Die Pressing

Table of Contents:

   1. INTRODUCTION

    2. STEPS INVOLVED IN DIE PRESSING

        2.1 FILLING OF THE DIE

        2.2 COMPACTION OF POWDERS

        2.3 EJECTION OF THE POWDER COMPACT

    3. DIE PRESSING PROCEDURE

    4. ADVANTAGES

    5. DISADVANTAGES

    6. APPLICATIONS

Introduction

    Die pressing or Uniaxial Pressing is defined as the compaction of powders into a rigid shape by the application of pressure along a single axial direction make use of by punch and piston. Uniaxial Pressing is known as "Die Pressing". In this article, we are going to see the step by step process in uniaxial pressing, process, advantages, disadvantages and applications of Uniaxial pressing. 

Steps involved in Die pressing: 

    In uniaxial pressing, the powder particles or a granule undergoes a process called compaction. After the compaction process, powders are converted into a rigid shape. The overall procedure consists of three steps. They are as follows:

STEP 1 : Filling of powders in the die

STEP 2 : Compaction of the powders

STEP 3 : Ejection of the compacted powders

    Based on the relative motion of the die and punches, compaction is of three main modes. Here we will see the modes of compaction

Single Action Mode : Movement takes place in the top punch, but the bottom punch and die are fixed

Double Action Mode : The top punch and bottom punch moves, but the die is fixed

Floating Die Mode : The top punch and die moves, but the bottom punch is fixed

Feed material:

    Powder is the main feed material in the process of Uniaxial Pressing. It is blended with binder to provide a green strength of the sample. There are some factors which influencing the powder particles are: size, size distribution, shape, packing of the particles, particle packing homogeneity, hardness and surface friction.

Filling of the die:

    During die filling process, the flow of powder particles is enhanced by an extensive distribution of s spherical shape, sizes and a smooth surface. The uniformity of die filling is the important factor to be considered because this will affect the packing homogeneity of the green body. In addition to filling of the die and configuration of the die, the uniformity of die filling depends upon the ratio of size of powder to the die of diameter. The actual particle packing density during filling of the die is ~ 25-35%

Compaction of powders:

    After die filling step, compaction of powders takes place. The compaction step can be divided into two stages. The first compaction stage involves rearrangement of particle because; reduction of large voids takes place. Due to movement of particles, rearrangement take place in first compaction step which will results in improved packing. But in case of second compaction stage, deformation of the powder particles takes place due to reduction in small voids. Deformation takes place when pressure is applied.

Ejection of the powder compact:

    At the time of compaction, particle undergoes elastic compression. When the pressure is released, the stored elastic energy leads to the expansion of the compact. This kind of expansion/ effect is known as Spring Back Effect/ Strain relaxation/Strain recovery. Spring Back Effect depends upon five factors: powder characteristics, application of pressure, rate of pressing, gas permeability of the powder compaction and the amount of organic matter.

Powder characteristics: Spring back effect in hard granules is very lower because of hardness. In case of soft granules, spring back effect is higher because of the softness present in it.

Application of pressure: If you apply pressure on the powder particles, spring back effect will be higher. No pressure applied means there is no spring back effect.

Rate of pressing: Higher the pressing rate, higher the effect and lower the pressing rate, lower the effect

Amount of organic matter: Higher the amount of organic matter (additives), higher the spring back effect and lower the amount of organic matter, lower the spring back effect.

Die pressing procedure:

• The unaixial pressing apparatus consists of three main components: Top punch, bottom punch and die.
• The powder with appropriate composition is feed using fed shoe and then the upper punch compress the powders in the die.
• The upper punch pulls in and the sample is ejected.
• Again the above three steps were repeated.

Die pressing 

• The cycle repeats 6 to 100 times per minute.
• For dry pressing, the powders contains <2% of water and for semi dry pressing, <5-20% of water.
• If the die pressing is carried out in room temperature, it is called as cold pressing and if carried out at a higher temperature, it is called as hot pressing.  

Advantages of Die pressing:

• Simple process
• Investment cost is moderate
• Lesser skilled labour is sufficient

Disadvantages/ Compaction defects:

    After the accomplishment of die compaction, the green body should be free of macroscopic defects and also the density gradients should be as low as possible. Due to density gradient, there is a development of crack like voids in the fired or sintered body takes place. In addition to cracks, a change in density improves the formation of defects in the compact sample on ejection from the die. There are some ways to reduce the density gradients: 

• Uniform die filling during compaction process decreases the amount of internal movement of the powder
• Reduce the friction between the particles and die wall friction by the use of lubricants
• Increasing the ratio of L/D (length to diameter) of the compact, stress gradients are enhanced because of die-wall friction
• L/D should be <0.5 for the single action mode of compaction and for double action mode of compaction, L/D should be less than 1.

Applications of Die pressing:

• Uniaxial pressing is used in small and simple shape compaction such as spacers, capacitors dielectrics and bushings
• It is used in making complex shapes compaction like switches, transistors and bases or sockets for tubes.
• It is used in the manufacturing of tiles, crucibles, bricks, and grinding wheels.  

Conclusion: 

    

   Uniaxial Pressing or Die Compaction is the simple method to produce simple as well as complex shapes to test the sample for different applications and we have seen the steps involved in die compaction in a detailed manner.