Volume 5  Number 2                            Dennis R. Dinger                                1 December 2006

Updates

"... for Ceramists" Series Books

The paperback version of Characterization Techniques for Ceramists is available on the Books and Downloads page at the web site!    Retail price is $29.95 plus shipping and handling. The book has 256 pages and it covers 34 different characterization techniques that are commonly used by ceramists.  Order your copy NOW!

The book sets on the web site have also been revised to include this new book.  A 3-book set of paperbacks, including one each of Particle Calculations for Ceramists, Rheology for Ceramists, and Characterization Techniques for Ceramists, is now available for $64.85 plus shipping and handling.  This is a $10 saving off the total retail price of the 3 paperback books.  A 3-book set of downloads is also available for $52.85.  This, too, represents a $10 saving off the total retail price of the 3 downloadable books.  

The E-Book version of Characterization Techniques for Ceramists is available for downloading at the Books and Downloads page of the website for $24.95.  The download is a 2.889 Mb self-extracting Zip® file for the Windows® environment which unzips to the 2.998 Mb book in PDF file format.  Those of you who order the downloadable book will want to know that the PDF book is formatted to print on 5.5" X 8.5" paper (i.e., 8.5" X 11" sheets cut in half.)

The other two books, Rheology for Ceramists and Particle Calculations for Ceramists, continue to be available for purchase as downloadable E-books and as paperback books at the Books and Downloads page of the web site.

The E-zine

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Can You Connect the Dots?

Introduction

Have you learned about all of the factors that can and do affect your production bodies?  Do you understand all of the details of each process variable?  Do you understand all of the cause and effect relationships that control body properties?  

If you answered, "Yes!" to each of these questions, then there is only one more question to be answered:  When presented with a processing mystery, can you connect the dots?  Can you see (and explain) the linkages from raw materials properties to processing properties and on to solutions of processing problems?

Understanding all about particle packing, rheology, suspensions, and processing properties is one thing --- but are the linkages visible to you?  The following is an example which shows the linkages in one processing problem out of my past.

A Milling Example

Years ago, on a consulting visit, I was asked to solve a processing problem.  Dry and fired shrinkages were producing wares that were too large for the size specifications. That was the immediate problem.  In response to process changes to reduce shrinkages, green strengths of the wares had also deteriorated to the point that the green wares could hardly be handled without incurring major damage.  

After asking lots of questions, I learned that over several months prior to my visit, the process engineer had been "tweaking" the milling operation to improve "milling efficiency."  Nothing else, really, had been going on.  His definition of "efficiency" had to do with successfully putting more and more powder through the mills while continuing to feed successfully-milled product to the next process step.  As far as they were concerned, this had absolutely nothing to do with the current processing problems.

It was obvious that the parts were not shrinking sufficiently to fall into the proper size specification window -- so forming operations were altered to produce more internal porosity in the green ware.  (This produced the weak green wares.)  Further adjustments, however, could not be made.  The parts were falling apart as it was and the wares were still borderline with regards to the size specifications.

Can you connect the dots?

          Fired Sizes Out-of-Spec

Let's work backwards for a few moments.  Most ceramic parts have fired size specifications.  Some don't.  As long as a toilet bowl fits into its box, it is not really a big deal whether its size is exactly identical from piece to piece.  When making ceramic tiles or dinnerware, however, they should all be the same size.  If not, problems result.  One tile company, for example, routinely produced tiles that varied slightly in both size and color.  Their initial solution to these problems was not actually a solution which could solve the problem --- they used nine warehouses to hold three different sizes of tile with three slightly different colors to accommodate their process variations.  The "solution" was actually an "accommodation."  Later, after implementing PPC, they were able to eliminate both variations and return to one warehouse with one color.  

Many technical ceramics have very tight size specifications.  Unless fired machining will be used, dry and fired shrinkages must be tightly controlled.

What causes too much or too little shrinkage?  Green and dry compact densities that vary due to particle size distribution and particle packing variations can cause such variations.  Which produces which? 

When particle packing is poor, compact densities will be low and internal porosities will be high.  During firing, low density green and dry compacts (with overabundances of porosity) can shrink a lot to produce wares that are smaller than desired specs.  If firing is altered to produce properly sized wares, fired internal porosities can be too high and strengths can be low.

When particle packing is really good, compact densities will be high and internal porosities will be low.  During firing, high density green and dry compacts (with little porosity) can shrink too little, producing wares that are larger than desired specs.  Once full density is achieved during firing, the firing process will not produce more shrinkage.  Internal porosities will always be low in these cases, although strengths can be high.

          High vs Low Density Compacts

What factors affect the density of compacts?  Particle size distributions are a major contributor to compact densities.  Processing conditions also affect the density of compacts.  

                    Particle Size Distributions

Many articles in this e-zine have been devoted to the effects of particle size distribution on particle packing phenomena.  Some distributions (usually especially broad, continuous distributions) pack very well to produce low porosity compacts.  Monodispersions and other very narrow particle size distributions, however, pack poorly to produce high porosity compacts.

                    Processing (Forming) Conditions

Some forming processes also affect the densities of the compacted green wares.  During dry pressing, low pressing pressures can produce weak parts with high porosities, while high pressing pressures can produce strong, dense parts.  Highly flocculated suspensions can produce low density, soft casts while highly deflocculated suspensions can produce fairly dense, hard casts.  A range of processing conditions as well as processing chemistries can alter ware densities.   

Regarding variations caused by additive chemistries, one usually finds that other body properties will limit the range of flocculation/deflocculation that can be used in a particular process.  Highly deflocculated bodies tend to be dilatant, while highly flocculated bodies tend to be very shear-thinning. 

Solids contents are another variable that can be included in this category of additive chemistry.  As particle size distributions vary, interparticle spacings (the average distance between suspended particles) can vary, OR solids contents can be altered to produce constant suspension viscosities.  Solids content variations are then used in conjunction with additive chemical tuning to produce desired rheological properties.  All of these variables affect the green and dry porosities of wares.

          Milling Variables

What causes particle size distribution variations?  Milling operations can alter product particle size distributions.  There is a large body of literature available that discusses the various types of mills that can be used, and the variables that can be altered to control mill products in each different type of mill.  Variables such as media sizes and media contents in mills, powder feed sizes and powder contents in mills, dry vs wet milling, mill speeds, etc., all will cause changes in product particle size distributions.  Scaling mills up from lab to pilot-scale to production sizes frequently changes mill product sizes.  "Tweaking" mills to improve milling efficiencies can certainly also alter powder product sizes.

Connecting the Dots in the Example

Enough hints have been given that you should be able to explain what happened in the milling example described above.  Here's what happened:

1.  The original powders apparently were a narrow particle size distribution that packed poorly.

2.  In the process of "tweaking" the mill operating conditions to improve milling efficiencies, the particle size distribution was slowly becoming broader, and the broader particle size distribution was slowly packing better and better than the original powder.

3.  This produced denser green wares.

4.  Firing of the denser wares produced less shrinkage.

5.  Fired sizes became larger and larger until they were out-of-spec.

6.  To solve the denser green ware problem, the process engineer altered the forming operation to produce higher porosities with the new, broader, better packing particle size distributions.

7.  Green strengths then decreased in response to these changes, although shrinkages improved, and fired wares began to move back into the desired size specification ranges.

8.  It was not possible, however, to alter the forming process sufficiently to produce enough porosity to solve the oversized ware problems.

9.  Options:  retool (change all mold sizes) to accommodate the new shrinkages produced by the new, broader, particle size distributions, OR call in the consultant.

The Connected Dots:

When asked "When did these problems begin to occur?", the answers indicated that the porosity changes and the fired size problems both began to occur concurrently with the commencement of the "tweaking" of the mill efficiencies.

(1)  The "tweaking" to alter mill efficiencies also (2) altered particle size distributions which (3) altered compact densities (4) as well as green and dry porosities which (5) altered green strengths which then (6) altered fired shrinkages.  The process correction that had been made -- to try to correct the problem by altering forming process variables -- was a good choice.  That solution showed promise!  But those alterations could not be made sufficiently far-reaching to completely solve the problem.

The porosity and out-of-spec size problems went away almost overnight when the other variable was altered (actually -- when it was reset):  the mill was reset to its original (less "efficient") conditions which were present before the "tweaking" began.  The new (old) particle size distribution packed poorly and produced lots of porosity in the green wares, which produced sufficient shrinkages to put the fired wares back within their size specifications.

Connecting Dots?????

Can you walk through your own processes step-by-step and explain how each processing variable affects the next, and the next, and the next, ... all the way from individual raw materials properties through to fired product properties?  Hopefully the answer from all of you is "Yes!"  (That should be the answer for all process engineers.)

If not .... ?????

If you can't answer "Yes!" to that question, you should set it as your goal.  You should understand your process so well that you can connect the dots from raw materials properties, through processing variables, to final ware properties.  

If you couldn't answer "Yes!", send me your processing questions and we'll try to fill in the gaps and move you in the desired direction -- so in the future, your answer will be an unqualified "YES!".