Rubber moulding

In a conventional rubber moulding press the main ram travels upwards. In a down-stroking press the main ram is situated above the movable top platen to which pressure is applied by a downward movement of the ram. 

The mineral muscovite, mainly an orthosilicate of aluminium and potassium, finely ground and used as a lubricant in rubber moulding and as an extended filler in latex compounds. 

In rubber moulding, those areas which contact each other when the mould is closed. Also used in the usual sense of alignment. 

In rubber moulding, another name for light, i.e., a product where the mould has not been completetly filled. 

In the case of isostatic compression the same pressure is exerted on the ceramic mass on all sides. The mass is contained in a rubber mould and this is immersed in a liquid inside a pressure vessel. By pressurizing the liquid you can apply the same pressure on the mould from all directions (figure 9.15). This method is especially used for fanciful and asymmetrical objects. 

An alternative system of starchless moulding has come into use where the product is deposited into rubber moulds. This type of system is satisfactory where the product is deposited at the final solids, e.g. pectin jellies. If the product has to be dried ( stoved in the terminology of the industry - see below) to final solids then starchless moulding is unsatisfactory. 

Imitation scrimshaw is made of plastic - for example, epoxy -which is moulded in a silicone rubber mould that is flexible. This enables it to be pealed off the moulded object, and eliminates the risk of marks from joins in the mould. The plastic is weighted to simulate ivory (Figs 3.10 and 3.11). 

FIG. 168. Schematic diagram of isostatic pressing (a) pressure liquid, (b) rubber stopper, (c) rubber mould, (d) powdered mixture being compacted. 

Fondants are formed generally in moulds. The fondant is reheated to 65-70°C and poured into moulds made of starch powder in high capacity continuous machines. Silicon rubber moulds are used in certain types of these machines. Generally, the finished product can be taken out of the mould after 8 hours of cooling. 

Important processing and material variables for rubber moulding processes are... 

Mould material — Most moulds for rubber moulding are made of steel. The metal should be close grained and of uniform and fault free texture. A carbon content of 0.25% to 0.45% is preferable. Moulds are sometimes made of aluminium alloys or manganese bronze with a high silicon content. Moulds difficult to machine by conventional methods can be made using these alloys. 

Although rotational moulding is used for both plastics and rubber, pharmaceutical applications, other than for rubber mouldings, are unlikely. 

Blooms and visually similar phenomena are sometimes seen on the surfaces of rubber mouldings. These may originate from the limited solubility of a particular component of the rubber compound, or from contamination, for instance, by a release agent from the mould. Some blooms are deliberately contrived - hydrocarbon wax gives some protection from ozone attack. Loadman and Tidd give a useful account of the classification and analysis of blooms on rubber. 

Because temperature is critical in processing, it is important to exercise very tight control. Mould cavities should be within 2 °C of the experimentally determined optimum working temperature. For general rubber mouldings, where the rubber modulus has little bearing on service requirements, a wider tolerance may be acceptable, albeit at the expense of profit Components with high technical specifications need carefully controlled conditions. 

Hydraulic mechanisms have proved the favoured means of press operation within the rubber industry for a number of reasons. Historically, before the advent of the injection process, rubber moulding was accomplished using compression or transfer techniques with hydraulically operated presses. These presses could be manufactured cheaply and run using water as the hydraulic fluid, with a simple accumulator system to develop the necessary pressure. The same water supply was used to raise the high steam pressure that was used to heat the platens. The operators required to set up these simple presses could be easily trained and needed no special engineering skills. The rubber industry was therefore conditioned to the use of hydraulic presses, well before the advent of injection moulding. In time the hydraulic systems were refined and changed to oil. 

Axel claims that its latest external mould release agent XTEND 19MDR can increase the lifetime of the room temperature vulcanised silicone rubber moulds used in polyurethane and... 

Matched metal moulding techniques have limitations when used to process composite materials of this form. In addition, a complicated and costly split mould would be required to remove the male section from the Inside diameter of the bellows after moulding and curing the epoxy resin. A flexible, self-releasing silicone rubber mould core offers a simple solution. 

Shorter curing times in the silicone cavity lead to shorter contact times for the thermoplastic. The shorter the contact time, the higher the temperature of the silicone cavity that can be used without it being critical for the plastic substrate. In other words, the temperature of the silicone rubber mould strongly depends on the speed of curing of the liquid rubber used in the processing. 

FIGURE 2.16 The effect of minimum film-formation temperature (MFFT) of a dispersion on its drying behaviour. Primal (Rohm Haas) dispersions were allowed to dry at room temperature, =21 °C, in a silicone rubber mould. An MFFT above room temperature results in incomplete coalescence of particles and poor film formation. The cracks and distortion that occur in the films result from the movement of water and shrinkage during drying, (a) AC-34 MFFT, 12 °C. (b) AC-73 MFFT, 37 °C. (c) B-85 MFFT, 90 °C. 

MANUFACTURING OF RUBBER MOULDINGS ORIENTED TO QUALITIES MATERIALMOULDING-PROCESS... 

Screens may be fitted using either rubber mouldings with or without additional sealants or may be bonded to the body structure enabling a flush condition to be obtained on the outer surfaces, reducing wind noise and improving the aerodynamic performance of the vehicle. 

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