Testing with the lastometer

The lastometer machine can be used for important pre-production assessments.

Most footwear construction requires the upper material to be stretched in the process of forming the footwear shape. In the lasting process, upper material is rapidly pulled over a last. As the grain surface of leather is stretched, there is a risk that it can crack. This can be, for example, as a result of the leather being of poor quality or if it is too dry. The ‘lastometer’ test (sometimes referred to as a ‘ball burst’ test) assesses the ability of an upper material to withstand the process of lasting.

It does this by measuring how much a material can be stretched simultaneously (‘distended’) in two directions without the material sustaining physical damage – for example, grain cracking. In addition to assessing leathers, a lastometer can also be used to test woven materials. It is not typically used to assess nonwoven materials.

SATRA has been supplying lastometers since at least 1940, having developed the method some years previously and significant advances have been introduced since those early devices were produced. Nevertheless, the principle of a lastometer test has stood the test of time and is used both in laboratories and as a quality inspection method within the global footwear industry today.

Conducting the test

In a lastometer test, a circular specimen of the material to be tested is clamped around its edge and is gradually distended by forcing a small metal ball attached to a plunger through the specimen at a controlled speed. As the test proceeds, the displacement of the ball plunger is accurately recorded, along with the force on the plunger.

SATRA TM24:2017 – ‘Lastometer ball burst test’ sets out the details of a relevant method. When testing leathers, the first damage usually occurs in the finish or the surface of the grain. During a test, the surface should be continually observed at the centre of the specimen where the maximum distension is taking place. At the first sign of surface cracking, the force on the plunger and the distension of the specimen should be recorded. The test method sets out specific approaches which need to be taken when testing patent leathers. When assessing synthetic materials (such as a coated, woven fabric), the first damage usually occurs within the material substrate, with no visible damage on the surface.

Internal damage within the specimen can be detected by observing the load during the test – the force on the plunger either stops rising or falls. In this instance, the distension and the load should be recorded when this load variation is observed. The test can be continued beyond first failure signs to bursting point. The load and distension should be recorded when the spherical ball appears through the test specimen.

The SATRA STD 190 and STD 190L (extended throat) instant lastometers (figures 1 and 2) are intended to be used as simple pass/fail gauges, such as in a goods inwards area of a production facility. They allow a valuable quality control test to be carried out on incoming materials, thus identifying those unlikely to be suitable for lasting before they go into production. The STD 190 and STD 190L machines are pneumatically activated, and use a ball-ended rod to cause a set 6, 7 or 8mm distension in the specimen material. The operator checks to see if the surface of the material cracks or tears when the distension is applied. The STD 190L version allows the test to be conducted further into the material, away from the edge. To determine which pre-set distension value should be set, more detailed laboratory tests should be carried out – for instance, by using a SATRA STM 463 digital lastometer.

The SATRA STM 463 digital lastometer (figure 3) is designed to conduct tests to the SATRA TM24 test method. This machine provides a mechanised rate of loading in line with the requirements of the test method of 0.20+/-0.05mm/s. A high accuracy load cell is used to record the load applied to the specimen. An electronic menu-driven system displays a continuous output during the test on an LCD screen. A USB output allows the test data to be sent to a computer or a printer, providing a graphic readout. The test machine can also be set to test a series of specimens, and calculates an average load, distension and standard deviation.

 

Publishing Data

This article was originally published on page 48 of the March 2018 issue of SATRA Bulletin.

Other articles from this issue »

---