Indentation load deflection

There are several ways to measure the hardness of a flexible foam, and the chosen method appears to be mostly dictated by tradition or geographic location rather than by technical considerations. The indentation load deflection (ILD), as with all the hardness" determinations, is generally accepted to be a measure of the load-bearing capability of the foam. Up until about 10 years ago in the UK the hardness of a foam could be described with reference to BS 3667 [47]. This technique indented a foam of dimensions 10x10x3... 

Figur6 12 Measurement of (a) indentation load deflection (ILD) and (b) compression load deflection (CLD).

IBC internal bubble cooling ILD indentation load deflection... 

SD Standard Deviation ILD indentation load deflection SLM standard litres per minute ... 

In general, most thermoplastics are softer than your typical metal materials. Elastomers and foams also have very different behavior when it comes to hardness, since they inherently have a high amount of elasticity, and can be easily compressed without having any permanent deformation. Foam hardness is often measured based on the amount of compression during a given load. Known as indentation load deflection, this measures the ability of a foam to support a given weight. 

Test I3 determines (1) the loss of force support at 40% IFD (indentation force deflection), (2) loss in thickness, and (3) structural breakdown as determined by visual inspection. This vocedure describes tests that deflect the material by a flat-horizontal indentation exerting a force of 750 +/- 20 N in the test specimen. Test I3 is similar to ISO 3385 and BS 4443 Method 13 and is used jximaiily to determine the loss in thickness and in hardness of flexible cellular materials intended for use in load- bearing upholstery. The test involves repeated indentation of a specimen by an indentor smaller in area than the test piece. The flexible cellular materials tested are usually latex and polyether urethanes. 

Method A. Indentation force deflection (IFD) consists of measuring the load necessary to produce deflection (generally 25-65 percent) in the foam product. The test is carried out by pushing a flat circular indentor foot down into the foam specimen and measuring the force on the foot at various compression amounts. The test is widely used in the cushioning and bedding industry. Higher IFD values indicate a stiffer foam. 

FIGURE 33.6 Indentation tests of three different UHMWPE materials, (GUR 1020, Hylamer, and Marathon). Note the differences in loading slope and maximum load for approximately the same loading deflection. Moduli, hardnesses, and EDFs can be determined from these indents. 

Two particular test methods have become very widely used. They are the Vicat softening point test (VSP test) and the heat deflection temperature under load test (HDT test) (which is also widely known by the earlier name of heat distortion temperature test). In the Vicat test a sample of the plastics material is heated at a specified rate of temperature increase and the temperature is noted at which a needle of specified dimensions indents into the material a specified distance under a specified load. In the most common method (method A) a load of ION is used, the needle indentor has a cross-sectional area of 1 mm, the specified penetration distance is 1 mm and the rate of temperature rise is 50°C per hour. For details see the relevant standards (ISO 306 BS 2782 method 120 ASTM D1525 and DIN 53460). (ISO 306 describes two methods, method A with a load of ION and method B with a load of SON, each with two possible rates of temperature rise, 50°C/h and 120°C/h. This results in ISO values quoted as A50, A120, B50 or B120. Many of the results quoted in this book predate the ISO standard and unless otherwise stated may be assumed to correspond to A50.)... 

The Vicat softening temperature is the temperature at which a standard deflection occurs for defined test samples subjected to a given linear temperature increase and a compression loading from a defined indenter of a specified weight. The load used is often ION (Vicat A) or SON (Vicat B) and must be indicated with the results. In either case the polymer cannot be used under this compression load at this temperature. 

The deflection of the beam was monitored using an inductive displacement gauge (Mahr Pupitron) with a sensitivity of 0.06mmA7. The transducer measured the deflection at the bottom side of the beam, directly under the loading nose main axis. No extra correction for indentation was necessary. 

Two procedures described in BS4443, Part 2, 1988 [48] are suitable in particular for latex, PVC, and polyurethane foams. One is faster to carry out and can be used as a quality control method (Procedure A). Procedure B can be used to determine the load to give indentations of 25, 40, and 65% deflections and hence the sag factor can be determined. In addition, by measuring the load for specified indentations of the foam on loading (as with Procedure B), followed by measuring the indentations on unloading, a measure of the foam hysteresis can be determined. Hysteresis is a measurement of the energy absorbed by a foam when subjected to a deformation. 

However E must be obtained at the same temperature and time conditions as the fracture test because of the viscoelastic nature of polymers. In order to avoid the associated uncertainties, it is considered preferable to determine Gy or G. (subject to the validity criteria of AV) directly from the load versus deflection curve up to the same load point as used for Ay or A l. determination (see Fig. 3). The energy should be corrected for speeimen indentation (suffered under loading and support pins) by deducing the energy of indentation. ( j from t y. Therefore ... 

Two particular test methods have become very widely used. These are the Vicat softening point test and the test widely known as the heat distortion temperature test (also called the deflection temperature under load test). In the Vicat softening point test a sample of polymer is heated at a specified rate temperature increase and the temperature is noted at which a needle of specified dimensions indents into the polymer a specified distance under a specified load. 

Once the tip is in contact with the surface, cantilever deflection will increase as the end of the cantilever is brought closer to the sample. If the cantilever is sufficiently stiff, the probe tip may indent the surface at this point. In this case, the slope or shape of the contact part of the force curve can provide information about the elasticity of the sample surface. Extending the tip (along line C-D) results in loading (repulsive) forces to the surface. These repulsive forces are usually used as a feedback parameter for the AFM system to obtain surface morphology. 

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