Viscosity
If you take a Styrofoam cup and fill it up with water, poke a hole in the bottom and watch it drain out, then do the same experiment with honey, you will see a dramatic difference in flow rate through the hole between water and honey. Viscosity is defined as a fluid’s internal resistance to flow. The molecules in the liquid experience friction as they move past each other, and that interaction determines the amount of energy required to make a fluid flow. Lower viscosity fluids (water) will flow much easier compared to a high viscosity fluid (honey) using the same force. Viscosity is a very important number to know and understand when mixing and pouring casting materials into different molds and cavities.
How Do You Measure a Liquid’s Viscosity?
We use a piece of equipment called a Viscometer to measure the viscosity of a liquid. The machine applies a force by rotating a spindle through liquid materials and measures the torque on the shaft. We record the results in Centipoise, the unit of measure listed on our Technical Data Sheets. The lower the Centipoise number, the easier it flows and the higher a centipoise number is, the material will be thicker and exhibit reduced flow. See the Viscosity Chart to see some real world examples.
Why Does Viscosity Matter?
Viscosity can be a critical piece of information when choosing a casting material. If you are casting into a closed mold, the viscosity will determine how quickly the liquid material will flow through that cavity. For a thin-walled part, the material experiences more drag and can take longer to navigate through to fill the mold. If you choose a material that has a very fast work time, the material may not fill the cavity in time. With an open top mold, or a very thick-walled part, material will flow much easier through the cavity. Knowing the viscosity may allow you to choose a material with more mixing and pouring time for that part/mold configuration. If you are trying to choose a material to brush into a mold or over a surface, a higher viscosity material will hang on vertical surfaces better and create a better skin. Viscosity differences between the A & B components helps us determine how a material system may mix in a static mix tube when being dispensed out of equipment as well. A large difference in A & B viscosities can potentially cause under-mixing in a mix tube, or you may need more time and force in a mixing container to adequately combine A & B. Viscosity is also a factor when vacuum degassing a casting material since lower viscosity systems let go of trapped air easier than higher viscosity systems.
