EXERCISE 2 V.P. 27.10.09
Maximizing throughput rate
Course: Polymer processing
Teacher: Marko Voho
Laboratory assistant: Valeria Poliakova
Time: 4 h
Objectives
On successful completion of this exercise the student should be able to
safely operate the extruder
name different parts of the extruder
explain operation principles of the extruder
explain various purposes of the extruder
explain what is rotational speed, pelletizer speed, throughput rate and how
these parameters are interconnected
Introduction
A schematic drawing of extruder-water bath- pelletizer system is presented on a
picture below:
Figure 1 Extruder, water bath and pelletizer from above
The system, where strands are extruded trough a die, cooled in a cooling bath and
then cut by a strand cutter (pelletizer) is called strand pelletizing system. Despite its
relatively large space requirements, it is still widely used, mostly because of its
simplicity [1]. There are also more sophisticated die-face pelletizing systems; more
information on them can be found in [1].
In industrial environment, productivity of extruder is an important parameter that
usually has to be maximized.
Productivity of an extruder can be measured by measuring throughput rate, what is
calculated in kg/h.
The goal of the present exercise is to find out what parameters affect throughput rate
and to see how it is possible to maximize this rate.
Preparation
Chose a material from those available in the storage in plastic lab (check here
lab.arcada.fi plastic lab polymer datasheets). Please notice that hydrophilic
polymers like PC, PS or PET require drying prior to extrusion, so if you chose one of
them, inform the lab assistant (poliakov@arcada.fi) of your choice before the actual
exercise.
Find out more about the material of your choice. Minimum information required for
the exercise is melting temperature and bulk density of the plastic. Bring material data
sheets with you.
Agree on time for your group with Marko or Valeria.
Be on time for the exercise. Please inform the lab assistant (poliakov@aracda.fi) if
you can’t attend the exercise. Students late more than 10 minutes will not be admitted.
Procedure
1. Switch on the extruder and check that the water bath is connected. Open the
cooling tap.
2. Set up temperature for all 6 zones of the barrel following recommendations of
material data sheets and the lab assistant.
3. Wait for temperature to stabilize.
4. Check from the “extruder” notebook the previous settings and make sure that the
temperature of operation is higher or the same as for the previous plastic or that
extruder has been cleaned with Arguclean. If that is not the case, increase the
temperature and clean the extruder. Ask for help from the lab assistant.
5. If the extruder is clean, start operations.
6. Set rotational speed to 5 rpm and press “ON”. Follow the current readings, they
should be constant. If at any time during the exercise current reading suddenly
increases, press emergency stop button.
7. If the screw rotating smoothly and there are no unusual sounds, place some
material into the hopper and increase the rotational speed of the screw.
8. Set take-off speed (pelletizer speed) to 3 M/min.
9. Set screw rotational speed to 15 rpm.
10. Feed the strands coming out of the extruder to the pelletizer. Try to feed them
without wasting material. Collect all the waste! When feeding is successful, start
timing. Continue material collection for 5 minutes.
11. Store the collected material for the time being in a plastic bag. Store waste
material in a different bag.
12. Increase rotational speed to 30 rpm.
13. Repeat steps 10-12 and make sure that you will not mix the plastic bags later.
14. Increase rotational speed to 45 rpm.
15. Repeat steps 10-12.
16. Increase rotational speed to 60 rpm.
17. Repeat steps 10-12.
18. Increase rotational speed to 75 rpm.
19. Repeat steps 10-12.
20. Now some member of the group can proceed with steps 21-22, while others could
jump directly to step 23 to save time.
21. Set take-off speed to 5 M/min
22. Repeat steps 9-19.
23. Weight the content of all the bags, so that you can make a table (in excel) that
looks as follows:
Time Rotational Take-off Weight of Weight of
(example) speed of the (pelletizer) collected waste material,
screw, R, rpm speed, r, M/min material, m1, m2, kg
kg
13:15 15 3
13:20 30 3
13:25 45 3
13:30 60 3
13:35 75 3
13:40 15 5
…
24. Clean the extruder and switch it off. Close the cooling tab.
25. Measure throughput rate of extruder (TR) using the following formula
TR (R, r) = m1/t (t is the time interval)
26. Measure potential throughput rate (TRp) using another formula
TRp (R, r) = (m1+m2) / t
27. Measure a ratio of TR and TRp and express it in relative terms.
28. Add all the values you’ve calculated to the table (in excel).
29. Build a graph, where dependence of throughput rate on rotational speed becomes
obvious (there should be two curves – for 3 M/min and for 5 M/min).
Submission
Submit a .doc document with supporting excel sheet. The following questions should
be answered in the document.
1. What are the parameters that throughput rate depends on?
2. Can you think of some other parameters, not discussed in the exercise?
3. How does rotational speed of the screw affect throughput rate?
4. Does take-off speed make any effect?
5. What is the best combination of rotational speed and take-off speed for the
plastic in question?
6. What else can be affected by these speeds? Consider pellets size, shape,
threads shape, etc…
7. What is the difference between TR and TRp? Where this difference came from?
8. If you would have to specify throughput rate of the extruder in a specification
sheet, which throughput rate would you use- TR or TRp? Why?
Submit the document named PP_groupN_extr to poliakov@arcada.fi the latest two
weeks after the exercise.
References
[1] Hensen F. (Ed), Plastic Extrusion Technology, 2nd edition (1997), Germany: Kösel
Own notes: