Tablet Tooling – How To Get The Biggest Punch
As the influx of new products invade the tablet press industry today, tablet manufacturers together with tooling vendors are constantly innovating the tablet compression technology. The article below discusses how the industry uses technology in determining the maximum compression force for tooling equipment.
Majority of the industries that make use of tablet tooling are the pharmaceutical and the dietary industry; that said, the equipment is also used in other industrial sectors such as confectionery, chemical and energy. Each of these sectors has their own set of guidelines and specifications requiring specific tooling equipment. To ensure that these demands are met technological aids especially computers are used to replace old school standards -- one of which is how calculation is done to obtain the maximum compression force for different tablet configurations.
In creating a high quality punch, it is important to choose the tool steel material. As each industry produces different products, choosing the right type would ensure the intended result. In the US for example, the AISI (American Iron and Steel Institute) has set a standard for the tool steel and the AISI S7 has become the preferred standard as not only does it offer excellent shock resistance but it also has good resistance to splitting stress. The S7 can also withstand high compression forces.
Aside from S7, other types also have excellent wear resistance as they can attain a higher hardness due to the high carbide content (Examples: DC-53, AISI D2 and K340). For corrosive or sticky formulation, the AISI 440C and the M340 are excellent tools as they have high chromium content. As a rule under the Rockwell hardness – the higher the hardness of the material, the lower is the impact toughness and the higher the wear resistance. That said, some PM-class steels could be both wear resistant and have high impact toughness, which is due to the distinctive chemical composition, as well as the manufacturing process that made it. Highly reputable vendors would offer the tooling manufacturer several options and can help them choose the right steel, which would not only meet the requirements but can also, last longer.
Whatever the tool steel grade and the application that one is using, it is important to take into account several factors in computing the maximum tip force. Aside from the tensile strength, one should also take into consideration the yield strength, compressive strength, impact toughness and the tensile strength. To determine these properties, one should check the composition of the material and its hardness. The compressive strength deals with the resistance level of the steel as to deformation during the compression loading. The tensile strength on the other hand refers to the ability of the material to handle stress when the material is stretched or pulled.
Another indicator, the yield strength focuses on the max amount of stress that the material can handle before deformation occurs. In certain instances, impact toughness is also considered whereby it measures the max energy that the material can withstand during shock loading. As different steel types work for different materials or products, the maximum compression force would also differ.
During the compression stage, normal force is applied to the cup’s surface. The force would then result to stress which is a function of both the tablet’s geometric profile and cup area. The basic stress relates to the force being exerted and the surface where it is applied. To calculate the maximum allowable compression force, one would need to compare the max stress with the yield strength of the material. Aside from the area and force, one should also factor in the stress concentration level.
Stress concentration is actually a certain area where high stress is observed and is due to the sharp transition in the geometric profile of the punch face. An example would be a bisect and blend radius where the embossing would meet the cup radius. Punches that have stress concentration have low max compression force as compared to tooling equipment that produce plain tablets.
The punch’s face land is also another critical factor that should be considered in determining the max allowable compression force. The land is the flat edge at the side of the punch cup where the radius of the cup ends. Although it is smaller compared to others, it plays a crucial role in determining how much force the punch tip can withstand. Generally, the bigger the land, the higher the max allowable compression force. In certain cases, one can increase the land to increase the max force. That said, one should take note that as the tooling equipment wears, the land would erode and become smaller. This is the reason why new punches can withstand more force as they have unworn lands as compared with tooling equipment of the same model that has been used several times. Proper maintenance therefore is important as not only does it ensure quality product but also helps to maintain the tooling equipment in tiptop shape.
To understand better the effects of different tooling designs, tooling vendors have adopted a Finite Element Analysis (FEA), which is a tool that enables one to combine different forces, pressure to a specific model, and study the results later. The result would show the stresses, displacement, strains and other factors that may be used in studying the safety use of the tooling equipment. Using the results from the analysis would enable the company to analyze various geometric shapes and figure where the stress concentration is and use the data to develop better tool design.
The FEA works by identifying key factors in a large model and breaks it down to simple patterns. To start with, the model is broken down to various elements, which one can handle. This collection is also called as mesh. Once done, each property is defined and a material is assigned to the model; the desired force is then applied to all the surfaces.
The model would then undergo a real live stimulation where it is tested for its reaction level. Lastly, the model is subjected to another test where the model would likely fail. Once the test starts, the FEA software would then gather the elements and the result. A detailed map is then produced, pinpointing the exact max force and the resulting failure that would likely occur.
For materials that are ductile such as metals, the accepted failure criterion is the “Equivalent tensile stress” also called as “von Mises stress”. This works when an elastically deformable body is being subjected to a 3D loading, the material will develop a complex network of 3D stresses.
The von Mises stress can also be formulated using the yield criterion where it states that even if none of the 3 major stresses exceeds the yields strength, the yield may still be reached by combining several stresses. The criterion works very well especially when the FEA makes use of the mesh model as it tightens the infinite number of degrees of freedom. The von Mises stress combines all the stresses into an equal tensor value that one can compare to the materials yield strength.
Build Ups, Safety and Bent Tips
It is important to note that the max compression force indicated in the list that vendors input also accounts for the tooling fatigue, which is also used in making millions of tablets. Fatigue happens when the compression tooling goes through repeated loading cycles during production causing residual stresses to build up. The phenomenon also known as “fatigue stress” can affect the tooling life. It is in fact normal that the max force that a punch can withstand would decrease over time due to the wear and tear as well as due to the build-up of residual stress. This means that the tooling loses its maximum force with repeated use and the force that it
exerts is different from the first to the succeeding production. It is important that tablet manufacturers talk with their tooling vendor to determine if their equipment is optimized for high cycle loading.
As the compression tooling is used repeatedly, residual stresses would start building up and should be taken into consideration by the tablet manufacturer.
One last factor to consider is the bending of the lower punch tip straights. This would generally affect tablet manufacturers that would require tips of 4millimeters in diameter or smaller. When the tip is small, the cup geometry is unlikely to be considered but rather the tip’s propensity to bend when compression load is done.
In considering all these factors, the ideal compression force is determined by calculating the stress, force, and compare to the FOS. The FOS is just the ratio of the calculated stress to the material’s yield strength.
An FOS of 2 would therefore mean that the applied force generates a stress in the material that is ½ of the material’s yield strength. The FOS value depends on the industry and the application as it can vary from one industry to another, which is especially true among tablet compression companies where various vendors use different ma compression forces. This is due to whichever the vendor find acceptable.