Forming plastic articles having strands by stretching

In order to make a non-planar plastics material article, a starting material has at least one ring of holes or depressions and is pressed so that the part which is within the ring is moved generally at right angles to the plane of the ring with respect to the part which is outside the ring, thereby stretching, into orientated strands, zones between adjacent holes or depressions in the ring; in the finished article, the strands interconnect the two parts.

BACKGROUND OF THE INVENTION 
The invention relates to plastic material articles having openings therein, 
which may or may not be closed by film, and to a method of making the 
articles. 
There have been various proposals for making such articles by hot forming 
or moulding of mesh structures. For instance articles such as brassiere 
cups and fruit bowls have been made in this manner. British Patent 
Specification No. 1,027,691 and U.S. Pat. No. 3,642,967 describe methods 
of this general type. The articles so formed are relatively heavy and/or 
relatively weak, being formed primarily of unorientated plastic material. 
As a technique, vacuum forming is well known, and usually a flat piece of 
imperforate plastic sheet or film is drawn at an elevated temperature by 
vacuum into a non-planar shape. British Patent Specification No. 607,995 
describes a technique of this type. The articles so formed are relatively 
weak, being formed primarily of unorientated plastic material. 
British Patent Specification No. 2,035,191 A describes a new technique for 
making flat mesh structures by stretching a starting material which has a 
pattern of holes or depressions therein, forming strong structures with a 
high degree of orientation. This Specification however does not disclose 
non-planar articles. 
SUMMARY OF THE INVENTION 
This invention affords a method of making a plastic material article 
comprising providing a piece of plastic starting material having therein 
at least one ring of holes or depressions; and moving a part of the piece 
which is within the ring with respect to a part which is outside the ring, 
generally at right angles to a notional plane which generally intersects 
the holes or depressions of the ring, thereby stretching, into oriented 
strands, zones between adjacent holes or depressions in the ring, which 
strands interconnect the two parts. This invention also affords a plastic 
article, comprising a first part and a substantially unoriented second 
part having an inner margin generally in the form of a ring, the first 
part being considerably out of the plane of the second part and being 
connected to the inner margin of the second part by means comprising 
spaced side-by-side strands which are oriented in the direction of their 
lengths. 
It will be understood that if the ring of holes or depressions is around 
the sides of a non-planar piece of starting material, the notional plane 
need not intersect all the holes or depressions of the ring; the notional 
plane is only referred to indicate the direction of movement of the part 
within the ring of holes or depressions. It will also be understood that 
only the inner margin of the second part of the article need be in the 
form of a ring, for the second part may be much larger. 
The articles of the invention can be, for instance, tote (i.e. carrying) 
baskets in general, baskets for small agricultural or horticultural 
produce, shopping or supermarket baskets, fish baskets, freezer or 
supermarket containers, snack containers, gift baskets, lobsters pots, 
bottle crates, moulded furniture and lampshades. If the starting material 
is formed with depressions and the plastics materials in the depressions 
does not rupture during stretching but forms orientated film, the articles 
could be buckets, washing bowls, tubs, plant containers, drinks 
containers, and also generally as above, for instance moulded furniture, 
lampshades and food containers. 
The invention can give the following advantages: 
(i) Reduction in product weight as a result of molecular orientation. 
(ii) Improved physical strength as a result of molecular orientation. 
(iii) Very good physical properties, since the forming process can take 
place at the optimum orientation temperature. 
(iv) More suitable resins, i.e. tougher resins with higher density and 
lower melt flow index, can be used since complex mould channels are not 
required. 
(v) Elimination of high capital outlay for injection moulding machine 
having large daylight (i.e., large spacing between platens when 
disengaged). 
(vi) Elimination of high cost injection moulds. 
(vii) Quicker return on capital outlay. 
(viii) Elimination of high mould maintenance cost (normally higher for open 
mesh products). 
(ix) Elimination of flashing and subsequent deflashing operation. 
(x) Low cost sampling as a result of an ability to prepare preforms (i.e. 
starting material) by drilling or machining. 
(xi) Prepared preforms can be transported to major market areas for 
forming, thus reducing transport costs. 
(xii) Export of preforms in high bulk density for subsequent local forming. 
If the starting material has been injection moulded: 
(xiii) Low mould costs. 
(xiv) As a result of preform simplicity, lower melt flow index resins can 
be used. 
(xv) Preform mouldings can be automatically ejected (difficult or 
impossible with three dimensional mesh products). 
The holes or depressions in the starting material, and any other shaping of 
the starting material, can be produced, for instance, by embossing, 
punching or stamping, drilling or injection moulding. It will be noted 
that the part of the starting material which corresponds to the outer part 
of the article and the part which corresponds to the inner part of the 
article, need not be planar. 
The zone or zones in which the holes or depressions are formed is 
preferably planar, and if there are depressions, including a membrane, the 
membrane preferably lies on the median plane of the zone. However, for 
some articles, there are advantages in having a starting material which is 
distinctively non-planar, for instance itself being injection moulded. In 
this way, the use of the invention provides the advantages of increased 
strength and saving of material and also reduces the size of the dies 
required for forming the starting material. In general, however, those 
parts which are stretched preferably have parallel faces. 
The part which is within the ring of holes or depressions may simply be 
moved in one direction generally at right angles to said notional plane of 
the ring. Alternatively, the starting material can be stretched by parts 
thereof being moved in two opposite directions, preferably sequentially, 
with respect to the part which is outside the ring(s). 
The starting material can be any suitable thermoplastic material, for 
instance, high density polyethylene (HDPE) or polypropylene. The starting 
material can be of any suitable thickness, the preferred range being 0.2 
millimeters up to 12 millimeters through a preferred minimum is 1 
millimeter and a preferred maximum is 4.5 or 5 millimeters. 
The stretching is carried out at a temperature above the second order 
transition temperature of the plastics material but substantially below 
the softening point so that significant melt flow orientation is avoided 
during the stretching. For example, for HDPE, the preferred temperature 
range is 95.degree.-102.degree. C. and for polypropylene, the preferred 
temperature range is 96.degree.-104.degree. C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a planar parallel-faced starting material 1, which in this 
case is a flat disc of plastic material having therein a plurality of 
coaxial circular rings of holes or depressions 2 and a central hole 3. The 
holes or depressions 2 of any one ring are on the same (notional) radial 
lines as the corresponding holes or depressions 2 of the other rings, and 
annular zones 4 are left between the rings or holes or depressions 2. The 
sides of the holes or depressions 2 are equal distances apart in the 
circumferential direction, in all the rings. Thus, in any one ring, the 
sum of the cross-sectional areas of each zone 14 between adjacent holes or 
depressions 2 of the ring, as measured in the cross-sectional plane of 
each such zone which has the smallest area, is equal to that in the other 
rings. 
The starting material 1 is put in the tool illustrated in section in FIG. 
2. Only part of the tool is illustrated, but it has a fixed retaining 
surface or retainer plate 5 mounting a plurality of pins 6 which engage in 
the holes or depressions 2 of the outer ring, a clamping plate 7 which 
clamps the starting material 1 against the retainer plate 5, a movable 
presser plate 8 which engages the centre part of the starting material 1 
and an actuating rod 9. The central hole 3 is entered by a projection on 
the presser plate 8 and ensures that the starting material 1 does not move 
sideways during pressing. In general, the presser plate 8 could merely 
abut the centre part of the starting material 1 or could be of greater 
diameter and have pins, similar to the pins 6, for engaging in the holes 
or depressions 2 of the inner ring, in which cases the starting material 1 
need not have a central hole 3. If desired (as shown), a clamping disc 10 
can be provided for clamping the centre part of the starting material 1 
against the presser plate 8. The plates 5, 7 and 8 and the disc 10 are 
circular and concentric with the rings of holes or depressions 2. 
The starting material 1 is heated to a suitable temperature. The actuating 
rod 9 is moved downwards, i.e. at right angles to the plane of the rings 
of holes or depressions 2 (and to a notional plane which intersects the 
centres of any of the rings of holes or depressions 2), moving the central 
(first) part 11 of the starting material 1 with respect to the outer 
(second) part 12. This movement stretches, into spaced, side-by-side 
orientated strands 13, the zones 14 between adjacent holes or depressions 
2 in the original ring; the strands 13 are orientated in the direction of 
their length and in effect connect (or more generally afford means 
connecting) the inner part 11 to the inner margin of the outer part 12 in 
the finished, non-planar article. The inner margin of the outer part 12, 
and indeed the whole of the outer part 12, is in the form of a ring and in 
a general sense, surrounds the inner part 11, though it is considerably 
out of the plane of the inner part 11. The annular zones 4 twist round so 
that their faces are roughly aligned with the strands 13 on either side, 
the zones 4 forming hoops around the article. It will be seen that the 
strands 13 on one side of each zone 4 are aligned with those on the other 
side. It will also be seen that the strands 13 are in well-defined rings 
around the article, separated by the hoop zones 4, the strands 13 of one 
ring being distinct from the strands 13 of the adjacent ring. The outer 
part 12, which forms an uninterrupted zone around the plastics material 
which has been stretched and orientated, is not itself substantially 
stretched during the pressing, the outer strands 13 being connected 
directly to its inner margin. The inner part 11 has likewise not been 
substantially stretched or orientated, in this particular case. 
A detailed explanation of what occurs when stretching the zones 14 to form 
the strands 13, is given in British Patent Specification No. 2 035 191 A 
and FIGS. 4 and 5 of the present application correspond generally to FIGS. 
1 and 2 of the Patent Specification whilst FIG. 6 of the present 
application is the same as FIG. 10 of the Patent Specification. 
However, in simple terms, the stretching will begin at the narrowest point 
of the zones 14, and as stretching continues, the orientation will pass 
along the zones 14 and will then pass into the zones 4, passing beyond the 
notional tangent lines 15 (tangential to the holes or depressions 2); in 
this way, some of the material which was originally in the zones 4 is 
drawn out and forms the end parts of the strands 13. Pressing is 
preferably terminated when the orientation of the strands 13 has not 
passed right through the zones 4, and it will be seen that in any case, 
the zones 16 which, in the original material, were between adjacent holes 
or depressions 2 of different rings, are not substantially stretched 
during the pressing. The centre line of each zone 4 will thus preferably 
retain the thickness of the original starting material 1 while the edges 
of the hoops formed from the zones 4 will be of undulating section. It is 
believed that by having the sides of the holes or depressions 2 equal 
distances apart, all the strands 13 have approximately the same width 
(circumferential direction dimension) and are stretched to substantially 
the same stretch ratio. The symmetrical disposition of the holes or 
depressions 2 and of the parts of the tool ensures that all the strands 13 
in any one band are stretched to substantially the same stretch ratio, for 
example 7:1 on the strands (measured by measuring the distance moved by 
the respective ends of the holes or depressions on either side of the 
strand). 
In the finished article, the extra thickness of the hoop-like zones provide 
good compression strength whilst the orientation of the strands 13 
provides good tensile strength in the vertical direction. 
When the article has been formed, it is preferably allowed to relax at a 
temperature close to the pressing temperature whilst still held in its 
stretched position by the tool, thus reducing the possibility of the 
finished article shrinking and distorting in subsequent use. 
The holes or depressions 2 need not be circular. FIG. 6 shows various 
shapes. The elongated holes or depressions 2 can be elongated in the 
direction of pressing or normal thereto. Depending somewhat on the shape 
of the holes or depressions 2, their area in general is preferably less 
than 50% of the (plan view) area of the starting material 1, and more 
preferably less than 25% thereof. 
The starting material 1 can be printed in selected areas to give decorative 
effects or convey a message, such areas normally being areas which are not 
stretched during pressing. 
FIGS. 7 and 8 illustrate that the outer zone of the inner part 11 can 
itself be orientated, though it is preferred in these cases to provide an 
outer former into which the base of the article is pressed, and to shape 
the moving part of the tool accordingly. 
Various patterns can be used for the holes or depressions 2. The holes or 
depressions 2 in the various rings need not be on the same (notional) 
radial lines, and can for instance be in a "diamond" pattern, with the 
holes or depressions 2 of one ring staggered with respect to those of the 
next ring or rings. FIG. 9 illustrates an article made from holes or 
depressions 2 in a diamond pattern, the rings of orientated strands 13 
being distinct from one another and separated by junctions between the 
strands 13. Furthermore, other patterns can be used (see the description 
of FIG. 18, below) if desired, and also the starting material may have 
annular zones of holes or depressions 2 in different patterns in the 
different zones; the hoop zones 4 can be present, if desired. In general, 
it is believed desirable that the sum of the distances (as measured in the 
circumferential direction) between each adjacent pair of holes or 
depressions in one ring be equal to that in the other ring or rings if the 
strands are to be stretched out the same ratio in each ring, though this 
would allow having double as many strands in one ring as in another. 
The article need not have the truncated wedge shape in cross-section which 
is seen in FIGS. 3 and 7 to 9. Using an expanding tool, articles having 
the shapes shown in FIGS. 10 to 12, and other suitable shapes, can be 
made. FIGS. 20 and 21 (see below) show other ways of producing different 
shapes. 
The rings of holes or depressions 2 need not be circular, but they are 
preferably concentric or coaxial and their centre lines are preferably 
geometrically similar, the respective parts of the tool being concentric 
or coaxial and geometrically similar. FIG. 13 illustrates that the rings 
can be oblong and FIG. 14 illustrates that the rings can be polygonal. The 
holes or depressions 2 at the ends or corners are elongated so that they 
remain the same distances apart in the circumferential direction. 
Although the starting material preferably has parallel faces in the zones 
which are stretched, it may have protruberances which are not 
substantially deformed during the pressing, e.g. to form a lip or rim on 
the article, or a foot. FIGS. 15 and 16 show that the outer part 12 has 
been pre-formed as a lip and the inner part 11 has been provided with a 
domed foot 17. 
FIG. 17 illustrates that if the presser plate 8 is suitably shaped, the 
shape of the article can be altered, without the necessity for an 
expanding tool, the article of FIG. 17 being formed from the same starting 
material as that of FIG. 16. 
FIG. 18 shows yet another starting material. 
In FIG. 18, the holes or depressions 2 are shown in straight rows, but 
these will have the configuration of rings, e.g. generally as shown in any 
of FIG. 1, 13 or 14. The holes 2 can be formed for instance by punching or 
by injection moulding. The holes 2 of a given row are aligned with those 
of one row removed, but are staggered with respect to those of the next 
row. The holes 2 of one row are preferably halfway between those of the 
next row. When the starting material is stretched in the N-S direction of 
the sheet of drawings, orientated strands 13 are formed, as shown in FIG. 
19, the strands 13 being likewise staggered. Annular zones 4 are left 
between the rows or rings of strands 13, and these zones 4 can be as wide 
as is necessary, e.g. up to 6 mm, to prevent excessive distortion into a 
serpentine zig-zag shape (the width being considered the N-S dimension in 
FIG. 19). 
FIG. 20 shows an article when it has been stretched. The starting material 
had a centre zone 21 (which remains unaltered except perhaps at its 
edges), a first ring or plurality of rings of holes or depressions around 
the centre zone (which form(s) an inner conical zone 22), a ring-shaped 
base zone 23 itself having a ring of holes (the base zone 23 remains 
substantially unaltered, except perhaps at its edges), a second ring or 
plurality of rings or holes or depressions around the base zone 23 (which 
form(s) an outer conical zone 24), and a lip or rim zone 25 (which remains 
substantially unaltered, except perhaps at its inner edge), having a ring 
of holes near its inner edge. The starting material is placed over rings 
of pins 26, 27 on two annular tools 28, 29 which are initially positioned 
with their upper faces coplanar. A tool head 30, mounted on an actuating 
rod 31, is then moved down to engage the base zone 23 and move it out of 
its original plane, pushing the tool 28 down at the same time; the tool 
head 30 stops at a predetermined position. There is a second tool head 32 
mounted on an actuating rod 33, which is preferably moved up when the main 
tool head 30 has reached its bottom position, so as to engage the centre 
zone 21; however, as alternative possibilities, the second tool head 32 
could remain stationary so as to engage the centre zone 21 as the main 
tool head 30 is still moving down, or the second tool head 32 could be 
pushed up first, when the bottom face of the main tool head 30 is say 
level with the top face of the annular tool 29. 
The holes or depressions in the zones of the starting material which 
eventually form the conical zones 22, 24 can be in any suitable pattern, 
and in the finished article, orientated strands are formed, as described. 
FIG. 21 shows an injection-moulded starting material, and although the 
holes 2 can be punched in the injection-moulded material, they are 
preferably also injection moulded. The starting material has a rim 41 and 
a base or bottom 42 which can be perforated or unperforated as desired. 
The holes 2 are formed in the sides, and can be in any suitable pattern, 
not just as shown in FIG. 21. Although the rim 41 and base or bottom 42 
are above and below the rings formed by the holes 2, respectively, they 
also can be considered as being outside the rings (the rim) and within the 
rings (the base or bottom). Although the sides are shown as vertical in 
FIG. 21, they could be inclined. Furthermore the articles could have any 
other suitable outline, for instance circular. As shown in FIG. 22, in 
order to form the finished article, the sides are deepened and form 
orientated strands 13. 
EXAMPLE 
This Example is in accordance with the method described with reference to 
FIGS. 1 to 5, except that the starting material 1 had no central hole 3 
and a simple flat disc (corresponding to the plate 8) was used for 
pressing. 
The starting material 1 was a 330 mm diameter circular disc of 3 mm thick 
HDPE, with three concentric rings of punched circular holes at pitch 
circle diameters of 283, 235.6 and 197 mm, the hole diameters being 12.7, 
8.43 and 5 mm respectively; there were 35 holes in each ring. The starting 
material and product weight was 230 gms. The presser disc diameter was 165 
mm. Pressing was effected at 98.degree. C. in a hot water bath, the centre 
of the starting material disc being moved 235 mm out of its original place 
in 10 seconds (a wide range of speeds is possible and the speed is not 
considered critical), and then moved back 10 mm for relaxation, giving an 
average final stretch ratio on the strands 13 of 7:1 . The product and 
pressing tools were lifted out of the water bath, the product was allowed 
to cool while held in its stretched position, and the product was 
released. 
The average thickness and width of the mid-points of the strands 13 in all 
three rings of strands 13 were 1 mm and 5.4 mm respectively. The centre 
lines of the hoops or zones 4 were 65 and 140 mm respectively vertically 
above the lower surface of the base or part 11. Each hoop had a width 
(vertical dimension between openings) of 1.3 mm and the dip were the 
orientation of the strand penetrated into the hoop extended about 1.5 mm 
beyond the line tangent to the openings. The product was suitable for use 
as a waste paper basket. 
Corresponding materials, dimensions and procedures can be used to make a 
product in accordance with FIGS. 21 to 23, and any other Figures of the 
drawings.