Moulding vulcanization

Adhesion of mbber is limited because of its inherent nonpolar nature and the presence of additives in formulation (processing oils, moulding agents, antiozonant waxes, vulcanization aids). Although, unvulcanized mbbers are somewhat less difficult to bond, most of the mbbers used in industry are vulcanized mbbers. To improve their adhesion, a surface treatment is always necessary. 

Numerous methods of vulcanization are available for manufacturing a rubber product. In the case of moulded goods for process... 

Tyres are normally cured in a modification of the compression mould where a bladder or an inflated airbag forces and holds the green rubber stock of the tyre against the mould surface during vulcanization. This force reproduces the design of the tyre tread and the heat from the steam is introduced into the bladder to effect the vulcanization. Small size rubber expansion joints used in piping systems are moulded by compression moulding... 

This is due to over-vulcanization. The remedy is to reduce cure times and temperatures or modify the compound formulation to reduce the speed of cure. Tearing can also be due to method of removal of the product from the mould while it is in hot condition. Tearing can be eliminated by removing the products from the mould after sufficient cooling or by careful and slow removing. 

In view of the high cost of mould and relatively large consumption of steam and electricity, economy means the shortest possible vulcanization time. Moulding time is directly proportional to the thickness of the product and inversely proportional to the temperature. 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

All rubber products exhibit shrinkage after cure, mainly due to the thermal expansion which occurs at vulcanization temperature. Moulded rubber goods are never as big as the moulds in which they are cured. The difference between the dimensions at room temperature of the finished goods and of the mould expressed as a percentage is called the shrinkage from mould dimensions. 

The dithiocarbamates have the pentacoordinate binuclear structure (44). The diamyl- and diethyl-dithiocarbamate complexes have been found to inhibit the hardening of asphalt, but the effect appears too weak to be useful.127 The latter complex is an effective antioxidant for polyethylene,128 polypropylene,129 polystyrene,130 poly(methyl methacrylate)130 and an isoprene-styrene copolymer.131 The di-n-butyldithiocarbamate complex is important in the vulcanization and injection moulding of rubber,132 as a stabilizer against photolytic and thermal degradation. 

The production of foamed rubber by whipping a latex is known for a long time. Rubber mattresses are still being made in this way. The latex is whipped into a foamy substance and then poured into a mould. The water is evaporated, and the rubber is vulcanised with the aid of vulcanization ingredients (sulphur etc.) added before to the latex. The density of a rubber latex foam for mattresses is about 70 kg/m3. 

Styrene copolymer. The di-n-butyldithiocarbamate complex is important in the vulcanization and injection moulding of rubber, as a stabilizer against photolytic and thermal degradation. 

Wan and Isayev (1996) examined a hybrid approach of control-volume finite-element and finite-difference modelling of injection moulding of rubber compounds. The effect of vulcanization on viscosity and yield stress during cavity filling is reported. On comparing two versions of the modified Cross viscosity models - with and without the effect of cure - the use of a viscosity model that accounts for the cure was found to improve the accuracy of the cavity-pressure-prediction models. When the modified Cross model was further extended to include the yield stress and was implemented in the simulation program a significant improvement in the prediction of cavity pressure was obtained in the case of low injection speed. 

Nichetti (2003, 2004) examined the use of a cure model to predict mechanical properties of isothermally vulcanized moulded items. 

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