The production of products from extruded or blown plastics generates plastic scrap. It is conventional to grind or chop the scrap and mix it with virgin or base plastics so that the scrap can be reused in the production of new products. Apparatus suitable for mixing virgin and scrap materials is disclosed in U.S. Pat. No. 4,108,334.
One of the problems encountered in the mixing and feeding of virgin and scrap plastics is that of maintaining a selected quantity or mass of the mixture delivered to the extruder. This problem is due in large part to variations in the densities of the materials being mixed. The density of the virgin plastic is relatively easy to control because it commonly is composed of rigid, flowable pellets of substantially uniform consistency and size. However, the scrap can, and usually does, consist of particles of greatly differing size, form, and densities. As a consequence, the bulk density of the mixture and the mass thereof discharged to an extruder may vary within wide limits.
Variations in the quantity of material delivered during a given period of time to an extruder for extrusion is undesirable for a number of reasons, among which is the inability to maintain uniformly filled spaces between adjacent vane convolutions of a helical, extruder feed screw. For example, if a mixture having a given bulk density is fed at a constant rate to fill the space between adjacent convolutions of an extruder screw, a reduction in the bulk density of the mixture can cause such space to be underfilled. This can cause non-uniform pressure at the discharge outlet of the extruder and the production of so-called gels, i.e., unmelted plastic particles in the product. The presence of gels is undesirable inasmuch as they contribute to the generation of more scrap.
The problems in using a mixture of virgin and recycled scrap plastics materials have been recognized heretofore and a number of solutions advanced. One proposed solution involves slaving the rate of delivery of the virgin/scrap mixture to the speed of rotation of a variable speed extruder feed screw. As the speed of the extruder feed screw changes, the speed of the scrap delivery to the system is correspondingly changed. One difficulty with this proposal is that it does not necessarily respond to variations in the bulk density of the virgin/scrap mixture, nor does it enable a consistent feed force to be maintained on the mixture as it is delivered to the extruder. Consequently, the quantity of the mixture actually delivered to the extruder may experience wide variations.
Another proposal to solve the problems of feeding a mixture of virgin and scrap materials to an extruder is to premix the rigid pellets and the fluffy scrap with one another and convey the mixture to an upright, conical hopper having a vertical, motor driven feed auger which forces the mixture from the hopper into the extruder inlet. This proposal requires the use of a special, electric torque control on the feed auger drive motor, such control being operable to maintain a preset voltage on the drive motor to enable the speed of the motor to vary in response to changes in its torque. This system requires not only a special motor and drive assembly, but also an expensive torque responsive device. In addition, the time and equipment required to premix the virgin and scrap materials represents substantial expense.
An object of this invention is to provide a method and apparatus for feeding a mixture of different density virgin and scrap plastic materials to an extruder and wherein the feed rate of the mixture is varied in response to changes in the bulk density of the mixture to maintain substantially uniform the quantity of the mixture delivered to the extruder per unit of time, thereby overcoming the disadvantages of methods and apparatus heretofore proposed for making use of a blend of virgin and scrap materials.