Cannon Afros has developed a new technology for the production of carbon fiber reinforced composite (CFRP) hollow structural components with built-in metal inserts. This technology uses a high pressure injection resin process, at the appropriate polymerization temperature, by differential pressure, using a high resistance low melting point of the extractable metal core material to produce.

In general, HP-RTM(High pressure resin transfer molding) composite components that are irregularly shaped or hollow structures with inserts placed inside may have a layer of carbon fiber reinforced composite (CFRP) on the outside and an exportable metal core on the inside.

The patent for this technology is pending, and in the meantime, it is already available for industrial production.

Historical development

The process of infiltrating carbon fiber composites has advanced rapidly in recent years because BMW needs to produce tens of thousands of composite parts in an efficient and repeatable way to make electric cars very light enough to meet the needs of people in large cities.

As a result, BMW and its composite component suppliers have invested huge sums of money in the development of the high-pressure resin transfer molding (HP-RTM) process.

This process uses high pressure to inject resin into a mold to infiltrate the fibers.

The rapidly curable polyurethane or epoxy material is first precisely measured and then mixed under high pressure through a mixing head mounted on the mold.

In this way, the cavity of the mold is the shape of the molded part. Therefore, it is usually the production of parts with a certain shape of three-dimensional sheet structure, rather than parts similar to hollow structures, because if you want to make the shape of the part toward the hollow shape, the fiber is easy to wrinkle.

Resin fills the mold cavity and infiltrates the carbon fiber sheet in the mold cavity, where the carbon fiber sheet can be several layers of carbon fiber fabric stacked together the sheet, the mold tonnage is thousands of tons (400 ~ 4000t).

All the air is pumped out from the closed mold, forming a vacuum in both the mold cavity and the fiber fabric laminate.

The resin is then injected into the mold cavity by a high-pressure mixing head, and then flows between the textures of the fabric until all the fiber sheets are completely saturated.

To avoid bubbles in the final product, a small amount of resin is injected to allow it to flow out of the joint of the mold.

The resin will enter the mold from the injection port of the mixing head, overcome the flow resistance, and gradually fill the mold cavity until it is completely soaked into the texture of the fabric.

As the resin diffuses in the mold cavity, it can be monitored that the pressure of the resin near the injection port gradually increases to a certain value. In the past, when the resin is filled with the mold cavity, the peak pressure is usually 80 to 100 bar. Recently, some customers have used the peak pressure of 130bar, so they will require the peak pressure to 200bar.

The final pressure value will always be higher, one of the reasons is that the reaction time of the resin has been shortening, and a product can now be extracted every 100s(previously every 5 to 6min).

The second is that the peak pressure is higher, and the resin of the molded part can be more compact, so the amount of resin in the composite material has reached about 40% to 45%. This resin ratio has been comparable to the resin ratio of aviation composite parts produced by autoclave processing prepreg.

Problems to be solved

Therefore, in order to produce high-quality carbon fiber reinforced composite (CFRP) structural parts using the high pressure resin transfer molding (HP-RTM) process, a unit pressure of at least 80bar must be applied to the surface of the carbon fiber laminate.

If it is a sheet structure of carbon fiber laminate, a press with enough tonnage can be used to compress the upper and lower dies of the mold to ensure that the pressure required for the surface of the carbon fiber laminate is achieved.

However, if you want to produce a hollow structure part, you need to strengthen its structural strength without adding too much weight. One way is to make two halves of a part first, and then glue them together; Another method is to place a lightweight core material or a heavy extrapolate core material in the hollow position for pressure, so as to avoid squishing the hollow part.

A light core (either balsa wood or foam) should not be extruded and the pressure should not exceed 15 to 20bars.

Alternatively, the extrapolable core can be made of a hard porous conglomerate, which can decompose and dissolve in water after the final part is molded.

The decomposed conglomerate powder can be discharged through small holes drilled in the surface of the carbon fiber composite material. But even this "conglomerate" cannot be held under more than 30bars.

Another way is to use a soft pressure bag filled with liquid to resist the pressure of the liquid resin (the liquid pressure bag is brittle and easy to deform, the volume can not be large, otherwise it is not easy to take out).

Thick liquid pressure bag will be pre-pressurized with the required peak pressure, but it may be too heavy to be removed from the molded parts, or it is difficult to take out the irregular hollow inner cavity, and the liquid pressure bag is difficult to go to the narrow corners of the cavity.

Cannon Afros' solution

uses a metal core, the shape of which can be exactly the same as the inner cavity shape of the final carbon fiber reinforced composite (CFRP) hollow structural component, and the final carbon fiber component is made, and then the metal core is exported.

The core also makes it easy to accommodate metal inserts or insert dowel pins into the cast.

This metal core can be made from a low eutectic alloy, and its melting point temperature is also lower than that of the general carbon fiber reinforced composite (CFRP) structural resin can withstand the temperature (usually curing temperature of about 130 to 140 бу C, can withstand a higher temperature of 30 бу C without damaging the final part).

It has been found that the melting point of several metal alloys is from slightly above room temperature to 250 бу C.

The metal alloy is poured into a mold in which the upper insert can be pre-fixed.

To avoid problems such as bubbles, shrinkage and adhesion, the die is pre-heated to a temperature close to the melting point of the eutectic alloy. A layer of release powder is applied inside the mold, which has openings and comes with extraction pins and gaskets to avoid excessive rough edges or waste of metal melt.

As soon as the metal core material is formed, it is immediately demoulded, burred edges and remove the release powder, check the correct position of the insert, and ensure that there are no bubbles on the surface and inside of the core material.

First lay several layers of fiber sheet correctly in the mold of high-pressure resin transfer molding (HP-RTM), and then put the core material into the mold of HP-RTM, and then wrap the core material with fiber laminated material.

It is important to note that the fibers are guided (placed in the right direction to give the part better support) and the fiber laminates need to be preformed to fit into the mold.

The core material is pressed onto the fiber laminate below, which is also laid with fiber laminate. Then, the mold of the HP-RTM is closed, at this time the vacuum seal of the mold is checked, and pressure is pushed into the outer surface of the resin-impregnated fiber.

First preheat the mold to the resin curing temperature (about 130бц), and then inject the resin under high pressure. At least one sensor is required near the injection port of the mold cavity to monitor the diffusion pressure. Sensors can also be installed at several points outside the mold to monitor the diffusion pressure. Some capacitive sensors can also be added to detect whether the resin has flowed to the corners or thin positions of the parts to ensure that the resin is full of the entire mold cavity.

Once the desired peak pressure (210 bar) is reached, the mixing head closes and enters recirculation mode. Wait for the curing time to end before the press is turned on and the part is removed. After

, there are two production methods to choose from:

The first production method is reheating, putting the part into a template guard, and then sending it into the heating furnace, heating the core material and air to the melting temperature of the eutectic alloy by radiation or induction. The molten eutectic alloy is discharged from the form guard through pre-drilled holes to form the inner cavity of the hollow composite component, and the insert is left in place, making it part of the carbon fiber reinforced composite (CFRP).

The second method of production is to cool the part so that the metal core remains inside the part, which can serve to strengthen the structure during later machining. The hard metal core is more conducive to the placement of the whole part and the operation of cutting and deburring of the part, because the metal core can reduce vibration and avoid deformation of the part during machining. After the standby processing is completed, the inspected parts are placed in a formwork guard and sent to the heating furnace to melt the low-eutectic alloy core material so that it can be completely discharged from the small holes.

In addition, the melting temperature of the low eutectic alloy (135 ~ 220бц) is low, if necessary, the part can be rotated, and all the low eutectic alloy can be drained, even if the low eutectic alloy liquid in the corner position of the part can be completely drained.

And if the cavity is filled with pressure bags or other materials, the pressurized material may not reach the corner position, or can not be completely discharged.

After the molten hypoeutectic alloy liquid is discharged, it can flow into a storage tank under the hot furnace for recovery, and finally pour back into the crucible of the hypoeutectic alloy for recycling.

This technology can also be used to produce cavity cores for large components in place of the original porous gravel cores. Although the lost wax molding process of the porous conglomerate can resist very high temperatures, its compression resistance is poor (the porous conglomerate can finally be discharged by vibration crushing or water solution).

In summary, carbon fiber reinforced composite (CFRP) components of hollow structure can be used in many fields, such as moving parts of manipulators, transmission mechanisms, irregularly shaped structural parts, non-cylindrical tanks, and liquid and compressed gas storage tanks.

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