Wednesday, August 17, 2011

Thermal properties of rubber compound : thermal diffusivity

Thermal diffusivity can be conveniently determined by observing temperature change as a function of time for sample geometrical shapes under heating condition. Alternatively of course if k r and Cp are known or separately determine. Thermal diffusivity can be calculated from its definition as thermal diffusivity = k/r*Cp. As specific heat is an additive property, it is generally convenient to calculate specific heats of rubber compound than to measure them. Specific heat of a rubber compound is given by

Cp = w1*C1 + w2*C2 + w3*C3 + …..

where w1, w2 and w3 are weight fractions of the ingredients and C1, C2 and C3 are their specific heat. In general for rubber compound above the grass transition thermal diffusivity tends to decrease slightly with increasing temperature. This is attributed to an increase of specific heat as temperature increase. However some author stated that there is no significant change in thermal diffusivity over the temperature range from room temperature up to 140 *C.

Tuesday, August 16, 2011

How to increased thermal conductivity of rubber compound.

Thermal conductivity is the basic parameter for defining heat flow in a material. Normally the thermal conductivity of rubber compound is inversely propertional to temperature but in the range of 20-90*C, the value of thermal conductivity of both gum and carbon black filled compound are slightly changed, therefore it can be assumed that the thermal conductivity of rubber is independent of temperature without any significant error. The inclusion of filler has a marked effect on thermal conductivity of the rubber compound which postulated that 10 phr of carbon black may be expected to increase the thermal conductivity by about 17% at room temperature. In the meantime we founded that thermal conductivity was an additive property depending on the volume fraction of the ingredient by an appropriate conductivity and adding these product to the thermal conductivity of gum vulcanisate. A large dependence of conductivity on loading of carbon black was reported that thermal conductivity of rubber compound increased almost linearly with black content in the range of 10-50 phr. An experimental result clearly showed that thermal conductivity increased linearly with carbon black loading and therefore the mathermatical relations between thermal conductivity and carbon black loading were introduce as

k(w) = k(0) + 0.32w

or

k(j) = k(0) + 0.4jf

where w is the weight fraction and j is the volume fraction of crabon black.

Apart from carbon black loading carbon black structure seems to have an influence on the thermal conductivity of rubber as well. Effect of carbon black structure on thermal conductivity of NR compound we found that higher carbon black structure gives higher value of thermal conductivity. However the degree of change in thermal conductivity as a function of carbon black structure is small and is not straight forward.

Monday, August 15, 2011

Thermal Properties : Basic principal of heat transfer

Holman had proposed a difinition of heat transfer as “science which seeks to predict the energy transfer which may take place between material bodies as a result of temperature difference”. The energy transfer is known as heat . There are three mode of heat transfer : conduction, convection, and radiation.

1. Conduction heat transfer : When a single body is subjected to a temperature gradient heat will be transferred from the hight temperature region to the low temperature region. This mode of heat transfer is call “conduction” and from Fourier’s law for steady state condition, the heat transfer rate per unit area is proportional to the normal temperature gradient, as shown in equation 1

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when dQ/dt is the heat transfer rate and partial derivative of temperature respective to x is the temperature gradient in the direction of the heat flow. The constant k is called the thermal conductivity of the material which is define as the heat transport in a material per unit temperature gradient per unit area between two isothermal planes. In case of unsteady state (transient) conduction, where the temperature at any point in a body varies with both time and position, the basic Fourier equation becomes a patial differential :

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where C and r are specific heat and density, respectively. The expression k/(rC) is called “thermal diffusivity” of the material and is represented by a .A hight value of a could either result from a high value of thermal conductivity (k), which would indicate a rapid energy transfer rate, or from a low value of the thermal heat capacity rC . The low value of heat capacity indicate that less of the energy moving through the material would be absorbed and used to raise the temperature of the material thus more energy would be available for further transfer. Therefore, the larger the value of a , the faster will heat diffuse through the material.

Sunday, August 14, 2011

Rubber wedge mounting

When flat rubber sandwich springs are placed symmetrically to form a wedge, compressive and shear strains will be imposed in the rubber simultaneously under action of the load. This combined compression and shear stress in the rubber is specially advantageous to its strength. Additional bending stresses which are unavoidable in simple shear loading are reduced by the superimposition of the compressive stresses. Bending stresses can even be completely eliminated if the rubber bodies take the form of a parallelogram whose centerline follows the direction of the compression-shear resultant. The same compressive and shear stress will then arise at every point.

Wedge rubber spring are particularly interesting for the designer as they defect in three directions and at the same time possess very different stiffness in each of the directions of their three main axes. It is a wedge rubber spring designed with special care. The rubber is protected against mechanical damage and attack from oil. The rubber spring can easily be attached both to the machine and to the floor.

Wednesday, August 10, 2011

Rubber mould

Rubber product can be produced reliably and economiclly only if moulds are designed to suit the rubber moulding process. The three major moulding methods used in rubber product manufacture are compression rubber moulding, transfer rubber moulding and injection rubber moulding. rubber mould in general are made of metal such as steel, cast iron, aluminium and aluminium alloy. The moulds must be so designed that they can withstand large forces without deformation. Further there must be ready access for the introduction of blanks and the easy removal of finished rubber product. Apart from two-part rubber moulds, multi-plate rubber moulds with loose cores and split inserts, are also in use. Vertical face are given a small taper. Large undercuts are to be avoided. Ribs which protrude into the cavity should not be too thin as they may bend or be damaged during compression. Rubber mould must be provided with appropriate vents to avoid faults in rubber product because of incomplete filling of the cavity or voids in the rubber moulding caused by the entrapped air.

Rubber product are mostly produced by compression moulding. The numbers of cavities frequently amount to 1 to 12 or more. With transfer moulding up to 90 cavities may be served. This is possible because transfer moulding does not call for the insertion of prepared uncured blanks into cavities. Instead, the mixed blank is placed into transfer pot from where the mix flows through sprue and runner into single closed cavities under pressure of the transfer plunger. Rubber injection moulding of rubber product is as yet a comparatively new process. It draw on the processing of plastics and allow extensive automation with moulding times which are at the most up to a tenth of the time needed for compression or transfer moulding.

Monday, August 8, 2011

Rubber product : rubber fender

Quay structures are today successfully protected from impact by ships with the help of rubber fender. This involves large systems of rubber fender to make the fender elastic. Different designs are used such as bonded rubber ring or loose hollow or cylindrical rubber fender. In every case extraordinarily large overall measurements are encountered in individual spring. On the new quay in Puttgarden on the Fehmarnbelt the special form of a loose hollow rubber fender has been chosen. The rubber fender has an outer diameter of 402 mm, a height of 500 mm , a hardness of 70 +/- 5 IRHD and a progressively rising rubber fender characteristic. It can be compressed to maximum of abount 340 mm with an expended force of 700 kN and an expended energy of about 55 kJ.

Sunday, August 7, 2011

Rubber properties : Ageing

Rubber ageing is that process by which natural rubber grades are exposed over a period of time to the attack of oxigen, ozone and ultraviolet light. Under these conditions cracks develop on the exposed surface. In bonded rubber products the rubber remains largely protected by the vulcanised-on metal part and antioxidants. Temperatures over 350 K (abount 80*C) accelerate ageing. To retard ageing, antiozonants are added to the rubber mixing process. The application of lacquer to exposed rubber surface is not of great significance. Spring of natural rubber should be compounded with waxes and thus always protected from the action of oil or petrol because these cause swelling, loosening of structure and reduced strength. Rubber product in systhetic rubber show greater resistance to the factors mentioned than those made from natural rubber.

Friday, August 5, 2011

Rubber pad

The circular rubber pad.

The circular rubber pad is the oldest form of bonded rubber spring. In design it is characterised by the diameter and the height. In practice it is given the name of “metalatic” pad mounting. Its range of application is very large. There are manufacturer who make circular rubber pad in over 100 different size and each in five different rubber grade for loading ranges up to 100 kN per rubber spring. In the moulding process the rubber pad are bonded to adjoining metal insert etc. Some design the rubber pad have metal plates bonded on both ends or have plates only at one end.

The hollow rubber pad.

The ejector spring which are required in cutting and formimg press tools for sheet metal work are frequently designed with hollow rubber pad instead of metal spring. They are cheap and reliable in operation and make simple assemblies. The German AWF Committee of the press manufacturers has published a set of guidelines for the choice, calculation and arrangement of hollow rubber pad. Polychoroprene, nitrile and natural rubber have become the eatablished material. Nitrile rubber is stable under the influence of oil, polychoroprene are reasonably stable, and natural rubber is unstable but natural rubber does not tend to creep as much as nitrile rubber. Creep is understood to be the relaxation of the spring force under continuous stress.