Accelerated Curing and Drying of China Products

OBJECTIVE:
To determine the optimum parameters for drying complex ceramic products in order to significantly reduce production time. The products included single and multiple piece cast shapes. Products composed of multiple molded components, referred to as stick-ups, were assembled wet, prior to drying. Additionally, we were requested to investigate the minimum drying time limits of the material used for these products.

OVERVIEW:
Tests were performed over a three-day period in The Lanly Company’s Heat Process Technology Laboratory. The laboratory programmable air impingement dryer was used to demonstrate the capability of Lanly forced air equipment to dry the sample products. In accordance with our preferred development test procedures, representatives from the company were present to observe, and participate in, the testing.

The testing involved the actual casting and drying of all products under varying temperature and moisture environments. The products were inspected during and after each test to evaluate the drying process and determine the best process parameters for each piece. Pieces were examined both visually and by instrumentation.

EQUIPMENT:

• Lanly Dryer

Batch style, direct fired gas dryer with variable speed circulation fan. The system is designed to supply forced air to both the top and bottom of the product under test. Temperature and humidity are programmable.

• Balance

A & D model EK-12KA (with .05 oz. resolution)

• LumenX Audit X-Ray Inspection System - including

Thompson dual mode image intensifier
Sony XC-75 CCD camera

PROCEDURE:
The test procedure followed these steps:

1. The china company representatives cast various pieces and assembled the stick-ups using the following process:

• Check slip to confirm density of 1680 grams/liter
• Pour slip into molds to slightly overflowing
• Let set for specified time
• Invert molds and let drain for recommended time
• Remove pieces from molds
• Stick the pieces together where required

2. The items were weighed
3. The items were placed in the dryer (wait times prior to dryer insertion are noted when applicable)
4. The items were remove from the dryer and weighed periodically. The time periods were determined by prior test results
5. The items were returned to the dryer for continued drying
6. Steps 4 and 5 were repeated until the weight between two subsequent readings was unchanged. The dry time was then recorded as the first of these two times; the beginning of the period where the weight did not change.

TESTING:
Ivory and bone slip, provided by the company, were cast. One of the company’s representatives observed and directed the testing.

The shapes tested included:

• Holiday Santa Figure Votive, 3-piece stick-up
• Santa’s Toyshop (Nutcracker Santa), 2-piece stick-up
• Classic Creamer, 2-piece stick-up
• Debut Sugar Vessel
• Mothers Day Vase 1999
• Globe Vase

The Creamer and Sugar bowl were cast in bone only. All of the other shapes were cast in both bone and ivory.

TEST RESULTS:
Humidity was only induced for the first six pieces, three each of the Holiday Santa Figure Votive and Nutcracker Santa. The humidity reading of 32% was within the range of humidity readings (25% to 32%) obtained in a previous series of tests.

In previous production testing, humidity was used to obtain better results on the Holiday Santa Figure Votive. It was found, however, that if the parts were carefully cast and the assembly was out of the molds for at least one hour prior to entering the dryer, we were able to dry this difficult piece without any induced humidity.

Chart A

With induced humidity, both the Holiday Santa Figure Votive and the Nutcracker Santa, cast in ivory, were dried in 75 minutes. Following the new procedures, the ivory Votive dried in 30 minutes, after being out of the mold and under plastic for 4 hours prior to drying at 400° F.

Chart B

Using the same parameters, the Nutcracker was dry in 15 minutes. When the stick-ups were under plastic for 5 hours before being subjected to 500° F, the Votive was dry in 20 minutes and the Nutcracker in 16. The comparison of these dry times, along with others, is shown on Charts A and B.

All of the pieces were dried successfully without induced humidity at both 400° F and 500° F. The results can been seen in Charts A and B. The dry times listed, based on our test procedure, are the times at which we reached a weight that was not decreased with further time in the dryer. Some pieces, however, lost only .05 oz in the final 3 to 5 minutes prior to being totally dry. Depending upon the level of dryness required, these pieces could be considered dry at those times. This would shorten the cycle time even further.

X-rays were taken of the Holiday Santa Votive to examine it in greater detail. The equipment used included a high frequency source for fine resolution. Samples were inspected for internal stress cracks and material voids. The samples were imaged at 40 kV @ 6mA, magnified and then captured on the Sony CCD camera. The results were printed on a photographic image printer. For the most part, the pictures supported our visual findings; however, there were substantially more voids than anticipated. It was agreed that the voids were directly related to the pouring and setting of the molds and were a significant contributor to internal stress fractures.

SUMMARY:
With properly cast parts, it is clear that the company’s ware, both individual pieces and stick-ups, can be successfully dried in a relatively short time using Lanly impingement air equipment. It was demonstrated that even stickups could be dried at both 400° F and 500° F. Table I summarizes the results of the three-day testing period. Although each of the products requires a different drying profile, the average reduction in drying time for all products was greater than 600%.