Structure of punch and die in tablet press (P2)

The punch and die are two main parts in a tablet press, designed to perform the function of compressing and shaping tablets. The structure of the punch and die includes basic components such as punch head, punch neck, punch body, punch head and die, each detail plays a specific role in shaping and ensuring product uniformity. The importance of the punch and die not only stops at deciding the tablet shape but also directly affects properties such as hardness and dissolution of the tablet.

Introduction

• To achieve the  appearance requirements and properties of tablets, the structure and materials of the punch and die must be carefully selected. Depending on the case, stainless steel materials or special alloys are used to increase hardness and wear resistance. In addition, the design of details such as the punch surface (flat, concave or ridged) along with the non-stick coating plays an important role in limiting punch sticking and product damage during the production process. Therefore, choosing the appropriate structure, design, and materials needs to be carefully evaluated to ensure tablet quality, process performance, and investment costs .

Punch head

Figure 2. Head radius (HR), Head flat (HF) head thickness (HT)

• Head angle or head radius determines the punch’s ability to withstand force when impacted by the compression wheel. The flat part of the punch head (HF – Head flat) plays a role in determining the time to maintain compression force, meaning the amount of time the tablet is under maximum pressure (dwell time), this needs to be optimized to create tablets has high mechanical strength. The flat part of the punch head must always have a smaller diameter than the punch neck to ensure even pressure is transmitted to the entire punch, avoiding deformation or damage during compression. The thickness of the tip of the punch needs to be precisely machined to ensure that the punch can slide smoothly along the cam path and, for the lower punch, to control the consistent refilling of the granules into the die.
• Common types of punch heads:
  • Domed heads: The domed head structure brings many benefits compared to the standard TSM (Angled head). When the punch first makes contact with the pressure roll, the 5/8 inch radius on the domed head relieves the enormous pressure normally experienced with the smaller radius (5/16 inch) on the standard punch head. This high pressure can cause ‘head pitting’, identified by voids appearing on the surface of the punch head. The collision of the compression wheel and punch tip radius at high speeds and strong compression forces can cause a material hardening effect, leading to “head pitting”. This phenomenon is harmful to the life of the punch and press wheel. The domed punch head provides a smoother transition into the compression cycle, minimizing stress and premature wear of the compression wheel.

• • Rotating heads: The rotating punch head structure is a two-part punch structure, in which the punch head is separate from the body and punch head, making it easy to replace the punch head when worn. When compressing circular tablets, the punches rotate as they are moved around the cam rail through different stages. As the punches rotate, the wear and pressure at the back corner of the punch tip are evenly distributed around the entire contact surface. This reduces punch tip wear and prolongs punch life.

Figure 3. Rotating heads

• • Mirror-finished heads: Some high-speed tablet presses use heavy metal cams, such as copper and copper alloys. This material is good for reducing premature punch wear and extending tool life, but it has the negative effect of contaminating the lubricant, causing the lubricant to turn black or dark blue. Usually the tip of the punch is smoothed with sandpaper or fine grinding. This method leaves small scratches on the contact surface of the punch head, thereby discoloring the lubricant and prematurely wearing the camshaft. Polishing the punch head with a soft cotton grinding wheel and a fine polishing compound to a mirror shine helps keep the lubricant cleaner and prolongs the life of the camshaft.

Punch neck

• The shape and diameter of the punch neck are determined by the size of the camshaft.

The body of punch

• The punch body pin is a required component for the upper punch when compressing non-round tablets. The upper punch pin helps maintain the alignment of the punch head so that the punch can move precisely into the die hole for compression. Normally, a pin is not needed for the lower punch because the lower punch does not leave the die bore during the compression cycle, so it is not necessary to maintain alignment. However, a pin may also be necessary when compressing embossing round tablets to prevent rotation of the punch after compression, avoid damage to the embossed tablet and reduce the possibility of “double embossing” occurring. phenomenon that occurs when a tablet is embossed twice during the manufacturing process) on the tablet surface.

Compress head

• The compress head can be cast as a whole or installed into the punch body. Multi-tip punches designs increase productivity but reduce punch force. The monolithic compress head is molded from a single block of material and has the advantage of reducing the risk of cross-contamination, limiting rust and making it easy to clean and maintain. For assembled compress heads, the components of the compress head can be disassembled and replaced, which saves costs and is easy to customize for many different types of products. The compress head of the lower punch is longer than the upper punch, helping to push the tablets out of the punch after the tablets have been compacted.

• When compressing tablets, the lower punch head creates a huge amount of friction as it moves through the entire length of the die. Especially with products that are sticky or have a low melting point, the friction created by the lower punch head can cause the punch to jam. Reducing the contact surface of the lower punch tip reduces friction, allowing the punch to move more easily in the die and reducing operating temperatures.

Die

• Tapered die (die hole is not vertical but has a slightly inclined shape, with a larger diameter at the ends and a smaller diameter in the middle)

• Tapered die are more expensive than standard die and can be tapered at both ends for reversible use. The tapered die helps release trapped air, compresses harder tablets with the same pressure as a straight die, and reduces the risk of cracking and separating the tablet layer. The taper also prolongs the life of the punch and pushcam, reduces friction and helps the tablet press run smoother. However, the disadvantage is that the upper punch can get stuck and stick in the die, causing tablet spots or premature wear of the punch head.

References:

1. Y. Qiu, Y. Chen, G. G. Zhang, L. Yu, and R. V. Mantri, Developing solid oral dosage forms: pharmaceutical theory and practice. Academic press, 2016.
2. P. Anbalagan, S. Sarkar, C. V. Liew, and P. W. Heng, “Influence of the punch head design on the physical quality of tablets produced in a rotary press,” Journal of pharmaceutical sciences, vol. 106, no. 1, pp. 356-365, 2017
3. Terminology of punch and die structure in tablet press (P1)