Electrical cable for an appliance, appliance and method for producing an electrical cable

Abstract

The invention relates to an electrical cable (1) for an appliance, especially a vacuum cleaner. The cable (1) comprises a core bundle (21) comprising two core wires (10), each of the two core wires (10) comprising a center conductor (11) made of conductive strands and an insulation layer (13) on the outer periphery of the center conductor (11), the insulation layer (13) comprising a non-foamed softened polyvinyl chloride compound an inner sheath layer (14) arranged around the insulation layers (13), the inner sheath layer (14) comprising a foamed softened polyvinyl chloride compound wherein the foamed softened polyvinyl chloride compound of the inner sheath layer (14) contains a plurality of cells (16) and wherein each cell (16) is characterized by an equivalent diameter, in particular the diameter of a sphere having the same volume as the cell (16), an outer sheath layer (15) arranged around the inner sheath layer (14), the outer sheath layer (15) comprising a non-foamed, softened polyvinyl chloride compound. The invention further relates to an appliance with such a cable (1) as well as to an method of manufacturing the cable (1).

Claims

1. An electrical cable (1) for an appliance comprising a core bundle (21) comprising two core wires (10), each of the two core wires (10) comprising a center conductor (11) made of conductive strands and an insulation layer (13) on the outer periphery of the center conductor (11), the insulation layer (13) comprising a non-foamed softened polyvinyl chloride compound, an inner sheath layer (14) arranged around the insulation layers (13), the inner sheath layer (14) comprising a foamed softened polyvinyl chloride compound, wherein the foamed softened polyvinyl chloride compound of the inner sheath layer (14) contains a plurality of cells (16) consisting of gaseous inclusions in the polyvinyl chloride compound, and wherein each cell (16) is characterized by an equivalent diameter, wherein the equivalent diameters of the plurality of cells (16) form a distribution of equivalent diameters, the distribution having an average equivalent diameter, wherein the equivalent diameter of the cells located at an outer location of the inner sheath layer (14) is larger than the equivalent diameter of cells located at an inner location of the inner sheath layer (14), and an outer sheath layer (15) arranged around the inner sheath layer (14), the outer sheath layer (15) comprising a non-foamed, softened polyvinyl chloride compound and having a minimum thickness larger than the average equivalent diameter, and a maximum thickness of twice a maximum equivalent diameter of the plurality of cells (16).

2. The electrical cable (1) according to claim 1, wherein the average equivalent diameter is in the range of 110 to 200 micrometers.

3. The electrical cable (1) according to claim 1, wherein the equivalent diameters are smaller than 250 micrometers.

4. The electrical cable (1) according to claim 1, wherein the outer sheath layer (15) has a minimum thickness between 120 micrometers and 300 micrometers.

5. The electrical cable (1) according to claim 1, wherein the outer sheath layer (15) has an average thickness which is at least equal to a maximum equivalent diameter of the cells.

6. The electrical cable (1) according to claim 1, wherein the combined inner sheath layer (14) and outer sheath layer (15) have an overall density that is between 2% to 15% lower than a density of a combined non-foamed inner sheath layer and outer sheath layer.

7. The electrical cable (1) according to claim 1, wherein the inner sheath layer (14) and the outer sheath layer (15) combined have an overall density between 1.1 g/cm3 and 1.35 g/cm3.

8. An appliance with an electrical cable (1) according to claim 1, wherein the electrical cable (1) is used as a power cord.

9. The appliance according to claim 8, wherein the appliance is a vacuum cleaner.

10. The electrical cable according to claim 1, wherein the appliance is a vacuum cleaner.

11. The electrical cable according to claim 1, wherein each cell (16) in the foamed softened polyvinyl chloride compound of the inner sheath layer (14) has an equivalent volume.

Description

EMBODIMENTS OF THE INVENTION

(1) Embodiments of the invention are explained using the following figures:

(2) FIG. 1 shows an electrical cable according to the invention.

(3) FIG. 2 shows a setup for the production of an electrical cable according to the invention.

(4) FIG. 3 shows a typical jerk test setup for testing a cable's impact resistance.

(5) FIG. 4 shows a typical test setup for testing a cable's bending and flexing properties.

(6) FIG. 5 shows an SEM image of cells comprised in an inner sheath layer of an electrical cable according to the invention.

(7) FIG. 1 shows a cross-sectional view of an electrical cable 1 used as power cord for an appliance. The cable 1 comprises a core bundle 21 comprising two core wires 10, each of the two core wires 10 comprising a center conductor 11 made of a number of conductive strands 12, which are not shown in detail, and an insulation layer 13 on the outer periphery of the center conductor 11, the insulation layer 13 comprising a non-foamed softened polyvinyl chloride compound. The cable 1 further comprises an inner sheath layer 14 arranged around the insulation layers 13, the inner sheath layer 14 comprising a foamed softened polyvinyl chloride compound which contains a plurality of cells 16 and wherein each cell 16 is characterized by an equivalent diameter, in particular the diameter of a sphere having the same volume as the cell 16, and an outer sheath layer 15 arranged around the inner sheath layer 14, the outer sheath layer 15 comprising a non-foamed, softened polyvinyl chloride compound. The outer sheath layer 15 is also referred to as skin.

(8) FIG. 2 shows a setup 30 for manufacturing of the cable 1. The setup 30 comprises an extrusion head 32 which is a co-extrusion head with a first part 34 for extrusion of the inner sheath layer 14 and a second part 36 for extrusion of the outer sheath layer 15. Each of the parts 34, 36 is connected to an extruder, namely a first extruder 38 and a second extruder 40. The first extruder 38 is loaded with a combination of a first material M1, which here is a polyvinyl chloride compound, and a chemical blowing agent BA, which comprises an active component to generate the cells 16 inside the inner sheath layer 14. The second extruder 40 is loaded with a second material M2, which here also is a polyvinyl chloride compound. Both extruders 38, 40 are heated to a temperature in the range of 120 to 150 C.

(9) For production of the electric cable 1, a core bundle 21 is fed to the extrusion head 32 and processed, i.e. moved with a certain extrusion velocity V. Both materials M1, M2 are then subsequently applied onto the core bundle 21. More specifically, the first material M1 is applied in the first part 34 of the extrusion head 32 and directly onto the core bundle 21. In an alternate embodiment, a separating powder is applied onto the core bundle 21 prior to feeding it to the extrusion head 32. Directly after applying the first material M1 to form the inner sheath layer 14, the second material M2 is applied onto this inner sheath layer 14 and forms the outer sheath layer 15.

(10) Upon exiting the extrusion head 32 the cable travels towards cooling means 42, which here comprise a water bath at a low temperature, e.g. around 17 C. The cooling means 42 are placed a certain distance away from the extrusion head 32, whereby the distance corresponds to a travelling time T which a given section of the cable 1 needs for travelling from the extrusion head 32 to the cooling means 42. The travelling time T is preferably less than 1 second.

(11) A jerk test is carried out on a machine 44 as shown in FIGS. 3a and 3b. The machine 44 comprises a cord winder 46, e.g. a vacuum cleaner drum on which a tested cable 1 is wound up. The length of the tested cable 1 is preferably around 2 m. The machine 44 works in cycles as illustrated by the two FIGS. 3a and 3b. One cycle starts with winding up the cable 1 into the drum by a pneumatic engine, which is not shown, while a weight 48 of 1 kg is attached to the cable 1. A flatbed 50 supports lifting the weight 48 during the last third of a drop length. Upon release of the pneumatic force the weight 48 drops from a distance of about 0.3 m such that the cable 1 is fully unwound from the cord winder 46 and the weight 48 creates a potentially damaging load acting on a section of the cable 1 which is in contact with a sharp edge 52 of the drum body 46. Electrical current is applied to the cable 1 and monitored in order to detect breaking of the centre conductors 11.

(12) A flexing test for the cable 1 is carried out by means of the apparatus 54 shown in FIG. 4. This apparatus 54 consists of a carrier 56, a driving system for the carrier 56 and four pulley wheels 58A, 58B, 58C, 58D for each cable 1 to be tested. The carrier 56 supports two pulley wheels 58A, 58B which are of the same diameter. The remaining two pulley wheels 58C, 58D are fixed at the ends of the apparatus 54 may be of a different diameter than the pulley wheels 58A, 58B, but all four pulley wheels 58A, 58B, 58C, 58D are arranged in a way that the cable 1 is horizontal between them. The carrier 56 performs cycles of forward and backward motion over a distance of about 1 m at an approximately constant speed of 0.33 m/s between each reversal of the direction of movement.

(13) Furthermore, weights 60 are attached to the cable 1. These weights 60 stretch the cable 1 along the path between the pulley wheels 58A, 58B, 58C, 58D. To prevent the cable 1 from slipping out of the apparatus 54 an number of restraining clamps 62 is attached to the cable and limits a slipping movement and by being restrained by corresponding supports 64 of the apparatus 54. The distance from one of the restraining clamps 62 to its support 64 is at most 50 mm. The other clamp 62 is resting on its support 64.

(14) A current is applied to the cable 1. The electrical current, the weights and the pulley diameters are chosen with respect to the specific cable 1 construction. For a two-core cable 1 with a copper cross-section of 2 times 0.75 mm.sup.2 the following values are used: a current of 3 A, a weight with a mass of 1 kg and pulley diameters of 80 mm. A total of 30,000 bending cycles is required to pass this test.

(15) It has been observed that prior art non-foamed cables typically can perform an average of 150,000 bending cycles until cracks on the sheathing appear while a cable 1 with a foamed inner sheath layer can be subject to 170,000 bending cycles, which is an improvement of around 13%.

(16) FIG. 5 shows an SEM image from a cross-sectional area of a cable 1. Clearly visible is the inner sheath layer 14 comprising a plurality of cells 16 of various sizes. Also visible are the outer sheath layer 15 as well as the core bundle 21 which comprises two centre conductors 11, each in turn comprising a number of conductive strands 12 surrounded by an insulation layer 13. Particularly visible is the variation in the cells' 16 size. Starting from the centre, i.e. the location of the core bundle 21, the cells' 16 diameter increases towards the outer sections of the inner sheath layer 14.

(17) A reference sample cable and a number of cables according to the invention were manufactured and subsequently tested with respect to their elongation and their tensile strength at break. The various cables were manufactured varying the extrusion velocity and the composition of the inner sheath layer.

REFERENCE EXAMPLE

(18) A prior art non-foamed reference cable was manufactured with an extrusion velocity of v=160 m/min. The concentration of the chemical blowing agent was 0 wt. % to create a non-foamed inner sheath layer. The elongation at break of the sheathing was 322%, tensile strength at break 18 MPa and the density of the combined inner and outer sheath layer, i.e. the overall sheathing, was 1.38 g/cm.sup.3. The cable surface had a smooth appearance without any defects.

(19) In the following examples a chemical blowing agent with azodicarbonamide, abbreviated ADCA, as active component was added in various concentrations to the polyvinyl chloride compound for the inner sheath layer to create a foamed inner sheath layer.

First Example

(20) A cable with a foamed inner sheath layer was manufactured with an extrusion velocity of V=120 m/min. The concentration of the active component was 0.25 wt. %. The elongation at break of the sheath was 250%, tensile strength at break was 14.5 MPa and the density of the overall sheathing was 1.27 g/cm.sup.3. The cell size was from the range 150 to 170 m and the minimum skin thickness was 245 m. The resultant cable surface had a smooth appearance without any defects.

Second Example

(21) A cable with foamed inner sheath layer was manufactured with an extrusion velocity of V=120 m/min. The concentration of the active component was 0.15 wt. %. The elongation at break of the sheath was 249%, tensile strength at break was 14.7 MPa and the density of the overall sheathing was 1.26 g/cm.sup.3. The size of the cells in the inner sheath layer was in the range of 140 to 150 m and the minimum thickness of the outer sheath layer, i.e. the skin was 228 m. The resultant cable surface had a smooth appearance without any defects.

Third Example

(22) A cable with foamed inner sheath layer was manufactured with an extrusion velocity of V=120 m/min. The concentration of the active component was 0.23 wt. %. The elongation at break of the sheath was 263%, tensile strength at break was 15.1 MPa and the density of the overall sheathing was 1.21 g/cm.sup.3. The size of the cells in the inner sheath layer was in the range of 120 to 200 m and the minimum skin thickness was 228 m. The resultant cable surface had a smooth appearance without any defects.

Fourth Example

(23) A cable with foamed inner sheath layer was manufactured with an extrusion velocity of V=120 m/min. The concentration of the active component was 0.24 wt. %. The elongation at break of the sheath was 218%, tensile strength at break was 13.5 MPa and the density of the overall sheathing was 1.28 g/cm.sup.3. The size of the cells in the inner sheath layer was in the range of 140 to 200 m and the minimum skin thickness was 266 m. The resultant cable surface had a smooth appearance without any defects.

Fifth Example

(24) A cable with foamed inner sheath layer was manufactured with an extrusion velocity of V=120 m/min. The concentration of the active component was 0.09 wt. %. The elongation at break of the sheath was 282%, tensile strength at break was 16 MPa and the density of the overall sheathing was 1.28 g/cm.sup.3. The size of the cells in the inner sheath layer was in the range of 140 to 160 m and the minimum skin thickness was 114 m. The resultant cable surface had a smooth appearance without any defects.

(25) The following table 1 summarizes the production parameters and test results for the above described cable samples:

(26) TABLE-US-00001 TABLE 1 1.sup.st 2.sup.nd 3.sup.rd 4.sup.th 5.sup.th Parameter Unit Reference example example example example example Extrusion velocity [m/min] 160 120 120 120 120 120 Elongation at break [%] 322 250 249 263 218 282 Tensile strength [MPa] 18 14.5 14.7 15.1 13.5 16.0 at break Active component [wt. %] 0 0.25 0.15 0.23 0.24 0.09 concentration Density of overall [g/cm.sup.3] 1.38 1.27 1.26 1.21 1.28 1.35 sheathing Mean cell size, [m] 0 150 140 120 140 140 i.e. diameter Maximum cell size, [m] 0 170 150 200 200 160 i.e. diameter Minimum thickness [m] N/A 245 228 228 266 144 of outer sheath layer

(27) It can readily be seen that all five example cables fulfill the mechanical requirements according to EN 50525-2-11, namely a tensile strength at break of at least 10 MPa and an elongation at break of at least 150%.

(28) Furthermore, an impact and jerking test as described above was conducted. For this test, two samples were fabricated, one standard sample with a non-foamed inner and outer sheath layer, and one sample with the novel foamed structure according to the instant application. The impact and jerking test was conducted on ten different section along each sample cable and the number of cycles until breaking of the sample was recorded.

(29) The results are shown in the following table 2, in which the first column indicates the number of the test run, the second column shows the number of cycles until breaking of the standard cable in each single test run and the third column shows the number of cycles until breaking of the cable with the novel structure in each single test run. The average number of cycles until breaking for the standard cable is 514 with a standard deviation of 126.1 and the number of cycles until break for the novel cable is 686 with a standard deviation of 126.1. It can readily be appreciated that the novel foamed cable structure is more robust.

(30) TABLE-US-00002 TABLE 2 Test run Number of cycles until Number of cycles until number breaking of standard cable breaking of novel cable 1 409 569 2 345 910 3 483 440 4 438 646 5 466 870 6 695 868 7 621 699 8 457 670 9 497 699 10 730 489

REFERENCES

(31) 1 Electrical cable 10 core wire 11 center conductor 12 conductive strand 13 insulation layer 14 inner sheath layer 15 outer sheath layer, skin 16 cell 21 core bundle 30 setup 32 extrusion head 34 first part of extrusion head 36 second part of extrusion head 38 first extruder 40 second extruder 42 cooling means 44 machine for jerk test 46 cord winder 48 weight 50 flatbed support 52 sharp edge 54 apparatus 56 carrier 58A, 58B, 58C, 58D pulley wheel 60 weight 62 restraining clamp 64 support BA chemical blowing agent M1, M2 material T travelling time V extrusion velocity