Thermocontact welding method and apparatus, and welded product

An improved thermocontact welding method and apparatus for producing welded seams joining superposed thermoplastic sheets, and in particular for producing solid seams in polypropylene and other polyolefin sheets. The method includes moving a lower platen for engaging an upper die assembly for applying pressure to interposed thermoplastic sheets at areas thereof between the die surface and the platen while the die surface is at a temperature higher than the melting temperature of the sheets and while thermally isolating other areas of the sheets, and the welded sheets are coerced or ejected from the die surface during disengagement of the platen from the die assembly. A preferred product of the method is a polypropylene album leaf having pockets defined by solid weld seams for containing cards or photographs.

BACKGROUND OF THE INVENTION 
This invention relates to methods and apparatus for thermocontact welding 
of thermoplastic sheets and the product produced thereby, and more 
particularly to a thermocontact welding method and apparatus for producing 
welded seams joining polyolefin sheets (such as polypropylene or 
polyethylene) and the welded product produced thereby. 
The invention finds particular application in the fabrication as such 
articles of album pages or leaves for storing and displaying cards 
depicting persons of common professional interest, such as sports figures, 
as well as album pages or leaves for storing and displaying photographs. 
Such articles are commonly fabricated of sheets of polyvinyl chloride, or 
vinyl, whereby linear seams are produced for welding such sheets in a 
configuration for producing pockets for inserting such cards or 
photographs, usually by radio-frequency welding techniques. 
Polypropylene has certain advantages over vinyl as a sheet material for 
fabricating such articles. Polypropylene is substantially chemically inert 
and will generally not react with dyes and other surface components of 
sports cards and photographs, and is physically resistant to extreme heat 
and cold. Polypropylene is highly suitable for archival applications, and 
polypropylene leaves having pockets for containing cards or photographs 
are particularly suitable for the long term storage of such contents. 
However, unlike vinyl, attempts to weld polypropylene sheets (as well as 
other polyolefin sheets) by radio-frequency welding techniques have been 
in general unsatisfactory. Instead, thermocontact welding is generally 
employed, although attempts to produce a solid weld seam by thermocontact 
welding have previously caused the welded sheets to exhibit a tendency to 
curl or otherwise deform, thought to be a result of polypropylene's 
sensitivity to heat. In order to prevent curling or deformation, prior art 
thermocontact methods for welding polypropylene sheets have utilized 
discontinuous or intermittent die surfaces for producing discontinuous or 
intermittent welded seams--i.e. the welded seam is comprised of a sequence 
or series of welded dots or short dashes with unwelded material between 
successive dots or dashes. As used herein, the term "solid" as applied to 
a welded seam is meant to describe a non-intermittent seam, and as applied 
to a die surface is meant to describe a non-intermittent die surface. 
In prior art thermocontact welding, two superposed thermoplastic webs are 
suspended beneath a downwardly facing upper die which is configured with 
seam forming die surfaces. Both the upper die and the lower platen having 
upwardly facing die surfaces are heated, with the temperature of the lower 
platen and die surfaces being higher than the temperature of the upper die 
surfaces. When thermocontact welding is utilized for welding polypropylene 
sheets by means of intermittent welds, the lower platen and die surfaces 
are conventionally at a temperature higher than the melting point of 
polypropylene, while the upper die surface are conventionally at a 
temperature lower than the melting point of polypropylene. 
SUMMARY OF THE INVENTION 
The present invention provides an improved thermocontact welding method and 
apparatus for forming solid welded seams for joining thermoplastic sheets, 
particularly thermoplastic sheets which may not be successfully welded by 
usual commercial methods such as high frequency welding. The method and 
apparatus find particular application in the welding of polyolefin sheet 
materials, including polypropylene and polyethylene sheets, wherein 
attempts to join such sheets with solid seams by prior art thermocontact 
welding methods and apparatus have resulted in physical deformation of the 
finished article. 
Briefly described, the thermocontact welding method of the present 
invention comprises the steps of providing a thermocontact welding press 
having platen means and opposing die means, the die means including a seam 
producing die surface; interposing at least two superposed thermoplastic 
sheets between the platen means and the die means; moving the platen means 
for engaging the die means for applying pressure to the superposed sheets 
at areas thereof between the die surface and the platen means while the 
die surface is at a temperature higher than the melting temperature of the 
sheets, the platen means is at a temperature lower than the temperature of 
the die surface, and thermally isolating other areas of the sheets in the 
vicinity of the die surface; moving the platen means for disengaging the 
platen means from the die means; and coercing the sheets from the die 
surface. Preferably, the platen means is at a temperature substantially 
lower than the melting temperature of the sheets, the method includes 
clamping the sheets together at such other areas while thermally isolated 
during the engaging step, and the sheets are coerced from the die surface 
during the disengaging step. 
The apparatus of the present invention comprises, in combination, a 
thermocontact welding press having platen means and opposing die means, 
the die means including a seam producing die surface; means for 
interposing at least two superposed thermoplastic sheets between the 
platen means and the die means; means for moving the platen means to 
engage the die means with the superposed sheets therebetween for applying 
pressure to the interposed sheets at areas thereof between the die surface 
and the platen means, and to disengage the platen means from the die 
means; means for heating the die surface to a temperature higher than the 
melting temperature of the sheets; means for thermally isolating other 
areas of the interposed sheets in the vicinity of the die surface when the 
platen means and the die means are engaged; and means for coercing the 
sheets from the die surface during disengagement of the platen means from 
the die means. Preferred apparatus includes means for clamping the 
interposed sheets together at such other areas when the platen means and 
the die means are engaged. The means for thermally isolating the other 
areas preferably includes thermal insulation means which additionally 
provides such clamping. 
According to one aspect of the apparatus according to the present 
invention, a die assembly for a thermocontact welding press is provided 
which comprises, in combination, support means; die member means secured 
to the support means and having a downwardly facing die surface; means for 
heating the die member means; upwardly movable thermal insulation means 
coupled to the support means and having a downwardly facing flat surface 
adjacent the die surface and normally slightly lower than the die surface; 
biasing means for downwardly biasing the thermal insulation means; and 
stop means for cooperating with the thermal insulation means for 
precluding upward movement of the flat surface more than a predetermined 
distance higher than the die surface. 
A preferred configuration of the die assembly of the present invention 
includes a pattern of solid seam producing die surfaces and biased thermal 
insulation means for producing, by means of the thermocontact welding 
method of the present invention, a polypropylene album leaf comprising, in 
combination, a first flat, flexible polypropylene sheet; and at least one 
second flat, flexible polypropylene sheet superposed on the first sheet 
and secured to the first sheet by a plurality of solid welded seams to 
form at least one pocket, such pocket having three closed sides defined by 
three of the solid seams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning first to FIG. 1, there is represented a conventional thermocontact 
welding press apparatus 10, which may be of the type manufactured by Paul 
Kiefel Hochfrequenc-Anlagen GmbH of Germany, such as its model KSV 600 
high speed indexing automated welding unit with thermocontact welding 
capability. 
The thermocontact welding press apparatus 10 includes an upper die assembly 
12 having a downwardly facing die surface 14. A lower platen assembly 16 
includes an upwardly facing platen 18 which is vertically movable toward 
and away from the downwardly facing die surface 14 of the die assembly 12, 
by means of pressure apparatus 20. Heating means 22 is provided for 
controllably heating the die surface 14 through the die assembly 12, and 
heating means 24 is provided for controllably heating the platen 18 
through the platen assembly 16. 
During operation of the thermocontact welding press, at least two 
polypropylene webs or sheets 26, 28 from respective supply rolls 30, 32 
are advanced in superposed relation for being interposed between the 
platen 18 and the die surface 14. Holding clamps 34, 36 hold the 
interposed webs or sheets 26, 28 suspended above the platen 18 and beneath 
the die surface 14 when separated. During the welding operation, the 
platen 18 is caused to move upwardly for carrying the interposed 
superposed sheets 26, 28 into welding contact with the die surface 14, 
after which the platen 18 is caused to move downwardly for permitting 
release of the welded sheets. 
Turning to FIG. 2, a preferred embodiment of a polypropylene album leaf 38 
for storing and displaying such items as sports cards or photographs, 
includes a substantially rectangular, flat, flexible, transparent 
polypropylene sheet 40 having an upper edge 42 and a lower edge 44 (as 
viewed in the drawing) and a left edge 46 and a right edge 48 (as viewed 
in the drawing). Superposed on the sheet 40 are three strips or 
substantially rectangular, flat, flexible, transparent polypropylene 
sheets 50a, 50b, 50c, each having a height slightly less than one-third 
the height of the first sheet 40, and each having a width equal to the 
width of the sheet 40. The second sheets 50a, 50b, 50c are arranged such 
that their respective left edges are aligned with the left edge 46 of the 
first sheet 40 and their respective right edges are aligned with the right 
edge 48 of the first sheet 40. As viewed in the drawing, the uppermost 
sheet 50a is superposed on the sheet 40 with the upper edge 52a of sheet 
50a parallel to and slightly lower than the upper edge 42 of the sheet 40; 
the upper edge 52b of the middle sheet 50b is parallel to and slightly 
lower than the lower edge 54a of the uppermost sheet 50a; and the upper 
edge 52c of the lowermost sheet 50c is parallel to and slightly lower than 
the lower edge 54b of the middle sheet 50b, with the lower edge 54c of the 
lowermost sheet 50c being aligned with the lower edge 44 of the sheet 40. 
By the method of the present invention, the second polypropylene sheets 
50a, 50b, 50c are each secured to the first polypropylene sheet 40 upon 
which they are respectively superposed, by means of a plurality of solid 
seams to produce the album page 38 having a plurality of pockets for 
holding cards or photographs insertable through respective pocket openings 
provided by the unsealed edges 52a, 52b, 52c. Although other seam patterns 
are of course possible, the pattern disclosed in the drawing includes, as 
viewed therein, a left vertical solid seam 56 and a right vertical solid 
seam 58 extending from the leaf's upper edge 42 to its lower edge 44, 
horizontal solid seams 60a, 60b, 60c extending between the vertical seams 
56, 58 and along the lower edges 54a, 54b, 54c of the sheets 50a, 50b, 
50c. A solid seam 64 is preferably provided along the leaf's left edge 46, 
for providing a tab or margin 68 in which apertures 69 may be cut for 
permitting the leaf 38 to be retained by correspondingly positioned rings 
of a loose-leaf binder. 
In one example of the album leaf 38, the thickness of each of the 
polypropylene sheets 40, 50a, 50b, 50c was approximately 130 microns, 
although flexible sheets having other thicknesses may be employed. 
Although polypropylene album leaves of similar general configuration are 
available in the prior art having intermittent seams--i.e., each seam is 
comprised of a sequence or series of welded dots or short dashes with 
unwelded material between successive dots or dashes--the method and 
apparatus of the present invention permits production of flat (as opposed 
to curled) polypropylene album leaves with solid or non-intermittent 
seams. As noted earlier, prior art attempts to produce polypropylene album 
leaves with solid seams have been unsuccessful, resulting in curled and 
otherwise deformed products. The improved polypropylene album leaf 38 is 
made possible, and may be mass produced upon utilization of the 
thermocontact welding apparatus of FIG. 1 as improved by the apparatus of 
the present invention and in accordance with the invention's method as 
described below. 
The upper die assembly 70, for producing the polypropylene album leaf 38 of 
FIG. 2 in accordance with the thermocontact welding method of the present 
invention, is represented in FIGS. 3 and 4. FIG. 3 represents 
substantially one-half of the upper die assembly and produces a single 
leaf, the remaining substantially one-half of the upper die assembly not 
being shown but comprises the mirror image of the die assembly shown in 
FIG. 3 and is employed for simultaneously producing a second leaf 38. 
The solid seams sealing the superposed polypropylene sheets of the leaf 38 
of FIG. 2, are formed by cooperation between the upper die assembly 70 and 
the lower platen 72, and specifically as a result of contact engagement 
between suitably heated non-intermittent or solid die surfaces 74 arranged 
in a pattern (FIG. 3) corresponding to the desired seam pattern of the 
album leaf 38 (FIG. 2). The die surfaces 74 are the lowermost portions 
including the downwardly facing surfaces of thermally conducting bars 76, 
which are typically constructed of a metal such as brass which has been 
chrome plated for lubricity. The die members or bar 76 are secured to a 
thermally conductive stationary die support 78 (which may be constructed 
of a metal such as aluminum) by means of thermally conductive securing 
members or blocks 80 (which may also constructed of a metal such as 
aluminum) secured to the stationary support 78. The securing blocks 80 are 
situated on both sides of the die member 76, at the left side of FIG. 4; 
for clarity of illustration, the rightmost securing block is not shown at 
the right side of the drawing in order to illustrate one means of securing 
the bars 76 to the securing blocks 78, namely by means of screws 82. The 
securing blocks 80 may be secured to the support 78 by conventional means 
such as by screw means (not shown). Controllable heating means (such as 
the heating means 22 indicated in FIG. 1) are provided for controllably 
heating the bars 76 and hence the die surfaces 74, such as by heating the 
stationary die support 78 transmitting heat to the securing blocks 80, in 
turn transmitting heat to the die bars 76 and hence to the die surfaces 
74. 
The depending die bars 76 are arranged with their die surface 74 in a grid 
pattern, defining a plurality of generally rectangular cavities 84. 
Captively suspended in each of these cavities 84, and laterally adjacent 
to the die surfaces 74, is a refractory insulation member or block 86 
having a substantially flat generally rectangular lower or downwardly 
facing surface 88. Each of the insulation blocks 86 are coupled to the die 
support 78 such that each block's downwardly facing surface 88 is slightly 
lower (say one-eighth inch) than the downwardly facing die surfaces 74, 
and each block 86 is upwardly movable against a downward biasing force. In 
the example shown in FIGS. 3 and 4, each block 86 is secured to a metal 
(e.g. aluminum) plate 90 by means of fastening screws 92 within apertures 
94 through the block 86 and apertures 96 through the metal plate 90. 
The plate 90 contains further apertures 98 through which vertically 
directed shoulder screws 100 are loosely fitted. The shoulder screws 100 
are threadedly secured to the die support 78, and each screw 100 includes 
an upwardly facing head shoulder 102 engaging the lower surface of the 
plate 90 (peripherally of the aperture 98) for supporting the plate 90 and 
hence the block 86 when the block surface 88 is in its downward-most or 
normal position as shown in FIG. 4. The insulation blocks 86 are provided 
with vertical bores 104 within which the shoulder screws 100 are 
positioned. The height of the bores 104 (and accordingly the height of the 
insulation blocks 86) is greater (say by 1/4 inch or more) than the height 
of the contained screw head plus the maximum upward travel distance of the 
block surface 88. The insulation block 86 (and hence the block surface 88) 
is downwardly biased, for example by means of helical springs 106 
positioned about the screws 100 and biased against the lower surface of 
the die support 78 and the upper surface of the plate 90. 
Additional insulation blocks 86' are provided as shown in FIG. 3, one being 
situated laterally adjacent to the die surface 74' for producing the tab 
seam 64 shown in FIG. 2. Another additional insulation block 86' (one half 
of which is shown along the right hand side of FIG. 3) is situated 
laterally adjacent to the farthest right die surface 74 as shown in FIG. 
3, and extends into the other half of the upper die assembly not shown in 
FIG. 3 but which comprises the mirror image of the die assembly shown, as 
previously discussed. Notwithstanding their different shapes, these 
additional insulation blocks 86' are secured to the die support 78 in 
generally the same manner as the blocks 86 and operate in similar manner 
thereto. 
The insulation blocks 86 are preferably constructed of a rigid, high 
compressive strength, damage resistant, thermally insulating, refractory 
material such as calcium silicate containing inert fillers and reinforcing 
agents, an example of which is sold under the trademark Marinite P and 
manufactured by BNZ Materials, Inc. 
The method and operation of the apparatus according to the present 
invention will be described with respect to FIGS. 4-7, in which the 
polypropylene album leaf 38 of FIG. 2 is represented as being produced, by 
way of example. In the manner discussed above with respect to FIG. 1, the 
upper die assembly 70 of FIG. 4 is maintained stationary above and is 
initially separated from the vertically movable lower platen 72. A buffer 
sheet 108 of resilient material, such as silicone rubber of say one-eighth 
inch thickness, is situated upon the platen 72. The platen 72 is 
controllably heated (such as by the heating means 24 of FIG. 1) and is 
maintained at a temperature substantially lower than the melting 
temperature of the thermoplastic sheets to be welded, while the die bar 
members 76 and hence the die surfaces 74 are maintained at a temperature 
higher than the melting temperature of the thermoplastic sheets to be 
welded. In one successful example using polypropylene sheets, the die bars 
76 were maintain at 234.degree. Centigrade and the platen 72 was 
maintained at 40.degree. Centigrade. 
The superposed polypropylene sheets are interposed between the upper die 
assembly 70 and the lower platen 72. The polypropylene sheets or strips 
50a, 50b, 50c are respectively superposed on the lower polypropylene sheet 
40, so as to form the arrangement of pockets on the finished album leaf 38 
shown in FIG. 2. In the fragment shown in FIG. 4, the sheets are arranged 
so that two sheets are in superposed relation directly beneath the seam 
producing die surfaces 74. The interposed sheets or webs 40, 50a, 50b, 50c 
are suspended just beneath the upper die assembly 70 by the holding clamps 
34, 36 indicated in FIG. 1, the direction of web transport (between 
thermocontact stamping operations) being perpendicular to the plane of the 
FIG. 4 drawing. 
During the thermocontact welding operation, the platen 72 (with its buffer 
sheet 108) is caused to move upwardly (by implementation of the pressure 
apparatus 20 indicated in FIG. 1), lifting the superposed thermoplastic 
sheets 40, 50 against the downwardly facing insulation block surface 88 
and causing upward movement of the insulation block 86 along the shafts of 
the screws 100 and further compressing the springs 106; FIG. 5 indicates 
the condition when the die surfaces 74 contact the upper sheet 50 of the 
superposed sheets 40, 50. The hot die bars 76 are buried into the 
thermoplastic sheets as melting occurs with continued pressure applied by 
the platen 72. As shown in FIG. 6, the insulation block's upward movement 
is stopped when an upwardly facing land 110 of the insulation block 86 
contacts a downwardly facing surface 112 of a securing block 80. This in 
turn stops further upward movement of the interposed sheets 40, 50, 
thereby controlling the depth of penetration of the die surfaces 74 into 
the melting weld areas 114. 
In one successful example, the press pressure driving the platen 72 was 
approximately 40 psi. It is estimated that the springs 106 produce a 
downward pressure by each block surface 88 of the order of 10 psi. During 
upward movement of the platen 72, this biasing pressure assures that air 
has been removed from between the superposed sheets 40, 50 while the 
insulating blocks 86 clamp the superposed sheets against the warm platen 
72 and thermally isolate the clamped areas of the superposed sheets 40, 50 
from the hot die bar members 76 and die surfaces 74 (as well as from the 
hot support 78 and securing blocks 80, as needed during the thermocontact 
welding operation. 
When upward platen pressure is released and the platen 72 is caused to move 
downwardly, the biasing force provided by the springs 106 forces downward 
movement of the insulation block 86, in turn forcing downward movement of 
the superposed sheets 40, 50 and coercing or ejecting the welded sheets 
from the die surfaces 74, as shown in FIG. 7. This occurrence precludes 
the welded sheets 40, 50 from sticking to or being hung up by the hot die 
bars 76 at the weld areas 114, facilitating rapid removal of the sheets 
from the hot die surfaces 74 for implementing rapid cooling of the weld 
areas 114 for their solidification into the welded seams 60a, 60b (FIG. 
2). 
It may be noted that the air removal function of the insulation blocks 86 
may find general analogy to the "push-pads" associated with the 
radio-frequency welding art, i.e. to the downwardly biased panels (usually 
Masonite board) positioned in the vicinity of the die electrodes in 
conventional radio-frequency apparatus for producing weld seams in vinyl 
sheets. It may be appreciated, however, that the thermal isolation 
function of the insulation blocks 86 of the present invention, as well as 
their coercing or ejecting function, do not find analogy in the 
radio-frequency welding art. 
Further downward movement of the platen 72 results in the platen 72 being 
situated as shown in FIG. 4. The welded product is transported downline 
(i.e. it is removed from being positioned between the upper die assembly 
70 and the lower platen 72) where it is appropriately cut for removing 
excess material and forming the tab apertures 69 for defining the album 
leaf 38 shown in FIG. 2. 
A release sheet such as a silicone coated paper, although not shown in the 
drawing, may be situated between the resilient buffer sheet 108 and the 
thermoplastic sheet 40, in order to assist in release of the welded sheets 
40, 50 from the buffer sheet 108 and platen 72. 
It should be recognized that the entire thermocontact welding operation 
occurs very rapidly, engagement between the platen 72 and the upper die 
assembly 70 during the period when the superposed sheets 40, 50 are in 
contact with the hot die surfaces 74 being very short, such as of the 
order of 1.3 milliseconds. 
Thus there has been described an improved thermocontact welding method, and 
apparatus for practicing the improved method, for producing solid seams 
for welding polypropylene sheets, as well as a card or photo album leaf 
fabricated by the method utilizing the apparatus of the present invention. 
Although polypropylene and other polyolefin sheet material have been 
discussed above as being particularly adapted for the method of the 
present invention, other thermoplastic materials may be utilized in 
practicing the invention. Modifications of the apparatus and method 
disclosed may be developed without departing from the essential 
characteristics of the present invention. Accordingly, the invention 
should be limited only by the scope of the claims listed below.