Custom Rubber Moldings
Compression moldings are the simplest, cheapest, and probably the most widespread of the three basic molding techniques. They are ideally suited to custom rubber moldings, 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. For a ball, one might use an elliptically shaped extrusion, cut to an appropriate length from a Barwell. 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 preform is placed in the bottom cavity and the top mold section placed on it.
by hand. If a significant number of custom rubber moldings are to be made, it is often advantageous to fix the two halves of the mold to their respective press platens, thus reducing manual handling and therefore labor costs.
The mold is continuously heated to a temperature, typically between 120 ° C and 180 ° C. A cure time for a smaller part might be 20 minutes, at 150 ° C, for thin cross sections (6 mm). In this case, temperatures above 150 ° C could reduce the cure time to 10 minutes or less.
At independent custom rubber molders, the chemist plays his part in achieving a smooth flow of material in the mold, by striving to control the uncured compound viscosity. This needs to be high enough to create the backpressure required to expel air efficiently as the mold closes, and low enough to permit completion of flow into all parts of the cavity before vulcanization begins. If we look at a low cured-hardness rubber, it usually contains little or no filler (NR & CR), or alternatively fillers plus a large quantity of oil. This can often make its viscosity too low for successful compression moldings and the compounder may strive to increase its viscosity, by choosing a raw gum elastomer grade with a high Mooney viscosity.
At the other end of the scale, high vulcanized-hardness compounds with lots of highly reinforcing fillers will need specialized process aids and low Mooney viscosity raw gum elastomers, to reduce viscosity, in order to promote the flow of the compound in the mold.
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. This `shock 'treatment is called` bumping'. 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. They permit an alternative escape route for the trapped air (together with some rubber). 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 for the independent custom rubber molders.
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, ie, 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 area of the press rams, divided by the projected area of rubber and flash between the mold halves, multiplied by the line pressure at the press, will give the pressure exerted on the product in the mold at closure. The required pressure is typically 7-10.5 MPa and will vary according to such things as the viscosity of the compound and the complexity of the mold cavity. The mold is designed to take the high stress involved.
The area of projected rubber can be smaller at the beginning of mold closure, since the rubber has not yet fully spread over all of the mold cavity. More of the force from the ram might briefly act on delicate inserts or parts of the mold, depending on the exact set up involved. This sometimes has the potential to cause damage if not taken into account.
The flow of material in a mold is a complex process, especially in compression moldings. The rubber in the cavity is undergoing large temperature changes, which translate to viscosity variations thus continuously altering the flow characteristics of the compound. In recent years finite element analysis packages, which describe the material flow patterns in the mold, have become available to mold designers. The use of such design aids is at an early stage in most of the rubber industry.
Once the compression mold has closed, the compound continues to heat up and attempts to thermally expand. Its coefficient of expansion can be a least fifteen times greater than that of the steel mold. For custom moldings with large cross sections or high volume to surface area ratios, such as a ball, phenomena such as backrind can occur. When the product is taken out of the mold, it looks chewed up and torn in the area of the flashline; this is described as backrind. If this occurs there is likely to be a flurry of activity between the shift foreman, chemist and engineer. These are the skill of independent custom rubber molders.
Backrind is thought to be caused because as the rubber heats up (heat transfers first from the mold to the outside layers of compound) the outer layers of the molding cure first, while the colder uncured inner layers are still heating up and attempting to thermally expand . Since the inner layers are restricted by the closed mold and cured outer layer of compound, they develop a continuously increasing internal pressure. If this internal pressure exceeds that applied by the press, the mold will open for an instant, relieving the internal pressure and causing a rupture at the 'cured' parting line; the mold will then instantly reclose. If this occurs a number of times during the cure it is called chattering.
Another theory is that at the end of the cure time, at the instant the press is opened, the removal of the external clamping force instantaneously releases the internal pressure in the product, opening the mold slightly and causing a rupture at the parting line of the vulcanizate. Sometimes, only some areas of the parting line are affected, suggesting that in these cases the mold is opening unevenly.
Possible solutions that might alleviate the backrind and chatter problem are:
a) Pre-heating the preform.
b) Designing a 'sacrificial' section into the product at which backrind will occur between this section and the flash line. This section is then removed after cure, leaving only a small blemish where it is connected to the product.
c) A more intriguing idea is to drill 6 mm holes through the mold into the cavity, into a less important section of the product. As the compound heats up and expands in the heated closed mold, it freely extrudes through these holes; in a large product, uncured compound can extrude for quite some time, (this may be analogous to moving water not freezing in an otherwise frozen stream). The mold is designed so that there is still sufficient backpressure to allow air and product to flow into the flash grooves. This last method might be used for large products, 11 kg or more in weight, since backrind is a more serious problem in larger products.
d) For certain simple product geometries, it is possible to place in the mold an amount of rubber, which is actually slightly less than the amount required to fill the cavity at room temperature.
As it heats up in the closed mold it expands and completely fills the cavity without the consequent build up of to much internal pressure. This would need precise control of preform dimensions and assumes the closed mold is not totally airtight.
e) A compound formulated for long scorch time might delay curing of the outer layers during thermal expansion, thus preventing any rupture of these layers during the presumed instantaneous mold opening during cure.
f) Reduction of the temperature of cure would decrease thermal expansion or possibly, in effect, increase scorch time of the compound. This would be at the price of increased cure time.
g) Cooling the mold after cure, before reducing the pressure applied by the press, and then opening the mold, might reduce internal pressure and therefore possibly reduce backrind.