There are combinations of factors contributing to this problem:

Dwell Time

Press Speed

Punch Head Design

Formulation Compressibility

Punch Tip Cup

Cup Design

Cup Depth

Before we explain in more depth how to solve these problems, you should first have a basic concept of how powder is formed into a solid tablet. Understanding this basic concept, we can theorize what may or may not be occurring in order to solve our problem. To compress a tablet requires a rotary or single station press and three components that constitute one station of tooling: Upper Punch, Lower Punch and Die.

During the compression cycle the die cavity is filled with powder, then the upper and lower punch move together in a vertical direction under two compression rolls to form a solid tablet. The duration of time the punches are under these rolls is called dwell time. There may be only fractions of a second of dwell time to form the solid tablet due to variables in press speeds, punch head size or punch head design.

Within that fraction of a second the following must occur: The applied compression force must reach the apex or deepest point on the punch cup contour so that the tablet core and any particle in contact with any punch or die surface can achieve adequate bonding; thereby achieving uniform tablet hardness. Also any air within the powder must be displaced around the punches so as not to inhibit the material bonding process.

Although there are other reasons why we may have a Picking, Surface Abrasion / Erosion, and Excessive Compression Force, for this scenario we'll consider our required production press speed is too fast and is providing too short of a dwell time duration. We determined this by evaluating the results of reducing press speed and found tablet quality improved. Reducing press speed thereby increased the dwell time duration permitting sufficient time for the force to reach the deepest point on the cup. Reducing press speed may solve the problem, but it will also slow production. In this scenario, any solution effecting production speed was not acceptable. Our course of action is then to evaluate the punch tip cup and determine if it is contributing to, or creating an inconsistent surface pressure problem.

Keep in mind what is occurring on the punch tip cup as the upper and lower punch are moving together in a vertical direction to compress the powder. Compression force begins at the outer punch tip perimeter and progresses toward the deepest point on the cup as the punches move closer together. We'll evaluate why our existing cup contour is not providing a consistent surface pressure, and prove if altering the cup contour will improve the compression characteristics.

At Elizabeth Carbide Die we have a computerized method for evaluating force displacement across a punch tip cup contour. Force displacement can be defined as the cup contour's ability to produce uniform tablet hardness within a given dwell time. It is possible to use this method to evaluate any cup form, but for ease of explanation we'll use a round punch tip with a Tableting Specification Manual single radius extra deep concave cup.

By evaluating the vertical force pattern on the existing "T.S.M." extra deep concave punch cup form, we can compare any subsequent patterns and establish a theory to the cause of our problem. Notice the progression of force on the cup form is illustrated as different colored rings and is due to the spherical cup design and depth. Because compaction force had to build from the outer perimeter in a series of rings to the deepest cup area, consequently the insufficient dwell time did not allow pressure to reach the deepest area on the cup to promote material bonding.

Due to the amount of volume our deep concave cup has, we'll need to use a cup design that will hold as much volume and provide the force displacement characteristics to solve our problem. A compound radius cup design may be the best design, because of the three radii construction. The outside two small radii provide the elevation for volume and the larger center radius may improve our force displacement at the apex of the cup. Now we'll apply the same amount of force to this design as we did to the deep concave cup design, and evaluate the results.

When comparing two-force displacement plots, we're looking for any change in color or pattern indicating an increase in force or change in the distribution of force to the powder. Evaluating the compound cup design, we first see a coloration change in the outer yellow zone indicating a slight increase in force due to the two small radii used to form our contour. We can change the effectiveness of these radii by increasing the size or rotation of the radii. As we move to the center of the cup, there is a new spiral pattern formed by the larger radius forming the contour. Increasing or decreasing this radius will change its effectiveness. Although we can not prove these are the best combination of radii to solve our problem, we have proven the cup design and patterns formed will have some effect on the cup's ability to transfer force to the powder and reduce picking, surface abrasion / erosion. Tablet volume and size restrictions will dictate design variables and any subsequent effect the cup design will have on solving the problem. By changing the cup design / force displacement our goal is to reduce the force necessary to achieve tablet hardness, thereby increasing tool life.

At Elizabeth Companies, we use our technology to provide information necessary for our customers to make tooling decisions.

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