Volume drift is usually caused by changing process conditions rather than the valve itself. Check the following in order.
1) Supply pressure stability: a 0.01 MPa fluctuation is measurable in the shot volume. Verify the regulator, reservoir level and leak-free tubing first.
2) Needle condition: wear, deformation or partial clogging drifts the flow; replace on schedule and keep same-batch spares.
3) Fluid temperature and viscosity: viscosity can change by more than 10% for every 5 C shift, so a temperature-controlled supply markedly improves repeatability.
4) Air in the valve: incomplete de-gassing or air introduced during fluid changes causes intermittent dispensing; add de-gassing or venting if needed.
5) Cured residue: after long stops the flow path can cure locally and form a hidden blockage; set the flush interval by fluid behaviour.
6) Drive parameters: open time and suck-back on pneumatic valves, screw speed and turns on screw valves, must be re-calibrated to volume and cycle time.
Once these conditions are fixed, log the volume data and you can tell whether the cause is the process or the valve.
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