Rubber surface heat transfer coefficient

The surface heat transfer coefficient is the quantity of heat flow per unit time normal on the surface across unit area at the interface between two materials with unit temperature difference across the interface. Obviously, it depends on the nature of this interface, and especially on the nature of the material that is in contact with the rubber. 

It is immediately apparent that in many processes involving rubber heat flow across the interface between two surfaces has to be considered. This is true in mixing, moulding, cooling after processing and conditioning of test pieces but, nevertheless, very little attention has been paid to the measurement of the coefficient. The effect of the heat transfer coefficient on net heat flow is greatest with thin articles and where one of the materials is a gas. It is probably reasonable to assume a value of infinity for the transfer coefficient when rubber is pressed into intimate contact with a metal but in other cases it will be finite. 

A 12-gage (2 mm diameter) wire with a resistance of 0.05 ohms/m carries a 15-ampere current and is insulated with rubber (k = 0.013 W/m-K). The heat transfer coefficient for the outer surface is 10 W/m2-K. Consider the wire temperature to be uniform and the ambient temperature to be 25 °C. Find (a) the thickness of insulation which will result in the lowest operating temperature of the wire, (b) the temperature of the outer surface of the rubber insulation. 

The problem was solved by using a numerical method based mainly on the Dusinberre generalization of the increment method [9] applied to one-dimensional transient conduction. In fact, the heat transfer coefficient at the steel-rubber interface is very large, the surface rubber temperature changed very quickly, and consequently the initial temperature was taken as the arithmetic mean of the original surface temperatures of the mold and rubber. 

Thus, two parameters were especially considered, such as the value of the cure enthalpy [25] and the value of the temperature of the motionless air in which the post cure is achieved [26]. Thus the increase in the state of cure obtained at the mid-plane of the rubber sheet was found to vary linearly with the value of the cure enthalpy. Obviously the lowest value of the coefficient of surface heat transfer at the rubber surface during the period of postcure is desirable... 

A major problem in cooling rubber, in any process, is its inherently poor conductivity, which means that it is only cooled where it touches a cool surface. Thus, total heat removal depends on the area of the coohng surface and also on the way in which fresh rubber surfaces are moved into contact with that cooling surface. In internal mixers this depends mainly on the geometry of the rotors. Mixers with intermeshing rotors are less influenced by the friction or adhesion between the rubber and metal, and are, therefore, less sensitive to starting temperature. Rotor geometry also affects the overall heat transfer coefficient in a mixer. ... 

Heat transfer coefficients were evaluated for the overall heat transfer from the stock to the cooling water and for the heat transfer from the stock to the chamber wall. The data indicate that the compound was not uniform at the beginning of the mixing, in spite of the fact that a premixed powdered rubber was used. The non-uniformity must be in the degree of incorporation of carbon black rather than in the composition. There is also non-uniformity in the temperature. The heat transfer coefficient has a high value when the carbon black is on the surface of rubber but decreases to a constant value with the incorporation of carbon black into the rubber. 

Various solutions are obtained according to the assumptions made for the initial and boundary conditions. A few of them are presented, and calculation is made for the more simple system, even if it is not realistic and should be used with great care—for example, when the temperatnre is uniform initially, and the temperatures are kept constant on the surfaces. The two other cases considered are (i) when a finite coefficient of heat transfer is at the snrfaces of the solid, and (ii) when the rubber is heated by the mold on both surfaces. 

By cooling the rubber sheet in stirred water at 20°C with a high coefficient of heat transfer at the rubber surface h = 276 W/(m deg) (Figure 4.28). 

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