Hard Particles in Plastisol Inks

Have you noted small, hard particles of plastisol ink on the screen? Perhaps you have noted that the ink does not clear the mesh completely, and there seemed to be ink particles stuck in the mesh openings.


Bill Hood, ASDPT Fellow

This is not normal and signifies that you are on a downhill slope in your ink care. The ink is reduced to very fine particles by grinding in a mill at the factory. The ink passes through two rollers that are separated in a few microns to create consistency. The constant ink bashing on social media is nothing more than ignorance and a buyer’s pride in whatever ink the basher uses. It is quite impossible that the ink is to blame for your problem.

In truth, you did something incorrectly to cause your problem.

Causes

The small, hard particles are usually a sign that heat has been applied to the ink and are a result of semi-curing of the ink as a result of one more actions;

    1. Using a power drill that creates friction during mixing. The bit moving rapidly against the ink creates friction, just as rubbing two pieces of wood together creates friction and can create a fire. When the ink components move relative to each other, the friction converts kinetic energy into thermal energy. The first note of this is that the ink will move to a lower viscosity (become thinner) and then quickly move to a higher viscosity (thicker) before separating into small, solid particles. As the thermal energy is created in the center of the ink’s vortex, it is not easily visible to the person performing the task. The faster the movement of the stirring, the quicker the ink will become semi-cured. Eventually, and with enough kinetic energy, the entire container of ink can turn into a solid, cured ink.
    2. Setting a container of ink on top of the dryer. In this instance, the heat is coming from the hot metal casing of the dryer. The components at the bottom of the container will become prehardened long before the ink’s top surface is warmed. There will be small hard particles of ink at the bottom of the container. Once stirred, the semi-cured particles will be spread throughout the container.
    3. Setting the container under a flash gel unit. Here the result is the same as setting the container on top of the dryer, but the heat is coming from above. The ink at the top of the container will become semi-cured, and when stirred, the particles will again be spread throughout the container.
    4. Using a Flash Gel Unit. The flash heats up the platens, which in turn heat up the mesh and ink in every screen following the flash. This heat can cause the ink to become semi-cured in the screen. Always use a cooling station after flashing.

In short, any time you heat the ink in any fashion before being transferred to the substrate will create printing problems.

Stirring

There is no “Ready-For-Use” (RFU) ink. Just like any other viscous material, all inks require stirring before use. There are two reasons for stirring.

Related Article: There Are No RFU Inks

    1. Viscous materials settle during shipping and storage. The heavier components sink to the bottom of the container and stirring from the bottom to the top blends the components to the correct consistency. If you remove ink from the top of the container without stirring, the pigments that create the opacity will likely have settled to the bottom requiring multiple passes of ink to create opacity.
    2. Stirring at a slow and even pace allows the ink that has settled into a higher viscosity to return to its intended viscosity making it easier to print, while providing opacity, and doing a better job of blocking dyes from the substrate migrating to the surface. All ink should be stirred from the bottom to the top.

Use an industry-specific ink mixer, such as the M&R Turn-About or the AWT Tornado Ink Mixer, to stir inks. These units work at the proper speed to reduce the kinetic energy converting into thermal energy. These machines are equipped with blades that move the ink from the bottom of the container to the top. The action ensures evenly blended inks by rotating the container with a lifting and rolling action that lifts the heavier particles that sink to the bottom, creating a better ink consistency.

Without a proper ink mixer, you will need to use a heavy and durable stirring blade to stir the inks. You will no doubt find this difficult when stirring a high-viscosity (thick) ink in a gallon container, let alone a 5-gallon container. Remember: All ink should be stirred from the bottom to the top. This action may not create the consistency needed and is ultimately a waste of energy, time, and resources.



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