1. Application of Punches and Dies in Tablet Presses
Punches and dies are designed to produce tablets with specific physical properties (e.g., shape, thickness, weight, hardness). The lower punch determines the depth of granule filling in the die bore, controlling the material quantity per tablet. The upper punch, guided into the die, applies pressure to compact the material. Tablet shape is defined by the die and punch geometry, while thickness and hardness depend on the applied pressure. Tablet weight is determined by the granule volume filled into the die before compression. Structural diagrams of the upper punch, lower punch, and die are shown in Figures 1–3.
2. Classification of Tablet Press Punches and Dies
2.1 By Shape
Round Punches/Dies:
Concave Punches: Produce convex tablets. Subtypes include shallow, standard, deep, ultra-deep, and modified spherical concave punches.
Flat-Faced Punches: Subdivided into flat, flat with rounded edges, and flat with beveled edges.
Irregular-Shaped Punches/Dies: Used for non-circular tablets. These punches often include anti-rotation keys to maintain alignment during compression.
2.2 By Size
Common punch types include B, B2, and D series, classified by total length, shank diameter, and punch tip dimensions. Corresponding dies (BB, B, D) vary in outer diameters:
B-Type: Shank diameter 19.00 mm, tip diameter 25.40 mm.
B2-Type: Shank diameter 19.05 mm, tip diameter 125.40 mm.
D-Type: Shank diameter 25.40 mm.
3. Selection of Tablet Press Punches and Dies
3.1 Consequences of Improper Selection
Mismatched punches/dies reduce service life, impair tablet quality, and risk equipment damage.
3.2 Key Parameters
Multi-Tip Punches: Increase output per cycle but raise pressure on punches and cams.
Short Punch Shanks: Essential for sealing systems to isolate lubricants from granules.
Anti-Dust Cups and Bellows: Prevent contamination and ensure lubrication integrity.
Key Types:
Half-Round Keys: Standard or high-speed variants for anti-rotation.
Fixed Flat Keys: Secured with pins for stability.
3.3 Die Material Selection
Carbide-Lined Dies: Ideal for abrasive/corrosive formulations.
Ceramic-Lined Dies: High strength but unsuitable for pharmaceuticals due to non-conductivity.
Tapered Dies: Reduce capping and ejection pressure.
4. Procurement, Inspection, and Maintenance
4.1 Drawing Management
Establish protocols for drawing approval, archiving, and cross-departmental reviews.
Implement training programs to ensure compliance.
4.2 Procurement Checklist
Example for Vitamin C tablet production:
Tablet dimensions: 7 mm diameter, 3 mm thickness.
Punch/die material: S7 tool steel (punches), D2 steel (die).
Key type: High-speed half-round key.
4.3 Storage and Maintenance
Storage:
Environment: 15–35°C, 40–60% RH, dust-free, and UV-protected.
Organization: Classify by size, frequency of use, tablet shape, or special requirements.
Maintenance:
Daily Checks: Inspect for wear, cracks, or corrosion.
Lubrication: Use lithium-based grease or molybdenum disulfide.
Cleaning: Ultrasonic cleaning with non-corrosive agents.
5. Common Issues and Solutions
5.1 Fatigue Failure
Stress cycles near material limits cause cumulative damage. Design for ≥1 million cycles.
5.2 Coatings for Punches and Dies
5.2.1 Advantages of Punch/Die Coatings
Specific coatings and treatment processes enhance the performance of tool steel, significantly reducing material adhesion during production. Key benefits include:
Improved wear resistance of tool steel.
Enhanced resistance to abrasive particle wear.
Increased corrosion resistance.
Better lubrication for sticky formulations.
5.2.2 Coating Processes
Common coating methods include electroless plating, physical vapor deposition (PVD), and ion beam-enhanced deposition (IBED). Existing coatings are typically based on one of these processes. For example, gold-nickel and chromium coatings are applied via electroplating or electroless plating.
5.3 Punch/Die Maintenance
Routine repair procedures include repairing damaged punch tips, addressing edge defects or scratches, removing burrs, and polishing. Only trained operators should perform these tasks:
Repairing Punch Tips:
Secure the punch in a polishing chuck.
Use an 80-grit emery cloth to grind damaged areas, followed by a 180-grit emery cloth for polishing. Apply constant pressure until all imperfections are removed.
Fixing Edge Defects or Scratches:
Measure the punch diameter with a micrometer.
Grind the punch’s outer diameter with an emery disc, ensuring the disc surface remains parallel to the punch face.
Re-measure to verify material removal accuracy.
Removing Burrs:
Secure the punch in a chuck and use an Arkansas stone to remove burrs on rotating punches. For punches with embossed text or fracture lines, gently move the stone along the edge.
Die Maintenance:
Apply diamond paste to a wool polishing wheel and moisten with polishing fluid.
Insert the rotating wheel into the die bore, applying light pressure and moving the wheel back and forth.
Avoid extending the wheel beyond 1/4 of the die bore length to prevent uneven wear or mid-bore concavity (which causes ejection issues).
Inspection: Check the die bore for wear, corrosion, burrs, or pressure ring damage. Clean with a lint-free cloth.
Polishing:
Post-Polishing: Re-measure with a micrometer and repeat if surface finish is unsatisfactory.
5.4 Common Issues in Tablet Production
Problems may arise from material properties, press malfunctions, or punch/die defects. Often, one component’s failure leads to cascading issues. Most failures result from neglecting three principles:
Maintain the lowest possible pressure.
Clean and lubricate equipment properly.
Keep punches and dies in optimal condition.
Factors affecting punch/die lifespan:
Corrosive formulations.
Overloading.
Cam-induced punch wear.
Physical damage (e.g., cracks, internal flaws).
Excessive hardness.
Worn pressure rolls or cam tracks.
Poor lubrication or formulation self-lubricity.
Malfunctions in automatic lubrication systems.
5.5 Root Causes and Corrective Actions for Punch/Die Issues
Punch/die failures significantly impact tablet production. Proper press operation and maintenance can prevent most issues. Table 1 summarizes common problems, root causes, and corrective measures:
| Mold Problem | Reason | Calibration Method |
| Cracks appear between the projecting and recessed parts of the punch, or the recessed part of the die cracks | The punch is too hard; the punch bears too much pressure; stress concentration at the crack line | Discard the punch; if the punch is appropriate, it should operate under minimal pressure; the crack line usually occurs along the line of lowest resistance |
| Wear on the middle part of the punch | Continuous exposure to pressure causes wear on the inner surface of the punch hole, leading to normal wear of the middle part of the punch | If the problem is severe, wear on the middle part of the punch may cause issues such as chipping or excessive tightness of the punch; if it is a pressed abrasive material, the mold manufacturer may provide a more wear-resistant mold |
| Wear at the edge of the punch | Occurs when the punch is being fitted with the die and the pressure machine, causing accidental damage | Carefully remove the damage through polishing |
| Collision occurs in the pressure machine; a crack line appears when damaged | The upper and lower punches touch the pressure machine; manual rotation of the middle punch until the middle punch is completely filled with powder | Do not start the pressure machine in a powder-deficient state; manually rotate the middle punch until the middle punch is completely filled with powder |
| Metal fragments from the top surface of the punch fall off | Excessive pressure and wear of the pressure wheel; impurities are found between the pressure wheel and the punch head | Reduce the pressure of the pressure wheel; maintain the pressure wheel; thoroughly clean the pressure machine |
| Insufficient lubrication in the guide hole of the punch or impurities cause scratches on the punch | The tightness of the guide hole in the rotating disc causes scratches, as well as the adhesion of metal particles leading to increased tightness; poor lubrication | Clean the powder and metal particles; pay attention to the cleanliness of the punch base; check the working length before reworking the punch; ensure the lubrication system is clean |
| The punch does not rotate, and the pressure wheel presses too hard, causing wear on the top surface | The pressure wheel pressure is too high; insufficient lubrication | Appropriately adjust the pressure to ensure free movement of the punch and pressure wheel; check for damage on the bottom surface of the punch; polish the punch; ensure sufficient lubrication |
| The punch is broken in the pressure machine | Due to punch damage, when entering the middle punch may be obstructed; punch too tight | Discard the mold; monitor the mold usage status to avoid overtightening and excessive pressure |
| Burrs on the surface of the punch | Misalignment of the punch and middle die; misalignment of the guide hole of the punch and the middle die seat | Check the concentricity of the punch; ensure that the fit between the punch and the middle die hole meets the standards |
| Deterioration of surface smoothness of the punch | Pressing wear-resistant or corrosive powder | Choose an appropriate mold steel; ensure that the powder contains sufficient lubricant |
Conclusion
Optimal selection, advanced manufacturing, and rigorous maintenance significantly enhance punch/die performance. This study provides actionable insights to improve tablet quality, reduce costs, and drive industrial innovation.
