The core mold determines the basic geometric dimensions of the part, so during the winding and curing process, the core mold must be able to support the uncured composite material to deform it within the allowable accuracy range. Permanent mandrel: It is part of the manufactured part. For example, some storage tanks that require gas isolation can be prepared by winding fibers on thin metal walls, where the role of thin metal walls is mainly to isolate gases. The removable mandrel requires that it can be separated from the part without curing the part after curing. When the core mold is reused, the core mold can be separated from the part under the condition of maintaining the integrity of the part and the core mold after curing.
(1) Metal core mold
Permanent mandrels and reusable mandrels are made of metal materials. Usually permanent mandrels are used in high-pressure vessels. Since most of the composite materials cannot completely block gas, especially helium, the role of the metal core mold in the container is to prevent leakage. One advantage of this structure is that it eliminates the need to remove the mandrel after the part has cured. Metal materials that can be used as mandrels include nickel-based alloys and titanium alloys. Reusable metal mandrels are widely used in the solidified rocket engine industry. These mandrels are generally frame structures that provide precise internal dimensions and geometry for the solid rocket motor casing during the winding process. After curing, remove the mandrel and take it out through the large open end of the engine case. The metal core mold used for winding the fiberglass tube and the launch tube is a straight or slightly tapered metal tubular core mold.
(2) Expansion mandrel
During curing, all mandrels expand when heated. However, some mandrels, especially rubber mandrels, can expand significantly from the inside to provide shape or pressure to the composite parts during curing. An application example of the expansion mandrel is the preparation of petroleum storage tanks. The thick rubber skin is made into a spherical shape, blown to a predetermined diameter with air, sprayed with a release layer, and then wrapped with a polyester fiberglass layer by planar winding technology. After curing at room temperature, release the pressure in the rubber bag and remove it from the top opening. Another application is to make a thin rubber bag on a male mold. When the winding is completed, put it in a closed female mold. The compressed air is used to pressurize the interface between the male mold and the rubber bag so that the winding part The outer tightly closes the closed female mold, resulting in high dimensional accuracy and a smooth outer surface. However, the internal dimensions of the parts cannot be precisely controlled.
(3) Disposable mandrel
The earliest disposable mandrel was a gypsum mandrel. The biggest disadvantage of the gypsum core mold is that the cured gypsum absorbs water significantly. As the temperature rises during the curing of the resin, moisture will escape from the gypsum and enter the resin matrix, thereby reducing the performance of the composite. Sealing the insulating layer does not completely prevent moisture from escaping during resin curing.
Another disposable mandrel is a soluble mandrel. The initial soluble mandrel is obtained by pouring a paste salt into the mold and heating it to lose water and harden it. After the part is cured, the mandrel dissolves in water. The salt core mold is very easy to damage. Later, using ultrafine sand and polyvinyl alcohol (PVA) to make the mandrel. However, the relative density of this mandrel is relatively large; above 150 ° C, PVA reacts and becomes insoluble in water. Recently, sodium sulfide or water glass has been used as the binder instead of PVA, and the maximum operating temperature can reach 343 ° C. This type of mandrel has been used in the winding of polyimide resin systems.