GB2570797A - A shatter resistant ruler - Google Patents

Abstract

A ruler 100 comprising length scale markings to define a length scale, and a frangible portion 104. The breakable portion 104, provided to reduce he possibility of the ruler 100 shattering, may be provided at or near the centre of the ruler in a direction running along its length, and may comprise a reduced thickness portion or one or more indentations in the surface of the straightedge 100. The frangible portion 104 may extend to a depth of between seven and twenty percent of the depth of the ruler 100, and across the width of the ruler 100, for example by a distance of eleven to eighteen percent of that of the ruler 100. The fragile portion 104 may be arranged to break upon flexing of the ruler 100 to bring the distal ends of the ruler to a threshold distance apart.

Description

Figure 12

A SHATTER RESISTANT RULER

The present application relates to a ruler and more particularly to a shatter resistant ruler.

Rulers formed from plastics materials such as general purpose polystyrene (GPPS) are susceptible to shattering if they experience excessive flexing or twisting forces. Stresses can build up within the ruler and create an explosive break. Misuse of such rulers can therefore lead them to shatter into a large number of sharp pieces that can be forcibly ejected and cause injury. The resulting parts of such a broken ruler can also have sharp edges which can cause injury to the user.

This problem can be particularly important where rulers are to be supplied for use in education environments such as schools. In this case there is a greater likelihood of misuse of the ruler by bending or twisting that can cause it to break. Furthermore, in a school environment it is particularly important for any product not to shatter into sharp pieces that could cause injury.

It is known to solve this problem by providing a shatter resistant ruler that is formed from a tough shatter resistant material. Since the 1970s rulers have been available which are formed from a strong shatter resistant material such as a thermoplastic resin (e.g. Styrene Butadiene (SB) block copolymer). Prior art rulers such as these are strong enough to adequately withstand any abusive treatment and avoid dangerous shattering if excessively bent or twisted. They are however intended as premium or high quality products rather than being suitable for mass production.

There are a number of drawbacks with such prior art shatter resistant rulers. Despite the use of stronger materials the level of shatter resistance may still be limited and dangerous shattering can still occur if adequate force is applied. Prior art shatter resistant rulers are also more difficult and costly to produce. They typically require the use of stronger, more expensive materials, to provide the necessary level of strength and toughness. They are not therefore suited to efficient high volume production and use in mass markets such as education or the like.

Because of the problems associated with manufacturing high strength shatter resistant rulers, non-shatter resistant GPPS rulers have been known to be labelled incorrectly as “shatter resistant”. There are currently no formal standards for certifying the level of shatter resistance provided in rulers. This may lead to the inadvertent use of rulers having inadequate levels of shatter resistance. Prior art GPPS rulers however are much more suited to efficient mass production, and are the most usual choice for high volume manufacture despite the limitations of their shatter resistance.

In one aspect, the present application provides a ruler comprising a frangible portion, the frangible portion being arranged to break in a predictable manner when a threshold amount of force is applied.

In another aspect, the present application provides a ruler, comprising: length scale markings to define a length; and a frangible portion.

By including a frangible portion, the ruler of the present application provides improved shatter resistance without the need to use stronger and tougher materials. The ruler of the present application is adapted to break cleanly at the frangible portion when a predefined level of force is applied without the material breaking into multiple smaller sharp fragments.

A material is said to be frangible if through deformation it tends to break up into fragments, rather than deforming elastically and retaining its cohesion as a single object. The frangible portion is provided separately, and is different from, any notches or indentations which may provide a measuring scale or guide, which will not have a frangible property. The frangible portion is also different from a living hinge, which is intended to provide flexibility and elastic deformation, rather than a breaking point.

As discussed above, there is a strong technical bias in the field of the invention to use low strength materials that are efficient and cost effective for production in larger volumes, but which provide ineffective shatter resistance. Contrary to this, the inventors have provided a ruler having a frangible portion to improve shatter resistance, while allowing the use of materials that are suitable for efficient mass production. A shatter resistant ruler suited to efficient high volume production is therefore provided.

The frangible portion may be arranged at or near the centre of the ruler in a direction running along its length. This may allow the ruler to break into two approximately equal parts if excessive force is applied.

The frangible portion may comprise a reduced thickness portion of the ruler. This may provide a point of weakness in the body of the ruler at which it is encouraged to break in preference to fragmenting into multiple sharp pieces.

The reduced thickness portion may be formed by one or more indentations in a surface of the ruler.

The frangible portion may comprise a first indentation disposed on a front face of the ruler and a second indentation disposed on a reverse face of the ruler. By providing indentations in both faces of the ruler the risk of shattering may be reduced if the ruler is bent or stressed by bending its distal ends in either direction.

The frangible portion may comprise an indentation on only a single face of the ruler. Preferably the frangible portion may comprise an indentation on a reverse face of the ruler, the reverse face being opposite to a front face on which distance scale markings are disposed. This may allow the structure forming the frangible portion to be less visible during use of the ruler.

The frangible portion may comprise at least one groove in a surface of the ruler. The at least one groove may be a V-shaped groove in the surface of the ruler. This may help to provide a clean break rather than sharp fragments.

The depth of the frangible portion into the body of the ruler may be between 7% and 20% of the thickness of the ruler. Preferably, the depth may be 12.5 % of the thickness of the ruler. A shallower frangible portion that does not extend too deeply into the thickness of the ruler has been found to provide a suitable balance between shatter resistance and the ruler being too weak for normal use. The depth of the frangible portion may be between 0.15 mm and 0.35 mm. Preferably, the depth of the frangible portion may be 0.25 mm. This has been found to provide a suitable combination of strength and breakability.

The frangible portion may extend across a width of the ruler, and preferably may extend across the full width of the ruler. This may improve controlled breaking of the ruler.

The frangible portion may extend across only part of the width of the ruler. The frangible portion may extend across the width of the ruler a distance that is between 11% and 18% of the width. The frangible portion may extend across the width of the ruler a distance that is 15% of the ruler width. A shorter frangible portion that does not extend across the full width of the ruler has been found to provide a suitable balance between shatter resistance and the ruler being too weak for normal use. The frangible portion may extend a distance of between 3 mm and 5 mm across the width of the ruler. Preferably the frangible portion may extend a distance of 4 mm across the width of the ruler. This has been found to provide a suitable combination of strength and breakability.

The length of the ruler may be in a range between 10 to 50 cm. Preferably, the length of the ruler may be between 28 and 32 cm. Rulers of this length have been found to be particularly susceptible to dangerous shattering and so are advantageous in combination with a frangible portion.

The ruler may be formed from a plastics material. Shatter resistance is particularly important where the ruler is made from a plastics material that would otherwise tend to shatter into sharp pieces if the ruler is misused.

Preferably, the ruler may be formed from styrene acrylonitrile resin. This may provide suitable overall material strength even if a weakening frangible portion is provided.

The ruler may be formed from a mixture comprising styrene acrylonitrile resin and Styrene Butadiene. This may provide a balance of shatter resistance and adequate strength for normal use.

The ruler may be formed from a mixture of general purpose polystyrene (GPPS) and high impact polystyrene (HIPS); or general purpose polystyrene (GPPS) and Styrene Butadiene co-polymer. These mixtures have been found to provide a suitable balance of strength, breakability of the frangible portion and economic manufacture. The mixture containing GPPS and HIPS is suitable for an opaque bodied ruler, with the mixture containing Styrene Butadiene and GPPS being suitable for a transparent bodied ruler.

The frangible portion may be arranged to break in a predictable manner when a threshold amount of force is applied.

The frangible portion may be configured such that it is arranged to break upon flexing of the ruler to bring the distal ends of the ruler to a threshold distance apart. The threshold distance may be about one third of the length of the ruler. Preferably, the length of the ruler is 30 cm and the threshold distance is 11 cm. This has been found to provide suitable strength for the ruler to withstand normal use, but allow it to break during misuse.

In another aspect, the present application provides a ruler comprising a frangible portion, wherein:

the frangible portion comprises at least one groove in a surface of the ruler, the frangible portion extends across the width of the ruler a distance of between 11% and 18% of the width of the ruler;

the groove or grooves forming the frangible portion each have a depth of between 7% and 20% of the thickness of the ruler; and the ruler is formed from a mixture comprising: general purpose polystyrene (GPPS) and high impact polystyrene (HIPS); or general purpose polystyrene (GPPS) and Styrene Butadiene co-polymer.

Any of the statements above may be provided in combination with each other and in combination with any of the aspects above.

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

Figure 1 shows perspective view of a ruler according to an embodiment;

Figure 2 shows a cross section view through part of the length of the ruler shown in Figure 1;

Figure 3 shows a close up perspective view of the frangible portion of the ruler shown in Figure 1;

Figure 4 shows a second close up perspective view of the frangible portion of the ruler shown in Figure 1 with hidden edges shown as broken lines;

Figure 5 shows a perspective view of a ruler according to another embodiment;

Figure 6 shows a cross section view through part of the length of the ruler shown in Figure 5;

Figure 7 shows a perspective view of a ruler according to another embodiment;

Figure 8 shows a close up of the region marked X in Figure 7;

Figure 9 shows a cross section through the width of the ruler shown in Figure 7;

Figure 10 shows a close up top view of the ruler shown in Figure 7;

Figure 11 shows a cross section along the length of the ruler; and

Figure 12 shows a method of quantifying stress that can be applied to the ruler before the frangible portion breaks.

A perspective view of a ruler 100 according to one embodiment is shown in Figure 1. The ruler 100 comprises an elongate body 102 such that it is suitable for measuring length or marking straight lines.

Length scale markings (not shown in the Figures) may be provided on the body 102 of the ruler 100 so as to define a length scale such that it may be used for measuring distances. The length scale is provided separately from the frangible portion. The frangible portion is not therefore formed from notches or grooves in the ruler than are provided as measurement scales or guides. In other embodiments, such length scale markings may be absent, with any other suitable markings or no markings at all provided. The ruler 100 may be formed from a rigid material that is liable to shatter upon application of a sufficient breaking force. The ruler may, for example, be formed from a plastics material (e.g. GPPS) that is particularly susceptible to shattering. In other embodiments, the ruler may be formed from any other material that is susceptible to shattering, such as wood for example. By shattering, we mean breaking violently into multiple pieces that may be forcibly ejected or may have resulting sharp edges.

The ruler 100 may be formed by a generally flat strip of material as shown in Figure 1. In this embodiment, the ruler 100 comprises a front face 102a, which may carry length scale or other markings, and a reverse face 102b opposite to the front face. In other embodiments, markings may be provided on both faces. The shape of the ruler 100 shown in Figure 1 is only one such example. In other embodiments, the ruler 100 may be any suitable shape such that it can be used to measure distance and/or to draw straight lines. For example, it may be formed by a length of material having a square or round cross section or other more complex shape.

The ruler 100 comprises a frangible portion 104. The frangible portion 104 is arranged to break in a predictable manner when a predetermined or threshold amount of force is applied to the ruler. For example, as an increasing force is applied the frangible portion will fail before any other part of the ruler. In other words, the threshold force at which the frangible portion is adapted to break is less than the threshold force at which the remaining body of the ruler will break or shatter. The frangible portion 104 may therefore define a part of the body 102 of the ruler that forms a breaking point, or portion of weakness, within the body 102 of the ruler 100 at which the ruler may preferentially break under a certain level of applied force. This may allow the ruler 100 to break at the frangible portion 104 in a controlled manner rather than shattering into a large number of fragments that may otherwise cause injury.

By breaking in a controlled manner we may mean breaking at the frangible portion 104 rather than at any other part of the body 102 of the ruler 100. For example, this may mean the ruler 100 breaking into only two separate pieces if only one frangible portion is provided. The frangible portion is different from a living hinge that may be formed in a ruler. Such a living hinge is not intended to break upon the application of excessive force. A living hinge is provided to allow flexibility, and so does not provide a part of the ruler that is intended to fail within the meaning of a frangible portion.

In some examples of prior art rulers, imperfections caused during manufacturing may lead to unintentional weak points in the material from which the ruler is formed. For example, a moulding cavity injection point resulting from an injection moulding production process may lead to a weak point in the surface of the ruler. Such an injection point or other imperfection itself is not adapted to ensure the ruler will break in a predictable manner when a threshold amount of force is applied and so does not fulfil the requirements of the frangible portion of this application. The frangible portion may not therefore be formed from an injection point alone. Relying on an injection point alone would instead result in inconsistent breaking as opposed to the consistent controlled breaking of the frangible portion of the present application.

The level of force required to cause the frangible portion 104 to break may be determined by the properties of the frangible portion 104 and may be chosen to ensure preferential breaking before sufficient force is applied to cause the body 102 of the ruler 100 to shatter in other places. For example, the frangible portion 104 may be adapted to break under application of a threshold level of bending or twisting force applied to the distal ends of the ruler 100.

In the described embodiment, the frangible portion 104 may be provided at or near the centre of the ruler 100 in a direction running along its length. As can be seen in Figure 1, the frangible portion 104 may be provided at or near the centre of the ruler 100 i.e. the centre of the length of the elongate body 102 from which it is formed. This may help the ruler 100 to break into two approximately equal parts when sufficient force is applied to cause the frangible portion 104 to break.

In other embodiments, the frangible portion 104 may be provided at any other suitable point along the length of the ruler 100. In the described embodiment, only a single frangible portion 104 (e.g. to provide a single point along the length of the ruler at which it may preferentially break into only two pieces) is provided for ease of manufacture. Providing only a single frangible portion such that the ruler breaks into two parts may have further advantages. It may, for example, allow the person applying the breaking force to be in constant control of the resulting two parts of the ruler after breaking. If the ruler breaks, neither of the two resulting parts will be ejected and become a danger because they will both be held by the person applying force to the ruler.

In other embodiments, two or more frangible portions may be provided (e.g. so that the ruler may break at two or more separate points along its length). The number of frangible portions may be determined according to the size (e.g. the length) of the ruler 100 or it susceptibility to shattering.

The frangible portion 104 may comprise a reduced thickness portion of the ruler 100. The reduced thickness portion may be formed by one or more indentations in the surface or surfaces of the ruler as described below. This may provide a point of weakness in the body 104 of the ruler 100 at which it is more likely to break compared to the remainder of the body 102 of the ruler 100. By removing material from the body 102 of the ruler 100 in this way the frangible portion 104 may be conveniently and efficiently manufactured.

In the embodiment shown in Figures 1 to 4, the frangible portion 104 comprises indentations formed on both the front face 102a and the reverse face 102b of the ruler 100. As can be seen in more detail in Figures 2 to 4, the frangible portion 104 is formed by a first groove 104a disposed on the front face 102a of the ruler 100 and a second groove 104b disposed on the reverse face 102b of the ruler 100. By providing indentations in both faces of the ruler the risk of shattering may be reduced if the ruler is bent or stressed in either direction (e.g. it may provide shatter resistance if the distal ends of the ruler are bent in a direction both towards or away from the front face). This may reduce the risk of shattering even if the ruler is turned over and bent the other way.

In other embodiments, the frangible portion may comprise one or more indentations disposed only on a single face of the ruler. An example of such an embodiment is shown in Figures 5 and 6 (corresponding reference numerals are used compared to Figures 1 to 4 for ease of explanation). In this embodiment a ruler 200 is shown which comprises a frangible portion 204 formed by a groove 204a disposed on the front face

202a of the ruler 200. By using grooves or other indentations only on a single face of the ruler adequate shatter resistance may be provided whilst allowing easy manufacture.

In yet other embodiments, the indentation(s) forming the frangible portion may be formed only on the reverse face of the ruler rather than the front face. This may mean that the grooves or indentations forming the frangible portion are less visible during use.

In the described embodiments, the frangible portion 104; 204 comprises at least one groove in a surface of the ruler 100; 200. In the described embodiments, the groove or grooves forming the frangible portion may be V-shaped groove(s) in the surface of the body 102; 202 of the ruler 100; 200. This can be seen more clearly in the cross section through the body 102; 202 of the ruler 100; 200 shown in Figures 2 and 6. The Vshaped groove(s) may allow the ruler 100; 200 to break into separate pieces that do not have sharp edges.

The V-shaped groove or grooves shown in the Figures are only one example of a reduced thickness portion of the body 102; 202 of the ruler 100; 200 that may form the frangible portion 104; 204. In other embodiments, the groove or grooves may have any other suitable cross sectional shape which may provide the desired preferential breaking point. In yet other embodiments, the frangible portion 104; 204 may be formed from a series of grooves, holes or other shaped indentations or a series of indentations in the surface of the ruler 100.

The reduction in thickness of the ruler 100; 200 at the frangible portion 104; 204 may be determined according to the desired predefined threshold force that will cause breaking of the frangible portion 104; 204. For example, the greater the amount of material removed at the frangible portion 104; 204 the lower the amount of force that must be applied for it to break. The level of force required to break the frangible portion 104; 204 may be adapted to provide a suitable balance between reducing the risk of the ruler shattering and ensuring the ruler is not too easily broken if only a small level of force is applied (e.g. corresponding to normal use rather than misuse).

Each groove (or other suitable indentation) forming the frangible portion 104; 204 may extend approximately 1 to 25 % through the thickness of the body of the ruler 100; 200. This may provide an advantageous balance between providing adequate strength (e.g. the ruler will not fail too easily) and shatter resistance (e.g. excessive force is not required to break the frangible portion so that shattering in the body of the ruler may instead occur). In other embodiments, the groove may extend a greater amount through the thickness of the ruler, and may extend approximately 50% through the thickness of the body 102; 202 of the ruler 100; 200.

In other embodiments, the frangible portion 104; 204 may take other forms apart from a portion of reduced thickness. In some embodiments, the frangible portion 104; 204 may be a portion of the body 102; 202 of the ruler 100; 200 made from a different material to the remainder of the body 102; 202 of the ruler 100; 200. In such an embodiment, the frangible portion 104; 204 may be formed from a structurally weaker material compared to the surrounding body 102; 202 of the ruler 100; 100 so that it may break preferentially and/or in a controlled manner. In yet other embodiments, the frangible portion 104; 204 may be formed from one or more through holes that extend through the body 102; 104 of the ruler 100; 200.

The length of the ruler 100; 200 may be in a range between 10 to 50 cm. For rulers having a length outside of this range shatter resistance has been found to be less of a problem. A frangible portion is however particularly advantageous for rulers between these lengths as they may be more susceptible to harmful shattering. Below 10 cm a ruler will more likely have adequate structural strength to resist breaking and shattering if force is applied at its distal ends. Rulers over 50 cm in length are more suitably made from a stronger material and so a frangible portion may be less useful. Preferably the length of the ruler 100; 200 may be between 28 and 32 cm (e.g. it may have a length suitable for providing a 30 cm or 12 inch length scale). Rulers of this length have been found to be particularly susceptible to shattering and so the frangible portion may be particularly advantageous if used in combination with a ruler of this length. The frangible portion of the present application may however be used in combination with rulers of any shape or dimensions.

The frangible portion 104; 204 may extend across a width of the ruler as shown in the Figures. It may, for example, extend across the full width of the ruler. In the described embodiments, the groove or grooves forming the frangible portion extends between the longitudinal edges of the ruler. This may help ensure a clean break across the width of the ruler if adequate breaking force is applied.

In the described embodiment, the ruler 100; 200 may be formed from a plastics material. Such a material may be particularly susceptible to shattering. The frangible portion 104; 204 of the present application is therefore particularly advantageous when used with a ruler formed from a plastics material.

In one embodiment, the ruler 100 may be formed from styrene acrylonitrile resin (SAN). This may provide improved strength compared to materials such as GPPS, but may still be suitable for efficient large volume production and is suitable for use for transparent rulers. The improved strength of SAN may be advantageous in combination with the use of a frangible portion which otherwise acts to weaken the ruler to provide controlled breaking.

In other embodiments, any other suitable plastics material may be used, such as GPPS for example. In some embodiments, a mixture of GPPS and Acrylonitrile butadiene styrene (ABS) may be used. The addition of ABS may provide a tougher ruler which requires greater force to shatter, but in which shattering is still a problem and so is advantageous in combination with the frangible portion.

Figures 7 to 11 illustrate a ruler 300 having a body 302 and a frangible portion 304. The frangible portion 304 is formed by grooves 304a, 304b in the front and reverse faces 302a, 302b of the ruler body 302. Any feature described above may be used in combination with the embodiment of Figures 7 to 11, and vice versa.

The inventors have found that it is difficult to incorporate a frangible portion into a known ruler whilst still retaining enough strength for it to be suitable for normal use (i.e. it does not break too easily). In order to maintain suitable overall strength, the inventors have found that reducing the depth of a groove or other indentation forming the frangible portion and/or the distance which the frangible portion extends across the width of the ruler may provide a suitable degree of overall strength to withstand normal use, whilst still allowing the frangible portion to break without the ruler shattering.

In the embodiment illustrated in Figures 7 to 11, the grooves 304a, 304b forming the frangible portion 304 each have a depth which they extend into the ruler (i.e. they extend a distance d into the body of the ruler). The depth d may be between 0.25 mm and 0.35 mm. This may be for a ruler having a thickness of between 1.75 mm and 2.25 mm (but could be applied to similarly sized rulers). The depth may therefore be between about 7% and 20% of the thickness of the ruler (measured between the front and reverse faces). In the described embodiment, the groves 304a, 304b extend 0.25 mm into the body of the ruler, which has a thickness of 2 mm. The depth may therefore be 12.5% of the thickness of the ruler.

By reducing the depth of the groove in this way the overall strength can be increased so that the ruler does not break too easily, whilst still providing a frangible portion at which the ruler fails in preference to shattering. In the described embodiment, the frangible portion is located at the centre of the ruler along its width and its length. This may provide even breaking when excess force is applied. In other embodiments, the frangible portion may be off centre along either the length and/or the width. The grooves may have a V-shaped profile with side walls at 90 degrees to each other (labelled Θ in Figure 11). This may provide ease of manufacture and a clean breaking point. Other shapes are however possible.

In the embodiment illustrated in Figures 7 to 11, the grooves forming the frangible portion extend only part way across the ruler body. The length (labelled L in Figures 9 and 10) of the grooves may be between about 3 mm and 5 mm. This may be for a ruler having a width between 27.75 mm and 28.25 mm (but could be applied to similarly sized rulers). The length of the grooves may therefore be between about 11% and 18 % of the distance across the body of the ruler. In the described embodiment, the length of the grooves may be 4 mm and the width of the ruler body 28 mm. The length of the grooves in this embodiment may therefore be 15 % of the width of the ruler. This may also help to increase the overall strength of the ruler so that it does not break too easily, while still providing shatter resistance via the frangible portion.

To further control the overall level of strength so that it is sufficient for normal use the material from which the ruler is made can also be changed accordingly. The inventors have found that a mixture of styrene acrylonitrile resin (SAN) and Styrene

Butadiene (K-Resin) provides suitable overall strength along with shatter resistance via the frangible portion (e.g. compared to using 100 % Styrene Butadiene).

In one preferred embodiment, the ruler may be formed from a mixture of general purpose polystyrene (GPPS) and high impact polystyrene (HIPS). In another preferred embodiment the ruler may be formed from a mixture of general purpose polystyrene (GPPS) and Styrene Butadiene co-polymer (e.g. k resin®). This may provide a suitable level of strength and either a transparent ruler (by using GPPS and Styrene Butadiene co-polymer mixture) or an opaque (white) ruler (by using a GPPS and HIPS mixture).

The embodiment illustrated in Figures 7 to 11 is a preferred embodiment which, after performing tests, the inventors have found provides an advantageous balance of overall strength, fracture resistance and production efficiency. The inventors have found that a combination of a groove extending 15% across the width of the ruler at a depth of 12.5% of the thickness of the ruler and the ruler body being formed from a mixture comprising: general purpose polystyrene (GPPS) and high impact polystyrene (HIPS); or general purpose polystyrene (GPPS) and Styrene Butadiene co-polymer. In the sample tested by the inventors, the length of the groove across the ruler body was 4 mm and it was 0.25 mm deep, with the ruler length being 30 cm.

The stress that can be applied to the ruler before the frangible portion breaks can be quantified by flexing the ruler by bringing its distal ends together. An example of this is illustrated in Figure 12, where the ruler 302 is bent or flexed by bringing its distal ends 310, 312 together. This can be done with the distal ends 310, 312 placed against a surface 314 (e.g. a table top) with one end abutting a fixed block 316. The frangible portion is configured such that it is arranged to break upon flexing of the ruler 302 to bring the distal ends 310, 312 to a threshold distance (labelled t in Figure 12) apart.

In a preferred embodiment, the threshold distance is about one third of the length of the ruler. Preferably, the length of the ruler may be about 30 cm and the threshold distance may be about 11 cm.

Various modifications will be apparent to the skilled person without departing form the scope of the claims.

The frangible portion 104; 204; 304 is described above in use with a ruler. The present application may however not be so limited. In other embodiments, the frangible portion may be provided in other similar rigid drawing or measurement tools or 5 instruments that are susceptible to shattering such as set squares, protractors or the like.

Claims (25)

1. A ruler, comprising:

length scale markings to define a length scale; and a frangible portion.

2. The ruler of claim 1, wherein the frangible portion is arranged at or near the centre of the ruler in a direction running along its length.

3. The ruler of claim 1 or claim 2, wherein the frangible portion comprises a reduced thickness portion of the ruler.

4. The ruler of any preceding claim, wherein the frangible portion comprises one or more indentations in a surface of the ruler.

5. The ruler of any preceding claim, wherein the frangible portion comprises a first indentation disposed on a front face of the ruler and a second indentation disposed on a reverse face of the ruler.

6. The ruler of any of claims 1 to 4, wherein the frangible portion comprises an indentation on only a single face of the ruler, and preferably wherein the frangible portion comprises an indentation on a reverse face of the ruler, the reverse face being opposite to a front face on which distance scale markings are disposed.

7. The ruler of any preceding claim, wherein the frangible portion comprises at least one groove in a surface of the ruler.

8. The ruler of claim 7, wherein the at least one groove is a V-shaped groove in the surface of the ruler.

9. The ruler of any of claims 3 to 8, wherein the depth of the frangible portion into the body of the ruler is between 7% and 20%, and preferably 12.5 %, of the thickness of the ruler.

10. The ruler of claim 9, wherein the depth of the frangible portion is between 0.15 mm and 0.35 mm, and preferably is 0.25 mm.

11. The ruler of any preceding claim, wherein the frangible portion extends across a width of the ruler.

12. The ruler of any preceding claim, wherein the frangible portion extends across only part of the width of the ruler.

13. The ruler of claim 12, wherein the frangible portion extends across the width of the ruler a distance that is between 11% and 18% of the ruler width and may preferably be 15% of the ruler width.

14. The ruler of claim 13, wherein the frangible portion extends a distance of between 3 mm and 5 mm across the width of the ruler, and preferably extends 4 mm across the width of the ruler.

15. The ruler according to claim 11, wherein the frangible portion extends across the full width of the ruler.

16. The ruler of any preceding claim, wherein the length of the ruler is in a range between 10 to 50 cm.

17. The ruler of claim 16, wherein the length of the ruler is between 28 and 32 cm.

18. The ruler of any preceding claim, wherein the ruler is formed from a plastics material.

19. The ruler of claim 18, wherein the ruler is formed from styrene acrylonitrile resin.

20. The ruler of claim 18 or claim 19, wherein the ruler is formed from a mixture comprising styrene acrylonitrile resin and Styrene Butadiene.

21. The ruler of claim 18, wherein the ruler is formed from a mixture of general purpose polystyrene (GPPS) and high impact polystyrene (HIPS); or general purpose polystyrene (GPPS) and Styrene Butadiene co-polymer.

22. The ruler according to any preceding claim, wherein the frangible portion is arranged to break in a predictable manner when a threshold amount of force is applied.

23. The ruler according to any preceding claim, wherein the frangible portion is configured such that it is arranged to break upon flexing of the ruler to bring the distal ends of the ruler to a threshold distance apart.

24. The ruler according to claim 23, wherein the threshold distance is preferably about one third of the length of the ruler, and further preferably:

wherein the length of the ruler is 30 cm and the threshold distance is 11 cm.

25. A ruler comprising a frangible portion, wherein:

the frangible portion comprises at least one groove in a surface of the ruler, the frangible portion extends across the width of the ruler a distance of between 11% and 18% of the width of the ruler;

the groove or grooves forming the frangible portion each have a depth of between 7% and 20% of the thickness of the ruler; and the ruler is formed from a mixture comprising: general purpose polystyrene (GPPS) and high impact polystyrene (HIPS); or general purpose polystyrene (GPPS) and Styrene Butadiene co-polymer.

Intellectual Property Office

Application No: GB1820752.2 Examiner: Mr Patrick Lucas