2.5 HARDNESS TEST

2.5 HARDNESS TEST:

2.5.1 INTRODUCTION:

Hardness is defined as the resistance of the material to deformation, particularly permanent deformation, indentation, or scratching. Hardness is purely a relative term and should not be confused with wear and abrasion resistance of the plastic material. Polystyrene, for example have very high Rockwell hardness but have very poor abrasion resistance. Hardness tests can differentiate the relative hardness of different grades of particular plastic. However, it is not valid to compare the hardness of various plastic entirely based on one type of test, since elastic recovery along with hardness is involved. The test is further complicated by the phenomena of creep. Many tests have been devised to measure the hardness. Because plastic materials vary considerably with respect to hardness, one type of hardness test does not cover the entire range of hardness property encountered. Two of the most commonly used hardness test for plastic is Rockwell and the Durometer hardness test. The Rockwell test is applied for relatively hard plastics such as acetals, nylons, acrylic, and polystyrene. For softer materials like flexible PVC, thermoplastic rubber, and polyethylene, durometer hardness is used.

2.5.2 DUROMETER HARDNESS: (ASTM D 2240)

The test method is based on the penetration of a specified indentor forced in to the material under specified conditions. Two types of durometer are commonly used Type A and Type D. the basic difference between the two type is the shape and dimension of the indentor. The hardness number derived from either scale is just numbers with out any units. Type A durometer is used with relatively soft materials while Type D durometer is used widely with harder materials[2,12].

2.5.3 APPARATUS:

The durometer hardness tester consists of a pressure foot, an indentor, and an indicating device. The indentor is a spring-loaded device and the point of the indentor protrudes through the hole in the base. The basic difference between the two types is the shape and the dimension of the indentor. The test specimen is at least ¼ in inch thick and can be either moulded or cut from a sheet. The poor contact between the thin specimens may cause the result to vary considerably.

fig 2.2 Hardness Testing Machine fig 2.3 Durometer

2.5.4 PROCEDURE:

1. The test is carried out by first placing a specimen on a hard, flat surface.
2. The pressure foot of the instrument is pressed on to the specimen, making sure that it is parallel to the surface of the specimen.
3. The durometer hardness is read with 1 sec after the pressure foot is in firm contact with the specimen.
4. The readings are noted at various places and the average value is taken.

2.5.5 FACTORS AFFECTING THE TEST RESULTS:

TEMEPRATURE AND HUMIDITY:

The hardness of all the plastic material is directly affected by the changes in the temperature and humidity. Specimen tested at higher than specified temperature tends to indicate a lower value.

SURFACE CONDITION OF THE SPECIMEN:

The surface finishes of the specimen have high significance on the results obtained. A smooth surface yields a higher value than a machined surface.

FILLER:

The hardness value may vary depending upon the type and the amount of filler present in the resin. The variation in hardness may be caused by the difference in hardness between resin and the filler.

ANISOTROPY:

Plastic with anisotropic property may cause indentation hardness to vary with the direction of testing.