What does viscosity mean for dispensing?
What does viscosity mean for dispensing and why does it matter when talking about robotic dispensing applications? Merriam Webster defines viscosity as:
A: technical: the property of resistance to flow in any material with fluid properties
Water has a small but measurable viscosity, or “stickiness,” which results from the weak mutual attraction of water molecules.— Hans Christian von Baeyer
B: The mathematical ratio of the tangential frictional force per unit area to the velocity gradient perpendicular to the direction of flow of a liquid
— called also coefficient of viscosity
Basically, how thick is the fluid, adhesive, or sealant that you are trying to dispense.
Why does viscosity matter?
Different materials that have a different viscosity behave differently when dispensed and each has their own challenges.
A thin cyanoacrylate that has a very low viscosity (like water) will be very easy to dispense quickly. Cyanoacrylates are a challenge because they are so runny that can quickly make a mess. They also tend to drip from the dispense tip. This can make a mess of components and tooling. A cyanoacrylate or thin adhesive can be a good choice when a wicking action is needed to disperse the adhesive between two fluids.
A thick (high viscosity) adhesive or sealant will require more force or pressure to dispense. Thick materials tend to leave annoying strings at the end of the dispense path. High viscosity materials may require larger dispense tips, slower application speeds, or positive displacement dispensing valves or equipment. Viscous material can be a good choice when a large bead is needed to seal between mating components.
How is viscosity measured?
Centipoise (cP or cPs) is the most common unit noted when discussing viscosity of adhesives.
1 centipoise = 1 MPa s (millipascal second)
This measurement is referred to as the dynamic viscosity and is sometimes referred to as the absolute viscosity. Another measure of viscosity is the kinematic viscosity. The kinematic viscosity is the ratio of the absolute (or dynamic) viscosity to the material’s density. For our purposes we will skip over kinematic viscosity for now. A nice breakdown of kinematic viscosity is discussed at the Engineering Toolbox.
There are a variety of ways to measure viscosity. Two of the most common methods of determining a material’s viscosity are the viscosity cup and the rotational viscometer.
A viscosity cup relies on gravity to allow the material to flow through a hole in the bottom of the cup. A known amount of material is in the cup and the amount of material that flows through the hole in the cup can be measured over a length of time to calculate the viscosity.
A rotational viscometer is arguably the most common method of measuring viscosity. A spindle attached to the viscometer is placed into the material and rotated. The viscosity is measured by evaluating the torque needed to rotate the spindle at a fixed speed. A rotational viscometer can measure viscosity up into the millions of centipoise.
There are a variety of other viscometers available including the glass capillary viscometer and the consistometer.
It’s always helpful to have some type of reference when thinking about viscosity. Here are a few example viscosities for reference:
Motor oil SAE 20
Motor oil SAE 50
We hope that gives you a brief introduction to viscosity and helps to begin to answer the question: What does viscosity mean for dispensing? There are many types of adhesive and sealants to consider when setting up a manufacturing process. Careful consideration of viscosity will help choose the right material for both the design of your product as well as the manufacturing process required to build your product.
Additional Definitions that might be helpful:
Thixotropic fluid: A thixotropic fluid has a decrease in viscosity as the stress applied to the fluid increases over time. For example, a jar of mayonnaise can be turned upside down and the mayonnaise will not flow out of the jar. But, if you attempt to stir the mayonnaise in the jar, it moves very easily.
Newtonian fluid: A Newtonian fluid is neither thixotropic nor dilatant and can be called a true liquid. The viscosity of a Newtonian fluid remains the same if it is static or in motion.
Dilatant fluid: A dilatant fluid is one that increases in viscosity when shear is applied to the fluid. Silly putty is an example of a dilatant fluid. Dilatant fluids are not good candidates for automated or robotic dispensing.
Rheology: Rheology is the field of study that is related to the flow of materials.
Shear-thinning or Pseudo-plastic fluids: These types of fluids see a reduction in viscosity as shear is applied. Paint, cake batter, and applesauce are examples of fluids that see a reduction in viscosity as they are stirred or mixed.
Bingham plastic fluids: A Bingham plastic fluid is one that needs a small amount of shear applied to begin to flow. Most Bingham plastic fluids will flow they behave as Newtonian fluids with constant viscosity. Examples of Bingham plastic fluids are toothpaste, blood, and tomato paste.