Improving yield and reducing waste

Abstract

Substantial losses in usable cutting area and the drop-split weight can occur if samming, splitting and shaving operations are not closely monitored and controlled. Usable cutting area is determined by assessing factors including: compliance to thickness specifications, softness or hardness, physical strength, colour uniformity, break, looseness and grain quality.

A new thickness gauge, developed by CSIRO, can be used to rapidly make large numbers of measurements over whole hides. The use of these measurements and purpose written Sirosplit software enables graphical whole hide thickness maps to be produced.

When the gauge is connected directly to a computer, the thickness maps give immediate feedback to machine operators. The compliance to thickness specifications in splitting or shaving is shown together with variance from allowable tolerances enabling adjustment of machines.

Use of the thickness maps at splitting machines can result in increased yield through:

* More precise splitting

* An increase in the weight of drop splits

* A reduction in the amount of shaving required

Reduced shaving usually results in increased area. In some cases, where splitting is sufficiently accurate, shaving is unnecessary.

Elimination of shaving has been shown to have positive benefits in leather properties including: strength, particularly in the belly areas, veininess, looseness and general handle. These improvements in leather properties lead to an increase in cutting yield.

Introduction

It was recently reported¹ that the samming, splitting and shaving operations can result in substantial losses in usable cutting area and drop split weight if not closely monitored and controlled.

Usable cutting area is determined by assessing factors including: compliance to thickness specifications, softness or hardness, physical strength, colour uniformity, break, looseness and grain quality.

If wet-blue splitting is poorly controlled and heavy shaving is required, the shaving operation does not always have the degree of thickness correction expected. In addition, the physical stresses on wet-blue during the shaving operation have been shown to cause physical disruption to the fibre structure particularly in the belly areas of hides.

This damage lowers the physical strength of leather and can contribute to looseness. In matched side experiments, it was found that in the belly areas, an increase of 20% in tensile strength and a 30% increase in lastometer load was achieved if the wet-blue was not shaved. Tear strength increased by 45% measured perpendicular to the backbone and 90% measured parallel to the backbone.

It was also found that shaving causes damage to the structure around veins, making them more visually apparent.

Some modern splitting machines, if adequately controlled, should be capable of splitting to an accuracy of at least +- 0.25mm. This accuracy is comparable to many shaving operations and would be sufficient for certain types of leathers.

Control of splitting machines requires accurate thickness measurement and it has been shown that commonly used calliper thickness gauges as illustrated in Figure 1 are not always reliable.

The force produced by these gauges does not always comply with the standard², either due to mechanical variations in the gauge or by operator faults³. Poor thickness measurement is one cause for dispute between tanneries and manufacturers.

Electronic thickness gauge

CSIRO has developed a new thickness gauge3 that can be used to rapidly make large numbers of measurements over whole hides. The gauge shown in Figure 2 is a modification of a commercially available paint thickness gauge.

The gauge can be used for individual measurements or is capable of storing measurements for later evaluation. It can also be directly connected to a computer for on-line transfer and monitoring of measurements. The latest generation of the CSIRO thickness gauge is electronically capable of taking more than 60 readings per minute. In practice it was found that up to 30-40 readings per minute are physically possible.

Sirosplit software

Dedicated computer software, known as Sirosplit, has been written for use with the gauge to enable whole hide thickness maps to be produced and displayed for use by machine operators. The software display shown in Figure 3 has a number of features including display of compliance to thickness specifications, together with variance from allowable tolerances enabling operators to adjust machines and optimise performance. In addition, there are statistical options for monitoring long term trends in thickness of split and shaved leathers. The grid displays can be tailored to suit individual tanneries requirements. The software is also capable of displaying average thicknesses of the last five measured hides. This enables the operator to react to only definite trends in machine performance rather than individual single hide measurements. The use of the software in this way results in a smooth control regime.

Improving yield

Use of these thickness maps at splitting machines can result in increased yield through:

* More precise splitting

* Splitting closer to the thickness required for the finished leather and hence increasing drop split weight

* A reduction in the amount of shaving required

Reduced wet-blue waste from shaving and trimming would result in a large reduction in waste disposal costs. Shaving was shown to result in area losses especially around the neck and leg areas of hides as shown in Figure 4.

By not shaving it was shown that area increases could be achieved. Not all of the increase was useful area. However, an increase of at least 5% was shown to be possible.

The elimination of shaving has been shown to have positive benefits on leather properties including: strength, particularly in the belly areas, veininess, looseness and general handle¹.

These improvements in leather properties lead to an increase in cutting yield.

The gauge and Sirosplit software have been used in tanneries during trials to measure the actual thickness distributions from splitting and shaving operations, the effects of machines on wet-blue and on the yield of leather produced. The individual measuring point variation provided useful information on the effectiveness of machine settings to produce the thickness profile required by the tanneries. Of particular interest was the degree of variation from what was thought to be the thickness accuracy from various splitting and shaving machines.

Figure 5 shows some of the results obtained from different splitting operations using both older machines and more modern machines that had either been in use for some time or had recently been commissioned. It can be seen that results are very varied and were in some instances found to be different from results based upon use of normal calliper thickness gauges because measurements were able to be taken more evenly over the whole hide.

Figure 6 shows the thickness distributions of splitting on a modern splitting machine compared to the shaved results obtained during process benchmarking. In this example, no attempt was made to use the measurement results to control the splitting. It can be seen that the thickness distribution from the splitting machine was from 1.3mm to 2.1mm or 1.7mm +/- 0.4mm. This result was not very much greater than the range of thicknesses from shaving at 1.1mm to 1.7mm or 1.4mm +/- 0.3mm. The capability of the splitting machine was quoted by the manufacturer at standard deviation of +/- 0.2mm. The results show it is possible to improve splitting by utilising the new gauge results and it was possible to control splitting to achieve the similar results as shaving.

In another tannery trial, the results from splitting were monitored without any attempt to influence outcomes. Shaving outcomes were then measured to determine what changes had occurred. In the results shown in Figure 7, the splitting results have been displaced on the graph to align with the shaving results in order to compare the thickness distribution. It can be seen that the average thickness has been reduced to the desired thickness by shaving but the actual thickness range has not been significantly altered.

What was found from a detailed examination of the data was that shaving can follow the thickness profile established by splitting. This was a surprising result that demonstrated the need to control thickness in the splitting operation as shaving does not necessarily even out irregularities as was thought to be the case. The reason for this is thought to be related to the compression of wet-blue in the shaving operation. Thicker areas of wet-blue will compress more than thinner areas, so that after shaving there will be more regain of substance in the thicker areas.

When comparing the thickness of wet-blue from splitting with wet-blue after shaving, allowance must be made for stretching that occurs during shaving. It has been shown¹ that stretching of wet-blue during shaving can increase the area by up to 10% and that this is accompanied by fibre damage especially in the belly areas.

In Figure 8, note that the splitting results have been displaced to align with the shaving results in order to demonstrate the close correlation in the actual thickness range.

In further trials, the thickness of wet-blue in splitting was controlled to give an overall thickness that would relate to that normally obtained from shaving. When these hides were processed through to finished leather, it was shown that it was possible to process without shaving and achieve the desired thickness of 1.2mm with a standard deviation of 0.2mm as shown in Figure 8. In this case the finished leather was a corrected grain and some further evening out of thickness occurs during the roller operations. So for full grain footwear leathers, it is possible to produce medium thickness leathers with a standard deviation of 0.25-0.3mm without shaving.

The results can only be achieved when thicknesses are measured over the whole hide area using a reliable measuring device and rapidly enough to allow machine operators to be able to respond to the outcomes. Sirosplit thickness maps can be used to follow changes in machine outcomes due to mechanical drift or differences in operator handling.

The procedure requires considerable effort and consequently a prototype thickness measuring machine was developed to study possibilities for automation. This machine had an array of thickness measuring devices mounted on a beam under which the wet-blue hides were traversed. In this way, 49 point thickness maps were able to be generated in less than 15 seconds. Studies from use of the prototype predict that it is possible to construct a through-feed measuring machine that could measure all hides processed through a splitting machine.

Conclusions

When wet-blue was accurately split using data from the new thickness gauge and Sirosplit software, and then processed to finished leather without shaving, it was found that there were increases in yield when compared with leathers that had been split and shaved.

Modern splitting machines are capable of splitting to give similar results obtained by shaving machines, provided accurate thickness measurements are used. There is opportunity for revising shaving practices especially for leathers where the thickness tolerance is greater than +/- 0.25mm; in some cases it is possible that controlling a splitting operation could eliminate shaving.

Elcometer leather thickness gauges and Sirosplit software can be obtained through Elcometer Instruments Ltd, Edge Lane, Manchester M43 6BU, UK (www.elcometer.com). Enquiries may be directed to CSIRO, Textile and Fibre Technology, PO Box 21, Belmont VIC 3216 Australia. [http://www.tft.csiro.au]

The authors wish to acknowledge funding for this work from the Australian Department of Industry, Science and Resources and the generous support of Australian tanneries who made facilities and materials available for the trials.