GRAVITY FILLING MACHINE PRINCIPLES
Gravity filling is the simplest filling method. The uncomplicated construction and operation of gravity filling machines permits them to run with a minimum of maintenance. The supply tank (more properly called the filler bowl) is the upper, central part of the machine. Filling stems are attached to the bottom surface of the bowl at each container filling point. A vent tube extends upward into the filler bowl to a point above the liquid level. To begin the filling operation, the container is raised by the platform until it contacts the filling stem. The platform then continues to raise the container against the stem, opening the filling valve. With the filling valve open, the liquid drains into the container. The air in the container flows out trough the vent tube into the space above the liquid in the filler bowl. Although the container becomes filled, the liquid continues to flow in. The excess fluid rises in the vent tube until it reaches the same height as the liquid level in the bowl. Because the vent tube extends above the bowl liquid level, there is no overflow of liquid from the container into the bowl. If the product is foamy, the foam will rise in the vent tube above the liquid level in the bowl. If it is stable foam and will not break down, it will ultimately overflow into the bowl. For this reason, gravity fillers are not often used for foamy products. At the predetermined time after the container is filled, it is lowered from its filling position, closing the filling valve. Liquid left in the filling stem is removed from the vent tube in several ways. For most applications the liquid will fill drain into the next container. For high viscosity (thick) liquids, the vent tube is usually brought out beyond the side or top of the bowl. Here its outer end can be connected to a device that applies pressure or vacuum to the liquid in the tube to assist in the liquid removal. The total differential pressure that allows the fluid to flow is caused by the gravity head pressure in the bowl. This is usually no more than two or three feet of head, or about one psi. On this basis, it can be seen that these fillers will not permit rapid filling of viscous liquids unless they have larger diameter filling stems. To accommodate the stem, the container must also have a large neck opening; otherwise machine modifications have to be made. ElGravity 150×150 Gravity Filling Machine PrinciplesAnother type of gravity filler uses electronics. It consists of a fixed liquid reservoir or bowl with open-end filling stems. The containers are conveyed on the filling line with an intermittent motion, stopping beneath the filling stems. Inside each filling stem is a ball check connected to a long rod. A pencil shaped magnetic block is attached to the top of the rod and passes through a magnetic coil. As the container moves under the stem, it is detected by a sensing device such as a limit switch or electric eye. This device stops the conveyor, and energizes the magnetic coil. The magnetic field causes the magnetic blocks to lift, raising the rod and the ball check from its seat inside the stem. The rate and amount of fill is controlled by the size of the stem orifice and time delay relay connected to the magnetic coil. Because a direct insertion filling tube is not used on this type of gravity filler the filling stem orifice must be smaller than the inside diameter of the container being filled. On small size containers a more positive means for positioning the bottle beneath the filling stem is used. Fill Height Control In addition to controlling fluid flow, control of the filling height is also important. In general filling machines that elevate the container control the fill height from the bottom of the bottle to the liquid level. The rise of the container is positive, and variations in overall container height are compensated for by greater or lesser seal compression. On rising container machines, a compression spring is often built into the tray elevating mechanism. In this case, container height variations are compensated for by the spring, and the fill height is then controlled from the top of the bottle to the liquid level. Controlling the fluid level from the top can be important if the bottle to be filled has square shoulders, because even a slight under fill is noticeable. In rising stem fillers, variation in container height is taken up by the stem itself. It is usually lowered by gravity or light spring pressure, so the fill height is controlled from the top of the bottle to the liquid level. If the product contains a volatile liquid, such as alcohol, control of the fill height is especially important. In this instance, excessive headspace could allow dangerous vapors to form and the bottle would possibly burst if it were stored in a hot warehouse. Therefore, controlling the fill height is an important function of the filling machine. Normally, a fill height tolerance of 1/32″ is acceptable. Container Control There are several devices used to control the containers coming into the filling area. Included are star wheels, worm or screw sorters, and lug chains. They can be used independently or in combination, depending on the type of container, the filling machine, and the product being placed in the container. The majority of all liquid filling machines operate as continuous filling devices. In most applications the machine has a large rotating filling head, which must be constantly supplied with containers. This is accomplished by a continuously running flat top chain conveyor feeding a star wheel or lead screw device. From here the containers are fed into the filling section. Star wheels when used alone separate the containers so they will be properly located beneath the filling stem. They can be made to handle a variety of container designs, although in some cases, the containers may have to be guided into the star wheel to ensure proper separation. Worm sorters are often used to guide containers into a star wheel. They can be short in length and only located near the machine in feed, or they may be full length of the machine’s main conveyor. The amount of container control determines the worm length. In most cases, worm sorters are very much like a wood screw; starting out small in diameter and then increasing to full diameter. A continuous pocket is formed at the root between the raised portion or crest of the thread. This pocket carries the container into its position on the filling machine by a rotating action. Because each container is different in design, worm sorters are usually made for individual applications and are not an “off-the-shelf” item. Lugged chains are normally used with inclined conveyors and semiautomatic filling machines. These machines can be either continuous or intermittent motion devices. The chain lugs are spaced to match the filling nozzles or stems. For example, if the filling stems are on four-inch centers. The chain is adjustable at the drive sprocket for timing purpose only. Position adjustments are usually made by moving the filling heads. Whatever method is used for container control, it is an important part of proper machine operation.