Note: Descriptions are shown in the official language in which they were submitted.
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F-2914 (2915)
liINGED COVER CARTON
The present invention relates to cartons with
hinged covers which are formed from sheets of plastic
materials by a thermoforming process and to a method for
forminy apertures in plastic resin sheet materials for
forming an improved locking device therefor. The
invention is particularly applicable to egg cartons but
may be applied to other types of cartons~also,
especially those which are used to carry fragile
articles in separate packaging cells within the carton.
Egg cartons used in the retail'marketing of
eggs have, up to the present, usually contained one
dozen eggs packaged in two rows of six eggs each. These
cartons, which may be made of wood pulp or r more
recently, of thermoformed plastic, generally comprise a
bottom section containing the egg-receiving cells, a
cover which is generally in the form of an inverted
dish-like lid and a locking flap which engages with the
cover to hold it closed.
In some markets, eggs are sold in cartons of
eighteen eggs, rather than in the traditional carton of
one dozen. In the past, eighteen cell egg cartons have
been thermoformed from plastic sheets only in versions
which do not have complete protective covers although an
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eighteen cell carton with a protective cover has been
produced in molded pulp. It would be desirable to be
able to make an eighteen cell egg carton with an
integral protective cover but the existing covered
eighteen cell cartons such as the pulp carton presently
on the market, cannot be thermoformed from plastic
sheet. One reason for that is that the latch on the
pulp carton has long elements, which, if thermoformed,
would require a small, relatively narrow piece of the
plastic sheet to be drawn a long way beyond the sheet
line. In thermoforming, long extensions beyond the
sheet line are generally undesirable because they
produce considerable weakening.
On the other hand, it is not practicable to
enlarge thermoformed plastic cartons of existing types
by the inclusion of a third row of six egg cells, to
form an eighteen egg carton because the enlarged cartons
could not be accommodated on existing packaging
equipment, a large amount of which is in use.
Conventional packaging equipment is capable of accepting
either covered twelve cell cartons with conventional
locking flaps or the eighteen cell covered pulp cartons,
both of which are small enough to fit within the
equipment. If the twelve cell thermoformed plastic
cartons were enlarged to hold eighteen eggs, the carton
in the open position would exceed the maximum acceptable
dimensions of 30 x 30 cm (12 x 12 inches). The covered
pulp carton mentioned above can be used on conventional
equipment because its locking elements extend upwards
from the lower section of the carton rather than
sideways, as the locking flap does on the conventional
plastic cartons.
Divers kinds of cartons formed from sheets of
plastic resin materials, such as foamed polystyrene,
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include latching or locking devices which are used to
keep the cover or-lid of the carton in a closed-
position. Although, as mentioned above, egg cartons are
the most typical of this kind of carton, cartons for
other articles, for example, cartons for the so-called
"fast food" items such as hamburgers may also include
such locking devices. These locking devices usually
include a male member, often shaped like a button or a
hook which fits into a female locking aperture or slot
in the other part of the carton. Originally, the
locking slots or apertures were forrned by punching them
out of the pre-formed carton. Not only was this an
expensive procedure, requiring separate punching
equipment but problems arose from its use, mainly
because accurate registering of the pre-formed carton in
the punching equipment was required but could not be
consistently achieved in practice.
In the prior art, apertures may be formed in
the carton at the same time as the carton is formed. In
-this method an attempt is made to form the aperture by
tearing and scraping the plastic during the moulding
cycle.
In other methods for forming apertures, shear
members in the male and female moulding dies shear the
plastic during the closing of the mould to form the
holes in the plastic. Another improved method of
forming locking apertures employs a biased shearing key
in the mould to shear the softened plastic during the
moulding process.
In all of these methods, however, the apertures
which have been procduced are essentially planar
apertures, that is, apertures which, except for their
extent through the plastic sheet are purely two
dimensional Certain types of carton, however, require
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a three-dimensional type of aperture in order to achieve
the desired locking effect.
A stackable carton has now been developed which
can be ~oulded by thermoforming a plastic sheet and
which has an improved locking device. The locking
device can be used on divers types of cartons formed
from sheets of plastic resin materials such as foamed
polystyrene but is especially useful with egg cartons
because it enables covered eighteen cell egg cartons,
which are capable of beiny packaged on conventional
equipment, to be made from plastic sheet by
thermoforming. The locking device is, moreover, sturdy
and capable of providing a firm locking engagement
between the cover and the cell section.
The method for producing the improved locking
device comprises forming a three-dimensional aperture in
a sheet of plastic resin material during the moulding
process. By a "three-dimensional~ aperture, is meant an
aperture (in a material of finite thickness) having a
configuration upon the face of the material in which it
is formed which extends in three dimensions. Such an
aperture will be formed in a material which itself has
a surface extending in three dimensions, for example, in
two intersecting planes or in a single or compound curve.
The three-dimensional aperture is formed in the
plastic sheet during a thermoforming process in which
the sheet is moulded between male and female die members
into the desired shape. The die members include male
and female shear keys each of which has a shearing face
or faces which extend in the direction of movement of
the die members relative to one another and also in two
dimensions in a p~ane extending perpendicular to the
direction of die movement.
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By shearing the plastic sheet with shear keys
having shearing faces which extend in the direction of
movement of the die members, a clean cut is made in the
-carton material, as contrasted to prior techniques in
which the hole forming members move in an oblique
direction which scrapes the plastlc away from the torn
edges. By extending the shear keys so that they shear
ln two dimensions in a plane perpendicular to the
direction of die movement, it is possible to form slots
or apertures in fluted or recessed portions of the
plastic sheet, with the holes extending from the back
wall of the flute or recess to the unfluted remainder of
the sheet. The hole, moreover, can be situated entirely
between the ends of the flute so that its upper and
lower margins are within the length of the 1ute. In
contrast to this, prior art shear keys form a merely one
dimensioned aperture.
In the most common form, the aperture will be
formed in two substantially planar intersecting surfaces
of the plastic sheet so that the edges of the aperture
will extend in two dimensions in the plane which is
perpendicular to the direction of the relative movement
of the die members. An example of this is where the
aperture is formed in the side walls and the rear wall
of a rectilinear locking flute of a plastic carton, as
described below. In such cases, the male and female
shear keys attached to the respective die members will
have multiple shear faces which extend substantially in
the same planes as the surface of the sheet; in the case
of the rectilinear, genPral~y ~-shaped configuration
locking f~tes described below, the shear keys will have
three shear faces in a generally U-shaped configuration
so as to form the aperture in the rear wall as well as
both side walls of the flute. The apertures may,
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however, also be formed in plastic sheets which are
curved in the plane which is perpendicular to the
direction of die movement, by employing appropriately
shaped shear keys. The plastic sheet will generally be
formed into a configuration in which its surfaces are
inclined at an angle to the direction of die movement
(measured in a plane parallel to the direction of die
movement) in the region of the aperture so that, as the
die closes the aperture formed by the action of the
shear keys extends through only a limited portion of the
sheet.
The three dimensional apertures which are
formed in the locking flutes or recesses of cartons by
the present method are particularly suitable for locking
with a continuous, rigid locking member formed
integrally with the bottom section of the carton.
According to my invention, the carton, which is
moulded from a plastic sheet by thermoforming comprises
a bottom section for receiving the articles to be
packaged and a cover which has an inverted dishlike
configuration with front, rear and side walls. The rear
wall is resiliently hinged to the rear edge of the
bottom section, suitably by a single or double hinge
line. An integral, continuous locking member extends
upwardly in a fixed position from the front edge of the
bottom section and has a number of locking extensions at
the top which extend towards the inside of the front
wall of the cover when the cover is in the closed
position. The locking extensions engage with a number
of locking slots which are formed in locking recesses in
the front wall of the cover in order to provide the
desired locking effect when the cover is closed. The
integral locking member is sufficiently rigid that it
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remains in a fixed portion relative to the bottom
section; however, the plastic sheet material of which
the cart~n is formed permits a certain amount of
deformation in the looking member and the cover so that,
when the carton is to be opened, the locking bar can be
disengayed from the locking slots in the cover by moving
the front wall of the cover and the locking member away
from one another.
The locking device is particularly useful with
egg cartons because it enables eighteen cell plastic
cartons to be made which can be used on existing
packaging equipment, as its dimensions when open do not
exceed the permissible 30 x 30 cm. The cartons can
therefore be stacked in the required manner on the
equipment. However, the locking mechanism may also be
used to advantage on the more conventional twelve-cell
cartons. When the locking device is used on an egg
carton, the bottom section comprises the egg-receiving
cells, and these are conveniently dimensioned so that
the eggs are enclosed for rather less than their
vertical dimension in order to permit easy removal; the
cover section is dimensioned so that the eggs are
accommodated snugly within the carton.
The cellular configuration of the bottom
section of the egg carton contributes materially to the
rigidity of the integral locking member by providing a
buttressed config~ration in which curvilinear buttresses
extend down from the locking member to the divlsions
bet~een the cells.
The carton is suitably made by thermoforming a
plastic sheet, for example, foamed polystyrene, into the
desired configuration. During the forming operation,
the male and female mould members are closed on a
preheated sheet of plastic and the bottom section and
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the cover are formed by drawing the plastic into the
mould in one direction from the sheet line. The
integral locking member, however, is formed by drawing
the sheet in the opposite direction from the sheet
line. The ~ch~ng slots in the recesses in the cover
are formed during the moulding operation using a
suitable die~
In the drawinys appended to this specifica-tion:
Figure 1 is an isometric view of a known type
of eighteen-cell pulp egg carton
Figure 2 is an isometric view of an
eighteen-cell plastic egg carton having the improved
locking device;
Fi.gure 3 is a front view of the carton of
Figure 2 in the partly open position;
Figure 4 is a front view of the carton of
Figure 2 in the fully closed position;
Figure 5 is a front view of a portion of the
carton showing the locking device;
Figure 6 is a section along 6-6' of Figure 4;
Figure 7 is a section along 7-7' of Figure 4;
Figure 8 is a section along 8-8' of Figure ~;
Figure 9 is a section of the carton along 6-6'
of Figure 4 but with the carton in the partly closed
position;
Figures lOA, lOB and lOC are simplified
diagrammatic representations of the thermo~orming
process used to make the carton;
Figure llA is a fragmentary isometric view of
the male die used in the thermoforming process~
Figur~ llB is a ~ragmentary isometric view of
the ~emale aie ~sed in the thermoforming process;
Figure llC is an enlarged view of part of
Figure 11;
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Figure 12 is a vertical view, partly in section
of a portion of the male die in the direction shown by
arrow A in ~igure llA:
Figure 13 is a cross section of the male and
female dies in operation, along the line indicated by
13-13' in Figure 12;
Figure 14 ls an enlarged isometric view of the
rnale shear key used in the male die; and
Figure 15 is an illustration showing the manner
in which the aperture may be -Eorrned relative to the
direction of die movement and the surface of the plastic
sheet.
A known type of eighteen-cell moulded wood pulp
carton is shown in Figure 1. It has a bottom section 10
for holding the eggs in a conventional 6 x 3
arrangement. The cover 11 is formed integrally with the
bottom section at the rear of the cover. The front wall
12 of cover 11 has four roughly-formed indentations 13
with a locking aperture 14 formed in each of them, of an
approximately inverted - T configuration (only one is
designated for clarity). Four corresponding locking
projections 15 stand up from the front oE the bottom
section with the top of each locking projection being
formed into a hook-like latch 16 which engages with the
locking edge of each locking aperture 14 when the cover
is in the closed position.
Pulp cartons of this kind may be used in
conventional packaging equipment because they do not
exceed the permissi.ble 30 x 30 cm size (plan view) when
open. However, if attempts were made to fabricate these
cartons from plastic sheet by thermoforming, the severe
drawing of relatively narrow pieces of the plastic sheet
which would be necessary to produce the latching
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projections 15 would lead to grossly weakened regions
around the areas where the drawing occurs, mostly in the
projections themselves.
The plastic carton including the locking device
according to the present invention, overcomes these
disadvantages An eighteen-cell egg carton with such a
locking device is shown generally in Figure 2 of the
drawings and includes a bottom section 20 with the egg
receiving cells 21 (only one is designated) arranged
conventionally in three rows of six cells each. In
order to provide a high degree of protection for the
eggsl projecting tips 22 are provided (only two are
designated in Figure 2) between adjacent cells, both
transversely and longitudinally; however, at the center
of the carton (see Figure 8), the tips are omitted for a
purpose which will be explained below. In the carton
shown in the drawings, individual tips are provided
between each pair of cells, both transversely and
longitudinally, except at the center of the carton.
Between each group of four tips, a depressed region 23
is provided to permit easy removal of the eggs.
However, a different arrangement is used at the center
of the carton, as described below.
The cover 24 is of inverted dish-like
configuration and has a generally flat top 25, a rear
wall 26, a front wall 27 and side walls 28. The cover
is integrally formed with bottom section 20 and is
attached by rear wall 26 to the rear edge 29 of bottom
section 20 by a resilient hinge 30 suitably of single or
double fold configuration. Front wall 27 of the cover
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has a number of locking flutes or recesses 35 which are
of wedge-shaped (trapezoidal) configuration although
they may also be parallel-sided or even of curvilinear
configuration, if desired. Trapezoidal and curvilinear
flute configurations having narrower widths at the
bottom of the front wall are preferred because they will
help to align the cover on the bottom section as the
carton is closed. In this case, there i5 a locking
flute situated on the center line extending between each
pair of transverse, short (three cell) rows of egg
cells, so that there is a total of five locking flutes,
only one of which is designated for clarity. However,
the number may be varied; for example, two or three
symmetrically disposed flutes may be used athough,
naturally, to do so would not provide so much security
as the five shown.
The rear face 36 of each locking flute 35 is
offset inwardly from the remainder of the inner face of
front wall 27 and a locking aperture 37, here of
slot-like configuration, is provided in each flute,
extending along the rear face and into the two side
walls 38 of each flute 35. The locking aperture is
formed in the manner described below with a flat edge 39
on its lower margin, preferably with a clean, sharp edge
on the inside in order to provide firm, positive
locking. The offset of each locking flute 35 increases
from the bottom up to the locking aperture; i.e., in the
direction from the edge of cover 24 towards aperture 37
so that as the cover is closed onto the bottom section,
the sloping inner surfaces 40 of the locking recesses
(see Figures 6 and 9) slide easily over the locking
member on the bottom section until engagement occurs
between the locking surfaces. From the locking aperture
up to the top 25 of the cover, the flute is of constant
offset.
The locking apertures are of three-dimensional
configuration, that is, they are formed both in the rear
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face as well as the two side walls of each locking
flute. The side walls of the flute form two planes
which intersect with the rear wall of the flute to form
the three-dimensionally extended surface in which the
apertures are formed. Obviously, however, the locking
recesses could ~ of curved, e.g., of semi-circular
configuration instead of being rectilinear, as shown.
At the fron~ of bottom ~ection 20 a riaid,
integral locking member 45 is form~d by a fixed, upward
extension of the egg cells and the region between them.
The locking member has five locking extensions 46 (one
designated) which extends forwards toward the inner face
of front wall 27 of cover 24 when the cover is in the
closed position (see Figure 6). These locking
extensions, one for each locking flute, are formed by a
forward extension of the plastic sheet in the region
between each two short rows of egg cells; because the
curving wall of each egg cell is close to the edge of
the carton, there is no forward extension of the sheet
along the center line of each short row of cells but, in
principle, there is no reason why it should not be
provided there to provide a continuous locking extension
along the entire top edge of locking member 45, except
that it would enlarge the carton size, possibly beyond
the desired 30 cm in the open position.
Rectangular stacking lugs 49 are provided on
locking member 45 between the locking extensions (only
partial on the two end cells) to prevent the cartons
from becoming jammed together when they are stacked for
shipping. Corner stacking lugs 47 are also provided on
the corne~s ~f the carto~ fo~ a ~i~ilar purpose.
Stackin~ lugs 49 on locking member 45, however, help to
maintain the desired rigidity in locking member ~5. If
desired, stacking lugs 49 could be extended laterally so
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that they extend around the curve of the egg cells to
provide even greater rigidity and possibly also to
provide a greater degree of self-aligning ~or the
locking flutes as the carton is closed. The locking
member also derives additio~3al support from the
buttress-]ike configuration of the sloping wall 48 of
the locking member where it extends down from locking
extensions 46 and into ~he botto~ section 20 between the
short rows of cells tsee Figures 6 and 8). If a section
along the locking member just above the sheet line is
taken, it will be seen that the locking member has an
undulating configuration with portions which extend
inwardly between adjacent egg cells in the front row
that is, between adjacent transverse rows of cells, to
provide rigidity to the locking member. The inwardly
extending portions provide ribs which confer lateral
strength to the locking member and maintain it in its
fixed position. At the bottom of the extent, these ribs
merge into the protective tips between the egg cells in
the front longitudinal row -- except at the center o~
this row where there is a depression between the cells,
as discussed further below.
~ s can be seen in Figure 9, the desired locking
engagement between integral locking member 45 and
locking flutes 35 is obtained as the carton is closed.
The sloping inner surfaces 40 of the locking recesses
slide over locking extensions 46 until locking slots 37
come into line with locking extensions 46, the necessary
latera~ movement o~ the locking parts being provided by
t~e resilie~t ~ature of the plastic material. When
locking slots 37 come into line with locking extensions
46, the locking extensions snap into the slots, securing
the cover to the bottom section (Figure 6). When the
carton is to be opened, the locking parts can be
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separated by running a digital extremity under the edge
of front wall 27 of the cover so as to move the front
wall of the cover and locking extensions 46 are drawn
out of locking slots 37, until the cover can be opened
upwards in the normal way. When the carton is fully
closed, the outer edges of the locking extensions remain
within the locking flutes so that inadvertent release oE
the locking parts is unlikely.
If the number of locking flutes is reduced, for
example, to two or three symmetrically disposed ones, it
is desirable to omit either the flutes or the locking
extensions 46 in the regions between the short cell rows
where there are no locking parts, otherwise the cover
will be distorted when it is closed. However, it is
preferred that the inner face of the front wall of the
cover should be in contact with the locking member along
the entire length between the locking parts and so, it
will be preferred either to omit the flutes or the
locking extensions (but not both) in the areas where
there are no locking parts. However, if unslotted
flutes are provided for reasons of rigidity or
appearance, locking extensions 46 may be omitted in the
region facing these unslotted flutes.
It may be desirable to have shorter locking
extensions 46 at the two outer ends of the carton
because the flexing of the carton sometimes prevents the
locking extensions in these positions from sliding all
the way up the locking recesses and into engagement with
the locking slots. However, if the extensions at these
positions are trimmed back slightly, these extensions
will slide readily up the flutes and then into the
locking slots.
In order to provide support for cover 24 a
dependent support post 60 is formed in the cover. This
support post has two feet 61 which, when the carton is
closed, rest on two large, flat-topped protruding
support tips 62, 63 along the transverse center line of
the carton between adjacent egg cells. An alternative
arrangement would be to have a ridge-like foot at the
bottom of post 60 which would rest on a corresponding
ridge-like protrusion along the transverse center line
of the carton. However, this alternative arrangement
would be less desirable than the one shown, for reasons
explained below.
It has been found that although the provision
of the projecting tips 22 between adjacent egg cells is
desirable for the purpose of protecting the eggs, this
does tend to make the carton more flexible than is
desirable: the carton has less resistance to sagging in
the center when supported at the ends or to sagging at
the ends when supported in the center. This undesired
flexure may, however, be overcome by providing vertical
reinforcing elements in the carton between adjacent
cells in the longitudinal rows. These
vertically-extending, longitudinal elements, formed
during the moulding process, contribute to the beam
strength and rigidity of the carton and help prevent
sagging. In the carton shown, the vertical beam
elements are provided along the transverse center line
of the carton; it has been ~ound that this provides
sufficient rigidity and resistance to flexure. These
vertically-extending, longitudinal elements are provided
by shoulders 64 which extend down from support tips 62,
63 to depressed regions 65 between adjacent cells in the
longitudinal (six cell) rows at the transverse center
line of the carton. Because the vertical parts of the
rear wall of the carton together with the attached cover
provide considerable resistance to sagging of either
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kind at the rear of the carton, it is unnecessary to
provide a depression between the cells in the rear
longitudinal row and, accordingly, the support tip 62
between the two rear longitudinal rows has an extending
shoulder 66 between the middle two egg cells in that row
to provide cushioning for the eggs in those cells.
Similarly, the upwardly-extending locking member
provides additional rigidity at the front of the carton
but in order to confer sufficient total resistance to
- Elexure, the reinforcing vertically-extending elements
at the front of the carton are provided by the depressed
region 65 in the front row of cells, as shown.
Rear wall 26 of cover 24 has a number of
depressions 70 (one designated) of approximately
parabolic configuration which extend outwardly from the
general plane of the inner face of this wall. The
purpose of these depressions is to provide additional
clearance for the eggs in the rear longitudinal row of
cells when the carton is closed. It has been found
`that, in some instances, when these depressions are not
provided, the rear wall of the carton presses against
the eggs in the rear row of cells when the carton is
being closed, particularly with the larger egg sizes.
This may move the cover back just far enough to prevent
it closing easily over the locking member. The
provision of the generally parabolic depressions,
however, provides additional clearance when the carton
is being closed while, at the same time, holding the
eggs in place once the carton is fully closed.
Accordingly, there is one depression 70 in rear wall 26
aligned with each cell in the rear longitudinal row of
cells so as to provide clearance for each egg in the
row. Generally, the inner face of the depression should
be offset outwards by about 0.5 to 1 mm from the general
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plane of the inner face of the wall. The parabolic
configuration for the depressions is preferred because
it holds the eggs in place firmly when the carton is
fully closed as the top margin of the depression fits
snugly around the egg when the cover is closed.
However, other curvilinear or rectilinear depressions
may be used, if desired, although to possibly less
advantage.
The cartons are made by thermoforming a sheet
plastic resin material into the requisite shape. The
preferred plastic resin material for making egg cartons
is foamed polystyrene because it is cheap and has
satisfactory mechanical properties; it has adequate
stiffness, provides satisEactory protection for the eggs
and can be readily printed. However other plastic resin
materials could be used, depending upon the type of
articles to be packaged in the carton. The manufacture
oE the cartons will be described below by specific
-reference to the use of foamed polystyrene for making
egg cartons but similar methods may be used with other
sheet plastic resin materials with appropriate
modification of process conditions; e.g. sheet
thickness, mould compression and pressure, temperature,
mould cycle time and so forth.
In the thermoforming operation, a sheet of the
plastic resin material is moulded between male and
female dies or mould members into the desired
configuration. The temperatures encountered during the
moulding operation will tend to cause sheets oE foamed
polystyrene to expand but the mould configuration will
control the thickness of the sheet in the various parts
o the carton; some parts may be thinner than the
initial sheet and others thicker.
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The moulding cycle is shown in Figures lOA and
lOC, in simplified form. A male die 90 can be closed on
a female die 91 with the sheet of plastic resin material
92 between them. In practice, the mould members will
have a number of carton moulding cavities arranyed in
them so that a number of cartons can be formed at each
closing of the dies, but only one pair of carton
moulding dies are shown in the drawings, for clarity.
As the dies close, as shown in Figure lOB, the
plastic sheet is drawn down to stretch the plastic over
the die members until, when the dies are completely
closed, as shown in Figure lOC, the sheet is formed into
the desired configuration between the male and female
dies. At the same time as the sheet is shaped, the
locking slots are formed in the manner described in
detail below.
In order to form locking member 45, the male
and female dies are shaped so as to draw the sheet up
from the sheet line in the opposite direction to that in
which the bottom section and the cover are formed. To
do this, a former 94 on the female die 91 which produces
the front of the carton is extended upwards; and the
edge of the male die correspondingly cut away to form
cavity 95, so that when the dies are closed (Fig. lOC),
the plastic sheet is drawn upwards to form the locking
member, in the opposite direction to that in which the
cover and bottom sections are drawn. As shown in Figure
lOC, the cover and the bottom section are drawn upwards
to form the locking member, in the opposite direction to
that in which the cover and bottom sections are drawn.
As shown in Figure lOC, the cover and the bottom section
are drawn downwards Erom sheet line 96 and the locking
member is drawn upwards from the sheet line. Locking
extensions 46 on the locking member are formed by the
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outward extension ofthe sheet. Trimming of the
thermoformed sheet following ejection from the mould
provides the desired sharp edge to the locking
extensions.
As previously mentioned, the locking slots 37
in the locking flutes are three-dimensional slots which
extend along the rear face 36 of each locking flute and
into its side walls 38. In order to provide firm
locking engagement with locking extensions 46 on the
locking member these locking slots should be as sharply
edged and as well defined as possible. It has been
found that such slots can be made by shearing the
plastic sheet during the thermoforming operation rather
than going to the additional expense of a cutting
operation subsequent to the thermoforming step.
A partial view of the male and female moulding
dies is given in Figures llA and 11B respectively,
showing the die configurations in the areas where the
locking flutes are formed. The male die has a
configuration which conforms to the configuration
desired for the inside of the carton. The portion of
the male die used to form the inside of the locking
flutes with their attendant locking slots is shown in
Figure llA and Figure 12 shows a vertical view of one of
the depressions in the die which are used to form the
recess and its slot. The corresponding female die forms
the outside of the carton and Figure lOB shows the part
of the female die which mates with the part of the male
die shown in Figure llA. The shearing action of the
shear keys is shown in Figure 13.
The inner moulding surface of male die 90 has a
number of depressions in the surface used to form the
rear face of the front wall of the cover. These
depressions, 101, correspond in number to the number of
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locking flutes to be formed in the covex of the carton
(only one is designated in Figure llA for clarity).
Each depression has a rear wall 102 and side walls 103
which are joined to the rear wall by means of a curving
fillet. The depression is both deeper and wider at its
top end than its bottom (referring to the disposition of
the die shown in Figure llA, although it should be
remembered that in use the male die will be inverted
from this position), in order to yive the correct
trapezoidal, wedge-shaped configuration to the locking
flutes.
The male shear key 105 for forming the locking
slot is situated a little more than halfway down
depression 101, at the point where the locking slot is
to be formed. Male shear key 105 (shown by itself in
Figure 14) has a rear shear face 106 and two lateral
shear faces 107 which are inclined at an angle laterally
to the rear shear face. Rear shear face 105 projects
out from rear wall 102 oE the depression in order to
~engage with the shear key on the female die to form the
locking slot. The lateral shear faces on the male shear
key are inclined at an angle (with respect to the rear
face) which corresponds substantially to the angle of
the side walls (with respect to the rear wall) of the
locking flute and thus, the angle chosen will depend
upon the configuration of the flute. If the flute is
curvilinear in cross-section, e.g., arcuate, the male
shear key may have a corresponding contour and the
female shear key will have a like contour.
Male shear key 105 is retained in a slot-like
channel 10~ within the body of male die 90. It is held
within the channel by means of an abutment 109 at its
rear end which fits within a cavity 110 in the body of
the die.
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The shear key is pre-loaded by means of
- concavo-convex washers 111, although other suitable
biasing means such as springs, resilient pads or
-pneumatic ~r ~ydraulic devices could be used. The
simpler mec~anical de~ices such as the washers shown or
springs will normally be preferred for simplicity. The
pre-load washers are held in by backplate 112 which is
retained by means of screws 113.
The foremost faces 114 of the male shear key
are chamfered to provide a ramp down which the leading
corner of the female shear key may slide as the dies
close. For this purpose, a ramp angle, ~igure 13) of
from about 10 to 30, preferably about 15 to
20, will be suitable. For similar reasons, the shear
faces of the shear key may be inclined at an angle to
the direction of die movement although the angle in this
case should be much less, preferably about 1 - 4,
in order to promote good shearing action. The angle
selected should be such that when the shear keys come
together, the aperture is formed by what is a shearing
movement between essentially parallel shearing faces.
The most efficient shearing action and the cleanest
apertures are produced by minimizing any scraping action
of one shear key upon the other; to this end it would be
preferred for the shear faces of the two keys to be
wholly parallel but, for practical reasons, the very
slight incline is advantageously provided to allow for
mechanical inaccuracies and to ensure constant contact
between the shearing surfaces as the mould is closed.
The corresp~nding ~emale shear key 115 is
fitted in~ ~em~le die 91 ln flute-forming protrusion or
ramp 116 which has a configuration conforminy to that
desired in the outer surface of the locking flutes or
recesses. The female shear key may be positively fixed
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in female die 91 without providing for any movement as
the male shear ke~ will be capable of taking up any side
play during th~ ~o~ding cycle. However, a relief
-chamfer ~ay be p~ovided on the leading edge of the
female shear key (see Figure 13) in order to ease the
shear faces over one another at the beginning of the
shearing action and to prevent chipping of the keys at
their leading edges.
As the plastic sheet is drawn into the mould by
the movement of the dies, the mating die surfaces
approach one another more closely until the shear faces
of the male and female shear keys come into engagement
with one another, just before the dies close
completely. The front shear face 117 of the female shear
key then engages with rear shear face 106 of the male
shear key and the lateral shear faces 118 (only one
designated in Figure llB) with the lateral shear faces
107 of the male shear key. Further closing movement of
the dies then shears the heat-softened plastic away from
the shear faces of the shear keys, this process being
assisted by the inherent tendency of the softened
plastic to draw away from any puncture formed in it.
The exact mating bet~een the shear keys which is
provided by their configuration and the pre load on the
male key ensures that a clean, sharp-edged locking slot
is formed in the carton which gives the desired
efficient locking characteristics.
As may ~e appreciated from this discussion, the
loc~ing slots are formed in two dimel~sions in the plane
sectionin~ the rear a~ e faces of the locking
flutes~ Also, the top and bottom edges of the locking
slots are accommodated within the length (top to bottom)
of the flutes so that the slots are formed wholly within
the confines of the length of the flutes, although, of
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course, the slots may be continued laterally along the
front wall of the carton, if desired. Thus, the slots
are bounded upon two edges by margins which extend in
two dimensions in the plane perpendicular to the
direction of die movement and they have a finite extent
(fixed by the length of the shear faces of the shear
keys) in the direction of relative die movement. The
sloping configuration of the lower part of the locking
flute below the locking slot, i.e., its varying offset
from the inner face of the front wall of the cover also
contributes to defining the size and final shape of the
slot and should therefore be taken into consideration
when fixing the characteristic configuration of the
locking elements.
Figure 15 illustrates in schematic form the
directions in which the aperture may extend relative to
the direction of die movement and to the surface of the
plastic sheet. In Fig. 11, three mutually orthogonal
planes A-A-A-A (the A plane), B-B-B-s (the B plane) and
C-C-C-C (the C plane) are indicated, together with a
fourth plane, D-D-D-D (the D plane) which is parallel to
the B plane. The planes meet in the region in which the
aperture is formed in the plastic sheet 92. The plastic
sheet is sectioned along, respectively, the D plane, the
A plane and the B plane. The plastic sheet is shown in
its thermoformed condition i.e. after moulding has been
completed, in the region of the locking flute where the
aperture is formed. Accordingly, the inner face of the
front wall 27 of the cover slopes down from the A plane
at an angle a; the inner 40 of the locking flute slopes
down from the s plane at a greater angle, a', as this
face is inclined at a different angle than the front
wall of the cover (see Figure 9). The front wall of the
cover continues on above the A plane, as shown by the
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section along the B plane, at angle a. The direction of
die movement is indicated by arrows 120 and 121 for the
female and male die members, respectively. As can be
seen, a section of the plastic sheet extends in two
dimensions in the plane perpendicular to the direction
of die movement, i.e. the A plane. Obviously, if the
flute had a curvilinear cross section, the sheet would
still extend àcross the A plane in a similar manner,
except that the section in the A plane would differ from
the generally rectilinear U-shape shown on the Figure.
The shear keys will normally have a configuration which
conforms to that of the flute and therefore will
normally extend across the A plane in a similar manner.
As shown, the plastic sheet is also inclined at an angle
to the direction of die movement, as indicated by arrows
120 and 121. The angle of the sheet surface with
respect to the direction of die movement is measured in
the B plane which is parallel to the direction of die
movement and is equal to (90+a) for the surface of
~the front wall and (90+a') for the surface below the
locking aperture, measured in the clockwise sense from
vertically above.
