What Is Ink Viscosity?

Viscosity describes a material’s resistance to flowing. A high-viscosity ink resists movement and feels relatively thick. A lower-viscosity ink flows more easily.

However, viscosity should not be reduced to a simple choice between “thick” and “thin.” The correct viscosity depends on the ink formula, temperature, mesh count, artwork, garment, squeegee, and desired ink deposit.

Screen printing ink viscosity affects how easily the ink floods the screen, passes through the mesh, releases onto the garment, and holds the intended detail. Ink that is too resistant to flow may be difficult to print, while ink that flows too easily may spread, lose opacity, or penetrate too deeply into the fabric.

An ink may have the correct viscosity but need mixing because pigments or specialty ingredients have settled. Some inks may feel unusually thick because it’s cold rather than because its viscosity needs adjusting.

Why Plastisol Changes While You Print

Plastisol is generally shear-thinning. This means its viscosity decreases when it is stirred, mixed, or pushed with the squeegee blade.

Because of shear thinning, adding reducer before mixing is a common mistake. Ink that initially appears too thick may become excessively thin after it warms up and is worked on press.

A bucket of ink that feels very thick when it’s first opened may become creamy and much easier to print after thorough mixing.

Premixing the ink may help prevent its viscosity from thinning excessively during production as it continues moving across the screen. This helps printers achieve consistent coverage and detail throughout an entire run of garments.

Temperature Has A Major Effect

Cold ink becomes more resistant to flow. A container stored in a winter delivery truck, cold warehouse, or garage may feel dramatically thicker than it will at normal room temperature.

Allow the entire container to warm naturally before evaluating it. A five-gallon bucket may remain cold in the center long after the outside feels comfortable.

Do not place plastisol on a heater, heating pad, flash dryer, or conveyor dryer. Plastisol cures through heat, and direct heating can partially gel the ink along the container walls or bottom.

Press temperature also matters. Repeated flashing can heat the screen and ink, causing the viscosity to decrease during a production run. Excessive heat may eventually begin gelling the ink inside the mesh.

Different Inks Need Different Viscosities

There is no ideal viscosity that applies to every screen printing ink.

High-opacity whites and bleed-resistant polyester inks often have more body because they must provide coverage and control dye migration. Process inks and soft-hand colors are usually designed for thinner deposits and higher mesh counts.

Specialty inks create additional differences:

Do not thin one product merely to make it feel like another. Evaluate whether it performs correctly for its intended application.

How Viscosity Affects Printing

Ink with excessive resistance to flow may:

Ink that flows too easily may:

The correct viscosity allows the screen to flood completely and the image to clear with controlled pressure while preserving detail, opacity, and deposit thickness.

Mesh Count And Viscosity Must Work Together

Higher mesh counts have smaller openings and deposit less ink. Lower mesh counts have larger openings and normally deposit more.

A fine-detail ink may be formulated to print through high mesh, while an opaque white or specialty product may require a more open screen. Trying to force heavy ink through excessively fine mesh can cause poor clearance and unnecessary pressure.

Reducer cannot make glitter flakes or metallic particles smaller. When particles cannot pass through the mesh, the correct solution is a more appropriate screen mesh rather than thinner ink.

Emulsion thickness also affects the total ink deposit. Thick emulsion creates a larger ink reservoir and may limit the ink flow and reduce detail even when the viscosity is unchanged.

Squeegee Selection Changes How Ink Prints

Squeegee hardness affects how much the blade flexes and how much ink is deposited.

Our Squeegee Rubber is available in several durometers:

A soft blade may make thick ink easier to deposit, but it can also reduce detail. A harder blade shears ink more cleanly and provides better control, although it may not lay down enough ink for an opaque or specialty print.

Angle, speed, edge condition, and pressure also matter. Before changing the ink’s viscosity, make sure the squeegee is sharp and the printing technique is appropriate.

Mixing Ink Correctly

Bring the ink to room temperature and mix from the bottom upward. Scrape the sides and corners until the color and consistency are uniform.

For large containers, an Ink Mixing Drill Bit can help blend settled material. Its four-way counteraction moves ink in opposing directions and helps disperse ink evenly.

Use a controlled mixing speed. Whipping the ink aggressively can introduce air, which may create bubbles or irregular deposits. Allow heavily aerated ink to rest before printing.

Mixing should make the ink uniform. It should not be used to grind cured particles or crush glitter and metallic flakes.

Check The Setup Before Adding Reducer

If ink seems too thick, inspect the entire printing process before changing the formula.

  1. Bring the ink to room temperature.
  2. Mix it thoroughly.
  3. Confirm that the mesh suits the ink.
  4. Inspect screen tension and off-contact.
  5. Check stencil thickness.
  6. Inspect the squeegee edge.
  7. Use controlled angle, speed, and pressure.
  8. Test the ink on the production garment.

When To Use A Viscosity Reducer

A reducer may be appropriate when properly warmed and mixed plastisol still needs a small adjustment for the selected mesh or print deposit.

Auto Formula is designed to lower plastisol viscosity without unnecessarily harming pigment strength.

Our recommended mixing instructions for Auto Formula:

Auto Formula may be added to any plastisol ink made by One Stroke Inks. It is available in 8 oz. bottles and gallon buckets. We can mix Auto Formula Additive into your ink before we ship it to you.

What Excessive Reduction Can Do

Reducing viscosity changes more than the effort required to move the squeegee. Too much reducer may:

Never thin plastisol with water, screen wash, mineral spirits, paint thinner, or household solvent. Use only an additive designed for the ink chemistry.

Choose An Ink Designed To Print Easily

Sometimes the best solution is selecting an appropriate ink rather than modifying an ink designed for performance.

Our 480 Series is an easy-printing general-purpose plastisol for cotton and poly/cotton garments, particularly on light fabric or over a white underbase.

For shops printing polyester, blends, and heat-sensitive garments, the ELT-S Series combines easy printability with low-temperature curing, stretch, and bleed resistance.

Choosing an ink designed for the fabric and artwork usually produces more reliable results than heavily reducing a product intended for another purpose.