Introduction

The valve is the part of a dispensing system that decides whether your process works. You can buy the best controller on the market and mount it on a well-built robot, and if the valve is wrong for the fluid, you will still get stringing, drooling, inconsistent shot weights and rejects at the end of the line.

Most of the calls we get at Adofil about dispensing accuracy problems turn out to be valve selection problems. Somebody specified a valve for a 500 cP adhesive, then the R&D team switched to a filled epoxy at 60,000 cP, and nobody revisited the hardware.

So here is how the three most common valve families actually behave, and which fluids each one belongs on.

A quick word on viscosity before anything else

Viscosity is measured in centipoise (cP). Water sits at 1 cP. A thin cyanoacrylate is around 5 to 20 cP. Solvent-based adhesives land between 200 and 2,000 cP. RTV silicone is usually 20,000 to 100,000 cP. Solder paste and filled epoxies behave more like putty than liquid.

Two fluids can share the same viscosity number and still need different valves. Silver-filled epoxy and unfilled epoxy might both read 30,000 cP, but the filled one carries hard particles that will chew through a soft seal in weeks. Abrasion matters as much as thickness. So does whether the fluid cures on contact with air, moisture or metal.

Get a viscosity figure at your actual operating temperature, not the datasheet figure at 25 degrees C. A material that reads 40,000 cP in an air-conditioned lab may be 90,000 cP on a shop floor in Chennai in April.

Peristaltic valves: thin fluids, zero contamination

A peristaltic valve does not let the fluid touch any moving part. A roller or cam squeezes a flexible tube, and the pinch travels along the tube to push fluid forward. The fluid only ever touches the inside of that tube.

That single design choice solves several problems at once. There is no seal to wear, no dead volume where material sits and cures, and changeover takes about thirty seconds because you swap the tube instead of stripping the valve.

Best suited for: fluids from roughly 1 cP up to about 5,000 cP. Cyanoacrylates, UV-cure adhesives, solvents, oils, alcohols, reagents and inks.

Where it wins: anywhere contamination is unacceptable. Pharmaceutical filling, medical device assembly, diagnostic reagent dispensing. Also anywhere the fluid cures on contact with metal, which is exactly why cyanoacrylate users end up here. Instant adhesive in a conventional valve will eventually polymerise against the seat and lock the valve shut. In a peristaltic tube it simply cannot.

Where it falls down: thick material. Push viscosity past 5,000 cP and the tube stops recovering its shape fast enough between compressions, so shot volume drifts. Abrasive fillers wear the tube from the inside. The tube is also a consumable, so you carry a recurring cost and a maintenance interval.

If you are dispensing anything thin and expensive, or anything that must stay sterile, start here. Our dispensing valves range covers peristaltic configurations for both benchtop and inline use.

Diaphragm valves: the general-purpose middle ground

A diaphragm valve uses a flexible membrane, usually PTFE or a similar polymer, that lifts off a seat to open the flow path and presses back down to close it. Air pressure on one side drives the diaphragm; fluid sits on the other side.

The key feature is the positive snap-off. When the diaphragm returns to the seat it displaces a small amount of material back up the fluid path, which pulls the deposit cleanly off the tip. That suck-back is what stops tailing and stringing.

Best suited for: roughly 100 cP to 50,000 cP. Solvent-based adhesives, silicones, greases, primers, sealants, medium-viscosity epoxies and lubricants.

Where it wins: most general industrial work. It handles a genuinely wide viscosity band, it produces repeatable dot sizes down to around 0.5 mm with a good controller, and the wetted parts are chemically inert enough for aggressive solvents. In electronics assembly it is the default choice for conformal coating, underfill and bonding operations.

Where it falls down: heavily filled or abrasive material. Glass-filled and metal-filled compounds grind at the seat and the diaphragm face. You will see shot weight drift long before you see a visible leak, which is the dangerous part, because the process degrades quietly. Very high viscosity material also needs more pressure than the diaphragm can comfortably work against.

Diaphragm valves are the ones people over-specify least often and under-maintain most often. If you run one on filled material, put seat inspection on a fixed schedule rather than waiting for a failure.

Spool valves: thick, filled and abrasive material

A spool valve moves a cylindrical spool inside a close-fitting bore. As it shifts position it opens or blocks the flow path. It is a mechanically simple, physically robust design that takes high pressure without complaint.

Best suited for: roughly 20,000 cP and up. Filled epoxies, solder paste, thermal interface material, high-viscosity RTV, structural adhesives, greases and potting compounds.

Where it wins: material that would destroy the other two. Because the sealing surface is a long metal-to-metal fit rather than a soft face pressed against a seat, abrasive filler causes far less damage. Spool valves also give strong suck-back, which is what you need when thick material would otherwise sit on the tip and sag onto the next part. For battery manufacturing work with thermal gap fillers, this is usually the only sensible option.

Where it falls down: thin fluid. Below roughly 10,000 cP, material seeps past the spool clearance and you get drooling between shots. Spool valves also carry more internal volume, so purging on a colour or material change takes longer.

Selection table

Four questions that settle most decisions

What is the viscosity at line temperature? Not lab temperature. Measure it where the machine will run.

Does the fluid contain fillers? If yes, and if those fillers are hard, you are looking at a spool valve regardless of what the viscosity number says.

Does it cure on contact with air, moisture or metal? If yes, peristaltic is the safe answer, assuming viscosity allows it.

What is your smallest deposit? Sub-millimetre dots need a diaphragm valve and a controller that holds pressure and time stable to the millisecond. A dispensing controller with closed-loop pressure regulation matters more here than the valve itself.

The mistake worth avoiding

People buy a valve for the material they are running today and forget that materials change. Your adhesive supplier reformulates, the design team adds a filler for thermal conductivity, and suddenly a valve chosen for 8,000 cP is running 45,000 cP filled compound.

If you know your material spec is unstable, specify for the worst case you can reasonably foresee, or at minimum ask what the upgrade path looks like before you commit.

Adofil manufactures dispensing valves in all three families from our Bangalore facility, and we size them against your actual material rather than a category label. Send us a sample or a datasheet through the contact page and we will tell you what fits. If it is a valve we do not make, we will say so. Product datasheets are available on our literature page