Filter rod making machines
Filter rods, for making filter plugs for attachment to cigarette lengths, may be made by continuously forming a tow of filter material, e.g. cellulose acetate, into a rod in a filter rod making machine, e.g. Molins PM5N. Conventionally a so-called plasticiser (commonly glyceryl triacetate) is added to the tow before it is passed into the rod-forming device. When cured, the plasticiser improves the properties of the finished rod, e.g. by hardening it or providing a beneficial additional filtering effect. 
     It is desirable that the correct amount of plasticiser is added to the tow, i.e. an amount just sufficient to secure the required improvement in properties of the rod. The present invention is particularly concerned with controlling the supply of plasticiser or similar fluid additive to the tow. 
     Plasticiser is commonly added to the tow in an applicator booth or chamber in which the tow passes through a spray of plasticiser droplets. Plasticiser is normally continuously supplied to the booth at a rate which exceeds that at which it is retained by the tow and so removed from the booth. For example, only 60% of the plasticiser directed at the tow may be captured by the tow. The remaining 40% may be returned to a supply of plasticiser, together with other plasticiser (if any) supplied to the booth but not captured by the tow. 
     According to one aspect of the present invention apparatus for applying fluid additive to a moving filter tow includes means for supplying fluid additive to an applicator chamber for application to the tow, e.g. by spraying, means for collecting fluid additive not captured by the tow, and means for reapplying said collected fluid additive to the tow at a rate controllable independently of the rate of supply of fluid additive to the applicator chamber. 
     Preferably the means for supplying fluid additive is arranged to supply it to the chamber at a desired rate of application to the tow. The means for reapplying collected fluid additive may then be arranged so that the proportion of the reapplied collected fluid additive which is captured compensates for the uncaptured proportion of the additive supplied by the supply means. Since, for example, only 60% of the fluid additive may be captured, in that case 40% will pass to the collecting means. Since, however, the collecting means will also collect some reapplied collected additive which is again not captured the rate of collection of fluid additive may easily exceed that of the main supply. In order to achieve the required capture rate it may be necessary to supply collected additive at a relatively high rate; clearly this cannot exceed the rate of collection indefinitely but since the rate of collection will to a significant extent depend on the rate of reapplication of collected fluid additive the rate of reapplication can be varied to achieve a required total application rate. When the rate of supply from the supply means (i.e. from a source) is set at the desired capture rate of additive by the tow the latter rate is achieved when the quantity of collected fluid additive (including that circulating and not captured by the tow) is constant, since then the tow must be removing fluid additive from the chamber at the rate of supply to the chamber. In some cases, particularly where the proportion of supplied fluid additive which is initially captured is relatively low, it may be desirable to supply additive at a rate greater than that required to be collected by the tow; some of the uncaptured (i.e. collected) additive may then need to be returned to the supply if the flow path for reapplying collected additive has insufficient capacity. 
     The collected fluid additive is preferably reapplied to the tow by way of a flow path to applicator means within the chamber. The path may be external to the chamber and may include control means for varying the rate of flow. The path may include a reservoir. Collected fluid additive stored in the reservoir may be returned for reapplication at a rate which varies with the quantity stored in the reservoir. 
     The applicator means for collected fluid additive may be the same as applicator means for the main supply, e.g. from an external source. However, the applicator means for collected fluid additive may be separate from any such main supply applicator means, and may be separately located within the chamber. For example, different parts of the tow may be supplied by the separate applicator means so as to improve the distrubution of fluid additive in the tow. The respective applicator means may apply fluid additive to different sides of the tow, i.e. the applicator means may be laterally spaced or with one above and one below the tow. 
     According to another aspect of the invention a method of applying fluid additive to a continuously moving tow comprises supplying fluid additive to the tow, collecting fluid additive not captured by the tow, and controlling the rate of reapplication of the collected additive to achieve a required final application rate. Preferably the collected additive is reapplied at a rate which is variable independently of the rate of main supply of additive. Preferably the fluid additive is supplied at the rate at which it is required to impregnate the tow with the additive, the collected additive being recirculated until it is captured. The collected additive may be applied at a different location, selected for example to improve the distribution of additive through the tow. 
     According to a further aspect of the invention a method of applying fluid additive to a continuously moving tow comprises supplying fluid additive to the tow, collecting additive not captured by the tow, and reapplying said collected additive at a position spaced from the collection position. Thus collected additive may be supplied at a selected position to improve distribution of additive through the tow, e.g. to make it more uniform. Thus collected fluid additive and the main supply of fluid additive may be directed at opposite sides of the tow. 
    
    
     The invention will be further described, by way of example only, with reference to the accompanying diagrammatic drawing which is a part-sectional elevation of apparatus for applying plasticiser to a tow of filter material. 
    
    
     The apparatus is asssociated with a filter rod making machine, such as a Molins PM5N, which forms filter tow into a continuous rod and cuts it into individual filter rod lengths. Means including driven rolls 40 are provided for continuously withdrawing a web of filter tow 2 from a supply (e.g. a bale) and moving it through a chamber 4 at a speed normally related to the operating speed of the rod making machine. Subsequently the tow 2 passes to the rod making machine. 
     In the chamber 4 plasticiser is sprayed onto the tow 2 from a rotating brush 6. Plasticiser is supplied to the brush 6 from a driven roller 8 which receives a surface film of plasticiser from a manifold 10. The arrangement in the chamber 4 for impregnating the tow 2 with plasticiser is similar to that disclosed in British patent specification No. 2054343, the disclosure of which is hereby incorporated herein in full. Some or all of the additional elements contained in the plasticiser chamber described in that application may be embodied in the chambr 4. Alternatively, the chamber 4 may be replaced by any means capable of receiving plasticiser and applying it to a passing tow to achieve an acceptable degree of impregnation. 
     Plasticiser is supplied to the manifold 10 through a pipe 12 containing a metering pump 14 which may, for example, be of the constant displacement type. The pipe 12 receives plasticiser from a supply reservoir 16. 
     The pump 14 is driven through an infinitely variable gearbox 18 at a speed which is dependent on the speed of the tow or the rod speed of the rod making machine (which may be directly proportional to the speed of the tow 2) so that a fixed quantity of plasticiser is supplied by the pump 14 for each increment of tow passing through the chamber 4. The value of this quantity is set by the ratio of the gearbox 18, which is controlled by a unit 20 which detects the pump speed and the speed of the input shaft to the gearbox 18 and maintains the gearbox ratio at a predetermined value. This value, which may be preset in the unit 20, may be changed when using a different type of tow and/or plasticiser. 
     The value may also by varied in accordance with signals received on a line 21 connected to the unit 20 and leading to a beta radiation mass detector (e.g. Molins MODIC) which determines the weight of finished filter rods or to a bale weighing device (e.g. Eastman Continuous Rod Weight Monitoring System, as described in Eastman publication ETB-157) which continuously monitors the reducing weight of the bale from which tow 2 is drawn. Such signals provided on line 21 can vary the value in unit 20 so that the ratio of gearbox 18 is varied to maintain the ratio of plasticiser supply rate (via pump 14) to measured filter production rate or tow consumption rate (by weight) constant. 
     Not all the plasticiser supplied to the manifold 10 through the pipe 12 is immediately captured by the tow 2 after spraying upwards by the brush 6. The uncaptured plasticiser drains to the bottom of the chamber 4 from which it may pass through a pipe 22 containing a pump 24 for delivery to an elevated cylinder 26. The bottom of the cylinder 26 is connected by a pipe 28 to the manifold 10. A variable restrictor 30 is incorporated in the pipe 28. An overflow pipe 32 leads from near the top of cylinder 26 to the reservoir 16. 
     Plasticiser drains from the cylinder 26 to the manifold 10 through the pipe 28 and supplements the plasticiser supplied through the pipe 12. The flow of plasticiser through the pipe 28 varies with the height of plasticiser in the cylinder 26. Thus if the proportion of plasticiser uncaptured by the tow increases the level in the cylinder 26 will also tend to increase so that more plasticiser is supplied through the pipe 28 to the manifold 10. Conversely if the proportion of uncaptured plasticiser decreases, so that more is retained by the tow, the level in cylinder 26 falls so that the flow of plasticiser through the pipe 28 also falls. Thus, by providing a secondary supply which varies inversely according to the rate of capture of plasticiser a more even plasticiser application rate can be achieved. In other words the secondary circuit incorporating pipes 22 and 28 operates to damp and correct excessive variations in the supply rate of plasticiser. 
     The pump 14 is normally set so that it supplies the plasticiser at precisely the rate at which it is required to be applied to the tow. Therefore, when a steady state has been achieved with the level in the cylinder 26 neither rising nor falling plasticiser is being applied at the correct rate. Some adjustment of the metering valve 30 may be initially necessary to allow a steady state to be achieved. 
     The use of a metering valve 30 to vary the rate of flow through the pipe 28 according the level in the cylinder 26 has the advantage of being simple. At the expense of some simplicity it would be possible to use a system which is potentially more accurate. For example, uncaptured plasticiser may be passed to a secondary reservoir from which it is returned to the chamber 4 by a pump which is driven at a rate depending on the quantity of plasticiser in the secondary reservoir. This alternative arrangement, including a pump 34, pipe 36, and pressure sensor 38 (for determining quantity of plasticiser in reservoir 26) is indicated in dotted lines in the drawing; the portion of pipe 28, including valve 30, by-passed by pipe 36 would be replaced by the pipe 36 and pump 34. A servo-control unit could be provided to control the pump 34 to maintain the level in the secondary reservoir at a substantially constant value. Alternatively, all uncaptured plasticiser collected from the chamber 4 could be returned directly to the chamber 4 without the intermediary of a reservoir. 
     Collected plasticiser need not be reapplied by the manifold 10. Thus, as indicated in dotted lines in the drawing, the pipe 28 beyond the valve 30 could be replaced by a section 29 having an applicator 31 (e.g. comprising a spray head) which applies the collected plasticiser to the upper side of the tow. The brush 6 and applicator 31 could be laterally spaced relative to the tow path. The section 29 and applicator 31 could be used with the pipe 36 and pump 34. 
     The control unit 20 may incorporate a counter which displays the number of displacements made by the pump 14 during a preset period (preferably measured in machine time). Thus the unit 20 may display the number of displacements of the pump 14 for every one thousand rods produced by the machine; this number is then directly proportional to the quantity of plasticiser supplied in this period (since the pump is of constant displacement). The counter may be reset every one thousand rods by a pulse derived from the input shaft to the gearbox 18.