Control of Compaction Pressure using an Electrical Circuit 1 Introduction

Physical testing machines are widely used for preparing both specimen compacts and measuring mechanical properties such as tensile strength and Young’s modulus (Church and Kennerley, 1982; Kerridge, 1988). The preparation of replicate specimen compacts at identical compaction pressures is of paramount importance when measuring these mechanical properties.

In this study an Instron Physical Testing Machine was used to examine the reproducibility of 12mm diameter replicate specimen compacts (Section 2.3.1) prepared using the gauge length return (GLR) mechanism available. Having highlighted the disadvantages of this technique an electrical device was developed for use with the testing machine and a comparison of the two techniques was performed.

The only facility on the Instron allowing relatively reproducible compaction of specimens is the gauge length return which is a mechanical device that stops the crosshead after travelling a given distance. Specimens are therefore compacted to constant displacement and often show variation in their recorded compaction pressures. The setting of the GLR involves a lengthy trial and error procedure and results in large material wastage. During specimen preparation the GLR also requires continual readjustment as the gauge setting has a tendency to drift. An electrical circuit, CCT, was therefore developed to improve specimen compact reproducibility and is shown schematically in Figure 2.2. This uses a comparator circuit to compare the output voltage from the load cell with a pre­ calibrated value set to achieve the desired compaction pressure. The compaction pressure is readily selected using a high-precision multi-turn dial potentiometer which can be calibrated against the load cell. It is important that the load cell is calibrated with the circuit connected to the Instron as the circuit tends to drain some of the load cell output voltage. It is also advisable to calibrate the circuit with the material under investigation present in the die rather than performing a punch-to-punch calibration (Section 2.3.2.3). A mechanical relay disarms the crosshead motor upon attaining the desired compaction pressure thus specimens are compacted to constant pressure which is less dependent on the die-fiU weight.

Mains Reference voltage Crosshead arrested Instron crosshead motor Instron load cell output voltage Comparator circuit Electrical circuit Mechanical relay

2.3.2.2 Experimental

Ten 12mm diameter Avicel PH 102 specimen compacts were prepared using the punch and die assembly shown previously in Figure 2.1 at 50MPa using the GLR with a compaction rate of 0.5cm/min (GLRs). The variation in weight from specimen to specimen was less than 0.1% eliminating weight variation effects. A fiirther 10 compactions to 50MPa were performed with the specimen and die omitted (GLRo). Ten specimen compacts were then prepared as above using the CCT (CCTs) and a further 10 compactions were performed with the specimen and die absent (CCTo). Specimen weight and thickness were recorded for comparison between the two groups.

2.3.2.3 Results and Discussion

Table 2.3 shows the variance ratios (F-test) calculated using the variance of the compaction pressure, specimen weight and thickness. The F-ratios GLRw / CCTw and

CCTt / G LRt show that between the groups there are no differences in specimen weight or thickness allowing these to be eliminated as possible causative factors in the explanation of the results (F-ratios insignificant).

The GLR is effective at producing reproducible conditions of compaction in the absence of powder in the die. However, when a powder is introduced the reproducibility is significantly reduced resulting in specimen compacts of variable compaction pressure (F- ratio for GLRs / GLRo significant at p = 0.001).

In the case of the CCT the introduction of a powder in to the die did not affect the reproducibility of compaction (F-ratio for CCTs / CCTo insignificant). However, it was found that the CCT should be calibrated with a material present in the die because if a specimen is compacted to a desired pressure selected following punch-to-punch calibration, the actual recorded compaction pressure is usually lower. The presence of a material in the die which behaves both elastically and plastically seems to affect the rigidity of the compaction system.

Using the CCT it is possible to prepare reproducible specimen compacts because of the greater sensitivity of the technique and the compaction to constant pressure. Another advantage is that the use of the CCT produces specimens of constant thickness. The GLR uses a crude, insensitive method for preparing specimens by compacting only to constant thickness. The electrical circuit was used to prepare all 12mm diameter specimen compacts

Table 2.4 Variance ratios (F-test) for specimens compacted using the gauge length return (GLR) and electrical circuit (CCT)

GLRs/GLRo CCTs/CCTo CCTt/GLRt GLRw/CCT,

F-Ratio 20.7240 1.5786 1.8595 1.4594

Legend

GLRo variance in the compaction pressure when using the gauge length return with specimen and die absent

GLRs variance in the compaction pressure when using the gauge length return to control specimen formation

GLRt variance in the specimen thickness when using the gauge length return to control specimen formation

GLRw variance in the specimen weight when using the gauge length return to control specimen formation

CCTo variance in the compaction pressure when using the electrical circuit with specimen and die absent

CCTs variance in the compaction pressure when using the electrical circuit to control specimen formation

CCTt variance in the specimen thickness when using the electrical circuit to control specimen formation

CCTw variance in the specimen weight when using the electrical circuit to control specimen formation

87 in this thesis except for the work reported under preliminary investigations (Chapter 3) as this work was performed before the circuit was available for use.