1. Field of the Invention
This invention relates to a process of determining the weight rate at which plastic granules are received by an extruder from a feed container having an outlet which opens into the inlet of the extruder, in which process granules are continuously fed to a supply container and consecutive charges of granules are intermittently fed to the feed container, the feed container is weighed before or at the beginning of or after or at the end of its filling operation, the time is measured which between the filling operations is required to withdraw the material in the amount which has been determined by the weighing operations, and the flow rate of the material is calculated from the amounts of material which have been received from the feed container in the measured intervals of time. The invention relates also to an apparatus for carrying out that process.
2. Description of the Prior Art
Extruders for making, e.g., flat plastic sheeting are fed with plastic granules and/or regranulated plastic waste or shredded plastics through cyclonelike containers, from which the material is then withdrawn in dependence on the feed rate of the extruder. In such operation it is desired to determine the rate at which material is consumed in order to gain information on the output of the extruder or on the weight of the extruded plastic sheeting per linear meter. For instance, information on the weight of the extruded plastic sheeting per linear meter will permit a determination whether or not the thicknesses of the extruded sheeting are in the desired tolerance range. That determination is significant from an economical aspect because the plastic material employed is expensive and a production of sheeting having an excessive thickness will not only render the subsequent processing more difficult but will constitute an undesired waste of material.
A process of the kind described first hereinbefore will now be explained with reference to FIG. 2 of the drawing. In that process granules of different plastics are supplied through two metering devices 18, 19 to a cyclonelike container 16, which is provided with two level indicators 16' and 16". The upper level indicator 16' is associated with an upper limiting level and the lower level indicator 16" is associated with a lower limiting level. The metering devices 18, 19 consist, e.g., of screw feeders for withdrawing granules from supply containers and are turned on as soon as the lower level indicator 16" has indicated that the charge in the container 16 has decreased to the lower limiting level. The metering devices will be turned off as soon as the upper level indicator 16' has indicated that the upper limiting level has been reached. The output pipe of the container 16 protrudes into but does not contact a feed container 6. The outlet pipe contains a valve 12 for a controlled opening and closing of said pipe. By means of a balance 5, the feed container 6 is suspended from a bracket 3, which is connected to the extruder 1. Granules flow continuously from the feed container 6 through the inlet opening 8 into the extruder 1. The outlet pipe of the feed container 6 opens into the inlet opening 8 without contacting the extruder 1. As soon as the weight of the feed container 6 inclusive of its charge has decreased below a predetermined lower limit, the valve 12 is opened until a preset desired weight has been reached. Thereafter the valve 12 is closed. By a measurement of the time in which the amount that has been fed to the feed container 6 is withdrawn therefrom it is possible to determine the flow rate of granules through the extruder 1 in kg/ unit of time.
In the operation of that known apparatus, difficulties will arise if the cyclonelike container 16 is supplied not only with granules but also with marginal strip material, which has been obtained, e.g., in that the extruded sheeting has been trimmed at its edges and in a shredded form is recycled. That recycled marginal strip material is continuously obtained at a substantially constant rate as the extruded flat sheeting is continuously trimmed. But during the operation of the extruder the metering means 18, 19 for feeding granules to the container 16 will be turned off as soon as the level indicator 16' indicates that the upper limiting level has been reached. The metering devices 18 and 19 will remain turned off until the lower level indicator 16" indicates that the charge in the container 16 has been reduced to the lower limiting level. During that time the metering devices 18, 19 do not supply granules but marginal strip material is continuously supplied to the container 16 so that a cushioning layer of marginal strip material is formed above the descending granule layer. As a result, the marginal strip material cannot uniformly be mixed in the container 16 with the granules which have been supplied but in dependence on the different bulk densities and the different shapes of the material the withdrawing action of the screw conveyors and, as a result, the feed rates thereof will be different at different times.