Drive System

• Motor
• Speed reducer
• Thrust bearing

Extruder drive motors

• Source of power to turn the screw
• Turn the screw
• Minimize the variation in screw speed
• Permit variable speed control (typically 50 to 150 r/min)
• Maintain constant torque
• Relatively large due to high power consumption by the polymer around the screw

Sources of power consumption

• Melting of solids via frictional heat generation
• Conveying of high viscosity molten polymer along the barrel
• Pumping of high viscosity molten polymer through the die restriction

Rule of thumb for motor size Rule of thumb for motor size

Motor power (HP) =~ throughput (lb/hr) / 5

Factors in drive motors selection

• Base speed variation
  –Based on the maximum speed available for the motor
• Presence or absence of brushes
• Cost

Speed variation of drive motor

• Drive motor speed variation does not change when the speed is reduce
• Screw speed is generally 5 to 10% of the motor speed, varies more than the motor speed
  –0.1% base – speed variation on a motor with a max speed of 1750 r/min produces a speed variation of ±1.75 r/min
  –If max screw speed is 117 r/min (reduction ratio of 15:1), the screw speed variation is ±1.75 r/min or 1.5%

Types of drives

• Alternating current (ac)
  –AC adjustable frequency drives
• Direct current (dc)
  –DC silicon control rectified (SCR)
• Hydraulic
  –Normally used in injection molding machine to develop clamp tonnage

DC SCR Drive

• Regulate speed through voltage control
• A solid – state dc rectifier connected to a dc motor
• Base speed ~ 1%, reduces to 0.1% when a tachnometer is added to the drive
• Very reliable, can handle high starting torque, can maintain a constant torque through a speed range of 20:1, and easy to maintain
• The drives have brushes, limited to noncorrosive polymers

AC Adjustable Frequency Drive

• Consists of a solid – state power supply connected to an ac “high – efficiency” or “vector” motor
• Power supply converts 3-phase ac line voltage to variable voltage dc power and then back to controlled ac frequency
• Voltage-to-frequency ratio is adjusted to provide constant torque from the ac motor, speed-torque characteristics can be optimized by varying the voltage-to-frequency ratio
• Provides constant torque up to base speed
• Have operational ranges of 1000:1 with an encoder and 100:1 without one
• Give base speed variation of 0.01%, have higher power factor (than dc SCR) at low speeds, and are brushless
• More expensive than dc SCR drives