Definitions: Plasticating Capacity: the maximum rate at which an extruder or molding machine can melt room temperature feedstock and raise it to the temperature suitable for extrusion or molding.
Plasticate: to render a thermoplastic more flexible, even molten, by means of both heat and mechanical working.
In part 1 we discussed the recovery rate, plasticating rate of the machine. It was left with a set of equations on how to calculate.
An example would be the following:
Step 1 was to convert the current grams/ seconds to a volumetric figure.
This is done by knowing the following:
A = Shot weight in PS for the machine (703 grams)
B = Volumetric size of the barrel capacity (772 cm³)
C = Grams per seconds of the plasticating rate in PS (85 g/sec)
Our equation would be:
(B/ A) x C = cubic volume per second
(772cm³ / 703 grams) x 85 grams/sec = 93.4 cm³/sec.
Step 2: to covert to new material one uses the volumetric rate result of step one and multiplies by the melt density of the new material.
Example is that the new material is polypropylene and our melt density is 0.68 g/cm³.
Multiply 93.4 cm³/sec (volumetric rate of machine in PS) by the 0.68 g/cm³ (melt density of new material) which results in 92.72 grams/second plasticating rate for processing polypropylene in the machine for that part.
(93.4 cm³/sec) x (0.68 grams/ cm³) = 92.72 grams/sec plasticating rate
The issue is where we find the melt density of the materials to use in the above example. These can be hard to find but they are available some of the most common ones follow.
|
density values | |
|
material |
70 F Density (gr/cc) |
Melt density (gr/cc) | |
REF 2 grams/cc |
|
| | | |
|
ABS |
1.05 |
0.97 | |
.895 to 0.908 |
|
Acetal |
1.42 |
1.17 | | |
|
Acrylic |
1.16 |
1.04 | | |
|
Polycarbonate |
1.2 |
1.02 | | |
|
HDPE |
0.94-.96 |
0.72 | |
0.752 to 0.772 |
|
Polypropylene |
0.9 |
0.7 | | |
|
Polystyrene GP |
1.08 |
0.97 | |
0.886 to 0.901 |
|
Polystyrene HIPS |
1.04 |
0.96 | |
0.895 to 0.917 |
| | | | |
The final calculation is now using the data and math for your benefit.
A simple example would be a part whose information is as follows:
Shot weight in PS = 150 grams
Cycle time is 10 seconds
Thus:
The number of seconds in one hour x shot weight in grams
Cycle time x 1000 = grams / second
(3,600 x 150) / (10 X1000) = 54 grams / second
If the quoted plasticating rate of the machine is say 90 grams/second than it appears we are okay. But the mistake is that the screw only rotates in the cooling portion of the cycle above. The cooling time for the above cycle is 6 seconds and we wish a safety of 1 second that leaves only 5 seconds of rotation time. Thus substituting the 5 seconds in the above equation for the cycle time of 10 seconds equals 108 grams / second. This machine is not capable of producing our part in the quoted cycle time.
SL Silvey
360-882-3183
silveysplastics@hotmail.com
