Calculating Residence Time

I was recently asked the question...            

Kevin

"Do you have any way of easily calculating the residence time in screw / barrels?"

My Response

Most general purpose screws contain approximately 1-2 shots within the flights depending on the geometry. To calculate the residence time you will have to know your part volume, the volume of material in the barrel, and your cycle time. You can use a basic calculation for the barrel volume minus the screw volume (ignoring the flights because they are relatively small) to calculate the material your screw holds.

To calculate the volume of the barrel:

3.14 x (Barrel radius)^2 x (Barrel length)

To calculate the volume of the screw:

3.14 x (Average root radius of the screw)^2 x (Barrel length) 

Volume of material in the barrel/screw: Barrel volume-screw volume

As for the residence time:

If you calculate the volume of material in the barrel as 10 ounces and your part is 5 ounces your screw contains two shots of material. Multiply the number of shots in the barrel by your cycle time and you will get a residence time for the material.

For example: If you have a 25 second cycle time and your screw holds 2 shots the residence time is roughly 50 seconds (two cycle times).

The residence time will be a multiple of your cycle time plus or minus one cycle (depending on the ratio of part volume to barrel volume).

-Andy