Brief Discussion on 25 Molding Processes of Composite Materials

 

The process flow is as follows:

Production  Preparation

Site The size of the hand lay-up molding work site should be determined according to the product size and daily output. The site should be clean, dry, and well ventilated. The air temperature should be maintained between 15 and 35°C. The post-processing and finishing section should be equipped with exhaust dust removal and water spraying devices. Mold preparation The preparation work includes cleaning, assembly, and applying release agent. Resin glue preparation When preparing, pay attention to two issues:

① Prevent bubbles from mixing into the glue;

② The amount of glue should not be too much, and each amount should be guaranteed to be used up before the resin gels. Reinforcement material preparation The type and specification of the reinforcement material are selected according to the design requirements.

 

Pasting and Curing Laying Paste Hand laying Paste is divided into Wet Method and Dry Method:

 

① Dry lamination

Use pre-impregnated cloth as the raw material. First, cut the pre-learned material (cloth) into bad material according to the sample. Heat and soften it during laying, and then stick it to the mold layer by layer, and pay attention to remove the bubbles between the layers to make it dense. This method is mostly used for hot-pressing and bag-pressing molding.

② Wet Laying

Directly dip the reinforcing material into glue on the mold, and stick it to the mold layer by layer, remove bubbles, and make it dense. Generally, this method is used for laying layers in hand lay-up process. Wet laying is divided into gel coat layer paste and structural layer paste. Hand lay-up tools Hand lay-up tools have a great influence on ensuring product quality.

There are wool rollers, bristle rollers, spiral rollers, electric saws, electric drills, grinding and polishing machines, etc. Curing The curing of products is divided into two stages: hardening and maturation. Resin transfer molding is referred to as RTM (Resin Transfer Molding). It usually takes 24 hours from gel to triangulation.

 

At this time, the curing degree reaches 50% to 70% (Barcol hardness is 15) and can be demolded. After demolding, it takes 1 to 2 weeks of curing under natural environment conditions to make the product have mechanical strength, which is called maturation. Its curing degree reaches more than 85%. Heating can promote the aging process.

For polyester fiberglass, heat at 80℃ for 3h. For epoxy fiberglass, the post-curing temperature can be controlled within 150℃. There are many heating curing methods. Small and medium-sized products can be cured by heating in a curing furnace. Large products can be heated in the mold or by infrared heating.

 

Demolding and Trimming Demolding must ensure that the product is not damaged.

 

There are Several Demoulding Methods:

① Ejection demoulding

Pre-embed the ejection device on the mold. When demoulding, turn the screw to eject the product.

② Pressure demoulding

There is a compressed air or water inlet on the mold. When demoulding, press the compressed air or water (0.2MPa) between the mold and the product. At the same time, use a wooden hammer and a rubber hammer to knock to separate the product from the mold.

③ Demolding of large products (such as ships) can be done with the help of tools such as jacks, cranes and hardwood wedges.

④ Complex products can be demoulded manually. First, paste two or three layers of fiberglass on the mold, peel it off from the mold after solidification, and then put it on the mold to continue pasting to the designed thickness. After solidification, it is easy to remove from the mold.

 

Trimming There are two types of trimming: one is size trimming, and the other is defect repair.

① Size trimming After the molded product, cut off the excess part according to the designed size;

② Defect repair includes perforation repair, bubble, crack repair, hole reinforcement, etc.

 

Spray Molding Technology is an improvement of hand paste molding, with a semi-mechanized degree. Spray molding technology accounts for a large proportion of composite material molding technology, such as 9.1% in the United States, 11.3% in Western Europe, and 21% in Japan. At present, the spray molding machines used in China are mainly imported from the United States.

 

Principle and Advantages and Disadvantages of Injection Molding Process

The injection molding process is to spray two polyesters mixed with initiator and promoter from both sides of the spray gun respectively, and at the same time spray the cut glass fiber roving from the center of the spray gun to make it evenly mixed with the resin and deposited on the mold. When it is deposited to a certain thickness, it is compacted with a roller to make the fiber soaked in resin, remove bubbles, and solidify into a product.

 

Advantages of Injection Molding:

① Using glass fiber roving instead of fabric can reduce material costs;

② The production efficiency is 2 to 4 times higher than that of hand lay-up;

③ The product has good integrity, no seams, high interlayer shear strength, high resin content, good corrosion resistance and leakage resistance;

④ It can reduce the consumption of flash, cloth scraps and residual glue;

⑤ The product size and shape are not restricted.

 

Its disadvantages are:

① The resin content is high and the product strength is low;

② The product can only be smooth on one side;

③ Pollute the environment and harm the health of workers. The injection molding efficiency reaches 15kg/min, so it is suitable for large-scale ship hull manufacturing. It has been widely used in the processing of bathtubs, machine covers, integrated toilets, automobile body components and large relief products.

 

Production preparation

In addition to meeting the requirements of hand lay-up process, the injection molding site should pay special attention to environmental exhaust. According to the size of the product, the operation room can be built into a closed type to save energy. Material preparation The raw materials are mainly resin (mainly unsaturated polyester resin) and untwisted glass fiber roving. Mold preparation The preparation work includes cleaning, assembly and application of release agent.

 

Injection Molding Equipment Injection Molding Machines are divided into pressure tank type and pump supply type:

 

① Pump supply injection molding machine, which is to pump the resin initiator and accelerator into the static mixer respectively by pump, and then spray them out by spray gun after sufficient mixing, which is called in-gun mixing type.

Its components include pneumatic control system, resin pump, additive pump, mixer, spray gun, fiber cutting injector, etc. The resin pump and the additive pump are rigidly connected by the rocker arm. The position of the additive pump on the rocker arm can be adjusted to ensure the proportion of ingredients.

Under the action of the air compressor, the resin and additive are evenly mixed in the mixer, and droplets are formed through the spray gun, which are continuously sprayed onto the mold surface with the cut fibers.

This spray machine has only one glue spray gun, which is simple in structure, light in weight, and has less initiator waste. However, due to the internal mixing, it must be cleaned immediately after use to prevent spray blockage.

 

② The pressure tank glue supply spray machine is to place the resin glue in the pressure tank separately, and rely on the gas pressure entering the tank to make the glue enter the spray gun and spray continuously. The machine is composed of two resin tanks, pipes, valves, spray guns, fiber cutting sprayers, trolleys and brackets.

When working, connect the compressed air source, so that the compressed air enters the resin tank, glass fiber cutter and spray gun through the gas-water separator, so that the resin and glass fiber are continuously sprayed from the spray gun, the resin is atomized, the glass fiber is dispersed, and the mixture is evenly mixed and then sinks to the mold. This type of spray machine mixes the resin outside the spray gun, so it is not easy to clog the spray gun nozzle.

 

Spray Molding Process control Spray process parameter selection:

① Resin content

In the spray molded products, the resin content is controlled at about 60%.

② Spray pressure

When the resin viscosity is 0.2Pa and the resin tank pressure is 0.05~0.15MPa, s, the atomization pressure is 0.3~0.55MPa, to ensure that the components are mixed evenly.

③ Spray gun angle

The resin sprayed at different angles has different mixing distances. Generally, a 20° angle is selected, and the distance between the spray gun and the mold is 350~400mm. When changing the distance, the spray gun angle should be increased at a high speed to ensure that the components are mixed near the mold surface to prevent the glue from flying away.

 

Precautions for Injection Molding:

① The ambient temperature should be controlled at (25±5)℃. If it is too high, it will easily cause the spray gun to be blocked; if it is too low, the mixing will be uneven and the curing will be slow;

② No moisture is allowed in the spray system, otherwise it will affect the product quality;

③ Before molding, spray a layer of resin on the mold first, and then spray the resin-fiber mixed layer;

④ Before injection molding, adjust the air pressure first to control the resin and glass fiber content;

⑤ The spray gun should move evenly to prevent leakage, and it cannot go in an arc. The overlap between the two rows should be less than 1/3 to ensure uniform coverage and thickness;

⑥ After spraying a layer, immediately compact it with a roller. Pay attention to the edges and concave and convex surfaces to ensure that each layer is flattened, and bubbles are discharged to prevent fibers from being brought up and causing burrs;

⑦ After each layer is sprayed, it should be inspected and the next layer should be sprayed only after it is qualified;

⑧ The last layer should be sprayed thinner to make the surface smooth;

⑨ The spray machine should be cleaned immediately after use to prevent the resin from solidifying and damaging the equipment. ,

 

Resin Transfer Molding

Resin transfer molding is referred to as RTM (Resin Transfer Molding). RTM started in the 1950s and is a closed mold molding technology that improves the hand lay-up molding process. It can produce products with two sides of the light. In foreign countries, other processes that belong to this category include resin injection and pressure injection.

The basic principle of RTM is to lay the glass fiber reinforced material into the closed mold cavity, inject the resin glue into the mold cavity with pressure, soak the glass fiber reinforced material, and then solidify and demold the molded product.

From the current research level, the research and development direction of RTM technology will include microcomputer-controlled injection units, reinforced material preforming technology, low-cost molds, fast resin curing systems, process stability and adaptability, etc.

 

Features of RTM Molding Technology:

① Products with two sides can be manufactured;

② High molding efficiency, suitable for medium-scale FRP product production (within 20,000 pieces/year);

③ RTM is a closed mold operation, which does not pollute the environment and does not harm the health of workers;

④ Reinforcement materials can be laid in any direction, and it is easy to lay reinforcement materials according to the stress conditions of the product;

⑤ Less consumption of raw materials and energy;

⑥ Small investment in factory construction and fast start-up.

RTM technology has a wide range of applications and is currently widely used in industrial fields such as construction, transportation, telecommunications, health, and aerospace.

Developed products include: automobile shells and parts, recreational vehicle components, propellers, 8.5m long wind turbine blades, antenna covers, machine covers, bathtubs, showers, swimming pool boards, seats, water tanks, telephone booths, telephone poles, small yachts, etc.

 

RTM Process and Equipment

The entire RTM production process is divided into 11 steps. The operators, tools and equipment of each step are fixed in position. The mold is transported by a trolley and passes through each step in turn to achieve assembly line operation.

The cycle time of the mold on the assembly line basically reflects the production cycle of the product. Small products generally only take more than ten minutes, and the production cycle of large products can be controlled to be completed within 1 hour.

Molding equipment RTM molding equipment mainly includes resin injection machines and molds:

 

① Resin Injection Machine

The resin injection machine consists of a resin pump and an injection gun. The resin pump is a set of piston reciprocating pumps, and the top is an air-powered pump.

When compressed air drives the air pump piston to move up and down, the resin pump draws the resin in the barrel into the resin reservoir quantitatively through the flow controller and filter, and the lateral lever moves the catalyst pump to draw the catalyst quantitatively into the reservoir.

Compressed air fills the two reservoirs, generating a buffer force opposite to the pump pressure, ensuring that the resin and catalyst can flow stably to the injection gun head. There is a static turbulent mixer behind the injection gun, which can mix the resin and catalyst evenly in an airless state, and then inject them into the mold through the gun.

There is a cleaning agent inlet behind the mixer, which is connected to a solvent tank with a pressure of 0.28MPa. When the machine is used up, turn on the switch, the solvent will automatically spray out, and clean the injection gun.

 

②Mold

RTM molds are divided into three types: FRP molds, FRP surface-plated metal molds, and metal molds. FRP molds are easy to manufacture and have a low price. Polyester FRP molds can be used 2,000 times, and epoxy FRP molds can be used 4,000 times.

FRP molds with metal plating on the surface can be used more than 10,000 times. Metal molds are rarely used in the RTM process. Generally speaking, the mold fee of RTM is only 2% to 16% of that of SMC.

 

RTM raw materials The raw materials used for RTM include resin system, reinforcing materials and fillers.

Resin system The resin used in the RTM process is mainly unsaturated polyester resin. Reinforcement material Generally, the reinforcement material of RTM is mainly glass fiber, and its content is 25% to 45% (weight ratio); commonly used reinforcement materials include glass fiber Continuous Mat, composite mat and lattice cloth. Filler Filler is very important for RTM process.

It can not only reduce costs and improve performance, but also absorb heat during the exothermic stage of resin curing. Commonly used fillers include aluminum hydroxide, glass beads, calcium carbonate, mica, etc. Its dosage is 20% to 40%.

Bag pressing method, autoclave method, hydraulic kettle method and thermal expansion molding method molding

Bag pressing method, autoclave method, hydraulic kettle method and thermal expansion molding method are collectively referred to as low-pressure molding process.

Its molding process is to lay the reinforcement material and resin (including prepreg material) on the mold layer by layer according to the design direction and sequence by hand stacking. After reaching the specified thickness, the product is obtained by pressurization, heating, curing, demoulding and trimming.

The difference between the four methods and the hand lay-up molding process is only the process of pressurization and curing. Therefore, they are just improvements on the hand lay-up process, in order to improve the compactness and interlayer bonding strength of the products.

High-performance composite products made of high-strength glass fiber, carbon fiber, boron fiber, aramid fiber and epoxy resin by low-pressure molding have been widely used in aircraft, missiles, satellites and space shuttles. Such as aircraft doors, fairings, airborne radar covers, brackets, wings, tails, partitions, wall panels and stealth aircraft.

 

Bag Pressing Method

Bag pressing method is to apply gas or liquid pressure to the uncured products formed by hand lay-up through rubber bags or other elastic materials, so that the products are compacted and cured under pressure.

 

The advantages of bag pressing method are:

①The product is smooth on both sides;

②It can adapt to polyester, epoxy and phenolic resins;

③The product weight is higher than hand lay-up.

 

Bag pressing is divided into two types: pressure bag method and vacuum bag method:

① Pressure Bag Method

The pressure bag method is to put the uncured hand-layup product into a rubber bag, fix the cover plate, and then introduce compressed air or steam (0.25-0.5MPa) to cure the product under hot pressing conditions.

② Vacuum Bag Method

This method is to cover the uncured hand-layup product with a layer of rubber film, place the product between the rubber film and the mold, seal the periphery, and evacuate (0.05-0.07MPa) to remove bubbles and volatiles in the product. Since the vacuum pressure of the vacuum bag molding method is relatively low, this method is only used for wet molding of polyester and epoxy composite products.

 

Hot Autoclave and Hydraulic Autoclave Method

Both the hot autoclave and hydraulic autoclave methods are processes in which uncured hand-layup products are heated and pressurized by compressed gas or liquid in a metal container to cure and shape them.

 

Autoclave Method

The autoclave is a horizontal metal pressure vessel. The uncured hand-laid product is vacuumed with a sealing rubber bag, and then pushed into the autoclave together with the mold by a trolley. Steam (pressure is 1.5-2.5MPa) is introduced and vacuumed. The product is pressurized and heated to expel bubbles and solidified under hot pressing conditions. It combines the advantages of the pressure bag method and the vacuum bag method, with a short production cycle and high product quality.

The autoclave method can produce high-quality, high-performance composite products with larger sizes and complex shapes. The product size is limited by the autoclave. At present, the largest autoclave in China has a diameter of 2.5m and a length of 18m. The products that have been developed and applied include wings, tails, and satellite antenna reflectors