Tooling and Design Considerations
Latex bladders are produced on mandrels that are dipped in a dispersion of natural rubber in water. The dipping mandrels have been chemically treated to attract the rubber molecules and build up a thickness of about .4 mm to .5 mm fairly quickly. Bladders for curing most advanced composites laminates are generally .020" to .030" thick. Thicker bladders can be produced, but the time element increases drastically above .060" After the dipping process, the bladder is cured in an oven at about 220° F. The latex shrinks 3% to 5% after cure. Bladders are manually removed from the mandrels by stretching the open end, blowing off with compressed air, or with some mechanical device.
Dipping Mandrels - The dipping mandrels must be impervious to water (and resistant to cleaning and other processing chemicals) and be able to withstand repeated cycles of heating to over 240° F. They must take the shock of being dipped while still hot into a water-based solution that is at about 100°F. The mandrels must also be strong enough to withstand the force applied while stretching the rubber bladder to remove it from the mandrel. This force can be considerable in some cases. Aluminum or stainless steels are the preferred materials. Ceramics and glass mandrels are too fragile for use in a production environment. Copper and brass cannot be used; it contaminates the liquid latex. Plastics and high temp epoxy casting resins work well for development work.
Size of the part - The dipping method can produce a latex bladder of any size, but it gets impractical to produce very large bladders because of the size of the latex tanks, the amount of material required, and the equipment needed to support the process. The equipment at LTI will support lengths of about 58"; a reasonable size for many composite parts. The tanks themselves are about 30" x 30" at the top, and about 60" deep. Larger size bladders can be made by seaming or by bending or modifying the shape of the mandrel to fit into the tanks.
Elongation - Ultimate elongation for natural rubber is about 900%. Dipping mandrels are almost always male molds and the open end becomes the air inlet tube used for inflation by the customer. The inlet or inflation tube is usually kept small so the bladder must be stretched to remove it from the mandrel. A good rule for designing the air inlet for a bladder is to try to limit the stretch to around 400%. Stretching an opening 600% to 700% can be accommodated, if necessary but adds to the cost of the bladder.
Shrinkage - In production, a good average shrink factor for bladders is 3% to 5%. However, there are many variables inherent in the dipping process that can increase this range. It is good practice to design dipping mandrels so that they can be adjusted during prototyping.
Shape of the Bladder – The ability to uniformly apply pressure to complex shaped parts is a primary reason for using a latex bladder. It is best to fit the bladder as close to the inside of the composite lay-up as possible. The high degree of elongation seems to imply that the shape need only fit the inside of the part approximately. However, bladders do not expand evenly under pressure and they stick to random areas of the lay-up. This can result in some sections of the bladder expanding to fit small areas of the cavity and exceeding the 900% ultimate elongation. Narrow or thin areas on parts are the most critical, like the edge of a propeller blade or the end of a golf shaft.
Costs - Raw material and labor for making latex bladders is relatively low, so the most important cost factor is factory overhead. The process time limits a dipping station to a fixed number of cycles per day so costs are based on this number of cycles divided into the total factory overhead. Therefore, if multiple mandrels can be dipped together, the cost is indirectly proportional to the number of mandrels dipped at once. The dipping station can handle from one to over a hundred mandrels at a time, so as more mandrels are fitted into the dipping tanks, the bladder cost is reduced.
Air Supply Details – Molding latex by dipping produces a thin uniform bladder. It is not practicable or cost effective for most recreation parts to incorporate or vulcanize special seals, valves fittings, etc. into the bladder. The air supply features are best incorporated into the mold design. The two most common ways the bladder is pressurized is with either an o-ring sealing device or with a flanged termination. There are dozens of variations of these two basic concepts.
Most bladders terminate in a round tube, and that tube can be run through a bored hole in the mold. Figure 1 shows a diagram of a typical connection. This is usually done on the split line of the mold with the seal made with O-rings on a line-drilled rod or with the lay-up mandrel itself.
Fig. 1. O-ring method of sealing round parts
Another method is to terminate the bladder with a flange type surface that works like a gasket, as in figure 2. This gasket is clamped between a surface on the mold and a flat plate with an air fitting supplies air to the cavity. The seal can be improved with an o-ring or bulb seal attached to one of the mating surfaces. The flange method takes more time to seal the bladder to the mold, and it also makes the mandrel occupy more space in the dipping station, so the cost per bladder may be greater.
Fig. 2. Flange sealing method.
Alternate air supply methods include devices such as expanding rubber plugs in the mold or hoses clamped directly to the bladder and restrained under pressure. These are typically prototype methods since they are often less reliable and slow.
Pressure – Latex bladders have been used with composite curing up to over 300 psi., although standard compressed air line pressures are more common. Bladders most often burst because of fit problems, not from excess pressure. Obviously, molds and fittings used for the higher pressures must be designed accordingly.
Temperature - Latex Technology has developed a formulation for natural rubber that can be re-useable for many composite curing applications. The natural rubber is degrading at temperatures over about 220°F but this degradation has been slowed down by formulating the rubber for more short-term temperature resistance. This high temperature resistant latex will last about 25-30 hours at 250°F and it declines to about one hour at 350° F. Wear and tear in a molding environment is often more a factor for the number of repeat cycles than temperature alone.
Latex Bladder Molding – Lessons Learned
High temperature latex bladders were first used in high volume production for making "bubble" golf shafts. These were shafts that were contoured and could not be made in the usual fashion since fabricating them on hard mandrels would lock the part to the mandrel. Since then, latex has become a useful tool for molding other types of parts, often for parts that would be difficult to mold by other methods. A lot has been learned about how to use this tool effectively and what tooling and processing techniques are applicable. The discussion below is based on feedback from users producing a variety of bladder molded products.
Fig. 3 Bladder fitted to ID of part
Bladder Sizing - The shape of the part being molded has a lot to do with how forgiving the fit of the bladder is to the inside of that part. Parts with long thin edges or small diameter tips can cause problems because the bladder tends to get pinched and rupture before filling the cavity. This can happen in a very small area.
Fig. 4 bladder too short; at risk of bursting
As the bladder expands under pressure, it does so unevenly and the expanded portion sticks to the wall of the laminate. Figure 3 is a diagram of a properly fitted bladder. The bladder in Figure 4 is at risk of bursting when pressure is applied. As air fills the bladder, the rubber sticks to the laminate and only the small area at the tip is left to expand. It gets progressively worse until a small area is left to expand the entire 900% until failure. This whole destructive process can sometimes occur in a surprisingly small area. Over 90% of bursting problems are resolved by correcting this issue. These problems diminish as production rates increase, bladder mandrels are modified, and technicians are trained.
Fig. 5 Bladder too big. OK if resin ridges are acceptable
Fig. 6 Open shapes are less risky with smaller bladders.
Bridging, Pinching and Seams - Unlike other bladder molding materials, latex has little resistance to being over-stretched as the pressure increases. Without being restrained by the mold or the lay-up, latex bladders will expand to failure. They are essentially like toy balloons, and will not "bridge" like nylon bagging films and nylon tube bladders. If folds in the lay-up or cracks in the mold itself are in contact with the bladder, it can be forced into the opening and burst, especially at the higher pressures.
Fig.7 The bladder is too short; only the end will stretch and likely fail.
Fig. 8 Under pressure the latex expands into small cavities and bursts
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