ME-542 Manufacturing with Non-metallic Materials

EXTRUSION OPERATION MANUAL

By Brian Austin, Soraya Ghamaty, and Richard Haslim

 

TABLE OF CONTENT

TITLE PAGE #

I. INTRODUCTION 1

II. SAFETY 2

III. PRINCIPAL OF EXTRUSION 5

IV. OPERATING INSTRUCTIONS

A. DIE SELECTION 8

B. MATERIAL 8

C. MACHINE SETUP 9

D. OPERATION 10

E. SHUTDOWN 10

F. TROUBLE SHOOTING 11

V. RECOMMENDATIONS

A. ADDITIONAL EQUIPMENTS 12

B. MAINTENANCE 12

VI. REFERENCE 13

VII. APPENDIX 14

I. INTRODUCTION

Extrusion is commonly used for the production of continuous lengths of thermoplastics with a wide variety of profiles. These forms including sheet, rod, and tubes. Granules, pellets or powder are fed through a hopper into the heated barrel of the extruder then converted to melt. The melt is forced through a die of desired geometry by a rotating screw. The melt is cooled and cut to desired lengths. The figures on the following pages show an extruder and some of its components (see figures I-1 and I-2, on the following pages).

 

II. SAFETY

The extruder has the potential to cause harm people in the vicinity of the extruder when in use. Please observe the safety steps below:

1. The extruder has hot surfaces, including at the die. Very hot polymer exiting the die can stick to skin and cause serious burns.

2. Wear gloves, safety glasses and closed-toe shoes.

3. Never get close to the die, where hot polymer may, at times, sputter out unexpectedly.

4. Never remove the die while the extruder is operating or the indicated pressure is not zero.

5. Never put anything other than polymer in the hopper or feed zone. Never put a screw driver in the feed zone, for example. (If the screw is stuck there are other methods of getting the screw to rotate freely)

III. Principles of Extrusion, Controls & Heat Transfer

The extruder is a machine used to form polymer in a continuous manner, see figs. I-1,I-2. The raw material, typically in the form of pellets or chips, is placed in a hopper that feeds it into the barrel. This barrel has a screw driven by a motor and gear box. The screw transports and mixes the material along the length of the barrel which is heated to a temperature sufficient to melt the polymer. At the end of the barrel, the material is then forced out, through an orifice of some desired geometry.

The material, ideally, exits this die with the desired shape and being sufficiently cooled to retain its shape. This continuously-formed polymer is then cut to the specified length. Therefore, in manufacturing a part by extrusion, an extruder, a cooling device, and a cut-off machine are required. The die is often oversized to compensate for the shrinkage associated with the material being used. Furthermore, the die used to make a certain shape has a different shape, perhaps having curved surfaces in order to assure that the final parts have flat sides. The performance of an extruder depends on many aspects, some of which include the screw pitch, flight height, die geometry, and screw speed.

The temperature along the barrel is regulated by controllers that uses a thermocouple for feedback. There are three heaters and thermocouples along the barrel. Two more heaters and thermocouples are placed after the barrel on the die and the adaptor.

These controllers use closed-loop, PID (Proportional-Integrating-Differentiating gains). Each gain can be adjusted to optimize the controller's response to temperature fluctuations, see Appendix A or consult the Eurotherm Corp. Controllers' Installation and Operation Manual. The extruder's dynamic behavior, enviromental disturbaces and the material's properties, are considered in optimizing the controllers parameters in order to improve the system's response (including limiting the percent overshoot and error from the set temperature).

Figure III-1, found on page 7, shows a simplified block diagram of a PID controller. R(s)is the desired temperature, setpoint, for that section of the barrel. Y(s) is the actual temperature of the barrel at that section. H(s) is the feedback and is measured by the thermocouple for that section. The controllers are necessary because disturbances alter the temperature of the barrel, adaptor, and die. The disturbances include varying convective cooling and heat transfer to and from the material traveling through the extruder at various rates.

Heat transfer depends on several factors. The heat capacity and thermal conductivity determine how much mixing and time are required for the polymer to melt as well as what temperature profile is required along the length of the barrel, adaptor, and die. Furthermore, the extruder has its own heat capacitance and thermal conductivity that, in part, determines the thermal behavior of the barrel, adaptor, and die. Convective cooling to the ambient air and the amount of a certain material traveling through it also effects the extruder's performance.

IV. OPERATING INSTRUCTIONS

A. DIE SELECTION

The die is choosen so that its geometry will provide the specified dimensions(see Fig. I-2B, p.3). One must account for changes in the material upon for cooling, shrinkage and viscosity. The die is often oversized to compensate for the shrinkage associated with the material being used. Furthermore, the die used to make a certain shape has a different shape, perhaps having curved surfaces in order to assure that the final parts have flat sides. Because the die affects the extruder's performance, it must be choosen to provide an operation point suitable to the extruder ratings.

 

B. MATERIAL SELECTION

The design must call for a thermoplastic material because these polymers can be melted and reshaped unlike thermosetting polymers. One must know the material's melting temperature, coefficient of thermal expansion, heat capacity, thermal conductance as well as the mechanical properties. Additionally, olymers behave viscoelastically.

Some common thermoplastics used for extruding are:

Polyethylene (PE)

Polypropylene (PP)

Polybutylene (PB)

Poly 4-methypentene (PMP)

Polystyrene (PS)

Polyvinyl Chloride (PVC)

Polytetraflouroethylene (PTFE, Teflon)

Acrylonitrile Butadiene Styrene(ABS)

 

C. MACHINE SETUP

Refer to Figs. 1-1,1-2 for Nomenclature

READ SAFETY SECTION II

DO NOT PRESS THE START BUTTON!

1. Place the Breaker Plate and the Die on to the extruder barrel then tighten bolts sufficiently to prevent polymer leaks. Do Not Over-Tighten Bolts!

2. If cooling bath, feed cooling, and/or cut-off systems are available set them up as necessary.

3. Throw the main power switch to "ON"

4. Turn the controllers to AUTO for Barrel Heaters and to "ON" for the Die and the Adaptor Heaters.

5. Set the temperatures for each zone according to

material properties by using the "UP" and "DOWN" arrows. (see Figure III-1, below) NOTE: the upper number is actual temperature and the lower number is set temperature.

6. Verify Controller Parameters. If in doubt of the proper controller values, do not change them until you have consulted the Eurotherm Corp. controllers' "Installation and Operation Manual" and/or read

Appendix A. Press the PAR" bottom twice, quickly. You should see the value of the Proportional Gain, PROP. Use an arrow(up or down) to change values if necessary. Press the "PAR" button again to see Integrator gain, INT. Cycle thru other parameter by pressing the "PAR" button repeatedly, confirming all parameters, until the temperatures are shown. Note: If left too long, the display will return to the temperatures)

7. Wait for all indicated temperature to reach Set Temperature.

8. After all the zones have reached their proper temperature for the material being used, place material in the hopper and open the hoppers chute.

 

D. Operation

1. Turn the motor Speed Dial for screw to zero.

2. Press green Start Button to start the motor.

3. Slowly , and while watching the Tachometer

and Motor Ammeter, turn the speed dial up to desired speed. The Ammeter should not exceed 15 amps, max. Be sure the screw is turning before turning the dial to higher values.

4. Give Time for material to melt and proceed to the die.

5. Watch Pressure Indicator, it should not go near or above 5,000 psi. Note scale value is times by 1000 psi.

6. Verify extrudate is of desired quality. (Design Specifications, etc.)

 

E. SHUTDOWN

1. Before shutting down the machine make sure all

the raw material has been extruded out.

2. Turn the controller to "OFF" position.

3. Make sure that the pressure is below 100 psi. then turn off the main power.

4. Allow cooling time before removing the die.

 

F. TROUBLE SHOOTING

_ The screw will not rotate. WARNING: Do not allow the motors ammeter to exceed 15 amps. Do not turn the motor's speed dial up beyond 10 RPM if the screw is not rotating, as indicated by the tachometer.

Verify that temperatures have reached their set temperatures.

Raise the set temperatures if the proper zone temps. for the material being used does not free the screw.

NOTE: Some Different Material(with a higher melt temp.), from a previous run may be remaining solid, preventing the screw from turning freely.

_ No material exciting extruder, after some time;

Check temperatures, especially near breaker plate, and melt temp.

Check motor rpm, note high speeds may move material too fast, not allowing time to melt and thus clogging the breaker plate.

_ Pressure reading high (Note; do not allow pressure to exceed 5,000 psi.)

Check melt temperature, may be too low.

Check screw rpm, may be too high.

If still no flow out die;

Raise temperatures at die, adaptor and zone #3 (end of barrel).

 

V. RECOMMENDATIONS

1. Additional Equipment

Some additional equipment is necessary in order to use this machine in a production setting. The cut-off system is required to cut the parts to a specified length. Furthermore, a system of cooling the extrudate and transporting from the die to a cut-off system is required. Insulation may be placed around the die to help heating and maintain proper temperature. A cooling system which would circulate coolant around the feed-zone should be incorperated.

2. Maintenance

Period maintainance should take place for prolonging the life of the extruder. This should include lubing the bearings and verifying the oil level in the gearbox. It is further recomended that a record of maintainance be kept to assure proper care.

 

VI. REFERENCES

Crawford, R. J., Plastic Engineering, Pergamon Press, 1987

Throne, James L., Plastics Process Engineering, Marcel Dekker, Inc., 1979 (SDSU TP156.P6T44)

McKelvey, James M., Polymer Processing, John Wiley and Sons, Inc., 1962 (SDSU TP156.P6M25)

Miller, Edward, editor, Plastics Products Design Handbook,(SDSU TP1130.P56)

Parrich, A., editor, Mechanical Engineer's Reference Book, CRC Press (TJ151.P35 or TJ151.M43 Ref.)

Lynch, Charles T., editor, Handbook of Materials Science, Vol. III: Nonmetallic Materials and Applications, CRC Press (TA403.4.L94 V.3)

Dostal, Cyril A., editor, Engineered Materials Handbook, Vol.2, Engineering Plastics, ASM International,

(SDSU TA403.E497 V.2 ref)