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 Mills:
Raw gum elastomer is placed into the gap between the two mill rolls, the mill nip. It then bands, as a continuous sheet, onto one of the rolls. The speeds of the two rolls are often different, the back roll rotating faster than the front. The difference in speed between the two rolls is called the friction ratio and allows a shearing action (friction) at the nip to disperse the ingredients and to force the compound to stay on one roll, preferably the front one. A friction ratio of 1.25:l is common. Powders, liquids, etc ., are then added to the nip in a specific way. The process produces friction which creates heat. This excess heat needs to be removed, either by spraying or flooding the inside of the roll with cooling water or by passing water through drilled channels in the wall of the roll.
Compression molding: This is the simplest, cheapest, and probably the most widespread of the three basic molding techniques. It is ideally suited to small quantity production, say, from around fifty to a few thousand of each product annually. One of the keys to successful molding is adequate removal of air while the mold cavity is filling up with rubber.
The uncured pieces of compound placed in the mold are known-variously as preforms, billets or load weights. This shape is important and deliberately chosen so that air in the mold cavity will have a free path of escape when the mold begins to close. Normally the weight of this preform will be chosen to be a few percent (from two to ten percent) above the weight of the final product, to ensure a fully formed product and to give an extra 'push' for expulsion of any residual trapped air. The shape of the preform and also its placement in the mold is important. The uncured rubber, placed in the cavity, might be a single piece or a number of pieces. This method is very much an art.
As the press platens close the mold, excess compound begins to squeeze out into the flash grooves, taking air with it. Often, residual air remains and various methods have been devised to remove it. One method is to bring the mold pressure back down to zero and then return to full pressure by quickly lowering and raising the press platens a number of times. An additional line of attack is to find where air is being trapped in the final cured product and drill a small diameter hole through the mold cavity in the equivalent area; these are called bleeder holes.
Since flash often spills over the land during compression, it is possible that a large land area between the flash groove and the outside of the mold might 'fine tune' backpressure control. A large land distance restricts flow at the time when the mold is almost closed and thus might increase backpressure, which would be of assistance with low viscosity compounds. For high viscosity materials the opposite might apply, i.e., a small land area and deep flash grooves would be desirable. This would also promote greater pressure at the moment before full mold closure for the same force exerted by the press ram. Radial grooves connecting the flash grooves with the outside of the mold should also assist in high viscosity compounds exiting the mold.
The press needs to exert a certain amount of pressure to allow the compound to flow into the cavities and for the mold to properly close. The objective is to obtain a thin flash, 'ideally', around 0.05 mm.
The flow of material in a mold is a complex process, especially in compression molding. The rubber in the cavity is undergoing large temperature changes, which translate to viscosity variations thus continuously altering the flow characteristics of the compound.
Transfer molding:
If we take the top half of a compression mold, then drill transfer holes through it and place a metal collar (transfer pot) on the closed mold so as to surround all of the holes, we have in effect converted it into a transfer mold.
All that is needed now is to put rubber compound into the pot and force it through the holes by placing a piston (plunger) into the pot and using the press platens to force the piston to push the compound down through the pot into the closed mold cavity. This conversion is used in the rubber industry. Alternatively, the transfer pot can be designed to be an integral part of the mold and the piston can be fixed to the upper press platen.
Injection molding:
An injection mold consists of a cylinder (injection barrel) with a ram or screw inside it, so that the rubber compound can be moved towards a nozzle at its end. The nozzle is then pressed against a hole made in the top half of a closed mold. This hole is then connected to smaller holes (gates and runners) which enter the cavities of the mold.
The compound can be presented to the barrel as a continuous strip, or in granulated form through a hopper, as in plastics injection molding. A ram has a tighter fit in the barrel than a screw and therefore there is less leakage backwards through the barrel; it is also cheaper than a screw. The screw 'mixes' the compound as it moves towards the nozzle, creating more frictional heat and therefore higher temperatures which translate to easier flow and shorter cure times. A combination of ram and screw is popular.
Advantages:
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The cure temperatures used can be much higher than those used for compression or
transfer molding. For example, MRPRA literature [lo], mentions NR injection temperatures of 160 "C and mold temperatures of 180-190 "C. High temperatures mean shorter cure times; one or two minutes are possible for thin cross sections. |
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Since the temperature of the compound entering the cavity is closer to the molding temperature, there is much less thermal volume expansion of the rubber during cure, therefore much less internal pressure build-up, resulting in a much reduced tendency to backrind. |
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No complex preform is needed. |
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Flash is significantly reduced or eliminated. |
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Air entrapment is significantly reduced. |
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The system is capable of a high level of automation, reminiscent of plastics injection molding. |
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It is suited to fast, high quantity production. |
Disadvantages:
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The molds need to withstand very high injection pressures. This entails use of high hardness steel molds and higher precision tooling. |
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If gates and runners are added to the mold its cost becomes
significantly higher than compression or transfer molding. |
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