Test equipment used for assessing soles – part 1
SATRA equipment available for testing one of the most important footwear components – the sole.
by Peter Allen
SATRA produces a diverse range of test equipment which can be used to assess soling performance characteristics. This article considers the SATRA test machines used for assessing a number of standard properties applicable to a range of soling materials. Part 2 of this article will examine some of the tests and the associated test machines used to assess the performance of soles after integration into footwear – for example, slip resistance testing.
There are several tests which are applicable to particular types of soling materials, to assess such characteristics as resistance to water penetration, fuel oil, fat, hot contact, discolouration, colour transfer and leather grain cracking, as well as split tear strength. However, we will concentrate on the equipment needed to conduct a number of more general assessments.
Hardness test devices
There are different types of hardness testing meters used in the footwear industry, including analogue durometers, digital durometers, IRHD testers and Asker hardness testers. Durometers are widely used by footwear companies to determine soling material hardness. Tests are carried out with either a hand-held device or a stand-mounted unit. The most common durometer hardness ranges for solings are Shore scale ‘A’ (suited to flexible compact polymers) or Shore ‘D’, for testing hard plastics such as used in women’s heel top-pieces. The hardness of soft foams can be tested with a device having a Shore ‘OO’ range. SATRA manufactures both digital and analogue durometers (SATRA STD 226 and STD 227 – see figure 1) in a range of Shore hardness scales – including A and D – and an analogue durometer in a Shore OO range. SATRA also produces compatible stands for these hardness meters.
Tests conducted with a tensile testing machine
There are a number of soling tests which are carried out with a tensile testing machine along with the jaws appropriate to the particular test. SATRA produces a piece of equipment suitable for these tests – the 5KN capacity STM 566 (figure 2). This tensile test machine can also be supplied with an interchangeable 1KN load cell. The tests carried out with a tensile test machine for assessing soling materials include tensile strength and elongation at break, tear strength and stitch tear strength. This latter test is primarily intended for assessing the ability of soling materials to hold stitches, and is applicable for footwear with stitched-on soles. The article ‘Principles of footwear testing – part 8: strength of soling materials’, gives more details on the test methods, sample preparation and how these tests are conducted.
Another test carried out with the SATRA STM 566 tensile test machine is SATRA TM401 – ‘Peel strength of adhesive bonds’. While this test allows a wide range of different material-to-material bonds to be assessed, it is also used when assessing the bonds in soling constructions – for instance, the bond between an outsole and a midsole. The test is conducted with quick release jaws using specimens prepared according to the SATRA TM402 test method.
Flexing performance of sole materials
Another important soling material characteristic to be assessed is the material’s resistance to crack growth on flexing. This can be achieved by testing to SATRA TM60 – ‘Resistance to cut growth on flexing’, using a SATRA STM 141 Ross flexing machine (figure 3). This test is designed to test non-leather materials used for shoe soles. It can also be used to test materials for other applications. The SATRA TM60 test method is intended to determine the resistance of polymeric material to cut growth during repeated flexing cycles.
When the test is conducted on sheet materials, specimens of the material are cut to the prescribed size and one end is clamped to a flexing arm. The other free end of the specimen is supported on rollers and between pinch rollers. The flexing arm moves through an angle of 90°, bending the material around a flexing mandrel while the rollers allow the free end of the material to move. This allows the specimen to bend, but not to stretch around the mandrel. Before conducting the test, a cut of defined dimensions is made through the centre of the test specimen. The purpose of test is to see how resistant the material is to propagation of the cut. The test is stopped after the cut length has grown by 6mm or after 150,000 cycles (whichever is the sooner). Detailed measurements taken of the cut allow an average cut growth rate to be determined per thousand flexing cycles. SATRA TM60 also describes how to conduct the test using larger test specimens, such as sole foreparts.
SATRA also produces a variant of this machine (STM 141F) which allows the tests to be carried out at low temperatures. The low temperature option permits assessment of materials which may be vulnerable to increased rate of cut growth due to increased brittleness of the material at low temperatures.
The Ross flex method is best suited to smooth soles or those with a very shallow tread pattern (for example, below 2mm). The next part of this article will give details of other tests which can be used to assess the flex resistance of moulded soles with deeper tread patterns.
Soling abrasion tests
Using good abrasion-resistant material is one of the most important factors influencing the durability of soles. Assessment of soling material can be conducted under the SATRA TM174 test method, using the SATRA STM 602 machine (figure 4). A specimen of soling material of defined dimensions is weighed before being installed in the machine. A defined load is applied between the specimen and a drum covered with a specified abrasive paper. The drum is rotated under the specimen at a fixed speed, while the sample traverses along the drum for a set distance. The specimen is removed and re-weighed, allowing the amount of soling material lost to abrasion to be established. The abrasive resistance of the compound is then determined by relating the test results to those of a control compound (STM 469CR). The weight loss is converted to a volume loss, having first determined the density of the material.
Assessing thickness and density
Obtaining optimum soling thickness is an important consideration for footwear. Soles with excess thickness will increase the weight of the footwear and can reduce flexibility. By contrast, soles that are too thin can have insufficient ground insulation and be less durable in wear. A process for taking measurement of sole units is set out in SATRA TM136, which concerns the thickness and effective thickness of soling materials and sole units. Determining accurate sample thicknesses is important in a number of tests, including tensile strength, flexing, determination of density and compression set. SATRA produces a range of stand-based thickness gauges. These are fitted with different diameter foot sizes, depending on the material to be assessed. The applied foot pressure, as specified in the individual test methods, is varied by interchangeable weights which are applied to the gauge spindle.
To determine the density of cellular soling materials, a thickness gauge is used to conduct tests to SATRA TM68 – ‘Density of cellular materials’, along with a precision balance and vernier callipers. Examples of SATRA thickness gauges include STD 483 (for leather), STD 484 (for non-leather flexible materials) and STD 495 (for sole units), shown in figure 5.
‘Compression set’ can be defined as ‘the loss of thickness of a material, measured after a period of compression and recovery’. This is an important consideration for assessing sole materials, which are subject to static and dynamic compression loads in the course of standing or walking. This property can be assessed using the SATRA TM64 test method – ‘Compression set-constant stress method’. The test is performed with the SATRA STD 133 equipment (figure 6). Specimen discs are cut from the sole or sole material and are held between spring-loaded platens for a given time (24 hours) under a constant pressure. By using several spacers, tiers of specimens can be tested at the same time. The thicknesses of the discs are measured before and after the test, and the decrease in thickness provides an indication of the compression set of the material. Further information for preparing specimens is provided within SATRA TM64. The SATRA STD 495 thickness gauge is suitable for determining the specimen thickness.
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This article was originally published on page 42 of the January 2014 issue of SATRA Bulletin.