Why Layering can be Beneficial: Conductivity and Slurry Stability

Why Layering can be Beneficial: Conductivity and Slurry Stability

by | Apr 23, 2012 | Uncategorized

“How do I know when my slurry is going bad?”  I have heard that question many times and I imagine standing over the slurry tank admonishing it by saying, “Bad Slurry! Bad Slurry!”  But, let’s put humor aside, take a step back and understand the parameters that impact slurry stability.  Most people understand that low pH is an indication of compromised slurry health. But, conductivity is frequently overlooked as another important indicator.layering

Conductivity is a numerical expression of the ability of an aqueous solution to carry an electric current.  This depends on a number of factors including the presence of ions, total concentration and temperature.

In slurries using colloidal silica, the conductivity value reflects the ionic charge across the microscopic amorphous particles of silica dispersed in the aqueous solution.   Let me try to explain it in slightly simpler terms.

An aqueous solution means that water is the solvent.  As a reminder, water is made up of Hydrogen ions (H+) and Hydroxide ions (OH−).  Colloidal silica particles are spheres of silicon dioxide (SiO2)with a negative surface charge (SiOH−).  The water ions (H+ and OH−) form an electric double layer that results in a neutral charge surrounding the silicon dioxide molecules.

As the concentration of ions increase, i.e. conductivity increases, there is less space between the ions and the double layer begins to collapse and the silica particles are forced together forming the following permanent bond:

SiO2 – Si – O – Si – SiO2 + H2O

This reaction is what forms the gel network that holds the molds together when molten metal is poured into the shell.  However, we want this reaction to take place during the shell building process, not in the slurry tank.  If this reaction is occurring in the slurry tank, shell strength may be reduced.

Each colloidal silica has an optimal conductivity range so check with your supplier for specific information.

Now that you have a basic understanding of conductivity, next week we will discuss parameters that impact conductivity and how to address those issues.

Shark Teeth or Iowa Cornfield? Slurry Control Chart Tips

Shark Teeth or Iowa Cornfield? Slurry Control Chart Tips

by | Apr 16, 2012 | Process Control

If your slurry control charts look more like shark teeth than an Iowa cornfield, perhaps you have some opportunity to control your process better.  If you don’t have slurry control charts, do you know what is happening in your slurry over time?  Charting your key parameters provides a visual indication of variability in your process.  The easiest way to reduce variability is to control what is being added to the slurry.OLYMPUS DIGITAL CAMERA

By spending some time reviewing slurry additions, you can minimize variability of key parameters during the shell building process.  Here are a few tips to consider:

  • SiO2 will climb over time so, have your target SiO2 for your make-up slurry about 1 to 2 percentage points below the center point of the dip tank range.
  • Viscosity will also climb, so, target the make-up tank to be about 2 seconds below the dip tank.
  • If pre-wet is being used, target a higher viscosity for the make-up tank.  Do some experimentation to determine the optimal viscosity range of the make-up slurry to maintain the dip tank viscosity through slurry additions.  The goal is to avoid making flour additions directly into the dip tank.
  • Make sure that the pre-wet is being monitored and controlled for key metrics including SiO2, % polymer and bacteria.
  • Educate your operators on when they should use water, binder or a binder-polymer “cocktail” when adjusting viscosity of both the make-up slurry and the dip tank.
  • Monitor and control the make-up slurry as closely as the dip tank.  Optimize the make-up slurry formulation so adjustments to the dip tank are minimal.

Steven Covey might not be an expert on precision investment casting, but he was correct when he said, “Begin with the End in Mind. ” Following these few simple steps will go a long way towards minimizing process variability while saving time and money.