Abstract:
A granulator for reducing the size of plastic particles to facilitate the removal of contaminants therefrom. The granulator includes an upper chamber and a lower chamber defined by an exit screen. The upper chamber includes a rotary shear and an air handling system for creating a pressure drop across the screen to facilitate the transit of particulate material therethrough.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Granulators are employed in the plastics industry in particular to reduce the size of the plastic particles to facilitate the removal of contaminants therefrom during the production of recycled resin such as, for example, polyethylene terephthalate PET. 
     2. Description of the Prior Art 
     Typically, the recycling process involves a two-stage process, although the process can be carried out in a single stage. The first stage involves the grinding of the material to be recycled to a nominal size of three-eighths inch (⅜″). This is accomplished by introducing the material to be recycled into a chamber of the granulator which is provided with rotating rows of knives that continue to cut the material being processed until the material is small enough to exit the first through holes in a screen in the bottom of the first stage of the process. The diameter of the holes in the screen is ⅜″. 
     It has been found difficult to reduce the diameter of the flake being produced by merely reducing the size of the holes in the exit screen. Generally, the requirement for a smaller particle size has resulted in a reduction in the size of the holes in the exit screen, as well as an increase in the rotational speed of the cutting mechanism. These efforts have caused a number of production problems including an increased production of dust and fines which cannot be recycled. The dust and fines tend to clog the exit screens and require increased operating energy to operate the cutting mechanism. The increased usage of energy results in a build-up of heat energy which must be dissipated in order to avoid damage to the attendant equipment. It will be understood that a number of factors come into play when grinding plastic material to a small particle size. Traditional grinding requires the weight of the flake (material being treated), airflow through the granulator, and an external air moving system under the exit screen to perform a push-pull operation to get the material through the holes in the screen as quickly as possible. In the event the transit time of the material being processed increases, the material remains in the cutting chamber for longer periods of time resulting in the material being continuously cut into smaller and smaller pieces. The longer it takes for the material to transit through the cutting chamber and the exit screen, the lower the production rate, increased wear and tear on the cutting knives, and more dust and fines (unusable product) are produced. Conventional granulators and grinders and associated screens are not capable of achieving improved results because the smaller the particles become, the less pliable they become. These less pliable particles are not able to readily pass through the small holes in the exit screens. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to produce a granulator capable of producing flake of a diameter of four (4) millimeters or less to facilitate the decontamination of the material by removal of contaminants therefrom. 
     Another object of the invention is to produce a granulator capable of producing small diameter flake at production rates superior to those of the prior art. 
     Another object of the invention is to produce a granulator for comminuted plastic material in the recycling process whereas the wear and tear in the associated cutting knives is greatly minimized. 
     Still another object of the invention is to produce a granulator wherein there is reduced stress on the cutting mechanism and associated exit screen. 
     Another object of the invention is to produce a granulator having a reduced quantity of dust and fines. 
     The above, as well as other objects of the invention may be surprisingly achieved in a granulator including rotary knives, associated bed knives, a rotor for carrying the rotary knives for sequential shearing action between the rotary knives and the bed knives, an exit screen allowing the passage of comminuted material less than four (4) millimeters diameter, and an air handling system creating a pressure drop across the screen to facilitate the passage of the processed material therethrough, wherein the screen is provided with a plurality of spaced apart holes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the invention will become readily apparent to those skilled in the art from reading the following detailed description of the invention when considered in the light of the accompanying drawings, in which 
     FIG. 1 is a schematic illustration of a system incorporating the features of the invention; 
     FIG. 2 is a fragmentary perspective view of the rotor carrying the associated rotary knives as illustrated in FIG. 1; 
     FIG. 3 is a perspective view of the exit screen illustrated in FIG. 1 showing the disposition of the plurality of exit holes therein; and 
     FIG. 4 is a fragmentary sectional view taken along line  4 — 4  of FIG. 3 showing the cross-section of the exit hole. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, there is illustrated in FIG. 1 a schematic illustration of a system utilizing the granulating principles of the present invention. More specifically, FIG. 1 shows a granulator  10  for comminuting material being processed. Generally, plastic material, specifically polyethylene-terephthalate (PET), is reground from its molded shapes into three-eighth inch (⅜″) nominal flake for reuse. However, some applications require a smaller particle size in order to increase the surface area of the plastic material available for processing. The illustrated system is typically capable of achieving the smaller particle size. 
     The granulator  10  includes an outer housing  12 . The housing  12  has an inlet  14  coupled to a supply of plastic material to be processed through an associated feed pipe  16  provided with a driven auger  18  for feeding material to the granulator  10 . Typically, the material being fed to the inlet  14  by the auger  18  is plastic material which has been ground to a ⅜″ nominal flake size. The material being treated is introduced into the granulator  10  through the inlet  14 . 
     The granulator  10  is further provided with an exit screen  20 , bed knives  22 , a plurality of rotor knives  24  carried by radially extending arms  26  mounted on a rotor shaft  28 . The rotor  28  is generally driven by an electric motor (not shown). Although the rotor  28  is illustrated as having five arms  26  in the preferred embodiment, it will be understood that there may be more or less arms. It has been found that rotor knives  24  and associated bed knives  22  of the type manufactured and sold by Nelmor Company, Inc., North Uxbridge, Mass. may be employed to satisfactorily achieve certain objectives of the invention. It will be noted from an examination of FIG. 2, the rotor knives  24  extend the full length of the knive assembly and are mounted at a slight angle in respect of the longitudinal axis of the rotor shaft  28 . Such disposition of the rotor knives  24  causes the cutting edges to cooperate with the cutting edges of the bed knives  22  to create or produce a shearing action therebetween. This shearing action tends to overcome the beating or hammering of the material being treated. The material being treated is truly cut or sheared, greatly assisting in the reduction of the amount of dust or fines which might otherwise be produced. The reduction in the dust or fines greatly reduces the dust causing housekeeping problems and increases the productivity of equipment. The presence of dust and/or fines causes considerable problems in the efficient transfer of the material being treated, resulting in unnecessary energy consumption and waste. 
     The screen  20  is located directly below the knives  22 ,  24 . The screen  20  is in the shape of a half cylinder  30  of sheet metal containing a large number of perforations or holes  32  as clearly illustrated in FIGS. 3 and 4. The screen  20  is positioned below the knives  22 ,  24  to effect the size of the granulator particles in the cutting area prior to passage through the holes  32  in the metal sheet  30  of the screen  20 . 
     The holes  32  formed in the sheet metal  30  of the screen  20  are preferably formed with a predetermined diameter from the inlet end, closest to the cooperating knives  22 ,  24 , to a point typically past the midpoint of the hole. The outlet ends of the holes  32  are in certain embodiments flared outwardly. The flared configuration may be achieved by counter-sinking the outlet ends of holes  32 . It was found that satisfactory results are achieved by maintaining the diameter of the holes  32  greater than the thickness of the sheet  30  from which the screen  20  is fabricated. Such configuration maintains the structural integrity of the screen  20  while facilitating the flow of processed particulate material therethrough. In the preferred embodiment of the invention, the diameter of the holes  32  at the inlet thereof is four (4) millimeters. 
     The screen  20  divides the interior of the housing  12  into an upper cutting chamber  36  and a lower screen chamber  38 . 
     In order to facilitate the movement of the feed, stock is delivered into the cutting chamber  36  from the inlet pipe  16  through the inlet  14 , and to cause some turbulence of the material being processed, air under pressure is introduced through pipes  40  and  42 . The pressurized air introduced through the pipes  40  and  42  is preferably a high volume low-pressure delivery. The high volume low pressure delivery of air to the cutting chamber  36  effectively causes movement of the feed stock being treated to facilitate the transit thereof through the cooperating cutting knives  22  and  24 , and thence, through the exit screen  20 . 
     The small particles exiting the screen  20  enter the screen chamber  38  and are drawn out therefrom through an exit pipe  44 , to a storage bin  46 , through a cyclone type separator  48  by an exhaust fan  50 . It will be appreciated that the transit air moved by the fan  50  to a large extent enters the system through a filtered inlet  52 . 
     The cooperative effect of the introduction of pressurized air into the cutting chamber from the pipes  40  and  42 , and the air introduced into the system through the filtered inlet  52  creates a pressure drop across the screen  20  which can be controlled to effectively maximize the production of small plastic particles for use in the production of pelletized and recycled post consumer PET for direct sale and reuse back into consumer packaging applications. 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be understood that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.