Why Filled Materials Are Hard to Dispense
Short answer: it depends on the filler. If it is hard, like alumina or silica, the problem is wear, and you need a tungsten carbide rotor and low pump speed. If it is soft or dense, like boron nitride, glass beads or copper and silver powder, the problem is settling and separation, and you need to keep the material mixed all the way to the pump. A progressive cavity pump handles both because it meters by volume at low speed.
With a hard filler, you know the pattern. The equipment works on day one. A few weeks in, shot weights drift. A few months in, you are rebuilding pumps. Standard wetted parts wear, clearances open and output falls out of spec before anything visibly breaks.
With a soft or dense filler, the pump may barely wear, and you still get scrap. The filler drops out in the pail, so the top of the container is lean and the bottom is rich. Dense metal pastes can separate under pressure, and hollow glass spheres can crush. Volume is right; the material isn't.
Is My Filled Material Abrasive?
Look up the filler on your data sheet. If it is in the top two rows, plan for wear. If it is lower down, plan for settling and separation. Hardness is on the Mohs scale, where diamond is 10.
| Filler | Mohs hardness (approx.) | Typical materials | Abrasive? | Main problem | What we specify |
|---|---|---|---|---|---|
| Aluminum oxide (alumina) | 9 | Thermal gap fillers, filled epoxies and potting compounds | Yes, severe | Wear on rotor, stator and nozzle | Tungsten carbide rotor, 8 to 12 rpm |
| Silica / quartz | 7 | Filled epoxies, potting compounds and encapsulants | Yes | Wear | Tungsten carbide rotor, 8 to 12 rpm |
| Glass beads and spheres | 5.5 | Bond-line spacer adhesives, lightweight fillers | Mild | Beads must stay evenly spread; hollow spheres can crush under high pressure | Low-shear volumetric pump at low speed |
| Zinc oxide | 4.5 | Thermal greases | Moderate | Settling, some wear | Low speed, material kept mixed |
| Copper, silver and other metal powders | 2.5 to 3 | Brazing pastes, conductive adhesives | Low | Very dense, so fast settling; binder can separate from the powder under pressure | Low-shear volumetric pump; MFS or gyroscopic mixer to keep it mixed |
| Boron nitride | 2 | Thermal greases, gels and gap fillers | Low | Settling | MFS or gyroscopic mixer to keep it mixed |
Many materials blend fillers. If yours has any alumina or silica, treat it as abrasive.
How Do You Dispense Abrasive Filled Materials?
Build for the wear instead of pretending it isn't there: hard wetted parts, low speed and volumetric metering that doesn't drift as parts age.
Tungsten Carbide Rotor
The rotor is the hardest-working part in a progressive cavity pump, so we make it out of tungsten carbide. Rotors last up to 24 months on abrasive materials, and a pump rebuild takes under an hour.
Low-RPM Operation
Wear rises with speed. Because the pump meters by cavity volume, it is accurate at 8 to 12 rpm on filled materials. Less speed, less wear, less shear.
Volumetric Metering
A progressive cavity pump moves sealed cavities of fixed volume. What goes in comes out, regardless of viscosity swings, temperature or head pressure.
Matched Elastomer Stator
Selected for your fluid chemistry. FEPM is the workhorse for gap fillers, encapsulants, silicones, epoxies and urethanes. Stators typically last 6 to 12 months on abrasive materials.
How Do You Stop Filler Settling and Separation?
Keep the material mixed from the container to the pump, and don't push it harder than you need to.
- A Material Feeding System (MFS) agitates and recirculates the material so the filler stays in suspension all the way to the pump, and degasses it under vacuum.
- A low-shear progressive cavity pump at low speed moves dense pastes and glass-bead adhesives without working them hard.
- Volumetric metering means a dense metal paste dispenses the same volume as a light one; check shot weight to confirm the mix.
Restore a Settled Pail
If the filler has already dropped out in storage, the material entering the pump is lean at the top and rich at the bottom, so performance varies shot to shot even with perfect volume control.
A gyroscopic mixer re-homogenizes the material inside its original sealed container, with no air entrainment and no transfer step. See the StokesIt 360 & 400 mixers →
System Specifications
| Specification | Detail |
|---|---|
| Wetted parts | Tungsten carbide rotor for abrasive fillers; FEPM stator (standard); alternative stator compounds per chemistry |
| Metering principle | Progressive cavity: positive displacement, volumetric |
| Viscosity range | 1 to 1,000,000 cP |
| Specific gravity range | 0.9 to 3.5 |
| Operating speed | 8 to 12 rpm on filled materials |
| Dispense modes | Dots, beads, continuous; no metering break required |
| 1K / 2K capability | Both; 2K with electronic ratio control, ±1% per side |
| Service life | Stators 6 to 12 months, tungsten carbide rotors up to 24 months on abrasive materials; rebuild in under an hour |
What Filled Materials Do You Dispense?
- Thermal gap fillers, greases and gels: see the thermal interface material page
- Ceramic-, silica- and glass-filled epoxies
- Filled encapsulants and potting compounds
- Silver-filled conductive adhesives and metal-filled pastes
- Highly filled 2K silicones and urethanes
See the materials we've dispensed, with viscosity, package and the pump we used.
Experience Behind the Engineering
Gavin Petersen has spent 30+ years in industrial fluid dispensing, including senior roles at Graco. Filled compounds are the problem he has been asked to solve most often.
Learn more about our experience