Abstract:
A replaceable ink reservoir for an inkjet printer includes a bag of flexible plastic film. The film is stretched over a plastic plug formed as a thick disc, and an annular ring is pressed over the plug, trapping the film therebetween. The grip achieved by this mode of fastening is secure enough for mechanical strength, and tight enough to seal the ink contained in the bag. No adhesives etc are needed. A needle-pierceable membrane is housed in the plug.

Description:
BACKGROUND TO THE INVENTION 
     This invention relates to computer-operated printers of the kind that direct a jet of ink onto a passing surface. The invention is mainly intended for use on the kind of ink-jet printer in which the surface to be printed is a small distance away from the ink jet head, i.e the type that is used for printing onto cardboard boxes and the like. In such printers, the (liquid) ink reservoir is in the form of a flexible bag, and the bag is replaceable. The invention is aimed at providing an improved replaceable reservoir. 
     Previous designs of replaceable reservoirs for ink-jet printers have been based on the principle of sealing the ink in a container and of including a membrane as a portion of the wall of the container. A hollow needle fixed into the printer is arranged to mechanically pierce the membrane when the container, full of ink, is assembled into place in the printer. Ink flows into the printer through the hollow needle, and the membrane material is such as to seal itself around the needle, whereby no ink can leak out. 
     Both when the full container is assembled to the needle, and when the (nearly) empty container is removed from the needle, the membrane provides an adequate seal around the needle, and the membrane closes itself up, i.e seals itself, after the needle has been removed. The ink reservoir as described herein also is adapted to make use of the needle-and-membrane technology. 
     THE PRIOR ART 
     Some previous designs of ink reservoirs for ink-jet printers that might be considered relevant to the invention are: U.S. Pat. No. 5,359,356 (Ecklund, October 1994); U.S. Pat. No. 4,183,031 (Kyser, January 1980); and U.S. Pat. No. 4,053,901 (Skafvenstedt, October 1977). 
     THE INVENTION IN RELATION TO THE PRIOR ART 
     One previous design approach to the replaceable reservoir has been to use a container having fixed or rigid walls. Such containers have been expensive, especially since, as the ink is used, some means of compensating for the reduced volume in the container has to be provided. Movable pistons, which require to be sealed, have added to the complexity and expense. Of course, such systems can be engineered to be leakproof, even if the components are subjected to abusive treatment, but the expense of achieving robust leak-proof reliability has been high. 
     Another approach has been to provide a flexible bag, which can collapse as the ink is consumed. This addresses the moving-parts problem, but raises the problem of how the bag might be attached to the base component, or plug, in which the membrane is located. The problem of sealing a bag of thin flexible film to a solid, chunky plug, has not previously been resolved adequately, but inexpensive manner. Too often, the joint between bag and plug has been prone to leakage, even when (expensive) care and attention is given especially to the potentially-leaky region of the reservoir during manufacture. Previous designs have included welding, adhesives of various types, and so on, in the attempt to provide a reliable seal. 
     The invention provides a manner of attaching a bag of flexible plastic film to a chunky, solid plug. Basically, the bag is held tightly between the plug and a tightly fitting ring. The ring holds the film material trapped between itself and the plug. As will be explained, assembly of this manner of attachment, on the production line, is simple enough that even an operator who carries out the assembly with a less than ideal degree of care and attention can readily produce reservoir after reservoir in which the chances of leakage are virtually nil. This level of reliability of the seal is achieved despite the fact that no adhesives or welding are needed. 
     For an effective seal, the ring should grip the film of the bag very tightly around the plug. The designer should see to it that the ring is of such dimensions as to support the stresses induced by the tight fit. The designer should have it in mind that plastic materials tend to settle or creep over a period of time, to a new dimension in which the stress is reduced. Thus, even though a badly-designed ring might produce a tight fit at first, such a ring might gradually “give” slightly, and the tight fit would be relaxed. The stresses induced in the ring must be kept low enough that the tight fit can be maintained indefinitely. Therefore, the ring and the plug should be thick and chunky. 
     It is recognised that a plug and a ring of such suitable dimensions as to hold the tight fit more or less indefinitely can be provided inexpensively, and can be accommodated within the environment available for a replaceable ink-reservoir on an ink-jet printer. 
     SUMMARY OF THE INVENTION 
     The invention lies in an ink-storage reservoir apparatus for a printer that includes a bag of flexible plastic film. A plug serves to close a mouth of the bag, and thereby to define an ink container in the bag. The plug has an outward-facing surface, and the mouth of the bag is placed thereover, and the surface of the plug is so dimensioned, in relation to the mouth of the bag, as to cause the material of the mouth of the bag to stretch over the plug. The apparatus includes a ring, and the ring has an inward-facing surface, which is complementary to the outward-facing surface of the plug. The ring is a tight interference fit over the material of the mouth of the bag when the mouth of the bag is stretched over the outer surface of the plug. The ring is pressed over the plug and encircles the plug, and serves to grip the material of the mouth of the bag, tightly and securely, between the outward-facing surface of the plug and the inward-facing surface of the ring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     By way of further explanation of the invention, exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a partly-sectioned side-elevation of a replaceable ink reservoir for an ink-jet printer, which embodies the invention; 
     FIG. 2 is a view of some of the components of the reservoir of FIG. 1, shown at a preliminary stage of manufacture; 
     FIG. 3 is is a another view of the components of FIG. 1, shown at a later stage of manufacture; 
     FIG. 4 is is another view of the components of FIG. 1, shown at a still later stage of manufacture. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatuses shown in the accompanying drawings and described below are examples which embody the invention. It should be noted that the scope of the invention is defined by the accompanying claims, and not necessarily by specific features of exemplary embodiments. 
     FIG. 1 shows the replaceable ink reservoir  20 . A plastic plug  23  has an outer right-cylindrical surface  24  having a diameter of about 35 mm. The thickness of the plug  23  between the inside wall  25  and the outside wall  26  is 15 mm. The plug  23  is formed with a central through-hole  27 , which includes a step  28  between a small diameter  29  and a larger diameter  30 . The plug may be made of nylon, low-density polyethylene, etc. 
     A ring  32 , also made of LDPE, encircles the plug  23 . The ring has an inside right-cylindrical surface  34 , which is complementary to the surface  24  on the plug. The axial length of the ring is approximately the same as the axial length of the plug. The ring has an annular thickness of 3.3 mm 
     A bag  35 , made of flexible plastic film, is trapped between the ring  32  and the plug  23 . The tightly-gripping fit of the plastic film between the ring and the plug arises because of the interference fit therebetween. The fit is so tight that the film is mechanically locked very securely, and is so tight that liquid ink inside the bag  35  cannot escape. The bag  35  is sealed at its upper end by two heat-welded bands  36 . 
     To assemble the reservoir  20 , the following steps are carried out. First, the plug  23  is inserted into the open mouth  37  at one end of a length of plastic film tubing  38  (FIG.  2 ). The mouth  39  at the other end of the tubing  38  lies open also. 
     The plug  23  lies cross-wise in the tubing  38  at first. Upon assembly, the tubing and the plug are to be co-axial, i.e the outer right cylindrical surface  24  of the plug makes direct contact all around with the inside surface  40  of the tubing  38 , as shown. The plug  23  is a tight fit in the tubing  38 , whereby, in order for the inside surface  40  of the tubing to fit over the outer right cylindrical surface  24 , the tubing  38  has to be stretched. 
     This diametral stretching of the tubing  38  is accomplished as follows. The plug  23 , having been inserted into the tubing (FIG. 2) lies cross-wise inside the tubing. The plug is then placed on a pillar  42 . The plug is manipulated and orientated within the tubing until the plug lies laterally across the diameter of the tubing, whereby the plug is aligned with its axis co-axial with the axis of the tubing (FIG.  3 ). The force required to stretch the tubing around the plug, even though the tubing is being stretched over the surface  24  of the plug, can easily be supplied by a person&#39;s fingers, and the drawing down operation can be done by hand. 
     Next, the tubing  38 , stretched over the plug  23 , is inverted. (It can be convenient to retain the pillar  42  inside the tubing  38  while this is done.) The ring  32  is slipped over the tubing  38 . The ring is loose on the nominal diameter of the tubing, and falls down over the tubing, until the ring then lies against the portion  43  of the tubing that has been stretched over the plug  23  (FIG.  4 ). 
     An assembly sleeve  45  is brought down, which picks up the ring  32 , and forces the ring to stretch over the film  43  of the tubing and over the plug. The sleeve  45  presses the ring all the way down until the face  46  of the ring lies more or less flush with the outside wall  26  of the plug. 
     With the ring  32  fully assembled over the plug  23 , any portion of the tubing film that might be protruding beyond the wall  26  is trimmed off (with a knife). It can happen sometimes that virtually no film protrudes after assembly; sometimes several millimeters of tubing protrudes, and need to be trimmed. 
     The tubing having been filled with ink (through the mouth  39 ), the mouth  39  is sealed by heat-welding, at  36 , and the reservoir  20  is ready for shipment and sale. 
     The various corners and junctions of the plug and ring should be chamfered, as at  47 , to ease assembly and to enable the film to engage and slide thereover. 
     The amount of stretching of the plastic film of the tubing is important. The stretching has to be substantial enough that every hint of a wrinkle in the film material is taken out. It has been found that even the slightest wrinkle in the film can lead to a weakness or lack of tightness in the fit, at which a leakage might develop at some time in the future. The tubing should be so dimensioned that it undergoes a stretch of at least 5 or 10 percent of diameter. 
     Also, to ensure a reliably sealed and mechanically secure joint, it has been found that an interference fit of the ring over the diameter of the tubing stretched over the plug should be about 0.75 mm (on a diameter of 35 mm). That is to say; the inside surface  34  of the ring should be a 0.75 mm diametral interference on the overall diameter of the plug with the tubing film stretched thereover. That interference figure is the preferred minimum: the designer should provide additional interference to take account of the inevitable manufacturing tolerances and inaccuracies. 
     To assemble the reservoir into the printer, the ink-receiving needle (which is a fixture of the printer) is inserted through the hole  27  of the plug  23 . Contained in the larger diameter receptacle  30  is a pierceable membrane  48 , surmounted by a stainless steel washer  49 . These items are retained in the hole by an insert  50 , made of e.g brass. The insert  50  preferably is barbed on its outside surface, to ensure the components cannot come out of the hole  27 . The insert should be inserted firmly enough to ensure that the membrane  48  is held slightly compressed against the step  28 , since the membrane serves not only to seal around the needle, but serves also to seal the through-hole  27 . 
     The outward-facing surface  24  on the plug, and the complementary inward-facing surface  34  on the ring, have been indicated as right-cylindrical—that is to say: not tapered. However, while that particular configuration is preferred, other shapes, including tapered, are contemplated. The key requirement is that the ring remain tightly gripped around the plug, with the film nipped and gripped securely therebetween. But right-cylindrical is preferred, in that any shape other than right-cylindrical might compromise the effectiveness of stretching the tubing over the plug, aimed at smoothing out wrinkles. Only the right-cylindrical form gives an even stretch (assuming the tubing is itself right-cylindrical). 
     The coefficient of friction of plastic on plastic can be rather low, and since the grip of the bag between the plug and the ring depends on friction, it is important that the grip be very tight. It is important that the engagement length L, over which the bag material is gripped between the plug and the ring, be quite long: a length of 1 cm has been found to give an adequate grip. Any lead-in chamfers, as at  47 , are not included in the length L.