Abstract:
An apparatus for blending polymer powder and pigment inside of a blender that heats the polymer through agitation of the blending process. The apparatus includes a control scheme that monitors the condition of the powder and discharges the powder from the blender just before the powder reaches its melting point. The apparatus monitors the flowability of the powder as well as the temperature. As an additional method of controlling the temperature inside of the blender, the blending speed can be varied by virtue of a variable speed motor that is attached to the blender.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to the field of polymeric powder conditioning. More specifically, the present invention relates to an apparatus and system for polishing and coloring powdered resins, including polymer powders of all types.  
           [0003]    2. Description of Related Art  
           [0004]    Pulverized resins, particularly resins intended for use in thermoplastic molding and shaping operations, are typically sold as fine powders, with particle sizes between 200 to 500 microns. These polymer powders are further conditioned prior to use to improve their flowability and optionally add pigment. The conditioning process usually involves stirring or blending the powder (with or without pigment) inside of a large drum with rotating rods having spaced blades. The rotation of the blades not only agitates and mixes, but also heats the powder, sometimes approaching the melting point of the resin. Because it is undesirable to melt the powder inside the drum, the powder is monitored, usually manually. One of the many problems that exist with manual observation of this process is the uncontrolled results that can often occur when different personnel perform the monitoring process. Even the same person can produce different results on different days.  
           [0005]    When the polymer powder conditioning includes pigmenting, the traditional methods include admixing the polymer powder with the pigment which electrostatically clings the pigment to the outer surface of the polymer powder particles. However, electrostatically attaching the pigment to the polymer has some disadvantages. Pigment applied in this manner can be easily removed by contact with solvents or friction caused handling the pigmented powder. Further, admixing pigment with the polymer powders results reduced structural strength of objects formed from those polymers.  
           [0006]    Therefore, there exists a need whereby polymer powders can be conditioned to improve their flowability and ability for pigmenting in a repeatable fashion without the threat of overheating and thus melting the polymer. A need also exists for a pigmented polymer that does not easily shed its color and whose structural integrity is not compromised by coloring.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    Disclosed herein is a method and apparatus for conditioning thermoplastic resin. According to this invention, set point data is pre programmed into a controller, where the set point data comprises flowability data and powder outletflow data. During operation a specified amount of powder and a specified amount of pigment is directed into a container. The contents of the container is continuously mixed which heats the powder that is inside of the container. The powder has a flowability value that changes in proportion to changes in powder temperature. During the mixing process, the controller directs the system to discharge a fraction of the powder within the container onto a flowability meter where the flowability of the powder being discharged from the container is measured. The measured value of the flowability is then transmitted to the controller where it is compared to the flowability data pre programmed into the controller. The powder remaining in the container is discharged from the container when the evaluated flowability of the powder being discharged from the container is equal to the flowability data pre programmed into the controller.  
           [0008]    The pigment impregnates the outer surface of the polymer powder particles and adheres in such a way that the pigment cannot readily be separated from the powders.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0009]    [0009]FIG. 1 provides a schematic view of the Continuous Blending Apparatus of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    [0010]FIG. 1 depicts a schematic view of a Continuous Blending Apparatus 1 according to this invention. In general, the Continuous Blending Apparatus 1 comprises a powder feed  10 , a pigment feed  20 , a continuous blender  30 , a controller  40 , a motor  55 , and a flowability meter  50 . The continuous blender  30  is preferably an apparatus housing one or more drums having stirrers mounted therein for mixing together the drum contents. The powder feed  10  is operatively attached to the continuous blender  30  for transferring powder from the powder feed  10  to the continuous blender  30 . It is preferred that the powder feed  10  consist of a stationary tank raised above the continuous blender  30  to facilitate transfer of the contents of the powder feed  10  to the continuous blender  30 . Powder enters the continuous blender  30  from the powder feed  10  through the powder feed inlet  31 ; the powder flow is controlled by the powder feed control valve  11 . It is envisioned that the powder generally used in conjunction with this invention comprise polymeric powders such as polyolefins including polyethylene, polypropylene, and other like materials including all thermoplastic materials. However, the powder processed by the invention disclosed herein can include any powder whose flowability and ability to be pigmented is enhanced by heating.  
         [0011]    Likewise, the pigment feed  20  is also preferably situated above the continuous blender  30  for ease of entry of the pigment into the continuous blender  30 . The pigment feed  20  can comprise any type of container suitable for storing and transferring pigment without undue spillage or leakage. Pigment enters the continuous blender  30  from the pigment feed  20  via the pigment feed inlet  32  and control of the pigment flowing from the pigment feed  20  to the continuous blender  30  is achieved by the pigment feed control valve  21 .  
         [0012]    Mechanically connected to the continuous blender  30  is a motor  55  that provides rotational energy to the stirrers (not shown). It is preferred that the motor  55  be electric and have the capability to operate at more than one rotational velocity. Therefore, shown connected to the motor  55  is a motor speed controller  56  in conjunction with a power supply  57 . The motor speed controller  56  comprises a variable resistor for varying the electrical current supplied to the motor  55 , however one skilled in the art can readily appreciate that many alternatives exist for varying the rotational speed of the motor  55 . The power supply  57  can provide either alternating or direct current.  
         [0013]    A powder outlet  33  is included on the continuous blender  30  which provides for the transfer of the powder out of the continuous blender  30  to the flowability meter  50  via a transfer line. The flow of powder from the powder outlet  33  is controlled by the continuous blender outlet control valve  34 . The temperature of the powder exiting the continuous blender  30  is monitored by the temperature element  43 , and possibly others, which can be attached to the outer or the inner surface of the transfer line. Accordingly, the temperature element  43  can be a thermocouple adhered to the transfer line, or a probe inserted through the transfer line either at the outer edge of the powder flow or in the powder flow stream. Alternatively a temperature probe can be situated in various positions in the turbulent powder flow within the continuous blender  30 .  
         [0014]    The flowability meter  50  measures the ability of the powder to flow (the powder flowability) as it exits from the continuous blender  30 . The preferred flowability units are based on ASTM D 1895-96 which are based on the period of time required for 100 grams of powder to pass through a 10 mm funnel orifice. While a typical powder takes 25-35 seconds to pass through the orifice, a polished powder could be less than 20 seconds. One method to determine flowability of the present invention involves measuring the mass flow rate of the powder exiting the continuous blender  30  as an indicator of how well the powder is flowing. A low mass flow rate would intimate a low flowability, whereas a high mass flow rate indicates high flowability. The flowability meter  50  illustrated in FIG. 1 utilizes the correlation of powder mass flow rate to the flowability by detecting the powder mass flow rate with a load cell  51 . The load cell  51  creates a signal in response to the instantaneous mass of the powder flowing across the flowability meter  50 , that signal is transmitted to the controller  40  where the signal is processed into flow data. It is appreciated that one skilled in the art can develop a numerical scale whereby signals received and processed by the controller  40  from the flowability meter  50  provide a reading reflecting the flowability of the powder flowing across the flowability meter  50 . One of the many advantages of monitoring the powder flowability in addition to only powder temperature is that the melting temperature of the powder can change from batch to batch and grade to grade.  
         [0015]    After the powder flows across the flowability meter  50  it is deposited onto the fluid bed  60  for cooling. The fluid bed  60  is a fluidized air driven bed unit which can lower the powder temperature by 30° C. or more. Traditional powder cooling techniques involve passing the warm powder through an air stream, however the additional use of air in this environment increases the chances for color contamination of similarly located processes or devices. The powder sieve  65  illustrated is a simple vibratory sieve device used to screen out any melted lumps or hairs of powder that may have formed during the heating process. Located under the powder sieve  65  is a selector valve  70  that can alternatively direct powder flow from the powder sieve  65  to the cooled powder bin  80  or the off spec bin  90 .  
         [0016]    Before a particular batch of powder is conditioned, the controller  40  is pre programmed with set points that pertain to the batch being conditioned. As is well known in the art, the controller  40  can consist of a single microchip or an entire computer system, both of which are capable of receiving and storing data as well as transmitting signals to process equipment. The set points can comprise maximum powder temperature, flowability values, flow control ranges, and current/voltage values. These set points will vary depending on powder material, pigment type, batch quantities, and desired product. Therefore, the operation of preprogramming set points into the controller  40  will be determined by process operators and it is appreciated that it is obvious to those skilled in the art.  
         [0017]    In operation, the start up procedures involve a warm up step where powder from the powder feed  10  is dosed into the continuous blender  30  until a specified amount has entered the continuous blender  30 . The dosing procedure can be accomplished by programming the controller  40  to actuate the powder feed control valve  11  open until the desired amount of powder has flowed into the continuous blender  30 . The amount of powder added to the continuous blender  30  will depend on the size of the continuous blender  30  and the powder used. However, the amount of powder allowed to flow across the powder feed control valve will be apparent to one skilled in the art.  
         [0018]    While the powder is being dosed into the continuous blender  30 , pigment is added into the continuous blender  30  with the powder. The pigment can be in either powder form, or can be a liquid dispersed pigment. During this time the stirrers (not shown) continuously rotate and thoroughly mix the powder with the pigment. The stirring action not only mixes the powder with the pigment, but also adds heat to the powder because of friction forces produced by the mixing. As the powder is heated, the powder particles become softer which enhances pigment adhesion to the powder particles. While the pigment is mixed with the powder, the powder is constantly monitored to ensure that its temperature does not reach its melting point. Obviously, because the powders composed of different materials will in all likelihood have different melting points, the melting point for each specific powder must be known and the memory of the controller  40  must be adjusted accordingly. Flowability and temperature values of the powder both indicate when the powder is close to melting. One of the many advantages of the present invention provides continuous monitoring of the flowability and the temperature of the powder being colored in the continuous blender  30 . During mixing, small doses of powder are released from the continuous blender  30  through the continuous blender outlet control valve  34 . Actuation of the continuous blender outlet control valve  34  can be directed by signals from the controller  40  in response to set points pre programmed into the controller  40 . As described above, the flowability of the small doses of powder exiting the continuous blender  30  is measured by the flowability meter  50  and transmitted to the controller  40 . When the flowability data monitored by the controller  40  indicates that the powder is approaching its melting point, the controller  40  is pre programmed to activate the continuous blender outlet control valve  34  to empty the powder from the continuous blender  50 . Alternatively, the temperature of the powder can be tracked by the temperature element  43  in conjunction with the controller  40 . The temperature element  43  transmits a signal to the controller  40  representative of the powder temperature, the controller is designed and configured to convert the signal from the temperature element  43  and compare it to set point data stored in the controller  40 . When the controller  40  senses that the powder is close to melting, it can direct the continuous blender outlet control valve  34  open to empty the powder from the continuous blender  30 .  
         [0019]    If desired, the continuous blending apparatus 1 can affect the powder temperature in the continuous blender  30  by altering the rotational speed of the motor  55 . The controller  40  should be capable of being programed to adjust the motor speed, via its cooperation with the motor speed controller  56 , based on data input from either the flowability meter  51  or the temperature element  43 . The option of adjusting motor speed provides additional flexibility in establishing an optimum powder mixing temperature.  
         [0020]    Because some of the powder exiting the continuous blender  50  has not been adequately colored, some of the powder will be repigmented or reblended before it is passed on for further processing. For example, the powder that is dosed out of the continuous blender  50  before the powder has reached a satisfactory temperature for pigment adhesion will be considered off spec. This off spec powder is forwarded by the selector valve  70 , under control by the controller  40 , to the off spec bin  90 . Likewise, powder that has been successfully pigmented will be forwarded by the selector valve  70  to the cooled powder bin  80  where it is stored for further processing.  
         [0021]    Mixing the pigment with the powder, when the powder is close to its melting point, glues the pigment to the outer surface of the individual powder particles. Because the pigment is now actually embedded or partially encapsulated in the polymer, the pigment adheres to the polymer and will not easily be removed from the polymer, even with solvents. Other added benefits of this process include increased color strength and intensity of the pigmented polymer, and added structural strength of the objects formed by polymers pigmented by the process. Additionally, due to the enhanced coloring effect achieved by the process disclosed herein, less pigment is now required than what is required in prior art processes.  
         [0022]    While the control valves (powder feed inlet control valve  11 , pigment feed control valve  21 , and continuous blender outlet control valve  34 ) are disclosed as typical gate type valves with diaphram actuated controls, these can consist of other types of control devices obvious to those skilled in the art. Further, existing polymer processing apparatus that are modified to monitor flowability data of powder exiting a powder conditioning device; and use that data to determine when conditioning is complete, are considered a part of this invention. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.