Custom rubber moldings and their production for aerospace applications
custom rubber moldings
Compression molding is the simplest, cheapest, and probably the most widely used of the three basic molding techniques. They are ideal for custom rubber molding, producing small quantities, say around fifty to a few thousand of each product annually.
One of the keys to successful molding is proper air removal while the mold cavity is being filled with rubber. The uncured composite pieces placed in the mold are known as preforms, billets, or fill weights. For a ball, an elliptically shaped extrusion, cut to a suitable length from a Barwell, could be used. This shape is important and is deliberately chosen so that the air in the mold cavity has a free escape path when the mold begins to close.
Normally, the weight of this preform will be chosen to be a few percent (two to ten percent) above the weight of the final product, to ensure a fully formed product and to give additional “push” for the expulsion of any residual trapped air. . The preform is placed in the lower cavity and the upper section of the mold is placed on top of it.
manually. If a significant number of custom rubber moldings are to be made, it is often advantageous to clamp the two halves of the mold to their respective press plates, thereby reducing manual handling and therefore labor costs.
The mold is continuously heated to a temperature, typically between 120°C and 180°C. Cure time for a smaller part may be 20 minutes at 150°C for thin cross sections (6mm). In this case, temperatures above 150°C could reduce the cure time to 10 minutes or less.
In stand-alone custom rubber molders, the chemist plays his part in achieving a smooth flow of material in the mold, striving to control the viscosity of the uncured compound. This should be high enough to create the back pressure required to eject air efficiently as the mold closes, and low enough to allow full flow to all parts of the cavity before vulcanization begins. . If we look at a low curing hardness rubber, it usually contains little or no fillers (NR and CR) or alternatively fillers plus a large amount of oil. This can often make its viscosity too low for successful compression molding and the compounder may strive to increase its viscosity by choosing a raw rubber elastomer grade with a high Mooney viscosity.
At the other end of the scale, high hardness vulcanized compounds with many highly reinforcing fillers will need specialized processing aids and Mooney low viscosity raw rubber elastomers to reduce viscosity and promote compound flow in the mold.
As the press plates close the mold, excess compound begins to flow out of the rounded slots, taking air with it. Residual air often remains and various methods have been devised to remove it. One method is to reduce the mold pressure to zero and then return to full pressure by rapidly lowering and raising the press plates several times. This ‘shock’ treatment is called ‘bumping’. A further line of attack is to find where air is trapped in the final cured product and drill a small diameter hole through the mold cavity in the equivalent area; these are called bleed holes. They allow an alternate escape route for trapped air (along with some rubber). The shape of the preform and also its placement in the mold is important. The uncured rubber, placed in the cavity, can be in one piece or in several pieces. This method is very much an art for independent custom rubber moulders.
Since flash often spills onto the surface during compression, it is possible that a large undercut area between the flash groove and the outside of the mold can ‘fine tune’ back pressure control. A large ground clearance restricts flow by the time the mold is nearly closed and thus could increase back pressure, which would help with low viscosity compounds. For high viscosity materials the reverse could apply, ie a small area and deep flash grooves would be desirable. This would also promote a higher pressure at the moment before the complete closure of the mold by the same force exerted by the piston of the press. The radial grooves that connect the flash grooves to the outside of the mold should also aid in high viscosity compounds coming out of 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 close properly. The goal is to get a fine flare, ‘ideally’ around 0.05mm.
The area of the press rams, divided by the projected area of rubber and flash between the mold halves, multiplied by the line pressure in the press, will give the pressure exerted on the product in the mold at the time of closure. The required pressure is typically 7 to 10.5 MPa and will vary depending on factors such as the viscosity of the compound and the complexity of the mold cavity. The mold is designed to withstand the high stress involved.
The projected rubber area may be less at the beginning of the mold closing, since the rubber has not yet spread completely throughout the mold cavity. More of the ram force could act briefly on delicate inserts or mold parts, depending on the exact setup 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 molding. The rubber in the cavity is experiencing large changes in temperature, which translates into viscosity variations that continually alter the flow characteristics of the compound. In recent years, mold designers have made available finite element analysis packages, which describe material flow patterns in the mold. 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 at least fifteen times that of the steel mould. In the case of custom moldings with large cross-sections or high volume-to-surface-area ratios, such as a ball, phenomena such as backrind can occur. The product when removed from the mold looks chewed and torn in the area of the flash line; this is described as backrind. If this occurs, there is likely to be a flurry of activity between the shift foreman, the chemist, and the engineer. These are the skills of independent custom rubber moulders.
Backrind is believed to occur because as the rubber heats up (heat is transferred from the mold to the outer layers of the compound first), the outer layers of the molding cure first, while the cooler inner layers and uncured are still heated and attempt to thermally expand. . Since the inner layers are constrained by the closed mold and the outer layer of cured 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 break in the ‘cured’ parting line; the mold will instantly close again. If this happens several times during healing, it is called chattering.
Another theory is that at the end of the curing time, at the instant the press opens, the removal of the external clamping force instantly releases the internal pressure of the product, slightly opening the mold and causing a break in the parting line. of the mold. vulcanize. Sometimes only some areas of the parting line are affected, suggesting that in these cases the mold opens unevenly.
Possible solutions that could alleviate the barking and chattering problem are:
a) Pre-heating of the preform.
b) Design a ‘sacrificial’ section in the product in which a crust will be produced between this section and the evaporation line. This section is then removed after curing, leaving only a small imperfection where it connects to the product.
c) A more intriguing idea is to drill 6mm holes through the mold into the cavity, in a less important section of the product. As the compound heats and expands in the heated closed mold, it extrudes freely through these holes; in a large product, the uncured compound can be extruded for quite some time (this can be analogous to moving water not freezing in a frozen stream). The mold is designed so there is still enough back pressure to allow air and product to flow into the flash slots. This latter method could be used for large products, 11 kg or more in weight, as back crust is a more serious problem on larger products.
d) For certain simple product geometries, it is possible to put an amount of rubber in the mold, 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 too much internal pressure. This would require precise control of the preform dimensions and would mean that the closed mold is not completely airtight.
e) A compound formulated for a long burn time could delay the cure of the outer layers during thermal expansion, thus avoiding any rupture of these layers during the supposed instantaneous opening of the mold during curing.
f) Reducing the curing temperature would decrease the thermal expansion or possibly, indeed, increase the burn time of the compound. This would be at the cost of a longer curing time.
g) Cooling the mold after curing, before reducing the pressure applied by the press and then opening the mold, could reduce the internal pressure and thus possibly reduce the skin.