Patent Description:
Protective cover members for droplet forming units in a print head assembly are known, for example from <CIT> or <CIT>. The print head assembly comprises a support body from which at least one droplet forming unit at least partially protrudes in a first direction. The first direction generally being in the droplet jetting direction of the droplet forming unit during use. The print head assembly further comprises a cover member which surrounds the at least one droplet forming unit to protect it from coming into contact with a print medium, which is moved with respect to the print head assembly while forming the image. The cover member may further prevent ink contamination by at least partially providing a seal around the droplet forming unit. The known cover members are generally formed of single material, for example by injection molding or stamping.

It is an object of the invention to provide an improved cover member for use in a print head assembly, specifically a cover member with improved flatness and/or compatibility with the support body.

In accordance with the present invention, a print head assembly according to claim <NUM>, and a method according to claim <NUM> are provided.

In order to achieve the above described object, the print head assembly comprises a ceramic support body having a thermal expansion coefficient from which support body at least one droplet forming unit formed of silicon at least partially protrudes in a first direction, and the cover member, which cover member comprises:.

In an embodiment, the sheet is formed from a first material different from a second material, the circumferential wall being formed of the second material. Since the sheet and the circumferential wall are provided as separate components, these may be formed of different materials and by different manufacturing methods as well. This allows for a wider selection of materials for each respective component, as compared to an integral cover member as known in the state of the art. For example, the material and manufacturing method for sheet may be selected to optimize its flatness, and/or wetting properties with respect to the type of fluid applied. The material for the circumferential wall may be chosen based on different criteria, such as a match between the circumferential wall and the support body upon which it is mounted. Since the print head assembly heats up during use, the cover member is formed and/or mounted to compensate for thermal expansion. The structure and attachment of the cover member are such that its integrity is maintained during heating. The second material has a thermal expansion coefficient (CTE) resembling that of the support body. A close match in CTE ensures the cover member remains properly secured to the support body, reducing the chance of contamination by ink creeping into the cover member. Similarly, a CTE of the sheet may be matched to that of the circumferential wall in another embodiment. Additionally, the CTE of the sheet material may resemble that of the droplet forming unit (which may be formed of silicon by MEMS manufacturing). Any strain on the droplet forming unit due a thermal expansion coefficient mismatch is also minimized or prevented in this manner, which benefits the print head assembly's print quality and/or lifetime. Alternatively, a flexible and/or deformable adhesive may be applied to attach the circumferential wall to the support body, which adhesive is arranged to compensate and/or allow for differences in thermal expansion between the wall and the support body. Likewise, such an adhesive may be applied to attach the sheet. It will further be appreciated that thermal expansion can additionally or alternatively be compensated, even when the CTEs of the different materials in the support body, circumferential wall, and/or sheet are relatively far apart from one another, by a suitable arrangement of said materials with respect to one another and/or structural adjustments that allow for the release of strain due to thermal expansion. The first material of the sheet is a metal, preferably a transition metal, and very preferably comprises titanium (Ti). The second material of the circumferential wall is a plastic, preferably a liquid crystal polymer (LCP). Third material is a ceramic, preferably comprising silicon carbide (SiC).

Thereby the object of the present invention has been achieved.

More specific optional features of the invention are indicated in the dependent claims.

In an embodiment, the circumferential wall is provided with a support for attaching the sheet. The support provides a convenient attachment point for securing the sheet to the circumferential wall. Preferably, the support is a ridge provided circumferentially along the circumferential wall, for example on the inner side or face of the circumferential wall. The support ridge allows the sheet to be supported at several points along its circumference, ensuring the flatness of the sheet is maintained in the mounted state. It is further preferred that the support extends over the full circumference of the circumferential wall. This improves support of the sheet and further allows the interface between the sheet and the support to sealed, preventing any fluid from creeping underneath the sheet. This improves device lifetime and reduces contamination. Very preferably, the sheet is attached to the support by means of an adhesive. The adhesive provides easy attachment means, while also forming a fluidtight seal between the sheet and the support.

In an embodiment, the sheet is a flat sheet. The sheet has a high degree of flatness, both in its surface properties (e.g. roughness) as well as in its shape (e.g. planar or two-dimensional). The sheet forms (the majority of) the end face of the print head assembly facing the print medium. A very flat end face allows for a narrow spacing (a so-called print gap) between the print medium and the nozzles of the droplet forming unit. This is beneficial for the image quality of the printed image.

In an embodiment, the opening is a central opening remote from all outer edges of the sheet. The sheet comprises an inner edge or edges surrounding the opening, which inner edge is spaced apart from the outer edge or edges of the sheet. The outer edge is supported on the support, while the inner edge or edges are positioned inside the support, when viewed in the first direction in the mounted state. The opening in the sheet has a smaller area than an opening formed by the support. The latter opening is in turn smaller than an opening circumscribed by the circumferential wall.

In an embodiment, a plurality of droplet forming units are mounted spaced apart from one another on the support body, and wherein each droplet forming unit is individually surrounded by its respective cover member. The cover member is dimensioned to fit and/or cover a single droplet forming. Each single droplet forming unit may be defined by its own respective nozzle plate or main body prior to mounting on the support body. The cover members are also mounted at a distance from one another.

The present invention further relates to a method for assembling a print head assembly, comprising the steps of:.

In the mounted state, the droplet forming unit extends out of the top plane of the support body, like an island. The cover member is mounted over the droplet forming unit by attaching it to the support body, thereby aligning the opening in the sheet with the nozzles, such that the nozzles are free from the sheet. Prior to mounting, the sheet has been cut to size and provided with an opening in a manufacturing process separate from that of the circumferential wall. After their individual forming processes, the sheet and the circumferential wall are attached to one another to form the cover member. As explained above, this provides a large degree of freedom in material selection and manufacturing processes for each component, allowing for the creation for a relatively low-costs and highly flat cover member.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the present invention will become apparent to those skilled in the art from this detailed description.

<FIG> schematically illustrates an exploded, perspective view of a droplet forming unit <NUM> in combination with a cover member <NUM>. The droplet forming unit <NUM> may be one of multiple droplet forming units <NUM> provided together on a common support body <NUM> of a print head assembly <NUM>, as shown in <FIG>. The droplet forming unit <NUM> comprises a nozzle plate <NUM> provided with a plurality of nozzles <NUM>. The nozzles <NUM> in <FIG> are provided in straight rows, but may be provided in any suitable pattern, such as a staggered or zigzag pattern. Each nozzle <NUM> is in fluid connection to a pressure chamber, which pressure chamber is in fluid connection to a fluid reservoir. Each pressure chamber is provided with an actuator for generating a pressure pulse in the pressure chamber, which results in a droplet of fluid being jetted from the respective nozzle. Such droplet forming units are known, for example from <CIT>.

A cover member <NUM> is provided over the droplet forming <NUM> in the mounted state as shown in <FIG> with a cross-sectional view in <FIG>. The cover member <NUM> is formed of a base <NUM> comprising a circumferential wall <NUM>, which extends fully around the droplet forming unit <NUM> as an endless loop. The circumferential wall <NUM> extends in the first direction D. In the example shown, the circumferential wall <NUM> is slightly inclined with respect to first direction D. The first direction D is perpendicular to the nozzle plate <NUM>. During use, the first direction D is preferably parallel to the vertical direction and extends in the direction of gravity. The outer perimeter of the circumferential wall <NUM> is greater than the outer perimeter formed by the side walls of the droplets forming unit <NUM>, allowing the droplet forming unit <NUM> to fit and/or be received inside the circumferential wall <NUM>. In <FIG> the circumferential wall <NUM> tapers in the first direction D towards its top surface <NUM>, wherein top is defined with respect to the first direction D. The top surface <NUM> of the base <NUM> is provided with an opening <NUM>. The opening <NUM> is dimensioned to exceed the combined area of the nozzles <NUM>, such that the nozzles <NUM> are exposed in the mounted state. Adjacent the opening <NUM> a support <NUM> is provided for receiving the sheet <NUM>. The support <NUM> extends perpendicular to the first direction D on the inside of the circumferential wall <NUM>, the support <NUM> is formed as an endless ridge, which forms the full edge of the opening <NUM>. In <FIG>, the support <NUM> is formed as a recess in the top surface <NUM>, creating a step <NUM> on the top surface <NUM> of the base <NUM>.

The support recess or ridge in <FIG> is dimensioned to fit the sheet <NUM>. The sheet <NUM> is also provided with an opening <NUM>. The opening <NUM> is dimensioned and positioned, such that in the mounted state the nozzles <NUM> are exposed. This allows the nozzles <NUM> to jet ink droplets in the mounted state. The opening <NUM> of the sheet <NUM> is smaller in area than the opening <NUM> in the base <NUM>. The sheet <NUM> itself, specifically its outer circumference <NUM>, is larger than the opening <NUM> in the base <NUM>. The opening <NUM> in the sheet <NUM> is located centrally on the sheet <NUM>, remote from the outer edges <NUM> of the sheet <NUM>. This allows the sheet <NUM> to be supported on the support <NUM>, while creating an opening <NUM> for the nozzles <NUM>. Preferably, the edge <NUM> of the opening in the sheet <NUM> is positioned adjacent or near the outer nozzles <NUM> in the mounted state.

The sheet <NUM> is mounted on to the base <NUM> by means of suitable attachment means <NUM>. Preferably, the sheet <NUM> is bonded onto the support <NUM> in a fluidtight and/or airtight manner by means of an adhesive <NUM>. This prevents or reduces fluid accumulating inside the cover member <NUM> and contaminating the print medium when it is released. Other attachment means <NUM> may include clamps, click mechanisms, raised ridges, hooks, etc..

The sheet <NUM> is formed of a first material different from that of the base <NUM>. The sheet <NUM> is preferably formed separately from the base <NUM> with a manufacturing method suitable for achieving a high degree of flatness, such as rolling or pressing. The first material may be a metal, such as titanium, platinum, silver, or any other suitable metal or alloy. Alternatively, plastics may also be applied for forming the sheet <NUM>. The sheet <NUM> is further cut to size using an accurate cutter. A (laser) milling device may for example be applied to cut the inner and outer edges <NUM>, <NUM> of the sheet <NUM>. This allows for the formation of a sheet <NUM> with an opening <NUM> and a well-defined and very flat top surface <NUM>.

The base <NUM> is preferably formed separately from the sheet <NUM>. The tapered shape of the base <NUM> allows it to be conveniently formed by injection molding, deep drawing, vacuum drawing, stamping, though other suitable manufacturing techniques may be applied. The base <NUM> is formed of a different material than the sheet <NUM>. The material of the base <NUM> may thus be selected in correspondence with requirements for the base <NUM> and/or its preferred manufacturing process, such as a thermal expansion coefficient close to that of the support body <NUM>. Since the manufacturing method for the base <NUM> may be different from that of the sheet <NUM>, additional freedom in the material selection for both components is achieved. The material for the base <NUM> is preferably a plastic and/or comprises at least one polymer. It was found that a liquid crystal polymer (LCP) material for the base <NUM> resulted in a good match in thermal expansion coefficient a support body <NUM> formed of a ceramic, such as silicon carbide (SiC). Similarly, a sheet formed of titanium has a good CTE match with the silicon material used for forming the droplet forming unit. Additionally or alternatively, differences in thermal expansion between components can be compensated through a suitably thick adhesive layer bonding said components together and/or by other known method for compensating thermal expansion.

In <FIG>, the cover member <NUM> is mounted on the support body <NUM>. The base <NUM> of the cover member <NUM> may thereto be provided with one or more attachment means <NUM>, such a clamp, hook, magnet, click mechanism, adhesive, etc. Preferably, a print head assembly <NUM> comprises multiple droplet forming units <NUM> provided spaced apart from one another on the same support body <NUM>. The droplet forming units <NUM> are preferably mounted in a staggered pattern, such that neighboring droplet forming units <NUM> partially overlap when viewed in a direction wherein the print head assembly and the print medium move with respect to one another. The droplet forming units <NUM> each protrude from the support body <NUM> when mounted thereon. Though not show, it will be appreciated that the support body <NUM> is provided with a plurality of channels for distributing fluid to the different droplet forming units <NUM> mounted on it.

Although specific embodiments of the invention are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations exist. It should be appreciated that the exemplary embodiment or exemplary embodiments are examples only and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents. Generally, this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.

Claim 1:
A print head assembly (<NUM>) comprising:
- at least one droplet forming unit (<NUM>) comprising a nozzle plate (<NUM>) provided with a plurality of nozzles (<NUM>);
- a ceramic support body (<NUM>) having a thermal expansion coefficient from which support body (<NUM>) the at least one droplet forming unit (<NUM>) formed of silicon at least partially protrudes in a first direction (D), and
- a cover member (<NUM>) surrounding the at least one droplet forming unit (<NUM>), wherein the cover member (<NUM>) comprises:
- a plastic circumferential wall (<NUM>) mounted on the support body (<NUM>), extending in the first direction (D), and surrounding the at least one droplet forming unit (<NUM>), and having a thermal expansion coefficient resembling that of the support body (<NUM>); and
- a metal sheet (<NUM>) attached to the circumferential wall (<NUM>) and comprising an opening (<NUM>) through which the at least one droplet forming unit (<NUM>) is accessible in the first direction (D).