Screw Feeder Constant Flow Rate

Getting a screw to feed consistently is oftentimes not easy or not possible based on the material flowability and the required accuracy.

Is the feed bin properly designed such as does the whole bottom terminate at the screw inlet or are there dead spots? Are the tapered walls steeper than the angle of repose?

The screw must be long enough that when it is turned off the material stops immediately. Very flow-able materials do not stop quickly or at all. If overflow is a problem one thing that helps is a choke in the screw or a minimum clearance circular housing.

Putting a screw with two pitches may help. The slower pitch is under the bin and the faster pitch pulls it down the screw so all regulation happens under the bin.

You may get better repeatability with a smaller screw diameter. What is your mass flow rate and what is the rotation speed of the screw?

If the material in the bin settles over time such that its bulk density changes, accurate feed rates with a simple screw may not be possible.

thank you for your support

requested flow rate is 333g/sec and rotation speed is 30rpm. now i get the required flow rate. the problem is the fluctuation.

material fully terminate at the screw inlet after i fabricate the tapered shaft. there are some powder remain at the edges of the bin. the bin is square type. tapered wall has an angle of 65 degrees.

this powder is not a free flowing material. sometimes lumps flow out from feeder outlet. if i can get a costant flow rate,thats enough for me!!

The problem you have having is controlling the material cutoff from the discharge of the screw. Sometimes a "dam" or other method can be used to hold back material in the screw voids, however as you just discovered Zinc Oxide can be a difficult material to handle and will likely pack.

I recommend inserting a second screw flight within the existing flight. This will cut in half the screw void volume and potentially double your cutoff accuracy. The second flight does not necessarily have to run the full length of the screw, just the last two flights might be sufficient. You likely have some spare flights leftover from your original fabrication.

Extending the screw flight(s) beyond the screw outlet may have a slight benefit because it could holdback material dependant on where the screw flight stopped within the outlet.

Another issue to investigate is whether your accuracy changes while the hopper is refilled. This may be a symptom of material with entrained air flushing out the screw or arching / bridging within the hopper.

thanks, i will do that second screw flight thing. i also thought that, but i didn't have a sure of it. as you said; there is a change in accuracy after refilling and also having the "air flushing". what can i do for avoid that??

Put the load cells on the material hopper that has the feed screw.

Set it to dispence desired amount then it stops simple. densities clumping or any error doesn't matter. weight out of the bin is all that matters. when it reaches it's desired weight it stops. we do this in a plastics factory when blending various compounds, the screwes are also at an upward ohh 30 deg angle to keep any from possibly draining out of the screw.

i already fabricated the system as you mentioned. i also thought that "loss weight" system will be the solution. but it didn't solve my problem. i just don't know why. we build a software for loss weight system from "visual Basic" and using an inverter for controlling. we change the Deceleration, rpm.. but didn't work. then we remove the PC and check via a timer(to remove VB software delay). but there is a +/- 100g tolerance. i need a +/- 20g accuracy.