Abstract:
Extruder feed system. The system includes a pair of spaced-apart, internally and oppositely threaded rotatable elements for receiving and engaging a plastic filament material. An electric motor rotates the rotatable elements in opposite directions thereby to drive the filament into a liquefier chamber for subsequent discharge through a nozzle. The system provides very accurate layer-by-layer build up.

Description:
[0001]    This application claim priority to provisional application Ser. No. 61/863,110 filed on Aug. 7, 2013, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to an extruder, and more particularly, to an extruder used in an additive manufacturing device employing a screw drive. 
         [0003]    Additive manufacturing devices such as 3-D printers build up an object layer-by-layer by extruding a filament material onto a support surface. The quality of the object produced depends in large measure on tight control of the flow rate of filament material through the extruder in conjunction with control of the X-Y position of the extruder head as it traverses an area to build up a layer. 
         [0004]    A prior art extruder system is shown schematically in  FIG. 1 . Filament material  10  passes through a pinch roller feed system  12  that drives the filament material  10  downwardly into a liquefier chamber  14 . Thereafter, filament material is discharged through a nozzle  16  onto a scaffolding  18 . The pinch roller system  12  engages the filament material  10  on each side as it drives the filament material into the liquefier chamber  14 . The driving forces that can be achieved with the arrangement in  FIG. 1  is limited. Further, the arrangement shown in  FIG. 1  is not as accurate as desired because of step size limitations in motor systems driving the pinch rollers. 
         [0005]    It is also known to use an internally threaded nut to drive a filament into a liquefier chamber. In this case, the filament passes through an internally threaded nut which, upon rotation, drives the filament material linearly. However, the nut rotation puts an unwanted torque on the filament causing it to distort as it is driven linearly. 
         [0006]    An object of the present invention is a screw drive employing counter-rotating elements to substantially eliminate the unwanted torque while driving the filament into the extruder. 
       SUMMARY OF THE INVENTION 
       [0007]    The extruder feed system according to the invention includes a pair of spaced-apart, internally and oppositely threaded rotatable elements for receiving and engaging a filament material. A motor is provided for rotating the rotatable elements in opposite directions, thereby to drive the filament into a liquefier chamber for subsequent discharge through a nozzle. In a preferred embodiment, the system includes a gear train driven by the motor to rotate the rotatable elements in opposite directions. A suitable motor is a stepper motor or a DC motor. The filament material may be plastic. 
         [0008]    In a preferred embodiment, they gear train includes beveled gears driven by the motor. The gear train may include a belt or cable driven with pulleys. 
         [0009]    In yet another embodiment, the system of the invention further includes a control loop for controlling power of the motor, thereby to control the filament material extrusion rate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]      FIG. 1  is a cross-sectional view of a prior art additive manufacturing extruder system. 
           [0011]      FIG. 2  is a schematic illustration of a pair of counter-rotatable hex nuts for driving a filament material. 
           [0012]      FIG. 3  is a cross-sectional view of an embodiment of the invention disclosed herein utilizing a motor, a flow controller and bevel gears driving the counter-rotating bevel gears. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    With reference to  FIG. 2 , filament material  10  is seen passing through the interior of first and second hex nuts  20  and  22 . The hex nut  20  is internally threaded in, for example, a right-handed thread pattern. Similarly, the hex nut  22  is infernally threaded to have the opposite direction for the threads, such as a left-handed thread pattern. It is preferred that the diameter of the filament  10  be slightly oversized with respect to the tapped hole through the hex nuts  20  and  22 . As can be seen in the figure, the hex not  20  is rotated in a counterclockwise direction and the hex nut  22  is rotated in a clockwise direction. Because the threads of the counter rotating hex nuts  20  and  22  are oppositely directed, the filament material  10  is driven downwardly in  FIG. 2 . 
         [0014]    Importantly, because the hex nuts  20  and  22  are counter-rotating, material distortion resulting from torque between the two hex nuts is substantially eliminated as the counter-rotating nuts balance out the torque effects. 
         [0015]    An embodiment of the present invention is shown in  FIG. 3 . A frame  30  supports for rotation bevel gears  32 ,  34  and  36 . A motor  38  under the control of a flow controller  40  rotates the bevel gear  32 . The bevel gear  32  operatively engages the bevel gears  34  and  36  driving these bevel gears in opposite rotational directions. As will be appreciated, the interior of the bevel gear  34  is threaded in a first sense, such as right-handed, and the bevel gear  36  is internally threaded in the opposite sense such as left-handed. When the motor  38  is activated under control of the flow controller  40 , the filament  10  will be driven into a liquefier chamber as shown in  FIG. 1 . The motor  38  may be a stepper motor or a DC motor. The motor may be a pneumatic motor, internal combustion engine or an AC motor. The arrangement of the motor  38  shown in  FIG. 3  assures that the flow rate of filament  10  material through the system can be precisely controlled. 
         [0016]    Those of skill in the art will recognize that separate motors could be used to drive the rotatable elements if desired. It is also noted that the space between the bevel gears  34  and  36  should be made small to minimize distortion of the filament passing through the counter-rotating bevel gears. The gap in  FIG. 3  is exaggerated for clarity. 
         [0017]    The inventors herein have determined that driving the bevel gears  34  and  36  at the same speed in opposite directions isn&#39;t sufficient to ensure a constant extrusion rate due to variabilities in the diameter of the filament  10  and other physical inconsistencies. The inventors have analytically determined that there is a direct relationship between the extrusions rate and the input electrical power to the motor  38  using the screw drive of the invention. In particular, the inventors have determined that the extrusion rate Q=kIV. That is to say, filament flow rate Q is linearly proportional to power (IV) on the motor  38 . As an example, if one wishes to have a constant flow rate, the electrical power to the motor  38  is held constant (that is to say, the product of motor current (I) and motor voltage (V) is held constant). To increase or decrease the flow rate, the motor  38  voltage is controlled via a PWM control on the motor  38 . Thus, filament flow rate is controlled by controlling power to the motor  38 . The flow controller  40  may include a conventional control loop employing PID control for example. 
         [0018]    It is recognized that modifications and variations of the present invention will be apparent to those of ordinary skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims.