Squeegee Durometer: Matching Blade Hardness to Paste

Squeegee durometer is the Shore A hardness of the blade that pushes paste across the stencil, and it governs how that load reaches the apertures. A soft blade wraps around the paste and the foil and spreads its force over a longer arc. A hard blade carries the same force into a narrow line, which raises peak pressure at the aperture and increases stencil wear.

Hardness is often treated as a fixed property of whatever blade is fitted, yet polyurethane changes with age, with solvent exposure and with temperature. A blade that read 80 Shore A when it was installed can read 88 a year later, and the print will have drifted with it. This note explains what the figure controls and how to keep it in range.

What Durometer Measures on a Blade

Durometer is a measure of resistance to indentation, taken with a handheld gauge pressed against a flat section of material. Printing blades use the Shore A scale, and the practical range runs from about 60 for a soft blade to 90 for a hard one. The reading depends on the thickness under the gauge, so a measurement taken on a thin lip reads higher than the same material measured on a thick block.

A blade has a lip and a body. The lip contacts the paste and the body keeps the lip straight, and hardness is normally specified for the lip. A 70 durometer lip on an 80 durometer body is a normal build, so comparing blades by a single number taken from the wrong part of the profile leads to the wrong conclusion.

How Hardness Changes Paste Shear

A soft blade deforms as it meets the stencil, so contact pressure builds gradually and the paste is sheared over a longer distance. That fills coarse apertures well and leaves a generous deposit, which is why soft blades suit thick deposits on wide pads.

A hard blade keeps its angle, transfers pressure sharply and shears the paste in a short distance, so fine apertures fill more consistently. The penalty is a higher peak load on the foil. Used at the same print pressure as a soft blade, a hard blade accelerates stencil wear and pushes more paste under the foil.

Blade Materials and Their Hardness Range

Polyurethane blades dominate because they can be moulded to a controlled hardness and they conform to small irregularities in the stencil. Metal blades, normally stainless steel, are far harder and are chosen where paste must be sheared aggressively, such as dense deposits on a very flat foil.

Composite blades pair a polyurethane lip with a stiff carrier, so they transfer pressure more like metal while keeping the release behaviour of a polymer tip. Whatever the construction, the blade has to be matched to the stencil, because a metal blade running against a damaged foil concentrates wear on the high points and polishes them flat.

Durometer, Paste Release and Fine Pitch

Release is where hardness shows up in the deposit. A blade that is too soft leaves a smear of paste at the trailing edge of the stroke, and on a fine pitch device that smear bridges adjacent openings. A blade that is too hard leaves a clean stencil but can tear the edge of a deposit when the aperture is marginal.

The release conditions depend on area ratio as much as on the blade, since a low ratio aperture releases poorly whatever its hardness. That is why a hardness change should follow a review of aperture geometry and foil finish rather than replace it, and the working figures are set out in our paste release notes.

When Blade Hardness Drifts

Polyurethane absorbs flux solvents and slowly hardens, and the lip also takes a compression set in the middle of the print area. The working band becomes thinner and locally harder than the ends of the blade. Both changes are gradual and neither is obvious during a routine visual check.

Temperature adds to the effect. A blade on a machine that runs warm, or one that has just been cleaned in solvent, reads harder than the same blade at the start of a shift. Measure at room temperature after the solvent has flashed off, and file the result against the blade serial number.

Matching Durometer to Pressure and Speed

Print pressure and blade hardness both control fill, and they are not interchangeable. Raising pressure on a soft blade flattens the lip and grows the contact area, so the gain in fill is modest. Fitting a harder blade at the same print pressure raises peak pressure without changing the contact arc, which is a sharper change.

Speed adds a second variable. A fast stroke with a hard blade gives the highest peak shear, a slow stroke with a soft blade gives the gentlest. A blade change should therefore be handled as a process change, with the reasoning behind blade selection recorded and the print verified again. The SMTA publishes guidance on this kind of process characterisation work.

Inspection and Replacement Points

Inspect the lip for nicks, for a polished band across the middle and for paste creeping around the blade ends. A nick leaves a repeating streak of missing paste at the same place on every panel, and a polished band is evidence of abrasive wear that has also changed the durometer. Both faults bias the print in one direction.

Replace on measurement rather than on feel. Keep the initial hardness on file and change the blade once the lip has moved by more than about five points or the contact edge has worn back to the carrier. Where two blades are alternated, keep the pair matched so that the print does not change when the blades are swapped.

Running a Durometer Check on the Line

The check takes a few minutes and needs no more than a gauge and a flat bench. Press the gauge on the lip at both ends and at the centre, take at least five readings, and record the median with the room temperature. Avoid reading on the moulded corner, where the material is thicker and the number is meaningless.

Keep the history in the same file as the squeegee angle and pressure settings, so that a movement in the number can be compared with a movement in the deposit. In most cases the data shows the blade hardening months before the deposit evidence appears, which is the point at which the change is cheapest to make.

FAQ

Is a harder blade always better for fine pitch? No. Hardness raises peak shear, but it also raises the load on a foil that is thin and easy to stretch. Fine pitch printing depends first on area ratio and foil tension, and hardness is the last of the three to adjust.

How long does a polyurethane blade last? Service life depends on pressure, paste chemistry and cleaning method rather than on a fixed number of strokes. Blades used at high pressure with solvent based cleaning harden fastest, and those are the blades that should be measured most often.

Can a blade be re-cut or reversed? Some blades can be reversed to present a fresh edge, and some holders allow the lip to be trimmed. Treat both as a new blade: measure the hardness again and re-verify the print, because the geometry has changed even if the material has not.