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Switch to double-sided tablet press solves powder-sticking problem

Software experiments help Schiff Nutrition's processes, helping tablets better withstand downstream packaging and shipping processes.

Hp 19409 13623 Magnesium 250s
Schiff Nutrition International, Salt Lake City, Utah, develops, manufactures, markets, distributes, and sells vitamins, nutritional supplements, and sports nutrition products. The company recently embarked upon an engineering project to solve an intermittent problem with multimineral tablets sticking inside the dies of a high-speed rotary tablet press. They also hoped to improve the tablet's hardness in order to better withstand packaging and shipping processes.

The production staff varied press parameters without success. They then developed a statistically designed experiment. It was a mixture design that allowed specific constraints on each of the constituents. The experimental results indicated that the standard formulation provided close to optimal properties for the multimineral tablet.

“It was clear that the formulation itself was not causing the problem,” says Jarom Webster, research and development scientist for Schiff Nutrition. “So we searched for other factors that could be playing a role and discovered that when we moved the operation from a single-sided to a double-sided press the problem went away.”

Schiff's multimineral tablet production process involves a mixture of various powders that are weighed and blended and discharged into bulk polyethylene-lined sacks. Next, these sacks are moved to another processing area where the blended material is loaded into hoppers that feed the tablet presses from Fette.

Factors and responses

The primary active ingredients of the tablet are magnesium and chelated zinc. It requires three different binders that make up approximately half of its composition by weight. Additional ingredients include a lubricant, a glidant, and a disintegrant. The components experimented on by Webster were:

A. Binder 1 (from 0 to 49.3 weight % of total tablet weight)
B. Binder 2 (0 to 49.3%)
C. Binder 3 (0 to 49.3%)

The responses for the experiment included the tablet hardness at three different levels of compression in the tablet press as measured on a Dr. Schleuniger 6D hardness tester. Webster evaluated the time required for the tablets to disintegrate in a 37°C water bath. He also tested punch tightness, which measures the force exerted by the lower punch to eject the tablet from the die. This is important because under certain circumstances powder can stick to the die wall and potentially jam up the press.

Here is the complete list of responses:

1) Tablet hardness at 15 KiloNewtons (KN) compression force, measured in kilopons
2) Hardness at 30 KN
3) Hardness at 45 KN
4) Disintegration time at 45 KN
5) Punch tightness (Newtons)
6) Tablet weight relative standard deviation (RSD) (%)

Experimental design

After identifying the factors and responses, Webster used Design-Expert® software from Stat-Ease, Inc. to design the experiment.

Figure 1 demonstrates the response surface for critical hardness response. Specialized tools kept the three ingredients within their specified total. The program laid out 20 formulations, including a number of replicates for estimating experimental error. The test matrix evaluated all of the factors simultaneously and probed higher order and multiple component interaction effects.

Figure 2 is a color-coded graph displaying relative desirability of alternative formulations. Webster entered the experimental results into Design-Expert, which analyzed them, created statistically validated predictive models to describe each of the responses, and provided graphs to visualize the outputs. Webster specified what he desired and the relative weight to be placed on each of the tableting attributes. The software then identified the composition that would maximize the overall desirability and predicted the results.

Solving the problem

“Based on this experiment, the optimized mixture turned out to be very close to what we were already running,” Webster says. “The results also showed that there was a considerable amount of variability that was not accounted for by the experiment. So we began looking outside the box for other factors that we did not consider in our initial experimental design.”

Webster asked production to run an informal experiment by tracking whether the tablets were pressed on single- or double-sided presses and recorded the results. The problem went away when double-sided presses were used.

“While design of experiments did not formally solve this problem, it led to the solution by conclusively determining that factors outside the ones we had been considering were the cause of problem,” Webster says. “A key factor in the success at Schiff has been the outstanding technical support provided by Stat-Ease,” Webster notes. “They have responded quickly and with a high level of expertise whenever I have run into a problem.”

Article supplied by Stat-Ease, Inc.
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