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Building in strength in to your design

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28 July 2016 No comments

The tablet land is a slight level surface that is vertical to the tablet’s boundary creating a junction between the boundary and the cup of the tablet. There are three main reasons why one would incorporate a land into the tablet’s design.

  1. Increase the strength of the punch edge
  2. Increase the wear of the punch cup inner edges
  3. Lower the relentlessness of the nicks on the edges of the punch

Some would agree that the land is beneficial as it adds durability and strength to the punch tip; however, some would argue that the land makes it more difficult to coat the tablet.

The width of the land is dependent upon the following:

  1. Tip diameter
  2. Cup depth
  3. Cup configuration
  4. Punch steel selection
  5. Compression force
  6. Abrasiveness of granulation

The round punch tips are deeper than the average concave and as such designed with 0.002 to 0.008 land. Special punch tips on the other hand are designed with 0.002 to 0.012 land. In certain cases, the width of the land is designed with excess of 0.020 to add durability and longevity to the punch tip.

As the depth of the cup increases, the cup’s slope also increases at the boundary of the tip. This often leads to the possibility of having a deformed tip edge or fractures of the tip when heavy compression loads is applied and the tip has little or no land. As the punch starts compressing the tablet, the stress is placed higher at the tip edge. Compression stress is distributed as the granulation is forced to the middle of the cup. This is also what happens and describes the wear problem that often occurs at the outer edges of the cup face, as the granulation flows through this area of the cup. In adding land to the design, it reduces the possibility of tip deflection while increasing the durability and strength of the edges. It is important to note that the deeper the design of the cup, the wider the land should be to help strengthen the tip.

Punches that are made of stainless steel would also require wider land as stainless steel has low shock resistance compared to standard steels and to help aid the punches as heavy force is needed to compress nutritional supplement tablets.

Even though compression punches undergo stress tests and are heat treated for optimal wear resistance use, the punches still need proper care and handling especially during set up and production process. Contacts between punch tips or another surface would cause damages to the tip’s edge. It is important that punch tips design should also include the correct land to lower down the possibility of accidental damage as the land adds strength to the area.

The presence of the land results to a lip or a ridge around the boundary of the tablet, at the tablet face’s junction, belly band and side wall. The edge is usually prone to erosion bought about by the tumbling action in the coating pan. The junction of the land as to the cup face will also result to a sharp inside corner on the tablet area. To prevent any problems especially during the coating process, it is important that the land should have a huge blend radius at the junction area. This will remove the sharp corner as it adds a curved transition between both surfaces. One can also indicate in the design “blended” so that the land is created before the sharp corners are blended. Other specifications may also be included in the tablet design to indicate the land width after blending and the blend radius may be dimensioned as well.

The blending of the inner area is achieved by:

  1. Adding the requirement to the drawing so that it is incorporated during the assembly of the equipment
  2. Blending the punch cup during the final step

The slope of the cup adjacent to the interior edge of the land that has a compound cup added to the design is greater than single cups, which results in the reduction of tip force. That said, the land becomes more important than ever as it helps replenish the strength loss caused by the steeper slope. A wider land is usually suggested when a compound cup design is needed.

Due to the concentrated wear and tear on the land, it is important that the area is inspected regularly. To extend the longevity of the equipment and maintain tablet quality, it is important to rework the punch cup and re-establish the land.

There are many designs that have evolved to accommodate heavy land that provides strength and longevity to the tablet press equipment (especially in the TDP rage) without affecting the quality of the tablet. Nutritional supplement manufacturers usually use 0.020’’ wide land to coat tablets successfully. This design is often used by those who manufacture generic tablets as well.

An increase in land width would result to increased punch strength reducing the possibility of tip deflection while still allowing higher compression and extending the tool lifespan. If you have any questions, contact your tooling vendor and let them guide you through the entire design process to ensure a successful tablet design.