Relationship Between Tableting Motion and Tablet Hardness in

Compression Molding

Shinji Kobayashi

, Takahiro Sato

and Takanori Yamazaki

Graduate school of Science and Engineering, Graduate school of Tokyo Denki University,

Ishizaka, Hatoyama, Hiki, Saitama, 350-0394, Japan

Nagase & Co. Ltd., Tokiwabashi Tower, 2-6-4, Ote, Chiyoda, Tokyo, 100-8142, Japan

School of Science and Engineering, Tokyo Denki University, Ishizaka, Hatoyama, Hiki, Saitama, 350-0394, Japan

Keywords: Compression Molding, Tableting Motion, Tablet Hardness, Elastic Recovery.

Abstract: A tableting machine is used to form powders into tablets. It is well known that the quality of tablets formed

by tableting machines varies greatly depending on the compression conditions, such as compression velocity

and compressive force. It is of industrial importance to clarify how compression conditions affect the

properties of the formed tablets. In this research, we manufactured a tableting machine with an upper and

lower pestle that can be arbitrarily operated, and the aim of this research is to clarify the relationship between

the tableting conditions and the property of the formed tablets. In this experiment, we change the driving

pestle as the tableting condition, measure the formed tablet hardness, and discuss the relationship between

them.

1 INTRODUCTION

A tableting machine is a machine used to make tablets

from powder by compression molding. This machine

has been widely used mainly in the pharmaceutical

industry. It is expected to be applied in the food and

material industries as well, due to its advantages such

as reduced transportation and storage costs by

reducing the volume of powder to tablets (Kamiya,

2022), (Brewin, 2008). Typical performance

requirements for tableting machines include high-

velocity molding for improve productivity and high-

hardness molding to prevent disintegration. In

general, it has been confirmed that tablet hardness

increases when compression velocity is reduced

(Mohan, 2012). However, decreasing the velocity is

accompanied by a decrease in productivity.

Therefore, the development of a technology that can

both improve productivity and increase hardness is a

problem (Kamiya, 2022). Conventional researches

have attempted to solve this problem by using the

method of precompression followed by main

compression (Patel, 2006) or by adding additives

such as excipients to the powder (Kamiya, 2012).

However, it has been confirmed that additives lack

physical safety (Bharate, 2010).

We have developed a single-shot tableting

machine with an upper and lower pestles that can be

arbitrarily operated until now, and we have performed

compression molding with the upper pestle as the

driving pestle. We considered that the tablet hardness

could change by the direction of the compressive

force acting on the powder. In this research, the

compression molding was performed in the following

three patterns: the upper pestle compression using the

upper pestle as the driving pestle, the lower pestle

compression using the lower pestle as the driving

pestle and the double pestle compression using both

pestles as the driving pestle. The hardness of tablets

formed by three different motions of the driving

pestle is compared.

2 EXPERIMENTAL DEVICES

2.1 Tableting Machine

Figure 1 shows the overall structure of a tableting

machine. There are two types of tableting machines:

the single-shot type and the rotary type. In this

research, the single-shot type was adopted because

302

Kobayashi, S., Sato, T. and Yamazaki, T.

Relationship Between Tableting Motion and Tablet Hardness in Compression Molding.

DOI: 10.5220/0012230200003543

In Proceedings of the 20th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2023) - Volume 2, pages 302-306

ISBN: 978-989-758-670-5; ISSN: 2184-2809

Table 1: Performance of each servo motor of tableting machine and hardness tester.

Specification Items Upper Lower Hardness tester

Rating output [kW] 0.36 0.4 0.05

Rating current [A] 2.8 1.1

Rating Torque [N･m]

1.15 1.27 0.16

Instantaneous maximum stall torque [N･m]

4.8 0.48

Instantaneous maximum stall current [A] 10.8 4.7

Torque constant [N･m/A]

0.524 0.1021

Rating rotation speed [min

-1

] 3000

Figure 1: Tableting machine.

the purpose of tableting is prototyping and the

tableting conditions can be changed.

The tableting machine consists of a PMAC as the

host device, and commands are sent from the motion

program on the PMAC to the servo amplifier, which

drives the servo motor. The tableting machine is

controlled by a semi-closed-loop system using a

rotary encoder with a resolution of 17 bits installed in

the servo motor. Data capture is performed at a setting

of 4 kH (250 ms/data). Each servo amplifier is

connected to the Eather CAT network environment to

improve synchronization performance of each axis.

The tableting machine is installed with a load cell

between the pestle and the electric cylinder to

measure the compressive force. The load cells have a

capacity to measure compressive forces of 5 kN for

the upper pestle and 10 kN for the lower pestle.

2.2 Hardness Tester

In our laboratory, we have manufactured a hardness

tester and use it to measure tablet hardness. Figure 2

shows the overall structure of the hardness tester. On

the

compression side of the hardness tester, a servo

Figure 2: Hardness tester.

motor and an electric cylinder connected by a

coupling are installed, and a terminal for compressing

tablets is attached to the end of the electric cylinder.

On the measurement side, a load cell was fixed to the

stage, and a terminal for supporting the tablet was

attached to the end of the load cell. The side of the

tablet was pressed in with the compression terminal,

and the force at break was measured by the load cell.

The compression speed was 1 mm/s.

3 EXPERIMENT CONDITIONS

The sample is weighed using an electronic balance

with an accuracy of 1.9995 g to 2.0004 g. The sample

is placed in the mortar, and compression molding is

performed using a driving pestle. The motion method

of the pestle is a motion numerical control using a

host device. Figure 3(a)~(c) shows the compression

processes (the process before driving pestle

compression where the motion of the driving pestle

starts, the compression process where the distance

between the pestles is minimized, and the pressure

release process where the compressive force is

removed) for each of upper pestle compression, lower

pestle

compression

and

double

pestle

compression.

Relationship Between Tableting Motion and Tablet Hardness in Compression Molding

303

Figure 3: Compression process.

In the upper pestle compression, the lower pestle

is held at a position -20 mm from the upper edge of

the mortar and the powder is placed into the mortar

(before the driving pestle compression process). The

upper pestle is compacted downward direction from

a position 50 mm to a position -11.5 mm

(Compression process).

In the lower pestle compression, the lower pestle

is held at a position -30 mm from the upper edge of

the mortar and the powder is placed into the mortar.

The upper pestle is moved downward from a position

50 mm and held in place at a position -10 mm (before

the driving pestle compression process). The lower

pestle is compacted upward direction from a position

-30 mm to a position -21.5 mm (Compression

process).

In the double pestles process, the lower pestle is

held at a position -25 mm from the upper edge of the

mortar and the powder is placed into the mortar. The

upper pestle is moved downward direction from the

position 50 mm and held at the position -5 mm

(before the driving pestle compression process). The

upper pestle compacted at the same time from a

position -5 mm to a position -10.75 mm and the lower

pestle from a position -25 mm to a position -19.25

mm (Compression process).

The optional motion of the dynamic pestle adopts

a two-stage compression molding process, in which

compression molding is performed at a constant

velocity of 1 mm/s and 10 mm/s, respectively,

starting from a position where the distance between

the pestles is 10.5 mm. Pressure release velocity is a

constant velocity of 50 mm/s. After compression

molding, tablet height is measured using a micro laser

distance measurement sensor and tablet weight is

measured using an electronic balance. The tablets

were then compacted from the side using a tablet

hardness tester to break the tablets, and the hardness

was calculated by measuring the value at the time of

breaking using a load cell.

4 EXPERIMENTAL RESULTS

Table 1 summarizes the experimental results. Where,

U-01 means that the upper pestle was used as the

driving pestle at a compression velocity of 1 mm/s.

The table shows that slower compression velocitys

resulted in higher tablet hardness without depending

on the tableting motion.

The profile of the compression process (pestle

position and compressive force) is shown in Figure 4.

The amount of elastic recovery is an important data

during tableting. In this time, the amount of elastic

recovery is defined as X, which is the distance from

the maximum compression position until no force is

detected. Elastic recovery in compression molding

occurs either inside the mortar immediately after

compression

molding

outside

the

mortar

when

the

ICINCO 2023 - 20th International Conference on Informatics in Control, Automation and Robotics

304

Table 2: Three times average experimental results.

Test condition

No.

Powder weight

Tablet weight

Tablet height

Tablet hardness

Compressive

force

Elastic

recovery

U–01 2.0003 1.9989 8.88 13.7 2.65 0.283

L–01 1.9999 1.9989 8.90 15.8 2.68 0.273

D–01 2.0001 1.9950 8.87 12.8 2.80 0.287

U–10 2.0000 1.9977 8.80 10.8 2.56 0.286

L–10 2.0001 1.9973 8.89 13.2 2.57 0.277

D–10 1.9999 1.9926 8.92 10.9 2.67 0.290

*[U–01]→Upper pestle compression and compression velocity 1mm/s*

Figure 4: Compression process of elastic recovery (1 mm/s).

Relationship Between Tableting Motion and Tablet Hardness in Compression Molding

305

tablet is removed from the mortar (Rahul, 2010). We

measured the amount of elastic recovery inner mortar.

Table 1 also shows the amount of elastic recovery.

Regarding the relationship between the compression

velocity and this amount, it can be seen that the faster

the compression velocity, the larger the amount of

elastic recovery, and the powder cannot be compacted.

As for the tableting motion, this amount is smaller in

the order of double pestles, upper pestle, and lower

pestle, indicating that the smaller the amount of

elastic recovery, the higher the hardness of the tablet.

It is considered to be more compressed in this

sequence.

5 CONCLUSIONS

In this research, an experiment was conducted to

compare tablet hardness by three types of pestle

motions in powder compression molding. As a result,

it was found that the highest tablet hardness can be

obtained by lower pestle compression. Tablet

hardness is considered to be related to the amount of

elastic recovery of the tablet that occurs inside the

mortar during compression molding. In the future,the

relationship between tablet hardness and elastic

recovery will be further investigated by applying

various tableting conditions.

On the other hand, the relationship between

compressive force and hardness is not clear. It is

affected by the particle size, particle shape, and

particle size distribution of the powder (Changouan,

2005). In addition, various factors such as air flow in

the mortar during compression molding and

temperature and humidity have been reported to have

an effect (Kremer, 2006), (Casettari, 2016). We are

also going to discuss the relationship between

compressive force and hardness or elastic recovery.

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