Latex bladders are produced on mandrels that are dipped in a dispersion of natural rubber in water. Bladders for curing most advanced composites laminates are generally .030” to .040” thick. Thicker bladders can be produced, but the time element increases drastically. 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 is done in a water-based solution that is at about 100°F. Aluminum or stainless steels are the preferred materials. The mandrels must also be strong enough to withstand the force applied while stretching the rubber bladder to remove it from the mandrel.
Size of the part.
The dipping equipment at PIERCAN USA, Inc. will support lengths of about 58” ; a reasonable size for many composite parts. The tanks themselves are about 30” * 30” at the top, and about 60” deep. Larger size bladders can be made by seaming, by bending or modifying the shape of the mandrel to fit into the tanks.
Dipping mandrels are almost always male molds and the open end becomes the air inlet tube used for inflation by the customer. The inlet is usually much smaller so it has to be stretched to remove the bladder from the mold. A good rule for designing the air inlet for a bladder is to try to limit the stretch to around 400%.
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 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’s best to fit the bladder as close to the inside of the composite lay-up as possible. This greatly reduces failures.
Latex bladders have been used with composite curing up to over 300 psi., although standard compressed air line pressures are more common.
PIERCAN USA, Inc. has developed a formulation for natural rubber that can be re-useable for many composite curing applications. 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.