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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a process for molding plastic containers
and to the article produced by the process. Specifically, it relates to a
unitary, injection molded plastic basket produced in a two-piece mold and
having light weight, strength, and large carrying capacity.
Generally, the injection molding of a plastic article requires a mold
formed of a number of pieces, which when fitted together define a cavity
of the shape of the article. The mold is assembled and the cavity filled
by forcing liquid thermoplastic material into it. After the plastic has
cooled and set, the mold may be disassembled and the plastic article
removed, at which time it may require further finishing.
One type of injection molding apparatus is exemplified by Fisher U.S. Pat.
No. 3,152,365. This type of mold, called a sliding mold, has several dies
which move into and out of engagement with each other in several
directions and, possibly, at different times. In Fisher, for example, the
injection molding apparatus is designed to produce the handle of a
telephone handset. As shown in FIG. 1 of Fisher, in addition to a lower
molding die and an upper molding die which move vertically into and out of
engagement with each other, the mold includes two core members, forming
the transmitter and receiver housings, which are rotated in and out of
position along screw threads. The mold also includes two other core
members, forming the interior surfaces of the central tubular portion,
which must be slid in and out of position around the corners of the
transmitter and receiver housings. Because such a mold has multiple
members, it is capable of producing a plastic article of relatively
complex shape. However, it has the disadvantages that a good deal of time
is necessary to move the multiple members into and out of molding position
and that the apparatus itself, because it has many moving parts, is
relatively expensive. The sliding mold, therefore, is unsuitable for
fabricating simple articles such as shopping baskets which are intended to
be sold at low cost.
Another type of injection molding apparatus, exemplified by Long U.S. Pat.
No. 2,556,590, is the two-piece mold. As its name implies, the mold of
this apparatus consists of only two pieces: a core member and a cavity
member. Furthermore, in the two-piece mold the core and cavity members are
moved into and out of engagement in a single direction. The interior
surface of the cavity, and the exterior of the core, when the two dies are
placed together, define the shape of the plastic article. Because the
two-piece mold has no moving parts, it is inexpensive both in initial cost
and in operation and is therefore capable of producing an inexpensive
plastic article. Its chief disadvantage arises from the need to withdraw
the core die from the cavity die along a single direction. Because of this
feature, plastic articles produced in a two-piece mold generally may not
have vertical walls, that is, walls which extend in the direction of
separation of the two dies. When vertical walls are attempted, they are
torn from the rest of the article as the dies are separated. Typically,
therefore, to facilitate separation of dies, the walls of the plastic
article are designed with a considerable amount of draft, or taper. In
fact, it is generally considered that such walls may be no closer than six
degrees to the vertical without assuming substantial risk of destruction
of the plastic article when the mold is separated.
There is also a relationship between the thickness of the wall and the
amount of draft necessary to ensure its integrity upon mold separation.
Generally, thinner walls require a greater draft. The reason for this is
apparent. The thinner a wall, the lower the maximum shear force it can
sustain without breaking. Ideally, for a wall of infinitesimal draft, when
mold separation occurs the dies instantaneously separate from the molded
article and no shear forces are applied to the article. Practically,
however, there are attractive forces between the surface of the dies and
the surface of the article, forces which decrease with the distance
between the surfaces. The shallower the draft of the wall, the closer the
surfaces are for a given distance of mold separation and the greater the
force on the article tending to part it. Such shallow drafts, therefore,
have previously required thicker walls to withstand the forces of mold
separation. A problem develops, therefore, in designing a lighter weight,
lower cost basket by thinning the walls. If the only weight-reduction
measure taken is to reduce the thickness of the basket walls, their draft
must be simultaneously increased. For a given maximum size of basket,
thinner walls and a greater draft necessarily result in a smaller carrying
capacity.
To overcome this problem of the relationship between wall thickness, draft,
and weight, solid, thin walls have been replaced by thicker, slotted
walls, as shown in the previously-mentioned Long patent. Molding the
basket with slots in its walls has the added advantages that the contents
are more visible and that they are ventilated. On the other hand, slots
present their own set of problems, especially in molding a basket using a
two-piece mold. Not only is the draft of the basket walls important, but
the draft of the edges of the slots likewise must be controlled to avoid
destruction of the article as the mold is separated. The same reasons
apply here as were discussed above in connection with wall thickness.
Because of the attractive forces between the dies and the edges of the
slots, the slot edges must have sufficient draft to withstand mold
separation.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a unitary
plastic basket suitable for injection molding in a two-piece mold.
It is another object of the present invention to provide such a basket
whose side walls have a relatively small degree of draft or taper, thereby
enabling the basket to be constructed with a large interior volume.
Another object of the present invention is to provide such a basket with
thin, slotted side walls to decrease the weight and cost of the basket.
A further object of the invention is to provide such a basket with special
side wall construction to increase the strength of the side walls.
The present invention is a unitary plastic basket which may be used, for
example, to carry groceries by hand in a supermarket and which, because of
its design, is suitable for injection molding in a two-piece mold. The
side walls of the basket are tapered only three degrees; herefore, the
bottom has an area which is a comparatively large fraction of the area of
the top opening of the basket. This allows the basket of this invention to
hold a larger volume than achieved with prior art designs.
Light weight is achieved partly by making the side walls of the basket thin
and partly by forming slots in them; strength in the thin, slotted side
walls is preserved by corrugating the slotted cross-member of the side
walls and overlapping this corrugated, slotted cross-member with
substantially planar, unslotted cross-members to form a strong, light
weight side wall. Corrugation of the slotted side wall cross-member has
been found to increase its strength at least sufficiently to overcome the
effects of introducing slots into the member; overlapping it above and
below with a substantially planar, unslotted cross-member increases its
strength even further.
The corrugated cross-members of the side walls comprise interior and
exterior panels, and a slot is placed in each panel. A corrugated
cross-member is overlapped along both its upper edge and its lower edge
with a substantially planar, unslotted cross-member, the unslotted
cross-member along the upper edge being overlapped on the outer surface of
the corrugated cross-member while the lower unslotted cross-member
overlaps the inner surface of the corrugated cross-member. Slots in the
interior panels of the corrugated cross-member extend upward from the
lower, unslotted cross-member through the upper edge of the corrugated
cross-member. Slots in the exterior panels of the corrugated cross-member
extend downward from the upper, unslotted cross-member through the lower
edge of the corrugated cross-member. In other words, interior slots, those
in interior panels, have a bottom edge formed by the lower unslotted
cross-member, but no top edge. Exterior slots, on the other hand, have a
top edge formed by the upper unslotted cross-member, but no bottom edge.
This construction allows the basket of the present invention to be molded
in a two-piece injection mold. Instead of a substantially planar side
wall, the present invention uses a side wall of at least three
cross-members overlapped with each other so that higher cross-members
overlap the outside top edge of the cross-members below them. This layered
construction allows a wall with only a three degree draft to be formed
instead of the prior art's six-degree-draft wall. By dividing the side
walls into a number of sections according to this invention, mold
separation is made easier, for a given amount of draft, and draft can
accordingly be decreased, leading to a basket of larger interior volume.
Another feature which facilitates injection molding is corrugation of the
slotted cross-member. Interior slots are formed entirely by the core
(upper) die of the mold, which, when the mold is opened, is pulled up and
out of the basket. Mold separation is aided by the open-ended nature of
the tops of the interior slots--having no top edge, the interior slots
allow the core of the mold to be simply lifted out of the slots. The same
relationship exists between the cavity (bottom) die and exterior slots.
Since they are open-ended at the bottom and formed entirely by the cavity
die, the basket may simply be lifted from the cavity, sliding the exterior
slots from the slot-forming sections of the cavity. Side edges of the
slots have a one degree draft to permit mold separation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view of the basket of this invention;
FIG. 2 is an end elevational view of the basket;
FIG. 3 is a top plan view of the basket;
FIG. 4 is a horizontal sectional view of the basket taken along line 4--4
of FIG. 1, but with the addition of fragments of the dies that would be
used to form the basket;
FIGS. 5A and 5B are enlarged sectional views taken along the line 5--5 of
FIG. 1; and
FIG. 6 is a schematic side elevation of a fragment of the basket with the
slope of the sides of the trapezoidal openings exaggerated to illustrate
the fact that the sides of the inner openings diverge upwardly and the
sides of the outer openings diverge downwardly in order to facilitate easy
separation of the dies from the basket.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, the side view of the hand carrying basket of the present
invention, the relative steepness of the sides (small draft) which may be
achieved with the present invention is apparent. The basket, indicated
generally at 10, has a bottom wall 12 and side walls 14. As dimension A
shows, the side walls have a three degree draft, or inclination from the
vertical, to allow separation of the core die from the basket and the
basket from the cavity die upon conclusion of the injection molding
process. This small draft allows base 12 to have a comparatively large
area in relation to the opening at the top of the basket, and in
consequence of this, the interior volume of the basket is large. The side
wall visible in FIG. 1 is seen to be formed with six cross-members,
numbered 16 through 26. In addition, at the top of side wall 14, a rolled
top edge 28 is provided for strength and smoothness.
The uppermost cross-member 16 extends downwardly from rolled edge 28 to
cross-member 18 below it. Cross-member 16 contains slots 30 to decrease
the weight of the basket and ribs 33, integrally formed with rolled edge
28, to strengthen both cross-member 16 and edge 28. Below cross-member 16
are, in order, unslotted cross-member 18, slotted cross-member 20,
unslotted cross-member 22, slotted cross-member 24, and unslotted
cross-member 26. Slotted members 20 and 24 contain slots 32, also to
reduce the weight of the basket. As shown in FIG. 1, slotted members 20
and 24 are each bounded at their upper and lower edges by an unslotted
cross-member (e.g., 18, 22); this arrangement maintains the strength of
the basket's side walls. On top of two opposite side walls of the
preferred embodiment, a pair of eyelets 34 are integrally formed with the
basket. Eyelets 34 serve as attachment points for the carrying handles
(not shown) which enable this basket to be used for the hand carrying of
articles such as groceries. Beneath eyelets 34, a portion of side wall 14
is left unslotted and serves as an identification space 36. Here, the
owner of the baskets (such as a supermarket chain) may have its name
embossed in order to reduce the possibility of theft.
Bottom wall 12 of the basket, as shown in FIG. 3, is of lattice
construction to further reduce the basket's weight. The lattice is formed
of a plurality of intersecting ribs 38 defining a plurality of spaces 40
between them. Also visible in FIG. 3 is rolled top edge 28 extending
around the entire periphery of basket 10 to reinforce the top edge of the
basket. In FIG. 3, the details of the side walls are omitted for the sake
of clarity.
The special side wall construction which strengthens the slotted
cross-members of the sidewalls is shown in FIG. 4. A portion of basket 10
is illustrated in an injection mold consisting of core die 42 and cavity
die 44 together defining the space which, when filled with plastic,
becomes basket 10. This section through a slotted cross-member of two side
walls 14 clearly depicts the corrugation of the side walls which gives
them their strength. The corrugated cross-members have exterior panels 46
alternating with interior panels 48 in which are formed slots 32. Exterior
panels 46 contain exterior slots 32', while interior panels 48 contain
interior slots 32". It can be seen from FIG. 4 that exterior slots 32' are
formed by the cavity member 44 of the injection molding dies, whereas
interior slots 32" are formed by the core member 42 of the dies.
FIGS. 5A and 5B show how the various cross-members of the side walls 14 are
joined to form a strong wall having a small draft. Top cross-member 16 is
formed integrally with ribs 33 and rolled edge 28 to provide increased
strength to compensate for the formation of slots 30. Cross-member 16
overlaps, at its lower edge, with the upper edge of unslotted cross-member
18. Member 18, in addition to being unslotted, is substantially planar and
provides strength between slotted cross-members 16 and 20. Overlapping
with the bottom edge of cross-member 18 is the top edge of slotted
cross-member 20. Because member 20 is slotted, added strength is provided
by corrugating it in the manner shown in FIG. 4. Exterior slots 32' are
formed in exterior panels (not shown) of slotted cross-member 20, while
interior slots 32" are formed in interior panels (not shown) of member 20.
The bottom edge of member 20 is overlapped with the top edge of unslotted
cross-member 22. Like unslotted cross-member 18, member 22 is
substantially planar and provides strength between adjacent slotted
cross-members. The bottom edge of cross-member 22 overlaps with the top
edge of slotted cross-member 24. Like slotted cross-member 20, member 24
is corrugated to provide additional strength to compensate for its slots.
Cross-member 24 also has exterior slots 32' in the exterior panels of its
corrugations and interior slots 32" in the interior panels. Finally, the
bottom edge of member 24 overlaps with the top edge of unslotted
cross-member 26. Member 26 is a substantially planar, unslotted
cross-member which is integrally formed with bottom wall 12 of the basket.
FIGS. 5A and 5B illustrate how the design of the present invention
facilitates mold separation. Slots formed by the cavity die have a top
edge but no bottom edge (i.e., they are open-ended at the bottom). Because
of this, the basket may simply be raised from the cavity, and the
slot-forming portion of the cavity die can slide out of the bottom of the
slot. For example, substantially planar unslotted cross-member 22 forms
the top edge of slot 32' in slotted cross-member 24; however, no member
forms a bottom edge of slot 32', so that it is open-ended at its bottom.
In contrast, slots formed by the core die have a bottom edge but no top
edge (are open-ended at the top). Because of this, the core may simply be
raised from the basket, and the slot-forming portion of the core die can
slide out of the top of the slot. For example, substantially planar
unslotted cross-member 22 forms the bottom edge of slot 32" in slotted
cross-member 20; however, no member forms a top edge of slot 32", so that
it is open-ended at its top.
From FIGS. 5A and 5B, it can also be seen that the overlapping of side wall
cross-members is done in such a manner that upper cross-members always
overlap the outer face of lower cross-members.
FIG. 6 illustrates, in exagerated form, the draft of the side edges of
slots 32. The side edges of all slots are tapered one degree from the
vertical. The edges of exterior slots 32' are tapered so that the slot is
wider at the bottom than at the top because slots 32' will be lifted from
the cavity die. Slots 32", on the other hand, are tapered so that the top
is wider than the bottom because the core die will be raised from slots
32". The draft of the edges of slots 32 need only be one degree, shown as
dimension B in FIG. 6.
By means of the above-described construction, the Hand Carrying Basket of
the present invention accomplishes the stated objectives in a manner which
is unobvious over the prior art. The strength of the basket's side walls
is maintained both by including a corrugated cross-member and by
overlapping the corrugated cross-member, above and below, with an
unslotted, substantially planar cross-member. Weight reduction results
from thinning the side walls and from forming slots in them. This also
tends to reduce the cost of the baskets. Notwithstanding the thinness of
the side walls, a basket having a large interior volume may be molded,
using walls of relatively shallow draft, by means of the present invention
.
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