Table 1: Producing a test grid can show the optimum combination of screw-speed and throughput (shown in bold in this example) RPM 200 400 600
Rate 300 400 500
200 400 600
200 400 600
400 500 300
500 300 400
Tensile strength 27.2 29.4 29.6
28.5 32.5 30.1
26.7 28.9 30.4
Impact 16.2 16.0 15.6
16.1 17.3 16.8
16.3 15.8 15.9
Colour 7 7 8
8 9 7
7 8 8
The best way to handle warm-up is to plan the time
you really want to start running the extruder. Then start heating up only as far ahead of this time to allow for complete warm-up plus adequate soak time. Cool-down: There are preferred shutdown procedures also, to avoid problems with black specks. Before shutting the extruder down, it is good practice to fill the extruder and die to the maximum degree of fill with some inert polymer, such as HDPE, to “seal” the machine. The HDPE displaces oxygen, and coats the metal surfaces to greatly slow down oxidation and the formation of carbon. After the extruder has been “sealed”, it is better to
crash cool the machine, rather than let it cool from ambient air over the next 18 hours. A crash cool involves turning all the temperature set-points down to zero, to force the cooling solenoid valves to open. With the coolant pump running, this will force cool water through all the barrels, bringing the temperature down quickly. A quick cool-down does not give the polymer a chance to degrade and carbonize.
1 7 Key process indicators
Most extruder operators tend to be like the old-time pilots, preferring to fly by the seat of their pants. And many of them have excellent instincts for knowing when the process is running right, and when it is not. But as products become more complex with tighter processing windows, it is much better to have some kind of quantita- tive way of assessing how the machine is running. A common example of this is when the operator is
convinced something is different about the material. The line is just not running the same. If the material supplier is contacted, they are probably going to say their QC records show that the material is the same as
34 COMPOUNDING WORLD | June 2012
Surface 7 7 7
8 8 7
8 8 7
Degree of fill 0.68 0.45 0.38
0.91 0.57 0.23
1.14 0.34 0.30
Specific energy 0.165 0.200 0.287
0.142 0.178 0.255
0.262 0.197 0.259
its always been. Without some real numbers, how can you prove to the material supplier, as well as yourself, that the material is indeed different? Indicator 1: Specific throughput Specific throughput is the throughput divided by the screw speed (eg kg/h divided by RPM). It gives you a number which is representative of the degree of fill. This figure is useful especially if records are kept over a period of time for many different products, as it can help you predict how best to run a new product. It also helps you plan machine hours, as a product which needs to run at a low degree of fill will take longer to run a certain lot size. Finally, degree of fill is helpful in scaling up (or down) runs on different size extruders. Indicator 2: Specific energy Specific Energy is how much drive power is being expended to process each kg of material. It is a measure of how much mechanical work is being performed on the material. It has to be calculated in two steps: kW (applied) = kW (motor rating) x % running torque x (running RPM/max RPM) x 0.97
Specific energy = kW (applied) divided by the throughput in kg/h
Again, it is useful to keep records of this figure for
products over a long period of time. After a while, operators will get to know which processes are “energy hogs”, and which are not. This will help in planning runs for new products, and estimating the production capacity of any given line. Specific energy is also very helpful in pinpointing
problems. If a product is known to always run with a specific energy value of around 0.25, and one day it is only running 0.16 when all other conditions are the same, then this would be a reason to suspect a change in the material[3]