WO2013011304A1

WO2013011304A1 - An item of footwear

Info

Publication number: WO2013011304A1
Application number: PCT/GB2012/051709
Authority: WO
Inventors: Kevin Bann
Original assignee: Name Drop Sarl
Priority date: 2011-07-18
Filing date: 2012-07-18
Publication date: 2013-01-24
Also published as: GB2493036B; GB201119822D0; GB2493036A; GB201112362D0; US20140290097A1; CN103687506A; EP2734073A1

Abstract

The present invention relates to an item of footwear having a sole, comprising two or more materials of different compressive resistances, flexibility or densities arranged as a frontal/toe region, a mid foot region and a heel region. The item of footwear comprises: a securing means for securing the item of footwear to a foot of a wearer; and a sole having a midsole which has a frontal/toe region, a mid- foot region and a heel region, the mid-foot region of the midsole being formed from a material different to the material used for the heel region such that the mid-foot region has a lower compressive resistance and/or a lower density and/or a higher flexibility than the heel region, wherein the mid-sole is thin and tapered.

Description

AN ITEM OF FOOTWEAR

The present invention relates to an item of footwear having a sole, comprising two or more materials of different compressive resistances, flexibility or densities arranged as a frontal/toe region, a mid foot region and a heel region .

Most shoes are purchased because of their aesthetics or their comfort. Comfort shoes are designed to provide comfortable walking and standing for a user and often include reasonable internal padding, arch support and comfort soles. In normal use, prior art shoes provide a good stable base for a foot of a user and this,

consequently, reduces the amount of muscle force required for a user to stand still or walk, whilst maintaining good balance .

The inventor has previously found that, by providing a slight, but safely-measured, instability in the footwear, the amount of difficulty a user experiences balancing whilst walking or standing is increased and, therefore, more muscle activity is required to achieve equivalent good balance to when using prior art shoes. In particular, muscles such as tibialis anterius, tibialis posterior, rectus femoris muscles of the quadriceps and gluteus maximus have been found to work harder whilst wearing footwear having a slight instability. Accordingly, WO 2008/132478 discloses an item of footwear that aids muscle toning and strengthening whilst a user is standing or walking around as they would do normally . It has previously been believed that the sole of the above-described item of footwear would only achieve an instability sufficient to provide a muscle training effect if the sole was of a significant thickness (significantly greater than regular footwear) . The applicant has to date believed that a mid-sole with a thickness at the heel of at least 40mm is needed to provide the instability effect.

However thicker soles are heavier, more costly, and are often considered less attractive.

The present invention provides an item of footwear as claimed in claim 1.

The inventor has discovered that a mid-sole comprising heel, frontal/toe and mid-foot regions of differing physical characteristics and having a heel thickness of between 18 to 28mm, preferably from 23mm to 25mm and most preferably approximately 24mm, can provide an advantageous instability effect .

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a top plan view of a sole of an item of footwear according to the present invention, the figure having additional lines to illustrate features of the footwear below the top surface of the sole;

Figure 2 shows a cross-section through the item of footwear of Figure 1 taken along the line A1-A2 of Figure 5;

Figures 3 and 4 are side elevation views of the item of footwear of Figure 1, respectively showing the lateral view and medial views of the item of footwear; Figure 5 is a bottom plan view showing an outsole of the item of footwear; and

Figures 6, 7 and 8 are cross-section views through the item of footwear of Figures 1 to 5, respectively taken along the lines of B1-B2, C1-C2 and D1-D2 shown in Figure 5.

The present invention relates to an item of footwear, such as a sandal, a flip-flop or a closed shoe. In the figures the sole of the item of footwear is shown, but it should be understood that the sole will typically be

provided with a strap arrangement (not shown) or an upper to secure a foot to the sole.

As shown in figure 4, the preferred embodiment of the invention has a sole 2 formed from a topsock 2A, a midsole 2B and an outsole 2C. The topsock 2A is bonded to the midsole 2B and the midsole 2B is bonded to the outsole 2C to provide a unitary sole 2. The topsock 2A is provided to receive a foot of a wearer and is textured to provide a gripping surface for the foot of the wearer. The topsock 2A could be a coating applied to an upper surface of the midsole 2B, e.g. a textile coating, or a layer of fabric bonded to the upper surface of the midsole 2B; a more substantial insole could be used in place of the topsock. The topsock 2A (or insole) could be integral with the midsole 2B. The outsole 2C provides a plantar surface, i.e. a surface which contacts the ground, and may be provided with one or more ridges or grooves (not shown) for providing grip. The outsole 2C is preferably moulded as a single piece prior to bonding with the midsole 2B. Since it

contacts the ground, the outsole 2C is formed by a material having good wear characteristics. The outsole 2C preferably has a constant thickness of about 4.5mm.

The midsole 2B is preferably formed with three distinct regions, as can be seen in the cross-section of Figure 2. A frontal/toe region is indicated by reference numeral 4. A mid-foot region is indicated by reference numeral 5. A heel region is indicated by reference numeral 6. The three regions of the midsole 2B are formed from two or more materials of different compressive resistances and/or flexibilities and/or densities, with the mid-sole region 5 being provided from a material of least compressive

resistance and/or density and/or greatest flexibility. Thus the sole 2 is arranged such that application of a wearer's weight, during walking, causes instability in the sole which requires balance correction by a user. The amount of instability caused is slight, but enough for small balance corrections to be made by the muscles of the wearer's leg. The mid-foot region 5 of the midsole comprises a first material of a first compressive resistance and/or density and/or flexibility; the frontal/toe region 4 comprises a second material of a compressive resistance higher than that of the first material and/or a density higher than that of the first material and/or a flexibility lower than that of the first material. Further, the heel region 6 is of a third material - different from the first and second materials - of a compressive resistance higher than that of the first material and/or a density higher than that of the first material and/or a flexibility lower than that of the first material. It is preferred that the sole 2 has a heel region 6 and a frontal/toe region both harder than the mid-foot region 5. In also preferred that the frontal/toe region 4 is slightly softer than the heel region 6, but with both the heel region 6 and the frontal/toe region 4 harder than the mid-foot region 5. By way of example, the hardness of the regions of the midsole 2B can measured using the Shore scale, the heel region has a Shore hardness of 48, the mid^¬ foot region a Shore hardness of 28 and the frontal/toe region a Shore hardness of 31. In preferred embodiments of an item of footwear 1 according to the present invention, the frontal/toe region 4 extends over the forwardmost 15% to 24% of the length of the sole 2, preferably around 18%, the mid-foot region 5 extends over the middle 37% to 53% of the length of the sole 2, preferably over the middle 47% to 53% of the length of the sole 2 and most preferably around 50%, and the heel region 6 extends over the rearmost 29% to 39% of the length of the sole 2, preferably around 32%. The lengths of these regions are indicated respectively as A, B and C in Figure 2.

Whilst it is preferable that the material of the mid-foot region 5 has a lower compressive resistance and/or a lower density and/or a higher flexibility than the frontal/toe region 4, when the frontal/toe region 4 is thin (as it is in tapered soles), these relative properties are less

important. In tapered soles, however, it is still essential that the heel region 6 has a higher compressive resistance and/or a higher density and/or a lower flexibility than the mid-foot region 5.

The mid-sole 2B is preferably manufactured by a multi^¬ stage moulding process. Initially the mid-foot region 5 is moulded as an independent component with inclined front and rear surfaces, i.e. in a trapezium shape when viewed in side elevation. Then the mid-foot region is loaded in a mould with granules of material to form the frontal/toe region and the heel region and heat is applied to melt the granules and form the frontal/toe and heel regions in the mould, melded to the preformed mid-foot region. Alternatively liquid material could be injected into the mould to form the frontal/toe and heel regions. On removal from the mould the initially roughly formed midsole expands in volume. It is abraded on its upper and lateral surfaces and a topsock is applied to the upper surface. The assembly of topsock and midsole is placed in a second mould in which it is heated and maintained at an elevated temperature for a chosen period of time, then it is cooled in the mould by passing a cooling fluid, e.g. water, around the mould for a chosen time before release from the mould. The released combination of topsock and midsole has the required end shape and the desired surface features, and is ready for the outsole to be adhered to the lower surface of the midsole.

The thickness 19 of the heel region 6 is typically measured as a minimum thickness, i.e. the thickness of the heel region at the lowest point of the heel region, this being typically found in the central part of the heel region when viewed in plan view. The thickness of the heel region 6 of a mid-sole according to the present invention is in the range 18mm to 28mm, preferably from 23mm to 25mm, and most preferably around 24mm. It has been found that within this range it is possible to obtain a suitable training effect if the layer is tapered as described below. Above this range, the thickness of the layer will be great enough that such tapering is not required to achieve the effect. Below this range, the thickness of the layer will be too small to achieve a training effect. As can be seen from Figures 2 to 4, the sole tapers along its length from the heel region 6 to the frontal/toe region 4. This taper is not a linear reduction in depth along the length of the sole, but is an overall thinning of the sole from heel to the toe. In fact, the thinning will not be continuous and some portions of the sole will be of constant thickness. What is important is that the wearer's foot is angled by the shoe such that the wearer's toes are lower than the wearer's heel. The sole 2 has a lateral side as shown in figure 3 and a medial side as shown in figure 4. When worn by a wearer, the big toe of the wearer's foot is on the medial side of the sole 2. The taper of the multi-density material layer is typically described by reference to an effective heel height, this being the difference in thickness between the thickness at a centre of the heel region 6 and the thickness of the midsole where it supports the ball of a wearer's foot. The midsole will support the ball of the wearer's foot near the transition between the frontal/toe region 4 and the arch region 5. In fact, it may be the case that the ball of the wearer's foot extends over the transition between these regions. For preferred embodiments of the present invention the effective heel height is in the range 6mm to 18mm, more preferably between 6mm and 10mm or between 12mm and 16mm. Preferred values of the effective heel height are around 8mm or around 15.5mm.

In the preferred embodiment, the sole comprises a concave recess 15 formed in the midsole, seen in figures 1 and 5 and seen by comparing the cross-sections of figures 7 and 10, respectively taken through the frontal/toe region and the heel region of the sole, with the cross-section of figure 8, taken through the recessed mid-foot region. The recess 15 extends from a part of the heel region 6 just forward of a calcaneus of a foot of a wearer to a point in the frontal/toe region which aligns with the phalangeal metatarsal joint of the wearer's foot, otherwise known as the first ray of the foot. In the illustrated embodiment this means that the recess extends rearwardly along the medial surface of the midsole from a point roughly 40% aft of the front of the sole to a point roughly 20% forward from the rear of the sole. In the preferred embodiment of the invention the narrowest width of the lowermost surface of the midsole in the recess 15 will be 61mm to 67mm and compared with a width of the uppermost surface of the midsole measured at the same point of 67mm to 84mm and more preferably from 67mm to 75mm. Preferably, the recess 15 when measured across the lowermost surface of the outsole at its narrowest point has a width of 61.5mm, which compares with a width measured at the same point on the uppermost surface of the midsole of 71mm. A further embodiment is considered in which the recess 15 when measured across the lowermost surface of the outsole at its narrowest point has a width of 64mm, which compares with a width measured at the same point on the uppermost surface of the midsole of 81mm.

The width of the lowermost surface of the midsole at its narrowest point can be narrower than the width of the uppermost surface of the midsole at the corresponding point by between 10% and 30%, and preferably between 10% and 20% and most preferably by 13%, without adversely affecting the wearer of the shoe. The minimum slope of the side of the recess 15 will be in the range of 45° to 80°, and preferably 75° .

The inventor has discovered that a wearer will walk with a gait that during a normal stride results in pressure being applied first centrally in the heel region 6, then along the lateral side of the mid-foot region of the sole and finally centrally in the frontal/toe region 4. The mid^¬ foot region 5 of the sole does not therefore need to support the user's foot across the full width of the sole 2, but only along the lateral side of the mid-foot region 5. This means that the weight and the cost of the sole 2 can be reduced by using less material in the mid-foot region 5. Advantageously, the soles of the invention achieve the reduction whilst maintaining a normal plan view outline of the uppermost surface of the midsole, by reducing the width of the midsole from it uppermost surface to its lowermost surface such that the width of the uppermost is greater than the width of the lowermost surface, as described above.

Width in this context is defined as the distance across the sole from the lateral side to the medial side in a direction perpendicular to a longest line joining the tip of the frontal/toe region 4 to the tip of the heel region 6; shown as line 33 in figure 5.

The provision of the recess 15 in the medial side surface of the mid shoe region enhances the instability effect provided by the sole and thus permits a reduced heel width .

The concave recess in the medial surface of the midsole will typically be formed in the last stage of the moulding process formed above, i.e. it is provided as a feature of the second mould. The sole will have a block form until it is placed in the second mould.

As can be seen from Figure 5, in addition to the presence of the recess 15, the lowermost surface of the midsole 2B is narrower than the outermost surface of the midsole 2B, around its entire periphery. In some embodiments the lowermost surface may be narrower than the outermost surface by from 1.5mm to 2.5mm and preferably around 2mm.

More preferably, however, because the midsole 2B is tapered, this is more pronounced in the heel region 6 than in the frontal/toe region 4. In such embodiments, the periphery of the outsole is stepped in between 2mm to 6mm from the uppermost to lowermost surfaces in the heel region 6 and between 1mm and 2.5mm in the frontal/toe region 4. The outsole is the same size as the lowermost surface of the midsole .

As can be seen in Figure 2, the frontal/toe region 4 abuts the mid-sole region 5 along an interface 7. At the interface 7, the material of the frontal/toe region 4 is melded to the material of the mid-foot region 5. In a similar way, an interface between the mid-foot region 5 and the heel region 6 is provided at an interface 8. Although the interfaces 7,8 could be vertical in nature, it is preferred to have a tapered interface. In particular, a tapered portion 4A of the frontal/toe region 4 overlaps with a tapered portion 5A of the mid-foot region 5 with the mid^¬ foot region 5 extending over and above the frontal mid-sole toe region 4. Further, a tapered portion 5B of the mid-foot region 5 overlaps with a tapered portion 6A of the heel region 6, with the mid-foot region extending over and above the heel region 6. Preferably, the interface between each region is sloped so as to increase the surface contact between the regions. In which case, the size of the regions will vary between the upper and lower surfaces of the midsole 2B. It is preferable that the frontal/toe region 4 extends over the forwardmost 18% of the top surface and 24% of the lower surface, the mid-foot region 5 extends over the middle 52% of the top surface and 37% of the lower surface, and the heel region 6 extends over the rearmost 30% of the top surfaces and 39% of the lower surface.

The interfaces are preferably inclined 45° to the horizontal when the sole is on a horizontal surface.

Advantageously, as can be seen in the Figures, the sole 2 is anatomically-shaped. The materials of the frontal/toe region 4, the mid-sole region 5 and the heel 6 are arranged to provide a wobble effect - slight instability - such that the foot, when stepping across the sole from heel 6 to toe 4 wobbles slightly, altering the balance position of a user's foot. The wobble effect can be medio-lateral or anterior- posterior or a combination of anterior-posterior and medio- lateral instability. In particular, it is the progression of pressure from the heel 6 through the lateral side of the mid-foot region 5, and then to the frontal/toe region 4 that causes the mid-foot region to flex to produce a wobble and cause slight instability.

The embodiment of sole 2 shown in the Figures is preferably made of ethylene vinyl acetate (EVA) . According to the invention, three different grades of EVA are utilised - having different compressive resistances and/or densities and/or flexibilities - for the different regions of the midsole to provide the wobble effect. However, it will be understood that other materials will provide an equivalent effect and examples of those materials are other elastomers, silicones, natural or synthetic rubbers and/or

polyurethanes . It has previously been considered that the instability effect of the shoe requires a broad sole to support the user's foot as he corrects his balance. Previously, it was assumed that the maximum width in the upper surface of the mid-sole in the heel region must be at least 75mm.

Similarly, it was assumed that the maximum width in the upper surface of the mid-sole in the upper surface of the mid-sole in the frontal/toe and arch regions must be at least 98mm. However, the inventors have discovered that the user can compensate for the instability and achieve a suitable training effect even in narrower shoes. Indeed, it has been discovered that footwear having a maximum width in the upper surface of the mid-sole in the heel region in the range of 55mm to 68mm can still achieve the training effect. Similarly, it has been discovered that footwear having a maximum width in the upper surface of the mid-sole in the frontal/toe and arch regions in the range of 80mm to 92mm can still achieve the training effect. The tapered sole described above, is particularly

advantageous for use in a closed shoe (e.g. a shoe with an upper cover that encapsulates the toes and/or foot forward of the arch) . This is because is it aesthetically desirable for closed shoes to be narrow (as compared with open shoes) . Narrow shoes constrict the wearer's toes within the cavity defined between the sole and the upper. To enable such a narrow construction, it is necessary to increase the volume of the cavity; by tapering the sole an increase in volume at the forward end of the shoe is achieved.

Claims

CLAIMS :

1.) An item of footwear comprising:

a securing means for securing the item of footwear to a foot of a wearer; and

a sole having a midsole which has a frontal/toe region, a mid-foot region and a heel region, the mid-foot region of the midsole being formed from a material different to the material used for the heel region such that the mid-foot region has a lower compressive resistance and/or a lower density and/or a higher flexibility than the heel region, wherein :

the mid-sole in the heel region has a thickness in the range of 18mm to 28mm; and

the mid-sole has a thickness in a part of the region for supporting a ball of wearer's foot which is between 6mm and 18mm less than the thickness of the mid-sole in the heel region .

2.) The item of footwear of claim 1, wherein the

frontal/toe region is formed from a material having higher compressive resistance and/or higher density and/or lower flexibility than the mid-foot region.

3.) The item of footwear of claim 1 or claim 2, wherein the thickness of the mid-sole in the heel region is from 23mm to 25mm.

4.) The item of footwear of any preceding claim, wherein the mid-sole in the frontal/toe region in the part for supporting the ball of the wearer's foot is between 12mm and 16mm less than the thickness of the mid-sole in the heel region .

5. ) The item of footwear of any preceding claim further comprising a recess formed in a medial side surface of the midsole which reduces in width the midsole at a lowermost surface of the midsole, with the width of the midsole narrowing downwardly from an uppermost surface of the midsole to the lowermost surface of the midsole.

6. ) The item of footwear of claim 5, wherein the recess narrows the lowermost surface of midsole to a narrowest width that is up to 14mm less than a narrowest width of the uppermost surface of the midsole.

7. ) The item of footwear of claim 5 or claim 6, wherein the recess narrows the lowermost surface of the midsole such that the lowermost surface has a narrowest width which is between 80% and 90% of a narrowest width of the uppermost surface of the midsole.

8. ) The item of footwear as claimed in any one of claims 5 to 7 wherein the recess extends along the medial side of the midsole from a point in the frontal/toe region of the sole aft of a front of the sole rearwardly across the whole of the mid-foot region of the sole to a point in the heel region of the sole forward of the rear of the heel.

9. ) The item of footwear as claimed in any one of claims 5 to 8 wherein the recess is a concave recess.

10.) The item of footwear of any preceding claim, wherein the sole further comprises an outsole, for engaging the ground .

11.) The item of footwear of any preceding claim, wherein the sole further comprises a topsock provided on an/the uppermost surface of the midsole and engageable by a foot of the wearer.

12.) The item of footwear of any preceding claim, wherein the sole further comprises an insole provided on an/the uppermost surface of the midsole and engageable by a foot of the wearer.

13.) The item of footwear of any preceding claim, wherein mid-sole thickness of 18mm to 28mm in the heel region is the minimum thickness of the mid-sole in the heel region.

14. ) The item of footwear of any preceding claim, wherein the mid-sole thickness of 18mm to 28mm in the heel region is the average thickness of the mid-sole in the heel region.

15. ) The item of footwear of any preceding claim wherein the frontal/toe region of the sole is the forwardmost 15-24% of the sole, the mid-foot region is the middle 37-53% of the sole and the heel region is the rearmost 29-39% of the sole.

16. ) The item of footwear of any preceding claim wherein in the heel region, an/the uppermost surface of the midsole is of greater surface area than a/the lowermost surface of the midsole, a periphery of the midsole being stepped in between 2mm to 6mm from the uppermost to lowermost surfaces.

17. ) An item of footwear comprising:

a securing means for securing the item of footwear to a foot of a wearer; and

a sole having a midsole which has a frontal/toe region, a mid-foot region and a heel region, the mid-foot region of the midsole being formed from a material different to the material used for the heel region such that the mid-foot region has a lower compressive resistance and/or a lower density and/or a higher flexibility than the heel region, wherein the maximum width of the upper surface of the mid-sole in the heel region is in the range of 55mm to 68mm.

18. ) An item of footwear comprising:

a securing means for securing the item of footwear to a foot of a wearer; and

a sole having a midsole which has a frontal/toe region, a mid-foot region and a heel region, the mid-foot region of the midsole being formed from a material different to the material used for the heel region such that the mid-foot region has a lower compressive resistance and/or a lower density and/or a higher flexibility than the heel region, wherein the maximum width of the upper surface of the mid-sole in the frontal/toe and arch regions is in the range of 80mm to 92mm.

19.) The item of footwear of any preceding claim, wherein the securing means is a cover for encapsulating the toes of the wearer.

20.) An item of footwear comprising:

a securing means for securing the item of footwear to a foot of a wearer; and a sole having a midsole which has a frontal/toe region, a mid-foot region and a heel region, the mid-foot region of the midsole being formed from a material different to the material used for the heel region such that the mid-foot region has a lower compressive resistance and/or a lower density and/or a higher flexibility than the heel region, wherein :

the sole in the heel region has a thickness in the range of 18mm to 28mm; and

the sole in the frontal/toe region has a thickness in a part of the region for supporting a ball of wearer's foot which is between 6mm and 10mm less than the thickness of the sole in the heel region.

21.) An item of footwear comprising:

a securing means for securing the item of footwear to a foot of a wearer; and

a sole having a midsole which has a frontal/toe region, a mid-foot region and a heel region, the mid-foot region of the midsole being formed from a material different to the material or materials used for the frontal/toe region and the heel region such that the mid-foot region has a lower compressive resistance and/or a lower density and/or a higher flexibility than the frontal/toe region and the heel region, wherein:

the sole in the heel region has a thickness in the range of 18mm to 28mm; and

22. ) The item of footwear of claim 20 or 21, wherein the thickness of the sole in the heel region is 23mm.

23. ) The item of footwear of any one of claims 20 to 22, wherein the sole in the frontal/toe region in the part for supporting the ball of the wearer's foot is 8mm less than the thickness of the sole in the heel region.

24. ) The item of footwear of any one of claims 20 to 23 further comprising a recess formed in a medial side surface of the midsole which reduces in width the midsole at a lowermost surface of the midsole, with the width of the midsole narrowing downwardly from an uppermost surface of the midsole to the lowermost surface of the midsole.

25. ) The item of footwear of claim 24, wherein the recess narrows the lowermost surface of midsole to a narrowest width that is 14 to 20 mm less than a narrowest width of the uppermost surface of the midsole.

26. ) The item of footwear of claim 24 or claim 25, wherein the recess narrows the lowermost surface of the midsole such that the lowermost surface has a narrowest width which is between 70% and 80% of a narrowest width of the uppermost surface of the midsole.

27. ) The item of footwear as claimed in any one of claims 24 to 26 wherein the recess extends along the medial side of the midsole from a point in the frontal/toe region of the sole aft of a front of the sole rearwardly across the whole of the mid-foot region of the sole to a point in the heel region of the sole forward of the rear of the heel.

28.) The item of footwear as claimed in any one of claims 24 to 27 wherein the recess is a concave recess.

29.) The item of footwear of any one of claims 20 to 28, wherein the sole further comprises an outsole, for engaging the ground.

30. ) The item of footwear of any one of claims 20 to 29, wherein the sole further comprises a topsock provided on an/the uppermost surface of the midsole and engageable by a foot of the wearer.

31. ) The item of footwear of any one of claims 20 to 30, wherein the sole further comprises an insole provided on an/the uppermost surface of the midsole and engageable by a foot of the wearer.

32. ) The item of footwear of any one of claims 20 to 31, wherein sole thickness of 18mm to 28mm in the heel region is the minimum thickness of the sole in the heel region.

33. ) The item of footwear of any one of claims 20 to 32, wherein the sole thickness of 18mm to 28mm in the heel region is the average thickness of the sole in the heel region .

34. ) The item of footwear of any one of claims 20 to 33 wherein the frontal/toe region of the sole is the

forwardmost 15-21% of the sole, the mid-foot region is the middle 47-53% of the sole and the heel region is the

rearmost 20-35% of the sole.

35.) The item of footwear of any one of claims 20 to 34 wherein an/the uppermost surface of the midsole is of greater surface area than a/the lowermost surface of the midsole, a periphery of the midsole being stepped in between 1.5mm to 2.5mm from the uppermost to lowermost surfaces.