Volume 6  Number 8                            Dennis R. Dinger                                1 June 2008

Updates

"... for Ceramists" Series Books

          Requests for Multiple Copies

I have had several recent inquiries about the purchase of multiple copies of these books.  Here are my two suggestions:  

          (1)  If you purchase downloadable versions, purchase the required number of copies (please be honest about the number) from the Books and Downloads page of this website.  Then download a single copy and distribute it (or print it and distribute it) to the people for whom you purchased the copies. ... or ... 

          (2) Purchase the required number of paperback copies from the Books and Downloads page of this websiteand distribute them to your people.  My books are priced $19.95, $24.95, and $29.95 with this in mind.  You won't find many other good ceramics books in this price range.  Most others start at $80 to $100 each and prices rise from there.  For example, our PPC book (when it was available) was $195 per copy.  (I had no input when that price was set.  During one phone conversation, after they made sure I was sitting down, they simply told me the price.)          

          Spanish Language Books

For those of you who speak Spanish as your primary language, a downloadable PDF version of Rheology for Ceramists in Spanish is currently in progress.  Reología para Ceramistas is currently being edited to be made available as soon as possible.  Best estimate at this time is that it will be available sometime in 2008 because the editing process is proceeding slowly.  The PDF file will be set up so it can be printed on your printer if you prefer a hard copy.  Depending on the reception this version receives, I will then consider translating the Particle Calculations book as well.  I will also then consider translating it into Portuguese.  Any thoughts, comments, and/or suggestions will be appreciated.

          English Language 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.  Purchase a 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

If this is the first issue of the Ceramic Processing E-zine that you've seen, you can add your name to the mailing list by clicking HERE.  All back issues can be accessed from the Publications page at the web site.  For those of you whose e-mail programs don't properly show the figures in these E-zines, go to the Publications page of the web site using your web browser to open any and all issues.  All figures should open properly when issues are accessed from the web site.  Questions, suggestions, and/or requests for topics to be covered in future issues of this e-zine can be sent to QuestionsandComments@DingerCeramics.com   .

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This topic was suggested by a reader.  Part III next month will cover impurities and additive effects.

Raw Materials' Effects on Ceramic Processing:
Part II -- Casting Process & Product Properties

Introduction

As raw materials and properties change, casting rates and cast structures change.  On one job site, we struggled with low casting rates for a long time and the solution came down to levels of chemical additives, the order in which the chemicals were added, and particle size distributions (and specific surface areas.)  In this article, we will briefly discuss some of the fundamental adjustments that can be made, and properties that will result from the slip casting process.

Casting Rates and Associated Properties

As mentioned above, the first and possibly most important cast property is casting rate.  How does one adjust this property?

          Increasing Casting Rates

When casting rates are low, there are two adjustments that must be made in combination with one another to cause the rates to rise.  The solids content must be decreased AND the suspension must be flocculated.  Adding water dilutes the slip which causes the viscosity to decrease.  Adding flocculant raises the viscosity and it also causes the gelation rate to increase.  Typically by making both these adjustments, casting rate should be higher when the viscosity is returned to its original value.  

Some casting slips appear to have no gelation at all.  For such slips, the viscosity vs time plot resembles Figure A below.

                               Figure A.  A "Gelation" Curve Demonstrating Little Gelation

This type of "gelation" curve shows very little gelation.  If anything, the gelation that occurs takes place over a much broader Time axis than shown in Figure A.  Such a slip could take as much as 30-45 minutes, or more, to gel to the viscosity shown.  The casting rate from this type of slip will be low, and the cast itself will tend to be dense and brittle.  This type of cast does not work well with green finishing -- it is more appropriate to dry finishing.  To open up the gel structures in such slips and the cast structures in such casts, the slip must be diluted and flocculated.  To flocculate only will cause the slip's viscosity to rise --- which may make the slip too thick to use in the process.  

One would prefer the type of gelation behavior shown in Figure B. 

                            Figure B.  A "Gelation" Curve Demonstrating Strong Gelation

This gelation behavior is fast and it levels off to a constant viscosity with time (the 'normal' curve).  Whether the slip takes 20, 40, 60 seconds, or more, to reach its maximum viscosity does not matter very much.  This is the shape of the gelation curve that you want.  Especially important is the steady viscosity at the higher times.  If the slip gels too fast, some syneresis might occur at the high viscosities which will cause the limiting, stable viscosity value past 10 minutes of measurement to be slightly lower than the maximum value actually measured earlier in the test (shown in the 'syneretic' curve).  A little such syneretic behavior is okay.  Too much can be a problem.  

          An Open Gel Structure Produces An Open Cast Structure

The reason you want a fast gelation rate in the slip is that you want the structure of the gel to form quickly which will produce relatively wide, open pores, through which the water can flow very quickly and easily.  With a well-formed open structure, removal of the water into the mold will occur quickly and easily through the open structure.  When the slip gels quickly, the colloids will be tied into the gel structure where they will not be pulled along with the water and where they can not block the pores of the mold to slow further dewatering rates.  Another reason for wanting a fast gelation rate is that you want a relatively well-formed gel structure in the un-cast slip that will be collapsed against the mold as dewatering (casting) occurs.  This is the only possible way to produce a cast that has consistent properties across the cast.

When the slip does not gel quickly, gel will be forming as casting (and dewatering) takes place.  When the gel is still forming as the water is removed, the cast properties will be changing as the cast is formed.  This will produce different cast properties across the cast.  When cast properties differ from inside to outside, pores will change in size, drying rates will change, cast water contents will change from inside to outside, etc.  When the cast surface is firm and dense and the inside surface is muddy and soft, the conditions are perfect for differential shrinkage during drying and CRACKS.  

          During Pressure Casting, Delaying the Application of the High Pressure

One of the reasons for using pressure casting is that the casting occurs quickly due to the high pressures utilized.  The high pressure is OK, but when used, sufficient time should be allowed for the gelation to build (and complete) in the slip before the application of the high pressure.  When no time is allowed for gelation to occur, the colloidal particles can be jarred loose from the gel structure and caused to flow with the water to the mold surface.  When the first layer of particles at the mold surface are colloids, pore sizes will be incredibly small --- and all further water to be removed by the casting process must necessarily pass through these really small pores.  This will disrupt the casting process and the benefits for which pressure casting was used in the first place.

Allowing gelation time before casting should applied to any casting or dewatering process --- filter pressing is a good example.  Filling the chamber and allowing gelation to occur before increasing the pressure will cause the casting to occur more quickly because the first particles cast will form a relatively open structure which will facilitate much quicker casting of the rest of the particles.  Typically, as soon as the filter press is full, the high pressure is immediately applied.

          Water Content vs Casting Rates

Many companies have sacred water contents that they require for slip casting operations.  For example, "We will use SG = 1.78 for all of our casting!"  My advice that one must dilute and then also flocculate runs contrary to such a taboo.  When taboos like this exist, casting and cast problems will almost always occur because the water content in the casting slip is a necessary property that should be considered and adjusted as the slip is adjusted with chemicals.  

Another problem is that companies which have done plaster casting for years automatically assume that slip and casting properties should be identical for pressure casting as they were for plaster casting.  Not true!  The way slip casts into plaster molds is not necessarily the proper way to run pressure casting operations.  

          Speed of Casting vs Water Content

An easy experiment to perform in your lab is to use a lab filter press to test casting rates as water contents are varied.  Take a standard sample of slip, and dilute several smaller samples of it to a variety of lower solids contents.  For example, start with slip with SG = 1.80 and then dilute smaller samples of it to 1.78, 1.76, 1.74 ..... 1.62, and 1.60.  Flocculate them all back to a standard viscosity and then cast in the laboratory press.  Measure dewatering rates and casting rates for each of these.  Lower specific gravities (when flocculated back to a standard value) will cast faster than the higher SG slips.

We may think that "More water will cause casting rates to decrease" and that may have been true due to saturation of plaster molds in some processes, but with pressure casting, "More water will cast faster" when slips are tuned properly.

A plaster casting slip that is routinely set for SG = 1.8 should pressure cast well at SG = 1.6.  Try the test and see for yourself where fastest casting occurs.

          Decreasing Casting Rates

To decrease casting rates, the same two properties (solids content and chemistry) must be adjusted.  Solids contents should be increased to bring particles closer together and to raise viscosities, and then deflocculants should be added to reduce the viscosity to the desired process setting.  These two properties (water contents and additive chemistries) should routinely be used in conjunction with one another.  To increase casting rates, dilute and flocculate.  To decrease casting rates, raise solids contents and deflocculate.

Cast Properties with Additions of Flocculants and Deflocculants

When casts are too firm, one should add more flocculants to open structures and to soften casts.  When casts are too soft, one should add more deflocculants to tighten structures and firm up the casts.  

          Water Contents and Additives

As body particle size distributions change from batch to batch, one has relatively good control of slip and casting properties if one can use both solids content and appropriate additives.  It is impossible to perfectly control slip and casting properties.  We do our best, but perfect control is out of our reach.  

To limit ourselves by requiring a particular solids content only allows one variable to control these properties:  additive chemistry.  The firmness or softness of the cast, the gelation rate of the slip, and the ability (or lack thereof) to do green vs dry finishing are all affected by our ability (or inability) to adjust the slip properly.  Taking water content off the table is a severe limitation which typically leads directly to a greater inability to adjust slip properly.  

The property of the slip that is limited when we take any one of these adjustments off the table is our ability to control RHEOLOGY of the slip.  Here is the scenario:

          As particle size distribution (PSD) of the body changes from day to day (and IT WILL), the simple addition of water to the powders to achieve a particular SG will produce slip with a particular VISCOSITY and with a particular RHEOLOGY (shear-thinning, Bingham, dilatant, etc.)

          The easiest property to control is VISCOSITY.  But each adjusted viscosity will also have a corresponding RHEOLOGY that will span the range from totally dilatant to totally shear-thinning.  In most cases, we ignore RHEOLOGY, and control only VISCOSITY.  The reason for this is that it is simple to measure viscosity and it is difficult to measure rheology.  

          Only with the two variables (solids content and additives) can we adjust a slip to achieve BOTH the desired VISCOSITY at the desired RHEOLOGY.  

          Shear-thinning slips tend to produce open casts over a range of firmness/softness levels.  Dilatant slips tend to produce tight casts which also cover a range of firmness/softness levels.  Shear-thinning slips tend to be softer whereas dilatant slips tend to be firmer.  Soft casts (with sufficient firmness) can easily be green-finished.  Firm casts (which lack sufficient softness or plasticity) are not easily green-finished.  When casts are too firm, rather than flex during green finishing, they tend to crack.  This may force the plant to use dry finishing (which is a dusty mess) rather than green finishing.  

          If we cannot adjust both VISCOSITY and RHEOLOGY, the variations in the PSD of the slip are the ultimate cause which defines the behavior of the slip and the cast properties for each batch.  

          When such daily fluctuations in PSD and slip properties occur, the expertise of the casters comes into play.  The good casters will be able to figure out how to handle the slip and the casting process, and they will be able to cast wares and produce acceptable product.  The less experienced casters will have much more problems and they will typically produce lower yields.

          With all variables ON the table, we should ALWAYS attempt to control both VISCOSITY and RHEOLOGY.  Only in this way can we accommodate the property fluctuations caused by daily PSD fluctuations.

Ions in the Raw Materials & in the Interparticle Fluid

A major problem that we have not discussed in this article deals with the impurity chemicals and ions in the raw materials which ultimately appear in the interparticle fluid of slips and suspensions.  We will discuss these phenomena and their effects on slips, suspensions, and the slip casting process in Part III of this topic.  

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...