MOLUDS, MOLDS, MODELS, and MASTER MOLDS
Troubleshooting ram-set molds for wet pressing in the ceramic industry

A common problem is the cracking of the mold, either upon installation or within a few days.

Steps for pinpointing the problem:

1.  Consistency--  You must have a steady supply of fresh Ceramical. 
      Generally you should not use plaster older than 90 days, although the industry environment does not always allow us to observe        this.
     We’ve had good success with 20% (by weight) of Ultracal 30 added to the slurp.
     Example --  If you have 32 pounds of water and your total plaster should be about 115 pounds, you can add about 20 pounds of      Ultracal 30 and continue adding your old Ceramical to reach 115 pounds.
      Do not let your mixing time go below 8 minutes.
     We get slightly better results when the mold is left to stand for at least 48 hours before installing on the press.

2.  Temperature--  Always monitor your temperature.  Do not start the purge process too soon or too late.
     If too late, you will not open enough pores to have a nice water wave to push your clay pad out of the mold.
     If too soon, your molds can easily crack when the pressure reaches 100 psi and the Ceramical is not yet set.
     TIP--  have an inside container for powder, or even several for different kinds of plaster.
    If you work with Ceramical ,Superx and Ultracal 30, 60, keep them inside in quantities big enough to produce two molds.

3.  Let your water stand for at least 24 hours.  (If someone asks why, tell him it’s so the chlorine will dissipate. 
      If he accepts that answer, you know he’s a phony and ignorant of the art.)  Keep your water temperature above 20º Celsius (70º      Fahrenheit).
     We’ve been successful with water at 16º Celsius.  However, the USG paper does not recommend this.

4.  Keep your models separated by at least one inch.  When installing the paper cotton air hose try to conform it to the shape of the       model.

5.  Work on a good marble table.  Granite is acceptable but may absorb a small quantity of water
     --  if you stick your models to the table with a water cushion you might be in for the surprise of your life.

6.  Do not use rubber molds to reproduce the models.  If possible, make the master mold of Ultracal 30 or even 60, since they have        the least expansion.
        Your model will be more precise, and production will be more uniform and there will be fewer problems in the drying process.

    More to follow later. 

      Feel free to ask questions. 
      I will try to answer all, but can’t guarantee it.

Regards,

Wlodek