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Conventional silicone or latex bladders are not rigid enough to support composite lay-up before custom molding. Often material must be applied to the inside of a female mold first, an inflatable bladder inserted, the mold closed, and the bladder pressurized and inflated to consolidate the material against the mold surface. This process is labor intensive, can produce seams, wrinkles and bridging, and often results in inaccurate part shapes. These drawbacks have led to the theory of an inflatable bladder core used as the mandrel, but conventional bladder cores lack the structure required for the composite lay-up process.

Shape memory polymer (SMP) bladders operate first as rigid mandrels and later as inflatable bladder cores. First providing a rigid, durable surface for composite lay-up, SMP bladders then provide flexibility and inflatability when the part is formed against the interior of the final mold.

How SMP Bladders Work

When heated above their activation temperature, SMP bladders become flexible and can be molded into a near-net bladder shape. When cooled while still under pressure in the near-net mold, they become rigid in the new shape. The resulting rigid bladder is used as a mandrel for lay-up or filament-winding of a composite part.

The composite lay-up on the SMP bladder is then inserted into a clamshell mold and subjected to air pressure and heat. The heat softens the SMP bladder, so it becomes flexible and inflates, consolidating the composite material against the mold, ensuring even expansion and precise outer surface dimensions on the part. The part is then cured while pressurized. Since the cure temperature of the composite is higher than the rigid-to-flexible transition temperature of the bladder, the bladder remains flexible and provides consistent pressure on the interior of the part.

Once the part is cured, the flexible SMP bladder is depressurized and contracts from the inner surface of the part. Because of its flexibility when heated, it can be removed easily and reused. Since the mold was completely closed when forming the part, wrinkling, bridging and pinching are less likely. The resulting precision part is seamless and requires no taping, sealing or pleating.

Benefits

• Suitable for hand and mechanical lay-up, filament winding, fiber placement, over-braiding and more
• No waste, no EPA restrictions, clean process
• Eliminates the need for part-specific tool retention and storage
• Easily extracted
• Reusable
• Reduces cycle time

Specifications

How SMP Bladders Are Incorporated into a Tooling Process:

1) The process begins with a rigid SMP bladder which has been molded into near-net shape based on the dimensions and requirements of the final composite part.	2) The rigid SMP bladder is first used as a mandrel for composite lay-up. This includes mechanical lay-up, filament winding, fiber placement, over-braiding and others.	3) The wet lay-up on the SMP bladder mandrel is inserted into a clamshell mold that represents the final part shape. Heat and pressure are applied, and the bladder becomes flexible, inflating against the interior of the composite, and pressing it against the interior of the female mold.
4) The composite part is cured at a temperature above the bladder’s rigid-to-flexible transition temp. The cure process does not adversely affect the bladder.	5) After the composite part is fully cured, the hot, flexible SMP bladder is depressurized and contracts, pulling away from the part.	6) The complex composite part is complete. The SMP bladder can then be remolded and reused.