Note: Descriptions are shown in the official language in which they were submitted.
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PLAYING SURFACE AND METHOD OF MANUFACTURING A
PLAYING SURFACE
This invention concerns a playing surface and a method of manufacturing a
playing surface. The invention has particular, but not
exclusive,
application to playing surfaces for use in children's playgrounds or other
situations wherein a user could fall from a significant height, such as
situations wherein the Critical Fall Height is required to be 2.4 metres or
more.
The surface used for playgrounds and the like must be chosen such that the
Critical Fall Height matches or exceeds the maximum height from which a
user can fall from the play equipment. BS EN 1177 defines the Critical
Fall Height test. A 4.6kg spherical `headform' representing a child's head
is dropped from various heights onto a test surface, and the time the
headform is in contact with the surface and its peak deceleration is
measured. A stiffer surface reduces the time a head is in contact with the
surface but this has to be balanced with the peak deceleration, which
increases with increases in stiffness.
The Critical Fall Height is the height at which the Head Injury Criterion
(HIC) is 1000. HIC is a measure of the likelihood of head injury arising
from an impact. The HIC is calculated using:-
2.5
1 t,
HIC = (t ' __ a(t).dt]
2 - tl f f
t2 --ti t,
-m'
wherein ti and t2 are the initial and final times (t) of the interval during
which the HIC attains a maximum value, usually a 15ms interval, and a(t)
is the instantaneous acceleration over this interval in units of gravitational
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force (g). At a HIC of 1000, one in six people suffer a life threatening
injury.
It is known to increase the thickness of a foam layer of a playing surface in
5 order to increase the Critical Fall Height achieved by the playing
surface.
However, it has been found that increasing the thickness of the foam layer
only increases the Critical Fall Height up to a specified limit. For a
polypropylene foam tile having a foam density of 30 grams per litre, the
limit for the Critical fall Height that can be achieved through increasing the
10 thickness of the foam is approximately 2.4 metres. Further increases in
the
thickness of the foam layer do not significantly increase the Critical Fall
Height of the playing surface.
Furthermore, to keep the time and cost of installation down, the playing
15 surface should be kept as shallow as possible to reduce the amount of
excavating work that needs to be done to lay the playing surface.
According to a first aspect of the invention there is provided a playing
surface comprising a foam layer having a top surface and a plurality of
20 downwardly extending energy dissipating projections and a fibrous carpet
layer overlying the top surface of the foam layer, wherein the plurality of
the energy dissipating projections are arranged to dissipate energy of an
impact through deformation to significantly affect a Critical Fall Height of
the playing surface.
It has been found that providing appropriately arranged projections on the
bottom of the foam layer may increase the Critical Fall Height of the
playing surface without having to increase the thickness of the foam layer,
allowing a shallower playing surface to be used. Furthermore, Critical Fall
30 Heights above 2.4 metres can be achieved. The projections deform into
the
spaces between the projections during an impact to dissipate the energy of
the impact. Appropriately arranged projections can change the profile of
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how energy is dissipated by the foam layer to increase the Critical Fall
Height of the playing surface.
The arrangement of the plurality of the projections may result in the
playing surface having a Critical Fall Height significantly greater, for
example 10cm, 20cm or 1 m greater, than a Critical Fall Height that would
be achieved with a foam layer of the same material and foam density
having a flat bottom surface.
It has been found that the size and shape of the projections can affect the
Critical Fall Height of the playing surface. In particular, for the typical
foam densities that are appropriate for a foam layer of a playing surface,
the projections should have a height of at least 10mm and preferably,
between 10mm and 20mm. It has been found that the size of the
projections can affect the performance of the playing surface at different
fall heights and it is believed that projections having a height of between
10mm and 20mm give a good balance between the performance at the lower
and higher fall heights.
In one embodiment, the foam layer is expanded polypropylene. The foam
density may be at least 25 grams per litre and preferably, between 30 grams
per litre and 40 grams per litre. Foam densities below 25 grams per litre
have a tendency to come apart, in use. Foam densities above 40 grams per
litre may be too stiff resulting in a Critical Fall Height of the playing
surface of less than 2.4 metres or requiring thicker projections to achieve a
2.4 metre Critical Fall Height, if this is possible at all, resulting in a
playing surface having a thickness that is impractical to use.
In one embodiment, the foam layer has a thickness (excluding the
projections) of between 40mm and 65mm. The base needs to be stiff
enough to avoid "bottoming out" on the hard supporting surface, such as a
concrete surface, when a person falls onto the surface but kept as shallow
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as possible to reduce the amount of excavating that needs to be done to lay
the surface. A foam layer having a thickness of less than 40mm may be too
weak at the typical foam densities used for playing surfaces, which may
result in the foam layer breaking up in use. A foam layer having a
5 thickness of more than 65mm may be too stiff at the typical foam
densities
used for playing surfaces, the impact of the headform not penetrating much
further than 65mm and therefore, not being dissipated by the projections.
Each projection may have a free end that, in use, contacts a surface
10 supporting the playing surface, the projection having a shape that
narrows
towards the free-end. The shape of the projection can affect the profile of
how energy is dissipated by the projections during an impact and it is
believed that narrowing projections provide a profile that reduces peak
deceleration and therefore, the HIC value. For example, the projections
15 may be dome shaped. Dome shaped projections perform particular well in
use, however, other shaped projections may be used such as pyramid,
tetrahedron, cone, wedge shaped, a frustum of these shapes or any other
suitable shape. The disadvantage of projections that end in points (such as
pyramids, tetrahedrons and cones) that contact the supporting surface is
20 that the point can break off in use.
In one embodiment, the foam layer may comprise one or more holes
extending from the top surface to the bottom surface of the foam layer.
The holes may allow water to drain from the top surface of the playing
25 surface. In addition to dissipating energy, the projections provide
channels below the foam layer for water to drain from the area covered by
the playing surface.
The playing surface may comprise a geotextile layer between the foam
30 layer and the carpet layer. The geotextile layer may protect the foam
layer
during laying of the carpet, in particular, the carpet layer may be formed
from rolls/strips of carpet connected together using a hot melt adhesive and
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the geotextile layer protects the foam layer being damaged from the heat.
Furthermore, the geotextile layer may increase the load bearing capacity of
the playing surface, reducing damage to the foam layer during an impact.
5 It has been found that moulding a foam layer with a conventional mould
results in a smooth finish to the foam layer that allows the carpet layer and
geotextile layer to move relative to the foam layer, in use. In a playground
such movement of the surface is undesirable. Accordingly, in this
embodiment, the top surface of the foam layer may comprise a rough
surface that contacts the geotextile or carpet layer to provide high
frictional
resistance to relative movement between the layers. The rough top surface
may comprise a plurality of upwardly extending projections, for example
upwardly extending projections that produce a course top surface. It will
be understood that the upwardly extending projections are small relative to
the thickness of the foam layer and the height of the energy absorbing
projections, for example having a height of less than 0.5mm, the upwardly
extending projections having minimal impact on the energy absorbing
capability of the foam layer but increasing the frictional resistance of the
top surface. In one embodiment, the projections may be irregular.
In one embodiment, the playing surface may comprise two or more foam
layers, each foam layer having a top surface and a plurality of downwardly
extending energy dissipating projections. The foam layers may be stacked
consecutively on top of each other. It is believed that the provision of
multiple foam layers will further increase the Critical Fall Height of the
playing surface. Furthermore, in an embodiment in which holes are
provided in each foam layer from the top surface to the bottom surface, the
projections provide channels from the holes in one foam layer to the holes
in another foam layer for water to drain. Without the channel between the
projections, the drainage holes of consecutive layers would have to be
aligned, increasing the costs of installation.
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The foam layer may comprise a plurality of interlocked tiles, each tile
having formations for cooperating with formations on the other tiles for
interlocking the tiles together. Forming the foam layer from a plurality of
tiles simplifies laying of the carpet layer. The interlocking of the tiles
5 reduces relative movement between the tiles, in use.
According to a second aspect of the invention there is provided a method of
manufacturing a playing surface comprising forming a foam layer having a
top surface and a plurality of downwardly extending energy dissipating
10 projections, wherein the plurality of the energy dissipating projections
are
arranged such that, when the foam layer is used as part of a playing surface
with a fibrous carpet layer overlaying the foam layer, the plurality of
projections dissipate energy of an impact through deformation to
significantly affect a Critical Fall Height of the playing surface.
According to a third aspect of the invention there is provided a method of
laying a playing surface comprising laying a foam layer having a top
surface and a plurality of downwardly extending energy dissipating
projections and overlaying the foam layer with a fibrous carpet layer,
20 wherein the plurality of the energy dissipating projections are arranged
such that, when the foam layer is used as part of a playing surface with a
fibrous carpet layer overlaying the foam layer, the plurality of projections
dissipate energy of an impact through deformation to significantly affect a
Critical Fall Height of the playing surface.
The plurality of the projections may be arranged such that the playing
surface has a Critical Fall Height of at least 2.4 metres.
The method may comprise providing laying a geotextile layer over the
30 foam layer and laying the carpet layer over the geotextile layer, the
carpet
layer comprising a series of carpet strips connected together by a hot melt
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adhesive, the geotextile layer arranged to protect the foam layer form heat
used in applying the hot melt adhesive.
Embodiments of the invention will now be described, by example only,
with reference to the following drawings, in which:-
FIGURE 1 is a cross-section of a playing surface according to an
embodiment of the invention;
FIGURE 2 is a perspective view of a tile of the foam layer of the
playing surface shown in Figure 1 from above;
FIGURE 3 is a perspective view of a tile of the foam layer of a
playing surface shown in Figures 1 and 2 from below;
FIGURE 4 is a side view of a tile of the foam layer of the playing
surface shown in Figures 1 to 3;
FIGURE 5 is a blown-up perspective view of a tile of the foam layer
of a playing surface shown in Figures 1 to 4 from above;
FIGURE 6 is a blown-up perspective view of a tile of the foam layer
of a playing surface shown in Figures 1 to 5 from below;
FIGURE 7 is a cross-section of a playing surface according to
another embodiment of the invention; and
FIGURE 8 is a cross-section of a playing surface according to yet
another embodiment of the invention.
Referring to Figure 1, a playing surface comprises a foam layer 1 and a
fibrous carpet layer 2 overlying the top surface of the foam layer 1.
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Located between the foam layer 1 and the carpet layer 2 is a geotextile
layer 3.
The fibrous carpet layer 1 comprises a pile (e.g. a yarn that stands up from
the backing), which may be considered synthetic grass. For example, the
pile of the carpet layer may be polypropylene needle punched surface
having a 12 to 14 mm pile (14 to 16 mm total thickness including the
backing). The pile may have an open fibrous structure having randomly
oriented fibres, as described in European Patent No: 0174755.
The geotextile layer 3 may be 2mm thick needle punched polyester
material.
The foam layer 2 comprises a plurality of interlocked tiles 5 (one of which
is shown in Figures 2 to 6), each tile having connecting formations, in this
embodiment trapezoidal recesses 4 for cooperating with corresponding
trapezoidal protrusion 6 on the other tiles, for interlocking the tiles 5
together. Each tile 5 has a top surface 7 and a bottom surface 8.
The top surface 8 has a rough texture that contacts the geotextile layer 3 to
provide high frictional resistance to relative movement between the foam
layer 1 and the geotextile layer 3. The high frictional resistance may
prevent movement of the geotextile layer 3 or carpet layer 2 relative to the
foam layer 1 when subjected to lateral forces.
The foam layer 1 further comprises a plurality of downwardly extending
energy dissipating projections 9. The projections 9 are of a shape, in this
embodiment dome shaped, such that the projections 9 narrow towards a free
end 11 (in this embodiment an apex 11 of the dome) that contacts a surface
supporting the playing surface. It will be understood that in other
embodiments, other shapes may be used for the projections 9. The
projections 9 have a height (the vertical distance from a valley between the
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projections to the apex 11 of the projections 9) of approximately 15mm
and, in this embodiment in which the projections 9 have a dome shape a
diameter of approximately 30mm. The projections 9 are arranged in a
pseudo-hexagonal packing arrangement with each projection 9 being close
to but being spaced from adjacent projections 9 by a predetermined
distance, as indicated by rings 11 around each projection 9. It will be
understood that rings 11 are included in the drawings to illustrate that the
projections 9 do not meet and these regions are flat and are part of the
valley between the projections 9.
In this embodiment, the foam layer 1 is expanded polypropylene having a
foam density of 30 grams per litre and the thickness of the foam layer from
the top surface to a valley of the bottom surface is 40mm.
In one embodiment, the foam layer 1 may comprise one or more holes (not
shown) extending from the top surface 7 to the bottom surface 8 that allow
water to drain from the playing surface. The
projections 9 provide
channels below the foam layer 1 for water to drain from the area covered
by the playing surface.
A combination of foam density of the foam layer 1 and an arrangement of
the plurality of the projections 9 significantly affects a Critical Fall
Height
of the playing surface. In particular, the projections 9 are arranged to
deform into the spaces to dissipate energy during an impact. It has been
found that the combination of features of the foam layer 1 of the above-
described embodiment results in the playing surface having a Critical Fall
Height of up to 1.4 metres.
Referring to Figure 7, there is shown another embodiment of a playing
surface according to an embodiment of the invention. In this embodiment,
the foam layer 1 has a thickness of 65mm. The Critical Fall height of this
playing surface may be at least 2.4 metres.
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Figure 8 shows another embodiment of a playing surface according to the
invention. In this embodiment, the playing surface comprises two foam
layers l' and 1" stacked consecutively on top of each other. It is believed
5 that the provision of multiple foam layers further increases the Critical
Fall
Height of the playing surface.
To manufacture the playing surface, the carpet layer 2 and the geotextile
layer 3 may be manufactured in a conventional manner. The foam layer 1 is
10 manufactured by moulding the foam layer 1 in a suitably shaped mould,
for
example by injection moulding. During the moulding process, the top
surface 7 is formed with a rough texture of relatively small projections.
Laying of the playing surface is typically carried out by digging out a
recess in the ground and, optionally, lining the ground with stone, the stone
providing a level supporting surface. The tiles forming the foam layer 1
are then placed on the stone surface, the tiles being interlocked together by
recesses 4 and protrusions 6, such that at least the apexes (free ends) of the
projections 9 contact the stone. Sheets forming the geotextile layer 3 are
placed over the foam layer 1 and rolls of carpet layer 2 are placed over the
geotextile layer 3. The rolls of carpet may be secured together at the joints
by introducing using a hot-melt adhesive at the seams. It has been found
that the geotextile layer 3 advantageously protects the foam layer 1 from
the heat used in this process.
The playing surface of this embodiment is suitable for external use, such as
on children's playgrounds.
The table below shows the results of measurements of HIC for a playing
surface according to the invention having 15mm energy absorbing
projections and a playing surface having a foam tile with no energy
absorbing projections.
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Type of Foam Tile Fall Height HIC
50mm tile with no energy absorbing projections 2.10m 897
50mm tile with 15mm energy absorbing 2.3m 801
projections
Two 50mm tiles with no energy absorbing 2.45m 949
projections placed one on top of the other
Two 50mm tiles with 15mm energy absorbing 3.6m 767
projections placed one on top of the other
As can be seen from the table, the playing surfaces with foam tiles having
the 15mm projections can achieve a lower HIC value for higher fall
heights. Accordingly, these measurements show that the energy absorbing
projections affect the Critical Fall Height of the playing surface.
It will be understood that the invention is not limited to the above-
described embodiments, but modifications and alterations can be made
thereto without departing from the aspects of the invention as defined
herein.
For example, the foam layer may be made of other suitable materials, such
as polyurethane, expanded polyethylene and expanded polystyrene.
Rather than forming the foam layer 1 with an integral rough top surface 7, a
separate additional layer may be adhered to the top surface of the foam
layer, this additional layer having an upper surface that provides a high
frictional resistance to movement of the geotextile layer 3.
