Razor

Details Razor Razor

1999-01-29

2001-01-16

Payer, Hwei-Siu (Department: 3724)

Cutlery

Razors

Multiple blade

C030S057000

Reexamination Certificate

active

06173498

ABSTRACT:

This invention relates to safety razors, and in particular it is concerned with a blade unit for a safety razor. The blade units of the invention may be either cartridges which are mounted replaceably on a handle for use, or the shaving heads of disposable razors having blade carrying parts permanently attached to a handle. A safety razor cartridge as currently marketed comprises a generally rectangular moulded plastics frame with guard and cap surfaces on the lengthwise extending frame parts. Within the opening of the frame are a pair of blades arranged in tandem with their cutting edges parallel to each other and directed towards the guard surface. The blades are carried on respective supports to which they are fixedly attached, and the ends of the blade supports are slidably guided in grooves in the end walls of the frame to enable the blades to retract, under forces exerted on the blades during shaving. Springs act on the blades to urge them to a normal rest position.
It is well known that the so-called shaving geometry of a blade unit is important in determining the shaving performance of the blade unit. The shaving geometry defines the position and orientation of the blades in relation to other skin contacting parts, in particular the guard and cap, of the blade unit. One parameter of the shaving geometry is the blade exposure, which is the perpendicular distance by which the edge of a blade protrudes above a shaving plane tangential to the surfaces of the skin contacting parts immediately in front of and behind the blade edge. The known blade unit described above has the blades mounted so that the exposure is reduced when loads of sufficient magnitude to overcome the force of the springs are exerted on the blades. Another important factor in the shaving geometry is the blade tangent angle, which is the angle at which the central plane of the blade intersects the tangent plane, the tangent plane being the plane which is tangent to the edge of the blade and to the surface of the skin contacting part next in front of the blade. For example, in a twin blade cartridge, the blade tangent angle of the first or primary blade is the angle of the blade to the tangent plane tangential to the guard surface and the edge of the primary blade.
During shaving the blades of a blade unit are subjected to a combination of drag forces and load forces. Drag forces are those forces directed essentially parallel to the shaving plane, and load forces are those forces directed against the blade by the skin in the direction substantially perpendicular to the shaving plane.
It has been proposed for example in U.S. Pat. Nos. 4,709,477 and 4,774,765, to mount the blades so that the blades can move to increase both exposure and blade tangent angle in response to increasing drag forces. Strain gauge measurements have shown that drag forces do not vary substantially during shaving. Hairs do not usually grow perpendicular to the skin surface and for the majority of men beard hair is inclined downwardly with respect to the face. It is believed that, when shaving, men apply the razor against the face with less pressure during upstrokes, when the blades are moving essentially against the grain, than during downstrokes, and the net result is that drag forces are not substantially different during upstrokes and downstrokes. On the basis that by applying the razor against the skin surface under greater load pressure a person is seeking greater closeness of shave, a more aggressive blade geometry would be desirable under such conditions. The prior art razors with movable blades are not capable of responding to load forces in this way.
Accordingly, the present invention seeks to fill this gap left by the prior art and in accordance with the invention there is provided a safety razor blade unit having at least one blade mounted for independent movement relative to other skin contacting parts of the blade unit under forces exerted at the blade edge during shaving, wherein the or each blade is mounted to move to increase the blade tangent angle in response to load forces exerted on the blade during shaving.
In the preferred embodiments of this invention the blade unit includes a frame, and the or each blade is fixedly carried on a blade support, and the blade support is connected to the frame by means defining a fulcrum, the movement of the blade under load fulcrum being located closer to the blade edge than is the point where the tangent plane meets the skin contacting surface next in front of the blade. Also, the fulcrum lies in or closely adjacent to the tangent plane. This arrangement minimizes the changes in blade exposure as the blade tangent angle increases, and to the extent that the exposure of the blade does change it reduces as the blade tangent angle increases. Also, the angular displacement of the blade is largely insensitive to the drag forces on the blade, and the blade span, i.e. the distance between the blade edge and the skin contacting surface in front of the blade edge measured in the tangent plane, does not change significantly as the blade tangent angle increases. It is expedient for the fulcrum to be spaced from the blade edge at a distance in the range of 0.2 mm to 0.6 mm.
With a blade unit embodying the invention the blade or blades are initially set with a minimum blade tangent angle, and this angle increases under increasing load forces. As a consequence there is greater comfort as compared with other arrangements which provide the blade tangent angle to be initially set at a maximum.
The blade supports can conveniently be coupled to the frame so that the fulcrum is fixed with respect to the frame, but this is not essential and it is also possible to allow a degree of controlled movement of the fulcrum.


REFERENCES:
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patent: 4516321 (1985-05-01), Frabcus
patent: 4709477 (1987-12-01), Ferraro
patent: 4774765 (1988-10-01), Ferraro
patent: 4932123 (1990-06-01), Francis
patent: 5222300 (1993-06-01), Althaus et al.
patent: 5224267 (1993-07-01), Simms et al.
patent: 5251376 (1993-10-01), Althaus et al.
patent: 5313706 (1994-05-01), Motta et al.
patent: 5546660 (1996-08-01), Burout et al.

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