There are two clear possibilities. One - the powder is fluidising and running back through the helix and clearances of the screw. Two – the material is arching over the screw at the inlet hopper.

This powder is fine and tends to be cohesive when settled, so the geometry of the inlet is very important to sustain reliable and consistent flow. If material feeds from the hopper to the screw it will usually convey and prevent back-flow whilst there is product continuing to feed in from the supply hopper, so I suspect that the real problem is the second prospect due to a poor hopper design. Material that is loaded into the hopper in a loose condition will tend to fill the screw and a small amount may collapse to move along the screw when the machine is started. The flow could then cease as the screw bores a tunnel through the bed. It may be possible to shake the hopper contents with a vibrator or inject air to dilate the powder to a loser flow condition, but these are brute force solutions with attendant drawbacks and a better approach is to fit or adapt a better shape of hopper. Do not poke about in the material with a stick, as it is likely to get caught by the screw and cause damage or an accident.

If you can generate steady flow from the hopper the product should then convey as long as the infeed continues, but when the hopper runs clear of product there is a danger of the material in transit then dilating and becoming fluidised to the point of very low viscosity fluidity. You may even experience a curious phenomenon just after the hopper finally empties. The material suddenly ‘flushes’ back out of the tube into the hopper, to settle and start conveying again until the contents just disappear from the hopper, to flush back out again in a few seconds and continuously repeat this cycle.

If you care to send a sketch of the hopper to lyn@ajax.co.uk and describe the loading and operating proceedure I will try to advise further. ■