This invention relates generally to the field of humidity controlled containers, and in particular to a shipping or storage container for an inkjet printhead that includes a liquid therein, the container providing and maintaining a humidity level within a desired range.
An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector consisting of an ink chamber, an ejecting actuator and an orifice through which droplets of ink are ejected. The droplets are typically directed toward paper or other print medium in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the print medium is moved relative to the printhead. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the chamber in order to propel a droplet out of the orifice, or a piezoelectric device which changes the wall geometry of the chamber in order to generate a pressure wave that ejects a droplet. In general, whatever the type of ejecting actuator, it is powered and controlled electronically. In this sense, an inkjet printhead is one example of an electronic device.
Inkjet printheads are an example of a class of electronic devices that are designed to function with liquids. The presence of liquids, especially at elevated temperature presents reliability issues which need to be addressed for proper long-term operation of the electronic device. Conductive metal lines in electronic devices are typically made of metals that may include aluminum or copper. Such metals may be subject to corrosion if exposed to liquids, via chemical or electrochemical interactions, especially if ionic materials are present. This can cause electrical shorts or opens, particularly for microelectronic devices having small conductive lines and small spaces between adjacent conductive lines. The corrosion-sensitive metals are typically passivated with organic or inorganic materials. Still, manufacturing defects, or manufacturing processes such as trimming off the edges of a circuit board near the conductors, can expose the metals to corrosion. Elevated temperatures, such as those that may occur during shipping or storage, may accelerate the chemical interactions and significantly shorten the useful lifetime of the device. In addition, some materials which are effective in protecting against corrosion at lower temperatures and humidities are less effective at higher temperatures and humidity, as their permeability to moisture increases.
It has been known for many years that the reliability of an electronic device can be preserved by keeping it in a dry environment where a low humidity environment is provided by a desiccant. For example, U.S. Pat. No. 3,326,810 describes various forms of providing a silica gel desiccant for electronic devices or other applications and using the desiccant dry out the air in the vicinity of the sensitive device. Moisture from the air is adsorbed into the desiccant. While “the drier the better” is a good rule for most electronic devices, some electronic devices such as inkjet printheads are actually shipped with liquid in them, and too dry an environment can adversely affect the subsequent reliability of operation.
Inkjet ink includes a variety of volatile and nonvolatile components including pigments or dyes, humectants, image durability enhancers, and carriers or solvents. A key consideration in ink formulation is the ability to produce high quality images on the print medium. During periods when ink is not being ejected from an ejector, the volatile components of the ink may evaporate through the nozzle, or there may be other factors why the ink properties (such as viscosity) at the nozzle may change. Such changes can make the drop ejection process nonuniform, so that the image quality may be degraded. Image quality may also be degraded by the presence of manufacturing defects, particularly in the drop ejector region. Such defects may include mechanical defects that cause asymmetry in a nozzle, or contamination defects that partially obstruct a nozzle, or out-of-tolerance geometries, or electrical defects. As not all defects are easily detected during manufacturing, it is common to print test each printhead after manufacturing is completed. The print testing fluid typically contains the same type of components as inkjet ink, and may in fact be an inkjet ink. The print test may be evaluated by inspecting the presence or absence or position of resulting dots on a test medium. Following the print test, it is necessary to flush the print test fluid out of the printhead. It is found however that complete cleanout of all residual test fluid is very difficult. If some residual test fluid remains after flushing, the volatile components may dry out during subsequent shipping of the printhead, resulting for example in plugged nozzles that are very difficult to clean out. The ironic result then is that the very print test that was meant to ensure delivery of only high quality printheads may help to degrade those printheads. A common strategy is to ship the printhead with a shipping fluid in the ink path passages of the printhead. This shipping fluid is put into the printhead following the flushing of the printhead. The shipping fluid may be ink, but more typically the shipping fluid would not include some of the components (such as the colorants) which might be more likely to contribute to nozzle clogging as the volatiles evaporate. Shipping fluid could be as simple as water, although it might also include a humectant such as glycerine, as well as a biocide.
Printheads may be shipped or stored within a container such as a sealed bag which is somewhat resistant to moisture loss. Thus the shipping fluid is prevented from drying out and can keep the nozzles wet enough to prevent persistent clogging. Any remaining clogs can therefore be easily removed during printhead installation and maintenance in the printer. However, during shipping and storage of the printheads prior to sale to the user, the printheads may encounter elevated temperatures such as 60° C. in a warehouse for extended periods of time. Since the printhead container (sealed bag) contains liquid, such an elevated temperature may result in a humidity level of about 95% at the elevated temperature. As indicated above, such elevated levels of temperature and humidity can compromise the reliability of the printhead electronics and metal interconnection lines.
Using desiccants to keep shipping containers within a range of humidity levels, and not just as dry as possible is known. For example, U.S. Pat. No. 5,529,177 discloses the use of panels for trucks or railroad cars, where the panels include a desiccant. Furthermore, for cargo that requires an elevated humidity shipping environment (particularly when shipping through dry regions), water may be added to keep the humidity control panels moist. In addition, US Patent application Publication No. 2006/0144733 discloses hydrating a humidity control substance (i.e. a desiccant such as silica gel) to a desired moisture content prior to enclosing it in the storage container with a moisture-sensitive product. The hydration may be accomplished by exposing the desiccant to a high humidity environment. The hydrated desiccant maintains the humidity in the container within a desired range so that the moisture-sensitive material does not change its moisture content excessively.
What is still needed, however, is an inexpensive shipping or storage container for a liquid-containing device that includes a predetermined amount of desiccant relative to the amount of liquid within the device, in order to establish and maintain the humidity in the container within a desired range.
According to one feature of the present invention, a container assembly includes a container including an inside and an outside, the container being sealed to isolate an environment inside the container from an environment outside the container. An electronic device is disposed inside the container. The electronic device includes an internal cavity, the internal cavity including a liquid. A predetermined mass of desiccant material is disposed inside the container. An air path exists between the desiccant material and the liquid in the internal cavity of the electronic device. The predetermined mass of desiccant material and the liquid maintain a humidity level of the environment inside the container within a desired range.
According to another feature of the present invention, a method of maintaining a humidity level of an environment within a container including an electronic device disposed therein includes providing a container including an inside and an outside; providing an electronic device including an internal cavity; adding a liquid to the internal cavity of the electronic device; disposing the electronic device inside the container; disposing a predetermined mass of desiccant material inside the container; providing an air path between the desiccant material and the liquid in the internal cavity of the electronic device, the predetermined mass of desiccant material and the liquid maintaining a humidity level of the environment inside the container within a desired range; and sealing the container to isolate the environment inside the container from an environment outside the container.
Referring to
Offline testing of degradation of printhead reliability through observation of moisture condensation and corrosion or hydrolysis of electronic components at high humidity at elevated temperature, as well as extensive and persistent nozzle clogging at low humidity and elevated temperature can determine a desirable humidity range to maintain within the shipping or storage container. The elevated temperature in the test may be chosen to represent typical or extreme conditions encountered during warehouse storage, for example.
Once the desired target humidity level is known, as well as the quantity and composition of shipping fluid, the appropriate amount of desiccant may be calculated. Without being overly constrained by theory, the following analysis provides an understanding of how to calculate a predetermined amount of desiccant that is appropriate. In this embodiment, silica gel is the particular desiccant described, but it is straightforward to adjust for other desiccant materials, as described below.
Inside a low permeability enclosure, such as the printhead shipping bag, the moisture in the atmosphere within the enclosure will reach an equilibrium with that solution. The humidity c, and the equilibrium between solution and vapor can be described to an adequate extent utilizing Raoult's Law,
i.e. the partial pressure of water in the contained atmosphere PH
Silica gel has a porous structure that adsorbs moisture reversibly and exhibits little dimensional change with degree of moisture adsorption. By adsorbing moisture reversibly it is meant that water that is adsorbed is not held permanently, but may be driven off—for example by exposing the silica gel to elevated temperature at low humidity. If the desiccant material is considered to have a finite number of sites to which the water molecules can be adsorbed, the adsorption process is naturally limited. Perhaps the simplest model of adsorption of molecules onto binding sites was provided by Langmuir, as is described in standard textbooks on physical chemistry. In Langmuir's model of adsorption, it is assumed that a) all adsorption sites are equal, b) adsorbed molecules do not interact, c) all adsorption occurs through the same mechanism, and d) at the maximum adsorption only a monolayer is formed (i.e. only one water molecule per adsorption site). The binding rate of water molecules to the desiccant is determined by the number of sites present and the concentration of moisture vapor,
where w is the amount of adsorbed water relative to the amount of desiccant, c is the water vapor concentration (RH), d is a dimensionless number indicative of the binding site concentration, and k are the rate constants, where the subscripts b and u indicate binding and unbinding.
At steady state, the binding rate is zero and the behavior of the desiccant can be described by,
w=Kduc
where K=kb/ku. (3)
Since there is a one-to-one correspondence between sites and water molecules, and the total number of sites is constant,
Experimentally, K and d may be determined by measuring the equilibrium uptake amounts of moisture as a function of relative humidity c (RH) and applying linear regression to the following equation (where equation 5 is simply a rearrangement of the terms in equation 4),
With K and d known, the amount of adsorbent material needed to reach a desired equilibrium humidity level with a given solution may be calculated from equation (4). As a working example, we take a shipping fluid having a water mass fraction of xw, the total amount of water in the system is then
mw,t=xwS, (5)
where S is the mass of shipping fluid remaining in the printhead at the time of packaging. Note: the amount of moisture in the atmosphere is on the order of 0.00001 grams per mL of air, so that with about 100 mL of air in the printhead bag, the moisture in the air is only about 0.001 gram, which is negligible compared to at least 0.1 g liquid water in the printhead.
At equilibrium, the water is distributed between the mass of water mw,s in the humectant solution and the mass of water mw,d that is in the desiccant,
m
w,t
=m
w,s
+m
w,d. (6)
From equations (1), (6) and (7) it may be shown that at the desired humidity c0, the mass of water that must be adsorbed onto the desiccant is given by
From equation (3), the mass of desiccant md needed to achieve the desired humidity c0 is then given by
Note from equation (9) that the mass of desiccant required for a given humidity level is directly proportional to the mass of water in the shipping fluid. Suppose that the composition of the shipping fluid is unchanged (80% water and 20% glycerine), but the printhead has larger ink passageways or more ink ports 276 so that 1 gram of shipping fluid is required rather than 0.5 gram. In such a case, the mass of silica gel required would need to be doubled to 4 grams for 30% relative humidity and 2 grams for 80% relative humidity. Similarly, for a smaller printhead with only a single ink port 276, the mass of shipping fluid would be decreased to perhaps 0.1 gram and the mass of silica gel required for 80% relative humidity would be about 0.2 gram.
Comparison of
Testing of printheads under the shipping and storage conditions of the example of
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.