Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to an injection moulded plastic
cup-like or dish-like article.
In the field of manufacture by injection moulding and
in the merchandising of plastic articles, particularly thin-
walled cup-like articles such as containers for food and
beverages and dish-like articles such as container lids, trays ~-
and dishes which are all produced in mass quantities, the
minimization of the plastic content of the article and the
maximization of the allowable speed of moulding production of
the article are both of high economic desirability.
The general objective of my invention disclosed in
Canadian Patent No. 938,233 was to obtain a substantial
improvement over the prior art in these two areas of economy in
respect to thin-walled articles. The principal element in the
aforesaid invention is a cross-lattice of intersecting ribs on
the wall or walls of the article dividing the wall surfaces `
bearing the network of ribs into a plurality of discrete
rhomboidal web portions entirely bounded by said ribs, each of
said ribs being directed at an acute angle to the theoretical
line of shortest distance in the walls between the point of
injection and the moulding terminus of the article.
The rib lattice of my prior invention allows a `
reduction of plastic content of a thin-walled article such as
a beverage cup without significant sacrifice of strength by
enhancing the strength-to-weight ratio. The benefits which
have been found to accrue therefrom allowed a reduction in the
material content of an article such as a beverage cup or other
- type of container by as much as 35~ without, in some cases,
reducing the impact and flexing strengths of the article beyond
acceptable limits. In some articles an actual strength
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enhancement accompanied the material reduction.
The produced costs of a plastic article, however,
depends not only on the monetary value of the material content
but to an important extent on the allowable speed of production,
which, in the case of injection moulding, is the cycling speed
of the moulding machine. The main restricting factor in the -
allowable cycling speed of production of a given article is the
required mould dwell which is the time the mould must remain
closed to permit solidification of the plastic being moulded.
It has been found that my prior invention allows
increases in mould cycling speeds ranging from 15% to as high
as 35~ over the production speed for articles of conventional
smooth-walled design. The explanation of this benefit appears
to be not only that the thinner basic wall areas in the webs
~reeze faster but, as well, the ribs solidify very quickly
because of the increased area of contact with the chilled walls
of the mould presented by the angular or spherical cross-section
contour of the ribs. Cycling speeds as high as 35 shots per
minute with the mould dwell time reduced to 8/lOths of a second
are being attained in production of such thin-walled articles as
plastic coffee cups designed in accordance with my invention.
Whilst my prior invention has been found to be most
advantageous in the moulding of all types of plastics articles,
particular problems do exist in the moulding of articles having
relatively large bottom areas, such as wide bottoms of pails,
wide~diameter container lids, dishes, and trays in which the
bottom wall may often be the main wall of the article. It will
be evident that when the structure of the rib lattice of my
prior invention, with each rib directed at an acute angle to
the line of shortest distance, is applied to any shape bottom
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wall in which the point of injection is centrally located,
round, square or rectangular, that uniformity of web shape and
area of rib density over the entire area becomes increasingly
difficult, if not impossible, to achieve.
The problem encountered arises from the fact that
the area and shape of the rhomboidal webs delineated by the
intersecting ribs and, conversely, the spacing of ribs over
the entire area to be served by the rib conduits during mould
filling, are both highly critical to the proper functioning
of the rib lattice. An individual optimum area and shape for
the webs and an optimum density of rib coverage must be
determined for each application, these optimums varying in
accord with variations of such factors as the viscosity and
flowing characteristic during mould filling of the particular
plastic of which the article is to be moulded, the distance
the plastic must flow to fill the mould, the iniection pressure
capabilities of the moulding machine to be used and the
structural strength required of the finished article. Also
of great importance is that once these optimums have been
determined, they be maintained with little variation over the
entire area served by the lattice. More specifically it is `~
of considerable importance that the unsupported width of the r
webs defined by the rib lattice ti.e. the short axis of the ;
rhomboid of the webs to be described below) and the distances
between ribs remain constant over the entire area of a bottom
wall from the point of injection to the perimeter of the rib
lattice.
In achieving the foregoing structural characteristics
in the production of thin-walled plastic articles the following
considerations are involved.
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First, the passageways in the closed mould to allow
formation of the rib networ~, being of relatively large
cross-section as compared to the moulding spaces for formation
of the webs, serve as an interlocking network of conduits for
the inrushing molten plastic during the moulding of the article
and permit the filling of thinner basic wall sections than
heretofore practical since each discrete web is completely
surrounded by ribs and thus is "flash" filled from multiple
directions from the ever-freshened supply of hot plastic in
the rib conduits. It has been found that w~bs as thin as
0.025 centimeters can be filled successfully with the aid of
this rib lattice. This degree of economy is not attained in
containers of smooth wall design or in containers of the prior
art which strive to attain thinness of the basic wall by
provision of ribs parallel to each other and aligned parallel
to the line of shortest distance from the point of injection
to the moulding terminus. The explanation would appear to be
that the inflowing plastic has little tendency to deviate
from the non-intersecting parallel ribs into thin web areas.
In consequence the ribs must be closely spaced and the web
areas between the ribs must remain relatively thick in cross-
section if they are to be filled successfully, thus diminishing
any gain in strength-to-weight ratio to be achieved.
The network of intersecting ribs, in addition to
àiding the filling of very thin web areas, serves to conduct
the molten plastic to the terminus of the mould void more
readily than when the article being moulded has conventional
ribless walls, the plastic in the rib channels being slower to
yield up its heat to the chilled wall of the mould by reason
again of the greater cross-section mass of the ribs. This in
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turn is a considerable aid to easy filling of the relatively
massive rim structure which usually constitutes the moulding
terminus in articles of the container category.
Another contribution o~ the present rib lattice to
a high strength-to-weight ratio of a thin-walled article is
the high-performance structural wall resulting from the
reinforcing effect of the interlocked rib triangles. These
interlocked ribs effectively enhance, in the instance of thin-
walled articles such as shallow dish-like lids, the rigidity
of the wall bearing the lattice.
Still another significant contribution to an improved
strength-to-weight ratio of a thin-walled plastic article
appears to be a notable reduction in the residual internal
stress remaining in the finished product as a result of the
presence of the rib lattice on its wall or walls. The
explanation of this benefit would appear to be as follows:
In the case of the moulding of a conventionally designed thin- `
walled article in which the walls are smooth and of equal
thickness all around, the plastic solidifies in all areas at
the same instant while the article is still in the rigid
confines of the still-closed mould and a complex pattern of ;
internal stress arising from the shrinkage of the plastic
during its change to the solid state is locked into the wall
of the article having no direction in which to relieve itself.
However, a different situation prevails in a wall bearing a
lattice of intersecting ribs at the time of freezing of the
plastic in the mould. First the plastic in the thin webs
freezes and much of the shrinkage stress which would normally
remain in the webs relieves into the close-by ribs which are
still molten by reason of their greater cross-section mass.
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When the rib runs freeze they in turn relieve stress into the
adjacent rib intersections which are the last points to
solidify, again because of their still greater cross-section
mass. Such sequential freezing resulting in reduction of the
residual stress which would otherwise be present in the walls
of the article appears to enhance the impact and flexural
strengths of the walls of articles made not only of the more
brittle plastics but also appears to benefit articles made of
less brittle plastics such as polyethylene and polypropylene
by minimizing post-mould warpage and later environmental stress
cracking.
A further benefit of the rib lattice of my invention -
has been found to be the tendency of the angular ribs to induce
the plastic flowing into the mould to flow in a multi-oriented -
pattern thus resulting in a multi-oriented "grain" in the
plastic of the walls of the finished article. This appears to
minimize the susceptability of the wall to splitting when
stressed, in the instance of lid containers, in the main plane
of the article. ~;
The present invention seeks to provide a novel
structure for a rib lattice which, when applied to any
substantially flat surface or wall in which the point of ~-~
injection is, for example, centrally located, will provide a
plurality of discrete rhomboidal webs all of approximately the
same area and shape over the entire area served by the lattice,
the basic geometric structure being adaptable to suit all areas
and shapes of such flat surfaces or bottom walls.
In the production by injection moulding of such
divers articles as large pails or serving trays with large
bottom wall areas it is sometimes the practice to provide the
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i:
mould with two or more points of injection. Thus, the present :
invention seeks to provide a rib lattice structure adaptable
to interfacing with two or more duplications of the same
structure in configurations which will achieve the same :
advantages as when only one point of injection is utilized.
With the foregoing and other aspects in view, the
present invention generally concerns a plastic article, ~
injection moulded, of cup-like or shallow dish-like conforma-
tion comprising a continuous bottom wall and a continuous side
wall with a point of injection located in the bottom wall and
a moulding terminus located at the extremity of the side wall,
and having superposed on at least one side of a portion or all
of the said bottom wall a cross-lattice network of ribs integral
with the said wall and consisting of a plurality of trunk ribs
radiating outward from the point of injection at equal angles `
to each other, each rib directed along a theoretical line of :.
shortest distance through the walls of the article from the ;.~:.
point of injection toward the moulding terminus, said trunk
ribs being supplemented with ribs branching out in opposing ~
pairs at equal intervals along the trunk ribs, each branch -:
rib from a trunk rib proceeding on a line parallel to the
adjacent trunk rib on the same side and intersecting branched
ribs from the said adjacent trunk.xib whereby said trunk and
branch ribs interact to form a plurality of discrete rhomboidal
web segments, each web segment entirely bounded by ribs being
of substantially uniform size and shape. i ~ .
The invention will now be described further by way ' ,
of example with reference to the accompanying drawings, in
which:
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Figure 1 is a plan view of a plastic container lid,
the main bottom area of which bears a rib
lattice conforming to the rib lattice
structure defined in my prior invention.
Fi~ure 2 is a plan view of a plastic container lid
bearing the novel rib lattice configuration
of the present invention.
Figure 3 is an elevation, partly in section, of the
lid depicted in Figure 2, the broken line
representing the peak level of the ribs
comprising the rib lattice.
Figures 4 and 5 represent segmental fragments of two -
alternative rib lattices of the present '~
invention illustrating the variations in
web shapes and dimensions and in rib de~sity ;,
per unit of wall area possible within the ~
geometric formula of the new rib lattice. ;
Figure 6 is an enlarged cross-sectional view of one , `
of the trunk ribs of the invention and the
integral portion of underlying basic wall
representing reduction in cross-sectional
area of the rib towards the rib periphery.
Figure 7 is an enlarged cross-sectional view of one ';~of the trunk ribs of the invention and the ,~
integral portion of underlying basic wall.
Contrary to the decline in rib cross-
sectional area depicted in Figure 6, Figure 7
illustrates an alternative decline in basic
wall thickness towards the periphery of the
wall.
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Figure 8 represents a large scale cross-sectional
view of one of either the trunk or branch
ribs of the rib lattice of the present
invention and the integral portion of
underlying basic wall, illustrating one of
the many variations in rib shapes permissible
in the rib lattice of the invention.
Pigure 9 is an illustration of one of the many shapes
of bottom wall to which the rib lattice of
the invention can be adapted. In this illus-
tration a rectangular bottom wall is depicted
in small scale along with the rib lattice
configuration adjusted to suit the rectangular
shape.
Figure 10 illustrates the bottom wall of an article
provided with two points of injection and
with two identical rib lattices radiating
from the points of injection and the rib
lattices adjusted in configuration so as to
interface symetrically at points equidistant
from the two points of injection. ;
Figure 11 represents a fragment from the bottom wall
of an article bearing the rib lattice of the
invention with arrows indicating the lines of
principal flow of the molten plastic from the
point of injection to the periphery of the
rib lattice during moulding of the article.
Figure 12 represents a single isolated web portion
delineated by the rib lattice of the invention
with its bounding ribs and with arrows
inscribed to show the various directions in
which the plastic flows from the rib ducts
into the web areas during moulding.
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Figure 13 represents a single web portion with its
bounding ribs with arrows showing the
direction in which shrinkage stress is
relieved as the webs, the adjacent ribs and
then the rib intersections freeze in sequence
during moulding.
The plastic container lid depicted in Figures 1, 2 and
.: .
3 is typical of a large number of articles in the fields of con-
tainers, container closures, dishes and trays in which the bottom
wall is either the main wall or, at least, is of sufficient area
to represent a significant portion of the total wall areas of
the article. In Figure 1 the article depicted includes a point
of injection 1, a hub area 2 being a circular smooth area from
which all ribs emanate, the ribs 3, and a sidewall structure
which is a shallow U-shaped rim 4.
The rib lattice in Figure 1 is constructed in accor-
dance with my prior invention specifying that each rib of the rib
lattice be directed at an acute angle to a theoretical line of
shortest distance between the point of injection and the rim or
moulding terminus of the article. To be of maximum benefit to `
the article during its production and later utilization, the
form of rib lattice must enclose webs of approximately equal
size and shape over the entire wall surface served by the lattice -
and result in a rib coverage of approximately equal density per
unit of area. It will be observed from Figure 1 that such uni-
formity is geometrically difficult when the rib lattice of the
prior invention is applied on any essentially flat bottom wall 5.
The webs 6 furthest from the hub 2 define more than four times
the area of the webs 7 closest to the point of injection and the
density or frequency of ribs per square centimeter of wall
surface at the outer perimeter of the rib lattice would appear to
be less than one-tenth the density in close to the hub. In these
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circumstances the benefits of the rib lattice of the prior
invention on any flat bottom wall area are limited.
Figure 2 is a full scale underside plan view of a
similar container lid as depicted in Figure 1 but bearing on
its bottom wall 8 the rib lattice of the present invention.
The basie structure of the new rib lattice will be appreciated
through a study of this example application. It will be seen
that a plurality of "trunk" ribs, three o~ which are indicated
by the numeral 9, run in the direct line of shortest distance
from the point of injeetion 10 to the outer perimeter of the ;-
rib lattiee 11 whieh, in this example, is the commencement of
the sidewall structure 12 of the article. All trunk ribs are
at equal angles 13 to each adjacent trwlk rib.
Although there may be a greater or lesser number of -~
trunk ribs in other applications, in this example there are
sixteen trunk ribs and the angle of divergence~ from each other
is 22 1/2 degrees. I have found that an angle 9 of not less -
than 15 and not more than 45~, totalling 360 by completing a
circle, is preferred, and the geometric consequence of this is
that the short axis of the rhomboids ranges between about 13 and
41~ of the long axis.
It will be further observed that at equally spaced
intervals proceeding outward along each trunk rib 9 from the
point of injection 10 two branch ribs depart from eaeh trunk
rib at eaeh interval, one in a clockwise direetion designated
by numeral 14 and one in an anti-elockwise direction designated
by numeral 15. It will be further apparent that eaeh of the
ribs 14 branching clockwise proceed outward parallel to the
nearest trunk rib on the clockwise side, i.e. on the same
common side, and that eaeh of the anti-elockwise ribs 15
proceeds in parallel to the nearest trunk rib on the anti-
clockwise side, i.e. again on the same common side. To
illustrate this in Figure 2, the center line of branehes 14
and their related clockwise trunk ribs have all been projected
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to the outside of Figure 2 along imaginary lines 16. Similar
imaginary lines 17 have been projected for the anti-clockwise
ribs 15 and their commvn adjacent anti-clockwise trunk rib.
It will be seen that all lines 16 are parallel to each other
and all lines 17 are parallel to each other.
When the foregoing structure is followed the branch
ribs intersect with each other at equal angles at equal
intervals with the result that each discrete web enclosed by
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the ribs closest to the point of injection, identified with the
number 18, to those on the perimeter of the rib lattice,
designated by numeral 19, are of approximately the same area
and shape even though it may be expedient in some applications
of my invention to vary slightly the width and height of the
trunk and branch ribs as they proceed outward from the point
of injection. It will also be apparent from Figure 2 that
approximately the same density of ribs per square centimeter
is automatically maintained over the entire area served by the
rib lattice. ' '
, .
Figure 3, an elevational view of the container lid of
Figure 2 with haIf cutaway to show a section, illustrates the
relationship of the rib lattice to the total article. In this
example the rib peak level indicated by broken line 20 extends
0.018 centimeters beyond the basic wall 8. The basic wall 8,
in this example, is 0.038 centimeters in thickness, and the
sidewall 12, in this example a rim structure, are both shown in
cross-section. As shown in Figure 2 the rib lattice terminates
at the rise 21 toward the rim structure proper 12.
In this example embodiment shown in Figures 2 and 3,
the rib lattice is applied to the underside of the bottom wall 8
so as to leave the top of bottom wall 8 smooth for easier
printing. However, the rib lattice of this invention may be
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applied to either side of the wall with equal effectiveness.
Also, while the rib lattice illustrated in Figure 2 covers all
the bottom wall area except a small hub area 22, in other
applications the rib lattice coverage may be limited to only a
smaller portion of the wall. For instance it may be desired to
provide a larger smooth hub area or a smooth peripheral band to
provide adequate space for printing of the manufacturer's logo, -,
etc. Another optional feature of the rib lattice illustrated in
Figure 2 to be noted is that the equally spaced interval of
branching from the trunk ribs has been selected so as to result
in the final outer branching occurring a half interval short of
the lattice perimeter. This results in a close spacing of the
ribs at the lattice perimeter and provides for easiest possible
flow of plastic into the sidewall structure during moulding.
Figures 4 and 5, being fragmental segments of two
alternative rib lattice structures, illustrate shapes and areas
of the web portions and the rib densities which can be readily
àltered to suit the particular plastic flow and strength
requirements of each individual application without departure
from the basic rib lattice structure of the invention. In both
these segments the length of the trunk ribs from the point of
injection 10 to the perimeter of the rib lattice 12 is the
same but in Figure 4 the angle 13 between the trunk ribs 9 is
30 degrees while the angle 13 between the trunk ribs of
Figure 5 is 22 1/2 degrees. In Figure 4 the equally spaced
intervals of branching marked A, B and C have been selected to ;'
result in three branchings whereas in Figure 5 the interval
selected results in four branchings, A, B, C and D. It will be
apparent from a comparative study of these two Figures that the
length of the short diagonal of the web rhomboids may be
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controlled to suit each application by selection of an
appropriate trunk rib angle and the length of the long axis
of the unsupported web rhomboid may be controlled by selection
of an appropriate measurement for the intervals of branching.
Thus it is evident that the rib lattice is adaptable to suit
each individual application.
In the moulding of bottom walls of some articles when
the plastic being used is of a free flowing type in its molten ;
state and when the area to be covered by the rib lattice is not ,
excessively large, it is satisfactory for the cross-section
area of the ri~s and the thickness of the underlying basic wall
to remain constant all the way from the point of injection or
hub to the outer extremity of the rib lattice. However such is
not the case when a high viscosity plastic must be used or a
large area bottom wall must be filled during moulding so it is
sometimes desirable to provide for easing the initial flow of
plastic at the point of injection into the e~panding outer area ;~
of the bottom wall.
Figures 6 and 7 illustrate alternative methods of
accomplishing this. One of these is to provide trunk ribs 9
which start out from the hub with a relatively high cross-section ,~
area and decline at a steady rate to a lower cross-section area ,~
at the perimeter of the rib lattice. The other is to provide
for a relatively high basic bottom wall 8 having a thickness
at the point of injection tapering off in thickness to the
outer perimeter. And, of course, these two alternatives can be
used in combination.
Figure 6 is a large scale cross-section view of a trunk
rib 9 and the underlying portion of basic wall 8 with which it
is integrated. The arc marked 25 represents the contour of the
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rib adjacent the point of injection or the hub and the arc
marked 25' represents the decreased rib cross-section at the
outer limits of the rib lattice. ~ reduction of approximately
40% decrease in cross-section is represented. The thickness of
the basic wall remains unchanged. A decrease in thickness
within the range of from 20~ to 50% is preferred.
Figure 7 is a large scale cross-section view of a
trunk rib 9 and the underlying portion of basic wall 8 with
which it is integrated. In this illustration the wall thickness
decreases about 50% from Tl at the point of injection to T2 at
the rib lattice perimeter, the rib cross-section remaining
constant. A decrease of thickness within the range of from 20%
to 50% is preferred.
Since there is little possibility of the ribs located
on a bottom wall being of a shape which would constitute an
undercut which would interfere with stripping of the article
from its mould unless the bottom wall is of exceptionally high
concavity, the trunk and branch ribs can be of any desired
cross-section profile. In most bottom wall applications low
but wide ribs of a spherical cross-section profile are preferred
because such a profile presents a maximum area of surface
contact for chilling in the mould and thus allows the fastest
possible mould cycling. In articles such as the container lid
of Figure 2 both trunk and branch ribs may be of spherical
cross-section contour and of the order of only 0.018 centimeters
high and 0.229 centimeters wide. However, sometimes considera-
tions other than fast cooling must be taken into account.
Figure 8 is a cross-section view of a triangular rib 30
on its substrate basic wall portion 31. This shape will
contribute, when requlred, greater rigidity to the wall than
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will the low wide spherical contoured rib illustrated in
Figures 6 and 7.
Figure 9 illustrates the applicability of the rib
lattice of the present invention to any shape of bottom wall, ~`
the shape chosen for this example being a rectangle. It should
be noted that, unlike the equal length trunk ribs 9 provided in
the circular bottom wall of Figure 2, the trunk ribs 32 of -
Figure 9 proceed over the full distance required in each
instance to reach the rectangular perimeter of the rib latticed
wall. The significant feature to be observed in Figure 9 is
that without departure from the basic rib lattice structure of
the invention, the web shapes and areas and the rib densities `
remain virtually unchanged over the entire rectangular area
served by the rib lattice, thus achieving a principal objective -
of the invention. Specifically, in Figure 9 it will be noted -
that the trunk rib angle TR~l equals trunk rib angle TRA2 and
that all other trunk rib angles are the same. Further it will
. .
be evident that all ribs branching from the clockwise side of ,`
each trunk rib are parallel to the nearest clockwise trunk rib
and that the converse is true for all ribs branching from the
anti-clockwise sides of the trunk ribs. The result is that all
webs 33 adjacent to the point of injection 34 are equal in ~;
shape and area to the last complete discrete webs 35 approaching
the outer perimeter of the rib lattice.
Figure 10 illustrates the applicability of the rib
lattice of the invention to a bottom wall containing two or
more points of in~ection. Figure 10 is a scaled down illustra-
tion of the bottom wall of an article which could be a large
tray or box. Sidewalls have not been included in the illustra-
tion.
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It will be seen that two identical rib lattices
emanate from two points of injection 36 and that in each rib
lattice the basic rib lattice structure of the invention has
been followed, the only exception being that a special rib 37
has been provided along the line of interface between the two
lattices. It should be noted that the equal angles between
the trunk ribs 38 and the equal intervals of branching have
been carefully selected so that, with the aid of the special
rib 37, the web areas adjacent to the line of interface are, in
the main,-reasonably close to the web areas elsewhere in the
rib lattices and variations in density of rib coverages are
minimized.
It will be understood that the rib lattice structure
of the present invention can extend across an article bottom
wall onto the article sidewall. For example, this rib lattice
structure would be suitable for an article such as a dish with
a relatively large area bottom wall and a short upcurving side
wall with a high draft angle wherein the rib lattice could be
applied to both the bottom wall and the side wall, the side
wall rib lattice being a continuation of the bottom wall lattice.
Figures ll, 12 and 13 show the manner in which the rib
lattice of the present invention contributes to the bottom wall
of an injection moulded plastic article the principal benefits
of material content economy, strength enhancement and production
economy. Figure 11 is a fragmentary plan of the container lid
depicted in Figure 2 illustrating the point of injection 10, a
segment of rib lattice including two trunk ribs 9, a complement
of intersecting branch ribs 14,15 and a segment o~ the sidewall
rim structure 12 on the perimeter of the rib lattice. Arrows 22
emplaced along the center line of each trunk and branch rib show
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~C~45081
the many relatively low resistance pathways by which the molten
plastic can flow out from the point of injection to fill the
relatively massive rim structure during moulding despite the
fact a major portion of the total wall is composed of very thin
web areas 37. It will be evident from a viewing of Figure 11
that the interlocking rib structure provides an advantageous
flexural strength to the wall by inducing the plastic to flow
in such a pattern as to result in a multi-oriented grain to the
plastic in the finished article. It should be further apparent
that the pattern of interlocked ribs tend to inhibit cracking of
the wall under stress.
Figure 12 is a representation of a single rib-enclosed
web and is for the purpose of showing how exceptionally thin-
section webs are easily filled during moulding. Arrows 38
indicate the multi-directional flow of molten plastic from the
surrounding ribs 14,15 into each web 34 which occurs during
moulding. Because of the relatively large cross-section masses
of the ribs ~he plastic tends to retain its heat and to stay in
molten free-flowing condition all the way through the rib
lattice and the thin webs are filled from this ever-freshened ;
supply of hot plastic in the ribs.
Figure 13 is again a representation of a single web
with enclosing ribs and illustrates the mechanics of the
residual stress reduction effect of the rib lattice. Unlike
the situation which prevails in the moulding of a thin-walled
article with walls of equal thickness in all areas freezing at
the same instant in the mould and thereby locking patterns of
shrinkage stress into the article, the wall bearing the lattice
of intersecting ribs freezes at sequential intervals. In
Figure 13 the legends 1st, 2nd and 3rd indicate the order of
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freezing and the arrows indicate the directions in which the
shrinkage stress is relieved as the wall hardens in the mould.
First the thin webs freeze and stress relieves into the nearby
.: .
ribs which are still molten because of their superior mass.
,; Next the rib runs freeze and relieve stress into the nearest
: .
rib intersections which are the last to harden because of the
still greater plastic masses at these points.
^ The plastic container lid in Figure 2 is only one of
~ ~;s. .
~- many dlfferent artlcles whlch could have been used as an
example of the instant invention. ~Iowever, a citing of some of
the comparative statistics against those of the same lid in
conventional design should be useful in indicating the economic
benefits attainable.
~ Composed of polyethylene, this particular lid in
:. .
, conventional form has a basic wall 0.076 centimeters in
-~ thickness and a total material content of 12.25 grams. With
the benefit of the rib lattice the basic wall thickness is
reduced to 0.038 centimeters and the total material content
including the weight of the ribs is 7.1 grams, a total saving
of 42%. As well, while the lid of conventional design moulds
~' only at about seven shots per minute, production of the lid of
:::
the present invention can be as high as eleven or twelve shots
~ per minute, a production rate increase of 36%.
,; To minimize any tendency toward post-mould warpage
:, .
of the flat wall of an article bearing the rib lattice of
the present invention, it may be found advantageous to provide
- one or more annular ribs concentric with the hub of the rib
; lattice and spaced at the midway point or other intervals
between the point of injection and the perimeter of the rib
lattice.
.'' - 19 -
.
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