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- Jul 04, 2019-

Cannon Afros has developed a new technology to produce hollow structural parts of carbon fiber reinforced composite (CFRP) structures with prefabricated metal inserts.The technology USES a high pressure resin injection process, under the appropriate polymerization temperature, through differential pressure, using a high resistance low melting point can be derived from the metal core.

In general, hp-rtm (high pressure resin transfer molding) composite components with irregular shapes or hollow structures with embedded parts can have a layer of carbon fiber reinforced composite (CFRP) on the outside and an exported metal core on the inside.

The technology is still being patented and, in the meantime, is ready for industrial production.

Historical development

The process of impregnating carbon fiber composites has developed rapidly in recent years, as BMW needs to produce tens of thousands of composite parts in an efficient and repeatable way to make very light electric cars for people in big cities.

BMW and its supplier of composite components have therefore invested heavily in the development of the high pressure resin transfer molding (hp-rtm) process.

The process involves injecting resin into molds at high pressure to soak the fibers.

Rapidly curable polyurethane or epoxy materials are accurately measured and high-pressure mixed through a mixing head mounted on a mold.

Thus, the mold cavity is the shape of the molded part.So, it's usually a three-dimensional piece that has a shape, not a hollow shape, because if you want to make the shape hollow, the fibers tend to wrinkle.

The resin fills the mold cavity and infiltrates the carbon fiber sheet inside the mold cavity. In this case, the carbon fiber sheet can be a sheet of several layers of carbon fiber fabric stacked together. The mold tonnage is several thousand tons (400 ~ 4000t).

All air is removed from the closed mold, creating a vacuum in the mold cavity and in the fabric stack.

The resin is then injected into the mold cavity through a high-pressure mixing head and flows between the fibers until all the fibers are completely saturated.

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

The resin enters the mold from the injection port of the mixing head, overcomes the flow resistance, and gradually fills the mold cavity until the texture of the fiber fabric is completely saturated.

When the resin diffuses in the mold cavity, the pressure of the resin near the injection port can be monitored to gradually increase to a certain value. Previously, when the resin fills the mold cavity, the peak pressure is usually 80 ~ 100 bar.

One of the reasons why the final pressure value is always higher is that the reaction time of the resin has been decreasing, and a product can be extracted every 100s(up from every 5 to 6 minutes).

Second, the pressure peak is higher, molding parts of the resin can be more compact, so the amount of resin in the composite material has reached about 40% ~ 45%.This resin ratio is already comparable to that of aero-composite components produced by prepreg processing in hot-press tanks.

Problems to be solved

Therefore, in order to produce high quality carbon fiber reinforced composite (CFRP) structures using high pressure resin transfer molding (hp-rtm) processes, a unit pressure of at least 80bar must be applied to the surface of the carbon fiber laminates.

In the case of a CFRP laminate, a press of sufficient tonnage can be used to press the upper and lower dies of the die to ensure that the pressure required on the surface of the CFRP laminate is met.

However, if hollow parts are to be made, they need to be structurally stronger without adding too much weight.Another method is to place a light core material or a heavy core material that can be derived at the center of the core to bear pressure, so as to avoid flattening the hollow parts.

Light core material (light wood or foam) cannot be exported, and the pressure cannot exceed 15 ~ 20bars.

Or a hard porous conglomerate can be used as a core material that can be exported, and the final component is molded so that the conglomerate can decompose and dissolve in water.

The decomposed conglomerate powder can be drained by drilling holes in the surface of the carbon fiber composite.But even this kind of conglomerate cannot be more than 30bars.

Another option is to use a soft, liquid-filled bag that resists the pressure of a liquid resin. (liquid bags are brittle, prone to deformation, and cannot be large enough to remove easily.)

Thick liquid pressure bags are prepressurized at the required peak pressure, but may be too heavy to remove from molded parts, or difficult to remove from irregular hollow cavities, and may have difficulty reaching the narrow corners of the cavities.

Cannon's Afros solution

The shape of a metal core material is identical to that of the hollow structural parts of the final carbon fiber reinforced composite (CFRP). After the final carbon fiber parts are made, the metal core material is derived.

The core is also convenient for accommodating metal inserts or for embedding pins in casting.

The core material can be made from an alloy with a low eutectic temperature, which is also lower than that of a typical CFRP structural resin (usually curing at about 130 to 140 ° c, and can withstand a further 30 ° c without damaging the final component).

Several metal alloys have been found to melt from slightly above room temperature up to 250℃.

Pour the metal alloy into a mold that can be pre-fixed with the insert.

To avoid problems such as bubbles, shrinkage and adhesion, the mold is pre-heated to a temperature close to the melting point of eutectic alloy.Apply a layer of demoulding powder in the mold with holes and extraction pins and gaskets to avoid excessive burr or waste of metal melt.

As soon as the metal core material is formed, the mold is removed immediately, the rough edges are removed and the mold powder is removed. Check the correct position of the insert part and ensure that there are no bubbles on the surface and inside of the core material.

First lay several layers of fiber sheets in the high-pressure resin transfer molding (hp-rtm) mold in turn, then put the core material into the hp-rtm mold, and then wrap the core material with fiber stack.

It is important to note that the fibers are oriented (placed in the right direction to give the components better support), and the fiber stacks are preformed into shapes suitable for use in molds.

The core material is pressed on the lower fiber laminate, and the upper fiber laminate is also laid.The hp-rtm mold is then closed, the vacuum tightness of the mold is checked, and the pressure is pushed into the outer surface of the resin soaked fiber.

Mould preheating first to resin curing temperature (about 130 ℃), high pressure injection resin, need at least one sensor near the mouth cavity injection to monitor diffusion pressure, can also be equipped with sensors to monitor at several point diffusion pressure, you can also add some have capacitive sensors to detect the resin flow to the position of parts or thin at the edges, to ensure that the resin fills the cavity.

Once the required peak pressure (210 bar) is reached, the mixing head closes and enters recirculation mode.After the curing time is over, the press opens and takes out the parts.

After that, there are two production modes to choose from:

The first mode of production is reheating, putting the parts into a formwork guard, and then into a heating furnace, heating the core material and air to the melting temperature of eutectic gold by radiation or induction.The molten, low-eutectic alloy is removed from the formwork guard through predrilled holes to form an internal cavity in a hollow composite component that remains in place as part of a carbon-fiber reinforced composite (CFRP).

The second way is to cool the parts, leaving the metal cores inside, which then strengthen the structure during machining.A hard metal core is more conducive to the placement of the whole part and the cutting and unburring of the part, because during machining, the metal core reduces vibration and prevents deformation of the part.After the processing is completed, the qualified parts are put into a formwork guard and sent to the heating furnace to melt the low eutectic gold core material, so that it can be completely discharged from the holes.

In addition, the melting temperature of low eutectic alloy is low (135 ~ 220℃). If necessary, the component can be rotated to empty all the low eutectic alloy. Even the liquid of low eutectic alloy at the corner of the component can be completely discharged.

If the cavity is filled with pressure bags or other materials, the pressure material may not reach the corners or be completely discharged.

The molten low-eutectic liquid is discharged and can be recycled into a storage tank below the hot furnace, and then back into the low-eutectic crucible for recycling.

It could also be used to produce hollow core materials for large components, replacing the original porous gravel core materials.Although the porous conglomerate can withstand very high temperatures, its compression resistance is poor (the porous conglomerate can eventually be removed by vibration or water dissolution).

In conclusion, hollow carbon fiber reinforced composite (CFRP) parts can be used in many fields, such as moving parts of manipulator, transmission mechanism, irregular structure parts, non-cylindrical tank, and storage tank of liquid and compressed gas.