Hinged cover carton

A carton with a hinged cover formed from plastic sheet material has a locking device which comprises a continuous, rigid locking member formed integrally with the bottom section of the carton which extends upwards from the bottom section with a forward-facing extension on the top for engaging with corresponding locking apertures formed in the cover. The cartons are particularly useful for packaging eggs since they permit eighteen-cell carton to be packed on conventional packaging equipment normally used for twelve-cell cartons.

My co-pending U.S. patent application, Ser. No. 630,163, filed concurrently 
with this application, describes the method of forming the locking slots 
or apertures used in the present cartons, now U.S. Pat. No. 4,012,153. 
My co-pending U.S. Design patent application, Ser. No. 630,166, filed 
concurrently with this application, shows an ornamental design for a 
carton incorporating a locking device of the type described in the present 
application. 
BACKGROUND OF THE INVENTION 
This invention relates to cartons with hinged covers which are formed from 
sheets of plastic materials by a thermoforming process. 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, 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. Various locking devices have been previously described or used: 
U.S. Pat. Nos. 3,337,110 and 3,356,284, for example, describe cartons 
which have a locking flap on the bottom section with a locking detent 
which engages with a locking flange on the cover. U.S. Pat. No. 3,648,916 
describes a carton with a latching flap on the bottom section which has 
wedge-shaped recesses matching similar recesses on the cover; latching 
bars on the recesses engage detent on the cover to provide the desired 
locking. Similar locking devices are shown in U.S. Pat. Nos. 3,687,350; 
3,735,917, 3,817,441 and 3,908,891. 
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 sheet only in versions which 
do not have complete protective covers although an 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.times.30 cm (12.times.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. 
In my prior patent application, Ser. No. 481,512, I have described a 
thermoformed plastic carton which is capable of holding eighteen eggs 
while still being capable of being filled on conventional packaging 
equipment. The carton described in that application has latch elements 
which are formed on both the cover and cell sections by extending the 
plastic sheet beyond the sheet line during the thermoforming operation. In 
this way, the severe stretching of the plastic which would lead to 
weakening of the latch elements is avoided. The cover section on that 
carton extends down a considerable way and difficulties have occasionally 
occurred when the edge of the extended cover has caught on something and 
been forced open. It would be possible to reduce this risk by making the 
cover extension shallower but if this were done, the latching apertures 
would be brought too near the bottom edge of the cover, weakening the 
latching arrangement severely. 
I have now devised a stackable carton which can be moulded by thermoforming 
a plastic sheet and which has an improved locking device. The locking 
device can be used on divers types of carton but is especially useful with 
egg cartons because it enables covered eighteen cell egg cartons, which 
are capable of being 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. 
SUMMARY OF THE INVENTION 
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 remains in a 
fixed portion relative to the bottom section; however, the plastic sheet 
material of which the carton is formed permits a certain amount of 
deformation in the locking member and the cover that, when the carton is 
to be opened, the locking bar can be disengaged 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.times.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 configuration in which curvilinear buttresses 
extend down from the locking member to the divisions between 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 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 locking slots in the 
recesses in the cover are formed during the moulding operation using a 
suitable die.

DETAILED DESCRIPTION 
Prior Cartons 
A known type of eighteen-cell moulded wood pulp carton is shown in FIG. 1. 
It has a bottom section 10 for holding the eggs in a conventional 
6.times.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 of 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 permissible 30.times.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 projections 15 would lead to grossly weakened regions around the 
areas where the drawing occurs, mostly in the projections themselves. 
Present Carton 
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 FIG. 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 eggs, projecting tips 
22 are provided (only two are designated in FIG. 2) between adjacent 
cells, both transversely and longitudinally; however, at the center of the 
carton (see FIG. 8), the tips are omitted for a purpose which will be 
explained below. The use of projecting tips between the egg cells to 
provide additional protection for the eggs is described in U.S. Pat. Nos. 
3,563,446, 3,687,350, 3,817,441 and 3,908,891, to which reference is made 
for details of various arrangements for these tips, including tips 
extending in a ridge-like form between two pairs of cells, either in a 
longitudinal or transverse direction. Any of these arrangements may be 
used in the present cartons but 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 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 is 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 although, 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 FIGS. 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. 
At the front of bottom section 20 a rigid, integral member 45 is formed 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 
extend forwards toward the inner face of front wall 27 of cover 24 when 
the cover is in the closed position (see FIG. 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 corners of the carton for 
a similar purpose. Stacking lugs 49 on locking member 45, however, help to 
maintain the desired rigidity in locking member 45. If desired, stacking 
lugs 49 could be extended laterally so that they extend around the curve 
of the egg cells to provide ever greater rigidity and possibly also to 
provide a greater degree of self-aligning for the locking flutes as the 
carton is closed. The locking member also derives additional support from 
the buttress-like configuration of the sloping wall 48 of the locking 
member where it extends down from locking extensions 46 and into the 
bottom section 20 between the short rows of cells (see FIGS. 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 their extent, these ribs merge into the 
protective tips between the egg cells in the front longitudinal 
row--except at the center of this row where there is a depression between 
the cells, as discussed further below. 
As can be seen in FIG. 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 lateral movement of the locking parts being 
provided by the resilient nature 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 (FIG. 6). 
When the carton is to be opened, the locking parts can be 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 member 45 relatively 
away from one another so that 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 of 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, similar to that 
shown in U.S. Pat. Nos. 3,563,446 (FIGS. 5 and 6) and 3,817,441 to which 
reference is made for details of such cover support arrangements. 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 found 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 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 flexure, 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 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. 
Carton Manufacture 
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 satisfactory 
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 of 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 of foamed polystyrene to expand but the mould 
configuration will control the thickness of the sheet in the various parts 
of the carton; some parts may be thinner than the initial sheet and others 
thicker. 
The moulding cycle is shown in FIGS. 10A to 10C, 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 arranged 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 FIG. 10B, the plastic sheet is drawn down to 
stretch the plastic over the die members until, when the dies are 
completely closed, as shown in FIG. 10C, 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. 10C), 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 FIG. 10C, the cover and the bottom section are drawn downwards 
from 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 
outward extension of the 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 FIGS. 11A 
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 FIG. 11A and FIG. 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 FIG. 10B shows the part of the female die which mates with 
the part of the male die shown in FIG. 11A. The shearing action of the 
shear keys is shown in FIG. 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 locking 
flutes to be formed in the cover of the carton (only one is designated in 
FIG. 11A 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 FIG. 11A, although it 
should be remembered that in use the male die will be inverted from this 
position), in order to give 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 FIG. 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 of 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 108 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. 
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 or hydraulic devices could be used. The simpler mechanical 
devices 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, (FIG. 13) of from about 
10.degree. to 30.degree., preferably about 15.degree. to 20.degree., 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.degree.-4.degree., in 
order to promote good shearing action 
The corresponding female shear key 115 is fitted into female die 91 in 
flute-forming protrusion or ramp 116 which has a configuration conforming 
to that desired in the outer surface of the locking flutes. The female 
shear key may be positively fixed in female die 91 without providing for 
any movement as the male shear key will be capable of taking up any side 
play during the moulding cycle. However, a relief chamfer may be provided 
on the leading edge of the female shear key (see FIG. 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 FIG. 
11B) 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 between 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. 
The aperture forming process is described in greater detail in my 
co-pending patent application, Ser. No. 630,163, to which reference is 
made for further details of the process and for the dies which are used in 
it.