Venetian blind slat

An elongate venetian blind slat having a longitudinally extending transversely convex upper surface and an opposed longitudinally extending transversely concave lower surface and opposed longitudinally extending edge surfaces. The slat comprises a rigid thermoplastic resin having a cellular core and an integral solid skin along the longitudinally surfaces of the slats and a density of 0.4 to 0.6 g/cc. The upper and lower surfaces of the slat have a radius of curvature less than the width of the slat and, preferably, the upper and lower surfaces have substantially the same radius of curvature.

BACKGROUND OF THE INVENTION 
Early venetian blinds were made with wide flat wood slats, but presented 
some problems because of warping, breakage and deterioration of the 
surface finish. Venetian blinds were later formed with slats of thin 
transversely arcuate sheet metal stock and, more recently, venetian blind 
slats have been formed of solid plastic material by thermo forming strip 
plastic sheet material or by extrusion. 
U.S. Pat. No. 4,711,005 discloses thermo forming venetian blind slats from 
PVC strip material. As disclosed in that patent, thin PVC strip 2.5 cm in 
width and 0.05 to 0.06 cm in thickness is thermo formed to a shallowly 
curved 7.62 to 12.7 cm or greater radius in curvature. However, solid 
plastic materials such as rigid polyvinyl chloride commonly used in the 
formation of plastic venetian blind slats, have a density that is three or 
four times higher than the density of the woods used in wooden slats, and 
it was necessary to reduce the thickness of the solid plastic slats to 
control the lift weight of the blind, and to also reduce the spacing of 
the ladder tapes in order to prevent sagging or drooping of the plastic 
slats under the temperature and humidity conditions sometimes encountered 
in window areas. 
U.S. Pat. No. 4,818,590 discloses forming wood veneer covered structural 
rigid plastic foam elements including venetian blind slats, having a rigid 
plastic foam substrate and a wood veneer adhered to opposite surfaces of 
the substrate. U.S. Pat. No. 5,121,785 discloses forming a venetian blind 
slat using a metal core plate with a wood veneer adhered to opposite faces 
of the metal core plate, and synthetic resin layers overlying the wood 
layers. In one alternative construction, a fiber reinforced plastic is 
substituted for the metal core plate, and in still another embodiment, the 
slat is formed with a wooden core plate with reinforcements of non-woven 
fabrics and resin sheets mounted to both faces of the core plate, and a 
pair of wooden plates mounted to the faces of the reinforcements. The 
formation and laminating of the slats from multiple individual layers as 
disclosed in these patents, necessarily increases the overall cost of 
making such venetian blind slats. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a venetian blind slat 
that can be economically produced, which is light in weight and has 
adequate strength to accommodate wide spacing of the ladder tapes that 
support the slats in a venetian blind, and which has a smooth continuous 
lengthwise extending outer surface. 
Accordingly, the present invention provides an elongate venetian blind slat 
having a longitudinally extending transversely convex upper surface and an 
opposed longitudinally extending transversely concave lower surface and 
opposed longitudinally extending edge surfaces, the slat comprising a 
rigid thermoplastic resin having a cellular core and an integral solid 
skin,along the longitudinally extending surfaces of the slats, the slat 
having a density of 0.4 to 0.6 g/cc. Advantageously, the upper and lower 
surfaces of the slat have a radius of curvature less than the width of the 
slat and, preferably, the upper and lower surfaces have substantially the 
same radius of curvature.

FIG. 1 illustrates a venetian blind in a raised or open position. In 
general, the venetian blind includes a headrail 10, a bottom rail 11, and 
a plurality of slats 12 supported on conventional venetian blind ladders 
13 formerly formed of tape or string. As is conventional, the ladders are 
mounted in the headrail in a manner that enables raising one ladder string 
while lowering the other to control tilting of the slat when the blind is 
in an open or at least partially open condition, and the lower ends of the 
ladders are attached to the bottom rail 11. Lift cords 15 are attached to 
the bottom rail and extend upwardly through cord openings in the slats and 
into the headrail, and then lengthwise of the headrail and through a cord 
lock 16 with operating portions of the lift cords 15 extending downwardly 
as indicated at 17. As is conventional, the operating portions of the lift 
cords can be pulled downwardly to raise the bottom rail and move the slats 
to a raised condition, and the operating cords can be manipulated to 
release the cord lock and enable lowering of the blind. 
It is generally desirable to minimize the weight of venetian blind slats to 
reduce the overall weight of the blind and thereby facilitate raising and 
lowering of the blind. Solid plastic materials used in forming plastic 
venetian blind slats, have relatively high density, generally over three 
times the density of woods that were used in making wooden blind slats, 
and to reduce the overall weight of a venetian blind formed with plastic 
slats, it has been the practice to use relatively thin slats having a 
thickness in a range between 0.05 to 0.15 cm. However, such thin slats had 
a relatively low beam strength and it was generally necessary to reduce 
the spacing between the ladder tapes from about 60 cm as was customary in 
venetian blinds with wood slats, to about 20 or 30 cm for venetian blinds 
with plastic slats. 
In accordance with the present invention, the venetian blind slat 12 is 
formed of rigid thermoplastic resin and has a cellular core 12a and an 
integral solid skin 12b that extends along the longitudinally extending 
surfaces of the slat. The slat with the cellular or foamed core and 
integral skin is formed so to have a low density of from 0.4 to 0.6 grams 
per cc, which density is about 0.25 to 0.4 with respect to the 
corresponding solid thermoplastic resin. The rigid thermoplastic resin is 
advantageously selected to be of a type that can be foamed during 
extrusion to form a slat with a cellular core and an integral solid skin 
and having the desired slat size and configuration. As is well known to 
those skilled in the art, a blowing agent is mixed with the rigid 
thermoplastic resin either before or during melting of the resin in the 
extruder. The resin is heated in the extruder to a temperature above the 
decomposition temperature of the blowing agent and extruded under high 
pressure through a die having the desired shape, and the surface of the 
extrudate is chilled at a location close to the die opening to produce a 
thin high density skin around the extrudate. Rigid PVC has many desirable 
characteristics for use in slats including stiffness, stability, 
resistance to moisture and sunlight and the rigid thermoplastic resin is 
preferably a rigid vinyl chloride polymer. The rigid thermoplastic resin 
may, for example, be a polyvinyl chloride extrusion compound marketed by 
B. F. Goodrich under the trademark "GEON" 87600. 
The slat 12 is formed with a longitudinally extending transversely convex 
upper surface US and in opposed longitudinally extending transversely 
concave lower surface LS, and opposed longitudinally extending edge 
surfaces ES. In order to enhance the beam strength of the slat and 
resistance to bending or sagging, the upper and lower surfaces are formed 
with a short radius of curvature designated RC in FIG. 2, which is less 
than the width W of the slat. The radii of curvature CU and CL, of the 
upper and lower surfaces of the slats is preferably substantially the same 
so that the lower surface LS of one slat will substantially conform to the 
upper surface US of the subjacent slat, to minimize the stacking height of 
the slats when the slats are in a fully raised position as shown in FIG. 
4. In the embodiment illustrated, the slat 12 has a width W of about 5 cm 
(approximately 2 inches) and the radius of curvature is about 4 cm. The 
slat has a thickness T at the longitudinal center about 0.38 cm and a 
crown height CH, measured between the crest of the upper surface US and a 
plane through the lower edges of the lower surface LS, in the range from 
1.1 to 1.2 cm. With this configuration, it has been found that the foamed 
plastic slats have sufficient beam strength so that they can be supported 
by ladder tapes spaced apart a distance of the order of 60 cm without 
bending or sagging. As will be seen, configuring the slats with a radius 
of curvature less than the width of the slats enhances the crown height of 
the slat and hence the beam strength of the slat, and that using the same 
short radii of curvature for both the upper and lower surfaces provides a 
slat having a greater thickness along the longitudinal center line of the 
slat and such that the slats can stack in close nested relation as shown 
in FIG. 4. 
Cord openings 12c are provided in the slats 12 for the lift cords 15. In 
order to facilitate tight closing of the slats when the ladders 13 are 
operated to tilt the slats to a closed condition, the cord openings 12c 
are preferably elongated in a direction crosswise of the slat and offset 
towards the edge of the slat that is at the inner side of the blind, as 
best shown in FIG. 5. 
The venetian blind slat shown in FIG. 6 is formed and configured the same 
as the slat 12 illustrated and described in connection with FIGS. 1-5, and 
differs only in the provision of lengthwise extending depressions or 
grooves 12d in the upper surface US inwardly to the edges ES, for 
decorative purposes. The slat in FIG. 6 is otherwise formed and configured 
in the same as in FIGS. 1-5 and like numerals are used to designate 
corresponding parts.