Hello and welcome to a new blog post. In a previous post, I presented the residence time calculation in injection moulding. Today we will focus on the extrusion process.
Residence time in extrusion is referring to the time, which a polymer
pellet, and as consequence, polymer melt needs to travel from the melting zone
until the end of the metering zone.
The online calculation
For an approximate calculation of the residence time, building the quotient of the volume of the extruder filled with melt and the volume flow of the melt is possible (underlying basic laws of physics):
Equation 1 - extrusion residence time (average residence time): t=V/Q
V = volume of the extruder filled with melt
Q = flow rate of the melt
t = extrusion residence time
This is the background of the
online calculation tool shown below:
In literature, extruder output (flow rate; kg/h) can be calculated by the following equation
[3]:
The leakage flow is calculated by addition of mass flow rate (drag) and
pressure flow rate minus the extruder output.
2nd way to calculate the average residence time
average residence time t = V/Q= (2 * z) / (v_bz *(1-a)) =
z =dz = unwound channel length = L / sin φ
φ = outer slope angle f.e. 30.72°
L = screw length f.e. 14*D
D = screw diameter (mm)
n = RPM (1/min)
h = groove depth
b = π*D*sin φ-e
e = bridge width
dp = pressure drop f.e. 20 bar = 20*10^5 Pa
v_bz = circumferential speed in Z direction = n*π*D*cos φ
a = throttle coefficient = ((b*h^3)/(12*η))*(dp/dz)* (2)/(v_bz*b*h)
Example with following extrusion screw and process data
D=45 mm
L=14*D
h=3 mm
e=4.7mm
n=60 1/min
φ=30.72°
η = 800 PAs
1. calculate b = 67.52 mm
2. calculate v_bz = 121.53 mm/s
3. calculate z = 1233.26 mm
4. calculate a = 0.25
5. calculate t = 2*1233,26 / 121,53 (1-0,25) = 27,07 s
Also as a general rule, the higher the temperature, the lower should be the
melt residence time.
Thanks for reading and #findoutaboutplastics
Greetings
Herwig Juster
Interested to talk with me about your polymer material selection, sustainability, and part design needs - here you can contact me
Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.
Literature:
[1] https://www.findoutaboutplastics.com/2017/10/how-to-calculate-residence-time-in.html
[2] https://www.extrusion-training.de/extruder-dimensionierung/
[3] Rao, Natti S. - Design Formulas for Plastics Engineers
[3] N. Rao: Design formulas for plastics
engineers
No comments:
Post a Comment