Abstract:
A method for manufacturing pellets of hot-melt ink which includes the steps of filling molten ink into a mold cavity defined by a first die and a second die of a mold, allowing the ink to cool down and solidify in the mold cavity, and heating at least one of the first and second dies for re-melting the surface of the ink pellet to facilitate its removal from the mold cavity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and a mold for manufacturing pellets of hot-melt ink. 
     2. Background Art 
     Certain types of inkjet printers employ a so-called hot-melt ink i.e. a wax-like ink material that is solid at room temperature and has a melting point in the order of, for example, 80 to 150° C. In the printhead of the printer, the ink is heated above its melting point, so that droplets of liquid ink can be expelled through the nozzles of the printhead. In order to obtain a high quality of the printed image, the viscosity and hence the temperature of the molten ink in the printhead should be maintained essentially constant. However, since the ink is consumed in the course of the printing process, and the ink reservoir accommodating the liquid ink within the printhead is preferably of a limited size, it is necessary to supply and melt solid ink while the printer is operating. The latent heat required for melting the ink tends to decrease the temperature in the ink reservoir. For this reason, it is desirable that the amount of solid ink supplied to the ink reservoir is precisely controlled and metered, and, to this end, it is advantageous that the ink is supplied in the form of pellets having a predetermined size and shape, e.g. in the form of small spherical pills or pellets. 
     Since the hot-melt ink is a thermoplastic material, the pellets having the desired shape and size can be manufactured by means of a molding process similar to injection molding processes known for manufacturing articles from thermoplastic resins. The molding process however should be adapted to the specific properties of hot melt ink, which are, in certain respects, different from those of thermoplastic synthetic resins. Since the amount of shrinkage which the hot-melt ink experiences when it is solidified is comparatively low, and since a certain amount of shrinkage can be tolerated because the final appearance of the molded ink pellets is not critical, it is not necessary to apply high locking forces for keeping the mold closed during the molding process. On the other hand, since the hot-melt ink has a relatively high melting point, it tends to solidify immediately when it comes into contact with the walls of the mold cavity. This effect and the fact that the surface of the ink pellet is somewhat tacky, even when the temperature has dropped below the melting point, increases the tendency of the pellet to adhere to the walls of the mold cavity. This makes it more difficult to reliably and reproducingly remove the molded pellet from the mold die. Especially when the upper and lower dies of the mold are symmetrical, as must be the case for example when the pellet has a spherical shape, it is not predictable whether the pellet will adhere to the upper die or to the lower die when the dies of the molds are separated. This tends to reduce the productivity of the molding process and/or necessitates the use of complex mechanisms for ejecting the molded product from the die. 
     It is well known that the removal of a molded product from a die can be facilitated by employing a separating agent which reduces the adherence between the molded product and the walls of the mold cavity. In this case, however, a portion of the separating agent will inevitably be dispersed or diluted in the molten material, and this is not acceptable in the case of hot-melt ink because it deteriorates the quality of the ink. For example, even minute particles of the separating agent, when dispersed in the ink, tends to clog the extremely fine nozzles of the printhead or change the ink properties such as its surface tension or crystallization point. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for manufacturing pellets of hot-melt ink, in which the pellets can be reliably and reproducibly withdrawn from the mold cavity. 
     According to the present invention, this object is achieved by a method comprising the steps of: 
     filling the molten ink into a mold cavity defined between a first die and a second die of a mold, 
     allowing the ink to cool down and to solidify in the mold cavity, and 
     heating at least one of the first and second dies for remelting only the surface of the ink pellet to be removed from the mold cavity. 
     According to the present invention, the molded pellet is separated from the wall of the mold cavity by heating at least a portion of the mold, so that a surface layer of the pellet is remelted. This can be achieved within a very short time. Thus, it is possible to remove the pellet from the mold cavity efficiently and in a well-defined manner. Since it is not necessary to employ a separating agent, the quality of the hot-melt ink will not be degraded. 
     When the lower die of the mold is heated before the upper and lower dies are separated, it is possible to positively release the pellet from the lower die and to withdraw it from the lower die, taking advantage of the fact that the pellet tends to adhere to the upper die which is not heated. Then, the pellet is released from the upper die by any suitable means, thereby allowing the pellet to simply drop out of the upper die. The pellets dropping out of the upper dies may be collected by any suitable collection means such as a chute which is brought in position underneath the pellets that have been withdrawn from the lower dies. 
     Further, it is possible to release the pellet from the upper die by heating the latter. In a preferred embodiment, the method comprises the steps of first heating the lower die, then separating the upper and lower dies with the pellets adhering to the upper die, and heating the upper die, thereby allowing the pellet to drop out. The release of the pellet from the upper die may be assisted and accelerated by blowing air into the runner hole of the upper die. As an alternative, an ejector pin may be inserted through the runner hole. In this case, the ejector pin may be arranged stationary, so it enters into the runner hole and engages the pellet adhered thereto when the upper die and the pellet are lifted from the lower die. 
     A mold for manufacturing pellets of hot-melt ink in accordance with the method described above comprises first and second dies defining a mold cavity, wherein at least one of the first and second dies has a wall thickness which is smaller than half the diameter of the mold cavity. If the mold cavity is not spherical, the wall thickness of the die is smaller than half the average diameter. 
     Due to the small wall thickness, the die has a very low heat capacity, such that the surface layer of the molded pellet can be remelted very quickly by heating the die. The small heat capacity of the die has the further advantage that the molten ink in the mold cavity can be cooled and solidified more rapidly, so that the productivity of the molding process is increased. 
     Preferably, both dies of the mold have a small wall thickness and hence a small heat capacity and are made of a material having a high heat conductivity, e.g. aluminium. Also stainless steel is useable if the wall thickness is small enough. In a preferred embodiment, the wall thickness of the dies is smaller than a quarter of the diameter of the mold cavity. For example, if the mold cavity is spherical and has a diameter in the order of 10 mm, the wall thickness of the dies may be 1.5 mm or less. 
     Rapid cooling and re-heating of the dies may be achieved in a very simple manner e.g. by blowing cold and hot air or even a liquid against the dies. A number of other heating or cooling devices can be used. 
     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 invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will now be described in conjunction with accompanying drawings, in which: 
     FIGS. 1 to  5  illustrate successive steps of a process for molding hot-melt ink pellets and removing them from the mold cavity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a group of three molds  10 , each of which comprises an upper die  12  and a lower die  14  each of which have a semi-spherical cup shape and, together, define a mold cavity  16  which is filled with molten hot-melt ink  18 . The upper die  12  is integrally formed with a top flange  20  and has a runner hole  22  formed in the center of the flange  20 , so that molten ink can be poured into the mold cavity  18  through a nozzle  24 . 
     The lower die  14  is essentially mirror-symmetric relative to the upper die  12  and is supported on a bottom  26  formed integrally therewith. The lower edge of the upper die  12  and the upper edge of the lower die  14  are surrounded by circumferential flanges  28 ,  30  which are held in firm engagement with one another in order to sealingly close the mold cavity  16 . 
     When the ink  18  has been poured in, as is shown in FIG. 1, the molds are transferred to a cooling stage illustrated in FIG. 2, where cold air  32  is blown against the outer surfaces of the dies  12 ,  14  from above and below, so that the ink in the mold cavities is cooled and solidified to form spherical pellets  34 . 
     Then, the molds  10  are transferred to a first heating stage which is shown in FIG.  3 . This heating stage comprises a heating block  36  having a number of recesses  38  for accommodating the lower dies  14  of the molds. The recesses  38  have a flat bottom, which defines a large contact area with the bottom flanges  26  of the lower dies  14 . Hot air is supplied into a system of passages  40  formed in the heating block  36  and is evenly blown out against the circumferential walls of the lower dies  14  of each mold  10 , as indicated by arrow  42 . The dies  12 ,  14  of the molds  10  are made of aluminium and have relatively thin walls (at least in the portion defining the mold cavity), so that their heat capacity is low, but their heat conductivity is high. As a result, the hot air blown against the walls of the dies  14  rapidly raise the temperature of these dies, and surface layers of the pellets  34  facing the lower dies  14  are re-melted, so that the pellets  34  can easily be released from the lower dies  14 . However, since the upper dies  12  have not been heated, the solidified material of the pellets  34  still adheres to the upper die  12 . Since the heating block  36  is constantly maintained at a high temperature (e.g. by the hot air passing therethrough), heating of the lower die  14  is accelerated by heat radiation and thermal contact between the block  36  and the bottom flange  26 . 
     Then, as is shown in FIG. 4, the upper and lower dies of each mold  10  are separated from one another, either by lifting the upper dies  12  or by lowering the heating block  36  and the lower dies  14 . Since the pellets  34  stick to the upper dies  12 , they are withdrawn from the lower dies  14 . 
     Finally, the upper dies  12  with the pellets  34  held therein are transferred to a second heating stage shown in FIG.  5 . This heating stage comprises a heating block  44  which has essentially the same configuration as the heating block  36  described above, but is arranged in an inverted position so that the recesses  38  face downward for accommodating the top flanges  20  of the upper dies  12 . In addition to the system of passages  40  for blowing hot air against the outer surfaces of the dies  12 , the heating block  44  has another air supply system  46  through which air can be blown with a suitable pressure into the runner holes  22  of the dies  12 . Again, by blowing hot air, indicated by arrows  48 , against the dies  12 , surface layers of the pellets  34  are re-melted, so that the pellets will no longer adhere to the dies  12  but will drop down into a chute  50 . This process is assisted and accelerated by blowing pressurized air into the runner holes  22 . Thus, the molding process for manufacturing the pellets  34  is completed, and the upper and lower dies  12 ,  14  may be re-circulated for use in another molding cycle. 
     Although not shown in the drawings, the dies  12 ,  14  of the molds  10 , the total number of which may be significantly larger than three, may be mounted to an endless conveyor in any known manner allowing to hold the molds  10  closed in the step illustrated in FIGS. 1 to  3  and to move the upper dies  12  and the lower dies  14  relative to one another in vertical direction in the step illustrated in figures  4 . Thus, the process described above lends itself to an efficient mass production of hot-melt ink pellets  34 .