The Glue That Dripped (No Valve Control)
We dispensed glue (on a part, for assembly). The glue was in a syringe (pushed by air). The valve (a solenoid, article 90) opened (the glue flowed). But after the valve closed, the glue kept dripping (the syringe’s pressure bled). The part got too much glue (a mess). The problem: the valve didn’t suck back (the glue kept flowing). We added a suck-back (the valve retracts the plunger, pulling the glue back). The drip stopped. The mistake was not controlling the dispense (open/close, no suck-back).
Dispensing valve controls the glue (dot, bead, drip-free). This article covers the basics.
The Dispense Valve
A dispense valve (for glue, oil, paste) controls the flow. It opens (the glue flows) and closes (the glue stops). The valve is actuated (pneumatic, or electric). The dispense (the amount) is controlled by the open time (and the pressure).
Step 1: The Dot (or Bead)
For a dot (a small amount, e.g., 0.05 mL), the valve opens briefly (10 ms). The glue flows (a dot). For a bead (a continuous line), the valve opens (as the part moves, a bead). The dispense is consistent (the same amount each time).
Step 2: The Suck-Back (Anti-Drip)
After the valve closes, the glue might drip (the pressure bleeds). A suck-back (the valve retracts the plunger) pulls the glue back (into the nozzle). No drip. For a low-viscosity glue (watery), the suck-back is essential (it drips otherwise).
The dispensing rule: Control the open time (and the pressure) for the amount. Add a suck-back (anti-drip). The glue that dripped had no suck-back. The valve opened (10 ms), dispensed, closed, sucked back. No drip.
Step 3: The Pressure Regulator
The glue’s flow depends on the pressure (the air pushing the syringe). A regulator (article 119) sets the pressure (e.g., 2 bar). The dispense is consistent (the same pressure, each time). If the pressure varies (the glue runs out, the syringe empties), the dispense changes. Monitor the pressure (a gauge).
Step 4: The Nozzle (Tip)
The nozzle (the tip) controls the glue’s path. A small tip (Ø0.5 mm) dispenses a small dot. A big tip (Ø2 mm) dispenses a bead. Pick the tip for the dispense (dot, or bead). Clean the tip (the glue cures, it clogs).
| Dispense | Valve |
|---|---|
| Dot (0.05 mL) | Short open (10 ms), small tip |
| Bead (line) | Open while moving, big tip |
| Anti-drip | Suck-back (retract) |
A Dispensing Checklist
- What is the dispense? (Dot? Bead?)
- Is the open time set? (ms?)
- Is the pressure regulated? (Article 119?)
- Is there a suck-back? (No drip?)
- Is the nozzle right? (Tip size?)
- Is the dispense consistent? (Weight?)
- Is the tip clean? (No clog?)
- Does the glue drip? (Suck-back?)
- Is the glue type right? (Viscosity?)
- Is the cure right? (Time?)
The Bottom Line
Dispensing valve controls the glue (dot, bead, drip-free). The glue that dripped had no suck-back. Control the open time. Regulate the pressure. Add a suck-back. The glue that didn’t drip wasn’t the best glue — it had a suck-back valve.
