TECHNICAL FIELD

The present invention pertains to the general field of human-machine interfaces, and more specifically human-machine interfaces that are integrated in plastic parts.

Generally speaking, the invention finds applications in any field of industry for which plastic surfaces require a human-machine interface. In particular, the invention may be applied to any interface for which mechanical push buttons are replaced by touch surfaces, for example for a central console of a vehicle. In this case, the integration of buttons generating a vibration when touched by the user transforms the plastic part constituting the central console into an “intelligent” object making it possible to interact with the driver or the passenger. The invention also finds applications for household electrical appliances for which the entirely smooth control panels require the addition of a control validation.

More specifically, the invention relates to a method for manufacturing a plastic part forming a human-machine control interface using a specific injection mould.

PRIOR ART

In a large number of electronic devices, such as household electrical appliances, automobile dashboards, etc., a plastic facade contains buttons enabling the user to interact with the electronic device. For example, in the case of an automobile dashboard, it is possible to position buttons to start the air conditioning, to lock the doors or instead to actuate the raising or lowering of a window, among other things.

Historically, these buttons were mechanical push buttons. These mechanical push buttons required assembly steps and were not perfectly integrated in the plastic facades. In particular, a slight amount of daylight always remained around the buttons. For this reason, more and more designers now prefer to integrate capacitive buttons, such as for example described in the U.S. Pat. No. 9,112,505 B2.

The use of capacitive sensors to replace mechanical push buttons makes it possible to have a completely smooth surface of the facade, which is thus more aesthetic, and also easier to clean and more robust, thanks to the absence of moving parts. In addition, users are today very familiar with capacitive interfaces thanks to the generalised use of touch screens in smartphones or tablets.

These capacitive sensors are particularly well suited to being integrated in facades decorated by a plastic injection technology such as the so-called IMF (In Mould Forming) technology. The principle of this technology is to print a decorative layer on a plastic sheet. This sheet is next cut, thermoformed if necessary, then over-moulded, to obtain a decorated plastic part.

However, a major drawback of these capacitive buttons is that the user does not have at his disposal tactile feedback when the button is actuated. A frequently employed solution to circumvent this difficulty is to generate a sound, for example a click, or a luminous signal, when the user actuates the capacitive button to confirm to him that the instruction has been taken into account by the electronic equipment. However, in noisy environments for example or in cases where the user is looking elsewhere, for example for an automobile driver who actuates a button while driving, these two solutions are not suitable.

Also, other solutions exist to ensure feedback to the user, for example by the triggering of a vibration that his finger is going to perceive, such as for example described in the patent application US 2007/229455 A1.

The actuators conventionally used to achieve haptic feedback, such as voice coil actuators, piezoelectric actuators, eccentric mass actuators and/or moving magnet actuators, have a bulk which, most of the time, do not make it possible to associate them individually with each button. Thus, as described in the patent application US 2007/229455 A1, for a keyboard comprising several keys, this results in an actuator that is going to make the whole of the keyboard vibrate.

This principle is well suited in the case of quite rigid, small sized electronic devices, such as smartphones. On the other hand, it comprises limits: firstly, when the facade is composed of a material that considerably dampens vibrations, it will be necessary to generate a lot of vibrations so that they are not too dampened and remain perceptible for the user; in addition, when the facade is completely embedded, vibrating it will be complicated; next, in the case of multi-key use, it would be preferable to carry out a localised vibration rather than a vibration of the whole of the part.

Faced with these constraints, piezoelectric actuators formed in thin films may be used such as described for example in the international application WO 2016/024066 A1. The interest of these actuators is that they are very thin and that they can thus be integrated directly in the facade. It may notably be thought to print them directly on the decorative film, and to over-mould them. However, if this is done, the actuators are completely embedded in the plastic part, and no vibration is perceived when they are supplied.

DESCRIPTION OF THE INVENTION

The aim of the invention is thus to resolve at least partially the aforementioned needs and drawbacks relating to embodiments of the prior art.

The subject matter of the invention is thus, according to one of its aspects, a method for manufacturing a plastic part forming a human-machine control interface, characterised in that it comprises the following steps:formation of a decorative film intended to form the visible interface,formation of a technical film, or haptic film, including a layer of haptic actuators, and a layer of capacitive sensors,assembly of the decorative film and the technical film to form an interface film,positioning of the interface film in an injection mould, including an upper portion, also called counter mould, with which the interface film is maintained in contact by suction and a lower portion, also called matrix, including a moveable support pad, if needs be several moveable support pads, capable of coming into contact with the interface film at the level of the layer of haptic actuators such that a thickness is left free between the interface film and the lower portion of the injection mould outside of the support pad,injection of a plastic material into the free thickness on either side of the support pad,opening of the injection mould and ejection of the plastic part formed.

The manufacturing method according to the invention may further comprise one or more of the following characteristics taken alone or according to all technically possible combinations thereof.

The decorative film may have a thickness comprised between 100 μm and 2 mm, preferentially between 250 μm and 1 mm.

The method may comprise a step of thermoforming the decorative film.

Furthermore, the decorative film and the technical film may be assembled together by means of an adhesive layer.

The support pad may be configured to form a free zone, without plastic material in the plastic part, of circular shape and having a diameter comprised between 5 and 40 mm.

Moreover, the support pad may be mounted on an elastic return member, notably a spring, within the lower portion of the injection mould.

The injection mould may be made of tempered steel.

Furthermore, before the step of injection of the plastic material, the method comprises the step of positioning an anti-deformation part, notably an annular part, in contact with the interface film in superposition with the support pad, said anti-deformation part being intended to be situated in the free zone of the plastic part formed by the support pad.

The support pad may comprise a machining to receive at least partially said anti-deformation part.

In addition, at the end of the step of ejection of the plastic part, the method comprises the step consisting in filling the free zone, without plastic material and formed by the support pad, with a flexible material, notably a rubbery material, having a Young's modulus less than 1 GPa, notably less than 100 MPa.

Moreover, at the end of the step of ejection of the plastic part, the method may comprise the step consisting in inserting into the free zone, without plastic material and formed by the support pad, a mechanically robust wall at a distance from the interface film. The distance separating the interface film and the mechanically robust wall may be comprised between 0.3 and 1 mm.

Furthermore, the haptic actuators of the layer of haptic actuators may be configured to operate as haptic sensors such that the interface formed by the decorative film comprises one or more pressure sensitive buttons.

The method may further comprise the cutting of the layer of haptic actuators and an optional adhesive layer between the layer of haptic actuators and the layer of capacitive sensors.

In addition, the method may comprise the step consisting in inserting a zero insertion force (ZIF) type sensor, at the level of the cutting of the layer of haptic actuators.

