding stock and flange. The 
plate or ring, in plan view, has a peripheral face of the general shape 
and size of the flange so that when the face is forced against the stock 
the sealing layer of the stock will soften the entire distance around the 
flange and adhere to the flange surface to form an effective seal between 
the stock and flange. The peripheral, sealing face of the plate or ring 
extends in a direction perpendicular to the plane of the plate or ring. 
In sealing lidding material to the flange of a container, it is desirable 
that the head or plate structure that effects the seal soften and indent 
the flange to form a more effective seal. However, head structures 
presently in use and designed to effect indentation of the container 
flange employ an indentor that forces the sealing layer of the lidding 
stock outwardly from the sealing area beneath the indentor and, similarly 
moves the softened (molten) flange material from the sealing area, as 
explained in detail hereinafter, in effect cutting through the sealing 
film, as it is pressed against the container flange. When the consumer 
attempts to peel back the lid from the flange, the sealing material of the 
stock tends to remain intact on the container, at least in part, while the 
lid is removed. The reason for this is explained below. The manufacturers 
and suppliers of packaged items using lidding stock require the entire 
seal to be removable from the flange. 
As explained hereinafter, the problem with heat sealing techniques 
currently in use is that they do not care for the rheology of the heat 
sealing and container material and the non-uniformity of heat transfer 
involved in the sealing process. (Rheology refers to the manner in which 
materials deform.) 
BRIEF SUMMARY OF THE INVENTION 
The present invention is directed to a plate structure that seals lidding 
stock to a container flange in a manner that controls the profile of heat 
and contact pressure to minimize the flow of sealing and flange material 
from the sealing area. This is effected by a special contoured face and 
edge portion of the plate structure. The contoured face and edge includes 
an indentor section that protrudes slightly from the plane and face of the 
remaining body of the edge portion, the remaining body having flat faces 
or curved recesses located on each side of the indentor section. The 
indentor section is effective to soften the container flange by the 
application of heat and permanently indent the same while the flat or 
recessed faces control penetration of the indentor and limit the 
application of contact pressure. The effect of limiting contact pressure 
is that the flow of both the sealing layer of a lidding stock and the 
molten portion of a container flange out of the sealed area is controlled. 
The benefits of this method are realized when using either conduction heat 
or induction heating of the lidding stock if the stock contains metallic 
foil. In either case, a robust, peelable seal is made between the lidding 
stock and the flange. 
In another embodiment of the invention, the head is bimetallic, with the 
indentor section of the sealing edge and face being made of a material 
having high thermal conductivity while the remaining structure of the head 
is made of a material having substantially lower thermal conductivity. The 
low thermal conductivity portion of the sealing face provides less heat 
transfer from the plate to the lidding stock at the areas of the seal 
adjacent the indentor. This provides lower effective temperatures and thus 
increased viscosity of the sealing layer under the flat or recessed faces 
of the plate adjacent to the indentor. The net effect of this 
"differential heat transfer" technique is that control of material flow is 
enhanced. 
The embodiment of the invention employing recesses on each side of the 
indentor, as opposed to the flat configuration, offers superior mechanical 
resistance to abuse of the seal in a manner explained in detail below.

PREFERRED EMBODIMENTS 
Referring now to FIG. 1 of the drawings, FIG. 1 shows the process of 
peeling lidding stock 10 from a flange 12 of a container (not otherwise 
visible in FIG. 1). Arrow 13 in FIG. 1 shows the general direction of 
peel. Lidding stock is comprised of an upper or outer layer 14 supporting 
a lower, inner layer of sealing material 16, such as a suitable organic 
polymer adhesive. The material of supporting layer 14 can be a metal foil, 
such as aluminum or an aluminum alloy, or other suitable material, such as 
an organic polymer. Such a polymer should be relatively strong and able to 
retain its mechanical integrity at sealing temperatures. The material of 
the container and flange 12 can include, but is not limited to, 
polypropylene or a high density polyethelene. Further, the container can 
be single layered, as shown in certain of the figures, or multi-layered. A 
barrier layer may be one of the layers of a multi-layered construction. 
There are several binding forces active in the peel of lidding stock that 
has been heat and pressure sealed to a container surface. The first of 
these is the adhesion of the sealing layer 16 of the lidding stock 10 to 
the container. This is indicated by opposing arrows 1 in FIG. 1 located at 
the interface between the sealing layer 16 and the flange 12. The second 
force is the adhesion of the sealing layer to the foil outer layer 14 of 
the stock, this being shown by opposing arrows 2 located at the interface 
between the outer layer and the sealing layer. A third force involved in 
the peeling process stems from the tensile strength of the sealing layer 
itself. This is shown by a curved line 3 in FIG. 1, which line extends 
lengthwise of the layer. Forces 2 and 3 serve to assist sealing layer 16 
to peel away from container flange 12. In practice, lids can be peeled 
from a container surface when force 1 is greater than force 2, evidently 
because the effect of tensile strength 3 of the sealing film, in addition 
to force 2, are able to overcome the force of 1. 
FIG. 2 of the drawings depicts the problem discussed above in connection 
with a prior approach for indenting and sealing lidding stock 10 to a 
flange 12 of a container not otherwise depicted in FIG. 2. The prior 
method employed a somewhat narrow face 18 of a plate structure 20 to 
effect a seal between the lidding stock and flange by the application of 
pressure and heat via plate 20 and its face 18. (In sealing lidding stock 
to a container, appropriate amounts of heat and pressure are typically 
applied for a time period sufficient to effect the seal.) As shown in 
FIGS. 2 and 3, sealing material 16 is forced out of the sealing area 
between lidding stock 10 and container flange 12 by the indentor, such as 
face 18, to leave a gap 16a in sealing layer 16. Similarly, the flange 
portion beneath gap 16a, being molten, is forced outwardly from the seal 
area. 
As shown further in FIG. 3, sealing film 16 is divided along foil 14 into 
sections 16b and c as the result of the sealing process described above in 
connection with FIG. 2. Further, in the process of forcing out sealing 
layer 16, the face of 18 indents flange 12 and produces outwardly facing 
protrusions 12a, the sharp ends of which form softened "pinning points" 
that receive the sealing material of layer 16 beneath such protrusions. 
When the lidding stock is subsequently pulled from the container flange, 
the pinning point, shown on the left in FIG. 3, mechanically holds sealing 
layer 16b of the stock to the flange, as does the adhesion force 1 (FIG. 
1) between layer 16 and flange 12. Further, because of the gap 16a between 
sealing layers 16b and c, the tensile force 3 provided by a continuous 
sealing layer is not available for pulling a complete layer 16 from flange 
12. 
FIG. 4 of the drawings shows in cross section apparatus 22 for effecting a 
heat and pressure seal between a container flange and lidding stock. 
Apparatus 22 comprises an upper portion 24 that, in turn, includes a large 
heating block 26 adapted to support a plate 28 of the present invention, 
plate 28 performing a sealing process that does not suffer from the 
problems of FIGS. 2 and 3. 
Beneath plate 28 is located a fixture 30 having a recess 32 for receiving a 
flanged container 34 to be sealed to lidding stock 10 in a peelable manner 
when the stock is disposed on a generally planar flange 36 of the 
container. (Flange 36 corresponds to flange 12 in FIGS. 1 to 3.) The 
fixture has a generally planar surface 38 located about recess 32 for 
supporting flange 36 against the force of plate 28 in the process of 
sealing the lidding stock to the flange. 
A typical sealing process involves, first of all, heating block 26 to a 
temperature suitable to heat lidding stock 10 to a temperature at which 
sealing layer 16 will soften and stick to container flange 36 in a 
peelable manner. This can be accomplished by an electrical resistance 
heater (not shown) located inside of the block, though other internal or 
external heating means can be used. 
The sealing process includes the steps of disposing a flanged container 34 
in recess 32 of fixture 30. A sheet of the lidding stock 10 (or a lid cut 
to the breadth of flange 12) is located in the space between plate 28 and 
planar surface 38 of fixture 30, and disposed on flange 36 of the 
container. The upper portion 24 of apparatus 22 is now moved toward 
fixture 30, and a peripherally located face 40 of plate 28 presses the 
lidding stock and flange between the upper surface 38 of fixture 30 and 
face 40 of plate 28 about the circumference of the flange 36. The heat of 
block 26 is transferred to the lidding stock via plate 28 and its face 40. 
The heat and pressure is maintained for a time sufficient to effect a 
peelable seal between the stock and flange. 
The configuration of face 40 in cross section (see FIGS. 6 to 9) is such 
that it indents stock 10 and flange at 50, as shown in FIG. 5, in the 
process of sealing the stock to flange 36. After the seal is made, and if 
pre-cut lids are not used, a cutting die (not shown) descends from the 
upper portion of apparatus 22 to sever outer peripheral portion of the 
sheet of lidding stock from that portion sealed to flange 36 and extending 
across the opening of container 34. In this manner, a peelable lid 10A 
(FIG. 5) is provided for the container. Lid 10A can also be pre-cut before 
it is sealed to a container flange. 
Plate structure 28 avoids the problems discussed with the structure and 
method of FIG. 2. More particularly, plate 28 has a peripheral face or 
edge 40 that is effective to minimize the flow of sealing material from 
beneath the face of the plate and to prevent cutting of the sealing 
material. As shown in greater detail in FIGS. 6 to 9, sealing face 40 of 
plate 28 includes a narrow, rounded protrusion 44 and laterally disposed, 
inner and outer flat areas 46 (FIGS. 6 and 8) or, in the case of the 
embodiments of FIGS. 7 and 9, curved recess areas 48. The distance d (FIG. 
6) between the tip of the protrusion 44 and the plane of the flat or 
recessed areas 46 or 48 is such that the indentor is an effective means 
for indenting both the lidding stock and flange, while the lateral face 
portions of 46 or 48 limit travel of the indentor, thereby preventing 
cutting of the sealing layer by simultaneously minimizing the flow of 
sealing and flange material from the area beneath the peripheral face of 
plate 28. This latter function is effected by the lateral faces 46 or 48. 
With less heat, the viscosity of sealing material 16 and flange 12 is 
greater in the areas under faces 46 or 48 such that the materials do not 
flow in any substantial amount from the sealing area between the lidding 
stock and container flange, even though indentor 44 is able to indent at 
50 (FIG. 5) the stock and flange. 
Indentation 50 and the seal effected between the flange and stock extends 
in a line around the entire periphery of the flange to provide a seal that 
is completely peelable from the flange, thereby leaving the flange free of 
sealing material 16. 
A typical force exerted by apparatus 22 on the lidding stock and flange is 
on the order of ten to thirty pounds per linear inch (or two to six 
kilograms per linear centimeter), which is dictated by the size of the 
container, depending upon the composition and gauge of the stock and 
flange. A typical temperature is on the order of 300 to 400 degrees F., 
with the force and temperature being applied for a time of say 0.5-1.5 
seconds, again, depending upon the material and gauge of the flange and 
lidding stock. 
In the case of the recess areas 48 of the FIGS. 7 and 9 embodiments, the 
recesses allow more of sealing material 16 to flow from the area beneath 
the indentor than the flat areas 46 of FIGS. 6 and 8. However, the amount 
of flow, which is controlled by the ratio of the recess volume to indentor 
volume is such that additional flow, with respect to the FIG. 6 
configuration, will not have a detrimental effect on peelability. The 
recesses, on the other hand, provide an advantage that the flat areas 46 
do not, i.e., when lid 10A is pulled upwardly from flange 36, it is pulled 
under a tension force. However, when the lid material reaches indentation 
50, the lid material now must move in the plane of the incline provided by 
the indentation so that the action and force of tensile pulling is 
converted to a shear action. This additional effort is helpful if the 
container is dropped or otherwise abused. With the recesses provided in 
the lid and flange by the structures of FIGS. 7 and 9, additional shear 
resistance to lid removal is provided, thereby increasing the resistance 
of the seal to abuse. 
In the embodiments of FIGS. 8 and 9, the indentor 44 is made of a material 
different from the remaining body of seal plate 28, as indicated by the 
different cross hatching, i.e., the material of 44 has a coefficient of 
thermal conductivity substantially greater than that of the material of 
the plate itself, including the material adjacent planar and recessed 
areas 46 and 48. As a consequence, lateral faces 46 and 48 of the 
embodiments of FIGS. 8 and 9 will provide less heat to the areas along 
side the indentor 44, because of their lower thermal conductivity. Since 
less heat is input into the area under 46 or 48, the viscosity of the 
sealing layer 16 beneath 46 or 48 will be higher so that the flow of the 
sealing layer from the area beneath the indentor will be minimal. 
A preferred range for the ratio's of the relative thermal conductivities of 
indentor 44 and plate body 28 is 5:1 to 15:1, though the effect of the 
bimetallic configuration will be more pronounced the higher the thermal 
conductivity of the indentor is in relation to material of 46 or 48. 
As shown in FIG. 4, plate 28 is secured to block 26 by a suitable threaded 
fastener 52 inserted through an opening in the plate and threaded into a 
tapped opening provided in the block. The structure of plate 28, however, 
need not be solid, i.e., plate 28 can be an open ring, as indicated by 
phantom lines 54. In such a case, the ring would be provided with openings 
(not shown) to receive means for securing the ring to block 26. 
In addition, the shape of the ring or plate, as well as that of the block, 
in plan view, is generally that of the container flange. 
While the invention has been described in terms of preferred embodiments, 
the claims appended hereto are intended to encompass all embodiments which 
fall within the spirit of the invention.