Speed Reducer

Why speed reducer is needed?

• Electric motors operate most efficiently at high rotational speeds
• Typical maximum speed for an extruder motor is 2,000 rpm
• Screw speed this high would be detrimental to polymeric materials (eg. May result in excessive shear heating and polymer degradation)
  Reducer or gearbox
• High-speed drive motor is coupled to the low-speed screw using a reducer or gear box
• Typical reduction ratios, 10:1, 15:1 or 20:1
• Maximum screw speeds: 100 to 200 rpm
• Helical gears
• Worm gear for older or very small machines
• Forced lubrication system allows oil to cool the bearings and gears; this oil is water-cooled by a heat exchanger in high-load machines
  Advantage of gearbox
• Increased torque
• Due to high power consumption of polymeric materials, high torque is needed to maintain screw speed
• Most drive systems are designed to keep screw speed constant even if the torque requirement changes, which could be created, for instance, by a change in material viscosity

Types of drive systems

• Input shaft of the motor may be either directly or indirectly connected to the speed reducer
• Direct drive
  –Hard – coupled directly through gears
• Indirect drive systems
  –Utilize belts and sheaves to connect the motor to the speed reducer

Advantages & disadvantages

• Direct drive systems have better speed control and are more efficient, but may be more expensive and time consuming to repair in the case of a system breakdown
• Indirect drive systems allow more flexibility in motor location and are easier to repair if the problem simply requires a new belt

Belt as drive coupling

• For small- and medium sized extruders
• Speed – torque relationship:
  –P = NT (P=power, T=torque, N=screw speed)
• Larger gears increases the torque but reduces screw speed
• For larger drives (P>225 kW), the drive motor is coupled directly to the screw

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

Single Screw Extruders

• Consists of a screw in a metal cylinder or barrel



• One end of the barrel attached to the feed throat while the other end is open



• A hopper is located above the feed throat and the barrel is surrounded by heating and cooling elements



• The screw itself is coupled through a thrust bearing and gear box, or reducer, to a drive motor that rotates the screw in the barrel



• A die is connected to the “open” end of the extruder with a breaker plate and screen pack forming a seal between the extruder and die.

During extrusion…

• Resin particles are fed through the hopper, through the feed throat of the extruder, and into the extruder barrel


• The resin falls onto the rotating screw and is packed in the first section or feed zone of the screw


• The packed particles are melted as they travel through the middle section (transition or compression zone) of the screw, and the melt is mixed in the final section or metering zone.


• Pressure generated in the extruder forces the molten polymer through the die.

Components of the extruder hardware

• Drive system


• Feed system


• Screw/ barrel system


• Head/ die system


• Instrumentation and control system
  
  

Plastics extrusion

Plastics extrusion is a high volume manufacturing process in which raw plastic material is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weather stripping, window frames, plastic sheeting, adhesive tape and wire insulation.

Process Overview

In the extrusion of plastics, raw thermoplastic material in the form of small beads (often called resin in the industry) is gravity fed from a top mounted hopper into the barrel of the extruder. Additives such as colorants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper.

The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (usually around 200 °C/400 °F). In most processes, a heating profile is set for the barrel in which three or more independently controlled heaters gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer. Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated.

At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt. The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5000 psi (34 MPa). The screen pack/breaker plate assembly also serves to create back pressure in the barrel. Back pressure is required for uniform melting and proper mixing of the polymer.This breaker plate also does the function of converting "rotational memory" of the molten plastic into "longitudinal memory".

After passing through the breaker plate, the molten plastic enters the die. The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile.

The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath. Plastics are very good thermal insulators and are therefore difficult to cool quickly. Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls.

Sometimes on the same line a secondary process may occur before the product has finished its run. In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot. Once the product has cooled, it can be spooled, or cut into lengths for later use.